Thicker Martian Atmosphere

NASA's rover Opportunity, as reported in this blog, has discovered two meteorites on Mars during its travels. The second one, found last month, is a big iron-nickel specimen. So big, in fact, that scientists say it could not have fallen through Mars' thin atmosphere and ended up where it is. A meteor that size would have roared through the atmosphere, slammed into the surface, been obliterated by the impact, and left a crater.

But there it sits, its bluish tint at odds with its reddish surroundings. The answer? At some point, Mars' atmosphere was thicker than it is today. A thicker atmosphere would have produced more friction on the rock during its descent, thus slowing it down enough to allow a big piece of the original body to reach the surface at a speed that left it intact.

Opportunity is continuing to study the rock, gathering data and looking for clues about when it felll. Scientists think early Mars did have a thicker atmosphere, but if the meteorite hit after that epoch it might mean Mars' atmosphere somehow cycles between thick and thin.