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We finally know how Mercury gets its dark gray color: an ancient carbon crust

It explains a long-standing mystery about the small planet

NASA

Mercury’s abnormally dark coloring has puzzled scientists for years — but a new study using NASA data has revealed the origins of the planet’s unique look. Patches of a carbon-rich material called graphite — the same stuff that’s in a pencil — cover Mercury’s surface, tinting it dark gray.

These patches are thought to come from an ancient carbon crust that's been hiding underneath Mercury's surface, a study published in Nature Geoscience says. The carbon comes up to the surface when asteroids or other objects hit the planet, leaving large impact craters that expose the ancient materials underneath.

"There are only a few things that can darken a surface."

This could finally explain the origins of Mercury's dark surface. Experts originally thought the planet's dark patches could be made of iron, since iron makes up similar dark patches on the Moon, according to NASA. But MESSENGER data indicated that Mercury's surface is very iron-poor, leaving astronomers stumped about where the planet's dark coloring comes from. "There are only a few things that can darken a surface," said Francis McCubbin, an astromaterials curator at NASA Johnson Space Center, who was not involved in the study. Some experts offered the idea that carbon could explain the color, but astronomers weren't sure if the carbon came from within the planet itself or if it was brought to Mercury by asteroids.

In this study, scientists used data from the final orbits of NASA's now defunct MESSENGER spacecraft, which orbited Mercury for four years. During MESSENGER's last few months in operation, NASA brought the probe closer and closer to Mercury and eventually let the spacecraft crash into the planet's surface. As MESSENGER got to lower altitudes, the spacecraft was able to analyze Mercury's surface more in-depth, allowing researchers to understand what the planet's dark patches are made of. "The investigation really happened with those last sets of measurements, on its death dive," said study author Patrick Peplowski, a research scientist at the Johns Hopkins University Applied Physics Laboratory. "It’s the spacecraft’s parting gift."

A dark halo of what is thought to be graphites surrounds a crater on Mercury. (NASA)

The researchers believe this carbon material must have formed on Mercury during its early days. Carbon minerals floated to the top of a global magma ocean that once covered the planet's entire surface, the study claims. There, the carbon hardened into a crust, and was eventually covered up by massive amounts of lava that spewed from Mercury's early volcanoes. The lava then cooled on top of the carbon, forming a secondary crust. But multiple exposures of the carbon crust mixed with the top layer, giving Mercury its dark shade.

In other words: the carbon is a very old material that's been part of Mercury since the planet first formed. "It offers us the opportunity to see an ancient surface," said Peplowski. "Studying this, we can try to better understand how terrestrial planets form."

"It offers us the opportunity to see an ancient surface."

For this study, NASA researchers used a MESSENGER instrument that can measure escaping particles found inside the nucleus of an atom, called neutrons. Neutrons escape from their atoms when high-energy cosmic rays hit a planet, busting apart nuclei on the planet’s surface. The number of neutrons fleeing a planet’s surface tells scientists what the surface is made of; iron, for instance, doesn't let as many neutrons escape as carbon does. MESSENGER found a lot of neutrons escaping from Mercury's dark patches, indicating a carbon presence known as graphite.

Peplowski and his team also noticed that this dark material is almost always found surrounding a crater. That led the researchers to believe the carbon material is in abundance underneath the crust; the carbon is then ejected or exposed whenever an object hits the top surface of Mercury.

This provides a clearer picture about how Mercury initially evolved, according to McCubbin. He also argues that this research provides more motivation to continue studying Mercury, since these minerals aren't found elsewhere in the Solar System. "We've never had a planetary surface as exotic as Mercury," said McCubbin. "Our next step should be to send something there to look at it or bring [some samples] back.