QUESTION:
How did carbon dioxide get in Mars' ice caps?

ANSWER from Mary Urquhart on April 2, 1997:
A good question.  When it is winter on the Earth in the northern
hemisphere water on the surface (like ocean water) and in the atmosphere
turns to ice in the arctic (because the temperature gets cold) making a
polar cap. Some of the ice stays frozen year-round. The same thing happens
at the south pole during its winter (when it's summer at the north pole).
 
During a Martian winter, the temperature at the pole is *very* cold (about
-200 degrees F or 145 K which is much colder than the north or south poles
of the Earth during winter).  Because it is so cold, carbon dioxide in the
atmosphere freezes (making dry ice) and accumulates on the ground near the
pole along with water ice.  The ice (dry ice and water ice) builds up into
a polar cap.  Like on the Earth, some of the ice in the polar cap stays
around even in summer. 

You can see a picture of the northern and southern polar caps of Mars 
compared with the Earth at 

http://cass.jsc.nasa.gov/images/sred/sred_S23.jpg

These images where taken during the summer for each hemisphere.


Mary Urquhart                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder