Here's why the surface of Mercury is so dark

The tiny planet is covered in a light-absorbing element.

NASA, Johns Hopkins University Applied Physics Laboratory, Carnegie Institution of Washington via AP

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.

The data might not just solve one riddle. Besides explaining some of Mercury's early history, it could also give a sense of the materials that were swirling close to the Sun as the solar system formed. The exact blend of minerals still isn't known, but this one insight could easily pay dividends. And given that Messenger collected much more data, it won't be shocking if there are more answers in the near future.