Is dust from Pluto’s moons landing on the planet (centre)? (Image: NASA, ESA and G. Bacon (STScI))

On far-flung Pluto, it may be raining moon dust. Models suggest that Pluto’s small moons are even now sprinkling dust on its equator, which could explain why Pluto’s middle is darker than its poles. A NASA spacecraft headed for Pluto’s neighbourhood should be able to check out the claim when it arrives next year.

Pluto and its moons lie in the Kuiper belt, a region beyond the orbit of Neptune filled with mostly small, icy worlds. While Pluto is only about half the size of Mercury, it boasts five known moons. The largest, Charon, is half Pluto’s size. The other four – Nix, Hydra, Kerberos and Styx – are much smaller. All five appear so similar that astronomers think a large object smashed into Pluto early in its history, ejecting debris that coalesced into moons.

The system is often bombarded by rocks flying through space, which would kick up dust. Since the smaller moons have low gravity, much of their dust should fly off into space, says Simon Porter at the Southwest Research Institute in Boulder, Colorado.


Dust ferry

Porter and William Grundy at Lowell Observatory in Flagstaff, Arizona, ran computer models that showed ejected dust moving at speeds of between 50 and 250 metres per second can get snared by the gravity of Pluto and Charon.

Charon receives more material per square centimetre because it is closer to the other moons. But Charon also lies in Pluto’s equatorial plane, so its gravity ferries some dust towards the larger world’s midsection. Over the past 3.5 billion years, the scientists calculate, Pluto’s equator may have built up several centimetres of dust.

Intriguingly, Pluto’s equator is darker than its poles, probably because it has bright polar ice caps. But Porter and Grundy suggest that moon dust may play a role as well. For instance, Saturn’s moon Iapetus is half dark and half white, and it owes its darker hemisphere to dust from another Saturnian satellite, Phoebe.

Active frosts

But William McKinnon at Washington University in St Louis, Missouri, doubts that such a small amount of moon dust affects Pluto much. “I think Pluto is a very active place, with an atmosphere and frosts that come and go,” he says. “You have to weigh this dust against every other geological process that happens on Pluto.” By contrast, Iapetus is smaller and inactive – perfect for collecting dust, which becomes more concentrated as sunlight vaporises its surface ice.

NASA’s New Horizons spacecraft will reach the Pluto system in July 2015. It will be able to test the idea by seeing whether the chemical composition of Pluto’s equator matches that of its small moons. That’s just one of many mysteries about Pluto waiting to be resolved during the close encounter, says McKinnon: “It’s not going to be a dull place.”

Reference: arxiv.org/abs/1403.4873, accepted for publication in Icarus