The comet dust acts like an invisible paint and lowers the reflective properties of the planet.
Comet dust is rich in carbon material and it is 25 percent made of carbon.
The planets that don’t have atmosphere regularly face impacts from micrometeorites and this generates a thin layer of tiny particles of nanophase iron that can darken the surface.
Spectral analysis shows that theses material concentrations on Mercury aren’t high enough to produce this thin layer of invisible paint.
According to Megan Bruck Syal of the Lawrence Livermore National Laboratory, Mercury’s low reflectance is due to some darkening agent.
Comets are frozen bodies and they break up as they approach the sun and mercury being so close to sun there are high chances that it is exposed to comet dust from billions of years.
Mercury is now made of 3 to 6 percent of carbon, from micrometeorites and comets.
They have used Ames Vertical Gun Range to prove this theory.
They have conducted experiments on how the surface has darkened with impacts and scattering of the material.
After the experiment they found that this process has reduced the reflectivity of comet by 5 percent similar to the darkest region on mercury.
It also revealed that the layer is undetectable.