WATER FLOWS across the Martian surface in an artist's rendering of how the Red Planet may have looked 2.5 billion to four billion years ago. Salt deposits along the water's edge appear purple in this twilight view. Image: Ron Miller
[12/13/06 Author's Update: Last week's announcement and publication of results on the discovery of evidence for current liquid water on Mars in the very shallow subsurface is extremely exciting news for planetary scientists and astrobiologists. As I discuss in the following article, abundant evidence from the recent rover and orbiter missions indicates that there was liquid water on Mars early in the planet's history. The water may have been persistent and long-lived on and near the surface. However, one of the key issues that I indicated still need to be resolved is the duration of that watery period. How long was liquid water stable on the surface or the subsurface? If it was a long time, then Mars could have been not only a place that was habitable for life, but a place where life could have thrived and even evolved as environmental conditions changed.
Michael Malin and his colleagues discovered gullies that appear to have had liquid water flowing in them sometime in the last decade. It provides further support for water having been stable at or near the Martian surface for very long periods of time--indeed, perhaps even for the entire history of the planet. One implication of this discovery is that if life got a foothold on an Earth-like early Mars, then subsurface regions where water remains stable could be "oases" where that life could still exist today.