ever greater extremes of temperature, pressure, pH, salinity, and so forth.4 One environmental factor that is almost universally accepted as necessary for life is at least episodic access to liquid water. This observation has implications for temperature but does not necessarily imply temperatures between 273 K and 373 K. Although there is theoretical evidence that metabolism can continue at temperatures at or below 233 K, there is no evidence of active cells below 253 K and no direct observation of cell replication below 248 K.5,6 At the high-temperature end, life has been cultured in the laboratory at 394 K,7 although evidence from deep-sea hydrothermal vents indicates that the upper temperature limit for life may be much higher than this.8 Apart from temperature, two other parameters critical to the survival of terran life are worthy of note—water activity and radiation resistance. The availability of liquid water to an organism is critical for its survival. Many organisms are resistant to desiccation; however, currently no organisms are known to survive at water activities lower than aW = 0.61.9 Therefore, temperature and water activity have recently been used to constrain regions on Mars associated with special planetary protection considerations.10

DOES MARS MEET HABITABILITY REQUIREMENTS?

The following discussion focuses on the availability of water. The committee assumes that solar energy, geothermal energy, and chemical energy, as well as nutrients, are available on Mars. Sources of energy and nutrients are present on Mars as they would be on any geologically active planet, although questions about the availability of nitrogen have not been resolved.11 A major unknown is where and when liquid water was available to enable the assets present to be used for a possible origin or maintenance of life.

Present Environmental Conditions

Conditions on the surface of Mars today are very inhospitable for life, but geological evidence suggests that conditions were more hospitable in the past, particularly the distant past. Liquid water is believed to be essential for life. With mean annual surface temperatures close to 215 K at the equator and 160 K at the poles, the ground is frozen on average to a depth of several kilometers to form a thick cryosphere. Any water present in this zone would be frozen. The cryosphere might be thinner locally in areas of anomalously high heat flow, but no such areas have been identified. The atmosphere is thin, with an average surface pressure of 5.6 mbar, and composed largely of CO2. Because the atmosphere is so thin, the Sun’s ultraviolet radiation passes almost unattenuated through it to the surface. Surface temperatures fluctuate widely during the day. On a clear summer day they may exceed 273 K close to noon. However, the fluctuations damp out rapidly at depth to converge on the average daily temperature, which is everywhere well below freezing, so that temperatures above freezing are restricted to the upper few centimeters. The ground is permanently frozen down to a kilometer or so below these depths.

Under present conditions, and probably under conditions that have prevailed for the last few billion years, weathering rates have been extremely low. Rocks in the Gusev crater have a millimeter-thick, oxidized rind, rich in volatiles such as sulfur, chlorine, and bromine.12,13 Soils have highly variable volatile contents and may contain an oxidizing agent. The fraction of organics in the soil is unknown. Although it was anticipated that the Viking gas chromatograph-mass spectrometer would detect some complex organic compounds, none were detected at the parts per billion level, and the Mars Pathfinder APXS measurements of soil could not detect carbon. The nitrogen content of the soils is also unknown.

Both poles have a residual water ice cap that is exposed when the CO2 cap recedes in the summer (see Figures 3.1 and 6.1), although in the south only small areas are exposed even at midsummer.14 Because of the cold polar temperatures, only minute amounts of water vapor are present in the atmosphere.15 Observations of seasonal frost and water fog in some areas on Mars demonstrate that the water content of the atmosphere varies both spatially and seasonally. However, if all the water vapor were precipitated out, it would form a global layer only about 10 μm thick. 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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