Mohawk Valley Astronomical Society

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Luck of the Rings

by Perry Pezzolanella

If someone were asked, “What is the most beautiful planet of all?”, the answer is easily Saturn. Who can deny those beautiful bright rings that adorn the planet unlike any other? They are bold, bright, and huge, completely circling the globe, which easily beats out the smoky, ghostly rings of Jupiter; the dark, ropey rings of Uranus; and the frail ring arcs around Neptune. Saturn’s rings seem invincible, like they have been around forever and will last forever, but they are actually frailer than thought, and short-lived.

The Cassini spacecraft orbited Saturn from June 30, 2004 through September 15, 2017 making many great discoveries along with detailed observations of the rings. During the final year, Cassini flew between the rings and Saturn’s cloud tops. It discovered that the rings are decaying at a rapid rate, and based on this rate, the rings could not have been around for more that 100 million years. It is most likely that Saturn was ringless during the age of the dinosaurs. Ring particles are raining into Saturn’s clouds at such a rate that they will be gone in about another 100 million years. The rings are mostly chunks of water ice ranging in size from microscopic particles to boulders several feet across. The ring particles are caught in a balancing act between the pull of Saturn’s gravity, which draws them towards the planet, and their orbital velocity that wants to fling them out into space. The ring particles can become electrically charged by ultraviolet radiation from the Sun or micrometeoroid bombardment. When the ring particles become charged, they feel the pull of Saturn’s magnetic field, which curves inwards towards the planet. The charged ring particles will then be drawn into Saturn’s cloud tops. There the particles vaporize allowing the water to react chemically in Saturn’s ionosphere creating charged hydrogen particles, which energized by sunlight, glow in the infrared. This creates a glowing band in the stratosphere that can be analyzed to determine the amount of ring particles raining from the rings.

The new research from the Cassini data indicate that the faint inner C-ring most likely was as thick and bright as the prominent B-ring at one time, but that it has been eroding away, raining inward onto Saturn, and fading. Gradually, the rings will become increasingly narrow, thinner and fainter as the B-ring decays, and finally, the bright, outer A-ring will succumb to Saturn’s gravity. In their final stage of existence, the rings will appear like a brighter, thicker version of those around Uranus, and then there will come the time when they will be gone forever. Through telescopes at public star parties, Saturn will appear as nothing more than a ringless, golden globe with Titan attending it, not much of a showpiece.

The rings are almost certainly a brief, recent, and temporary addition to Saturn, probably existing no more than 300 million years in the 4.5 billion-year lifetime of Saturn and the Solar System. We are lucky to be alive during this era where we are able to observe them in their full glory.