A couple of years ago they found a meteorite from Mars. How can they tell it came from only Mars and how?

The whole story of how this hypothesis came about and what the supporting evidence is can be foudn here:

http://scienceworld.wolfram.com/astronomy/SNCMeteo...

Basically the radioactive dating gave these rocks too young an age to be from anything smaller (smaller bodes like asteroids cool too quickly to remain geologically active for very long - since the dating of the SNC meteorites gave ages of ~1-1.5 billion years, it had to be from a planet.

It is easier to eject a rock from Mars than Venus (because it is smaller, so has lower gravitational field, and has a very thin atmosphere). So a big meteorite impact on Mars could eject sizeable chunks of Mars.

Then comparison to analyses by landers etc, and gases trapped in the rocks were compared to Mars atmosphere and so on. Read the link - it's a pretty cool story

When another big meteorite hits mars surface,lot of rocks are going up and few of them will reach mars escape velocity which is much less than that of earth. This will be in solar orbit and ultimately earth goes near by path. The mineral combination is perfectly comparable with marsand the trapped gases match the martian atmosphere

actually 34 have been found

it was determined by the minerals and gases stuck in the rocks. The composition matched the results gathered from the 1976 viking landers experiments

they have different oxygen-isotope ratios compared to the

Earth and other meteorites.

all explained here in this nifty little PDF (adobe reader needed)

http://www.lpi.usra.edu/expmars/poster/fullpacket....

an asteroid hit mars, sending chunks of mars flying through space....(this could have happened along time ago, 200 years thru millions of years ago)

this was expected again, earlier this year when an asteroid was headed close to mars. unfortunatly it didn't hit mars.

Much too early. Curiosity could be pottering about the Martian landscape for 15 years or so if we're lucky, so don't be so impatient. The first week or so after landing will be spent in checking that the various parts of the lander have survived their time in space and the violence of the landing in good shape, and are working properly. Only then will the actual science project start.

Very tiny pockets were contained within the meteorite (ALH 84001) from which samples of air were taken. Analysis of that air were consistent with what we know the composition of air on Mars is.

I totally agree with mercury, An asteroid hit mars ejecting materials out of its atmosphere and then collided with earth, Then of course the dating process tell of its age,

SG

Different chemical composition from earth rocks, different isotopes.

i have the same question, nobody saw where the meteorite come from

it was apparently wrapped in the usual black with gold and red lettering