A meteorite found on Mars!

The Mars Rover Opportunity found a little while ago a decent-size meteorite while cruising around in Mars. This is an extraordinary find for the things it tells us. One of the most important clues is that it points to a thicker atmosphere on Mars long ago, which strongly hints at the presence of water. I could explain some more, but I'll let Ethan Siegel, a bona-fide astronomer tell the story.