A team of astronomers using observations from the NASA/ESA Hubble Space Telescope has found that two of Pluto’s small moons, Nix and Hydra, are not neatly rotating on their axes but are in chaotic rotation while orbiting around Pluto and its largest moon, Charon. According to the scientists, Nix and Hydra have bright surfaces similar to that of Charon, while another Pluto’s moon, Kerberos, has a mysterious jet-black coloring.
This image shows the scale and comparative brightness of Pluto’s small satellites. Charon 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. Image credit: NASA / ESA / A. Field, STScI.
Almost every moon in the Solar System, including our Moon, rotates on its axis at the same speed as it orbits its parent body. It is for this reason that we always see the same side of the Moon facing us on Earth.
On Pluto, however, scientists have now discovered that there are no hidden sides to its moons.
In a new study, it has been found that two of Pluto’s small 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.
“Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system. Our research provides important new constraints on the sequence of events that led to the formation of the system,” said team member Dr Mark Showalter of the SETI Institute in Mountain View, California.
This set of illustrations of Pluto’s moon Nix shows how the orientation of the moon changes unpredictably as it orbits the Pluto-Charon system. This illustration is based on a computer simulation which calculated the chaotic movement of the four smaller moons in the Pluto-Charon system. Image credit: NASA / ESA / M. Showalter, SETI Institute / G. Bacon, STScI.
Dr Showalter and his colleague, Dr Doug Hamilton from the University of Maryland at College Park, also found Styx, Nix and Hydra are presently locked together in resonance – meaning there is a precise ratio for their orbital periods, which is reminiscent of the resonance linking Jupiter’s moons Io, Europa and Ganymede.
“If you were sitting on Nix, you would see that Styx orbits Pluto twice for every three orbits made by Hydra,” Dr Hamilton explained
Hubble data also reveal the moon Kerberos is as dark as a charcoal briquette, while the other frozen moons are as bright as sand.
It was predicted that dust blasted off the moons by meteorite impacts should coat all the moons, giving their surfaces a homogenous look, which makes Kerberos’ coloring very surprising.
“Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm,” said Dr John Grunsfeld of NASA’s Science Mission Directorate in Washington, who was not involved in the study.
“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.”
The scientists said that a combination of Hubble data monitoring and New Horizon’s brief close-up look, as well as future observations with NASA’s James Webb Space Telescope will help settle many mysteries of the Pluto system.
“Pluto will continue to surprise us when New Horizons flies past it in July. Our work with the Hubble telescope just gives us a foretaste of what’s in store,” Dr Showalter said.
The results will appear in the June 4 issue of the journal Nature.
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M. R. Showalter & D. P. Hamilton. 2015. Resonant interactions and chaotic rotation of Pluto’s small moons. Nature 522, 45–49; doi: 10.1038/nature14469
