Water, Water Everywhere on Mars
By Phil Berardelli
ScienceNOW Daily News
13 December 2006

SAN FRANCISCO, CALIFORNIA--Spirit and Opportunity are painting a considerably different picture of Mars from what NASA's mission scientists had expected when the twin rovers set down separately on the red planet in January 2004. Designed expressly to look for signs of water, both craft have found such signs in the rocks and soil lying in their paths. But what's surprising and disappointing is the water seems only to have lurked beneath the Martian surface. Lakes, rivers, and oceans seem to have been almost totally absent from the planet's history.
Reporting here today at the annual meeting of the American Geophysical Union, members of the Mars Exploration Rover science team declared both machines basically healthy--although Spirit remains partially crippled by a right-front wheel drive motor that failed nine months ago. So far, Spirit has logged 1047 sols, or Martian days, and Opportunity has been operating for 1026 sols. The Mars rovers' project manager, John Callas of NASA's Jet Propulsion Laboratory in Pasadena, California, quipped that crossing the 1000-sol threshold created a problem, because the rovers' software was not designed to handle four-digit dates. "But it's a good problem to have," Callas adds.

Perhaps the biggest surprise of those 1000-plus sols for both rovers is the way the Martian terrain has been unfolding, says Steve Squyres of Cornell University, the mission's principal investigator. "We have seen evidence for surface water at Eagle, Endurance, and Erebus craters," where Opportunity has been exploring, Squyres says. "But the fact that we see these so rarely indicates this was an arid environment on the surface, with plenty of water under the surface. On the surface, there were lots of sulfate salts that were windblown."

The mission's deputy principal investigator, Ray Arvidson of Washington University in St. Louis, Missouri, says Spirit's explorations have produced the same result. "For first 150 sols, there was very little evidence for water at Gusev," the large crater in which Spirit landed. That situation changed when Spirit drove toward the higher elevations of the Columbia Hills, Arvidson says. There, the rover encountered direct evidence of water-bearing sulfates, although the primary evidence is "for water at subsurface," he says. "Nothing we have seen at Gusev demands surface water."

Squyres likens the geological structures seen by the rovers to a "bathtub ring," in which rocks produced by the action of water accumulated at a layer just below the surface, and then were slowly moved, shaped, or wiped away by wind erosion. That has been particularly evident with Opportunity, he says. The rover, which has been driving slightly uphill for about six miles since it landed, has encountered the leading edge of the bathtub ring intermittently. If and when the rover drives down into Victoria Crater--its current location--the team hopes it will encounter the layer again, Squyres says. "This [would support] the idea that water rose only to a certain level."

Bruce Murray, a planetary geologist at the California Institute of Technology in Pasadena, agrees. "We have lots of history, lots of evidence of when water was available on Mars, but it wasn't a wet planet." If water flowed on the surface in significant amounts, Murray says, it must have "happened early, and the evidence isn't well preserved."