A large boulder spied by the Mars Exploration Rover in July has been confirmed as the largest iron meteorite found on Mars to date.

The rock in question was first spotted on 18 July, and stood out because of its impressive size against the smaller rocks scattered across the barren terrain of Meridiani Planum, Opportunity's home for the last five years.

The object, dubbed Block Island, is around 60 centimetres wide and 30 centimetres tall, and has now been confirmed as the biggest meteorite found on Mars to date. Opportunity studied Block Island with its Alpha Particle X-ray Spectrometer (APXS) last weekend and determined its composition as an iron-nickel meteorite similar to the meteorite found near the lander's heat shield back in 2005 (and subsequently nicknamed Heat Shield Rock).

"There are, however, subtle differences in the chemical composition that we are attempting to characterise and understand," Opportunity team member Dr Albert Yen tells Astronomy Now. "The shiny specks that we see on Block Island are clean metallic surfaces that reflect light well. These spots are not rusted, indicating that the environment at Meridiani has been dry ever since the arrival of this meteorite."

One big question is how a rock of this size could have survived impact with the surface, given that the current thin Martian atmosphere can't do very much to slow it down. "The possibility that this meteorite landed at a time when the atmosphere was thicker needs to be considered," says Yen.

The team are planning more analysis with the APXS instrument, as well as with the Mossbauer spectrometer and Microscopic Imager (MI) instruments, through the coming weekend. The Mossbauer instrument lends itself to close up investigations of iron-bearing minerals, while the MI is used for obtaining high resolution images of rocks and soils.

The forthcoming analysis could help planetary scientists determine how long the meteorite has been lying on the red planet's surface and whether it is related to Heat Shield Rock, or even to the 22 kilometre wide crater called Endeavour, where Opportunity is headed. The rover took a 250 metre detour to examine Block Island.

"When you're driving in a sandy desert, any opportunity to stop and analyse a big rock is welcome!" says Yen.

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