In one of its final acts before plunging to a lonely death in April last year, NASA's Messenger probe sniffed Mercury's dark surface, which it had been orbiting for four years, and found carbon. But whether that carbon hitched a ride on comets and asteroids, or was within the planet all along, has had planetary scientists stumped.
A slew of data from Messenger's last days, published in Nature Geoscience by an American team led by Patrick Peplowski from the Johns Hopkins University Applied Physics Laboratory, appears to show the carbon darkening Mercury was buried in a layer below the current crust, and has been unearthed bit by bit by comet and asteroids shaking up the planet.
At first glance, Mercury looks a lot like the Moon. With a dark grey hue, the planet closest to the Sun is peppered with impact craters.
But its unusually dark colour has had planetary scientists mystified: iron was thought to be the blackening agent, as it is on the Moon, yet Messenger detected almost no iron on Mercury's surface.
Carbon then became the main suspect.
Last year, scientists proposed that Mercury’s dark hue was the result of carbon-rich comets brushing past. But this wasn't based on observations; rather, the scientists came to that conclusion after running experiments in the laboratory and using computer modelling.
It wasn't until Messenger's final few days, where it skimmed less than 100 kilometres above the surface of the planet, that it could get a fix on the carbon content.
Measurements were made using particles – in particular, neutrons – streaming from the Sun, which interact with atoms in the top few centimetres of the planet's surface.
As a neutron collides with, say, a carbon atom, it transfers a little of its energy to the carbon, much like a billard ball striking another. Sometimes they rebound.
The neutron spectrometer on board the probe measured the numbers and energies of neutrons bouncing off Mercury as it flew close by. It found more low-energy neutrons, a signature of carbon, as it passed particularly dark regions – such as the Basho 'halo' below.
The carbon-rich areas were linked to large craters, suggesting the carbon was brought up from within the crust and exposed by impacts.
So how did the planet get this carbon-rich crust?
More than four billion years ago, when the Solar System was very young, much of Mercury was so hot that it contained a global “ocean” of molten magma. Laboratory experiments and modelling show that as this ocean cooled, most solidified minerals sunk.
Except graphite – a form of carbon.
More buoyant than magma, it floated to the surface to form Mercury's original dark crust.
And although this primordial crust was obscured by later volcanic eruptions and other geological processes, some of the carbon-rich material mixed into the overlying materials to cause a global darkening of Mercury’s surface, as well as even darker patches where comets and asteroids unearthed it more.
"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," Peplowski says.
Comets paint Mercury black