New study explains why Mercury is black

LIVERMORE, Calif., March 31 (UPI) -- Mercury and the moon look a lot alike -- barren and riddled with craters. But whereas the moon appears a cream white in the light of the sun, Mercury is dark and dirty.

A new study reveals the source of color differentiation. Mercury, it turns out, has been regularly peppered by carbon from passing comets over the course of its planetary lifetime -- slowly but steadily darkening its surface.

Typically, iron is responsible for diminishing a planet's reflectivity. But for Mercury, it is its outer coating of carbon that makes it the least reflective planet in the solar system.

Researchers at Lawrence Livermore National Laboratory pinpointed comet carbon as the most likely source of Mercury's dark color using a computer model. Astronomers knew comets are most likely to break apart and shed their materials as they near the sun, and Mercury is the closest planet to the sun.

Their model showed most of the carbon dust would remain on the planet's surface -- and not bounce back into space -- calculating that Mercury's carbon likely accounts for 3 to 6 percent of its surface composition.

Researchers then tested the darkening effects of engrained carbon, mimicking the impacts of small meteorites on Mercury's surface at NASA's Ames Vertical Gun Range. To mimic meteorites, scientists encapsulated organic comet compounds inside a ball of sugar. Using a cannon, they blasted the sugar balls at a lunar basalt-like substance.

"We used the lunar basalt model because we wanted to start with something dark already and see if we could darken it further," study co-authro Peter Schultz, a geological science professor at Brown University, told Voice of America.

The sugar burned up in the blast and impact, while the organic compounds (including the carbon) embedded in the surface materials -- creating a darkening effect that matched Mercury's appearance.

"Understanding the role of micrometeorites in delivering dark material to Mercury provides new ways of interpreting observations of the planet," Megan Bruck Syal, a postdoctoral researcher at the lab, explained in a press release. "Additionally, we are now working on how micrometeorites may have delivered other materials of interest to Mercury, including water."

The new research was published in the journal Nature Geoscience.