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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 50
Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 52
Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 53
Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 54
Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 55
Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 56
Suggested Citation:"2 New Frontiers Mission Options." National Research Council. 2008. Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity. Washington, DC: The National Academies Press. doi: 10.17226/12175.
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Page 57

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2 New Frontiers Mission Options The National Research Council’s (NRC’s) 2003 solar system exploration decadal survey, New Frontiers in the Solar System: An Integrated Exploration Strategy, specified five mission candidates and ranked them according to priority: • Kuiper Belt Pluto Explorer, • South Pole-Aitken Basin Sample Return, • Jupiter Polar Orbiter with Probes, • Venus In Situ Explorer, and • Comet Surface Sample Return. To date there have been two New Frontiers missions selectedthe New Horizons mission to Pluto and the Kuiper Belt and the Juno mission to orbit Jupiter. Three missions remain from the original decadal survey list of potential New Frontiers missions: • South Pole-Aitken Basin Sample Return, • Venus In Situ Explorer, and • Comet Surface Sample Return. The committee believes that all three of these remaining missions are viable candidates for the New Frontiers Program; the committee also recommends expanding the list of mission candidates to include the five additional medium-size missions that were mentioned in the decadal survey but not ranked: • Network Science, • Trojan/Centaur Reconnaissance, • Asteroid Rover/Sample Return, • Io Observer, and • Ganymede Observer. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Wash- ington, D.C., 2003. 15

16 OPENING NEW FRONTIERS IN SPACE For consideration in the next New Frontiers Announcement of Opportunity, the committee offers descriptions of eight candidate missions and an innovative mission option. The eight mission options, described below in the same order as they appeared in the decadal survey, are followed by a description of the innovative mission option. No science prioritization is implied by their order. South Pole-Aitken Basin Sample Return The South Pole-Aitken Basin Sample Return mission, as described in the decadal survey, is an inner-solar- system mission to study basin-forming processes and impact chronology by returning samples from the deepest, most heavily crateredand, hence inferred to be the oldestimpact structure preserved on the Moon (Figure 2.1). Another goal of the mission is to use these returned samples to understand the nature of the Moon’s deep crust and upper mantle and the planetary processes that produced these features. These goals are to be accomplished through the intensive study of the returned materials in Earth-based laboratories. Heavy bombardment in the very early history of the solar system is a paradigm established from analysis of the samples returned by the Apollo and the Soviet robotic Luna missions. Careful site selection and the study of new samples of the Moon will result in detailed verification and extension of this central concept for the formation and early history of the terrestrial planets and its implications for the earliest appearances and evolution of life. In particular, this mission would allow a test of theories that have been proposed about the early impact history of the inner solar system, notably the “lunar cataclysm” model. FIGURE 2.1  South Pole-Aitken Basin. The basin is clearly visible in both the topography and iron projections, illustrating how a large impact affected the Moon. The black area at the pole was not imaged. SOURCE: Courtesy of Clementine Science Group, Lunar and Planetary Institute.

NEW FRONTIERS MISSION OPTIONS 17 Melting of planetary surfaces (magma oceans) during the early accretion process of planetary bodies in the inner solar system is an important concept resulting from detailed analysis of the Apollo and Luna samples in Earth-based laboratories. The detailed analysis of samples from the South Pole-Aitken Basin Sample Return mis- sion should verify and extend this central concept for the differentiation of the early planetary body into crust and mantle. Sample return allows samples to be analyzed with the most sophisticated instruments on Earth (many of which cannot be transported to the sampling location). And it has other benefits as well, providing the ability to share samples with many research teams for broad-based experimentation and the archiving of samples for analysis in the future when better instrumentation will exist. It is possible, for example, to conduct far more sophisticated analysis of Apollo samples today than it was when they were first returned to Earth. Background A South Pole-Aitken Basin Sample Return mission would directly address the following crosscutting themes and key questions identified in the decadal survey (numbering is taken from the decadal survey):  The First Billion Years of Solar System History 1. What processes marked the initial stages of planet and satellite formation? 3. How did the impactor flux decay during the solar system’s youth and in what way(s) did this decline influence the timing of life’s emergence on Earth? Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? In addition, the decadal survey identified this mission as providing scientific return in three categories: highly significant scientific return, very useful scientific return, and supporting scientific return:  • Highly Significant Scientific Return Past: What led to the unique character of our home planet? a. What are the bulk compositions of the inner planets and how do they vary with distance from the Sun? 1. Determine elemental and mineralogic surface compositions. 4. Determine interior (mantle) compositions. b. What is the internal structure and how did the core, crust, and mantle of each planet evolve? 2. Determine compositional variations and evolution of crusts and mantles. c. What were the history and role of early impacts? 1. Determine large-impactor flux in the early solar system and calibrate the lunar impact record. 3. Investigate how major impacts early in a planet’s history can alter its evolution and orbital dynamics. • Very Useful Scientific Return Past: What led to the unique character of our home planet? b. What is the internal structure and how did the core, crust, and mantle of each planet evolve? 3. Determine major heat-loss mechanisms and resulting changes in tectonic and volcanic styles. New Frontiers in the Solar System, p. 3, Table ES.1. New Frontiers in the Solar System, pp. 56-57, Table 2.1.

18 OPENING NEW FRONTIERS IN SPACE c. What were the history and role of early impacts? 2. Determine the global geology of the inner planets. Present: What common dynamic processes shape Earth-like planets? b. How do active internal processes shape the atmosphere and surface environments? 2. Determine absolute ages of surfaces. Future: What fate awaits Earth’s environment and those of the other terrestrial planets? d. What are the resources of the inner solar system? 2. Assess mineral resources. • Supporting Scientific Return Past: What led to the unique character of our home planet? a. What are the bulk compositions of the inner planets and how do they vary with distance from the Sun? 3. Measure oxygen isotopic ratios of the unaltered surface and atmosphere. b. What is the internal structure and how did the core, crust, and mantle of each planet evolve? 1. Determine horizontal and vertical variations in internal structures. d. What is the history of water and other volatiles and how did the atmospheres of inner planets evolve? 2. Determine the composition of magmatic volatiles. Present: What common dynamic processes shape Earth-like planets? c. How do active external processes shape the atmosphere and surface environment? 3. Quantify regolith processes on bodies with tenuous atmospheres. Future: What fate awaits Earth’s environment and those of the other terrestrial planets? b. How do varied geologic histories enable predictions of volcanic and tectonic activity? 1. Assess the distribution and age of volcanism on the terrestrial planets. c. What are the consequences of impacting particles and large objects? 1. Determine the recent cratering history and current flux of impactors in the inner solar system. Developments Since the Decadal Survey Significant advances have been made in modeling the early history of our solar system, in particular the timing of accretion of the large planets in the outer solar system and the dynamical effects of their subsequent orbital evolution. Recent work has shown that the earliest crust on Earth may have formed very early, potentially provid- ing a foothold for the early development of life. However, the intense bombardment during the earliest history of the solar system may have prevented or delayed the development of life until more quiescent times. It has long been known that the major basins on the Moon that have been dated are all around 4 billion years in age. A major question is whether all major basins formed in a 200-million-year time perioda “lunar cataclysm”or whether the basins that have been dated simply represent the end of a declining flux of large impacts starting around 4.5 billion years ago. Argon-argon dating of impact-produced glasses in lunar meteorites, which plausibly sample the entire lunar surface, suggests that there was a lunar cataclysm. If this is correct, some mechanism must be found that would explain how asteroids were dislodged from the main belt 500 million years after solar system formation. Two models that are not necessarily incompatible have been proposed. One model for the evolution of the outer solar system postulates that the eccentricities of Jupiter and Saturn were pumped up as they passed through 2:1 orbit:orbit resonances, sweeping resonances through the main belt and dislodging main belt asteroids. These asteroids then produced cataclysms on all terrestrial planets and satel- lites, including the Moon. See R. Gomes, H.F. Levison, K. Tsiganis, and A. Morbidelli, Origin of the cataclysmic Late Heavy Bombardment period of the ter- restrial plants, Nature 435:466-469; K. Tsiganis, R. Gomes, A. Morbidelli, and H.F. Levison, Origin of the orbital architecture of the giant planets of the solar system, Nature 435:459-461, 2005; and A. Morbidelli, K. Tsiganis, A. Crida, H.F. Levison, and R. Gomes, Dynamics of the

NEW FRONTIERS MISSION OPTIONS 19 The other model notes that the size-frequency distribution of the highland craters on the Moon is the same as that in the main belt and distinct from the modern population of near-Earth asteroids. The near-Earth asteroids seem to be responsible for cratering the lunar maria, i.e., more recently than 3.9 billion years ago.  Strom et al. postulate that Neptune, and possibly Uranus, were formed 500 million years after the formation of the other plan- ets, around 4 billion years ago. The resultant migration inward of Jupiter caused resonances to sweep through the main belt with the same consequences. Conclusions The committee concludes that the South Pole-Aitken Basin Sample Return mission remains a very scientifi- cally important mission that should be considered for the New Frontiers Program. Although the committee is concerned that NASA should not be too specific in defining how New Frontiers missions should be conducted, it has concluded that in this case, given the maturity of the science questions and the precise design of the mission as stated in the decadal survey, studying new samples from the Moon is a reasonable and irreducible requirement of the mission. Furthermore, the South Pole-Aitken Basin is the preferred lunar region to target for this mission. Other sample return sites may exist that can address the preponderance of the objectives for this mission; however, it is the responsibility of the proposer to convincingly defend the merits of an alternative site. After exhaustive and extended laboratory analysis of the Apollo and Luna lunar samples and meteorites from the Moon, no evidence of water in any form has been found in lunar rocks or soils. While the search for water on the Moon is not a science objective for this mission, returned samples from the deep crust or upper mantle may contain trace water. Discovery of water in returned lunar samples, even in the minutest quantities, would constitute a major scientific discovery. Mission-Specific Recommendations A South Pole-Aitken Basin Sample Return mission is tenable under the New Frontiers Program and can address many decadal survey objectives. The committee recommends that the South Pole-Aitken Basin Sample Return mission as described in the decadal survey remain a high priority for the New Frontiers Program. The committee has no changes to the decadal survey’s scientific objectives or engineering implementation of this mission. How- ever, the committee recommends that NASA not be overly prescriptive about specific approaches to address the scientific objectives. Instead, NASA should allow proposers to develop their own innovative approaches. The committee believes that the following science goals, not in priority order, should be established for this mission: • Elucidate the nature of the Moon’s lower crust and/or mantle by direct measurements of its composition and of sample ages; • Determine the chronology of basin-forming impacts and constrain the period of late, heavy bombardment in the inner solar system, and thus, address fundamental questions of inner solar system impact processes and chronology; giant planets of the solar system in the gaseous protoplanetary disk and their relationship to the current orbital architecture, The Astronomical Journal 134:1790-1798, 2007. See R.G. Strom, R. Malhotra, T. Ito, F. Yoshida, and D.A. Kring, The origin of planetary impactors in the inner solar system, Science 309(5742):1847-1850, 2005. See R.G. Strom, R. Malhotra, T. Ito, F. Yoshida, and D.A. Kring, The origin of planetary impactors in the inner solar system, Science 309(5742):1847-1850, 2005. As the committee was finishing its report, it learned of a presentation at the fall meeting of the American Geophysical Union that may indicate the presence of significant water in lunar volcanic glasses. See A.E. Sael, E.H. Hauri, M. Lo Cascio, J. Van Orman, M. Rutherford, and R. Cooper. Volatiles in the lunar volcanic glasses: Evidence for the presence of indigenous water in the Moon’s interior, AGU Fall 2007 Meeting.

20 OPENING NEW FRONTIERS IN SPACE • Characterize a large lunar impact basin through “ground truth” validation of global, regional, and local remotely sensed data of the sampled site; • Elucidate the sources of thorium and other heat-producing elements to understand lunar differentiation and thermal evolution; and • Determine the age and composition of farside basalts to determine how mantle source regions on the Moon’s farside differ from the basalts from regions sampled by Apollo and Luna. Venus In Situ Explorer A Venus In Situ Explorer mission would address fundamental unanswered questions of the history and current state of Venus through a characterization of the chemical composition and dynamics of the atmosphere of Venus, and/or measure surface composition and rock textures. While it is unlikely that all of the objectives delineated in the decadal survey could be addressed within the New Frontiers cost constraints, a mission that addresses a subset of these objectives would provide critical information about the present state and history of Venus. The current European Space Agency (ESA) Venus Express mission has greatly expanded knowledge of the upper atmosphere and exosphere of Venus, and has contributed to understanding of regions of the atmosphere nearer to the planet’s surface. However, characterization of the noble-gas and isotopic signatures of the well-mixed lower atmosphere would greatly expand understanding of the formation and evolution of the atmosphere of Venus, illuminate impor- tant elements of the current climate, including the drivers for the Venus greenhouse effect, and potentially provide insight on the early tectonic evolution of the planet. Prior landed missions of Soviet Venera and Vega spacecraft (Figure 2.2) have provided some information on crustal compositions and textures, but they have been confined to lowland areas composed of basaltic lava flows. Landed missions in highland regions or on older terrains could answer questions related to presence of silicic rock compositions or earlier phases of tectonism, but they present significant technological challenges. FIGURE 2.2  Image taken of the surface of Venus by Venera 13 in 1982. The Soviet Union successfully conducted several Venus lander missions with 1980s era technology. These images depict the distorting effects of the thick Venusian atmosphere. SOURCE: C.M. Pieters, J.W. Head, W. Patterson, S. Pratt, J.B. Garvin, V.L. Barsukov, A.T. Basilevsky, I.L. Khodakovsky, A.S. Selivanov, A.S. Panfilov, Y.M. Getkin, and Y.M. Narayeva, The color of the surface of Venus, Science 234:1379-1383, 1986. Reprinted with permission of AAAS.

NEW FRONTIERS MISSION OPTIONS 21 Background The science questions targeted by a Venus In Situ Explorer (VISE) mission directly address the following crosscutting themes and key questions identified in the decadal survey:  Volatiles and Organics: The Stuff of Life 6. What global mechanisms affect the evolution of volatiles on planetary bodies? The Origin and Evolution of Habitable Worlds 9. Why have the terrestrial planets differed so dramatically in their evolutions? Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? As noted in the decadal survey, a New Frontiers VISE mission should address a number of the following objectives, which were not prioritized: Science mission objectives for VISE are as follows: •  etermine the composition of Venus’s atmosphere, including trace gas species and light stable D isotopes; • Accurately measure noble gas isotopic abundance in the atmosphere; • Provide descent, surface, and ascent meteorological data; • Measure zonal cloud-level winds over several Earth days; • Obtain near-infrared descent images of the surface from 10-km altitude to the surface; • Accurately measure elemental abundances and mineralogy of a core from the surface; and • Evaluate the texture of surface materials to constrain weathering environment. The mission objectives are provided in the decadal survey in the context of atmospheric and surface science objectives:10 Atmospheric Science Objectives: The composition of the lower atmosphere of Venus is unknown. Without this knowledge, comparisons of the factors that affect climate on Earth and on Venus, including photochemistry, clouds, volcanism, surface-atmosphere interactions, and the loss of light gases to space, are impossible. VISE will mea- sure the abundance of trace gas species in the lower atmosphere of Venus to parts per million accuracy, enabling an understanding of how these processes affect terrestrial planetary climates. A fundamental quest is to understand how and why Venus, roughly the same size, composition, and distance from the Sun as Earth, has evolved to such a different state. The record of planetary atmospheres is contained in the isotope ratios of the most inert gases­—xenon, krypton, argon, and neon. Are planetary atmospheres the remnants of gases that were originally solar in composition but then suffered massive hydrodynamic escape, or did they require atmospheres from volatiles that had already been differentiated? What was the role of impacts on the ultimate compositions and evolution of the terrestrial planets? Discrimination New Frontiers in the Solar System, p. 3, Table ES.1. New Frontiers in the Solar System, p. 58. 10New Frontiers in the Solar System, p. 59.

22 OPENING NEW FRONTIERS IN SPACE between these events for each of the inner planets is possible if noble gas isotopic ratios can be measured with a state-of-the-art neutral mass spectrometer. Previous spacecraft measurements have been inadequate to address these issues. VISE will determine the noble gas abundances and isotope ratios to sufficient accuracy to distinguish between hypotheses of the origin and evolution of Venus’s atmosphere. A meteo- rological package will measure atmospheric pressure and temperature profiles down to the surface, and pressure, temperature, and winds at the surface. Cloud-level winds will be determined by tracking the ascent balloon during its 3.5-day lifetime, providing improved data on atmospheric dynamics and the origin of Venus’s mysterious atmospheric superrotation. Surface Science Objectives: The former Soviet Union’s Venera landers returned basic elemental chemistry and images of four sites on the surface, and Magellan data provided evidence of possible evolved volcanic deposits. However, we lack sufficient information on surface elemental abundances and mineralogy to determine the degree of crustal evolution on Venus. The VISE mission would measure elemental compositions at a surface site complementary to those of the Veneras. Mineralogy of a surface sample core will be obtained for the first time, allowing analysis of any weathered layer and testing for depth of alteration and occur- rence of unaltered material. Textural analysis of the sample using a microscope imaging system would provide information on the formation and nature of surface rocks. These data will be used to constrain questions outlined above. Despite global radar coverage of Venus by Magellan, little is known of the surface morphology at scales of 1 to 10 m. Without such information, it is difficult to determine how the plains formed and to understand the nature of mobile materials on the surface. A descent camera on the lander will provide the first broadscale visible images of the surface, with images returned from about 10 km altitude to the surface. These images will enhance interpretation of the Magellan radar images by providing ground-truth data on the surface texture of the lava flows that make up Venus’s plains. The morphology and texture of these flows can be related to emplacement rate, volatile content, and rheology, which are needed in order to understand the role of volcanism in shaping the atmosphere and surface of Venus. Images of Venus’s surface will also be returned from the lander, with filters chosen to provide compositional information. These images will help to determine the recent geological history of Venus and will resolve differences in the interpretation of Venus’s resurfacing history. Developments Since the Decadal Survey Since the decadal survey, NASA’s Venus Exploration Analysis Group (VEXAG) has worked with the Venus science community to develop the following Venus exploration goals, with prioritized objectives. 11 Goal 1: Origin and Early Evolution of Venus: How did Venus originate and evolve? The highest priority objectives are: 1.  etermine the elemental and isotopic composition of the atmosphere to identify earlier epochs of Venus’s D history, and clues to Venus’s origin, formation and evolution. 2.  ap the mineralogy and chemical composition of Venus’s surface on the planetary scale for evidence of M past environmental conditions and for constraints on the evolution of Venus’s atmosphere. 11The committee has reproduced only the top three VEXAG goals but notes that the VEXAG committee has produced a valuable document that can be used as a reference on Venus science objectives. This document, Venus Exploration Goals, Objectives, Investigations, and Priori- ties: 2007, is available at http://www.lpi.usra.edu/vexag/vexag_goals_2007.pdf.

NEW FRONTIERS MISSION OPTIONS 23 3.  haracterize the history of volatiles in the interior, surface and atmosphere of Venus, including volatile C additions due to cometary impacts, degassing and atmospheric escape, to understand the planet’s geologic and atmospheric evolution. Goal 2: Venus as a Terrestrial Planet: What are the processes that have shaped and still shape the planet? The highest priority objectives are: 1.  onstrain the coupling of thermochemical, photochemical and dynamical processes in Venus’s atmosphere C and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles. 2.  onstrain the resurfacing history of Venus, and the nature of the resurfacing processes, including the role of C tectonism, volcanism, impacts of asteroids or comets, sedimentation/erosion, and chemical weathering. 3.  onstrain the nature and timing of volcanic activity on Venus, including thermal evolution, current and C past rates of volcanic activity, and the effects of outgassing on atmospheric and interior processes. Goal 3: What does Venus tell us about the fate of Earth’s environment? The highest priority objectives are: 1.  earch for evidence of past global-climate changes on Venus, including chemical-and-isotope evidence S in the atmosphere, as well as rock chemistry and characteristics of surface weathering. In particular, seek evidence for the presence or absence of past oceans. 2.  earch for evidence of past changes in interior dynamics, volcanics and tectonics, including possible evo- S lution from plate tectonics to stagnant-lid tectonics, which may have resulted in significant changes in the global climate pattern. 3.  haracterize the Venus greenhouse effect, including the interplay of chemistry, dynamics, meteorology, C and radiative physics in the atmosphere, especially in the clouds. In addition, ESA’s Venus Express has entered Venus orbit and has returned new data since the decadal survey. Venus Express has expanded understanding of the upper atmosphere and exosphere and has contributed to knowl- edge of the mid- to lower atmosphere. However, most of the science objectives from the decadal survey require in situ measurements that are beyond the capabilities of an orbital mission such as Venus Express. 12 Conclusions The committee concludes that a VISE mission remains a very scientifically important mission that should be considered for the New Frontiers Program. The VEXAG goals and objectives align well with the New Frontiers Venus mission objectives, further validating the selection process in the decadal survey. Although these objec- tives address fundamental science themes for Venus exploration, it is unlikely that they can be fully addressed in a single mission. Cost and technology risk factors may preclude a single VISE mission proposal from addressing all of the objectives. Consequently, a mission that addresses a major subset of the objectives would be consistent with the recommendations of the decadal survey. For example, a successful mission might not have to include a landed component if it addressed the major atmospheric objectives. In addition, an interpretation of the decadal survey’s science objectives should prescribe the important data to be collected, rather than dictate measurement techniques or mission scenarios. While no attempt is made here to prescribe or define implementation strategies, potential challenges related to the Venus environmentsuch as high temperatures, high pressures, and a corrosive atmosphere in the near-surface environmentmay require the use of nontraditional (though previously demon- 12New Frontiers in the Solar System, p. 58.

24 OPENING NEW FRONTIERS IN SPACE strated) mobility systems such as balloons (a technology that also has some applications on other atmospheric bodies). The committee also notes that most of the technologies required to address the decadal survey objectives have been demonstrated on prior missions. For instance, Soviet-era Venus missions not only successfully reached the surface, but also operated there for up to an hour, proving that surface missions are possible. In the decadal survey, the VISE mission concept was discussed in terms of what it could contribute to a future flagship-class Venus sample return mission. While such an approach has significant merit, the committee warns that placing a technology demonstration for a future mission in the critical path of VISE mission success is unwise, particularly given the technical challenges for Venus sample return. Nonetheless, future Venus exploration beyond a VISE mission would require major technology development and demonstration, so that the inclusion of demonstration technologies in a VISE mission on a non-interference, non-critical-path basis is justified. Mission-Specific Recommendations The committee concluded that a VISE mission that addresses a significant number of the decadal survey objectives is tenable. Such a mission would make use of technologies that have been successfully demonstrated in prior missions to the Venus surface and near-surface environment. The committee also concluded that several of the VEXAG goals should be included with the goals established in the decadal survey, particularly the VEXAG goals concerning understanding the thermal balance of the atmosphere and gathering global mineralogic data. The challenges associated with landing in a region not previously sampled, collection of a sample, and lofting to a more clement altitude are the source of greatest technology and cost risk. Consequently, the New Frontiers announcement of opportunity should not preclude a mission that addresses the major goals for chemical sampling of the mid- to lower atmosphere on Venus and characterizing atmospheric dynamics, but lacks a surface sampling component. On the other hand, a mission that only addressed surface sampling would not be acceptable. The science goals for this mission, which are not in priority order, should be to: • Understand the physics and chemistry of the atmosphere of Venus through measurement of its composition, especially the abundances of sulfur, trace gases, light-stable isotopes, and noble-gas isotopes; • Constrain the coupling of thermochemical, photochemical, and dynamical processes in the atmosphere of Venus and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles; • Understand the physics and chemistry of the crust of Venus, for example, through analysis of near-infrared descent images from below the clouds to the surface and through measurements of elemental abundances and mineralogy from a surface sample; • Understand the properties of the atmosphere of Venus down to the surface through meteorological mea- surements and improve understanding of zonal cloud-level winds on Venus through temporal measurements over several Earth days; • Understand the weathering environment of the crust of Venus in the context of the dynamics of the atmo- sphere and the composition and texture of surface materials; and • Map the mineralogy and chemical composition of the surface of Venus on the planetary scale for evidence of past hydrologic cycles, oceans, and life and constraints on the evolution of the atmosphere of Venus. COMET SURFACE SAMPLE RETURN Scientific community interest in a Comet Surface Sample Return (CSSR) mission has been very high for many years. The advantages of such a mission have been stated in many documents including the decadal survey. Flyby missions to comets are fairly simple, and the Deep Space-1, Stardust, and Deep Impact missions have produced remarkable data. Rendezvous missions such as the ESA’s Rosetta mission (Figure 2.3) are more challenging, and a sample return mission can take twice as long as a rendezvous mission, thereby increasing cost and risk. The decadal survey concluded that bringing back a warm (i.e., non-cryogenic) sample was within a New Frontiers mission budget. While cometary science goals make the return of a cryogenic core sample highly desirable, such

NEW FRONTIERS MISSION OPTIONS 25 FIGURE 2.3  The European Space Agency’s Rosetta mission to a comet. This is a rendezvous and landing mission. Sample return would double the length of such a mission and add additional risk. SOURCE: Courtesy of European Space Agency. a mission may not fit within the fiscal limits and programmatic timescale of the New Frontiers Program. The sci- ence yield from a warm sample return mission will have to be strongly defended by proposers. Background The decadal survey recommended that a comet mission be included in the New Frontiers Program: 13   The Comet Surface Sample Return (CSSR) mission will collect materials from the near surface of an active comet and return them to Earth for analysis. These samples will furnish direct evidence on how cometary activity is driven. 13New Frontiers in the Solar System, p. 6.

26 OPENING NEW FRONTIERS IN SPACE Information will be provided on the manner in which cometary materials are bound together and on how small bodies accrete at scales from microns to centimeters. By comparing materials on the nucleus against the coma’s constituents, CSSR will indicate the selection effects at work. It will also inventory organic materials in comets. Finally, CSSR will yield the first clues on crystalline structure, isotopic ratios, and the physical relationships between volatiles, ice, refractory materials, and the comet’s porosity. These observations will give important information about the building blocks of the planets. The rationale for such a mission was well described in “Primitive Bodies,” Chapter 1 of the decadal survey. The key questions addressed by this mission were summarized in the decadal survey and are given below: 14 The First Billion Years of Solar System History 1. What processes marked the initial stages of planet and satellite formation? Volatiles and Organics: The Stuff of Life 4. What is the history of volatile compounds, especially water, across the solar system? 5. What is the nature of organic material in the solar system and how has this matter evolved? Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? Developments Since the Decadal Survey The nucleus images of Halley, Borrelly, Wild, and Tempel 1 have shown the diversity of comets. The Deep Impact mission showed that one portion of the surface of Tempel 1 contained very fine particles and gave clues to the organic composition. Consideration of the diversity of comets has added a new dimension to cometary research. In addition, the ESA Rosetta mission, launched in March 2004, will reach its target, 67P/Churyumov-Gerasi- menko, in 2014. The Stardust mission was launched in February 1999 and returned to Earth with a sample from a comet in January 2006. The returned samples from these two comets show that comet dust has minerals that formed near the Sun or other stars, which indicates that material ejected by the early Sun can travel to the outermost reaches of the solar system where comets formed. Stardust data also indicate that comets may resemble asteroids more than scientists previously believed.15 Finally, comets have been discovered in the asteroid belt. Unlike other comets previously studied, these main- belt comets appear to have formed in the much warmer inner solar system where they are found today and might, therefore, contain ice that is quite different in chemical and isotopic composition from that in other comets. Conclusions The committee concluded that the CSSR mission remains a highly scientifically important mission that should be considered for the New Frontiers Program. The questions posed in the decadal survey remain extremely relevant. The committee notes, however, that it would be a challenge to choose the “right” cometwith an effi- cient rendezvous and return trajectorythat is compatible with the fixed timescale dictated by the New Frontiers announcement of opportunity. 14New Frontiers in the Solar System, p. 3, Table ES.1. 15H.A.Ishii, J.P. Bradley, Z. Rong Dai, M. Chi, A.T. Kearsley, M.J. Burchell, N.D. Browning, and F. Molster, Comparison of Comet 81P/Wild 2 Dust with interplanetary dust from comets, Science 319(5862):447-450, 2008.

NEW FRONTIERS MISSION OPTIONS 27 Mission-Specific Recommendations For the next New Frontiers announcement of opportunity, the committee recommends that for the CSSR mission the decadal survey’s science goals should be used (not those of the 2003 New Frontiers announcement of opportunity). Although not all of them must be answered by a single mission, a CSSR candidate mission should seek to answer the following science questions, which are not in priority order: 16 • What is the elemental, isotopic, organic, and mineralogical composition of cometary materials? • How is cometary activity driven? • How do small bodies accrete? • What are the scales of physical and compositional heterogeneity? • How are the particles on a cometary nucleus bound together? • What are the macroscopic mineralogical and crystalline structure and isotopic ratios in cometary solids? The committee further recommends that the New Frontiers announcement of opportunity should leave the choice of target comet to the proposer and that the choice of target should be a major factor for evaluation. Finally, the committee notes that proposers for warm comet sample return missions must demonstrate that significant progress toward the goals of the decadal survey will be achieved by noncryogenic sample return. Network Science Network Science missions to study the interiors of Mercury, Venus, and the Moon, with geophysics measure- ments as a primary objective, would address fundamental science questions. Although remote measurements of a planet’s gravitational field, magnetic field, and rotational state provide information on its internal structure, only seismologic observations can definitively determine the nature of a planet’s interior, including the size and physi- cal state of its metallic core, the thickness of the crust, spatial variations in crustal thickness, and the occurrence and locations of subsolidus phase changes and regions of partial melting in the crust and mantle. The required seismic data can be obtained only with a globally distributed seismic network (Figure 2.4). The interiors of Mer- cury, Venus, Mars, and the Moon are poorly characterized, and geophysical network missions to these bodies are needed to learn what is inside them. A geophysical network can also be supplemented to measure planetary heat flow, magnetic field, atmospheric properties and winds, climate variations, surface-atmosphere interactions, and surface mechanical and thermal properties. A variety of developments since the decadal survey, when combined with the strong initial rationale, elevate the Network Science mission concept to one for consideration. Background Discussions of network-based science missions appeared throughout the decadal survey. The decadal survey recommended the Mars Long-Lived Lander Network as its second-highest-priority Mars medium-class mission after the Mars Science Laboratory: “The highest-priority objectives for network science on Mars are the determina- tion of the planet’s internal structure, including its core; the elucidation of surface and near-surface composition as well as thermal and mechanical properties; and extensive synoptic measurements of the atmosphere and weather. In addition, atmospheric gas isotopic observations (to constrain the size of currently active volatile reservoirs) and measurements of subsurface oxidizing properties and surface-atmosphere volatile exchange processes will be valuable.”17 (The committee also notes that a Mars network mission was strongly endorsed in another National Research Council report, Assessment of NASA’s Mars Architecture 2007-2016.)18 Although specific network missions were not recommended for the other terrestrial planets besides Mars, 16This text is taken from several sections in the decadal survey. See New Frontiers in the Solar System, pp. 25, 180, 182-183, and 195. 17New Frontiers in the Solar System, p. 7. 18National Research Council, Assessment of NASA’s Mars Architecture 2007-2016, The National Academies Press, Washington, D.C., 2006.

28 OPENING NEW FRONTIERS IN SPACE FIGURE 2.4  Artist’s conception of a possible Mars Network Mission. Such a mission would involve placing several spacecraft on the surface of the planet which would send data through orbiting spacecraft that would relay it back to Earth. Mars is one possible site of a geophysical network, but not the only one. SOURCE: Courtesy of Jet Propulsion Laboratory and Centre National d’Études Spatiales. the decadal survey was quite clear about the importance of geophysical network science for these bodies. For Mercury, the decadal survey stated that “[b]asic information is needed on surface composition, internal structure, and distribution of mass, each of which provides important constraints on bulk major-element composition.” 19 The decadal survey advocated the emplacing of “a geophysical network (seismic, heat flow) to determine internal structure, distribution of heat-producing elements, lateral and vertical heterogeneity of crust and mantle, and the true density of the core. Geophysical network science would address how small bodies differentiate and how the bulk composition of Mercury is related to the composition of the terrestrial planets.” 20 For the Moon, the decadal survey said that “seismic data would resolve the internal structure, permitting a much-improved estimate of bulk composition.”21 For Venus, the decadal survey recommended “geophysical network science (seismic, heat flow) to determine internal structure, distribution of heat-producing elements, lateral and vertical heterogeneity of crust and mantle, and the possible existence of an iron-rich core. Geophysical network science would address how small planetary bodies differentiate, how the bulk composition of the Moon is related to the composition of Earth, and how planetary compositions are related to nebular condensation and planetary accretion processes.”22 These studies would contribute to understanding the South Pole-Aitken Basin. Seismology that provides both the structure of the pristine highland crust and the after-basin crust should aid in computing the energy, and probably the velocity, of the meteorite. For Venus, the decadal survey noted that “[g]eothermal heat flow measured at multiple locations to 19New Frontiers in the Solar System, p. 42. 20New Frontiers in the Solar System, p. 62. 21New Frontiers in the Solar System, p. 42. 22New Frontiers in the Solar System, p. 62.

NEW FRONTIERS MISSION OPTIONS 29 determine rates of heat flow within the planet and between the surface and atmosphere . . . [would] lead to better understanding of volcanism and tectonics of the crust and mantle.” 23 The Mars Long-Lived Lander Network satisfied some of the crosscutting themes identified in the decadal survey:24 The Origin and Evolution of Habitable Worlds 9. Why have the terrestrial planets differed so dramatically in their evolutions? Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? It is the committee’s view that in general a geophysical network could satisfy many of these crosscutting themes and key scientific questions. The committee merely notes that the decadal survey was more detailed in its discussion of network missions for Mars than it was in its discussion of network missions elsewhere in the solar system. Without the geophysical data from a network mission, the “foundation question” put forth by the Inner Planets Panel of the decadal surveyi.e., How do the compositions, internal makeup, and geologic history of the planets explain the formation and sustainment of habitable planetary environments? cannot be answered.25 The impor- tance of learning about the internal structure of the planets is a pervasive theme of the Inner Planets Panel. For example, the panel noted that “knowledge of the internal structure of the planets is fundamental to understanding their history after accretion. Key issues include dissipation of internal heat, core formation and associated issues concerning magnetic-field generation, distribution of heat-producing radioactive elements, and styles and extent of volcanism.”26 Under “Future Directions” the panel emphasized that seismic data for each of the inner planets are ultimately needed to constrain the structure, mineralogy, and composi- tion of the deep planetary interiors. Key investigations that address evolution of the crust, mantle, and core include the following: • Determination of the horizontal and vertical variations in internal structures, • Determination of the compositional variations and evolution of crusts and mantles, • Determination of the major heat-loss mechanisms and resulting changes in tectonic and volcanic styles, and • Determination of the major characteristics of iron-rich metallic cores (size and the nature of liquid and solid components).27 In the decadal survey section titled “A Long-Term Exploration Strategy for the Inner Planets,” the Inner Planets Panel recommended a focus on “essential network science” involving “the establishment of multiple surface sta- tions operating concurrently on a planet.”28 In addition, the decadal survey identified geophysical network science as providing scientific return in three categories: highly significant scientific return, very useful scientific return, and supporting scientific return:29 23New Frontiers in the Solar System, p. 62. 24New Frontiers in the Solar System, p. 3, Table ES.1. 25New Frontiers in the Solar System, p. 40. 26New Frontiers in the Solar System, p. 42. 27New Frontiers in the Solar System, p. 43. 28New Frontiers in the Solar System, p. 63. 29New Frontiers in the Solar System, pp. 56-57, Table 2.1.

30 OPENING NEW FRONTIERS IN SPACE • Highly Significant Scientific Return Past: What led to the unique character of our home planet? b. What is the internal structure and how did the core, crust, and mantle of each planet evolve? 1. Determine horizontal and vertical variations in internal structures. 4. Determine characteristics of Fe-rich metallic cores (size; liquid and solid components). Present: What common dynamic processes shape Earth-like planets? a. What processes stabilize climate? 1. Determine the general circulation and dynamics of the inner planets’ atmospheres. 3. Determine how sunlight, thermal radiation, and clouds drive greenhouse effects. b. How do active internal processes shape the atmosphere and surface environments? 1. Characterize current volcanic and/or tectonic activity and outgassing. • Very Useful Scientific Return Past: What led to the unique character of our home planet? a. What are the bulk compositions of the inner planets and how do they vary with distance from the Sun? 4. Determine interior (mantle) compositions. b. What is the internal structure and how did the core, crust, and mantle of each planet evolve? 2. Determine compositional variations and evolution of crusts and mantles. 3. Determine major heat-loss mechanisms and resulting changes in tectonic and volcanic styles. c. What were the history and role of early impacts? 2. Determine the global geology of the inner planets. 3. Investigate how major impacts early in a planet’s history can alter its evolution and orbital dynamics. Present: What common dynamic processes shape Earth-like planets? a. What processes stabilize climate? 4. Determine processes and rates of surface/atmosphere interaction. b. How do active internal processes shape the atmosphere and surface environments? 3. Characterize magnetic fields and relationships to surface, atmosphere, and the interplanetary medium. Future: What fate awaits Earth’s environment and those of the other terrestrial planets? a. What do diverse climates of the inner planets reveal about the vulnerability of Earth’s environment? 1. Characterize the greenhouse effect through meteorological observations. c. What are the consequences of impacting particles and large objects? 1. Determine the recent cratering history and current flux of impactors in the inner solar system. 2. Evaluate the temporal storage and record of solar-wind gasses. • Supporting Future: What fate awaits Earth’s environment and those of the other terrestrial planets? b. How do varied geologic histories enable predictions of volcanic and tectonic activity? 1. Assess the distribution and age of volcanism on the terrestrial planets. d. What are the resources of the inner solar system? 1. Assess volatile resources. 2. Assess mineral resources.

NEW FRONTIERS MISSION OPTIONS 31 Developments Since the Decadal Survey With the exception of the detection of a partially molten core in Mercury from Earth-based radar observations,30 there is no new information on the interiors of the terrestrial planets. New observations of the atmospheres of Mars and Venus have been made by several Mars missions and the Venus Express mission, several orbital missions to the Moon are underway or planned, and the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft is planned to orbit Mercury in the near future. But there are no atmospheric measurements planned at or near the surface and at multiple surface locations at the same time, nor are there geophysical network missions planned. At the same time, significant technological advances have been made in seismometer design for planetary missions, long-lived power supplies, data management and storage, telecommunication systems, and landing concepts that may improve the readiness of missions for flight. Conclusions The committee concluded that Network Science missions remain scientifically important and should be con- sidered for the New Frontiers Program and that the primary goal of any Network Science mission is geophysics. But for Mars, atmospheric measurements near the surface are an important secondary goal. Mission-Specific Recommendations The committee recommends that a network science mission be included in the forthcoming NASA New Fron- tiers announcement of opportunity. The decadal survey identifies a Network Science mission’s primary objective as geophysics. For Mars, atmospheric measurements near the surface are a valuable supplement to the geophysics measurements, but cannot be a substitute for them. In light of the decadal survey’s recognition of the importance of network science on all the terrestrial planets and the Moon, the committee recommends that Network Science missions to the Moon, Venus, and Mercury be con- sidered as candidate missions for the New Frontiers announcement of opportunity in addition to a Mars mission. The scientific objectives of a Network Science mission should be drawn from a subset of the objectives (not in priority order) described in the decadal survey, as follows. 31 For the Interior • Determine the internal structure including horizontal and vertical variations in the properties of the crust and mantle, and evaluate implications for how the core, mantle, and crust evolved. • Determine the characteristics of the metallic core (e.g., size, density, and presence and distribution of liquid) and explain the strength or absence of a present-day magnetic field. • Determine the heat flow and the distribution of heat-producing elements in the crust and mantle. • Determine interior composition and compositional variations to elucidate differentiation, crust-mantle evo- lution (plate tectonics, basin formation by impacts, conditions for life), and how the bulk composition relates to that of the Earth and other terrestrial planets and how planetary compositions are related to nebular condensation and accretion processes. For the Surface/Atmosphere • Measure the surface winds and their time variability and the near surface global circulation. • Measure the temperature, pressure, humidity, and radiative flux. • Measure the atmospheric, elemental, and isotopic compositions. 30J.L. Margot, S.J. Peale, R.F. Jurgens, M.A. Slade, and I.V. Holin, Large longitude libration of Mercury reveals a molten core, Science 316:710-714, doi:10.1126/science.1140514, 2007. 31This list is culled from several places in the decadal survey. See New Frontiers in the Solar System, pp. 7, 42, and 62.

32 OPENING NEW FRONTIERS IN SPACE • Understand the relationship between the near-surface general circulation and the physical processes that force it. • Determine how the near-surface general circulation controls the exchange of dust, water, CO2, etc., between the atmosphere and surface. • Begin to establish a weather monitoring infrastructure to support future robotic and crewed missions. • Provide an enhanced assessment of year-to-year atmospheric mass exchange between the atmosphere and polar caps and regolith. • Determine the mineralogic composition of the surface and its thermophysical properties. Trojan/Centaur Reconnaissance The decadal survey’s Primitive Bodies Panel recommended “reconnaissance of the Trojans and Centaurs,” 32 and the decadal survey listed this as a deferred medium-class mission.33 A variety of developments since the decadal survey, when combined with the strong initial rationale, elevate this mission concept to one for consideration. The Trojans, initially discovered in the early 20th century, tend to be spectral type-D asteroids (with a low albedo and a featureless, reddish spectrum) and are thought to be primitive leftovers from early solar system for- mation, possibly captured during giant planet formation, although this remains a question of debate. At the time of the decadal survey, Trojans were thought to have formed in place and sample the region of the accretion disk from which Jupiter formed. (See Figure 2.5, which illustrates the Trojan orbits.) These objects, now known to number well over 1,000, are aggregated about the L4 and L5 equilibrium points along Jupiter’s orbit. The Centaurs occupy positions farther from the Sun than the Trojans. Centaurs are generally described as objects with semimajor axes and perihelia lying between the heliocentric distances of Jupiter and Neptune. Although not as distant as Kuiper Belt objects (KBOs), they are still sufficiently small and distant to be difficult to study remotely. They exhibit a range of sizes and colors; 10199 Chariklo is the largest currently known, with an approximately 225-km characteristic diameter. Water ice has been unambiguously detected on some of the larger Centaurs (e.g., 2060 Chiron, 5145 Pholus, and 10199 Chariklo).34 While the Trojans are spectrally homogeneous, the Centaurs are diverse, ranging from spectrally neutral, but displaying cometary outbursts (like 2060 Chiron), to inactive and spectrally very red (e.g., 5145 Pholus). The orbits are dynamically unstable, and the current accepted theory is that these objects originated as KBOs, and thus provide a more accessible sample than KBOs much further out in the solar system. As a result of Cassini observations made since the last decadal survey, there is also a working hypothesis that the moon of Saturn, Phoebe (Figure 2.6), may be a Centaur captured in the Saturn system. Fewer than 100 of these objects are currently known. Background The decadal survey stated that a Trojan Asteroid/Centaur Reconnaissance mission “would send a KBP [Kuiper Belt-Pluto]-like flyby reconnaissance spacecraft equipped with imaging, imaging spectroscopy, radio science, and, potentially other instruments to make the first explorations of both a Jovian Trojan asteroid and a Centaur. Beyond simply opening up these two new classes of primitive bodies to exploration, this mission has deep ties to understanding the origins of primitive bodies.”35 In addition, the decadal survey stated that “the Trojan flyby would sample primitive material from the Jovian accretion region of the nebula; it would also allow an important recalibration of the bombardment flux on objects in the Jovian system and would offer new insights into space weathering and other processes affecting asteroids, 32New Frontiers in the Solar System, p. 35. 33New Frontiers in the Solar System, p. 197. 34A. Delsanti and D. Jewitt, The solar system beyond the planets, pp. 267 in Solar System Update, P. Blondel and J. Mason, eds., Springer Praxis, Berlin, 2006. 35New Frontiers in the Solar System, p. 25.

NEW FRONTIERS MISSION OPTIONS 33 FIGURE 2.5  Illustration of asteroids in the solar system showing how the Trojan asteroids (also referred to as “Greeks” in this image) occupy positions in front of and trailing Jupiter’s orbit around the Sun. SOURCE: Courtesy of NASA. particularly in the main belt. The Centaur flyby would provide insights into the nature of the Kuiper Belt, the nature and origin of short-period comets and their parent bodies, and activity in distant comets.” 36 The decadal survey identified a number of crosscutting science themes that have been addressed by the New Horizons mission and could be addressed by a Trojan/Centaur Reconnaissance mission. As listed in the decadal survey, these include:37 The First Billion Years of Solar System History 1. What processes marked the initial stages of planet and satellite formation? 3. How did the impactor flux decay during the solar system’s youth, and in what way(s) did this decline influence the timing of life’s emergence on Earth? The decadal survey also identified several other areas where a Trojan/Centaur Reconnaissance mission could answer key questions: 36New Frontiers in the Solar System, p. 25. 37New Frontiers in the Solar System, p. 3, Table ES.1.

34 OPENING NEW FRONTIERS IN SPACE FIGURE 2.6  NASA close-up image of Saturn moon Phoebe by the Cassini spacecraft. Phoebe is roughly spherical and has a diameter of 220 kilometers. Some scientists believe that Phoebe is a captured Centaur object. SOURCE: Courtesy of NASA/ JPL/Space Science Institute.

NEW FRONTIERS MISSION OPTIONS 35 Volatiles and Organics: The Stuff of Life 4. What is the history of volatile compounds, especially water, across the solar system? Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? 12.  What does the solar system tell us about the development and evolution of extrasolar planetary systems, and vice versa? Trojans and Centaurs both formed beyond the solar system ice line and likely represent the transition from inner main belt, ice-free asteroids and outer solar system comets. Detailed study of these objects should greatly expand understanding of the history of volatiles in the solar system. Trojans and Centaurs exist in regions of the solar system that are likely fundamentally different, with respect to impact flux and solar energy, from those regions sampled by main belt asteroids. Examination of these objects will expand knowledge of the interaction of small bodies with the space environment throughout the solar system. The Primitive Bodies Panel of the decadal survey identified the following high-level science questions as sig- nificant for being addressed by the Trojan/Centaur Reconnaissance mission, dividing these into paradigm-altering, pivotal, and foundation-building observations for each of two themes, “Building Blocks of the Solar System” and “Organic Matter in the Solar System: Materials for the Origin of Life”: 38 Paradigm Altering • What is the nature of the KBOs (Kuiper Belt objects)? • Where in the solar system did building blocks form; which were transported and which were not? • How is organic matter distributed throughout the solar system? Pivotal • What processes modify the surfaces of all categories of building blocks? Foundation Building •  ow do colors and albedos of small bodies relate to their compositions and histories of alteration H by various processes since their origin? •  hat roles did various dynamical processes play in the origin and evolution of the primitive bodies W in the solar system, and what were the time scales? Given recent experience with both comet and asteroid missions, it is clear that each body contributes to only part of a synoptic view. Clearly, a single KBO encounter by New Horizons will not elucidate the full diversity of KBOs, but an encounter with a Trojan asteroid or Centaur (which, counter to previous opinion, could be a scattered KBO) could provide valuable insights into the diversity of KBOs. A Trojan asteroid could prove particularly intriguing given the possibility that these asteroids either sample the Jovian accretion zone or could be asteroids captured during giant-planet formation. Both Trojan asteroids and Centaurs could prove to have sampled regions of the nebula and be rich in the types of organics not sampled on Earth or by previous missions. Basic characterization of these asteroids, coupled with knowledge gained about the alteration history of these objects, should strengthen the ability to relate ground-based spectra to geologic history. 38New Frontiers in the Solar System, p. 34, Table 1.1.

36 OPENING NEW FRONTIERS IN SPACE Study of Trojans and Centaurs in the context of both early (e.g., asteroid migration and capture) and late (e.g., space weathering) solar system evolution will elucidate these physical and dynamical processes through time. The Primitive Bodies Panel further listed the following specific questions deserving of investigation: 39 •  here in the solar system are the primitive bodies found, and what range of sizes, compositions, W and other physical characteristics do they represent? •  here in the solar nebula did the classes of primitive bodies form? Which were subsequently W transported, and which remain in place? •  hat are the basic physical properties (mass, density, size) of Kuiper Belt objects, Centaurs, and W comets? •  hat roles did various dynamical processes play in the origin and evolution of the primitive bodies W in the solar system, and what were the time scales? •  hat are the surface properties and compositions of these bodies, and how do endogenous and W exogenous processes affect them? •  hat are all of the space weathering processes that operate on the surfaces of bodies without W atmospheres, and how have these processes varied over time? •  hat is the time-history of collisional events and their consequences at various distances from the W Sun? In many respects, the reconnaissance of Trojans and Centaurs is only beginning. The increased sensitivity of ground-based telescopes is increasing the discovery rates not only of asteroids, but also of Trojans and Centaurs. While the basic properties of the Trojans and Centaurs are emerging, little is known about individual objects, and the current view of the population as a whole is changing rapidly. Both Trojans and Centaurs are now considered to be primitive objects, but knowledge of their origins remains uncertain. Centaurs are known to be in orbits that are unstable over periods of tens of millions of years. Knowledge of properties of both the Trojans and the Centaurs is typically limited to single-pixel photometry and spectrometry because of their relatively small sizes and large distances. 40 To determine the homogeneity, heterogeneity, mass, and volume of these bodies in general, and Trojans and KBOs in particular, close flybys are required at a minimum. That flybys can accomplish such objectives to a level better than that provided by remote measurements aloneobjectives that can be met in no other wayis demonstrated by the Cassini observations of Phoebe (see Figure 2.6) and the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker observations of the main belt asteroid 253 Mathilde (Figure 2.7). Interpretations of the spectra of D-type asteroids, such as the Trojans, retain both ambiguities and uncertain- ties that will remain unresolved until these objects themselves can be studied more completely, at least in terms of their major features. Close-up images and spectral observations of Trojans and Centaurs can address the effects of “space weath- ering” in the short term, and of impacts in the long term. The observations of Phoebe by Cassini are a case in point. Developments Since the Decadal Survey In the past several years, understanding of small bodies has been substantially enriched with the analysis of data from the NEAR mission, the successful encounter of asteroid Itokawa by the Japanese Hayabusa mission, and the encounters of comets with the Deep Impact and Stardust missions, as well as the initiation of the New 39New Frontiers in the Solar System, pp. 14, 15, 17, and 18. 40A. Delsanti and D. Jewitt, The solar system beyond the planets, pp. 267-294 in Solar System Update, P. Blondel and J. Mason, eds., Springer Praxis, Berlin, 2006.

NEW FRONTIERS MISSION OPTIONS 37 FIGURE 2.7  The asteroid Mathilde was studied by the NEAR spacecraft during a flyby, providing better data than could be obtained by remote measurements alone. Mathilde is approximately 66 by 48 by 46 kilometers in size. SOURCE: Courtesy of NASA. Horizons mission to Pluto and the Kuiper Belt. In addition, since the decadal survey was written, there is now an appreciation that Trojans could have formed elsewhere in the solar system and been captured by Jupiter during significant orbital migrations of the gas and ice giant planets, strengthening the need to understand where these objects formed and how they reached their current positions.41 Conclusions Asteroidal bodies offer some of the best clues to the materials and processes that were dominant in the early history of the solar system. The importance of these bodies was clearly recognized by the decadal survey. A Trojan/Centaur Reconnaissance mission would explore two classes of bodies that are thought to have formed in place, thus preserving samples of the solid materials that were present in the region where they formedthe solar nebula in which Jupiter formed. The discovery of more of these asteroids and the recognition that they sample volatile-rich regions of the solar nebula strengthen interest in these objects. Coupled with the idea that large-planet migration might perturb Kuiper Belt objects into Centaur-like orbits, this mission is necessary to fully inventory the primitive bodies of our solar system and understand their origins. Mission-Specific Recommendations Coupling new science emerging in the past several years with the decadal survey, the Trojan/Centaur Recon- naissance mission originally described in the decadal survey should be modified so that NASA informs potential proposers of the kind of science questions that should be answered and does not prescribe how the mission should actually be accomplished. The mission requirements should also permit orbital encounters and state that main belt asteroid flyby is not considered critical to this mission. Such a mission should have the following science objectives: 41A.Morbidelli, H.F. Levison, K. Tsiganis, and R. Gomes, Chaotic capture of Jupiter’s Trojan asteroids in the early solar system, Nature 435:462-465, doi:10.1038/nature03540, 2005.

38 OPENING NEW FRONTIERS IN SPACE • Determine the physical properties (e.g., mass, size, density) of a Trojan and a Centaur. • Map the color, albedo, and surface geology of both a Trojan and a Centaur at a resolution sufficient to distinguish important features for deciphering the history of the object (e.g., craters, fractures, lithologic units). Each of these objectives can be addressed by appropriate imagers, spectrometers, or spectrographs that resolve the target. For example, spacecraft tracking could obtain a mass, so that a density can be determined. A full dis- cussion of appropriate error bars on such science objectives that are achievable under this program require studies of trade-offs that are yet to be made and are beyond the scope of this study. If the target Centaur for a proposed mission has suspected cometary or quasi-cometary activity, then in situ instrumentation capable of sensing such activity and its constituents should be considered. Asteroid Rover/Sample Return Asteroids are planetesimals, found largely in the orbit between Mars and Jupiter, that escaped the melting and differentiation that shaped the larger terrestrial planets, although short-lived metamorphism and aqueous altera- tion occurred on many of these bodies. Asteroids offer a unique record of the complex chemical evolution that occurred in the early solar nebula, which has since been obliterated from the larger planets. In studying asteroids, there is the substantial advantage of having abundant asteroid samples in the form of meteorites, although these samples lack the geologic context that would allow examination of, for example, radial heterogeneity in the solar system. The decadal survey’s Primitive Bodies Panel recommended an Asteroid Rover/Sample Return mission as its fourth-ranked mission priority;42 however, the mission was ultimately deferred from consideration by the decadal survey.43 A variety of developments since the decadal survey, when combined with the strong initial rationale, prompted the committee to elevate this mission concept to one for consideration. The primary motivations for an asteroid sample return mission is the desire to both acquire samples with known geologic context and to return materials that are either unlikely to survive passage to Earth (e.g., friable, volatile-rich material) or would be compromised by terrestrial contamination upon their fall (e.g., extraterrestrial organics). Background The decadal survey emphasized the value of a return to asteroid 433 Eros, which has already been the subject of global characterization during the Near Earth Asteroid Rendezvous (NEAR) mission that ended in early 2001. The NEAR mission emphasized the complexity of asteroidal regolith and the need to understand surface processes if remotely sensed spectra of asteroids are to be interpreted. The decadal survey identified a number of crosscutting science themes that were addressed by the New Hori- zons mission and would be addressed by a Asteroid Rover/Sample Return mission: 44 The First Billion Years of Solar System History 1. What processes marked the initial stages of planet and satellite formation? 3. How did the impactor flux decay during the solar system’s youth and in what way(s) did this decline influence the timing of life’s emergence on Earth? Organic-rich, water-rich asteroids have the potential to yield fundamental information about the source of water and prebiotic organics that contributed to the accretion of the terrestrial planets. 42New Frontiers in the Solar System, p. 35. 43New Frontiers in the Solar System, p. 197. 44New Frontiers in the Solar System, p. 3, Table ES.1.

NEW FRONTIERS MISSION OPTIONS 39 In addition to being the major source of impactors to the early Earth, the asteroid belt preserves a unique record of collisionsbreakup and cratering over the past 4.5 billion years. Each asteroid has been shaped by these processes and provides insight into the influence of cratering through time. Individual asteroids preserve a record of the evolution of organics from the birth of the solar system to interactions of organics at the surface with the space environment. The decadal survey identified several questions relevant to this as well. In addition, an asteroid mission would address several other questions asked in the decadal survey: 45 Volatiles and Organics: The Stuff of Life 4. What is the history of volatile compounds, especially water, across the solar system? 5. What is the nature of organic material in the solar system and how has this matter evolved? Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? 12.  What does the solar system tell us about the development and evolution of extrasolar planetary systems, and vice versa? Little is known about the surfaces of organic-rich asteroids and their interaction with the space environment, largely resulting from their near-featureless reflectance spectra. Returned samples may provide the only means for assessing these processes. The Primitive Bodies Panel of the decadal survey identified the following high-level science questions for which an Asteroid Rover/Sample Return mission could provide breakthrough advances or significantly address, dividing these into paradigm-altering, pivotal, and foundation-building observations: 46 Paradigm Altering • What are the compositions and origins of the organic and volatile materials in primitive bodies? • How is organic matter distributed throughout the solar system? Pivotal • What processes modify the surfaces of all categories of building blocks? •  id organic matter from comets and meteorites provide the feedstock for the origin of life on D Earth? Foundation Building •  ow do colors and albedos of small bodies relate to their compositions and histories of alteration H by various processes since their origin? •  hat are the processes by which organic material forms on the surfaces of icy and other primitive W bodies in the current epoch? •  hat is the thermal and aqueous alteration history of the parent bodies of the organic-rich primi- W tive meteorites? 45New Frontiers in the Solar System, p. 3, Table ES.1. 46New Frontiers in the Solar System, p. 34, Table 1.1.

40 OPENING NEW FRONTIERS IN SPACE The relationship of the colors and albedos of small bodies to their compositions and histories of alteration since their origin is essential information that would allow scientists to interpret remotely sensed spectra of asteroids and, by extension, develop a geologic map of processes occurring in the early solar system. Furthermore, very little is known or can be directly ascertained about the interactions of volatiles and organics without sample return. The Primitive Bodies Panel further listed the following specific questions deserving of investigation: 47 •  here in the solar system are the primitive bodies found, and what range of sizes, compositions, W and other physical characteristics do they represent? • What processes led to the formation of these objects? • Since their formation, what processes have altered the primitive bodies? •  hat is the composition, origin, and primordial distribution of solid organic matter in the solar W system? • What is its present-day distribution? •  hat processes can be identified that create, destroy, and modify solid organic matter in the solar W nebula, in the epoch of the faint early Sun, and in the current solar system? Developments Since the Decadal Survey In the past several years a number of developments have strengthened the case for asteroid sample return. The most important of these is the complete analyses of the data on 433 Eros (Figure 2.8) returned from the NEAR mission and the successful encounter of the Japanese Hayabusa spacecraft with asteroid Itokawa. These encounters demonstrated the diversity of asteroids and their complexity both internally and on the surface. The recognition of the diversity of organics in meteorite samples has prompted a re-evaluation of the astro- biological importance of asteroids.48 A sample return from an organic-rich asteroid may offer insights into the distribution of biogenic compounds and allow an evaluation of whether the delivery of precursor exogenous materi- als might have contributed to the origin of life. The scientific consensus derived from the studies of carbonaceous chondrite meteorites is that abiotic syntheses may lead to prebiotically relevant organic molecules but only in complex mixtures where the prebiotically “desirable” compounds are but a small fraction of the total. 49 Because life’s biomolecules are, by contrast, the product of strict compositional selection and are function-specific, cur- rent findings do not resolve the question of whether the extraterrestrial material of meteorites is, in fact, prebioti- cally relevant. Attributes such as the molecular asymmetry described for some amino acids of the Murchison and Murray meteorites appear to offer such relevance, but their range and concentration need to be determined in a contamination-free sample.50 The committee recognizes that the exact correspondence between meteorites and possible parent asteroids is not well established. It is hoped that renewed effort in linking meteorite and asteroid spectra and new findings (e.g., see below about main-belt comets) would better direct proposers toward an organic-rich target. 51 The final major development is the detection of main-belt comets. There are currently three known main-belt comets (Figure 2.9): 133P/(7968) Elst-Pizarro (EP), P/2005 U1 (Read) (P/Read), and asteroid 118401 (1999 RE70). Orbiting completely within the main asteroid belt, main-belt comets present a distinct contrast with other periodic comets, such as the Jupiter-family and Halley-family comets, which originate in the cold outer solar system in the Kuiper Belt or Oort Cloud and are later perturbed into highly eccentric orbits while passing through the inner solar 47New Frontiers in the Solar System, pp. 14, 15, and 20. 48S. Pizzarello, G.W. Cooper, and G.J. Flynn, The nature and distribution of the organic material in carbonaceous chondrites and interplan- etary dust particles, pp. 625-651 in Meteorites and the Early Solar System II, D.S. Lauretta and H.Y. McSween Jr., eds., University of Arizona Press, Tucson, Ariz., 2006. 49National Research Council, The Astrophysical Context of Life, The National Academies Press, Washington, D.C., 2005, pp. 41-42. 50J.R. Cronin and S. Pizzarello, Enantiomeric excesses in meteoritic amino acids, Science 275:951-935, 1997. 51T.H. Burbine and R.P. Binzel, Small main-belt asteroid spectroscopic survey in the near infrared, Icarus 159(2):468-499, 2002.

NEW FRONTIERS MISSION OPTIONS 41 FIGURE 2.8  The S-type asteroids 433 Eros (top) and 25143 Itokawa (bottom). These asteroids were the targets of the NEAR and Hayabusa missions, respectively, which revealed the complexity in asteroid surfaces. Eros is approximately 33 by 13 by 13 kilometers in size, whereas Itokawa is much smaller, at 535 by 294 by 209 meters. SOURCE: Eros image courtesy of NASA, Itokawa image courtesy of JAXA (Japan Aerospace Exploration Agency).

42 OPENING NEW FRONTIERS IN SPACE FIGURE 2.9  Images of known main belt comets (MBCs) from UH 2.2-meter telescope data. All MBCs are clearly cometary, at least in the observational and physical sense (i.e., they appear “fuzzy” and all indications are that this fuzziness is caused by the ejection of dust by the sublimation of volatile material, most likely water ice). SOURCE: Courtesy of Henry H. Hsieh. Available at http://www.ifa.hawaii.edu/~hsieh/mbcs.html. system where researchers observe them. Unlike the Jupiter-family and Halley-family comets, main-belt comets appear to have formed in the much warmer inner solar system where they are found today, and so they likely contain ice that is quite different in chemical and isotopic composition from that in other comets. The committee notes that the distinction between comets and asteroids is narrowing. Because of this, comet sample return and asteroid sample return missions will share many characteristics. Conclusions Asteroidal samples offer a unique record of the complex chemical evolution that occurred in the early solar nebula. Despite the abundance of samples in the form of meteorites, these samples lack geologic context. An Asteroid Rover/Sample Return mission would acquire samples with known geologic context that are either unlikely to survive passage to Earth or would be compromised by terrestrial contamination upon their fall. Successful encounters of the NEAR and Hayabusa missions have demonstrated the diversity of asteroids and their complexity both internally and on the surface. This recognition of the diversity of organics in meteorite samples, coupled with the detection of main-belt comets that appear to have formed in the much warmer inner solar system, strengthens the importance of this mission. Mission-Specific Recommendations The committee recommends that although the Asteroid Rover/Sample Return mission should be included as a candidate mission for the New Frontiers Program, the mission objectives should be changed to reflect new

NEW FRONTIERS MISSION OPTIONS 43 scientific information acquired since the decadal survey. Specifically, the unique scientific value of organic-rich targets may elevate them for consideration when compared to the type of asteroid visited by the NEAR mission emphasized by the decadal survey. Such a mission should have the following science objectives, which are not prioritized: • Map the surface texture, spectral properties (e.g., color, albedo), and geochemistry of the surface of an asteroid at sufficient spatial resolution to resolve geologic features (e.g., craters, fractures, lithologic units) neces- sary to decipher the geologic history of the asteroid and provide context for returned samples. • Document the regolith at the sampling site in situ with emphasis on, e.g., lateral and vertical textural, mineralogical, and geochemical heterogeneity at scales down to the submillimeter. • Return a sample to Earth in an amount sufficient for molecular (or organic) and mineralogical analyses, including documentation of possible sources of contamination throughout the collection, return, and curation phases of the mission. Io Observer The decadal survey Large Satellites Panel recommended an Io Observer as a potential medium-class mission.52 As the most volcanically active body in the solar system, Io remains a unique target for study (see Figure 2.10). Notably, the causes and consequences of such active volcanism demand closer investigation. The decadal survey noted the salient, scientifically significant features of Io that emerged largely from the Voyager era, including new understanding of large-satellite tectonics, the discovery of eruptive activity, the effect of the eruptions on the global sulfur-rich chemistry of this body and its unique and little-understood atmosphere, the subsequent loss of volatiles (mostly sulfur and oxygen) to Jupiter’s magnetosphere, and conducting satellite/plasma interaction, best illustrated by Io and Europa. While Io is one of the four Galilean satellites of Jupiter, all of which lie close to the planet and are of roughly similar size, it has a significant metallic core with a silicate mantle and lacks the layer FIGURE 2.10  The turbulent surface of Io. The lack of visible craters is one indication that Io’s surface is very young—any impact craters have been filled in due to volcanism and volcanic structures are readily visible. The colorful surface is also an indication of various materials being distributed by volcanic action. SOURCE: Courtesy of U.S. Geological Survey Astrogeol- ogy. Available at http://astrogeology.usgs.gov/Projects/JupiterSatellites/io.html. ��New Frontiers in the Solar System, pp. 133 and 197.

44 OPENING NEW FRONTIERS IN SPACE of water ice and/or water that plays a significant role with the other three satellites. The uniform coloring of the entire surface and the lack of impact craters result from the continued eruptions of sulfurous and silicate materials from the interior.53 Galileo data were too sparse to allow a full understanding of Io, and so a return mission with modern remote-sensing instruments and a high data rate would revolutionize understanding of this complex and dynamic body. Background The decadal survey identified top-level, crosscutting science themes that are relevant to the study of Io. 54 Although the survey did not specifically address how these crosscutting themes and key questions would be addressed by an Io Observer mission, it is possible to map such a mission against many of the themes, such as those reprinted below. The First Billion Years of Solar System History 1. What processes marked the initial stages of planet and satellite formation? Io’s volcanic activity continues to resurface the large moon by depositing new material from the hot interior to the surface. While this activity has obscured the original surface, it does provide a unique view of interior materi- als at this location in the solar system. The tidal processes that dominate Io were probably important in the early evolution of many planetary satellites, and Io provides the only current example of how a planetary body responds to the very high heat flow that was a likely factor in the early evolution of most solid bodies in the solar system. Volatiles and Organics: The Stuff of Life 4. What is the history of volatile compounds, especially water, across the solar system? 6. What global mechanisms affect the evolution of volatiles on planetary bodies? Io is a water-poor world unlike its Jovian neighbors, especially Europa. Io currently loses volatiles such as sulfur and sodium at the rate of about 1 ton per second via magnetospheric processes and may have lost its water earlier by similar processes. Volatiles are transported rapidly across Io’s surface by volcanic and atmospheric pro- cesses and are then lost from the moon when they are transported outwards to the other moons. Io’s volatiles are ultimately transported out of the system by complex and poorly understood plasma processes. Volatile loss and redistribution processes are likely to have wide importance in the history and evolution of solar system volatiles and are easily studied at Io because they operate there with unique rapidity and intensity. The Origin and Evolution of Habitable Worlds 7. What planetary processes are responsible for generating and sustaining habitable worlds, and where are the habitable zones in the solar system? Tidal heating, a process that can greatly expand the habitable zones in the solar system and elsewhere, is best studied at Io because Io provides the most extreme example of this process in the solar system. 53New Frontiers in the Solar System, pp. 121-129. 54New Frontiers in the Solar System, p. 3, Table ES.1.

NEW FRONTIERS MISSION OPTIONS 45 Processes: How Planetary Systems Work 11.  How do the processes that shape the contemporary character of planetary bodies operate and interact? Io provides the best place in the solar system, beyond Earth, to study volcanism, a process of fundamental importance on many planetary bodies. Io also provides some of the most dramatic, freshest, and most easily studied examples of fundamental geologic processes such as mountain building and mass wasting. The volcanic activity on Io drives interlocking processes on a variety of time scales. While volcanic activity resurfaces and recycles the surface, it also contributes volatiles to the Jovian sulfur and sodium nebulae via a time-varying atmosphere and exosphere. By providing approximately 1 ton per second of material deep within the magnetosphere of Jupiter, Io is a primary driver for most magnetospheric activity. With transport of material to Europa and the rest of the system, re-energization processes, and the Alfvénic interaction between Io and the upper atmosphere of Jupiter itself, sci- entists know that multiple, nonlinear feedback processes are present on many spatial and temporal scales. Drawn from Table 5.2 of the decadal survey, the material below indicates more specific questions that would be addressed by an Io Observer mission:55 • Questions on which an Io Observer could offer a breakthrough-level advance A. Origin and Evolution of Satellite Systems 2. What affects differentiation, outgassing, and the formation of a thick atmosphere? Characterization of internal heat sources 3.  what extent are the surfaces of icy satellites coupled to their interiors (chemically and To physically)? Geologic processes/history (including impacts) Tectonics/volcanism D. Understanding Dynamic Planetary Processes 1. What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Heat flow and tidal heating Global volcanism and tectonism Secular variations of magnetic field 2.  What are the currently active endogenic geologic processes (volcanism, tectonism, diapirism) and what can we learn about such processes in general from these active worlds? Observations of dynamic processes with high spatial and temporal resolution Search and discovery of new types of activity 3.  What are the complex processes and interactions on the surfaces and in volcanic or geyserlike plumes, atmospheres, exospheres, and magnetospheres? Dynamic of plumes, geysers, atmospheres, exospheres, and magnetospheres History of volatiles Atmospheric loss (fields and particles) 55New Frontiers in the Solar System, pp. 140-143.

46 OPENING NEW FRONTIERS IN SPACE • Questions on which an Io Observer could offer a major advance A. Origin and Evolution of Satellite Systems 1. How do conditions in the protoplanetary nebula influence the compositions, orbits, and sizes of the resulting satellites? Characterization of magnetic fields in satellites 2. What affects differentiation, outgassing, and the formation of a thick atmosphere? (Why is Titan unique?) Atmospheric composition Production/loss rates 3.  what extent are the surfaces of icy satellites coupled to their interiors (chemically and To physically)? Geologic processes/history (including impacts) Map surface composition B. Origin and Evolution of Water-Rich Environments in Icy Satellites 2. What combination of size, energy sources, composition, and history produce long-lived internal oceans? Heat flow • Questions on which an Io Observer could offer a significant advance A. Origin and Evolution of Satellite Systems 1. How do conditions in the protoplanetary nebula influence the compositions, orbits, and sizes of the resulting satellites? Interior structure and composition of (major) satellites Secular variation of orbital parameters 2. What affects differentiation, outagassing, and the formation of a thick atmosphere? (Why is Titan unique?) Atmospheric composition Interior structure and composition 3.  what extent are the surfaces of icy satellites coupled to their interiors (chemically and To physically)? Subsurface sounding 4. How has the impactor population in the outer solar system evolved through time, and how is it different from the inner solar system? Observation of craters (on many different bodies) Geology/modification B. Origin and Evolution of Water-Rich Environments in Icy Satellites 1. What is the chemical composition of the water-rich phase? Remote and in situ composition observations 2. What is the distribution of internal water, in space and in time? Elemental and isotopic composition 4. Can and does life exist in the internal ocean of an icy satellite? Characterization of surface radiation environment Transport processes C. Exploring Organic-Rich Environments 1. What is the nature of organics on large satellites? Composition (elemental, isotopic, and molecular), remote and in situ Production/loss (radiation, degassing, escape, lightning, and exogenic/endogenic) Physical state

NEW FRONTIERS MISSION OPTIONS 47 Optical properties Reaction rates/kinetic information 2. What are the processes currently affecting organic-rich surfaces? Cryovolcanic processes Tectonic processes D. Understanding Dynamic Planetary Processes 1. What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Interior structure Although the decadal survey outlined the ways in which an Io mission could contribute to important questions about planetary satellites, it did not detail Io-specific science goals for an Io orbiter mission. 56 Similarly, although the decadal survey lacked a detailed mission description, it did summarize the mission concept. The mission concept for Io could involve either a Jupiter orbiter dedicated to multiple close flybys of Io or a multirole mission with part of the mission and payload being devoted to magnetospheric space physics goals and/or atmospheric and auroral observations. The assumption that this mission could achieve the stated goals within the New Frontiers cost category rests partially on an assumption that heritage from the Europa Geophysical Explorer would allow significantly reduced costs. Although the Europa Geophysical Explorer was not pursued, significant studies of the Jupiter radiation environment were performed as part of the Jupiter Icy Moons Orbiter program, and some radiation-hardened electronics have been developed in the interim. Nevertheless, an Io Observer spacecraft would definitely benefit from future studies and technology development, including work currently underway for the Juno mission. More Io-specific goals can be drawn from the white paper provided to the decadal survey by the Io community.57 Developments Since the Decadal Survey The decadal survey report was written mostly in 2001 after most of Galileo’s Io flybys. Remote analyses of Io have continued, using Earth-based assets. Near observations of Io have been limited to the few, but spectacular, views provided during the New Horizons flyby of Jupiter on its way to Pluto in February 2007. These additional views provide insights into the time variations in and extent of the eruptions on Io. Study of volcanic activity on Io—the presumed prime driver of phenomena throughout the Jovian magnetospheric systemthus has conse- quences for understanding the dynamics and evolution of the Jovian system on time scales stretching from mere hours to the age of the solar system. The New Horizons Io images (see Figure 2.11) underscore the importance of temporal coverage, and they open a new window on plume dynamics by showing how plume structure can be tracked to reveal rapid motions, while ground-based adaptive optics images underscore the importance of long-term temporal coverage in tracking volcanic eruptions. Continuing work on Io’s atmosphere, such as the recent identification of sodium chloride, hints at the atmo- sphere’s likely chemical complexity, and its physical complexity is revealed in the large regional variations in atmo- spheric density revealed by new Hubble Space Telescope and ground-based observations, and global circulation patterns made available by disk-resolved millimeter-wave atmospheric observations. New Horizons’ exploration of Jupiter’s magnetotail also sheds new light on the processes of magnetospheric volatile loss from Io. Observations from the ground and from spacecraft such as Hubble, Cassini, and New Horizons can help pro- 56Other top-level notes for such a mission are included in Table 5.3 and Table 7.1 of the decadal survey, New Frontiers in the Solar System, pp. 144 and 176. 57J.R.Spencer et al.,����������������������������������������������������������������������������������������� The Future of Solar System The future of Io exploration, community white paper for the Planetary Decadal Survey in Exploration, 2003-2013, M. Sykes, ed., Astron. Soc. Pac. Conference Series 270:201-216, 2002.

48 OPENING NEW FRONTIERS IN SPACE FIGURE 2.11  Image of Io taken during Jupiter flyby in February 2007. Io has a diameter of 3,642 kilometers. SOURCE: Courtesy of NASA. vide clues to the Jovian system. Juno will also provide magnetic field data when it reaches Jupiter in 2015. But the lack of dedicated, targetable observations makes any approach to understanding the system extremely piecemeal. Resolution, or even significant advances in understanding, of the processes at work on Io is problematic without nearby, focused observations. There have been technological developments since the decadal survey that may make an Io Observer mis- sion more feasible than it was 5 years ago. In particular, radiation-hardened electronics have been developed that would be vital to an Io mission. The 2007 flagship-class mission studies for the Europa Explorer and Jupiter System Observer (JSO) demonstrate the spacecraft longevity that is possible with modern radiation-hardened electronicswhich is especially important in the intense Jovian radiation environment. In particular, Figure 4.4-4 of the JSO report shows that a spacecraft would accumulate only 10 percent of its allowable radiation dose during its first three Io flybys, suggesting that a dedicated Io mission could survive a large number of Io flybys. 58 In addition, the Juno mission demonstrated the feasibility of using radiation shielding and a solar-powered satellite at Jupiter’s distance from the Sun. Conclusions The Galileo mission to Jupiter provided relatively limited information on Io for several reasons: Galileo had very limited ability to provide high-resolution spatial coverage of Io because of the low data rate, and its instru- mentation was limited (e.g., with almost no ability to study Io’s molecular atmosphere, and very limited spatial coverage possible during each flyby). Galileo was also limited to seven Io flybys (many of which were late in the 58National Aeronautics and Space Administration, Jupiter System Observer, Mission Study: Final Report, November 1, 2007, available at http://solarsystem.nasa.gov/multimedia/downloads.cfm.

NEW FRONTIERS MISSION OPTIONS 49 extended mission and were compromised by spacecraft problems), which was not nearly enough to characterize Io’s internal structure and determine if it has a magnetic field or to investigate temporal variability of the surface with high spatial resolution. Several new technology developments have occurred which make an Io mission more feasible than it was 5 years ago. The committee, like the decadal survey, envisions a possible mission concept involving a Jupiter orbiter in eccentric orbit with multiple Io flybys and extensive monitoring at other times in its orbit. Mission-Specific Recommendations An Io Observer mission that addresses fundamental goals for solar system exploration may be possible. Consequently, an Io Observer mission should be included in the suite of possible missions included in the next New Frontiers announcement of opportunity. The mission should address some of the following science questions, which are not listed in order of priority. The committee, however, acknowledges that there are more objectives here than can be included in a single New Frontiers mission and leaves it to potential competitors to choose their science goals and defend their choices. Science objectives that could be addressed with an Io Observer mission can include the following: • Determine the magnitude, spatial distribution, temporal variability, and dissipation mechanisms of Io’s tidal heating. • Determine Io’s interior structure, such as whether it has a magma ocean. • Determine whether Io has a magnetic field. • Understand the eruption mechanisms for Io’s lavas and plumes and their implications for volcanic processes on Earth, especially early in Earth’s history when its heat flow was similar to Io’s, and elsewhere in the solar system. • Investigate the processes that form Io’s mountains and the implications for tectonics under high-heat-flow conditions that may have existed early in the history of other planets. • Understand Io’s surface chemistry, including volatiles and silicates, and derive magma compositions (and ranges thereof), crustal and mantle compositions and implications for the extent of differentiation, and contribu- tions to the atmosphere, magnetosphere, and torus. • Understand the composition, structure, and thermal structure of Io’s atmosphere and ionosphere, the domi- nant mechanisms of mass loss, and the connection to Io’s volcanism. These objectives are probably best addressed within a New Frontiers budget by a Jupiter-orbiting spacecraft with multiple Io flybys. It is possible that such a mission may exceed the New Frontiers cost cap—considering the results of NASA’s 2007 billion-dollar-box study of missions to Titan and Enceladus, which found that a Saturn orbiter with Enceladus flybys would probably cost well in excess of a billion dollars. 59 Nevertheless, innova- tive approaches might be able to circumvent these problems and enable a capable New Frontiers Io Observer mission. Ganymede Observer Large icy satellites may hold the key to answering many fundamental questions about the solar system, and Jupiter’s largest moon, Ganymede, is of particular interest because of its unique internal magnetic field and its interaction with Jupiter. Ganymede is the only icy body in the solar system known to generate its own magnetic field, thus providing a unique window into its interior and, moreover, shedding light on how internal magnetic fields are generated else- where in the solar system. Ganymede also provides a laboratory for the study of plasma effects on satellite surfaces: 59See K. Reh, J. Elliott, T. Spilker, E. Jorgensen, J. Spencer, and R. Lorenz, Titan and Enceladus $1B Mission Feasibility Study Report, JPL D-37401 B, Jet Propulsion Laboratory, Pasadena, Calif., January 30, 2007.

50 OPENING NEW FRONTIERS IN SPACE FIGURE 2.12  Ganymede, Jupiter’s largest moon, which has a mean radius of 2,631.2 kilometers. Like Europa, which was the highest-rated outer planets priority in the decadal survey, Ganymede is also believed to have a large subsurface ocean. SOURCE: Courtesy of NASA. the decadal survey notes that “Ganymede’s magnetic field is strong enough that it creates a mini-magnetosphere of its own in Jupiter’s magnetosphere, partially shielding the satellite from plasma bombardment. The interaction between Ganymede’s magnetosphere and Jupiter’s magnetosphere is similar to the interaction between Earth’s magnetosphere and the solar wind, where magnetic reconnection plays a key role.” 60 Ganymede also exhibits evidence for a subsurface ocean. In contrast to Europa, an ocean in Ganymede may be bounded both above and below by ice rather than rock; nonetheless, it is likely to illuminate processes that may produce habitable environments elsewhere in the solar system (or maybe on Ganymede itself). Ganymede’s surface suggests a complex geologic history (see Figure 2.12) with similarities to those of Miranda and Enceladus. Moreover, some of its geologic terrains may be analogous to terrestrial features, thereby providing a bridge between silicate and icy bodies that could well provide fundamental information regarding the behavior of ice in geologic processes. Ganymede’s geologic activity and magnetic field are probably powered by tidal heating. The decadal survey states that “Ganymede’s differentiated interior and actively convecting core (required to generate its magnetic field) may be a consequence of its passage into resonance, while Callisto has not experienced this history” (pp. 121-122). Thus, better understanding of Ganymede could provide information about the tidal history of the entire Jovian system. 60New Frontiers in the Solar System, p. 129.

NEW FRONTIERS MISSION OPTIONS 51 Background The decadal survey identified top-level, crosscutting science themes. While the survey did not specifically address how these crosscutting themes and key questions would be addressed by a Ganymede Observer mission, it is possible to map such a mission against many of these themes: 61 The First Billion Years of Solar System History 1. What processes marked the initial stages of planet and satellite formation? 2. How long did it take the gas giant Jupiter to form, and how was the formation of the ice giants (Uranus and Neptune) different from that of Jupiter and its gas giant sibling, Saturn? 3. How did the impactor flux decay during the solar system’s youth, and in what way(s) did this decline influence the timing of life’s emergence on Earth? Volatiles and Organics: The Stuff of Life 4. What is the history of volatile compounds, especially water, across the solar system? 5. What global mechanisms affect the evolution of volatiles on planetary bodies? The Origin and Evolution of Habitable Worlds 7. What planetary processes are responsible for generating and sustaining habitable worlds, and where are the habitable zones in the solar system? Processes: How Planetary Systems Work 1. How do the processes that shape the contemporary character of planetary bodies operate and interact? The following excerpts from Table 5.2 of the decadal survey62 indicate questions that would be addressed by a Ganymede Observer: • Questions on which a Ganymede Orbiter could offer a breakthrough-level advance A. Origin and Evolution of Satellite Systems 1. How do conditions in the protoplanetary nebula influence the compositions, orbits, and sizes of the resulting satellites? Characterization of magnetic fields in satellites 5. What does the magnetic field of Ganymede tell us about its thermal evolution, and is Ganymede unique? Internal magnetic fields B. Origin and Evolution of Water-Rich Environments in Icy Satellites 3. What combination of size, energy sources, composition, and history produce long-lived internal oceans? Intrinsic magnetic field (past/present) D. Understanding Dynamic Planetary Processes 1. What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Secular variations of magnetic field 61New Frontiers in the Solar System, p. 3, Table ES.1. 62New Frontiers in the Solar System, pp. 140-143.

52 OPENING NEW FRONTIERS IN SPACE 3. What are the complex processes and interactions on the surfaces and in volcanic or geyserlike plumes, atmospheres, exospheres, and magnetospheres? Atmospheric loss (fields and particles) • Questions on which a Ganymede Orbiter could offer a major advance A. Origin and Evolution of Satellite Systems 2. What affects differentiation, outgassing, and the formation of a thick atmosphere? (Why is Titan unique?) Production/loss rates 3.  what extent are the surfaces of icy satellites coupled to their interiors (chemically and To physically)? Geologic processes/ history (including impacts) Tectonics/volcanism Map surface composition Subsurface sounding B. Origin and Evolution of Water-Rich Environments in Icy Satellites 1. What is the chemical composition of the water-rich phase? Remote and in situ composition observations 2. What is the distribution of internal water, in space and in time? Geology/stratigraphy Subsurface sounding Internal structure 3. What combination of size, energy sources, composition, and history produce long-lived internal oceans? Composition Internal Structure 4. Can and does life exist in the internal ocean of an icy satellite? Characterization of surface radiation environment Characterization of chemistry of surface and ocean Life in extreme environments (Earth analogues) Transport processes C. Exploring Organic-Rich Environments 1. What is the nature of organics on large satellites? Composition (elemental, isotopic, and molecular), remote and in situ D. Understanding Dynamic Planetary Processes 1. What are the active interior processes and their relations to tidal heating, heat flow, and global patterns of volcanism and tectonism? Interior structure Heat flow and tidal heating Global volcanism and tectonism • Questions on which a Ganymede Orbiter can offer a significant advance A. Origin and Evolution of Satellite Systems 1. How do conditions in the protoplanetary nebula influence the compositions, orbits, and sizes of the resulting satellites? —Interior structure and composition of (major) satellites —Secular variation of orbital parameters

NEW FRONTIERS MISSION OPTIONS 53 2. What affects differentiation, outgassing, and the formation of a thick atmosphere? (Why is Titan unique?) —Atmospheric composition —Interior structure and composition —Characterization of internal heat sources 3.  what extent are the surfaces of icy satellites coupled to their interiors (chemically and To physically)? Transport processes 4. How has the impactor population in the outer solar system evolved through time, and how is it different from the inner solar system? —Observation of craters (on many different bodies) —Geology/modification 5. What does the magnetic field of Ganymede tell us about its thermal evolution, and is Ganymede unique? —Plasma/ionospheric observation of external field —Transport processes B. Origin and Evolution of Water-Rich Environments in Icy Satellites 2. What is the distribution of internal water, in space and in time? —Elemental and isotopic composition 3. What combination of size, energy sources, composition, and history produce long-lived internal oceans? —Geology 4. Can and does life exist in the internal ocean of an icy satellite? —Search for evidence of biology and organic compounds at surface and in the deeper interior C. Exploring Organic-Rich Environments 1. What is the nature of organics on large satellites? —Production/loss (radiation, degassing, escape, lightning, and exogenic/endogenic) —Physical state —Optical properties —Reaction rates/kinetic information 2. What are the processes currently affecting organic-rich surfaces? —Impact processes —Tectonic processes —Chemical (and radiation) processes D. Understanding Dynamic Planetary Processes 2. What are the currently active endogenic geologic processes (volcanism, tectonism, diapirism) and what can we learn about such processes in general from these active worlds? —Observations of dynamic processes with high spatial and temporal resolution —Composition of recent surface deposits, plumes or geysers, and atmospheres —Search and discovery of new types of activity 3. What are the complex processes and interactions on the surfaces and in volcanic or geyserlike plumes, atmospheres, exospheres, and magnetospheres? —Dynamic of plumes, geysers, atmospheres, exospheres, and magnetospheres —History of volatiles

54 OPENING NEW FRONTIERS IN SPACE Developments Since the Decadal Survey No white paper on Ganymede was submitted for the decadal survey; however, the science definition team for the Jupiter Icy Moons Orbiter mission (which was canceled in 2005 but included a long stay in Ganymede orbit) discussed Ganymede science goals in some detail.63 More recently, an extensive review of the science that could be accomplished at Ganymede by a flagship-class Ganymede orbiter has been published in the Jupiter System Observer Science Definition Team’s 2007 report, which established several goals relevant to a Ganymede Observer mission that are reprinted below: 64 Goal, Magnetospheres: Understand the magnetospheric environments of Jupiter, its moons and their interac- tions • Objective A. Moon Interior Structure. Establish internal structure of icy moons including presence and proper- ties of putative conducting layers, measurement of higher harmonics and secular variations of Ganymede’s magnetic field and set limits on intrinsic magnetic fields for Europa and Callisto • Objective B. Ganymede’s Intrinsic Magnetosphere. Investigate the magnetic field, particle populations, and dynamics of Ganymede’s magnetosphere • Objective C. Moon-Magnetosphere Interactions. Determine the effect of the Jovian magnetosphere on the icy moons. Understand effects of the moons on the magnetosphere and Jupiter’s auroral ionosphere Goal, Satellites: Understand the mechanisms responsible for formation of surface features and implications for geological history, evolution, and levels of current activity • Understand geologic history, potential for current activity, and the implications for Jupiter’s satellite system • Understand the processes responsible for the observed geologic features • Understand heat balance and tidal dissipation Goal, Satellites: Determine the surface compositions and implications for the origin, evolution and transport of surface materials • Understand composition, physical characteristics, distribution, and evolution of surface materials Goal, Satellites: Determine the compositions, origins, and evolution of the atmosphere, including transport of material throughout the Jovian system • Understand the sources (sublimation, surface sputtering) and sinks (freezing out, plasma pickup/sputtering, thermal escape) of atmospheric components • Understand the temporal, spatial, and compositional variability of the atmosphere Goal, Interiors: Determine the interior structures and processes operating in the Galilean Satellites in relation to the formation and history of the Jupiter system and potential habitability of the moons. • The study also established several goals at the investigation level for the interiors section: Characterize the formation and chemical evolution of the Jupiter system Place bounds on the orbital evolution of the satellites Determine the sizes and states of the cores of the moons Determine the presence and location of water within these moons Determine the extent of differentiation of the three icy satellites Establish the presence of oceans Characterize the extent and location of water (including brines) in 3D within Europa, Ganymede and Callisto Determine the thickness of the ice layer for all Icy Satellites Characterize the operation of magnetic dynamo processes in the Jovian system and their interaction with the sur- rounding magnetic field Globally characterize Ganymede’s intrinsic magnetic field and search for temporal variability in the field 63R. Taylor, Prometheus Project Final Report, 982-R120461, NASA Jet Propulsion Laboratory, Pasadena, Calif., 2005. 64National Aeronautics and Space Administration, Jupiter System Observer, Mission Study: Final Report, November 1, 2007, available at http://solarsystem.nasa.gov/multimedia/downloads.cfm.

NEW FRONTIERS MISSION OPTIONS 55 Characterize the interaction of Ganymede’s magnetosphere with Jupiter’s magnetosphere Identify the dynamical processes that cause internal evolution and near-surface tectonics of all four moons Determine the extent of differentiation of all four satellites Characterize the near-surface tectonic and volcanic processes and their relation to interior processes The committee notes that there have also been technological developments since the decadal survey that may make such a mission more feasible now than it was 5 years ago. In particular, radiation-hardened electronics have been developed that would be vital to a Ganymede mission, and the radiation environment in the Jovian system is much better understood now that data from Galileo have been fully analyzed. Also, the Juno mission to Jupiter demonstrates the feasibility of using a solar-powered satellite at Jupiter’s distance from the Sun, and NASA’s development of a Stirling engine could also help enable this mission. Conclusions The Galileo mission to Jupiter provided relatively limited information on Ganymede’s internal structure and magnetic field. Numerous fundamental questions about Ganymede remainquestions that bear on essential scien- tific objectives identified in the decadal survey. Thus, a mission to explore Ganymede in depth has great potential for substantial science return. Furthermore, the results from Galileo’s radiation data and the Jupiter System Orbiter demonstrate the spacecraft longevity possible at Ganymede’s distance from Jupiter; therefore, a platform located at Ganymede could also provide potential for long-term monitoring of other high-priority targets in the Jovian system. Finally, the success of the Juno mission illustrates the feasibility of solar power at Jupiter. A Ganymede orbiter was identified as a potential medium-class mission in the decadal survey, which stated: “No detailed studies are yet available, and the assumption that this mission could achieve the stated goals within this cost category rests partially on assuming that the lesser radiation environment and heritage from the Europa Geophysical Explorer mission would allow significantly reduced costs.”65 The development of the Juno mission and the more recent NASA Science Definition Team investigation of the flagship-class Jupiter System Observer could produce a mission that ultimately would achieve orbit around Ganymede, characterizing its surface in detail as well as its gravity and magnetic fields, thereby accomplishing a multitude of science objectives. However, the committee is concerned whether a spacecraft orbiting Ganymede would be feasible under New Frontiers budgetary constraints, given the results of NASA’s billion-dollar-box study in 2007. 66 Nonetheless, such a rich array of fundamental science questions can be addressed at Ganymede that a New Frontiers mission that focuses on answering a subset of these questions would be very worthwhile. The commit- tee concluded that a spacecraft going into Ganymede orbit may not be required. If a significant number of such questions can be addressed without the spacecraft going into Ganymede orbit, significant cost savings may ensue, which would more easily accommodate the New Frontiers cost caps. In addition, such a mission would enable broader goals within the Jovian system. Mission-Specific Recommendations A Ganymede Observer mission that addresses fundamental goals for solar system exploration may be possible and would also enable broader goals within the Jovian system. Consequently, such a mission should be included in the next New Frontiers announcement of opportunity. Because the Ganymede Observer was not described in significant detail in the decadal survey, the committee chose to list science objectives that such a mission could address, but stresses that this list should not be exclusive. In no case should these science questions be considered to be mission requirements—they are merely options for such a mission. This list includes far more science than can be included in a single New Frontiers mission and the committee stresses that it fully expects those proposing 65NewFrontiers in the Solar System, p. 133. 66See K. Reh, J. Elliott, T. Spilker, E. Jorgensen, J. Spencer, and R. Lorenz, Titan and Enceladus $1B Mission Feasibility Study Report, JPL D-37401 B, Jet Propulsion Laboratory, Pasadena, Calif., January 30, 2007.

56 OPENING NEW FRONTIERS IN SPACE such a mission to choose among these science objectives. It will be up to the proposers to make the case as to why some science objectives are more important than others. These objectives, which are not prioritized, include: • Understand Ganymede’s intrinsic and induced magnetic fields and how they are generated, and characterize their interaction with Jupiter’s magnetic field. • Determine Ganymede’s internal structure, especially the depths to and sizes or thicknesses of the probable metallic core and deep liquid water ocean, and the implications for current and past tidal heating and the evolution of the Galilean satellite system as well as ocean chemistry. • Understand Ganymede’s endogenic geologic processes, e.g., the extent and role(s) of cryovolcanism, the driving mechanism for the formation of the younger, grooved terrain, and the extent to which Ganymede’s tectonic processes are analogs for tectonics on other planetary bodies (both icy and silicate). • Document the non-ice materials on Ganymede’s surface and characterize in detail the connection between Ganymede’s magnetosphere and its surface composition (e.g., polar caps). • Document the composition and structure of the atmosphere, identifying the sources and sinks of the atmo- spheric components and the extent of variability (spatial and/or temporal). Under a New Frontiers budget it is likely that the most feasible way to address these objectives is by a Jupi- ter-orbiting spacecraft with multiple Ganymede flybys—in other words, the spacecraft may not have to enter Ganymede orbit. Even so, it is possible that such a mission may exceed the New Frontiers cost cap. Nevertheless, innovative approaches might be able to circumvent these problems and enable fundamental Ganymede science under New Frontiers constraints. Innovative Mission Options During the course of this study, the committee was impressed with the abilities of those competing in both the Discovery and New Frontiers programs to develop innovative ideas about how to accomplish their proposed missions. Missions that were considered nearly impossible less than two decades ago—like a solar-powered spacecraft at Jupiter or a Mercury orbiter—can now be done because clever solutions have been developed by principal investigators. The committee believes that this is a strength of the competitive process that will increase the probability that NASA will receive New Frontiers proposals that are realistic and doable within the constraints of the program. The committee was also impressed with arguments it heard about the importance of innovation and the risks of being overly specific regarding how to accomplish the goals of the decadal survey. Thus, in addition to the eight missions identified by the committee in this report, NASA should offer an additional option for other missions in the same size class that offer compelling answers to high-priority science questions from the decadal survey. The committee heard of several proposals for missions in the New Frontiers class that were not explicitly drawn from the decadal survey. Although the committee is not recommending any of these missions for the next New Frontiers announcement of opportunity, it is unwilling to explicitly rule them out. In order for the New Frontiers Program to remain healthy over the long run, it must maintain an influx of new ideas and a growing applicant pool for new missions. Finally, as the sections above on the eight proposed missions demonstrate, scientific understanding of the solar system has continued to advance since the decadal survey. Thus, there may be new science to be explored that was not included in the survey and may be viable as the basis for a New Frontiers mission. Thus, the committee concluded that NASA’s next New Frontiers announcement of opportunity should not be strictly limited to the eight mission options discussed in detail above, but should also be open to proposals with extraordinary justification and inventiveness. This was the foundation for the committee’s third top-level recommendation: Recommendation 3: NASA should consider mission options outside the three remaining and five additional medium-size missions described in the decadal survey that are spurred by major scientific and technologi- cal developments made since the decadal survey. As with any New Frontiers mission, these proposals must

NEW FRONTIERS MISSION OPTIONS 57 offer the potential to dramatically advance fundamental scientific goals of the decadal survey and should accomplish scientific investigations well beyond the scope of the smaller Discovery Program. Both mission- enabling technological advances and novel applications of current technology could be considered. However, NASA should limit its choices to the eight specific candidate missions unless a highly compelling argument can be made for an outside proposal.

Next: 3 Summary of Mission-Specific Recommendations »
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