Hubble observes chaotic dance of Pluto’s moons

This illustration shows the scale and comparative brightness of Pluto’s small satellites, as discovered by Hubble over the past years. Pluto’s binary companion, Charon — discovered in 1978 — is placed at the bottom for scale. The image illustrates that two of the moons are highly oblate and that the reflectivity among the moons varies from dark charcoal to the brightness of sand. As Hubble cannot resolve surface features on the moons the textures seen here are purely for illustration purposes

In a new study, scientists have gathered all available NASA/ESA Hubble Space Telescope data on the four outer moons of Pluto to analyse the system in more depth than ever before. 

It has been found that two of Pluto’s moons, Nix and Hydra, are in a chaotic rotation. This means that an observer on Pluto would not see the same face of the moons from one night to the next. For visitors on the moons themselves, things would get even more confusing, as every day would be a different length to the one that preceded it.

The other two moons studied, Kerberos and Styx, will likely be found to be chaotic too, pending further study.

The chaotic motion of the moons is caused by the system’s two central bodies, Pluto and Charon. “These two bodies whirl around each other rapidly, causing the gravitational forces that they exert on the small nearby moons to change constantly,” explains Doug Hamilton, co-author of the study. “Being subject to such varying gravitational forces makes the rotation of Pluto’s moons very unpredictable. The chaos in their rotation is further accentuated by the fact that these moons are not neat and round, but are actually shaped like rugby balls!”

Clues to the Pluto system chaos first came when astronomers measured variations in the light reflected off of the two small moons. Looking at images taken by Hubble between 2005 and 2012, the brightness was found to change unpredictably — instead of following a regular cycle — in a way that could only be explained by chaotic movement.

These images also showed that the moon Kerberos is charcoal-black in colour, a stark contrast to the bright white of the other moons. It was predicted that pollution from dust blasted off the satellites by meteorite impacts would coat the moons, giving their surfaces a homogenous look, but why Kerberos is black is a mystery.

The chaotic rotation of the moons was not the only surprise that arose from the study; Hubble’s monitoring also revealed a connection between the orbits of the three moons Nix, Styx, and Hydra.

“Their motion is tied together in a way similar to that of three of Jupiter’s large moons,” noted Doug Hamilton of the University of Maryland, co-author of the study. “If you were sitting on Nix, you would see Styx go around Pluto twice every time Hydra goes around three times.”