Why are some meteorites from Mars?
When big space rocks hit Mars, they can blast pieces of Mars into space. A few of these pieces, after drifting for millions of years, land on Earth.
It sounds amazing, but it is true: some rocks found on Earth come from Mars.
Here is how it happens. Every so often, a large asteroid crashes into Mars. The impact is so powerful that it throws rocks out of the crater at enormous speeds. Mars has weaker gravity than Earth, so if a rock is thrown faster than about 5 kilometres per second, it escapes from Mars completely and goes into orbit around the Sun.
These pieces of Mars travel through space, sometimes for millions of years. Gradually, the pull of the planets changes their paths. A few eventually cross Earth's path, fall through our air and land as meteorites.
Scientists know these meteorites came from Mars because tiny bubbles of gas trapped inside some of them match the air of Mars, measured by the Viking spacecraft that landed there in 1976. They also have special chemistry that matches Mars and not Earth.
About 400 Martian meteorites have been named, many of them pieces of the same falls. They are the only pieces of Mars that scientists on Earth can study, until future spacecraft bring samples home. So if you see a Martian meteorite in a museum, you are looking at a real piece of another planet.