If you think your life is chaotic, imagine living on Pluto's moons Hydra or Nix. True, at temperatures of -220°C (-364°F), things probably move pretty slowly—next month's New Horizons flyby is likely to be the most exciting thing that has happened since being captured in orbit around Pluto. Still, at least on Earth there are some things you can rely on, like the rising and setting of the sun. Not so on these tiny moons.
Hubble Space Telescope observations, published in Nature, suggest the two moons are tumbling chaotically, so much so that it is impossible to predict their movements long-term. While we know the dates of eclipses hundreds of years into the future, any residents of Nix or Hydra would not know more than a day in advance when the sun would be up. Geography would be hard too as the north and south poles frequently reverse.
Modeling of Nix's rotation; Credit Showalter and Hamilton. Nature.
The authors think that the same is true of Pluto's other two small moons, Styx and Kerberos, but need more data before they can confirm this. Hydra and Nix are modest-sized worlds, estimated around 100 kilometers (62 miles) across, but Styk and Kerberos are about a fifth of that.
Most moons in the solar system—including our own—have synchronous orbits, always keeping the same face to their planet. However, these are all orbiting a relatively straight-forward body. The center of mass of a system like Jupiter, for example, moves slightly with the orbits of the Galilean moons. Be that as it may, the mass of these moons is so tiny compared to that of Jupiter that the effect is negligible. To a small outer moon, the point they are orbiting is as good as fixed.
Not so for the Pluto-Charon system. As we've shown before, the difference in mass between these two is sufficiently small—a ratio of less than 9:1—that they dance around each other. The four outer moons experience constantly changing gravitational fields and since moons this small are not spherical, the combination of an unbalanced shape and an endlessly changing torque results in chaotic tumbling, at least for the two that are large enough for the Hubble Space Telescope to get a good look at.
Each moon is close to resonance with ratios of 1:3:4:5:6. However, unlike similar resonances for the moons of Jupiter and Saturn, the outer four are out of resonance by between 0.3% and 5.4%, which the authors explain as a remnant of a time when Charon's orbit was more eccentric.
"Prior to the Hubble observations nobody appreciated the intricate dynamics of the Pluto system," said first author Dr. Mark Showalter of the SETI Institute. However, the observations were so confusing that Showalter needed to do extensive modeling to explain how the orbits relate.
The authors express the hope that New Horizons will explain why Kerberos appears dark as coal, while the other moons are very bright.