An Astrobiology Strategy for the Search for Life in the Universe (2019) / Chapter Skim
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2 Dynamic Habitability
2 Dynamic Habitability
Pages 17-43
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From page 17... ...
(See Box 4.1 for a discussion of the role viruses may have played in the transition from abiotic to biotic processes.) The broad strokes of this paradigm are still recognizable in much of modern prebiotic chemistry, which is laid out in detail in Chapters 1 to 3 of the 2015 Astrobiology Strategy.
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: • A means to sustain thermodynamic disequilibrium; • An environment capable of maintaining covalent bonds, especially between carbon, hydrogen, and other atoms; • A liquid environment; and • A self-replicating molecular system that can support Darwinian evolution. Whether or not life elsewhere in the solar system or beyond might be similar to terran life remains to be determined.
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From page 19... ...
At the planetary scale, characteristics of dynamic habitability include stellar evolution and its impact on the presence of liquid water over time; the evolving structure of the planet interior influencing, among other things, the magnetic field and plate tectonics; and the state of the atmosphere, which might, for ex ample, redistribute sources of metabolic energy. On more local scales, dynamic habitability encompasses changing environmental conditions driven not only by planetary-scale dynamics, but also by more local processes like fluid flow and mixing or water-rock reactions, which can alter thermal and chemical gradients and therefore control not only available energy sources but also energy gradients and fluxes.
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The global extent and temporal ambiguity of these conditions further limit the extent to which prebiotic chemistry research can truly reflect early Earth environments. The importance of constraining environmental conditions to better characterize early Earth prebiotic chemistry is emphasized in Chapters 1 and 2 of the 2015 Astrobiology Strategy (NASA 2015)
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From page 21... ...
. In addition to the variety of mineralogies available on the early Earth, the effects of individual parameters, including pH, salinity, fluid chemistry, and pressure on a range of prebiotic processes -- from CO2 reduction to RNA polymerization -- are being investigated to discriminate those physicochemical conditions that could or could not give rise to viable prebiotic chemistry (e.g., Cody et al.
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From page 22... ...
" Constraining the specific location for the emergence of life on early Earth is a robust area of investigation and is still hotly debated. However, investigations of Earth's potential prebiotic chemistry have highlighted the overall global planetary conditions and specific types of environments that might prove to be viable targets for the rise of life elsewhere.
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Finding: Planetary conditions that may be habitable today or in the past are not necessarily the same as those that could have fostered the emergence of life. Both are important for the search for life.
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Efforts to consider their impact are tied to a better understanding of the thermodynamic drivers of the ecosystems -- whether abundant energy is available or not, and if the life exhibits vigorous, slow, or even dormant metabolic processes. With increasing interest in extreme surface and subsurface environments on other planets, understanding the role of these systems and their impact on planetary evolution and life detection is critical, making them a frontier of astrobiological research for which a number of notable examples are provided here.
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Since publication of the 2015 Astrobiology Strategy, however, attention has been turning increasingly to the idea of the modern martian surface as one existing on the edge of habitability (Ehlmann 2018)
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The timing of the boundaries between Mars's three major named geological periods is highly uncertain because of the absence of an absolute calibration of the ages of martian surface features. Moreover, the geological record of the earliest events in martian history, those of the so-called pre-Noachian era, has been largely erased by subsequent events, including the heavy bombardment that took place during the Noachian era.
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For example, salts increase the range of environmental conditions under which liquid water may exist (Fischer et al.
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2016) and the role of water-rock interactions in potential habitability of ocean worlds and icy bodies such as Europa and Enceladus (Bouquet et al.
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DYNAMIC HABITABILITY 29 FIGURE 2.4 Brine habitats are found around the world in the surface and subsurface.
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Marine Subsurface and Ocean Worlds Deep submarine oases, such as hydrothermal vents (Figure 2.5) , highlight the potential for extensive ecosystems in highly dynamic tectonic settings.
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2014a,b) , which is insufficient to fracture Europa's ice shell (Goldreich and Mitchell 2010)
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SOURCE: Kevin Hand, Jet Propulsion Laboratory, "On the Habitability of Ocean Worlds," presentation to the Workshop on Searching for Life across Space and Time, December 5, 2016. Enceladus, too, likely possesses a subsurface ocean (Iess et al.
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, versus the potentially slow rate of metabolic consumption in subsurface environments (Lin et al. 2006; Hoehler and Jorgenson 2013; Sherwood Lollar et al.
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2015. A prebiotic chemistry experiment on the adsorption of nucleic acids bases onto a natural zeolite.
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Geophysical Research Letters 43:4172-4179. Biondi, E., Y
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2014. Experimental evidence for the formation of liquid saline water on Mars. Geophysical Research Letters 41(13)
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Journal of Geophysical Research 106(E10)
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2016. Surface interaction of ribonucleic acid constituents with spinel ferrite nanoparticles: A prebiotic chemistry experiment.
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2016. The evolution of the surface of the mineral schreibersite in prebiotic chemistry.
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Geophysical Research Letters 31(L01701)
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Geophysical Research Letters 40:245-249. Schmidt, B., D
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Geophysical Research Letters 43(10)
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From page 43... ...
Geophysical Research Letters 42:712-719. Walker, J.J., J.R.
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