Sites And Samples For Recent Life

All biochemical reactions that sustain life require water. Hence, water (and by extension ice) is the primary focal point in site selection for the search for present life. Sites where liquid water may have been present recently at the surface include numerous gullies,16 the ends of geologically recent outflow channels,17 and sites of recent glaciation.18 Water is, however, not the only target in the search for present life. Gas anomalies are also of great interest. For example, metabolic end products, including methane, carbon dioxide, and hydrogen sulfide, might indicate the presence of active local microbial communities.

Sites Containing Liquid Water

On Earth, life is active in aqueous systems over broad ranges of physicochemical conditions. Therefore, the presence of liquid water is a strong signal to follow in searching for present life.

Near the Surface

To date, there is no evidence for standing water on the surface of Mars. In fact, water is unstable under present surface conditions. However, it may persist in the shallow subsurface as part of an extensive groundwater system. Target environments where subsurface water—and hence present life—may be most likely include the following:

• Sediments and sedimentary rocks where porosity and permeability are characteristic of aquifers;

• Areas below surface features related to recent aqueous processes (e.g., gullies and young outflow channels); and

• Areas with significant concentrations of minerals such as clay or evaporates that are diagnostic of low-temperature aqueous processes and that appear to be very recent.

Surface Water in the Recent Past

Recent outflow channels and recent gullies, if they formed by groundwater seepage, could have brought subsurface organisms to the surface. In this scenario, sediments and/or ice associated with gully and channel formation could harbor dormant life as spores or cells in vegetative states. On Earth, for example, spores have been shown to survive for very long periods of time.19 Sites of particular interest are fluvial deposits, evaporites, rocks, and minerals indicative of hydrothermal systems (e.g., hematite), and permafrost.20 Deposits at the ends of gullies, common in mid-southern latitudes, and deposits at the ends of young outflow channels, such as those in Cerberus and to the southeast of Olympus Mons, are thus possible sites to search for dormant, but extant, life.

Water Ice

Recent Earth-based studies have demonstrated that microorganisms can survive, grow, and metabolize in frozen sediments and ice cores, where pockets of liquid water are maintained.21 This could occur if salts are concentrated in liquid water or at the grain contacts where the freezing temperature is depressed. Relevant water-ice deposits include the following:

• Ground ice. Ground ice appears to be ubiquitous a few centimeters below the surface at latitudes above ~50°N and S.22 While martian permafrost temperatures are much lower than terrestrial permafrost temperatures, these ice-rich environments have some similarities to permafrost environments found on Earth that contain many metabolizing microorganisms.

• Glacial ice. Deposits left by cold-based glaciers have been identified on Mars, particularly on the northwest flanks of the large Tharsis volcanos.23 They probably formed in recent geological times during periods of high obliquity and might therefore still contain dormant lifeforms or their remnants.24

• Polar ice caps. Thick (up to several km), layered sediments of dust and ice are exposed in the polar ice caps, which extend to ~80°N and S latitude (Figure 4.3). Owing to chaotic oscillations of Mars's obliquity,25 the

FIGURE 4.3 Layered deposits near Mars's north pole. The intricate patterns in the middle of the image are caused by gypsum-rich dunes exposed in a cliff. The smooth areas to the upper left and lower right are more typical of the deposits that surround the north pole. Image obtained by the High Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter spacecraft and provided courtesy of NASA/JPL/University of Arizona.

FIGURE 4.3 Layered deposits near Mars's north pole. The intricate patterns in the middle of the image are caused by gypsum-rich dunes exposed in a cliff. The smooth areas to the upper left and lower right are more typical of the deposits that surround the north pole. Image obtained by the High Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter spacecraft and provided courtesy of NASA/JPL/University of Arizona.

polar regions may experience periodic temperatures near the melting point of water ice, making the cap environments potentially habitable to extremely psychrophylic (cold-loving) microorganisms.

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