A new study reveals that Mercury, the planet closest to the sun, is apparently covered by a thin layer of black dust from billions of years of deposits accumulated from passing comets.

This comet dust is made from carbon rich materials and acts like an "invisible paint" that lowers the reflective properties of the tiny planet. Comet dust is made of 25 percent carbon.

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Planets that don't have an atmosphere like our moon experience regular impacts from micrometeorites. Since these worlds are airless, this process generates a thin layer of tiny particles of nanophase iron that can darken surfaces.

Spectral analysis has proven these material concentrations on Mercury aren't high enough to produce this type of thin layer of "invisible paint".

Mercury's low reflectance can be attributed to some mysterious darkening agent, according to Megan Bruck Syal of the Lawrence Livermore National Laboratory. This is the first time scientists consider material from passing comets dumped on Mercury.

Comets are essentially frozen bodies that break-up when they approach the Sun. There is a high chance Mercury would be exposed to comet dust during the course of billions of years from comets and other passing bodies.

Syal estimates Mercury is now made of three to six percent of carbon, originating from comets and micrometeorites.

To prove their theory about this mystery thin layer, scientists used the Ames Vertical Gun Range to run experiments on how these impacts and the scattering of this material darkened the surface of Mercury.  Here, a 14-foot cannon fires projectiles at speeds of up to 16,000 miles per hour.

NASA uses this facility for small scale simulations of high speed celestial body impacts. This gun helps with scenarios that can happen to entry vehicles, asteroid impacts on planets and micrometeoroid impacts on spacecraft.

After conducting the projectile impact experiments, researchers found this process reduced reflectivity by five percent, which is similar to the darkest regions of Mercury. Spectroscopic analysis revealed this coating was undetectable in the dark coating, which suggests the material found in Mercury acts like an invisible paint.

This study ws published in the journal, Nature Geoscience.

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