of salts of the right composition and in sufficient quantity can lower the freezing point enough to allow a liquid solution to exist, although such a liquid is unstable with respect to evaporation (Clark and Van Hart, 1981). Alternatively, ice crystals trapped in closed pores in rocks or regolith grains could melt under certain circumstances, and the resulting liquid water could be prevented from evaporating by virtue of being enclosed.

Analytical experiments carried aboard the Viking landers indicated that the surface environment of Mars is highly oxidizing, although the exact nature of the oxidants was not determined (Hunten, 1979). It is possible that the martian soil contains oxidants, such as hydrogen peroxide, which are postulated to form photochemically from atmospheric water vapor and to diffuse readily into the soil. If present, such oxidants would react with, and destroy, organic molecules or biota and could be effective in sterilizing the surface environment. Their presence may be responsible for the absence of organic molecules in the soil.

The Viking lander experiments found no organic substances in the soil despite the fact that organic molecules are being added continually from meteorite impacts (Biemann et al., 1977).

The atmosphere is relatively thin, averaging about 6 millibar pressure, and consists primarily of carbon dioxide. Owing to the low concentration of atmospheric ozone, ultraviolet light from the sun can reach the surface of Mars almost unattenuated. Winter-hemisphere atmospheric ozone can absorb some of the ultraviolet, but only during a fraction of the year and only over a fraction of the planet. The attenuation is much less than that due to the ozone layer on Earth. Thus, throughout the martian year the entire surface of the planet is subject to an intense flux of ultraviolet radiation.

THE ANCIENT MARTIAN ENVIRONMENT

The surface environment of Mars may not always have been so hostile to life. Early in the planet's history, the average temperature almost certainly was warmer and the atmosphere more dense, and liquid water may have existed at the surface. Evidence for the presence of surface water on early Mars comes from interpretation of the geomorphology of the planet's surface. A substantial fraction of the surface of Mars is older than about 3.5 billion years, based on the number of impact craters, which provide a window into the planet's early history.

Two aspects of these older surfaces suggest that the climate prior to about 3.5 billion years ago was different from the present climate (Squyres and Kasting, 1994). First, impact craters smaller than about 15 kilometers in diameter have been obliterated on these older surfaces, and impact craters larger than this have undergone substantial degradation, whereas younger impact craters have not been altered significantly. This suggests that erosion rates were up to 1,000 times larger early in martian history. The style of erosion that is seen on some of the remaining larger impact craters is indicative of water runoff, and water erosion is



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