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	2009 Sky Almanac McDonald Observatory logo cap

A chunk of iron and nickel the size of a watermelon has provided new evidence that the Martian atmosphere was once much warmer and denser than it is today.

Opportunity gets close to the meteorite. [NASA/JPL/Cornell]

The Opportunity rover discovered the meteorite during its long trek to Endeavour Crater. The meteorite is so big and heavy that it should have been pulverized when it hit the ground. Since it wasn't, scientists concluded that it must have hit Mars at a time when the planet's atmosphere was much thicker than it is today. The air slowed the rock's descent enough to keep it from disintegrating when it hit the ground.

Scientists aren't sure when the meteorite hit, however. It could have been billions of years ago, at a time when lakes and seas covered the Martian surface. Or it could have been hit more recently, at a time when the planet warmed up for a while, releasing gas from its ice caps to create a thicker atmosphere.