[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.
The jury is still out on this discovery and its implications, however. Confirmation is needed from other measurements, and planetary scientists and mission planners are scrambling to target these small regions with many of the other instruments currently in orbit around Mars. (Unfortunately, these gully sites are too far away from the Mars rovers Spirit and Opportunity to be visited by them.) Still, the exciting possibility of liquid water so close to the surface of Mars today has once again invigorated the Mars and astrobiology communities, further motivating the continuing study of the Red Planet's watery past--and present!]
By February 2005 the Mars Exploration Rover named Spirit had already spent more than a year in Gusev Crater, a two-kilometer-deep, Connecticut-size hole in the Red Planet's surface. Because Gusev lies at the end of an ancient, dry river valley longer than the Grand Canyon, many of us on the rover's mission team had expected Spirit to find evidence that the crater had been filled with water billions of years ago. On the flat plains where the craft had landed, however, the rover found neither lake deposits nor other preserved signs that water had once flowed inside Gusev. The rover's photographs showed only dust and sand and bone-dry volcanic lava rocks.
But everything changed once Spirit reached the slopes of the Columbia Hills, about 2.6 kilometers from the landing site. (Each of the hills is named after one of the seven astronauts who died in the space shuttle Columbia disaster in 2003.) As Spirit struggled to climb the western slope of Husband Hill, its wheels dislodged rocks and dug deep tracks in the Martian soil. At one patch of particularly slippery soil, an area dubbed Paso Robles, the wheels accidentally uncovered some exotic, whitish deposits that were unlike anything we had seen before in Gusev. Actually, Spirit had driven well past the Paso Robles soils before the mission team noticed them; when we saw what we had uncovered, though, we did the rover equivalent of slamming on the brakes and pulling a U-turn.
On further inspection, we determined that the deposits were hydrated sulfate minerals, rich in iron and magnesium, concentrated just below the dusty surface. On Earth these kinds of deposits are found in places where salty water has evaporated or where groundwater interacts with volcanic gases or fluids. Either process could have also taken place on Mars. (Although scientists have found no active volcanoes in Gusev or anywhere else on Mars, eruptions certainly occurred earlier in the planet's history.) Regardless of which hypothesis was right, we realized that these buried sulfate salts could be remnants of a past watery environment in Gusev.