[nightdriver]

"Opportunity has landed in an area of Mars where liquid water once drenched the surface," said Edward Weiler, associate NASA administrator for space science, at a news conference. "This area would have been a good, habitable environment."

I call Bee Ess!

The planet is in equilibrium after being in existence for umpty-ump millions of years, and it's atmospheric pressure is below the saturation pressure of water whenever the surface temperature ever gets over freezing. That means water would just boil off.

The present atmospheric water content is only 250 parts per million. That says that there is just plain no water there, at least not on the surface.

[cartoonistx]

One theory is that once Mars had a dense atmosphere and oceans but lost most of the atmosphere after a very close encounter with another planet (which may have been torn apart by the encounter and now is the asteroid belt) or by a large meteor impact.

[RightWhale]

Mars would present conditions near the triple point of water. Under the surface, sealed from the partial vacuum they laughingly call an atmosphere, ice would be present and would sublimate off only where the ice were exposed by erosion of the surface. They are searching for geysers and hot springs using remote sensing from orbit right now.