Saturn’s Bulge Tells Us That Some Of Its Moons Are Younger Than Thought

Astronomers knew that the gravity of the larger moons of the system was squishing the planet’s rocky core (which is 18 times the size of Earth), but they underestimated the effect. The bulging is more severe and the moons are moving away faster than they thought.

Since the moons are so close to the planet, this has a big implication. They must be younger than previous estimates, otherwise they would be further away by now. The findings are published in the journal Icarus.

“All of these Cassini mission measurements are changing our view of the Saturnian system, as it turns our old theories upside down,” said co-author Radwan Tajeddine, a Cornell research associate in astronomy, in a statement. "It takes one good spacecraft to tell us how wrong we were in the past."

For this discovery to be possible, the researchers had to measure the dissipation factor, which is related to the speed at which the moons are moving away, and the Love number, named after August E.H. Love, which is a measurement of the rigidity of a planet.

The two values are difficult to study because they are interconnected, since the moons are both moving away and exerting tidal forces on the planet. They found a way around the issue by studying four of the smallest moons, which are being pushed out but don’t influence the planet much.

These objects are Telesto, Calypso, Helene, and Polydeuces. They are unique objects trapped in a specific location by the balancing forces of the planet and the larger moons Tethys and Dione. Their orbit is subtly influenced by the bulging planet, and Cassini was able to determine the size of this interaction.

“By monitoring these disturbances, we managed to obtain the first measurement of Saturn’s Love number and distinguish it from the planet’s dissipation factor,” Tajeddine added. “The moons are migrating away much faster than expected.”

The team also discovered that another moon Rhea is moving away 10 times faster than other moons, a potential indication that the dissipation factor might change with distance, although there’s no conclusive evidence of that.

Cassini is a collaborative mission by NASA and the European and Italian space agencies. It has been orbiting Saturn since 2004 and it’s gearing up for its final months around the ringed planet. It will take a swan dive into Saturn in September 2017.