Scientists have found out that it is indeed carbon that causes the darkening of the planet, but not from impact with comets. Using MESSENGER, an orbiting spacecraft from NASA, the scientist believes that the color is from the remains of the Mercury's ancient surface.
During the formation of the planet, the surface of the planet remained in lava form for longer than in other planets. But as it cooled most minerals sunk under the crust. But, heating of carbon at certain temperatures lead to the creation of granite. The granite is light and did not sink like the rest of the minerals. It remained as dust on the surface of the planet, causing the dark appearance of the globe.
"The previous proposal of comets delivering carbon to Mercury was based on modeling and simulation. Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence," said Larry Nittler, an investigator with MESSANGER. "If so, we may be observing the remains of Mercury's original, 4.6-billion-year-old surface."
There is a saturation of the dark color along the natural craters' on the planet. This is evidence that the occurrence is natural and is as a result of planetary evolution rather than comet impact.