Origins, Worlds, and Life A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2022) / Chapter Skim
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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 last 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.
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Q10.1a What Are the Environmental Characteristics Required for Habitability? The principal habitability criteria at the planetary scale are "the presence of liquid water, conditions favorable for the assembly of complex organic molecules at some time during the planet's history, and energy sources to sustain metabolism" (NASEM 2019)
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FIGURE 13.2 Question 10. Some of the major components of what constitutes a habitable environment.
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Q10.1b How Is Habitability Sustained, Changed, or Lost Over Time? The solar system is rife with examples of changing habitability over time.
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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.
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Venus Venus holds critical clues for understanding the role of climate change for a world becoming uninhabitable. Venus' 740 K surface is clearly uninhabitable today.
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raising atmospheric temperature and pressure above what we observe today and potentially making ephemeral occurrence of liquid water possible. Orbiters have identified ground ice, ice sheets, and glaciers distributed from the poles to the mid-latitudes within 1 m of the surface (Morgan et al.
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could deliver methane from the interior to the atmosphere (Lopes et al. 2013 and references therein)
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● Determine the distribution of past and present subsurface oceans –fully liquid and muddy– and their historical evolution through detailed investigations using detailed geological/geophysical investigations and modeling efforts coupled with a search for oceans by remote sensing. ● Determine the evolution of the climate of Mars and Venus and the timing of changes by measurement of atmospheric gases, chemistry and isotopes in the atmosphere and rocks, and climate modeling.
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Q10.3b What Are the Long-Term Endogenic and Exogenic Controls on the Presence of Liquid Water on Terrestrial Planets? Aside from Earth, we have the most information to disentangle processes controlling availability of water on Mars (Figure 13.3)
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Volcanism supplies water and other gases to terrestrial planet surfaces and atmospheres through much of their history. The release of volatiles is directly correlated with the amount of volcanism, but the amount of water supplied depends critically on the amount of water incorporated during accretion, the redox state of the interior, and styles of volcanism, particularly temperature and degree of partial melting.
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Callisto, for example, experiences essentially no tidal energy dissipation, but it has a sufficiently high interior rock mass so as to sustain an ocean by radiogenic decay. Triton may have experienced considerable internal heating from tides during the early evolution of its orbit around Neptune.
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Q10.4 ORGANIC SYNTHESIS AND CYCLING: WHERE AND HOW ARE ORGANIC BUILDING BLOCKS OF LIFE SYNTHESIZED IN THE SOLAR SYSTEM? Organic molecules are an essential component of planetary habitability, as they enable a variety of chemical structures and specific reactivity that is foundational to the biochemistry of life as we know it.
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oxygen sources are available, even more complex compounds can be formed by the incorporation of these elements into organic structures, leading to the formation of prebiotic molecules. FIGURE 13.4 Question 10.
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predicted to be richer in prebiotically relevant nitriles than at Titan (Wong et al.
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amorphous carbon. Select locations on the surface are enriched in organic chains of atypical composition or abundance, thought to be generated from endogenous hydrothermal processes (De Sanctis et al.
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tend to form species that are solid only at low temperature (e.g., CO2, NH3, CO, CH4, N2, H2S) , these compounds were depleted in inner solar system solids and enriched in outer solar system materials (as observed in comets today)
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FIGURE 13.6 Question 10. Plumes and overturn of a fractured ice shell are means of exchanging matter between surface and ocean and volcanism, vents, and infiltration of fractured crust are means of exchange between ocean and core within icy satellites.
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can generate disequilibrium, e.g., subsurface buildup of reduced gases from water-rock reactions and periodic release into a more oxic atmosphere. Strategic Research for Q10.5 ● Determine if life-supporting chemical species, including reduced carbon- and phosphorus bearing molecules, are present in the atmosphere of Venus, via in situ atmospheric measurements.
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reactions with carbon dioxide. On Earth, oxygenic photosynthesis evolved relatively late, but anoxygenic photosynthesis likely sustained the earliest photosynthetic life forms, including early Archean shallow-water stromatolites (Q9.3)
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(e.g., precessional) cycles.
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Strategic Research for Q10.6 ● Geochemically characterize past and present environments with liquid water to determine whether there is/was energy to sustain metabolic processes of life by in situ or sample analysis of waters and preserved water-formed mineral and chemical species to determine concentrations of major ions, electron donors and acceptors, mineral products, and other relevant chemical species. ● Characterize the compositional and geological heterogeneity of potentially habitable worlds at progressively smaller scales (km- to cm-scale)
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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.
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SUPPORTIVE ACTIVITIES FOR QUESTION 10 ● Improved radiative transfer modeling and photochemical modeling in planetary atmospheres for models of climate and organics production by fundamental laboratory measurements and computational work to obtain photochemistry reaction coefficients and gas absorption parameters. ● Improve the characterization of diverse ices and the mineral and organic products of water-rock interaction that are fingerprints of habitability by fundamental laboratory measurements to obtain and compile libraries of ice, mineral, and organic spectra and optical constants at ultraviolet to far infrared wavelengths.
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Coustenis, A
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Kasting, J.F.
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Publications of the Astronomical Society of the Pacific 132(1014)
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Wong, M.L., Y.L.
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Q11 PLATE: A true-color "selfie" of the Perseverance rover, accompanied by the Ingenuity helicopter, on the surface of Mars in 2021. SOURCE: NASA/JPL-Caltech/MSSS.
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