The carbon is thought to have been transported to the surface through impact craters left after large bodies smashed into the planet.
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NASA Messenger Mercury
Mystery behind Mercury's deep dark surface may have been solved
A view of the planet Mercury from the MESSENGER spacecraft, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. (Photo via NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
This artist's rendering provided by NASA shows the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft around Mercury. On Thursday, April 16, 2015, NASA announced that after years of orbiting the planet, the spacecraft will crash into the planet at the end of the month. (NASA/JHU APL/Carnegie Institution of Washington via AP)
Ten years ago, on August 3, 2004, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft blasted off from Cape Canaveral, Florida, for a risky mission that would take the small satellite dangerously close to Mercury’s surface, paving the way for an ambitious study of the planet closest to the Sun. (Photo via NASA)
A solar flare erupted on the far side of the sun on June 4, 2011, and sent solar neutrons out into space. (Photo via NASA/STEREO/Helioviewer)
South America and portions of North America and Africa are shown in this false-color image from NASA's MESSENGER spacecraft, taken on an Aug. 2, 2005 Earth flyby to adjust the spacecraft's path to Mercury. (Photo via NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
This artist's rendering provided by the Johns Hopkins University Applied Physics Laboratory shows the sunshade on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (Messenger) around the planet Mercury. The sunshade shields the spacecraft's instruments from heat and solar radiation. (Johns Hopkins University Applied Physics Laboratory via AP) Image converted using ifftoany
This Oct. 6, 2008 image provided by NASA shows previously unseen terrain on Mercury as the Messenger spacecraft approached the planet during its second flyby. In the foreground is a region of rough, heavily cratered terrain with a large, ancient two-ring impact basin at the bottom center of the image. In the distance is a region of younger, tectonically modified smooth plains that have been pockmarked by small craters. (NASA, Johns Hopkins University Applied Physics Laboratory, Carnegie Institution of Washington via AP)
This color image, taken on May 1, 2013 by the Wide Angle Camera (WAC) instrument aboard NASA's MESSENGER spacecraft orbiting Mercury, features Hovnatanian crater, named for Armenian painter Hakop Hovnatanian. (Photo via NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
This Oct. 6, 2008 photo provided by NASA shows Mercury during the Messenger spacecraft's second flyby of the planet. (NASA, Johns Hopkins University Applied Physics Laboratory, Carnegie Institution of Washington via AP)
This new, high-resolution view of Cunningham crater was recently acquired by MESSENGER. What you can't see in this image, which shows striking details of the crater's interior, is the extensive set of rays associated with Cunningham. The bright rays of Cunningham indicate that the crater is relatively young, having formed on Mercury likely within the last billion years. In this new view, the preserved terraces of the crater walls, the well-defined central peak, and the limited number of overlying small craters are also all signs of Cunningham's relative youth. (Photo via NASA/Johns Hopkins University Applied Physics)
In this undated photo provided by NASA, technicians with The Johns Hopkins University Applied Physics Laboratory in Titusville, Fla., prepare the MESSESNGER spacecraft for a move to a hazardous processing facility in preparation for loading the spacecraft's hypergolic propellants. (NASA via AP)
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER's primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up Mercury's surface. (Photo via NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
The crater at the center of this scene is named for choreographer George Balanchine, as the beautiful swath of diffuse blue ejecta emanating from the crater might remind one of the famous blue tutus in one of Balanchine's most well known ballets, Serenade. (Photo via NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
This image of Mercury, acquired by the Mercury Dual Imaging System (MDIS) aboard NASA's MESSENGER mission on April 23, 2013, allows us to take a step back to view the planet. Prior to the MESSENGER mission, Mercury's surface was often compared to the surface of Earth's moon, when in fact, Mercury and the moon are very different. (Photo via NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
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As such, Mercury's surface is estimated to contain "weight-percent levels of carbon, likely in the form of graphite."
This concentration is much higher than that found on Earth or Mars.
The researchers hypothesize that, in Mercury's early history, the hot magma that covered it eventually cooled, leaving carbon to form the crust that lies underneath.