An Astrobiology Strategy for the Exploration of Mars (2007) / Chapter Skim
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2 The Present State of Knowledge About Mars and Possible Life
2 The Present State of Knowledge About Mars and Possible Life
Pages 19-40
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From page 19... ...
However, current understanding of the limits of terran life continue to expand as living forms are found in FIGURE 2.1 The layers in Candor Chasma mimic those created by sedimentation in underwater environments on Earth. This image from the Mars Orbiter Camera on the Mars Global Surveyor spacecraft is courtesy of NASA/JPL/Malin Space Science Systems.
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From page 20... ...
Abundant ground ice, however, may be present and available to interact with the atmosphere, and to enhance its water content should conditions change.16 Under present conditions, at depths greater than a few tens of centimeters below the surface at latitudes in excess of 50° north and south, water ice is stable. Consistent with these conditions, large fractions of ice have been detected just below the surface at these latitudes by orbital
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From page 21... ...
However, the young crystallization ages of most martian meteorites22 and the apparent youthfulness of some volcanic features suggest that Mars today is volcanically active, at least intermittently. Heat flow under volcanic regions such as Tharsis may be significantly larger than the average, and the cryosphere correspondingly thinner.
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From page 22... ...
These charac teristics can be interpreted as suggesting that a mixture of sediment and a fluid with the properties of liquid water emerged from the crater wall and ran down through a preexisting gully channel within the last 5 years. These images were taken with the Mars Orbiter Camera on the Mars Global Surveyor spacecraft and are courtesy of NASA/JPL/Malin Space Science Systems.
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From page 23... ...
Another possibility is that large impacts or large volcanic eruptions episodically altered surface conditions temporarily,28 thereby briefly stabilizing liquid water at the surface. At the end of the Noachian, the rate of formation of valley networks declined rapidly, although not to zero, erosion rates fell precipitously, and clay minerals appear to have stopped forming.29 There can be little doubt that a major change in surface conditions occurred at the end of the Noachian.
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From page 24... ...
Any terminal lakes or seas would thus have frozen, and ultimately the ice would have sublimated away or been buried. Observational evidence for any such oceans is sparse, although there is good evidence in the northern plains for burial of pre-flood craters and ridges by sediments, 34,35 and several features suggest the former presence of ice in the low areas at the ends of the channels (Figure 2.6)
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From page 25... ...
Several large channels also start in box canyons to the north of the main canyons, indicating that liquid water was present locally at elevations well above the floor of the main canyons. The central and eastern sections of the canyons contain thick stacks of layered sediments, rich in sulfates, which could have been deposited subaqueously.
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From page 26... ...
Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.
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From page 27... ...
Weathering and erosion rates declined rapidly to very low rates, which resulted in dominantly cold surface conditions and development of a thick cryosphere. Large floods episodically flowed across the surface leaving behind temporary lakes or seas, which could have proved temporary refuges.
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From page 28... ...
The branching ridges on the delta surface are former water courses left higher than their surroundings because of greater resistance to erosion. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.
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From page 29... ...
Faulting appears to have triggered massive release of groundwater. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.
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From page 30... ...
Image from the Mars Orbiter Camera on the Mars Global Surveyor spacecraft courtesy of NASA/JPL/ Malin Space Science Systems.
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From page 31... ...
Formation of the valleys may have been accompanied by hydrothermal activity. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.
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From page 32... ...
Despite the negative results from the life-detection investigations, the Viking mission returned invaluable information for future biological experiments, such as data on the oxidizing nature of the surface and the possible scarcity of organics. In hindsight, the Viking missions constitute a compelling argument in favor of the kind of systematic approach advocated in this report and in NASA's 1995 report An Exobiological Strategy for Mars Exploration.51 The Search for Life on Early Earth and in the Martian Meteorite ALH 84001 The lack of a conclusive set of criteria for life detection and preservation has been illustrated recently by two debates: the search for the oldest evidence of life on Earth and the raging debate on the claims for life in ALH 84001.
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From page 33... ...
announced that they had found evidence for life in the martian meteorite ALH 84001,74 a coarse-grained igneous rock (an orthopyroxenite) that crystallized 4.5 billion years ago.
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From page 34... ...
While initial excitement focused on the significance of methane as a potential biomarker, what is known about the planet is consistent with a variety of possible origins for methane. 87 Geological sources of methane include volcanic emissions and production of methane via low-temperature rock-water reactions as well as thermogenic gas from recycling of buried organic remnants from putative past life.
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From page 35... ...
. Terran organisms, from Earth Organisms not present on the craft measuring them, but previously transferred from Earth by either meteorite impact or contamination of previous spacecraft; target molecules could include individual genes, membrane constituents, specific enzymes, and co-enzymes that would be expected to be overexpressed or adapted in martian conditions Terran organisms, evolved on Mars Organisms that utilize terran biochemistries and have evolved on Mars; target molecules could include individual genes, membrane constituents, specific enzymes, and co enzymes that would be expected to be overexpressed or adapted in martian conditions or organisms using metabolisms that would not be present on a spacecraft contaminant such as methanogens, or psychrophiles with endolithic survival mechanisms Non-terran organisms Organisms that utilize an array of molecules for information storage, information transfer, compartmentalization, and enzymatic activity that differ from those used by extant terran life; examples include the use of novel amino acids and nucleotides or the use of novel nitrogen utilization strategies Fossil biomarkers Established terrestrial fossil biomarkers such as hopanes, archaeal lipids, and steranes for detection of the diagenetic remains of terran life; characterization of potential breakdown products that can be reasonably extrapolated from the detection of molecules constituting an extant martian life form; detection of the diagenesis products of fossil martian organisms based on carbon compositions consistent with biological fractionation of a narrow range of abiotic precursors many of the characteristics potentially conducive for life.
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From page 36... ...
Peulvast, "Formation of Recent Martian Debris Flows by Melting of Nearsurface Ground Ice at High Obliquity," Science 295:110-113, 2002.
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From page 37... ...
Shaw, and R.G. Strom, "Evidence for Ancient Continental Glaciation in the Martian Northern Plains," Journal of Geophysical Research 100:5351-5368, 1995.
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From page 38... ...
Maechling, and R.N. Zare, "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH 84001," Science 273:924-930, 1996.
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From page 39... ...
McKay, "Evidence for Exclusively Inorganic Formation of Magnetite in Martian Meteorite ALH 84001," American Mineralogist 89:681-695, 2004.
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