Being the closest to the sun, you'd think Mercury would shine the brightest among the inner planets of the solar system. In truth, however, it reflects even less light than our own moon. Scientists have long speculated on why that is so and what leads to Mercury's very dark facade. Thanks to data gathered by the dearly departed MESSENGER probe, they confirm that carbon is indeed to blame. But interestingly enough, the carbon didn't come from outside forces but right from the planet's crust itself.
Mercury's darkness has always been a puzzle to scientists, especially as they compare it with our moon. Just as the moon has dark spots, scientists theorized that an overabundance of certain elements cause the small planet's gloomy appearance. But unlike the moon, Mercury's darkness isn't caused by iron-rich minerals. Instead, it is due to carbon, particularly in graphite.
Yes, graphite, like the ones used in pencils, which can be a bit amusing if you think about it. But although scientists have already speculated at this possibility, they haven't completely figured out the origin of that much carbon. Some even posited that the carbon is alien in origin. No, not extraterrestrials, but meteorites that come crashing into the planet, scattering carbon all around.
However, thanks to the MESSENGER's Neutron Spectrometer and X-rays, scientists now believe that the origin of that much carbon is closer to home. During Mercury's youth, it was so hot that it was practically covered by an ocean of molten magma. As it cooled, most minerals solidified and started to sink. Except graphite, which instead floated and would form the crust of the planet. You could almost say that the sun burned Mercury to a crisp, hence the darkness.
This new model of Mercury's composition was all thanks to NASA's MESSENGER probe. Sadly, the probe didn't live to celebrate that contribution. After orbiting the innermost planet 4,104 times, MESSENGER crashed, purposely, into Mercury in May last year.