Why is Mercury so Dark?

Mercury, the smallest planet in the solar system, and closest to the Sun, is only a little bit bigger than Earth’s Moon.  But the Moon is comparatively reflective object.  Mercury is thought to be made of the same rock as the Moon, so what is the difference? Why do objects in our Solar system have different brightnesses?

Mercury. Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The key is in a property called albedo.  It’s basically how much light an object reflects, measured as a fraction.  For example, the Moon reflects 12% of the light the Sun shines on it, so it has an albedo of 0.12.  The albedo of materials on Earth is shown below.

Albedo of Earthbound Materials. Credit: Hannes Grobe under CC 2.5

The highest albedo in the solar system is the bright ice moon of Enceladus, at 0.99.  The smooth, icy surface of Enceladus reflects almost all of the Sun’s rays back out into space, making appear very bright when viewed from Earth or from the Cassini spacecraft.

Another planet with a very high albedo is Venus, or rather the cloud layers of Venus.  Venus has an albedo of 0.65, which is why is appears so magnificently bright in night time skies.  Compared to the Earth, with an albedo of 0.306, Venus is bright and reflective.

Mercury is quite dark, with an albedo of 0.106.  Why is it so dark? Often with planets and Moons, nanoparticles of Iron result from impacts of meteorites and spread across the planet causing a darker appearance, but this is not the case with Mercury, there is not much Iron present on the surface.  It turns out it may be another dark powder that is the culprit: Carbon.

There is a significant amount of Carbon present in comets, about 25% by weight.  The majority of comets have long elliptical orbits that take them far out into the Oort cloud and then back close to the Sun.  As they approach the powerful gravitational field and radiation of the Sun, they can break apart, unleashing a significant amount of dust.  Since Mercury is so close to the Sun, the idea is that the leftover comet dust has coated Mercury like soot, giving the entire surface a darker appearance.

To study this theory, a team of scientists from Brown University launched projectiles at a material used to simulate the Lunar basalt.  The projectiles were coated with sugar, the burning of which releases a substantial amount of Carbon dust.  This is a good representation of an impact where organic molecules in the comet would burn under the heat of the collision, releasing the dust.  In the experiment, the tiny Carbon particles became embedded in the material, reducing their albedo by 5%, which is about the same difference between the Moon and Mercury’s albedo!  This comet dust perfectly accounts for the darkening of Mercury.

Although more data is required to confirm the result, it’s pretty amazing to perform an Earthbound experiment and figure out what has happened for Billions of years on another planet.

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