Why are many large craters on Mercury relatively smooth inside? Images from the MESSENGER spacecraft that flew by Mercury in October 2008 show previously uncharted regions of the planet that have large craters with an internal smoothness similar to Earth's own moon, and are thought to have been flooded by lava floes that are old but not as old as the surrounding more highly cratered surface.

MESSENGER will buzz past Mercury late in 2009 before entering orbit in 2011.