How do they know some rocks are from Mars?

` This has been bothering me for some time: How do they know that some meteorites (burned bits of meteors) come from Mars? Of course, I've never looked into that - I've been busy looking into other things. However, good old Unca Cecil has gotten to it, so I don't necessarily have to.

` Ah, laziness. But I might as well show anyone who accidentally wanders in here...

` There are 34 Martian meteorites to date, and these special rocks are from the SNC ("snick") group, which is short for the shergottite, nakhlite and chassignite group. Why such strange names? The type specimens of each category landed in Shergotty, India (1865), Nakhla, Egypt (1911), and Chassigny, France (1815).
` In 1979, Hap McSween and Ed Stolper put these facts together about the SNC meteorites: All of these space rocks were volcanic, meaning they had come from a planet with volcanoes! They all had solidified 1.3 billion years ago or less. The moon had cooled off 3 billion years ago. Asteroids had cooled off abour 4.5 billion years ago. Venus, which is the next planet toward the sun, is highly volcanic and has a thick atmosphere and very strong gravity. Mars, which is the next planet away from the sun, has had volcanic activity about 1.3 billion years ago, and is more likely to give up some of its rocks due to its lower gravity.

` (Unless somehow, the rocks came from Earth and then fell back again, but it gets more interesting...)

` Other scientists, who of course thought this was pretty unlikely, measured the noble gases trapped inside the pores of a shergottite rock, and it matched the atmosphere signature of Mars sampled by Viking.
` Also, a meteorite found in 1982 on Antarctica is from the moon (it matches exactly moon rocks taken by Apollo astronauts), meaning that something which hit the moon - probably causing one of its many craters - knocked a chunk off.
` So, if a chunk of rock can get knocked from the moon, why not a nearby planet? Like Mars?

` This conclusion at least makes the most sense. Of course, it's more complicated than that, but that's the gist of it.
` Now, that 'Martian life-forms' thing, on the other hand...