Analysis of Hubble data shows that two — and possibly all four — of the Pluto-Charon system’s smallest moons are wobbling in a wildly unpredictable fashion. What’s more, one moon, Kerberos, appears to exhibit a dark charcoal-like surface that’s radically distinct from other Plutonian moons.

Above: Computer animation showing the chaotic tumbling of Nix.

The Pluto-Charon binary is proving to be one of the most interesting, and certainly unique, celestial environments in the Solar System. Work done by Mark Showalter of the SETI Institute in Mountain View California and Doug Hamilton of the University of Maryland at College Park has shown that two of the smaller moons, Nix and Hydra, are caught in a chaotic tumble. The astronomers, who used Hubble data taken from 2005 to 2012, also suspect that the other small moons, Kerberos and Styx, are stuck in the same wild waltz as well. The details of their work can be found in the June 4 edition of Nature.

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The reason for the chaos has to do with the moons’ position within a dynamically shifting gravitational field — one exerted by the system’s most prominent bodies, Pluto and Charon, as they spin around each other. The resulting variable gravitational field is producing torques that are causing the smaller moons to spin in unpredictable ways. And the fact that these moons are shaped like footballs adds to the effect.

The astronomers also discovered that Nix, Styx, and Hydra are currently locked together in resonance where there’s a fixed ratio among their orbital periods.

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“This ties together their motion in a way similar to that of three of Jupiter’s large moons,” noted Hamilton in a statement. “If you were sitting on Nix you would see that Styx orbits Pluto twice for every three orbits made by Hydra.”

More analysis is required to determine if the system is inherently unstable and if it’s at risk of flying apart.

“Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm,” said Grunsfeld. “When the New Horizons spacecraft flies through the Pluto system in July we’ll get a chance to see what these moons look like up close and personal.”

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The team also learned that Kerberos is not like the others. It features a surface that’s incredibly dark, while the other moons appear to as bright as white sand. This now presents a puzzle to astronomers who thought that all the moons would have a consistent look.

Read the entire study at Nature: “Resonant interactions and chaotic rotation of Pluto’s small moons”.

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Email the author at george@io9.com. All images by NASA, ESA, M. Showalter (SETI Institute), and G. Bacon (STScI)