NASA's MEcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER) may have burned up last year, but the data it gathered before its demise is still being put to good use. Scientists have used the information to reveal the secret behind the dark appearance of Mercury's surface, with the results partially contradicting a recently proposed theory on the matter.

The new research addresses a theory proposed last year, in which scientists suggested that the dark appearance of Mercury's surface as due to the presence of large volumes of carbon, and that the material had been deposited on the surface by impacting comets.

The researchers set about solving the mystery, using data collected by MESSENGER's Neutron Spectrometer to map the distribution of carbon, then comparing it with the areas of dark material visible on the surface. The information was collected during the last year of the MESSENGER mission, during numerous orbits, where the spacecraft passed less than 60 miles (100 km) above the surface of the planet.

Not only did the data confirm that the dark patches are indeed carbon, but also provided information that describes where the material likely came from. The Neutron Spectrometer information was combined with X-ray and reflectance spectra data, allowing the team to determine exactly how much carbon was present in the dark areas. Once the calculations were complete, the results showed that the rocks in the target areas were made up of as much as a few percent of graphitic carbon, which is a much higher concentration than found on other planets.

The project scientists report that the data, combined with laboratory experiments and modeling, indicates that Mercury once played host to a global ocean of magma. When this eventually cooled, most minerals sunk, with the exception of graphite, which is a more bouyant material. The dark areas that are visible today are the remnants of that ancient graphite-rich crust.

"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 mission principal investigator Larry Nittler.

The results of the research were published in the journal Nature Geoscience.