A Scientific Rationale for Mobility in Planetary Environments (1999) / Chapter Skim
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2 The Role of Mobility in Solar System Exploration
2 The Role of Mobility in Solar System Exploration
Pages 12-27
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From page 12... ...
As a result the atmosphere completes one rotation every 4 days, but the planet itself rotates much more slowly. Although the fastest winds are found near the cloud tops, most of the atmosphere' s angular momentum is concentrated in the very dense lower atmosphere well below the clouds.
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From page 13... ...
The boundary layer can be strongly affected by the nature of the surface, and this may vary considerably on Venus from place to place. Necessary Observations The following measurements are needed to achieve a better understanding of the circulation in Venus' s lower atmosphere: · Multiple, geographically dispersed, simultaneous, and temporally extended measurements of pressure and temperature as a function of altitude and horizontal location to determine the basic structure of the lower atmosphere; · Multiple, geographically dispersed, simultaneous, and temporally extended measurements of the winds in the lowest one to two scale heights, with sufficient accuracy and sampling rate to enable the definition of the circulation itself and the determination of horizontal- and vertical-momentum fluxes as a function of altitude and horizontal location; and · Multiple measurements of radiative fluxes, both solar and infrared, as a function of altitude and at different locations to determine the radiative forcing of the atmospheric circulation.
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From page 14... ...
In summary, a small fleet of balloons could obtain the needed measurements, offering major advantages over multiple entry probes and remote sensing from orbiters. In particular, balloons could reveal the full extent of horizontal and vertical variations in composition, structure, and wind velocity, including the important but currently poorly understood planetary boundary layer.
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From page 15... ...
Rift-like features, called chasmata, were first imaged with Earth-based radar.~5 Their dimensions, global extent, and possible tectonic significance became more apparent in Pioneer Venus altimetry datable i7 Chasmata are long, narrow troughs; some are many thousands of kilometers long but only about 100 kilometers or less wide, and several to nearly 10 kilometers deep. Some chasmata are associated with major volcanic highlands, such as Alla and Beta Regiones.
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From page 16... ...
along traverses across chasmata for distances greater than 100 km; and · Deployment of seismic stations, measurements of rock compositions, and sample collection for compositional analysis could be accomplished by rovers capable of traversing tens of kilometers, or by an aircraft or balloon that can touch down at intervals while conducting traverses across chasmata. These requirements pose significant technical challenges because of the hostile surface environment of Venus (pressures greater than 90 bars and temperatures near 730 K)
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From page 17... ...
[Mars] for extinct or extant life, including evidence of the accumulation of a reservoir of prebiotic organic compounds and the extent of any subsequent prebiotic chemical evolution." This objective is a direct outgrowth of recommendations and conclusions reached in previous reportsi9-2i and of the inference that the putative martian meteorite ALH84001 may contain evidence for past life on Mars.22 This case study is also directly relevant to one of the campaigns that NASA's solar system exploration roadmap outlines, the Evolution of Earthlike Environments, and is indirectly relevant to another, the Formation and Dynamics of Earthlike Planets.
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From page 18... ...
Sojourner provided initial experience with rover operations on Mars, and subsequent rovers in the Mars Surveyor program will, according to current plans, perform more complex activities, such as extracting core samples from rocks and soil and caching them for later return to Earth. PHYSICAL AND CHEMICAL HETEROGENEITY WITHIN SMALL BODIES Importance Comets and chondritic asteroids are thought to consist of relatively primitive materials and, thus, their study is relevant to addressing issues encompassed by the campaign, Building Blocks and Our Chemical Origins, outlined in NASA's solar system exploration roadmap.
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From page 19... ...
Geographic variations in Vesta's topography determined from Hubble Space Telescope images29 reveal the presence of a very large impact crater that apparently excavated to depths of many kilometers into the crust and possibly the mantle, in the process ejecting kilometer-sized fragments that now can be dynamically linked to Vesta through their orbital properties.30 Reflectance spectra from the floor of this large crater suggest that there are significant mineralogical differences between the surficial crust and deeper units.3i The small Vesta-like asteroids liberated from the larger parent body also have spectral properties that link them to the HED meteorites and to Vesta itself.32 Understanding the heterogeneity within individual bodies such as Vesta provides a geologic context for these meteorites. Documenting the diversity among small solar system bodies, both heavily processed like Vesta and those that may have experienced less severe processing, can address the following objectives previously identified in NRC reports 33-35 and · Determination of the record of early solar system processes and history retained by small bodies; · Constraining the nature and composition of planetesimals, such as those that accreted to form the planets; · Recognition of relationships among asteroids, comets, and extraterrestrial samples (meteorites and interplanetary dust particles)
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From page 20... ...
Need for Mobility Chemical variations for minor and trace elements that carry much information on interior source regions and on melting and crystallization processes cannot be measured remotely. Remote measurements also cannot determine radiogenic isotope compositions and absolute ages of various units, data that provide essential detailed information on the evolutionary history of the body.37 38 A more fundamental understanding of the geologic context for the HED meteorite samples from Vesta's ancient volcanic surface will first require measurements from orbit to identify both ancient flow sites and units exhibiting the most extreme diversity either through a complex volcanic history or from deep impact excavation and mixing.
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From page 21... ...
Champollion will land on the nucleus of Comet Tempel 1 in 2005, drill into the surface to the depth of approximately 1 meter, extract some 100 cc of material, and package it for return to Earth. ZONAL WINDS IN THE JOVIAN ATMOSPHERE Importance The recent Galileo mission provided strong evidence that the composition3940 and structure4~42 in some regions of the Jovian atmosphere differ greatly from those that exist in the bulk of the planet.43 The Galileo probe appears to have descended into a "desert" a region known as a Alum hot spoilt in which the temperature increases with depth along a dry adiabat (neutral stability)
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From page 22... ...
Necessary Observations To gain an improved understanding of the dynamics and compositional variations of Alum hot spots and the dynamics of the zonal winds in the Jovian atmosphere, the following measurements are needed: · Abundances of condensable species (e.g., H2O, NH3, NH4S) as functions of altitude and horizontal location in Alum hot spots and in the background atmosphere; · Compositions, abundances, and altitudes of cloud layers; · Temperatures, pressures, and winds as a function of depth and horizontal location; and · Radiative fluxes versus depth and horizontal position.
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From page 23... ...
In particular, balloons could reveal the full extent of horizontal and vertical variations in composition, structure, and wind velocity in the Jovian atmosphere. EUROPA'S INTERNAL STRUCTURE Importance The nature of Europa's internal structure and, in particular, the possibility of a liquid water ocean beneath the ice cover are crucial to the nest or present existence of life on this satellite of Juniter.54 The surface of Eurona is _ ~ ~ ~ _ ~ .
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From page 24... ...
Necessary Observations The goals outlined above for the characterization of Europa require the following observations: · Determination of the geologic structure and tectonic history of the ice crust; · Estimation of the ages of different segments of crust based on differences in impact-crater densities; · In situ analysis of young, near-surface ice to determine its chemistry, including salts, particulates, organic constituents, and possible isotopic indicators of biological activity; · Measurements of local and global values of the geothermal gradient in the ice crust within boreholes and by remote sensing using microwave techniques; · Geodetic measurements of the response of the crust to tidal forces; · Analysis of the chemical and physical properties of near-surface liquid, if present; and · Identification of the geological processes occurring at the interface between a liquid layer, if it exists, and the surrounding ice or rock. Need for Mobility Detailed measurements of Europa' s shape and gravitational field are priority goals of NASA' s Europa Orbiter mission.62 These data should provide the critical evidence needed to determine if a subsurface ocean actually exists.
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From page 25... ...
11. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, p.
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From page 26... ...
M.J. Gaffey, "Asteroid 6 Hebe: Spectral Evaluation of the Prime Large Mainbelt Ordinary Chondrite Parent Body Candidate with Implications from Space Weathering of Gaspra and the Ida-Dactyl System," Lunar and Planetary Science Conference XXVII, Lunar and Planetary Institute, Houston, Tex., 1996, pp.
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From page 27... ...
G Neukum et al., "Cratering Chronology in the Jovian System and Derivation of Absolute Ages," in Lunar and Planetary Science Conference XXIX, Abstract 1742, Lunar and Planetary Institute, Houston, Tex., 1998 (CD-ROM)
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