It has also been suggested that Mars had an early atmosphere -- around 4 billion years ago -- that was 100 times more massive than its present atmosphere. This early, massive atmosphere could have been entirely driven off through multiple collisions of asteroids onto Mars.

From Melosh and Vickery 1989 -- http://cmex-www.arc.nasa.gov/CMEX/data/FrontPg/Missions/SamRet/Haberle/RMH_Abs.htm

However, some recent thoughts suggest that Mars' liquid water had to have existed within the last 1 billion years. This evidence includes the smoothness of the northern plains as well as the lack of craters there.

Some MGS images imply very recent liquid water flow (or seepage).

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