Don said:

>Today, the martian atmosphere is 10,000 drier than the atmopshere of earth.
>That is incredibly dry. By all accounts, it appears to have been this way for a
>very, very long time.

The Martian atmosphere is also "saturated" with water. This is not a contradiction to Don's comment. The surface pressure of the Martian atmosphere gets up to about 6 millibars (sea level pressure on Earth is about one bar). The atmosphere of Mars is so thin that it can be saturated with water vapor and still have almost no water in it. Another gee-wiz fact concerning Martian thermodynamics is the triple point of water. The Martian analogue to Death Valley is the Hellas Basin (lowest elevation on the planet). Pressure and temperature of the Martian atmosphere in the Hellas Basin can on a warm summer day actually approach the triple point of water. So there's liquid water in Hellas, right? Nope, wrong again. We know from measurements taken by the Mars Odyssey spacecraft that Hellas is one of the driest places on the planet. The higher temperature there acts like a reverse cold trap. Maybe a zillion years ago there was liquid water in Hellas but long ago it turned to vapor and then later condensed to ice somewhere else on Mars. Not suprising, the most ice rich places on Mars are also the coldest.

Gary