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Mercury's surface is darker than it should be. (NASA)
Like a goth kid from the '90s, Mercury has an unusually dark, crusty exterior. Last year, scientists proposed that the strange darkness could be explained by carbon dust shed during comet impacts. Now, using data from the final orbits of NASA's MESSENGER spacecraft, researchers led by Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory say they've confirmed that carbon is to blame.

But instead of carbon shed from cometary encounters, they say, it might be the remains of Mercury's ancient surface – a layer of the planet buried by other materials over the course of billions of years. Comets hurtling into Mercury's surface may have actually unearthed dark carbon that was already there.

The new findings were published Monday in the journal Nature Geoscience.

Mercury has always seemed darker than it should be. Because the planet has no atmosphere to speak of — and is the closest thing to the sun, where solar winds are violent and space debris impacts are most common — it's expected to acquire a lot of tiny iron particles, which are known to absorb light and make planets look darker. The moon gets its dark appearance from an abundance of iron. But even though Mercury has a lower concentration of iron than the moon, it absorbs more light.