QUESTION:
How do you know that the meteorite came from Mars?

ANSWER from Cheick Diarra on September 19, l996:
Most Martian meteorites are 1.3 billion years old or less, much younger
than typical igneous meteorites from asteroids which are 4.5 billion
years old. They also have higher contents of volatiles than igneous
meteorites. The conclusive evidence that this meteorite originated on
Mars comes from the measurement of gases trapped in its interior. The
trapped gases match those that Viking measured in the Martian
atmosphere. 

For more information, go to URL 
http://www.jsc.nasa.gov/pao/flash/marslife/index.html

ANSWER from Jeff Plescia on March 10, 1997:
There are now about a dozen meteorites we believe came from Mars.
All of them are igneous rocks (which means they formed by the
cooling of molten rock - either lava erupted onto the surface or
injected into the subsurface).

We believe they are from Mars for a number of reasons:

1. Age. These meteorites solidified from a liquid state between about
200 Million years ago to over 4 billion years ago. The younger age
(200 Million years) indicates they come from a body that was able
to produce lava that recently. Asteroids and the Moon are geologically
inactive 200 million years ago, which suggests are large planet (but
not necessarily Mars).

2. Trapped Gas. When they rocks were ejected from Mars, some of
the atmosphere was trapped in the rocks. When the meteorites were
studied on earth it was discovered that the gas had a composition
identical to that of the Mars atmosphere (as indicated by the measurements
from the Viking lander missions in 1976).

3. Isotopic Composition. Each element has a specific set of cousins which
are very similar, have the same basic chemical properties, but are slightly
different in mass (in this case the number of neutrons) - these different
cousins are called isotopes (e.g., oxygen 16, oxygen 17, oxygen 18). The
ratio of the various isotopes in the martian meteorites was different from
any earth rocks or any of the lunar samples or any of the other meteorites.

All of this led to the conclusion that these rocks came from Mars.

ANSWER from Bruce Jakosky on April 27, 1997:
There are twelve different meteorites that are thought to have come
from Mars.  Two of them have strong evidence tying them to Mars.  In 
these two, there is a glassy component of rock that is made up of rock 
that was partly melted by the force of the impact that must have 
ejected them from Mars.  There is a gas trapped within this glass, and 
this gas has a composition that is identical to the composition of the 
martian atmosphere and different from any other source of gas in the 
solar system.  This gas is thought to have been "implanted" into the 
glass, also during the impact that ejected it from the martian surface.  
The presence of this gas is a very convincing tie to Mars.  In fact, if 
they turn out not to be from Mars, then there is no place in the solar 
system where they can be from--there are only a few planets big enough 
to have had active volcanism so late in history (and these rocks are 
pieces of volcanic rock), the Earth and Moon are ruled out by the 
meteorites' chemical behavior, and Venus (the only other possible 
place) has a very different atmosphere.

So, what about the other ten meteorites (since only two of the twelve 
have the trapped gas)?  They are tied to the first two by their 
composition. It is very likely that they come from the same planet, 
but not absolutely certain.