Saturn Moon’s Wobble Could Be Clue to What’s Inside

Posted October 17th, 2014 at 8:59 pm (UTC-4)
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This mosaic of Saturn's moon Mimas was created from images taken by NASA's Cassini spacecraft during its closest flyby of the moon on Feb. 13, 2010. (NASA/JPL-Caltech/Space Science Institute)

This mosaic of Saturn’s moon Mimas was created from images taken by NASA’s Cassini spacecraft during its closest flyby of the moon on Feb. 13, 2010. (NASA/JPL-Caltech/Space Science Institute)

Scientists with NASA’s Cassini Mission, which has been studying Saturn since 2004, were trying to figure out just how much one of its moons, Mimas, wobbled as it orbits the ringed planet.

After reviewing and analyzing copious images snapped by the robotic probe and running several models, the scientists think the moon’s wobble could be caused by either a frozen core shaped like an American football, or an ocean of liquid water.

The study outlining the scientist’s research was published in a recent issue of the journal Science.

“The data suggest that something is not right, so to speak, inside Mimas,” said study lead author Radwan Tajeddine, a Cassini research associate from Cornell University in a press release. “The amount of wobble we measured is double what was predicted.”

Tajeddine thinks that either possibility suggested by their models would be interesting, especially since the appearance of Mimas’s heavily cratered surface alone doesn’t indicate that something unusual might be lying deep inside.

Artist's concept of Saturn's rings and major icy moons. (NASA)

Artist’s concept of Saturn’s rings and major icy moons. (NASA)

The scientists pointed out that since Mimas, which many think resembles the ‘Death Star’ from the Star Wars movie, formed over 4 billion years ago, its core should have settled down and relaxed into pretty much a spherical configuration by now.  So, if the core does turn out to be frozen and oblong shaped, as some of the models suggest, the researchers think it could provide a ‘frozen in time’ glance at the early history of the moon’s formation.

But, if the moon instead does have a liquid ocean inside, as other models demonstrated, Mimas would become a member of an elite group of “ocean worlds” that includes two of its siblings the Saturn moons, Enceladus and Titan as well as several of the moons that orbit Jupiter.

Like how our own moon always shows its same side to those of us on Earth, as well as others throughout the solar system, Mimas’ own axis rotation is in sync with its orbit around Saturn, which is something called a spin-orbit resonance.  Just like Earth’s moon, Mimas also takes the same time to completely turn around on its own axis as it does to orbit its parent planet.

Many people think that Mimas, one of Saturn's moons, totally looks like the Death Star! (Paul T via Flickr/Creative Commons)

Many people think that Mimas, one of Saturn’s moons, totally looks like the Death Star! (Paul T via Flickr/Creative Commons)

Mimas’ doesn’t orbit around Saturn in an exact circle; instead it extends out a little into more of an elliptical orbit.

Because of this type of orbit if you were to observe Mimas from the surface of Saturn you’d notice that moon would look like it had a little bit of a wobble as it orbits around the planet.  This little wobble or variation effect is called libration and believe it or not our own moon does the same thing.

“Observing libration can provide useful insights about what is going on inside a body,” said Tajeddine. “In this case, it is telling us that this cratered little moon may be more complex than we thought.”

If Mimas does have a liquid water ocean, models developed by the researchers show that it would sit about 24 to 31 kilometers beneath its surface.

Since it’s only about 396 kilometers wide, scientists figure that Mimas is probably too small to have been able to keep and store internal heat that was generated from its formation.  So, in order to for the underground liquid ocean concept to work another source of energy would be from its formation, so some other source of energy would be required to maintain an underground ocean.

Artist's concept of the Cassini spacecraft during Saturn orbit insertion.  (NASA/JPL/Caltech)

Artist’s concept of the Cassini spacecraft during Saturn orbit insertion. (NASA/JPL/Caltech)

The researchers found enough evidence to suggest that Mimas’ elliptical orbit may have extended over a wider area in the past, something they said could have generated enough orbital and rotational energy to create tidal heating that could produce an ocean.

Working with the model that suggested an oblong frozen core, the scientists thought that if it did contain such an object of that size, Mimas should also have a different shape than what they observed.

The researchers recommended that Cassini take more measurements of Mimas so that additional models could be developed that could better explain the libration that they observed.

Rick Pantaleo
Rick Pantaleo maintains the Science World blog and writes stories for VOA’s web and radio on a variety of science, technology and health topics. He also occasionally appears on various VOA programs to talk about the latest scientific news. Rick joined VOA in 1992 after a 20 year career in commercial broadcasting.

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