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​Mercury Has Been Painted Black By Comet Dust

I see this small world, and I want to paint it black.
​Composite of Mercury’s north pole. Credit: NASA

Mercury is the least explored terrestrial planet in our solar system, having only been visited by the Mariner 10 probe in 1975, and the current MESSENGER mission. As a result, scientists have been left in the lurch on some fairly basic details about this innermost world, including the composition of its surface, which is inexplicably dark and non-reflective.

Now, a new theory suggests that Mercury is dim because comet dust is literally painting it black, Rolling Stones style. This compelling idea was proposed by a team led by Megan Bruck Syal, a planetary scientist at the Lawrence Livermore National Laboratory. The results are published to​day in Nature Geoscience.

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"Carbon, delivered by cometary dust, helps to explain a longstanding mystery," Syal told me over email. "Why is Mercury's surface so dark, yet the iron content at the planet's surface is so low?"

Indeed, planetary darkening is usually caused by microm​eteorite bombardment, which produces a thin iron coating on airless worlds. But spectroscopic analysis conducted by the MESSENGER orbiter has revealed that Mercury has far less surface iron than the Moon, despite being several times darker. Clearly, some other substance is responsible for its shadowy hue.

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Syal and her team suggest that this mystery paint is runoff from passing asteroids and comets—material that has been peppering Mercury with a carbon-rich layer of cosmic runoff for eons. This conclusion is supported by an improved understanding of the complex dynamics of the inner solar system, where the Sun's intense heat causes comets to sweat off massive chunks of debris.

"Previous work used observations of how the spatial density of dust increases towards the Sun, combined with analytical models for how the dust flux at Mercury should compare to the dust flux at the Earth," Syal explained.

"Newer work uses computer models to numerically simulate the evolution of dust particles in the inner solar system, allowing a flux estimate for Mercury to be derived from our knowledge of the dust flux at the Earth," she continued.

In other words, as the tools for studying dust runoff evolved, so did the understanding of its complicated relationship with our own planet, and with Mercury. Over the last 20 years or so, scientists have discovered that Earth actually receives much more dust bombardment than originally assumed, as does Mercury.

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Moreover, Syal said that several subtle forces in the inner solar system—including solar wind drag and the gravity of the terrestrial planets—have conspired to sprinkle Mercury with about 170 times more d​ust than previously estimated. She and her colleagues collated all of this new information, then tested the theory out for themselves using the NASA Ames Vertical Gun ​Range—a planetary impact simulator.

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The team fired projectiles laced with sugar, which is a great stand-in for cometary dust, out of the 14-foot canon at 5.68 km​/s. As a result, the target was spangled with tiny carbon particles, which darkened the impact zone by about five percent, a figure that supports the team's theory that carbon paint accounts for Mercury's dark surface.

Syal hopes that these findings will be corroborated by data from MESSENGER, or other future Mercury-focused missions. "Carbon is difficult to definitively detect with most remote sensing methods," she told me.

"However, I believe that the Gamma Ray and Neutron Spectrometer instruments on-board the MESSENGER spacecraft have been looking for carbon," she continued. "The final days of the MESSENGER mission will include low-altitude data, which could possibly shed light on the carbon abundance at Mercury's surface."

In addition, an upcoming ESA/JAXA mission called BepiColombo, slated to launch in 2017 and for arrival at Mercury in 2024, may also shed light on this dark planet. And though no Mercury landers are on the docket at this time, Syal is optimistic that a sample return mission to the planet will come to fruition eventually.

"Sample return is very expensive," she said. "But that would be wonderful; someday we'll get there!"

Fortunately, there's plenty of ground-based research to delve into in the meantime. If Mercury really is being rained down upon with loads of carbonaceous dust, it could have major consequences in our understanding of planetary science.

"One broader implication is that micrometeorites can efficiently deliver all sorts of interesting materials to Mercury," Syal told me.

"For example, we are currently studying how much water could be delivered to Mercury by micrometeorites, and whether this amount is consistent with detected reservoirs of water ice at the poles of Mercury," she said. "The idea that recently accreted material from impacting micrometeorites can directly affect observations of planetary surfaces provides new ways of interpreting data from past, current, and future planetary missions."