Planetary flotsam

The Mars rover Opportunity has boggled me again. As it trundled across a stretch of Meridiani Planum examining some scrap from its discarded heat shield, NASA announced, it spotted the unmistakable sight of a nickel-iron meteorite as big as your head. If a random stroll of 2 kilometers turns up something like this, the surface of Mars must be teeming with stones from space. But the rover hasn't been checking all the stones it meets. Now that it's happened, the idea of finding abundant meteorites on Mars is blindingly obvious. On Earth, meteorites are quite rare--and an even rarer fraction of them are of Martian origin.