The Mysterious Moons of Pluto

Pluto’s largest moon Charon is revealed in this image from New Horizons’ Long Range Reconnaissance Imager (LORRI), taken late on July 13, 2015 from a distance of 289,000 miles (466,000 kilometers). This moon is one of five that orbit in the Pluto system. The others are very small and orbit much farher away from Pluto. NASA-JHUAPL-SWRI

Planet Pluto continues to tell a fascinating story as scientists pore over the data taken by the New Horizons mission in 2015. Long before the tiny spacecraft passed through the system, the science team knew there were five moons out there, worlds that were distant and mysterious. They were hoping to get a closer look at as many of these places as possible in an effort to understand more about them and how they came to exist.

As the spacecraft whizzed past, it captured close-up images of Charon — Pluto's largest moon, and glimpses of the smaller ones. These were named Styx, Nix, Kerberos, and Hydra. The four smaller moons orbit in circular paths, with Pluto and Charon orbiting together like the bulls-eye of a target. Planetary scientists suspect that Pluto's moons formed in the aftermath of a titanic collision between at least two objects that occurred in the distant past. Pluto and Charon settled into a locked orbit with each other, while the other moons scattered out to more distant orbits.

Charon

Pluto's largest moon, Charon, was first discovered in 1978, when an observer at the Naval Observatory captured an image of what looked almost like a "bump" growing out the side of Pluto. It's about half the size of Pluto, and its surface is mostly grayish with mottled areas of reddish material near one pole. That polar material is made up of a substance called "tholin", which is made up of methane or ethane molecules, sometimes combined with nitrogen ices, and reddened by constant exposure to solar ultraviolet light.

The ices form as gases from Pluto transfer from and get deposited onto Charon (which lies only about 12,000 miles away). Pluto and Charon are locked in an orbit that takes 6.3 days and they keep the same face toward each other all the time. At one time, scientists considered calling these a "binary planet", and there is some consensus that Charon itself could be a dwarf planet.

Even though Charon's surface is frigid and icy, it turns out to more than 50 percent rock in its interior. Pluto itself is more rocky, and covered with an icy shell. Charon's icy covering is mostly water ice, with patches of other material from Pluto, or coming from under the surface by cryovolcanoes.

New Horizons got close enough, no one was sure what to expect about Charon's surface. So, it was fascinating to see the greyish ice, colored in spots with the tholins. At least one large canyon splits the landscape, and there are more craters in the north than the south. This suggests that something happened to "resurface" Charon and cover many old craters.

The name Charon comes from the Greek legends of the underworld (Hades). He was the boatman sent to ferry the souls of the deceased over the river Styx. In deference to the discoverer of Charon, who referenced his wife's name for the world, it's spelled Charon, but pronounced "SHARE-on". 

The Smaller Moons of Pluto

Styx, Nyx, Hydra and Kerberos are tiny worlds that orbit between two and four times the distance that Charon does from Pluto. They're oddly shaped, which lends credence to the idea that they formed as part of a collision in Pluto's past.

Styx was discovered in 2012 as astronomers were using Hubble Space Telescope to search the system for moons and rings around Pluto. It appears to have an elongated shape, and is about 3 by 4.3 miles.

Nyx orbits out beyond Styx, and was found in 2006 along with distant Hydra. It's about 33 by 25 by 22 miles across, making it somewhat oddly shaped, and it takes nearly 25 days to make one orbit of Pluto. It may have some of the same tholins as Charon spread across its surface, but New Horizons didn't get close enough to get many details.

Hydra is the most distant of Pluto's five moons, and New Horizons was able to get a fairly good image of it as the spacecraft went by. There appear to be a few craters on its lumpy surface. Hydra measures about 34 by 25 miles and takes about 39 days to make one orbit around Pluto.

The most mysterious-looking moon is Kerberos, which looks lumpy and misshapen in the New Horizons mission image. It appears to be a double-lobed world about 11 12 x 3 miles across. It takes just over 5 days to make one trip around Pluto. Not much else is known about Kerberos, which was discovered in 2011 by astronomers using Hubble Space Telescope.

How Did Pluto's Moons Get Their Names?

Pluto is named for the god of the underworld in Greek mythology. So, when astronomers wanted to name the moons in orbit with it, they looked to the same classical mythology. Styx is the river that dead souls were supposed to cross to get to Hades, while Nix is the Greek goddess of darkness. Hydra is a many-headed serpent thought to have battled with the Greek hero Heracles. Kerberos is an alternate spelling for Cereberus, the so-called "hound of Hades" who guarded the gates to the underworld in mythology.

Now that New Horizons is well beyond Pluto, its next target is a small dwarf planet in the Kuiper Belt. It will pass by that one on January 1, 2019. Its first reconnaissance of this distant region taught much about the Pluto system and the next one promises to be equally interesting as it reveals more about the solar system and its distant worlds.