Though it is nearest to the sun, Mercury’s surface is immensely dark. So, what is the ‘darkening agent’? About a year ago, scientists proposed that Mercury’s darkness was due to carbon that gradually accumulated from the impact of comets that travelled into the inner Solar System. Now, the scientists, led by Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory, have used data from the MESSENGER mission to confirm that a high abundance of carbon is present at Mercury’s surface.
MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun.
However, they also have also found that, rather than being delivered by comets, the carbon most likely originated deep below the surface, in the form of a now-disrupted and buried ancient graphite-rich crust, some of which was later brought to the surface by impact processes after most of Mercury’s current crust had formed.
MESSENGER obtained its statistically robust data via many orbits on which the spacecraft passed lower than 60 miles (100 km) above the surface of the planet during its last year of operation.
Repeated Neutron Spectrometer measurements showed higher amounts of low-energy neutrons, a signature consistent with the presence of elevated carbon, coming from the surface when the spacecraft passed over concentrations of the darkest material.
When Mercury was very young, much of the planet was likely so hot that there was a global “ocean” of molten magma. From laboratory experiments and modeling, scientists have suggested that as this magma ocean cooled, most minerals that solidified would sink. A notable exception is graphite, which would have been buoyant and floated to form the original crust of Mercury.
The study has been published in Nature Geoscience journal.