QUESTION:
What will you do if you get to Mars, collect the rock samples and return
safely to Earth, only to discover that the Mars rocks that you collected
were actually Earth rocks deposited on Mars due to a Meteor which struck the
Earth with such force that huge boulders and chunks of the Earth crust were
heaved up out of our atmosphere and came down on Mars?

ANSWER from Mary Urquhart on November 15, 1997:
I don't think we have to worry about your scenario actually happening. The
probability that any random rock picked up and returned the Earth would be
an "Earth meteorite" is **extremely** low.  Meteorites that originated as
rocks on the Earth are sure to exist on Mars, but they would be no more
common than Mars rocks on the Earth. If you pick up a random rock on the
Earth the chances are also **extremely** low that it will be a meteorite
(unless you are in a very specially location like an Antarctic ice sheet
where all rocks are meteorites because there isn't another source for them),
much less one from Mars.  We will also select the rock or rocks to be
returned with much more care than a random sample...we want it to be
representative of the area where the sample is collected from or to have a
composition that makes the rock particularly interesting for detailed study
on the Earth.

Mary Urquhart                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder