Researchers have uncovered the mystery of Mercury 's 'stealth ' colour scheme . The say a steady dusting of carbon from passing comets has slowly painted Mercury black over billions of years . It solves the mystery of Mercury 's unusual colouring - which has baffled scientists . The say a steady dusting of carbon from passing comets has slowly painted Mercury with a dark 'stealth ' coating over billions of years . Cometary dust is composed of as much as 25 percent carbon by weight , so Mercury would be exposed to a steady bombardment of carbon from these crumbling comets . Using a model of impact delivery and a known estimate of micrometeorite flux at Mercury , researchers calculated suggest that after billions of years of bombardment , Mercury 's surface should be anywhere from 3 to 6 percent carbon . 'It 's long been hypothesized that there 's a mystery darkening agent that 's contributing to Mercury 's low reflectance , ' said Megan Bruck Syal , a postdoctoral researcher at Lawrence Livermore National Laboratory who performed this research while a graduate student at Brown University . 'One thing that had n't been considered was that Mercury gets dumped on by a lot of material derived from comets . ' On average , Mercury is much darker than its closest airless neighbour , our Moon . Airless bodies are known to be darkened by micrometeorite impacts and bombardment of solar wind , processes that create a thin coating of dark iron nanoparticles on the surface . But spectral data from Mercury suggests its surface contains very little nanophase iron , certainly not enough to account for its dim appearance . As comets approach Mercury 's neighborhood near the sun , they often start to break apart . Cometary dust is composed of as much as 25 percent carbon by weight , so Mercury would be exposed to a steady bombardment of carbon from these crumbling comets . Using a model of impact delivery and a known estimate of micrometeorite flux at Mercury , Bruck Syal was able to estimate how often cometary material would impact Mercury , how much carbon would stick to Mercury 's surface , and how much would be thrown back into space . Her calculations suggest that after billions of years of bombardment , Mercury 's surface should be anywhere from 3 to 6 percent carbon . The next part of the work was to find out how much darkening could be expected from all that impacting carbon . For that , the researchers turned to the NASA Ames Vertical Gun Range . The 14-foot canon simulates celestial impacts by firing projectiles at up to 16,000 miles per hour . Experiments show that impact material is significantly darkened when impacts occur in the presence of complex organics - shown here on the right . For this study , the team launched projectiles in the presence of sugar , a complex organic compound that mimics the organics in comet material . The heat of an impact burns the sugar up , releasing carbon . Projectiles were fired into a material that mimics lunar basalt , the rock that makes up the dark patches on the nearside of 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 Peter Schultz , professor emeritus of geological sciences at Brown and a co-author of the new research . The experiments showed that tiny carbon particles become deeply embedded in the impact melted material . The process reduced the amount of light reflected by the target material to less than 5 percent -- about the same as the darkest parts of Mercury . Importantly , spectroscopic analysis of the impact samples revealed no distinctive spectral fingerprints , again similar to flat spectral signatures from Mercury . 'We show that carbon acts like a stealth darkening agent , ' Schultz said . 'From the standpoint of spectral analysis , it 's like an invisible paint . ' And that paint has been building up on Mercury 's surface for billions of years . 'We think this is a scenario that needs to be considered , ' Schultz said . 'It appears that Mercury may well be a painted planet . '
Dusting of mercury from direct sunlight has made Mercury darker . Mercury 's composition is anywhere from 3 to 6 percent carbon . Has gives the mercury an almost black appearance .