# Iapetus equatorial ridge fell from the skies!

1. Apr 19, 2014

### PhysicoRaj

Moons of Saturn have always been mysterious. The Iapetus equatorial ridge is now seen as exogenic, according to these people:Discover_Magazine-Iapetus.
What makes a ring system unstable?
Also, the wiki says that equatorial ridges are unique to the Saturnian system.

2. Apr 19, 2014

### Bandersnatch

There is viscous spreading(http://arxiv.org/abs/1006.0633), which is basically the rings getting gravitationally flattened like a pancake and falling onto the surface or escaping past the Roche limit(to eventually form moons), depending on whether it's the inner or outer edge.
There's also tidal friction, which causes the material in the rings to either spiral outwards(like our Moon does) or inwards, depending on the orbital velocity.

3. Apr 19, 2014

### PhysicoRaj

Thanks Bandersnatch. And what factors affect the Roche limit? Like if a planet is extraordinarily huge but has weak gravitational field, the limit may lie within the planet's surface..etc. I found this formula on wiki but it does not include the rigidity factor of the orbiting body
$d=1.26R_m{\Big({\frac{M}{m}}\Big)}^{1/3}$

4. Apr 19, 2014

Staff Emeritus
Like almost all the other moons in the solar system. (Hyperion is an exception, as may be Jupiter's Himalia group). I'm not sure I would take this as a good source.

5. Apr 19, 2014

### PhysicoRaj

Hmm..Yes.. like all other moons.. you are right. But because Iapetus is further away from saturn than other moons, they might be thinking of low tidal forces incapable of reducing the rapid spinning(?) and the large impact craters like Turgis and Engelier, which might be signs of devastating collisions that slowed the moon down. The source actually says that the findings were published in arXiv:http://arxiv.org/abs/1404.2337

6. Apr 19, 2014

### Bandersnatch

In the idealised scenario, as can be glanced from the equation, the limit depends only on radius and the ratio of masses, or equivalently, on the ratio of denisties(with radius changing to that of the primary rather than of the satellite). As you say, for a high denisty satellite and low density planet, the limit may lie under the planet's surface.

On the wiki page, you can find the derivations for two extreme cases of an ideal rigid and fluid body. The two differ by the numerical factor in the equation - 1.26 and 2.44. The actual non-ideal bodies will lie somewhere in-between.

Additionally, the Roche limit as expressed by the above equation concerns only bodies held together solely by their gravity. It does not take into account any other forces that may affect cohesion, so, for example, a human being within Roche limit of Earth(~27 000 km for a ~1g/cm^3 dense human body) does not disintegrate, as other forces than gravity hold his body together.

I've never seen the actual calculations for real-world cases, but I hear they're not trivial.

7. Apr 20, 2014

### PhysicoRaj

That really convinced me. So there is room for speculation that Iapetus ridge may have been formed by gravitational influence of Saturn or other moons. It orbits beyond Titan but it might have once orbited much close, so tides pulled down its spin, then something might have ejected it out.

Are the rings of saturn (or any other planet with ring system for that matter) present inside the Roche limit of the planet?

8. Apr 20, 2014

### Staff: Mentor

For Saturn, some of the rings are inside and some are outside. I didn't check the other planets.

9. Apr 20, 2014