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6 The Search for Life in the Coming Decades
Pages 101-143

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From page 101...
... THE PHYSICAL AND CHEMICAL EVOLUTION OF THE EARLY SOLAR SYSTEM AND PREBIOTIC EARTH The stage for the emergence of life was set long before the rise of prebiotic chemistry. Condensation of the solar nebula, disc formation, stellar activity, planetary accretion and differentiation, and the composition and impact frequency of asteroids and comets all determine the conditions within which life might emerge and survive.
From page 102...
... Many phenomena related to the early evolution of the solar system were critical to the habitability of the terrestrial planets in general, such as Venus and Mars, as well as Earth and probably a number of the satellites around the gas giants, including Europa, Callisto, Enceladus, Titan, and possibly others. Thus, understanding the creation of a habitable environment for Earth in terms of the emergence of life is relevant for the other bodies in the solar system and beyond as well.
From page 103...
... The fundamental chemistry of life is based on oxidation/ reduction reactions -- that is, the chemistry of electron transfer. The oxidation/reduction reactions that drive prebiotic chemistry rely either on chemical and thermal disequilibria generated as Earth cools or by solar ultraviolet radiation.
From page 104...
... This coevolution can be direct and causative, indirect, fundamentally disconnected, or stochastic. While Earth is presently our singular example of planetary and biosphere coevolution, nearby terrestrial planets have their own geologic history.
From page 105...
... Studying such periods will not only inform whether or not a continuous biosphere maintains a planet's habitability, but also will help identify potential biosignatures by which such organisms might be recognized. Periods of Catastrophic Change How do periods of catastrophic change reflect the balance of influence between planetary dynamics and the biosphere?
From page 106...
... DIVERSE HABITABLE CONDITIONS AND SUBSURFACE WORLDS Earth may represent only one end-member of a life-hosting planet, even among bodies with similar initial conditions and geophysical processes. The deep subsurface on Earth, Mars, terrestrial planets in other systems, and the ocean worlds all have a diversity of environmental conditions that share some degree of similarity, and could be habitable in similar ways.
From page 107...
... • Habitable environments in the martian subsurface -- What is the spatial and temporal distribution of subsurface water, the sources and sinks for methane and other reduced gases such as hydrogen, and the relevant water-rock reactions capable of sustaining habitable environments in the subsurface on Mars? • Habitability of ocean worlds -- What are the chemical inventories and sources of energy that could generate habitability on ocean worlds, and what processes sustain these inventories?
From page 108...
... Astrobiology is not just about the search for extraterrestrial life, but also the broader scientific understanding of how habitable planets form, what makes them habitable, and the processes that sustain life. Habitable Environments in the Martian Subsurface What is the spatial and temporal distribution of subsurface water, the sources and sinks for methane and other reduced gases such as hydrogen, and the relevant water-rock reactions capable of sustaining habitable environments in the subsurface on Mars?
From page 109...
... Small satellites delivered as secondary payloads are capable of monitoring large areas of the planet at lower costs and can complement rover exploration missions by accessing sites that rovers cannot reach. Looking to the future, an integration of the multiple approaches described above could address the role of subsurface processes in governing habitability, the preservation of habitable environments, and the preservation of biosignatures.
From page 110...
... Habitability of Ocean Worlds What are the chemical inventories and sources of energy that could generate habitability on ocean worlds, and what processes sustain these inventories? The ocean worlds of the outer solar system are compelling both due to their potential for extant life as well as for exotic life as we do not know it.
From page 111...
... Therefore, the ocean worlds of the outer solar system are the likeliest to answer questions about what alternative biospheres might look like on another planet and what processes of energy cycling they use. Europa Clipper, currently slated to launch in early- to mid-2022, is the first systems-level mission to an ocean world, and the first motivated primarily by characterizing the moon's potential for habitability.
From page 112...
... The nature and habitability of these alien worlds will be a key step in understanding the probability of life in the universe. Future direct imaging missions would allow us to probe the surface environments of terrestrial planets orbiting stars like the Sun to search for oceans and signs of life.
From page 113...
... • Evolution of terrestrial planets -- How do terrestrial planets evolve around different stellar types? • Nearly habitable exoplanets -- Do nearby stars host habitable planets?
From page 114...
... Planets? Transiting Exoplanet Survey Satellite Nearly all-sky precision Demographics of Multiplanet sys- Demographics of Frequency of Identify optimal relative photometry to nearby exoplanetary tems, dynamical terrestrial planets potentially ter- targets for JWST detect transiting planets systems for relatively interactions for a broad range of restrial planets follow up to short-period planets stellar host types orbiting M search for the po dwarfs tential habitability of terrestrial plan ets in the habitable zones of nearby, bright M dwarfs James Webb Space Telescope Transit spectroscopy Atmospheric compo- Variation of atmo- Atmospheric Existence and Search for sition, volatiles spheric composi- composition for composition of biosignatures tion of low-mass M-dwarf terrestrial atmospheres for a handful of planets with planet planets as a function for M-dwarf M-dwarf habitable mass, distance of orbital distance terrestrials zone terrestrials from star, and host star properties Secondary eclipse and Atmospheric com- Atmospheric Possible Search for thermal phase curves position, and day- composition and measurements biosignatures for night temperature day-night tempera- of atmospheric M-dwarf planets contrasts for more ture contrasts for composition massive planetary hotter and larger via near- and companions terrestrials mid-infrared spectroscopy for a handful of M dwarf habitable zone planets High-contrast photom- Disk demograph- Disk dynamics, Variation of disk etry and spectroscopy of ics and composition volatile delivery properties with stel protoplanetary disks of young stars and lar age and mass nascent planetary systems Ground-Based <10 m Facilities Precision radial Mass measurements, Architectures and Detection, mini- Detection of ter velocities long-period planets dynamics of multi- mum masses, and restrial planets planet systems orbits of low-mass, around nearby habitable-zone plan- low-mass stars ets around nearby and possibly low-mass stars with Sun-like stars.
From page 115...
... around planets orbiting nearby low-mass the smallest stars stars Sub-millimeter imaging Protoplanetary disks, volatiles, ices Mini- and Nanosatellites (CubeSat) Time-resolved photom- Demographics of Stellar activity Environments of Stellar activity etry exoplanets exoplanets as a and ultraviolet function of host- photometry for star mass and age, M dwarfs as a detection of transits function of stel of low-mass planets lar mass and age; identified via radial mass, radius, and velocities density measure ments for transit ing low-mass planets Wide Field Infrared Survey Telescope Direct imaging spectros- Circumstellar disks, Volatile deliveries Atmospheric With starshade, copy jovian planets characterization of detection of jovian planets habitable-zone (Test-bed technologies)
From page 116...
... High resolution spectros- Stellar-activity Planetary mass Potential biosigna copy (including radial indicators and char- for habitable- tures for M-dwarf velocities) acterization zone planets planets from M-dwarf to Planetary mass Sun-like stars Atmospheric com- Atmospheric position of terrestri- composition als in transmission for M-dwarf and reflected light habitable-zone planets in transmission and reflected light Direct imaging and Young planet obser- Disk dynam- Atmospheric char- Nearby reflected Biosignatures for spectroscopy vations, solar system ics, multiplanet acterization light habitable- select targetsa analog jovian planets dynamics zone planets (M dwarfs)
From page 117...
... Indeed, because the host star has a significant impact on planetary habitability, and the star's activity and luminosity evolve considerably, it will be important to determine and observe stellar activity indicators in systems of all ages and to understand evolutionary pathways, particularly for M-type stars, to feed back into the overall picture of the evolution of habitable terrestrial planets. Nearly Habitable Exoplanets Do nearby stars host habitable planets?
From page 118...
... Terrestrial planets, which are small, are more readily observed and characterized when orbiting small M-dwarf stars, and yet M-dwarf stars, although the most common type of star in the galaxy, present many challenges to habitability for their planets. To understand whether these planets are habitable will require a coordinated effort between modelers and exoplanet and stellar observers to determine if M-dwarf planets can retain their atmospheres and oceans, and to understand the composition of M-dwarf planet atmospheres.
From page 119...
... FIGURE 6.3  Artist rendition of the Proxima Cen b, a possibly rocky planet orbiting the red dwarf Proxima Centauri, and the Alpha Centauri binary in the distance.
From page 120...
... In the 2035 timescale, all three giant segmented mirror telescope (GSMT) programs have planned second generation instruments that will be capable of direct imaging of terrestrial planets as well spectroscopic capabilities for exploring biosignatures.
From page 121...
... (Right) The James Webb primary mirror ­ and folded secondary mirror at NASA Goddard Space Flight Center getting ready for testing.
From page 122...
... . More generally, JWST will help us better understand the evolution of terrestrial planets orbiting M dwarfs, including putting constraints on atmospheric and ocean loss processes, and potentially providing an observational test of the habitable zone concept via observations of the seven planets spanning the habitable zone in the TRAPPIST-1 system.
From page 123...
... In addition, instruments are currently being designed for near-infrared direct imaging coupled with high resolution spectroscopy using Keck Observatory with the Keck Planet Imager and Characterizer (Mawet et al.
From page 124...
... Simulations have shown that the mission could be photometrically sensitive to a few nearby super-Earth planets. The accomplishment of these goals requires significant advances in space coronagraphic design, serving as a precursor to larger space missions to measure the atmospheres of terrestrial planets.
From page 125...
... . First light for these telescopes are predicted in 2028-2030, and all three programs are currently exploring second generation direct imaging capabilities for studies of exoplanets.
From page 126...
... These missions will then be able to image and obtain direct imaging spectra of nontransiting terrestrial planets within the habitable zones of a handful to several dozen more Sun-like (F, G, K) stars.
From page 127...
... SOURCE: (top) NASA Goddard Space Flight Center; (bottom)
From page 128...
... UNDERSTANDING BIOSIGNATURES IN THE CONTEXT OF THEIR ENVIRONMENT Although significant progress has been made since publication of the 2015 Astrobiology Strategy, those advances have revealed that much more work needs to be done before the majority of biosignatures are well enough understood to resolve outstanding controversies regarding the earliest evidence of life on this planet, let alone on planets, moons, or exoplanets beyond Earth. There is a pressing need for a comprehensive set of standards to guide the evaluation and testing of remote and in situ biosignatures in their environmental context and to take into account the probabilities of false positives, false negatives, and levels of uncertainty.
From page 129...
... . On planetary bodies within the solar system, these chemistries have led to a wide variety of complex organic molecules that may be preserved in the lithosphere and can be detected directly, although issues of false negatives, false positives, and differentiation from signatures produced by nonbiological processes remain a critical challenge.
From page 130...
... The potential value of a biosignature for life detection derives from a combination of the above considerations and a comprehensive framework for biosignature assessment. It reflects not only the intrinsic value of the biosignature, but also the associated propensity for both false negatives and false positives, which together create an uncertainty and likelihood for detection unique to each biosignature.
From page 131...
... The challenge for biosignature science is to strive for a comprehensive, quantitative foundation that uses multiple lines of evidence and environmental context to provide the most robust life detection framework involving interdisciplinary laboratory, field and modeling work, as well as community efforts to develop a consensus on assessment to apply to the search for life beyond Earth. Biosignature Searches in the Solar System Missions to Mars, Venus, and the ocean worlds will provide opportunities to search for biosignatures, and provide insight into the planetary processes that may also lead to biosignature false positive and negatives.
From page 132...
... ; second, retrieves the cached samples and transports them to a stable orbit about Mars; and third, collects the samples from Mars orbit and returns them to Earth. If suc cessful, the return of Mars samples to Earth for study in terrestrial laboratories will realize one of the top-level astrobiology goals identified in the 2007 NRC report, An Astrobiology Strategy for the Exploration of Mars, and the highest priority solar system science goal identified in the 2011 Vision and Voyages planetary science decadal survey.
From page 133...
... There will also be significant advances in instruments for in situ evaluation of drilled samples, improvements in contamination control together with a much more ­ ophisticated understanding of the martian regolith drawn from the experience with Curiosity and current orbiting s spacecraft. Last, but not least, international interest and participation in a Mars sample return campaign means that such an expensive undertaking would be more realistic for the individual partners thereby allowing bold science objectives to be realized and planetary protection responsibilities (Box 6.2)
From page 134...
... . Meanwhile, in the upcoming decades, SpaceX is expeditiously moving forward with ambitious Mars missions that could deploy fleets of small satellites as secondary missions, while Blue Origin has been developing concepts for lunar missions that could also carry small satellites.
From page 135...
... The following sections examine some small-scale activities that may have big payoffs in the coming decades. Small Satellite Technologies Knowledge of the atmospheric density and wind profiles as a function of altitude is critical to the planning of entry, descent, and landing of Mars missions requiring precise landing for the collection of optimum samples for astrobiology studies.
From page 136...
... SOURCE: NASA/JPL. the giant planets and Titan, where traditional radio occultation to Earth is limited to dawn or dusk, crosslink radio occultation measurements could provide the first coverage of the diurnal cycle.
From page 137...
... Comparably modest investments can transform these commercial instruments into spaceflight hardware capable of addressing key astrobiology science goals, ensuring appropriate planetary protection requirements (Box 6.2) are met, and providing the computational and memory resources needed to support on-board data analysis.
From page 138...
... . The 1967 United Nations Outer Space Treaty, to which the United States is signatory, states in Article IX that all states party to the treaty "shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination, and also adverse changes in the environment of the Earth resulting from the introduc tion of extraterrestrial matter." In addition, Article VI of the same treaty specifies that States Parties "shall bear international responsibility for national activities in outer space, including the Moon and other celestial bodies, whether such activities are carried on by governmental agencies or by non-governmental entities." Technical aspects of planetary protection are developed through the Committee on Space Research (COSPAR)
From page 139...
... • The recent transfer of NASA's Office of Planetary Protection (OPP) from the Science Mission Directorate to the Office of Safety and Mission Assurance is generally regarded as a positive change.
From page 140...
... 2016. The sustainability of habitability on terrestrial planets: Insights, questions, and needed measurements from Mars for under standing the evolution of Earth-like worlds.
From page 141...
... 2015. NASA Astrobiology Strategy 2015.
From page 142...
... 2015. The Transiting Exoplanet Survey Satellite: Simulations of planet detections and astrophysical false positives.
From page 143...
... 2010. Prediction of uncertainties in atmospheric properties measured by radio occultation experiments.


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