I have heard about the clues of life found on a meteorite from Mars. but how do they know it came from Mars. and how did it get knocked off Mars?
Mineral composition of the rock would be identical to Mars. (not just chemically, but in particle formation, age, proportions, etc) It would have got knocked off by a big meteorite strike on Mars surface.
IANAM(Martian), but, a large asteroid or other large mass hitting Mars may drive some material into the atmosphere and it could reach escape velocity from Mars, thereby becoming a meteor.
I believe they determine that it is from Mars by checking its composition - due to the inherent nature of Mars, there are certain patterns of radiation or actual amounts of different materials unique to Mars.
(Hey! Wait a minute! I am a Martian! The Great Gazoo is from Mars!!! 
)
It is generally accepted that the structures found meteorite ALH 84001 could have been formed by non-biological processes, though not everyone agrees, and recently there’s been some more analysis that supports the assertion that some of these sturctures were formed by bacteria.
Here’s a page on the many meteorites believed to have come from Mars, with links (many dead, sadly): http://www.jpl.nasa.gov/snc/index.html
And here’s a very good page detailed page on ALH84001: http://www.lpi.usra.edu/lpi/meteorites/life.html
I thought the consensus on that meteorite was that it was
spewed (spewn?) out of a volcano when Mars was still
geologically active.
Oh, and at escape velocity, obviously.
No, bup, that’s not a reasonable scenario. It was almost certainly thrown up by an impact, which can easily eject material, even from a large planet like Earth or Mars.
thanks a lot. the NASA site has all the scoop. never too old to learn.
I always thought that the meteorites could have come from Mars, but they seem to have proven it.