QUESTION: 
I read with interest  the MTO  question archives  on precipitation on
Mars, particularly the idea of carbon  dioxide snow.   My question is
why  can't  this  phenomenon also  occur  on  Antarctica,  since  the
temperatures at (e.g.) Vostok regularly approach -109 degrees F  (the
freezing point  of CO2) and the  lowest recorded  temperature is well
below that (-124 degrees F)?

Additionally,  the  atmosphere  high   above  these   regions  should
approximate the Martian climate even more  accurately, since the  air
pressure is lower. Have any simulations of this nature been done,  to
determine the behavior of such unconventional precipitation?

ANSWER from Bob Haberle on May 11, 1999:
The vapor  pressure of  CO2 in  earth's atmosphere  at the  surface is
about 0.3 mb.  At that pressure, CO2 would begin to condense at  about
-229F.    It  never  gets  this  cold  anywhere in earth's atmosphere.
However,  it  does  in  the  martian  atmosphere.  Pathfinder measured
temperatures approaching almost -300F at about 80km above the surface.

I believe  the coldest  it gets  in earth's  atmosphere is  about 190K
(-117F)  at  the  tropical  tropopause  (~17km)  where  the  CO2 vapor
pressure would be about  10-15 times less than  it is at the  surface.
While cold, its not cold enough for CO2 to condense, and it  certainly
doesn't  approach   the  frigid   conditions  in   the  Martian  upper
atmosphere.