ALH84001 is a famous meteorite found in Antarctica. Scientists have since claimed that (1) this meteorite came from Mars (along with several other meteorites), and (2) that it contained evidence of life.
The focus here is on the first claim, that the rock comes from Mars.
A game of cosmic pinball is involved. The rocks have to escape Mars gravity by some event, orbit around in space, and hit Earth before hitting something else.
The NASA website explanation seems to be, more or less, "gas bubbles":
How do we know the meteorite came from Mars?
Meteorite ALH84001 is a softball-sized igneous rock weighing 1.9 kilograms (4.2 pounds). It is one of twelve meteorites discovered on Earth which are thought to be from Mars. Most meteorites formed early in the history of the solar system, some 4.6 billion years ago. Eleven of the twelve martian meteorites have ages less than 1.3 billion years, ALH84001 at 4.5 billion years old being the only exception. All twelve are igneous rocks crystallized from molten magma in a way which suggests they formed in a planetary-sized body, not an asteroid. They have similar oxygen isotope characteristics to each other and higher concentrations of ferric iron, water, and other volatiles than other meteorites. All twelve also show evidence of shock heating, presumably as a result of the impact which ejected them into space. Gas bubbles trapped in one meteorite, EETA79001, have a composition which matches the current martian atmosphere as measured by the Viking Landers, compelling evidence that this meteorite and by association the others, including ALH84001, came from Mars
An article in Wired http://www.wired.com/wiredscience/2010/04/allan-hills-meteorite-age/ notes that the estimated age of the rock had to be adjusted in light of new evidence.
Why does it matter that the gas in the meteorite matches a modern Martian atmosphere if the rock was ejected from Mars in a cataclysm over a billion years ago? Would the atmosphere still be the same? Aren't there other solar system objects, such as comets, that contain gas whose composition we either haven't yet observed, or whose gas might otherwise mimic what scientists saw in the Mars probe observations? As a meteorite enters Earth's atmosphere, isn't it subject to thermal stresses that could produce or change gas bubbles?
Since we now have rovers on Mars whose primary job is to examine rocks, shouldn't there be more evidence when comparing the meteorites than "gas bubbles"?
As a number of meteorites are now classified as being from Mars, how does this number compare to what might have possibly landed on Earth? Has anyone estimated the probability that ejecta from Mars will occur, and eventually land on the Earth (as opposed to orbiting the sun, or again landing on Mars, exiting the solar system, or impacting another larger body such as Jupiter).
The doubt for me would be that there are few ejecta events that produce material that exits Martian orbit, whose trajectory subsequently crosses Earth's atmosphere before impacting some other body, whose material also is large enough to survive atmospheric entry, to finally rest on land and (probability 1/3 with 2/3 ocean), specifically land untouched in a desolate area like antarctica.
This seems doubtful because we keep having to multiply what would seem like small probabilities. Taking just the first item, how often do events on the Earth occur that launches rocks at escape velocity?
Should we believe the claims that these rocks can be traced to Mars?