[ex-snook]

Maybe they had global cooling.

[flashbunny]

They did in a sense. The core cooled, stopped spinning, the planet lost its magnetosphere, and without it the atmosphere fell victim to the sun's rays.

[Laserman]

"Maybe they had global cooling."

Actually, not a facetious statement. If Mars once had flowing water, then it must have been much warmer. The only realistic reason Mars would have cooled is that the Sun has cooled. Perhaps our current Earth climate is only a temporary warm period on a trend of Solar System cooling.

[Calvin Locke]

Probably caused by it's most prominent volcano, Olympus Mons.

3X the height of Mt. Everest on a planet half the diameter of Earth.

Talk about getting stuff high up in the atmosphere.