"OK, so the rock is a meteorite.
"Why should we think it is from Mars rather than the asteroid belt, where most meteors arise?
"We have a detailed analysis of the Mars atmosphere from the Viking landers.
–The gas bubbles trapped in ALH 84001 have the same composition.
"The mineral composition of ALH 84001 puts it in a category (SNC meteorites) we think is most likely from Mars.