- Mercury was "painted black" by millions of years of micrometeorites, researchers say
- The research could solve the puzzle of why the planet appears so dark
Not even NASA spends much time there, making it the least-explored inner planet in the solar system.
But one question has long perplexed scientists: Why's the darned thing so dark? Compared with our own moon, which is about the same size, Mercury reflects much less light.
Scientists at Lawrence Livermore National Laboratory, Brown University and the Planetary Sciences Institute say they may have found the answer: It's been "effectively painted black," in the words of Megan Bruck Syal, a postdoctoral researcher at Lawrence Livermore.
Over the eons, the researchers argue in a paper published Monday in the journal Nature Geoscience, microscopic meteorites spawned from carbon-rich comets may have contributed enough dark-hued carbon to reduce the amount of reflected light.
The pieces seem to fit together: There's a lot more carbon dust thrown off from comets close to the sun, where Mercury orbits -- about 50 times as much for Mercury as for our moon, the researchers say.
And tests using a big NASA gun made to simulate planetary impacts on a small scale seem to show the theory is plausible, the authors say.
"We show that carbon acts like a stealth darkening agent," said Peter Schultz, a professor emeritus of geological sciences at Brown University. "From the standpoint of spectral analysis, it's like an invisible paint."