Mercury may have comet dust to thank for its dark appearance. The smallest planet in the solar system (sorry Pluto) has curious coloring: It’s darker than it should be, and scientists aren’t sure why. But a new study published in Nature Geoscience suggests an answer.

Because Mercury has basically no atmosphere – 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.

But the moon (which is also what we’d call “airless”) has more iron than Mercury and still appears lighter.

Carbon dust may be the answer: In the study, researchers calculated how much material passing comets might shed on a defenseless planet close to the sun. It turns out that the answer is probably lots and lots – enough to leave Mercury with a surface that’s as much as 6 percent carbon after a few billion years of existence, which is about 50 times more than has been left on the moon – and it turns out that could account for Mercury’s unusual murkiness.

The coolest part? The researchers, led by Lawrence Livermore National Laboratory post-doc Megan Bruck Syal, tested their theory by launching some projectiles across the NASA Ames Vertical Gun Range – a 14-foot canon that shoots up to 16,000 miles per hour to simulate impacts that happen in space.

By launching sugary missiles (Sugar mimics the organic materials in comets) at a target that simulates the darker parts of the surface of the moon (the closest analogy we have to Mercury, given their shared airlessness), the study authors showed that comet carbon has serious sticking power. The heat of impact made tiny particles of carbon break free, and these were then embedded into the melted impact crater of their new home.

The impact spots then absorbed a lot more light than they had before. Only 5 percent of light shown on those spots was reflected. But the carbon didn’t reveal itself in spectral analysis, which could explain why similar readings on Mercury haven’t revealed the cosmic paint job.

“We show that carbon acts like a stealth darkening agent,” study co-author Peter Schultz, professor emeritus of geological sciences at Brown, said in a statement. “From the standpoint of spectral analysis, it’s like an invisible paint.”