For years, scientists have wondered why Mercury is so dark. It doesn't have as much iron and titanium as the Moon, so it should be brighter. A team at Johns Hopkins' Applied Physics Laboratory finally has the answer, though. Thanks to a spectral analysis of color images from NASA's Messenger probe, they've determined that carbon (specifically, graphite) is the probable culprit behind Mercury's dim look. Most likely, what you're seeing is the aftermath of the tiny planet growing up. As the young Mercury's magma ocean cooled 4.6 billion years ago, the graphite would have floated to the top and formed the original crust -- there just happens to be enough of it left to affect visibility.