Mercury is the planet closest to the dazzling light of the sun. So why is its colour so dark? Scientists think they've finally figured out the answer.

Billions of years ago, Mercury was black all over, coated by a crust of graphite — the form of carbon found in pencil leads.

That "primordial crust" has long since been buried, but when asteroids and other objects smash through Mercury's surface into the buried carbon, they sometimes blow its remains back up to the surface and scatter it over the new crust, proposes a new study led by Patrick Peplowski at the Johns Hopkins University Applied Physics Laboratory.

"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.

Dark material Mercury

Dark material on Mercury’s surface appears blue in this colour-enhanced image. Dark areas are associated with impact-excavated material like the young rayed crater Degas (near the centre), and the crater Akutagawa (far left ). (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Mercury's darkness had puzzled researchers because previous research showed its surface didn't have a lot of iron or titanium — elements blamed for darkening other solar system objects like our moon.

Peplowski and his team looked at measurements made by NASA's Messenger spacecraft, which confirmed that the darkest parts of Mercury's surface contain unusually large amounts of carbon — levels hundreds of times higher than on the surfaces of other planets such as Earth. Those levels are certainly high enough to explain Mercury's dark colour.

It's not the first time scientists had proposed that carbon might be Mercury's darkening agent. A study published last year suggested that such carbon might come from comet dust falling on the planet's surface gradually over time.

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. That suggests that the material is normally buried deep under the surface and only blown up to the surface by major impacts.

Messenger over Mercury

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. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

But where did that buried carbon come from? There's evidence that when Mercury first formed about 4.6 billion years ago, it was covered in an ocean of magma full of dissolved minerals. Those minerals gradually cooled, solidified and sank, except for the graphite, which would have floated, forming a crust over the surface up to a kilometre thick, the paper says.

Over time, volcanic eruptions and other geological processes would have buried that original crust, but bits of it would have later been resurfaced by events like meteor impacts.

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. The spacecraft ended its mission on April 30, 2015, when it was intentionally smashed into the surface of Mercury.