Carbon from cometary material that bombards Mercury, the first planet from the Sun, may be the reason the planet’s surface is heavily dark, says new research published in the journal Nature Geoscience.
Color differences on Mercury are subtle, but they reveal important information about the nature of the planet’s surface material. A number of bright spots with a bluish tinge are visible in this image; these are relatively recent impact craters. Some of the bright craters have bright streaks emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. This image was taken on January 14, 2008, when the spacecraft was moving from a distance of roughly 12,800 to 16,700 km from the surface of Mercury. Image credit: NASA / JPL.
Why Mercury is much darker than its closest airless neighbor, our Moon, has long been a mystery to planetary researchers.
Airless bodies are known to be darkened by impacts of micrometeorites and solar wind bombardment, processes that create a thin coating of dark iron nanoparticles on their surface.
But spectral data from Mercury suggests its surface contains very little nanophase iron, certainly not enough to account for its dim appearance.
“It’s long been hypothesized that there’s a mystery darkening agent that’s contributing to Mercury’s low reflectance. One thing that hadn’t been considered was that Mercury gets dumped on by a lot of material derived from comets,” said study lead author Dr Megan Bruck Syal of Lawrence Livermore National Laboratory.
As comets approach Mercury’s neighborhood near the Sun, they often start to break apart. Cometary dust is composed of as much as 25 percent carbon by weight, so the planet would be exposed to a steady bombardment of carbon from these comets.
Using a model of impact delivery and a known estimate of micrometeorite flux at Mercury, the scientists were able to estimate how often cometary material would impact the planet, how much carbon would stick to its surface, and how much would be thrown back into space.
The scientists estimate that 50 times as many carbon-rich micrometeorites per unit surface area are delivered to Mercury, compared with the Moon, resulting in approximately 3 – 6 percent carbon (in graphite, amorphous, or nanodiamond form) at the planet’s surface.
The next part of the work was to find out how much darkening could be expected from all that impacting carbon.
For that, Dr Syal and her colleagues from Brown University and Planetary Science Institute in Tucson, Arizona, turned to NASA’s Ames Vertical Gun Range. They launched projectiles in the presence of sugar, a complex organic compound that mimics the organics in comet material. The heat of an impact burns the sugar up, releasing carbon. Projectiles were fired into a material that mimics lunar basalt, the rock that makes up the dark patches on the nearside of the Moon.
The experiments showed that tiny carbon particles become deeply embedded in the impact melted material. The process reduced the amount of light reflected by the target material to less than 5 percent – about the same as the darkest parts of Mercury.
_____
Megan Bruck Syal et al. Darkening of Mercury’s surface by cometary carbon. Nature Geoscience, published online March 30, 2015; doi: 10.1038/ngeo2397
