[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.

[nightdriver]

"One theory is that once Mars had a dense atmosphere and oceans....."

It's OK for a theory, but remember that Mars has a gravitational attraction of only one-third that of the earth. It cannot hold any more atmosphere than it now has.

[JimRed]

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.

"Worlds in Collision", circa 1950, by Immanuel Velokovsky, expounds part of this this theory. It is an interesting read.