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Pages 339-366

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From page 339...
... 13 Question 10: Dynamic Habitability Where in the solar system do potentially habitable environments exist, what processes led to their formation, and how do planetary environments and habitable conditions co-evolve over time? The past several decades of exploration of Earth's biosphere have expanded our knowledge of the range of environments with liquid water, nutrients, and energy sources that sustain life.1 Simultaneously, the past several decades of planetary exploration have revealed multiple ancient and modern potential habitable environments across the solar system.
From page 340...
... 340 ORIGINS, WORLDS, AND LIFE At present, with a single example of an inhabited world (Earth) , we lack the information to fully understand the conditions that lead to -- or prohibit -- the origin and sustenance of life on worlds in the solar system.
From page 341...
... QUESTION 10: DYNAMIC HABITABILITY 341 FIGURE 13-2  Question 10. Some of the major components of what constitutes a habitable environment.
From page 342...
... 342 ORIGINS, WORLDS, AND LIFE For example, any early ocean and atmosphere on Earth would have been sterilized by the Moon-forming impact. Subsequently, as Earth cooled and water oceans became reestablished, early Earth may have had a CO2- or a CH4-rich atmosphere akin to Titan today, depending on the balance of input from volcanism and impacts.
From page 343...
... QUESTION 10: DYNAMIC HABITABILITY 343 TABLE 13-1  Question 10. The Factors That Govern Planetary Habitability, and Whether Those Factors Are Present for Select Planetary Bodies Across the Solar System NOTE: Cells with a check and question mark signify likely/probable.
From page 344...
... 344 ORIGINS, WORLDS, AND LIFE exoplanetary systems (Question 12, Chapter 15)
From page 345...
... QUESTION 10: DYNAMIC HABITABILITY 345 around the giant planets. The habitable zone, as defined by the presence of liquid water (Q10.1)
From page 346...
... 346 ORIGINS, WORLDS, AND LIFE Q10.2c What Can Small Bodies Reveal About Habitability, and Are There Habitable Dwarf Planets Today? Results over the past decade by NASA's Dawn and New Horizons missions revolutionized our understanding of habitability related to small solar system bodies, such as asteroids, comets, and trans-neptunian objects (TNOs)
From page 347...
... QUESTION 10: DYNAMIC HABITABILITY 347 a contribution has been suggested for highly hydrated dust grains resulting from the interaction of the solar wind with planetesimals (Daly et al.
From page 348...
... 348 ORIGINS, WORLDS, AND LIFE The availability of liquid water at the surface of the terrestrial planets is determined in part by surface temperature, which itself is a function of distance from the Sun and atmospheric greenhouse warming. Although only small amounts of water are present in the clouds today, early Venus may have had a much more clement climate before a greenhouse effect drove liquid water into the atmosphere then away to space (Q6.2b)
From page 349...
... QUESTION 10: DYNAMIC HABITABILITY 349 antifreeze in the oceans themselves. Ammonia and salts can facilitate melting by freezing point lowering, but an interior heat source is still required.
From page 350...
... 350 ORIGINS, WORLDS, AND LIFE Mars, chondrite parent bodies, Ceres, Enceladus, Titan, Pluto, comets, the atmospheres of the giant planets, and possibly Venus (Table 13-1)
From page 351...
... QUESTION 10: DYNAMIC HABITABILITY 351 FIGURE 13-4  Question 10. Atmospheric chemistry leads to formation of complex organics on Titan.
From page 352...
... 352 ORIGINS, WORLDS, AND LIFE FIGURE 13-5  Question 10. Oxidation and hydration of silicate rocks in reactions that produce iron-bearing phases, serpentine, and other phyllosilicates also generate hydrogen, which can serve as an energy source for life.
From page 353...
... QUESTION 10: DYNAMIC HABITABILITY 353 preservation in the presence of H2S. So far, the martian organics can all be explained by abiotic processes, although our understanding of organic compounds on Mars is nascent, and strategies are being developed to discriminate between abiotic and biotic organics (Question 11)
From page 354...
... 354 ORIGINS, WORLDS, AND LIFE H2O and H2S. Whether there is phosphorus in the Venus atmosphere remains unclear: P sourced from volcanic eruptions could exist as P4O6 in the lower atmosphere, before being lofted to middle- and upper atmospheric levels and converted to PH3; this latter phenomenon was recently tentatively observed.
From page 355...
... QUESTION 10: DYNAMIC HABITABILITY 355 of ice may transport oxidants to Europa's ocean (e.g., Hand et al.
From page 356...
... 356 ORIGINS, WORLDS, AND LIFE Q10.5c How and Why Have the Inventories, Forms, and Distribution of Life-Supporting Elements Changed Through Time? The availability of CHNOPS on a planet varies with time by mechanisms similar to those controlling the availability of water (Q10.3)
From page 357...
... QUESTION 10: DYNAMIC HABITABILITY 357 Q10.6a What Are the Available Energy Sources for Life? All life as we know it exploits energy via thermodynamic disequilibria.
From page 358...
... 358 ORIGINS, WORLDS, AND LIFE only sustain life in a dormant state against environmental stressors but to support metabolism, mobility, growth, and reproduction (Hoehler 2007)
From page 359...
... QUESTION 10: DYNAMIC HABITABILITY 359 hydrological parameters that control fluid circulation. In some cases, such processes might become transiently self-sustaining: for example, volumetric expansion associated with mineral hydration, as observed in serpentinization systems, can lead to fresh fracturing and, hence, further introduction of fluid.
From page 360...
... 360 ORIGINS, WORLDS, AND LIFE Q10.7b What Endogenous Factors Control the Continuity of Habitability? The duration of habitability may be most strongly affected by the processes associated with heat transfer and loss over time, which are themselves a function of planetary size and starting composition (e.g., Ehlmann et al.
From page 361...
... QUESTION 10: DYNAMIC HABITABILITY 361 • Develop technologies for in situ measurement of light and radiogenic isotopes that trace the fluxes of key CHNOPS species and geological evolution in absolute time by miniaturization and maturation of instruments for isotope measurement on landed missions to terrestrial and ocean worlds. • Increasingly high-fidelity analyses of organics and metal isotopes that determine abiogenic versus biogenic organic production as well as reservoirs and fluxes of key elements for biology by development of sample return technology for silicate, ice, and atmospheric samples and advanced facilities for returned sample analysis and curation.
From page 362...
... 362 ORIGINS, WORLDS, AND LIFE Glavin, D.P., C.M.O'D. Alexander, J.C.
From page 363...
... QUESTION 10: DYNAMIC HABITABILITY 363 NASEM (National Academies of Sciences, Engineering, and Medicine)
From page 364...
... 364 ORIGINS, WORLDS, AND LIFE Webster, C.R., P.R. Mahaffy, J
From page 366...
... Q11 PLATE: A true-color "selfie" of the Perseverance rover, accompanied by the Ingenuity helicopter, on the surface of Mars in 2021. SOURCE: Courtesy of NASA/JPL-Caltech/MSSS.

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