QUESTION:
Are the polar ice caps on Mars smaller or larger than the polar ice caps 
of Earth?

ANSWER from Mike Mellon on February 12, 1997:
Are the polar caps of Mars smaller or larger than Earth's? 
I don't know. Let's compare them. 

The north polar cap on Mars is roughly 1000 km across and about 4
to 5 km thick at the deapest point. The southern martian cap somewhat
thinner but covers a wider area. Keep in mind that the bright areas of
the poles of Mars comprise only a small fraction of the total polar ice 
cap. Most of the ice is covered by some dust and sand which makes it
look rather like soil, so the bright pole itself is a smaller region
that is absent of dust and sand.

For comparison, the Antarctic ice cap at our south pole is about 
4500 km across  and about 3.5 to 4 km deep at its deepest. 
At the Earth's north pole is the arctic ocean and the ice cover there 
is a few meters thick. The nearest thing to an ice cap, for comparison,
is Greenland. Greenland is about 900 km by 2100 km in size and has 
about 2 to 3 km of ice piled on top of it. 

So it appears that Mars and Earth have polar ice deposits that are 
similar in thickness, but the Earth's ice caps cover a much larger area.

                        - Mike Mellon