How Mars Lost its Atmosphere

NASA is planning to send a new orbiter to Mars, dubbed MAVEN, to test the most widely accepted theory of how the red planet lost most of its atmosphere. The theory is that when Mars' molten core cooled and solidified, its magnetic field withered away, leaving it exposed to the solar wind, which stripped away most of the air. The new probe will test this by measuring the rate at which the solar wind is currently tearing away gas molecules. Launch is scheduled for November. (Unless there is government shutdown, of course.)

There is a second theory out there should this one fail: that it was a great bombardment by asteroids that blasted off Mars' atmosphere.

Either way, it's a depressing thing to contemplate.