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The Mars Express orbiter has detected water ice in the south polar cap of Mars. It seems that this is the first direct detection of water on Mars, though there were many hints from previous data (like the presence of hydrogen, the canyons, etc).

Any comments or thoughts ?

One thing that's changed over the past few years (though the change started prior to Mars Express, as a result of such things as Mars Odyssey data) is the consensus view of the composition of the south polar cap. It used to be thought that the north cap was mostly water ice with a thin layer of CO2 on top, but that the south cap was almost all carbon dioxide ice ("dry ice"). Now it's thought that both of them are mostly water, with some carbon dioxide ice on top; the carbon-dioxide layer is somewhat thicker on the south cap, but it's still only several meters thick.

That seems to go against an old theory about the atmospheres of the inner planets. It used to be believed that Venus, Earth, and Mars started out with about the same amount of carbon dioxide in their atmospheres. On Mars, most of it eventually froze into the south polar cap, and the removal of its greenhouse warming effect from the atmosphere made Mars colder. On Earth, most of it ended up locked into carbonate rocks, but a little more stayed in the air (though Earth's atmosphere is much thicker overall, so it is still a minor component). On Venus, it all stayed up in the air and turned Venus into a hellish oven.

But now, it appears that there was never quite that much carbon dioxide on Mars in the first place. So the challenge is to reconcile this with the various theories about Mars having been considerably warmer in the past (if indeed it was!) Water vapor is a greenhouse gas too, but I don't know if it can play the same role.

Well let me throw in a few things here. A few years back I used to work as security guard at a very large name brand corporation in the San Francisco Bay area. I will not name them. But they had many temperature controled rooms for testing how there products held up to various temperatures nad conditions. The temps went from -20 all the way up 120 degrees F with 100% humidity. I had to check these rooms twice a night to make sure their temps didn't drift off from there set perameters. You may ask where I going with this. Well one thing I can tell you is that water in its gaseous for maks a very good green house gas. Take a temp room that is 100 degrees and turn off the heat and then do the same to the room with 100 degrees and 100% humidity. Now which one do you think is going to cool down first. The dry one of course. CO2 is a great green house gas but it is a dry gas and by itself can loose heat just as fast as regular air does. Now add water vapor to that and see what happens. Water in its gaseous form acts like a thermal blanket.Just think of warm summer nights when the humidity is high. It feels like the night is never going to cool down. Same thing on Mars. If Mars had a great deal of water on its surface and in gaseous form along with the CO2 even at lower levels I think there should have been enough heat to maintain liquid water for some time. It is all a mater of the most simple of thermal dynamics. Water vapor is a very powerfull green house gas when it is available.
Just my 2 cents.
Don. Edwards