The planet Mercury is particularly dark. It is considerably darker than the Moon, which is the next closest airless body to the Sun. Exactly why it is so dark, however, has remained a bit of a mystery.

Airless bodies tend to be darker than others because of micrometeorite impacts and solar wind which create a thin coating of iron particles on the surface. Spectral data from Mercury doesn’t show enough nano phase iron to account for the lack of reflectiveness from the planet’s surface.

Now, a new theory has emerged which could help to explain the peculiar blackness of the closest planet to the Sun. In a paper published in the journal Nature Geoscience suggests that the heavy comet traffic in Mercury’s neighborhood has left a steady dusting of carbon in its wake.

“It’s long been hypothesized that there’s a mystery darkening agent that’s contributing to Mercury’s low reflectance. One thing that hadn’t been considered was that Mercury gets dumped on by a lot of material derived from comets,” said Megan Bruck Syal, a postdoctoral researcher at Lawrence Livermore National Laboratory who performed this research while a graduate student at Brown University in a statement.

When comets approach the Sun, they warm up, ice melts and they begin to break apart. Carbon makes up as much as 25 percent of the dust released by the comets.

The team tested the theory, first, by estimating the frequency with which carbon from cometary dust would impact mercury, and how much would stick to the surface. Bruce Syal’s estimates show that after billions of years of dusting, three to six percent of the planet’s surface should be carbon dust.

Next, the researchers set out to simulate the carbon dusting so that the decrease in reflectiveness could be measured.

Using a 14 foot canon that simulates celestial impacts at the NASA Ames Vertical Gun Range the researchers lauded projectiles of an organic compound that mimics comet dust. The heat of the impact would result in the burning of sugar in the compound, releasing carbon from the material.

To simulate the surface of Mercury, the team used material designed to mimic lunar basalt which can be seen in the dark patches on the light side of the moon.

“We used the lunar basalt model because we wanted to start with something dark already and see if we could darken it further,” said Peter Schultz, professor emeritus of geological sciences at Brown and a co-author of the new research.

The simulations showed that when the impact material melts, tiny carbon particles become deeply embedded and the amount of light reflected is drastically reduced. Ultimately, the reflectiveness was reduced to less than five percent which matches the darkest patches on Mercury.

Additional spectroscopic analysis of the simulated impact sites showed no distinctive spectral fingerprints, which is similar to the signatures studied on Mercury.

“We show that carbon acts like a stealth darkening agent. From the standpoint of spectral analysis, it’s like an invisible paint. We think this is a scenario that needs to be considered. It appears that Mercury may well be a painted planet,” said Schultz.