Saturn’s moons are recent companions, but they’re drifting away

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Saturn’s moon Tethys seems to hang above the planet’s north pole in this view from NASA’s Cassini spacecraft, taken 26 January 2015. New calculations show the gas giant’s pushing its moons away.
NASA / JPL-Caltech / Space Science Institute

Did Saturn’s bevy of moons form at the same time as the gas giant 4.5 billion years ago, or are they recently acquired companions?

New research suggests the latter, and that they were likely born from the gas giant’s famous ring system.

Valery Lainey from the Paris Observatory in France and colleagues in Europe and the US pored over century-old photographs and recent Cassini spacecraft data and found Saturn’s moons are moving away from the planet much faster than expected.

If they all formed at the same time, they write, they should be much, much further away than they are today. The work will be published in the January issue of Icarus.

Saturn is a gas giant, but it’s not completely gaseous. A mammoth layer of liquid hydrogen and helium shrouds a rocky core, thought to be around 18 times the size of Earth.

And just like Earth, which bulges slightly thanks to our moon’s gravitational tug, Saturn’s core is warped by its major moons. 

Geophysicists quantify the rigidity of a planet with its “Love numbers” – named after a British mathematician who studied elasticity. There are three Love numbers; for a perfectly rigid body, they all equal zero.  

In 1977, a pair of Russian physicists calculated one of Saturn’s Love numbers as 0.341.

The value’s been used as standard since, but it relies on assumptions and models of the planet’s internal structure that have improved greatly in intervening decades.

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And it’s difficult to untangle from another planetary measure, the dissipation factor. This describes the speed at which moons migrate away from their planet.

To calculate a more accurate Saturnian Love number and untangle the dissipation factor, Lainey and her colleagues examined the orbits of four tiny moons – Telesto, Calypso, Helene and Polydeuces – and two larger moons, Tethys and Dione.

The small moons are too insignificant to exert any tidal forces on Saturn’s core, but their orbits are buffeted by its massive pull.

“By monitoring these disturbances, we managed to obtain the first measurement of Saturn’s Love number [slightly higher at around 0.390] and distinguish it from the planet’s dissipation factor,” study co-author and Cornell University astronomer Radwan Tajeddine, said. 

And they found the moons are drawing away from Saturn faster than previously thought. If they’d been whizzing away at that rate for 4.5 billion years, they’d be far beyond their current orbits.

This favours a theory that the moons formed from Saturn’s rings, which astronomers think are made from bits of comets and asteroids that broke up in orbit around the gas giant and spread out.

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