Mercury’s surface is pockmarked with craters, showing astronomers that it has been bombarded by meteors and asteroids for four billion years. Although these craters are in themselves utterly spectacular, they have recently revealed the answer to a longstanding question about the geology of this distant rocky world: Why is Mercury’s surface so dark?
As a new study in Nature Geoscience concludes, this darkness is due to a hidden subterranean layer of graphite. This graphite occasionally coats the surface whenever a meteor or asteroid smacks into it, forcing parts of this layer up into the sky. Essentially, Mercury is covered in pencil shavings.
A team led by the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland looked at measurements taken of the darkest regions of Mercury by the Messenger probe. Messenger spent five years investigating the geological history of Mercury, and vast patches of an unidentified dark chemical compound were observed that could not be explained by the planet’s evolution or overall chemical composition.
Researchers have cataloged the various physical attributes of a plethora of compounds, including their ability to reflect and absorb different wavelengths of light. This means that chemical compounds unreachable by chemists can be identified remotely, including the dark patches on Mercury. To this end, the research team concludes that the only possible candidate for these patches, based on its reflectance, is elemental carbon, which in this case is graphite.
NASA’s Messenger spacecraft purposefully crashed into Mercury in 2015, but not before collecting boatloads of data. NASA
Notably, the appearance of this surface graphite is strongly related to the positioning of the largest impact craters. This implies that the graphite is being gouged out from below the surface whenever a powerful enough asteroid or meteor careens into Mercury. This finding, although revelatory, gave the researchers a new conundrum – namely, where did the graphite come from in the first place?
Like the Moon, early Earth and Venus, and to some extent the Jovian moon of Io today, Mercury once had a global magma ocean. In the ancient past, when these rocky bodies formed, they were still incredibly hot from their fiery formations, and they hadn’t yet cooled enough to form complex layers of solid rock and deeper, molten magma.
Mercury was clearly rich in carbon in a way that Earth was not. When different minerals began to crystallize and form from the cooling magma ocean, denser ones would have descended towards the core, and lighter ones – including graphite – would have floated up to the surface. At one point, this graphite may have even formed the original crust of Mercury.
Over time, more complex volcanic activity formed, and the graphite crust was buried in epic lava flows that formed the younger, basaltic rock we can see today. “If we've really identified the remains of Mercury's original crust, then understanding its properties provides a means for understanding Mercury's earliest history,” Patrick Peplowski, a researcher at APL, told BBC News.