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Slide 14 of 16
Water is another issue important to the search for life on Mars. Mariner and Viking missions of the 1960s and 1970s and the Pathfinder mission of 1997 all demonstrated that large quantities of liquid water flowed along the Martian surface in the distant past. Shown on the left is an image of Gusev Crater, which was once a lake. Connecting to this ancient lake is Ma'adim Vallis, through which water once flowed, depositing mineral sediments into Gusev that may have fossilized lifeforms that might have been living there.
Based on humidity, temperature and pressure, scientists have calculated that minute quantities of liquid water can actually exist temporarily at the surface during part of the Martian day even at the two Viking landing sites. Nevertheless, Mars missions have begun to focus heavily on searching for locations on the planet that might have larger quantities of liquid water and, if such locations are confirmed, life detection missions may target those areas.
On June 22, 2000 NASA announced the discovery, using the Mars Global Surveyor, of signs of water seeping into what appear to be young, freshly-cut gullies and gaps in the Martian surface. This means that liquid water (most likely a very concentrated brine) has been making it to the surface of the planet in the very recent past and that we may eventually find it in accessible locations. On the right is an image of these gullies. Many such formations have been confirmed throughout the planet.