QUESTION:
How many gases are in Mars' ice caps, and what are their names?

ANSWER from Bruce Jakosky on May 10, 1997:
The martian polar caps consist of a couple of different ices (which are
just gases that have frozen out into solid form).  One of them is
water (H2O), and the other is carbon dioxide (CO2).  The water ice
is the same ice that we get from our freezer.  Carbon dioxide ice is
used on Earth; it is known as "dry ice" because it does not melt but goes
directly from a solid to a gas.  On Mars, both water and carbon dioxide
ices will go directly from the ice to the gas when they are heated; at
the low temperatures and pressures that occur on Mars, even water will not
melt before it becomes a gas.  We see this same thing occurring on the
Earth when it snows during the winter.  The snow accumulates on the ground,
and some of it will disappear even thought the temperature has never gotten
above freezing; it "sublimes" directly from a solid to a gas.

There may be some other gases trapped in small quantities in the ice.  However,
they are not important for understanding the current martian climate.  Besides,
they have not been observed directly.