A team of scientists have an explanation for Mercury's dark surface - passing comets dusting the planet's surface with carbon dust which slowly painted Mercury black, according to a press release from Brown University in Providence, R.I. The new findings are published in the journal Nature Geoscience.
Mercury's dark surface is even darker than its airless neighbor, our moon. Airless bodies, like the moon, are darkened by micrometeorite impacts and bombardment of solar wind, both processes that create a thin coating of dark iron nanoparticles on the surface. Spectral data from Mercury suggests its surface contains very little nanophase iron.
"It's long been hypothesized that there's a mystery darkening agent that's contributing to Mercury's low reflectance," said postdoctoral researcher at Lawrence Livermore National Laboratory Megan Bruck Syal, according to the press release. Bruck Syal performed this research while a graduate student at Brown University. "One thing that hadn't been considered was that Mercury gets dumped on by a lot of material derived from comets."
As comets arrive in Mercury's neighborhood near our sun, they usually break up. Dust from the crumbling comets is made up of 25 percent carbon by weight, so the dust falling onto Mercury's surface would be largely made of carbon. Bruck Syal's calculations suggest that after billions of years, Mercury's surface should be 3 to 6 percent carbon.
How dark does all that carbon make the planet? Researchers used the NASA Ames Vertical Gun Range to find out by using the 14-foot canon that simulates celestial impacts by firing projectiles at up to 16,000 mph. The substance launched: sugar.
The heat from an impact burns sugar, releasing carbon. The projectiles were fired into a material that mimics lunar basalt (the material found in the dark patches on the moon). "We used the lunar basalt model because we wanted to start with something dark already and see if we could darken it further," said co-author Peter Schultz, a professor emeritus of geological sciences at Brown, according to the press release.
The experiments resulted in tiny carbon particles embedded in the melted impact material and light reflection dropped to less than 5 percent - about the same as the darkest parts of Mercury, according to the press release. "We show that carbon acts like a stealth darkening agent," Schultz said. "From the standpoint of spectral analysis, it's like an invisible paint. We think this is a scenario that needs to be considered. It appears that Mercury may well be a painted planet."
The research was supported by NASA's Planetary Geology and Geophysics program (NNX13AB75G) and the NASA Earth and Space Science Fellowship program (NNXC12AL79H). Miriam Riner from the Planetary Sciences Institute was a co-author on the paper.
