Why is the surface of Mercury dark? It’s a question that has stumped scientists for years. The closest planet to the sun doesn’t have an abundance of minerals, like iron and titanium, that is known for giving other bodies a darker appearance.

Patrick Peplowski, the lead author of a new paper, explains carbon became the primary suspect.

“A process of elimination led prior researchers to suggest that carbon may be the unidentified darkening agent, but we lacked proof,” Peplowski said. “Spectral modeling of MESSENGER color imaging data suggested that weight-percent levels of carbon, likely in the form of graphite, would be required to darken Mercury’s surface sufficiently. This level is unusually high, given that carbon is found at typical concentrations of only ~100 parts per million on the Moon, Earth and Mars.”

How would Mercury have so much carbon? One theory took aim at comets. Over time, comet impacts would have caused carbon deposits to slowly accumulate.

During MESSENGER’s last days, the spacecraft confirmed a lot of carbon (in the form of graphite) is on Mercury’s surface. But, the comet theory was wrong. MESSENGER’s Neutron Spectrometer shows the carbon on Mercury “most likely originated deep in the crust,” according to co-author Larry Nittler.

“Moreover, we used both neutrons and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface,” Nittler added.

MESSENGER answered the mystery of Mercury’s dark surface during orbits of less than 60 miles above the surface. Many of the most important measurements happened just a few days before its scheduled impact in April 2015.

Mercury’s carbon surface

With the comet theory out the window, scientists turned to other modeling. During Mercury’s early years, most of the planet was probably covered in an ‘ocean’ of molten magma. Modeling shows as this molten ocean cooled, most minerals solidified and sank. With one exception. Yep, graphite. It would have floated to the top and formed Mercury’s original crust.

“The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury’s original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface we see today. This result is a testament to the phenomenal success of the MESSENGER mission and adds to a long list of ways the innermost planet differs from its planetary neighbors and provides additional clues to the origin and early evolution of the inner Solar System,” said Nittler.

We can see how the planet’s ancient crust mixes with today’s crust by looking at impact craters. Here’s a picture of Basho crater.

Mercury basho crater

The dark material around the impact area stands out in contrast to the rest of the surrounding surface. This graphite was brought to the surface as a result of the impact. Scientists believe this graphite could be parts of Mercury’s primordial crust. We could be looking at pieces of the planet’s surface dating back 4.6 billion years.

Why is this discovery a big deal? The more we know about Mercury’s original surface, the more we know about how it formed. There’s still plenty of questions about Mercury, though. While we now know about carbon, we still don’t know what other minerals make up the planet’s crust.

Did you know: The MESSENGER spacecraft orbited Mercury 4,105 times during its more than four years at the innermost planet. MESSENGER’s mission came to an end on April 30, 2015. But not before sending more than 277,000 images back to Earth. And this is probably my favorite image. The first (left) and last image (right) of Mercury captured by MESSENGER.

Mercury first and last image from MESSENGER

Image credits: NASA