Saturn’s Moons May Be Younger than Previously Thought

New data from NASA’s Cassini spacecraft reveal that Saturn’s moons may be younger than previously thought.

A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA’s Cassini spacecraft. Image credit: NASA / JPL-Caltech / SSI.

“All of these Cassini mission measurements are changing our view of the Saturnian system, as it turns our old theories upside down,” said Dr. Radwan Tajeddine, a researcher at Cornell University and the Observatory of Paris, France, and co-author of the paper reporting the results in the journal Icarus.

Dr. Tajeddine and his colleagues provided two key measurements: (i) the rigidity of the tidal bulge, or the Love number – named for Augustus E.H. Love, a famed British mathematician who worked on elasticity and wave theories; and (ii) the dissipation factor, which controls the speed at which moons move away.

While Saturn is mostly a gigantic shroud of liquid hydrogen and liquid helium, it contains a rocky core – about 18 times the size of Earth, which responds to tidal forces from all of Saturn’s major moons by bulging. The forces of the bulging core, in turn, push the moons slightly away.

“Those two parameters are difficult to separate,” Dr. Tajeddine said.

So the authors detected and examined the orbits of four tiny Saturnian moons associated with the larger moons Tethys (Telesto and Calypso) and Dione (Helene and Polydeuces).

While these small moons do not affect the tidal forces on Saturn, their orbits are disturbed by Saturn’s core tidal bulges.

This is an artist’s concept of Saturn’s rings and major icy moons. Saturn’s rings make up an enormous, complex structure. From edge-to-edge, the ring system would not even fit in the distance between Earth and the Moon. The seven main rings are labeled in the order in which they were discovered. From the planet outward, they are D, C, B, A, F, G and E. The D ring is very faint and closest to Saturn. The main rings are A, B and C. The outermost ring, easily seen with Earth-based telescopes, is the A ring. The Cassini Division is the largest gap in the rings and separates the B ring from the A ring. Just outside the A ring is the narrow F ring, shepherded by tiny moons, Pandora and Prometheus. Beyond that are two much fainter rings named G and E. Saturn’s diffuse E ring is the largest planetary ring in our solar system, extending from Mimas’ orbit to Titan’s orbit, 621,370 miles (1 million km). The particles in Saturn’s rings are composed primarily of water ice and range in size from microns to tens of feet. The rings show a tremendous amount of structure on all scales; some of this structure is related to gravitational interactions with Saturn’s many moons, but much of it remains unexplained. One moonlet, Pan, actually orbits inside the A ring in a 200-mile-wide (330 km) gap called the Encke Gap. The main rings (A, B and C) are less than 300 feet (100 m) thick in most places, compared to their radial extent of 38,600 miles (62,120 km). The main rings are much younger than the age of the Solar System, perhaps only a few hundred million years old. Image credit: NASA / JPL.

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

According to the team, if Saturn’s moons actually formed 4.5 billion years ago, their current distances from the parent planet should be greater. Thus, the moons are younger than 4.5 billion years, favoring a theory that they formed from Saturn’s rings.

The researchers also found that Rhea, the second largest moon of Saturn, is moving away 10 times faster than the other moons, which is the first evidence that a planet’s dissipation factor can vary with its distance in relation to the moon. The scientists said that they have no definitive explanation.

“What we believe about Saturn’s moons history might still change in the coming years with the finale of the Cassini mission,” said lead author Dr. Valery Lainey, from the Observatory of Paris.

“The more we learn about Saturn, the more we learn about exoplanets.”

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Valery Lainey et al. 2017. New constraints on Saturn’s interior from Cassini astrometric data. Icarus 281: 286-296; doi: 10.1016/j.icarus.2016.07.014

This article is based on a press-release from Cornell University.