But instead of carbon shed from cometary encounters, they say, it might be the remains of Mercury's ancient surface – a layer of the planet buried by other materials over the course of billions of years. Comets hurtling into Mercury's surface may have actually unearthed dark carbon that was already there.
The new findings were published Monday in the journal Nature Geoscience.
Mercury has always seemed darker than it should be. Because the planet has no atmosphere to speak of — and is the closest thing to the sun, where solar winds are violent and space debris impacts are most common — it's expected to acquire a lot of tiny iron particles, which are known to absorb light and make planets look darker. The moon gets its dark appearance from an abundance of iron. But even though Mercury has a lower concentration of iron than the moon, it absorbs more light.
"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," Carnegie Science's Larry Nittler, deputy principal investigator of the MESSENGER mission and a co-author on the new study, said in a statement.
To figure out whether carbon was the culprit, Nittler and his team used MESSENGER's neutron spectrometer to analyze the distribution of the element. They found that the parts of the planet with higher concentrations of carbon were also the darkest.
The researchers believe the carbon is in the form of graphite (think pencil tips). Because the darkest material is only present around impact craters, they think its source may sit deep beneath the surface.
Based on modeling, scientists are fairly certain that a young Mercury once hosted a hot, global ocean of magma. As the planet aged and cooled, most of its elements would have sunk down below the surface. But graphite is light enough that it would have floated up to the top, crystalizing to form a dark, dark crust over half a mile thick.
Volcanic activity and the refuse of cosmic collisions would have covered up this crust, leaving it hidden from sight until comets massive enough to form deep impact craters came along to reveal it.
"If so, we may be observing the remains of Mercury’s original, 4.6-billion-year-old surface," co-author Rachel Klima, a planetary geologist at APL, said in a statement.
If that's the case, scientists may have pinpointed the best way to look into the planet's early history. One day, this crust could help reveal how Mercury – and other planets in the inner solar system – came to be.