Meteorite on display in Exploring The Planets - Loaned to the National Air and Space Museum by Division of Meteorites, National Museum of Natural History

Planetary Origin

Radioactive dating techniques reveal that almost all meteorites solidified from molten rock about 4.5 billion years ago, when the solar system was formed. The SNC meteorites, however, are only about 1.3 billion years old. Their relatively young age suggests that SNCs cannot have come, like other meteorites, from the small asteroids. They must have solidified on a planet--a body large enough to retain heat and produce molten rock even billions of years after it formed.

Why from Mars?

The SNC meteorite exhibited here was found to contain some gas trapped in its interior. Analysis showed that the composition of this gas was nearly identical to that of the atmosphere on Mars, as measured by the Viking Landers in 1976.
Each point on the graph [115k GIF]shows the concentration of one of the gases in the Martian atmosphere (vertical axis). For a perfect correlation, the points would all fall precisely on the diagonal line. The close correlation is persuasive evidence that this meteorite actually came from Mars.

Mars Impact Ejecta

Most of the debris produced by crater-forming collisions on Mars falls back to the surface of the planet. Some of the rocks strewn about the Martian surface may have been ejected long ago by such impacts elsewhere on Mars. But such cratering events may also eject some rocks with such great speed that they escape from the planet. Some of these escaped Mars rocks, orbiting in interplanetary space, may in turn eventually collide with Earth, resulting in SNC meteorites, such as the sample exhibited here. A few meteorites appear to be lunar rocks that escaped from the Moon in the same way.

A Speculation

Similar collisions of asteroids or comets with Earth could have blasted terrestrial rocks into escaping orbits that eventually carried them to Mars. Some of these rocks could have been large enough to shield any dormant spores or microbes inside them for millions of years against the sterilizing efects of solar and cosmic radiation. Some of the rocks could have broken apart when they landed on Mars.
This leads to an intriguing speculation: A few billion years ago, when Mars still had water on its surface, could the planet have been "seeded" by organisms carried in meteorites from Earth?