Pluto’s Moons Just Can’t Stay Still
Pluto's tiny moons

New Horizons latest data dump brings curious observations of Pluto’s four tiny moons. Most moons in the solar system are in synchronous rotation (tidal locked) with their planet. Meaning, one side always faces toward the planet. Like our moon. Every night, we all see the same side of the moon. It wasn’t until 1959 that we saw the dark side of the moon thanks to images from the Soviet spacecraft Luna 3.

Four of Pluto’s tiny moons are not in synchronous rotation. Instead of rotating once for every orbit of the planet, they rotate several times. And in Hydra’s case, it’s an astounding 89 times for every trip around the planet. “If Hydra were spinning much faster, material would fly off its surface due to the centrifugal force,” said Mark Showalter, a Co-Investigator on the New Horizons Mission.

The other three moons rotate more than once per orbit, but nothing compared to Hydra’s 89 times. Kerberos and Styx spin 6 times per orbit while Nix spins 13 times.

What’s going on? Showalter is eying Pluto’s largest moon, Charon. The New Horizons science team believes the gravitational influence of Charon is preventing each small moon from entering a synchronous rotation with Pluto. Showalter and Douglas Hamilton (from the University of Maryland) predicted this, but not quite to this level.

“There’s clearly something fundamental about the dynamics of the system that we do not understand,” said Showalter. “We expected chaos, but this is pandemonium.”

It’s not just Hydra that’s acting weird either. Nix is tilted on its axis and spinning backwards. Showalter likens the moons to “spinning tops.” Here’s a video showing how much each of the tiny moons rotate. (Styx = Red, Nix = Yellow, Kerberos = Green and Hydra = Blue)

More than meets the eye?

The latest data dump also indicates that at least two and maybe all four of Pluto’s small moons are the result of mergers between smaller moons.

Pluto's merged moons

“We suspect from this that Pluto had more moons in the past, in the aftermath of the big impact that also created Charon,” said Showalter.

Every new release of data from New Horizons yields more surprises about Pluto and its moons. Something tells me we are still at the beginning, and the data will throw even more questions towards scientists.

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