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SPACE DAILY
Washington - Apr 28, 2004
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. 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 considered to be responsible for removing the smaller craters. Second, many of the same older surfaces contain networks of valleys that form dendritic patterns similar to terrestrial water-carved stream channels.
There is continuing debate as to exactly how these valleys were formed the process may have involved runoff of precipitation, seepage of subsurface water in a process termed "sapping," or erosion by water-rich debris flows. Independent of the exact process, their formation must have involved the presence of liquid water at or very near the surface during these earlier epochs.
Thus, geological evidence suggests that the martian climate prior to about 3.5 billion years ago was somehow warmer than the present climate and that liquid water flowed on the surface in a way that is not observed today. Unfortunately, the observations do not allow a unique determination of what the temperature, atmospheric pressure, or partitioning of liquid water between the subsurface, surface, and atmosphere were at that time.
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