The indefatigable Opportunity rover, still motoring across the Red Planet five years into its mission, recently came across what may be a large meteorite sitting on the Martian surface.

The 0.6-meter rock, dubbed Block Island [detail below], would not be the first meteorite discovered on Mars by the rover, but would be notable for its size. Block Island is nearly twice as long as the meteorite known colloquially as Heat Shield Rock and formally designated Meridiani Planum, which Opportunity spotted in 2005. That meteorite was the first to be found on another planet and remains the only one formally accepted by the Meteoritical Society. (Meridiani Planum was found on a plain of the same name; the Meteoritical Society's convention is to name meteorites for a nearby geographic feature.)

Several other candidates have been discovered on Mars by Opportunity and its twin rover Spirit over the years: In 2006 Spirit photographed two likely suspects, of roughly equal size to Meridiani Planum, but could not inspect them up close because of the steepness of the terrain and the fact that the rover was dragging a broken wheel. Opportunity has come across several much smaller cobbles in its travels that also may be of meteoritic origin.

Investigating meteorites on Mars may provide a clearer glimpse into the asteroid belt—some have speculated that asteroids impacting Mars may be fundamentally different, in both orbital characteristics and composition, than those that strike Earth. And signs of weathering, or lack thereof, can provide clues to the presence or absence of liquid water on Mars.

Opportunity is checking out its new find using the onboard Alpha Particle X-Ray Spectrometer to assess the rock's chemical composition and further unwind its origins.

Image Credit: NASA/JPL-Caltech