The Mercury MESSENGER probe folks just released a new cool image, and this one shows one of the larger craters on the planet that has a dark ring around it. Emily has lots of details, of course, with an explanation... but I know the real cause of these rings. You'd have dark circles around your eyes too if the Sun were constantly shining in your face.
P.S. My buddy Chris Lintott is hosting the Carnival of Space this week, too.