QUESTION:
How long would you actually survive if you were to walk out into the Mars
surface on a nice 50 degree day? Say you had a canister of air so lack of
oxygen is not an issue. How soon would the radiation kill you and would you
implode as you would in space?

ANSWER from Bruce Jakosky on July 16, 1997:
You've hit on the right issues--supply of air and the total pressure. The
total atmospheric pressure is about 1% of that at the surface of the Earth,
and this is not really very different from the zero pressure in space.  I
would guess that someone could survive for a matter of minutes, but not much
longer.

And, if neither of these got you right away, certainly the cold night would
be pretty deadly.