QUESTION:
If microbes were to enter the Martian atmosphere, would they survive,
and should we care?

ANSWER from Geoff Briggs on April 18, 2000:
I assume that you are asking about microbes from Earth -- carried to 
Mars on robotic landers or on astronauts.

There are some microbes which have been flown in space (on a European 
experiment carried on the outside of the Russian space station) that 
were able to survive for weeks or months.  Such microbes would 
probably survive in the martian atmosphere but, lacking liquid water, 
they would not be able to grow and replicate.

We would care if we were carrying out an experiment to look for 
evidence of life on Mars and found ourselves discovering a microbe 
that we had carried from Earth.   That is why NASA went to great 
lengths to clean and sterilize the Viking landers that were launched 
to Mars in 1975 (they arrived in 1976).  Similar care will be taken 
in the future when we send spacecraft to Mars with life detection 
experiments.