QUESTION:
What would happen if you melted the ice caps on Mars?

ANSWER from Wendy Calvin on February 11, 1998:
Well, it wouldn't be as spectacular as if you melted the ice caps on Earth. 
On Earth, it's expected that melting the Antarctica and Greenland ice sheets
could raise sea level several 10s to 100s of feet - that is most coastal
cities in the US would be under a lot more water.  On Mars, one cap is made
of carbon dioxide (dry) ice and the other water ice.  The CO2 would just go
into the atmosphere, but there isn't enough that it would change the
pressure.  That is, distributing the CO2 around the entire planet wouldn't
increase the pressure that much. The water ice, if melted, doesn't have an
ocean to add to so it might run across the surface as streams or rivers for
a while, but it would eventually percolate down into the porous rocky ground
and would make permafrost or aquifers (underground collections of water).
Some of the water would also evaporate into the atmosphere, but the thin CO2
atmosphere can't hold very much water so most of it would find other places
to go.