This striking, oblique image of Basho crater shows the distinctive dark halo that encircles the crater. The halo is composed of so-called Low Reflectance Material (LRM), which was excavated from depth when…
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The more we know about Mercury’s original surface, the more we know about how it formed. These are known to be rare at Mercury’s surface, so what is the “darkening agent” there? The researchers suggest that millions of years of being bombarded by meteorites and comets led to the graphite dust being kicked up from beneath Mercury’s surface topsoil, painting the planet black.
Mercury, the small planet that’s closest to the sun, may have once had a surface made of graphite, the material used in making pencil lead.
Patrick Peplowski from the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland and colleagues analysed measurements of the darkest parts of Mercury’s surface taken by Messenger at the end of its mission.
“We may be observing the remains of Mercury’s original, 4.6-billion-year-old surface”, said Rachel Klima, a planetary geologist who works with Peplowski and co-authored the study published this week in Nature Geoscience.
The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation. Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence.
Information sent from Nasa’s Mercury Messenger spacecraft suggests the dark areas are caused by graphite.
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 a year ago of operation.
On Earth graphite is used in industry to make bricks that line refractory furnaces and increase the carbon content of steel.
According to the data – gathered in the final days of MESSENGER’s mission as it was performing low-altitude flyovers – Mercury’s surface could contain a few percent carbon, but more than other planets in our Solar System. It is an indication of high levels of carbon coming from surface and graphite is most likely the material that produced it.
Researchers explain that planet Mercury was once very hot and hosted a global ocean of molten magma. The ones being most buoyant in a planet’s magma ocean floated up and formed the planet’s crust and the other minerals buried beneath the surface.
But Peplowski and his team noticed that the darkest material on Mercury tended to be found in large impact craters and the material surrounding them. One day, this crust could help reveal how Mercury – and other planets in the inner solar system – came to be.
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The Messenger spacecraft, which orbited Mercury from 2011 until 2015, was equipped with cameras and instruments that could scan the surface and look for the chemical fingerprints of different elements.
