QUESTION:
What is the atmospheric pressure on mars?

ANSWER from Bob Haberle on July 20, 1998:
The average pressure at the surface of Mars is about 7 mb, but it flucuates
by several mb during the course of a year. The reason for this fluctuation
is because it gets so cold in the polar regions during winter that the main
constituent of the atmosphere, CO2, condenses out onto the surface. Then
during spring and summer it sublimes back into the atmosphere. Because this
occurs in both polar regions, there is a pronouneced semi-annual variation
in surface pressure. This was theoretically predicted way back in 1966, but
was not observationally verified until the Viking mission in the mid 1970's.