Saturn's Rings Could Be the Shreds of a Giant Moon

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Astrophysicist Robin Canup loves to smash things together.

No, she hasn’t joined the team on Mythbusters. Canup’s smash-ups are strictly extraterrestrial. Her computer simulations crash a Mars-sized protoplanet into primeval Earth to recreate the birth of our moon. As if Pluto’s not beaten up enough, she has also computationally slammed a companion Kuiper belt object into Pluto to spin off its three moons: Charon, Nix and Hydra.

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But the mother of all planetary bang-ups has been staring us right in the face all along, according to Canup. At last week’s American Astronomical Society’s Division of Planetary Science meeting in Pasadena, California, she proposed that Saturn’s magnificent ring system is the result of a moon at least the size of the planet Mercury that plunged into the gas giant.

If this is the case, we missed the fireworks by a long shot. Observations by the NASA/ESA Cassini orbiter show that the rings must be billions of years old.

Conventional wisdom has been that the rings formed from a much smaller icy moon getting gravitationally ripped apart into zillions of pieces.

But Canup says that the rings would be more of a rocky-ice mix if a small moon was pulverized. In fact the rings are almost purely water ice. The merely ten percent rocky composition of the rings probably came from micrometeorites that dusted the icy rings over billions of years.

Another popular thought was that a wayward comet got caught in Saturn’s gravitational Cuisinart. But Jupiter, Uranus, and Neptune should have icy rings too. Instead, they all have dark rings fed by dust being blown off their moons by meteorites.

The hypothetical doomed moon would have been at least as large as Saturn’s giant moon Titan, which is so big and meteorologically/geologically complex it has been dubbed “Earth II.”

The doomed primeval moon would have spiraled in toward Saturn due to drag in a vast gaseous disk that accompanied satellite formation around the gas giant. Think of it as a small-scale version of the huge circumstellar disk that encircled our sun to form the planets. Hubble Space Telescope observations of a young gas giant planet orbiting the star Fomalhaut suggest it is unusually bright because it is surrounded by a large gas disk.

Sound crazy? Not at all considering that a Hubble deep galactic survey showed that at least seven percent of the stars in the Milky Way have “Hot Jupiters.” These worlds must have spiraled into very close to their stars, perhaps due to drag in a viscous disk.

This could happen on a planetary scale too, says Canup. Titan remained far enough away not to be dragged in. But its sister moon fell so close to Saturn that tidal forces stripped off the moon’s icy outer layers. The dense, rocky inner core did a cannonball plunge into Saturn’s atmosphere. That would have been a sight to behold! Canup’s proposes that once the massive ring formed the icy inner moons including Enceledus, Dione and Tehys that were spawned off the ring’s outer rim. 

It seems that the solar system’s most opulent planet could really be a cannibal, wearing a brilliant tiara made of the satellite-planet it swallowed. Such is life in a universe where creation, destruction and rebirth are an endless cycle.

Image credits: NASA, ESO

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