The planet Mercury has a peculiarly dark complexion, and new information from NASA's MESSENGER spacecraft seems to have solved the mystery of its gray-black surface composition. According to a study published today in Nature Geoscience, Mercury's carbon-based, graphite-splattered face may be the exposed remnants of a thick carbon crust that formed in the aftermath of an ancient lava ocean. Prior to these new findings, researchers were confused about the dark color of the innermost planet, especially in the absence of high concentrations of iron or titanium, partialy focusing on carbon as the culprit but not understanding the complete story of how that came to be. A hypothesis was offered that Mercury was once bombarded by graphite-rich comets during its early formation. This notion stood until these new discoveries by NASA’s MESSENGER mission (2011-2015), the first spacecraft to conduct a comprehensive geologic survey of Mercury’s strange surface.
“We used MESSENGER’s Neutron Spectrometer to spatially resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust,” said study co-author Larry Nittler, the Deputy Principal Investigator on the MESSENGER mission.
The graphite dataset was collected during Messenger's low-altitude flyovers near the end of its mission, indicating that Mercury’s surface could be made up of a much higher percentage of carbon, unusual when compared to other rocky planets in our solar system and likely to have been sourced from deep inside the planet's own mantle.
“This was really a huge surprise,” declared Patrick Peplowski, a planetary scientist at The Johns Hopkins University Applied Physics Laboratory and lead author on the new study. “The question is: if there’s several percent carbon on Mercury’s surface and not on the other planets, what process could have concentrated it?”
During its more turbulent younger days, Mercury's scorching surface was an intense ocean of magma. Researchers now believe that graphite might have crystallized out of that fiery magma ocean, forming a primordial crust, the slate-colored remnants of which lurk beneath the planet’s surface today as a darkening agent, only exposed during the ancient battering of harsh cosmic impacts. While MESSENGER's datasets are further dissected, these intriging new discoveries will continue to be explored.
“We’re still refining our understanding of how the Earth-Moon system formed after numerous missions,” Peplowski said. “To think we really understand the early history of Mercury is naive. MESSENGER returned a huge dataset and we’ve only looked at a fraction of it. I think we’ll find there’s rich data to be mined for years to come.”