Monday, December 28, 2015

What's With Pluto's Smaller, Rapidly Spinning Moons?

Image from an animation showing Pluto and its relatively large moon Charon near the center  of the Pluto system, with the smaller, rapidly rotating moons further out - spinning as they orbit the larger bodies.

There is a problem, a big one, with Pluto's outer moons. Like disobedient children these four small moons are apparently rotating at inexplicably high rates defying the gravitational influence expected from their mother planet, and its larger moon companion, Charon.

These four moons, Styx, Nix, Kerberos and Hydra (in order of proximity to Pluto) all spin much faster than the 20 to 38 Earth days the moons take to orbit the Pluto-Charon system, as planetary astronomers recently reported. Kerberos spins the slowest, once every 5.33 Earth days, whereas Hydra rotates once every 10 hours and 19 minutes.

Make no mistake these are fast rotations for moons, which usually keep one face pointed at their central planet (synchronous rotation), rotating just once per orbit of that planet. Earth's Moon, for example, rotates once every 27 days 8 hours, the time it takes to orbit our planet once. Charon also plays by the rules, rotating once per Plutonian orbit. But the four small moons are renegades by comparison.

According to Mark Showalter of the NASA New Horizons mission put it this way (Eos Earth & Space Science News, Dec. 15, p. 3):

"These Pluto moons are essentially spinning tops and that radically changes the way we understand how they operate. This is unlike anything we've seen elsewhere in the solar system. No one has ever seen a moon, like Hydra, that rotates 89 times during a single orbit."

Recall last July, the New Horizons mission carried out the first flyby of Pluto. The analysis of images recorded by the spacecraft in the weeks leading up to the flyby led to the discovery of the moons surprising rotation rates.  This was unveiled November 9th at the annual meeting of the American Astronomical Society's Division of Planetary Sciences.

Showalter said these fast spin rates are so surprising because even if the moons originated as rapid rotators, the push and pull of the gravitational tides of Pluto and Charon ought to have slowed down that motion. So the question becomes: Why didn't they?

As Showalter put it (ibid):

"It's as if the moons just picked some random rate of rotation and Pluto and Charon have no part in any of that."

One explanation offered is that the gravitational tides of Pluto and Charon worked against each other, thus leaving the moons to maintain a high spin.

Among the category of so-called "regular moons", i.e. those that form as a result of collision between a planet and another body  - say large asteroid - Hydra ranks as the ninth fastest rotator.

Because all the moons in question are tiny - ranging from ten to a few tens of kilometers across, any collision with a large impactor could easily have imparted a lot of angular momentum to the objects.

Interestingly prior to the flyby and analysis of the moons, the leading theory was that they rotated chaotically, varying their spin rate and tumbling as they orbited Pluto and Charon. This work was published by Showalter and Douglas Hamilton of the University of Maryland in the June 4 issue of Nature.  Interested readers can read more about that study here:

The theory - perhaps more a hypothesis - held up fairly well until last July when New Horizon detected brightness variations showing clear signs of periodicity. These revealed the four small moons were not so much tumbling as spinning in a regular fashion - just much faster than anyone expected. The images also revealed that Nix is rotating backward compared to the rest of the Pluto system - and also on its side.

More interesting findings from this system are sure to come.

To see the model animation from which the graphic displayed at the top came, go to:

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