The Limited Reign of Saturn's Rings

Saturn's rings

NASA

Saturn’s rings—stretching tens of thousands of miles above its equator but no more than a few hundred yards thick—mark an ancient debris field of orbiting ice shards, the remains of a moon-sized object that strayed too close and was torn to pieces by Saturn’s intense gravitation. Astronomers have debated when the rings formed and how long they will stay in orbit. Recent observations from large, land-based telescopes and orbiting spacecraft reveal that Saturn’s rings are remarkably young and are dissipating at a rapid rate.

In 2011, a research group led by James O’Donoghue, then a doctoral candidate at the University of Leicester, using the Keck II Telescope on Mauna Kea, Hawaii, observed the spectrum of charged water molecules cascading from the rings into the atmosphere of Saturn under the influence of the planet’s magnetic and gravitational fields. Commenting on a detailed analysis published in the 6 November 2018 issue of the journal Icarus, O’Donoghue, now at NASA’s Goddard Spaceflight Center in Maryland, estimated that “this ‘ring rain’ drains the equivalent of an Olympic-sized swimming pool from Saturn’s rings in half an hour.”

Those 2011 observations augment measurements made by the Cassini Saturn orbiter as it flew inside the rings during a final “death dive” into the planet in 2017. As reported in the October 5, 2018 issue of Science, instruments aboard Cassini detected a flow of neutral material from the rings that its lead author, Hunter Waite of the Southwest Research Institute, characterized as “more like a ring downpour.” Detailed tracking of how Cassini’s final nosedive was affected by the gravitational pull of the rings has yielded their total mass, which had only been roughly estimated in the past. The result, announced in the January 17, 2019 issue of Science by lead author Luciano Iess of Sapienza University, Rome, Italy, and colleagues, is about 40 percent of the mass of Saturn’s moon Mimas (which is 2000 times smaller than our Moon). Models based on this mass yield a relatively young age for the rings, somewhere between 10 and 100 million years. If they were older, they would have been significantly darkened by encounters with interplanetary dust. O’Donoghue estimates that, from his observations alone, “The entire ring system will be gone in 300 million years . . . Add to this the Cassini-spacecraft detected ring-material falling into Saturn’s equator, and the rings have less than 100 million years to live."

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