QUESTION:
Does it snow on Mars?

ANSWER from Mike Mellon on March 31, 1997: 
Probably, but we have not observed it. We do know that each winter carbon
dioxide (the primary gas in the atmosphere) condenses on the polar caps 
to form a thick layer of carbon dioxide frost. This frost may fall 
from the sky as snow, landing on the polar surface where it accumulates 
in a layer that can be as much as a couple of meters thick. 

Snow as we know it on Earth (made from water) is unlikely. Although it 
is certainly cold enough on Mars, there simply isn't enough evaporated 
water in the atmosphere to form any significant amount of snow to 
fall to the ground. In other words, air on Mars is too dry.
There is, however, lots of water ice in the polar caps, but this
water is probably more like glacial ice than snow.

                - Mike Mellon