One of the unusual characteristics of Mercury is that it’s quite dark in comparison with the other planets and rocky bodies of the solar system. Mercury’s albedo — a measure of how much light the body reflects — is below that of Earth, Venus, and Mars. Depending on which definition of albedo you use, it’s reflected light is even lower than the Moon.
This would be easy to explain if Mercury outer layer were simply rich in a dark element like iron. But the Moon’s outer crust actually contains more iron than Mercury’s (most of Mercury’s iron is thought to be within the liquid core of the planet). So why is Mercury so hard to see?
The answer, based on data beamed back from the now-smashed Messenger spacecraft and published in Nature Geoscience, is carbon, in the form of graphite. Current thinking from the science team is that Mercury may once have had a graphite “flotation crust” from an enormous, planet-wide magma ocean.
Is Mercury the remains of a much larger planet?
Mercury is odd in a number of ways. Its core is believed to account for 42% of its mass (the Earth’s core, in contrast, is just 17% of its mass). Its crust is much thinner than ours, and it has this puzzling layer of graphite layering its exterior. So far, it’s the only body in solar system known to have a carbon layer of this sort.
There are various theories that explain these events, but they all start with a Mercury containing roughly 2x the current planet’s mass. It’s possible that this additional material was vaporized by the early sun as it condensed, or that a giant impact smashed the early planet, driving a significant amount of its material into the sun. Either way, the Mercury we know is thought to be a fraction of the planet we ought to have had.
Messenger’s data suggests that the carbon on the surface of Mercury originated from within the planet, which suggests that the carbon could have precipitated out from a roiling magma “ocean.” Such an ocean could have been formed in the aftermath of massive impact — the great impact that may have created the Moon would have liquefied the surface of the Earth as well.
Mercury remains unique for another reason as well: Many of the extrasolar planets we’ve detected have been so-called “Hot Jupiters,” gas giants with extremely close orbits to their parent stars. While it’s true that we can’t see Mercury-sized planets that close to a parent star, the orbits of many of these gas giants leave no room for a Mercury-type planet to exist.
We’ve discovered a great many planets since extrasolar research began decades ago — but none of the solar systems discovered to date look very much like ours.
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