Mercury's dark secret
NASA's MEcury Surface, SpaceENvironment, GEochemistry and Ranging spacecraft (MESSENGER) may haveburned up last year, but the data it gathered before its demise is still being put togood use. Scientists have used the information to reveal the secretbehind the dark appearance of Mercury's surface, with the resultspartially contradicting a recently proposed theory on the matter.
The new research addresses a theoryproposed last year, in which scientists suggested that the darkappearance of Mercury's surface as due to the presence of largevolumes of carbon, and that the material had been deposited on thesurface by impacting comets.
The researchers set about solving themystery, using data collected by MESSENGER's Neutron Spectrometer tomap the distribution of carbon, then comparing it with the areas ofdark material visible on the surface. The information was collectedduring the last year of the MESSENGER mission, during numerousorbits, where the spacecraft passed less than 60 miles (100 km) abovethe surface of the planet.
Not only did the data confirm that thedark patches are indeed carbon, but also provided information thatdescribes where the material likely came from. The Neutron Spectrometer information was combined with X-rayand reflectance spectra data, allowing the team to determine exactlyhow much carbon was present in the dark areas. Once the calculationswere complete, the results showed that the rocks inthe target areas were made up of as much as a few percent ofgraphitic carbon, which is a much higher concentration than found onother planets.
The project scientists report that thedata, combined with laboratory experiments and modeling, indicatesthat Mercury once played host to a global ocean of magma. When thiseventually cooled, most minerals sunk, with the exception ofgraphite, which is a more bouyant material. The dark areas that arevisible today are the remnants of that ancient graphite-rich crust.
"This result is a testament to thephenomenal success of the MESSENGER mission and adds to a long listof ways the innermost planet differs from its planetary neighbors andprovides additional clues to the origin and early evolution of theinner Solar System," said mission principal investigator LarryNittler.
The results of the research werepublished in the journal Nature Geoscience.
Source: Carnegie Institution