As the closest one to orbit the sun, Mercury is a difficult planet to study.

The first spacecraft to visit Mercury was Mariner 10, which flew by the planet in 1974 and 1975. Not even half of the planet was observed, so it had remained a mystery for a long time.

About 30 years later, NASA sent its MESSENGER spacecraft to fly by and study Mercury up close. The spacecraft had transmitted countless new data back to Earth, until it crashed into Mercury and ended its mission.

Mercury's surface is quite similar to Earth's moon as the planet is covered with holes that are called impact craters. Like the moon, Mercury exhibits a complete range of phases while it moves around its orbit relative to Earth.

In one hemisphere of the planet, MESSENGER had detected concentrations of potassium, uranium, chlorine, iron, magnesium, calcium, sodium, aluminum, silicon, thorium and sulfur. Scientists were able to create low-resolution maps out of the data.

But the maps were very limited. Last year, new maps of the planet's surface were created, revealing the chemical composition of areas that have not been studied before.

Aside from the planet's chemical composition, what has puzzled scientists for so long is the reason behind Mercury's dark surface. Comparing it to the moon's iron-rich surface, Mercury's surface reflects less sunlight. But iron is rare on Mercury's surface, so what is the "darkening agent" here?

Why Planet Mercury's Surface Is Dark

Like the moon, Mercury is often plummeted by impacts from micrometeorites because there is no atmosphere.

In 2015, scientists had proposed that Mercury's dark surface was due to carbon that slowly accumulated from the impact of comets that traversed into the inner Solar System.

The carbon-rich comet dust supposedly acts like an invisible paint which lowers the reflectivity of the small planet, scientists said. The proposal was based on stimulation and modeling, so there was no direct evidence for the matter.

Now, a group of scientists led by Patrick Peplowski of Johns Hopkins University studied data from the MESSENGER mission and confirmed that there is indeed high abundance of carbon on Mercury's dark surface.

Instead of comet dust, however, the carbon most likely originated from deep below the surface of the planet. It is now in the form of a buried and disrupted ancient graphite-rich crust. Some of the crust was brought to the surface by impact processes after most of Mercury's current crust was formed.

Thanks to MESSENGER's Neutron Spectrometer, the team was able to spatially resolve the carbon distribution on the surface and discovered that the element was correlated with the darkest material on the planet.

The team also used neutrons and X-Rays to confirm that the material is not rich in iron.

Remnants Of Mercury's Ancient Crust

MESSENGER acquired its data via many orbits in which the spacecraft passed lower than 60 miles above the planet's surface during its last year of operation.

To identify carbon, scientists used data and measurements taken days before MESSENGER impacted the planet in April 2015.

During Mercury's early years, much of it was possibly so hot that there may have been a global "ocean" of molten magma, scientists said.

From modeling and lab experiments, experts suggested that as the magma ocean likely cooled down, most of solidified minerals would sink. One exception is graphite, which would have buoyed to form the original crust of the planet.

Study co-author and principal investigator Larry Nittler said that the presence of abundant carbon on the planet's surface indicates that it may be remnants of the planet's ancient crust, which was mixed into the ejecta from volcanic rocks and meteoritic explosion that has formed the current surface.

"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," added Nittler.

The study is featured in the journal Nature Geoscience and was funded by NASA.