Meteorite on Mars.

Opportunity, one of the twin rovers on Mars, has discovered the first meteorite on a planet other than Earth. Initial observations, taken from a distance with the rover's thermal-emission spectrometer, indicated that the pitted, basketball-size body is a metal-rich meteorite. Driving close enough to use its X-ray and gamma-ray spectrometers, Opportunity confirmed the object's meteorite status and revealed that it's made mostly of iron and nickel, NASA announced on Jan. 21.

The metallic composition indicates that the rock came from an asteroid or planetary chunk large enough for its mixture of minerals to have separated into a dense, metallic core and a lighter, rocky mantle, notes rover researcher Steve Squyres of Cornell University.

Scientists have dubbed the meteorite Heat Shield Rock because it lies near debris from Opportunity's heat shield in Meridiani Planum, the cratered plain where the rover landed on Jan. 24, 2004. Metal-rich meteorites are relatively rare on Earth, and scientists have hypothesized that Mars is also bombarded by many more rocky meteorites than metallic ones, notes Squyres.

Other clumps of material seen at the surface of Meridiani Planum may be rocky meteorites, he suggests.

Given the multitude of meteorites that scientists have already studied on Earth, meteorites on Mars are more interesting for what they might reveal about the Red Planet than about the rocks themselves. For example, determining the number of exposed meteorites at Meridiani Planum could indicate whether the flatland is gradually eroding or being built up by ongoing geophysical processes.