Earlier this week, NASA announced a press conference for Wednesday morning (US Pacific time; Thursday morning in eastern Australia) with "important news" about Mars. The news are: they have found evidence of liquid water on Mars. The important part is: not in the distant past, but in the last five years.
Two images of the same area of the planet taken by the Mars Global Surveyor in 1999 and 2005 (seen here) show changes that indicate a recent flow of liquid water on the surface; images of a different area show similar activity happening after mid-2002. The supposed flows of water left behind lightly-colored deposits, which are very rare on Mars (disturbances of the soil usually show the darker material that is underneath).
It is presumed that the water flowed from underground deposits, but it's not clear whether the water is permanently liquid (thus providing good conditions for underground habitats for local life-forms) or just becoming liquid for a short period and spurting out of the ground when that happens. When exposed to the thin atmosphere of Mars, liquid water doesn't last very long; it quickly becomes either solid ice (due to the low temperature) or vapour (due to the low pressure).
The Mars Global Surveyor recently stopped sending data and was declared lost, but it clearly brought very important information to Earth; analysis of its images will almost certainly bring more discoveries over the next years. And, of course, the discovery of the presence of liquid water on the planet, even temporarily, brings a boost to the idea of sending people there in the near future.
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