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Comets paint Mercury black

The riddle of Mercury’s dark complexion is almost solved. James Mitchell Crow explains.

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An artist's impression of the Messenger probe in orbit around Mercury.

NASA’s Messenger probe to Mercury is heading for a violent end. In a few weeks from now, when its fuel tanks empty, Messenger will smash into the surface of the planet it has been studying since March 2011. Before it disintegrates, Messenger might also help solve one of the biggest enigmas about the planet closest to the Sun: its colour.

Mercury is similar in size and composition to our own Moon. Yet our Moon is a ghostly pale presence in the night sky. So a question mark has hung over the source of Mercury’s dark complexion.

Last month, Megan Bruck Syal from Lawrence Livermore National Laboratory in California and her colleagues published a paper in Nature Geoscience suggesting Mercury’s darkness could be caused by a layer of carbon deposited by passing comets. Bruck Syal says that before Messenger’s death crash “it might be possible” for the probe to measure the carbon in Mercury’s soil.

Iron has long been considered the reason Mercury is dark. The planet’s feeble gravity is too weak to hold an atmosphere, so even micrometeorites – less than 2 millimetres in size and so small they would burn up in the Earth’s atmosphere – frequently smash into Mercury’s surface. One theory was that iron from pulverised micrometeorites could form a nanoparticle that reflects almost no light, darkening the planet’s appearance.

Yet, in 2011, Messenger’s instruments showed Mercury’s soils contain little nano-iron. “It became really clear with the new Messenger data that this wasn’t enough – that there was some mystery darkening agent,” Bruck Syal says.

Mercury’s surface could be up to 6% carbon – comet soot accumulated over billions of years.

Bruck Syal and her colleagues realised that Mercury is also bombarded by comets – which can be rich in carbon. Their mathematical models showed most full-sized comets strike the planet so fast that they ricochet off again. Comets leave a dusty trail, which can be up to 25% carbon. This dust travels slowly enough to stick to the planet’s surface. The team’s calculations suggest Mercury’s surface could be up to 6% carbon – comet soot accumulated over billions of years.

Mercury’s sweltering orbit takes it through a neighbourhood rich in comets ensnared by the Sun’s gravity and which are beginning to break apart in the heat. This means Mercury receives 50 times more carbon dust than the Moon, the team calculated.

What form would this carbon take, and how might it be incorporated into Mercury’s sandy surface? To find out, Bruck Syal ran impact tests at NASA’s Ames Vertical Gun Range. She mixed carbon – in the form of sugar – with a fine grey basalt powder that approximated Mercury’s soil, then fired projectiles into the mixture at 5 kilometres per second to mimic micrometeorite bombardment. The heat created by the impacts vaporised the sugar to graphite, which mixed with the Mercury soil as it resettled on the surface, making it darker. When they repeated the experiment without sugar, the soil remained lightly coloured.

Experiments conducted at NASA's Ames Vertical Gun Range showed that soil incorporating carbon, right, was visibly darker than carbon-free soil, left. – NASA/AMES/BROWN UNIVERSITY

“This seems like a really nice, elegant solution to the problem of Mercury’s dark colour,” says Jonti Horner, a planetary astronomer at the University of Southern Queensland. “They did the mathematical modelling first and said, ‘hang on this can work, let’s see if you can actually get the darkening in the lab – oh yes, you do’. So they’ve brought these parts together in a really nice piece of work.”

And the icing on the cake would be if Messenger could confirm Mercury’s carbon-rich surface before the probe disintegrates – an event likely to take place in the next few weeks, depending on just how long Messenger’s final drops of fuel last.

Messenger’s first look at Mercury’s previously unseen side. The planet closest to the Sun is strikingly similar in size and composition to our Moon. – NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Messenger measures the different elements in Mercury's soils by analysing the echoes of high-energy radiation called cosmic rays that pelt the planet's surface, thanks again to its lack of atmosphere. When a cosmic ray strikes an atom in Mercury's soil, it can knock loose a neutron from the atom's nucleus - which carries a certain energy characteristic of the element that emitted it. Swallowing a cosmic ray can also cause the atom to burp a gamma ray, also with a characteristic energy.

Whereas iron happens to emit high-energy neutrons and gamma rays that Messenger’s instruments could detect from a high orbit, carbon’s emissions are harder to pick out from afar – hence the hope that skimming over the surface will give Messenger a much stronger signal from which to gauge carbon levels.

Even if Messenger can’t get a fix on the quantity of carbon on Mercury’s soil, future missions might confirm Bruck Syal’s theory: “The next mission to Mercury is a joint European-Japanese mission that will launch in 2017 and arrive in 2024, and that will have a lot of instruments for surface composition analysis, so that mission could answer the question.”

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James Mitchell Crow is a freelance writer and editor.
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