QUESTION:
What is the atmospheric pressure at the lowest point on the surface of Mars?
How long could a person conceivably survive there without benefit of a
pressure suit?

ANSWER from Nathan Bridges on November 30, 1999:
At the lowest point on Mars' surface, which is 7-8 km below the arbitrary
mean datum, the atmopsheric pressure can get as high as ~ 17 mbars.  To put
this in perspective, Earth's pressure at sea level is 1 bar, or 1000 mbars.
With this low pressure and a CO2 composition, a person would survive no
more than a few seconds without a pressure suit.