QUESTION:
How do you prove a meteorite came from Mars?

ANSWER from Bruce Jakosky on November 11, 1998:
There are about a dozen meteorites that have been collected on Earth
that
we believe came from Mars.  The evidence that they are martian is
relatively straightforward:

1) All but one of the rocks are relatively young.  This means that they
crystallized from melted rock recently, within the last billion years. 
A
planet would need to be big in order to be geologically active this
recently.  This means that the only possible planets from which it could
have come are Earth, Venus, or Mars.

2) The composition of the oxygen that is contained within the different
minerals in the rocks absolutely rules out the Earth as their place of
origin.

3) The most convincing evidence is that two of the dozen rocks contain
a gas trapped within the rocks, and that gas is identical in composition
to the martian atmosphere and different from any other known source of
gas in the solar system.  It is thought that the gas was "implanted"
into
the rock by the force of the asteroid impact that also ejected the rock
into space.

If the rocks aren't from Mars, then there is no other place in the solar
system that they could be from.  And, various arguments rule out the
possibility that they came from outside our solar system.