Why Is Planet Mercury So Dark?

Mercury, along with samples of test dust particles similar to those which astronomers think might be turning Mercury so dark. Brown University/Proktor.

The planet Mercury has one of the darkest planetary surfaces in the solar system, and astronomers may have finally figured out why. It seems that comets may have played a role in painting Mercury a dark charcoal gray.

Essentially, Mercury has picked up some kind of "darkening agent" that turned it a blackish color. It's darker than the airless Moon, which has a volcanic surface darkened by micrometeorites slamming into the surface.Interaction with charged particles in the solar wind has also played a role. These have created a thin coat of dark iron nanoparticles on the lunar surface. (The Moon isn't the only world to be bombarded. Early Earth was too, along with the other planets.) So. could the same things have occurred at Mercury?

How Mercury Got Its Dark Surface

The material that turned Mercury's rugged, cratered and cracked surface into a dark wasteland wasn't the same as the stuff that darkened the Moon. Astronomers suspect something literally even cooler: comets.

The secret ingredient is part of a comet's chemistry. These orbiting chunks of ice, rock, and dust regularly cross Mercury's orbit as they make their way around the Sun. They originate many millions of kilometers away, in the Oort Cloud or Kuiper Belt. Out there, water, carbon dioxide, methane, ammonia, and other ices exist without danger of sublimating (as dry ice does in the sunlight). 

It's not a safe trip in from the outskirts, by any means. The heat of the Sun softens a comet's ices, and the gravitational strain can break them apart. This leaves chunks of ice and cometary dust spread out through the orbital path of the former comets. Cometary streams can also cross Earth's orbit, too, which is how we get meteor showers.

Cometary dust can be as much as 25% carbon. As Mercury moves through its orbit, it encounters this cometary dust, and experiences a steady bombardment of carbon from crumbling comets. By some estimates, Mercury's surface could be anywhere between 3 to 6% carbon, simply from comet bombardment alone. 

Finding Evidence of Comet Dust Bombardment

This bombardment hasn't been observed directly, so astronomers used a special firing range at NASA's Ames Research Center called the Vertical Gun Range to simulate comet darkening of Mercury. Projectiles were fired into a material that mimics lunar basalt, the volcanic rock that makes up the dark patches on the nearside of the Moon. The experiments showed that tiny carbon particles got deeply embedded in the impact melted material. The process reduced the amount of light reflected by the target material to about the same as the darkest parts of Mercury. It seems that carbon acts like a stealth darkening agent, which further supports the "carbon-rich dust particles turning Mercury dark" idea. 

More About Mercury

Mercury is the closest planet to the Sun, orbiting at an average distance of just 69,816,900 kilometers (43,385,221 miles), and takes 88 Earth days to make one trip. The planet has a next-to-nothing atmosphere, and its surface temperatures range from -173 C, -280 F at night to 427 C, 800 F during the day). Thanks to ongoing measurements made by the MESSENGER spacecraft, we have very detailed maps of the planet's volcanic plains and hills, scarred by craters.

Mercury has the highest iron content of any world, and astronomers are still working out why. The best ideas so far: that Mercury was more of a metal-silicate type of world (more similar to Earth) in the early days of the solar system. Not long after it formed, the infant Mercury may have been in a collision with another planetesimal. That shattered Mercury's silicate crust, sending it to space, and leaving behind a planet with a very high concentration of iron.

Or, the young Sun destroyed much of the planet's rocky content. Possibly conditions in the solar nebula did not permit Mercury to gather much of a rocky crust. Further studies by MESSENGER seem to show that Mercury didn't lose all of its heavier elements, which might indicate that the planet simply did not gather up enough of the necessary rocky materials as it formed, creating an iron-rich Mercury.