Mercury has been covered with a thin layer of black dust from billions of years of deposits from comets passing through the orbit of the innermost planet of the solar system, according to a new study.
Comet dust, loaded with carbon-rich materials, acts like an invisible paint, lowering the reflectivity of the tiny world. Astronomers have long wondered why the surface of the smallest planet in the solar system is so dark.
Airless worlds, such as our own moon, are regularly pummeled by impacts from micrometeorites. This process releases a thin layer of tiny particles of nanophase iron, which can darken the surfaces of these bodies. However, spectral analysis reveals that concentrations of this material on Mercury are not high enough to account for the observed color.
"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 Megan Bruck Syal from the Lawrence Livermore National Laboratory. Bruck Syal performed this research while a graduate student at Brown University.
Cometary dust is composed of around 25 percent carbon, and the frozen bodies often break apart as they travel closer to the sun. This suggests that Mercury would likely be exposed to large quantities of the material over the course of billions of years.
Bruck Syal calculated that between 3 percent and 6 percent of the material on the surface of the tiny world is carbon, delivered by comets and micrometeorites.
The Ames Vertical Gun Range (AVGs) was used to run experiments to test how such impacts and scattering of material would darken the world. This facility houses a 14-foot cannon capable of firing projectiles at up to 16,000 mph. NASA uses the facility to simulate high-speed, celestial body impacts on a small scale to check what may happen with entry vehicles, meteor or asteroid impacts on a planet or moon, and micrometeoroid impacts on a spacecraft.
Researchers fired projectiles into a material meant to replicate basalt, like that found on the moon. The shots were carried out in the presence of sugar, meant to simulate the organic material within comets. The study showed the process reduced reflectivity of the target material to around 5 percent, roughly the same as the darkest regions of Mercury. Spectroscopic analysis of this coating was unable to detect the dark coating, suggesting the material on Mercury may also be hiding from astronomers, acting as a form of invisible paint.
"We used the lunar basalt model because we wanted to start with something dark already and see if we could darken it further," said Peter Schultz of Brown University.
Enceladus, one of the dozens of moons of Saturn, is one of the most reflective bodies in the solar system as light bounces off the ice-covered satellite.
Analysis of the possible role played by comets and micrometeorites in darkening the surface of Mercury was published in the journal Nature Geoscience.
Photo: NASA Goddard Space Flight Center | Flickr
