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How to rip a moon apart

December 14, 2010 2 comments

artist concept of ice particles in saturn rings


We know that Saturn’s rings are ice particles orbiting the planet like a zillion tiny moons. But we’re not so sure how they got there. Surprised?

This week, a researcher at the Southwest Research Institute in Colorado, Robin Canup, published a new and intriguing hypothesis for what built Saturn’s rings as well as its inner moons. In a nutshell, Canup says multiple icy moons spiraled to their doom early in Saturn’s history, leaving behind the ice and rock that formed the rings and Saturn’s small inner moons.

Canup’s idea solves a few key problems for Saturn ring theorists. For more details, see the press release or a story by Discovery News writer Irene Klotz.

It’s hard to imagine a moon perhaps the size of Titan — around 5,150 kilometers (3,200 miles) across — getting torn apart. But it can happen, and the cause of it all is called tidal disruption.

Terry Hurford is a planetary scientist at Goddard who studies tidal disruption as it relates to Jupiter’s moon Europa and Saturn’s moon Enceladus. Gravitational tides alternately stretch and compress those bodies, causing cracking at the surface and interior heating.

On Europa, the degree of distortion is about 1 kilometer (0.6 miles) in extent. It’s not as clear about Enceladus, but Hurtford roughly estimates it could be around 500 meters.

But if a moon strays too close to its planet, the tides can stretch it to the breaking point. The threshold is known as the Roche limit. Once a satellite gets within that distance to its planet, the planet’s gravity is in charge and the moon literally can’t hold itself together.

“The body gets more and more distorted tidally,” Hurford explains. “The gravity from Saturn makes it more like a football shape, where you have this kind of a point of the football pointed toward Saturn. As you get closer, this distortion can grow really large. If you get close enough, you can distort the body so much that it no longer can hold onto its mass.”

And that’s exactly what Canup shows in her new computer simulation. Multiple moons stray inside Saturn’s Roche limit. Tidal flexing — the same thing that today occurs on Europa and Enceladus — melts the moon’s watery ices and separates them from rocky material. Eventually the ice gets stripped off entirely to form the rings and inner moons, and the rocky stuff plummets into Saturn.

Pretty dramatic! And it’s intriguing to think that Saturn might have once had several large moon, not just one. I think they deserve to at least be named, these lost satellites of Saturn.

How about: “Going, “Going,” and “Gone”?


Tidal Disruption of a Moon
[These illustrations appear in the Wikipedia entry for “Roche limit” and were created by Theresa Knott of English Wikipedia. The illustration shows a top view of a planet and its moon.]


illustration of tidal disruptionFar from the planet’s Roche limit (curved white line), the moon’s gravity molds it into a near-perfect sphere.





illustration of tidal disruptionAs the moon approaches the Roche limit, the planet’ s powerful gravity stretches and distorts the smaller satellite.





illustration of tidal disruptionAt the Roche limit, tidal forces overwhelm the moon’s own gravity, tearing the satellite into pieces.





illustration of tidal disruptionParticles inside the Roche limit orbit faster than those outside the limit. This causes the particles to spread out and form rings.
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OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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