New analysis conducted by Mars Exploration Rover Opportunity on a large meteorite that it discovered last month reveals more details on the red planet's environmental history.

Last week we reported on the first analysis that Opportunity performed on the rock known as Block Island using its Alpha Particle X-ray Spectrometer (APXS), confirming the watermelon-sized boulder as an iron-nickel meteorite. Block Island is the largest meteorite known to have fallen on Mars, and scientists say that it is too massive to have hit the ground without disintegrating unless Mars' atmosphere was much thicker at the time the rock fell. A thicker atmosphere would have acted as a brake, slowing the meteoroid's descent before it struck the ground.

"Consideration of existing model results indicates a meteorite this size requires a thicker atmosphere," says rover team member Matt Golombek of NASA's Jet Propulsion Laboratory. "Either Mars has hidden reserves of carbon dioxide ice that can supply large amounts of carbon dioxide gas into the atmosphere during warm periods of more recent climate cycles, or Block Island fell billions of years ago."

The microscopic imaging camera on the rover's arm also revealed a distinctive triangular pattern, identical to patterns common in iron-nickel meteorites found on Earth. "Normally this pattern is exposed when the meteorite is cut, polished and etched with acid," says Tim McCoy from the Smithsonian Institution in Washington. "Sometimes it shows up on the surface of meteorites that have been eroded by windblown sand in deserts, and that appears to be what we see with Block Island."

Earlier measurements had already identified variations in the composition across the meteorite's surface that could result from interaction with the Martian environment, such as rusting of metals in the rock exposed to water.

"We have lots of iron-nickel meteorites on Earth. We're using this meteorite as a way to study Mars," says Albert Yen, a rover team member at JPL. "Before we drive away from Block Island, we intend to examine more targets on this rock where the images show variations in color and texture. We're looking to see how extensively the rock surface has been altered, which helps us understand the history of the Martian climate since it fell."

Once analysis of Block Island is complete, Opportunity will continue its journey to Endeavour Crater.