Sound Science Bite: December 28. A New Necklace for Saturn

Well, if you consider 200 million years "new", that is. Many planetary scientists have thought Saturn's rings were as old as the solar system. For one thing, they were thought to have a mass greater than Saturn's moon Mimas, which implies that they had to have formed early in the solar system's history when a lot of mass from the proto-planetary nebula was available.

Results from the Cassini spacecraft mission, reported in a news article in the 22 Dec 2017 journal Science, provide convincing evidence the rings are mere solar system babies. For one thing the mass of the rings looks to be less than half that of Mimas. Also the influx of micrometeorites onto Saturn is much larger than previously thought. These particles darken objects they adhere to, including the particles of the rings. With the large influx of these particles now identified, the rings should be significantly darker than they actually are were they formed at the same time as Saturn. With these new results the age of the rings is estimated to be only a few hundreds of millions of years old.

This leaves us the question of how they managed to form in the first place. Possibly a collision between a Saturnian moon and a comet? I was surprised to see the absence in the news article of mentioning the possibility that a moon in a retrograde orbit spiraled in to Saturn close enough to be torn apart by Saturn's gravity. This seems to be the eventual fate of Neptune's retrograde moon Triton, which, once it gets too close to Neptune, will be torn apart and may form a spectacular set of rings. (A retrograde orbit occurs when a moon orbits opposite the direction in which the planet is rotating. Tidal forces then cause the moon to move closer to the planet on each orbit.) Of course, if this scenario is true, it brings up another question: How is it that Saturn came to have a moon in a retrograde orbit? At any rate, Saturn's rings continue to be a difficult puzzle to solve.