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Pluto: Observing chaos at solar system’s outer limits

The study, carried out by Mark R. Showalter of the SETI Institute and Doug Hamilton based at the University of Maryland, reveals that at least two of Pluto’s moons do not have an orderly rotation on their axes but instead orbit around Pluto and its companion Charon in a chaotic fashion. The same research also suggests that one of the moons has a mysterious jet-black coloring.

With the principal exception of the Saturnian moon, Hyperion, visited by NASA’s Cassini spacecraft as recently as last weekend, just about every moon in our solar system, including our Moon, orbits it parent planet in a neat and tidy, tidally locked formation. What this means is that most moons have the same period of rotation on their axis as the time they take to complete a single orbit of a planet. The result is that most moons keep the same face pointed towards their parent planet, a prime example being our own Moon and one of the reasons we can never see the dark side of the Moon from a terrestrial viewpoint.

But Pluto’s planetary system in miniature seems to be different, its moons having no hidden sides. This new study demonstrates that two of Pluto’s moons, Nix and Hydra, are in a chaotic rotation. The hypothetical observer positioned on Pluto, therefore, would see different faces of these moons from one night to the next.

But for another hypothetical observer, stationed on the moons themselves, matters would be even more ever-changing and confusing. For such an observer, each day last longer (or shorter) than the day before.
Pluto’s two other moons thus far discovered, Kerberos and Styx, will require further study but the likelihood is that they, too, will be found to have chaotic orbits.

After the discovery of Pluto’s companion body Charon in 1978, Hubble discovered Nix and Hydra in 2005, Kerberos in 2011 and Styx in 2012. These little moons, measuring just tens of miles across, were found as part of several Hubble searches for moons and rings to characterize the system in preparation for NASA’s New Horizons spacecraft flyby. New Horizons is now on its final approach to Pluto after a journey of over nine years.

Illustration showing the scale and comparative brightness of Pluto's small satellites. Pluto's b...

Illustration showing the scale and comparative brightness of Pluto’s small satellites. Pluto’s binary companion, Charon, is 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.Hubble can’t resolve surface features on the moons so textures are purely for illustration purposes.
NASA, ESA, A. Field (STScI)

“Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system,” commented Showalter, main author of this latest study which is published in Nature June 4.

Pluto’s moons’ chaotic motion is due to the movement of this minor system’s two central bodies, Pluto and its companion Charon. Pluto and Charon orbit around a common centre of gravity, located in the space between the bodies. At first glance Pluto and Charon’s cha-cha might appear similar to the Earth and Moon’s perpetual flamenco in space but there the similarities end. Whereas the Moon has just one-eightieth of Earth’s mass, Charon has one-eighth the mass of Pluto. Not only that but while the Moon takes roughly 28 days to orbit Earth, Charon whizzes around Pluto in just 6.3 days.

These differences cause the gravitational forces Pluto and Charon exert on the tiny moons to change constantly.

“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!” explained Hamilton.

How Pluto’s moons behave has implications for the study of how planets orbiting a binary star system might behave. Hamilton suggests that chaos might be the order of the day in such binary systems and that such chaotic movement of planetary bodies may even have consequences for how life might evolve on such planets.

An inkling of the chaos present in the Pluto system was gleaned by astronomers measuring variations in the light reflected by two of the small moons. An examination of images taken by NASA/ESA’s Hubble Space Telescope between 2005 and 2012 showed the moons’ brightness changed unpredictably instead of following a regular pattern. Such unpredictability pointed to the moons moving chaotically.

The same images also produced something of a puzzle in that the moon, Kerberos, was found to be charcoal-black in colour in stark contrast to the bright white of the other moons. The expectation was that over the eons, dust thrown up from the moons by meteorite impacts would “sugar-coat” the moons’ surfaces giving them a homogenous reflective look. So, just why Kerberos is jet-black presents something of a mystery.

From the Hubble images the astronomers also made a surprising discovery concerning the orbits of the three moons Nix, Styx and Hydra. Their jig around Pluto is somewhat more complicated, as Hamilton explained, “Their motion is tied together in a way similar to that of three of Jupiter’s large moons. If you were sitting on Nix, you would see Styx go around Pluto twice every time Hydra goes around three times.”

That the Pluto-Charon mini-system operates in a state of such chaos doesn’t mean that it’s likely to tear itself apart any time soon but the researchers say that more studies are needed to determine the long-term prognosis for the Pluto system.

As luck would have it, the lid may be lifted on some of Pluto’s mysteries this summer. NASA’s New Horizons space probe is scheduled to fly by the Pluto system in July 2015. Already New Horizons has returned some tantalizing images of Pluto. End April New Horizons detected surface features including what may be a polar cap on this distant, mysterious world.

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