Preventing the Forward Contamination of Mars (2006) / Chapter Skim
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4 Environments on Mars Relative to Life
4 Environments on Mars Relative to Life
Pages 41-68
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From page 41... ...
The rest of the chapter discusses the implications for the definition of special regions2 and the need for additional observations and studies. 1Organic molecules are molecules based on carbon bonded to hydrogen or nitrogen and perhaps other atoms: organic molecules can be produced by both biological and nonbiological processes.
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From page 42... ...
derive from photochemical processes in the martian atmosphere and from the interaction of solar ultraviolet radiation with surface minerals to form superoxides (Yen et al., 2000)
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Evidence for a water-rich Mars is provided by the geomorphic interpretation of a long list of landforms (e.g., Carr and Schaber, 1977; Rossbacher and Judson, 1981; Carr, 1986, 1996; Squyres et al., 1992; Malin and Edgett, 2000, 2003) and by geochemical and sedimentary evidence, recently acquired by the Mars Exploration Rover (MER)
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From page 44... ...
This distribution is likely to be influenced by an equivalent level of geologic complexity and the spatial variability of such crustal characteristics as lithology, structure, stratigraphy, porosity, permeability, ice content, mechanical strength, and thermal properties.5 The distribution of the two principal reservoirs of deep subsurface water on Mars -- ground ice and groundwater -- is thought to be determined by the thermal structure of the crust. Current mean annual surface temperatures on Mars range from ~154 kelvin (K)
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. The distribution of ground ice is expected to follow the thermal structure of the crust, whereas the distribution of groundwater, under the influence of gravity, will drain and saturate the lowermost porous regions present at depth.
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From page 46... ...
. Thus, the present state of deep subsurface water on Mars is bracketed by two extremes: one in which a small planetary inventory, combined with the progressive cooling of the crust, has eliminated any persistent reservoir of groundwater, and another in which the planetary inventory is sufficiently large that a sizable reservoir of liquid may still survive at depth over much of the planet.
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From page 47... ...
-- properties that may have a significant influence on the local distribution of near-surface ground ice. Thus, what appears as a uniform distribution of ice at a resolution of 105 km2 may exhibit considerable variability at a scale of 1 km2, such that regions possessing very high concentrations of near-surface ice may be interspersed with smaller regions that are ice-poor.
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From page 48... ...
. Atmospheric water shows strong seasonal variations associated with the holding capacity of the martian atmosphere and the condensation and sublimation of water from nearsurface reservoirs such as ground ice, adsorbed water, and surface ice.
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Because of the low partial pressure of water vapor in the martian atmosphere, liquid water in diffusive contact with the atmosphere will evaporate rapidly, and so the stability of liquid water at the surface today is at best transient. The relatively narrow temperature range at which pure water is transiently stable at the martian surface is controlled by the atmospheric surface pressure, which exhibits both geographic and seasonal variations (Haberle et al., 2001)
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and by the Mars Express Orbiter OMEGA spectrometer (Bibring et al., 2005) suggest the widespread distribution of sulfates and other evaporite minerals on the martian surface.
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However, such water would evaporate rapidly into the dry martian atmosphere, and it would have to be replenished in order to be present over many diurnal or annual cycles. Liquid water in the near-subsurface faces similar challenges, but the requirements for resupply could be significantly reduced by the presence of diffusive barriers such as fine-grained soil, duricrust, ice, or rocks.
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This situation is reversed at times of high obliquity, when summers of continuous illumination alternate with dark winters to produce both extreme seasonal variations and higher mean annual temperatures at the poles. Recent studies have demonstrated that the evolution of the martian obliquity is chaotic, varying from ~0o to 60° (Touma and Wisdom, 1993; Laskar and Robutel, 1993; Laskar et al., 2004)
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ENVIRONMENTS ON MARS RELATIVE TO LIFE 53 FIGURE 4.7 General circulation model calculations of current martian annual maximum surface pressures (top) and annual maximum surface temperatures (bottom)
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. There could also be widespread melting of the polar ice, as well as high-latitude snow packs and nearsurface ground ice (Pathare and Paige, 1998; Costard et al., 2002; Christensen, 2003)
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3. Segregated ground ice.
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high obliquities could result in the production of substantially more liquid water where segregated ground ice occurs than would be produced for dispersed ice in soil under equivalent environmental conditions.
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These investigations suggest that in some regions, under favorable conditions, fossil ground ice might survive at shallow depth even at the equator. The acquisition of high-spatial-resolution thermal data alone is not sufficient to allow the prediction of the present local distribution of ground ice, because the thermal properties of the regolith cannot be extrapolated with any confidence beyond the depth that experiences diurnal temperature variations (essentially the top 10 to 25 cm)
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From page 58... ...
Thus, plausible permutations of crustal diffusive and thermal properties can result in intricate combinations of low- and high-permeability strata that yield substantial differences in the local distribution of ground ice. Understanding of the subsurface distribution of ice is further complicated by the lack of knowledge regarding the nature and duration of processes involved in the geologic evolution of the local crust, among the most important of which are geographic and temporal variations in crustal heat flow.
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However, for the lower abundances of hydrogen detected at equatorial and midlatitudes (where mean annual temperatures exceed the frost point) , the association with ground ice is less clear.
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. But the geometry and contrast associated with the volatile targets of greatest scientific interest, such as liquid water and massive ground ice (MEPAG, 2004)
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The chief immediate concern in locating special regions is to identify where liquid water and massive ground ice are present within the top ~25 m of the regolith, the maximum depth to which currently envisioned investigations by robotic spacecraft will be capable of reaching over the next decade. In addition, because variations in regolith thermal and diffusive properties -- capable of preserving liquid water or ice at shallow depth -- can occur at small scales, an understanding of the distribution and state of subsurface H2O at as high a resolution as practically possible (ideally, at a scale equivalent to that defined by the operational activities and landing accuracy of the investigating robotic spacecraft)
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experiment to fly on the 2008 Lunar Reconnaissance Orbiter, or by measurements from lower-altitude airborne platforms. Surface Measurements Global orbiter measurements intended to identify special regions with good spatial resolution will require ground truth measurements of the electromagnetic, physical, and compositional properties of the top meter of the regolith at multiple locations around the planet.
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From page 63... ...
This fact makes it difficult currently to designate with confidence any specific region of the planet as special or nonspecial. Coming to grips with the many issues relating to Mars's current habitability will require a continued broad and sustained program of interdisciplinary scientific exploration of Mars, and also measurements specifically targeted to locating special regions, especially to identify where liquid water and massive ground ice are present within the near-surface.
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Workshop on Concepts and Approaches for Mars Exploration. Abstract No.
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1998. Mars: The effect of stratigraphic variations in regolith diffusive properties on the evolution and vertical distribution of equatorial ground ice.
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Nature 338: 487-489. MEPAG (Mars Exploration Program Analysis Group)
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1992. The thermal stability of near-surface ground ice on Mars.
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From page 68... ...
30th Lunar and Planetary Science Conference. Abstract No.
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