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So I've heard of meteorites "originating from Mars" (e.g. AH84001), but the phrase confuses me. I'm interested in what this means - have these rocks somehow escaped Mars' gravity and ended up here; or were they part of the material that Mars formed from, but did not end up as part of the red planet? Or another explanation?

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These are chunks of rock that existed as part of the crust of Mars but were ejected into interplanetary space by a very powerful impact and then eventually impacted the Earth. It wasn't until we had sent probes to Mars and began to understand the composition of martian minerals and atmosphere that we started realizing some of the meteorites we had already found were from Mars. The clincher has been the analysis of trapped gasses within meteorites. Nearly 100 Martian meteorites are known to have been found.

Interestingly, most Martian meteorites fall into only 3 mineralogical categories (Shergottites, Nakhlites, and Chassignites, or SNC meteorites) and had been identified as being unusual as meteorites even before they were confirmed to have originated on Mars.

Similarly, there are likely some number of Earth meteorites on Mars.

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Not as many Earth meteorites on Mars as vice versa; Earth is "downhill" from Mars. –  Andrew Aug 9 '11 at 0:27
    
So something else collided with Mars and blew Mars-debris into space; which ended up here? –  Smashery Aug 9 '11 at 23:01
    
@Smashery, exactly. –  Wedge Aug 10 '11 at 7:49
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