QUESTION:
Why do the cycles of the polar caps affect atmospheric pressure? Why
is there less condensation in the northern polar region during the 
winter than in the southern region?

ANSWER from Bruce Jakosky on March 27, 1999:
The Martian atmosphere is thin and does not hold heat readily.  As a
result,
the polar regions can get cold enough during the long polar night (that
lasts half a Mars year, much as the Earth's polar regions are in the
dark
for half an Earth year) that the major constituent of the atmosphere,
CO2,
can freeze out.  When it freezes, it turns directly to ice, and the ice
settles out as frost on the ground.  During the winter season, about a
third
of the Martian atmosphere will freeze out on the ground.  The CO2 ice
vaporizes again into the atmosphere during the following spring.  This
cycling
of CO2 into and out of the atmosphere happens at both poles, with frost
condensing onto the northern polar cap during northern-hemisphere summer
and then onto the southern polar cap during southern-hemisphere summer.
Because of this cycling of CO2 into and out of the atmosphere, the
amount
of CO2 in the atmosphere itself varies with the seasons.  With CO2 being
frozen onto and released from both polar caps, the seasonal cycle of
atmospheric CO2 has two "peaks", when the amount of gas is a maximum,
and two "valleys", when the CO2 in the atmosphere is at a minimum.

In addition, there is a difference between the two polar caps.  As seen
by spacecraft, the south polar cap does not appear to lose its covering
of
CO2 ice during the summer.  The north cap, however, does lose all of its
CO2 ice.  The differences are still not well understood, but certainly
involve the difference in elevation of the two poles (the north cap is
lower), differences in the brightness of the ice at the two poles (and
how
much sunlight is absorbed during spring), the amount of dust in the
atmosphere
when the CO2 ice is one the ground (with the atmosphere being dustiest
during southern-hemisphere summer), and the past history of CO2 ice.
We hope that new spacecraft measurements will help to unravel this
puzzle.