Scientists have learned a low amount of iron is behind he Mercury's lack of glow, a phenomenon stemming from the planet constantly getting hit with comet dust.

According to BBC News, authors of a study published in the journal Nature Geoscience determined Mercury's surface has an iron deficiency, which has caused the planet to appear darker than even the moon.

"It's long been hypothesized that there's a mystery darkening agent that's contributing to Mercury's low reflectance," study lead author Megan Bruck Syal, a postdoctoral researcher at Lawrence Livermore National Laboratory, said in a press release. "One thing that hadn't been considered was that Mercury gets dumped on by a lot of material derived from comets."

For their study, the research team created a model to simulate the dimming effects passing comets could have by sprinkling dust on a planet. They concluded that billions of years of dusting Mercury would have "painted [it] black."

"We used the lunar basalt model because we wanted to start with something dark already and see if we could darken it further," study co-author Peter Schultz, professor emeritus of geological sciences at Brown University, said in the release. "We show that carbon acts like a stealth darkening agent.

"From the standpoint of spectral analysis, it's like an invisible paint."

They found they did not have the same results when they mixed their solution with sand rather than salt, supporting the latter in their conclusion.

"We think this is a scenario that needs to be considered," Schultz said. "It appears that Mercury may well be a painted planet."