Titan, Iapetus, and the Great Crash

From Titan, you can't see the rings of Saturn very well. It's hard to see through the smoggy atmosphere and besides, Titan orbits in the plane of Saturn's rings, so all you see is the edge. If you try it from Titan's poles to get a better perspective, Saturn and it's rings are on the horizon and you are looking through more smog.

From airless Iapetus, however, you can get a great perspective from above the rings, or below. You can see the other moons, too. There's a nice artist's depiction on Wikipedia:

http://en.wikipedia.org/wiki/File:Saturn_from_Iapetus.jpg

Iapetus' orbital inclination to Saturn's equator:

http://en.wikipedia.org/wiki/Iapetus_(moon)

is 15.47 degrees. It's inclination to the ecliptic is 17.28 degrees, but to the Laplace plane it's inclination is 7.52 degrees. If it formed in the plane of Saturn's rings, it's made a long, strange trip indeed.

"Current triaxial measurements of Iapetus give it dimensions of 747.1 × 749 × 712.6 km, with a mean radius of 736 ±2 km.[2] However, these measurements may be inaccurate on the kilometer scale as Iapetus's entire surface has not yet been imaged in high enough resolution. The observed oblateness corresponds to a rotation period of 10 hours, not to the 79 days observed." That 10 hour period is shorter than inner moon Mimas' 22.6 hours, so it's a pretty sure bet that it didn't pick up that kind of rotation from Saturn.

Iapetus' semimajor axis of 3,560,000 km is far outside of Titan's 1,220,000 km. It's like a captured body. Yet, it's about the size of Saturn's other largest moons, except giant 5150 km diameter Titan. It's 1470 km diameter fits right in between Dione's 1120 km and Rhea's 1528 km. It's an inner moon - and yet it's an outer moon.

Titan has it's own anomalies. It's as big as a planet, almost the size of Jupiter's largest moons, yet the Saturn system has only one third the mass of the Jupiter system. It's like an intruder into the orderly mass distribution of a normal gas giant. It has an atmosphere, too.

That atmosphere has two inert gases, helium and argon, both decay products of radioactive materials. If Titan can hang on to even a little bit of helium, it should hang on to neon, but there is none. In fact, there's only one argon isotope, argon-40, and no krypton or xenon. It appears as though Titan lost it's original atmosphere. It must have collided with something big.

I propose it collided with itself.

Half the object formed in the Sun/Saturn Lagrange point L4 and the other half formed in the L-5 point. Having run plenty of objects on GravitySimulator, I can attest that objects orbiting one of these points but far from it spend a long time hanging around the L-3 point. With Jupiter stirring things up, a capture as a binary is possible (Jupiter makes it a non-Lagrange three body problem). Now we have a binary object in orbit around the Sun at the same distance as Saturn. It's too big to fit into a Lagrange point, so eventually it will approach Saturn.

When a small binary approaches a large object, there can be several results. One could be captured and the other thrown away at a considerable velocity. That occurs if, at closest approach, the three bodies are all in a straight line. However, I don't think that happened at Titan. If both objects are at the same altitude at closest approach, then their revolution with respect to each other is cancelled by the larger objects gravity and they attract each other and collide. I've said elsewhere that I think that's what happened to Triton and Haumea when they passed Neptune. That was a glancing blow that imparted impressive spin to Haumea.

But the Titan halves apparently made a head-on collision. This would release significantly more energy than if the halves were made of high explosives. It would turn into a blob of gas, highly vulnerable to tidal forces.

In spite of the explosion, Titan would fly on past Saturn unless something carried away some of it's angular momentum. If something comes down, something else must go up. I would nominate unfortunate Iapetus for this role. Iapetus would have whatever spin it would have from being tidally locked with Titan at it's closest approach.

A ten hour orbit around Titan would correspond to a distance of 6680 km, but Iapetus was likely moving at escape velocity so you would multiply that by the square root of two to get 9400 km, just outside Titan's Roche limit. So Iapetus did not explode but went sailing off into orbit around the Sun.

Meanwhile, the blob of gas that was Titan begins raining iron. Since we have a large object kicking a small object out of the system, Titan would have a highly eccentric orbit from being just barely captured. Being large it would be subjected to tidal forces which would let it easily get into the plane of Saturn's rotation. As it's eccentric orbit decayed, it's mass would spill out of it's Hill sphere and form a torus outside of it's eventual circular orbit.

Millions of years later, Iapetus returns, having been tossed around a little by the other gas giants. It plows into the torus of ground-up Titan remains and is aerobraked into a new orbit around Saturn. Iapetus gathers a ring from the torus and it decays into the Voyager mountains. Iapetus clears it's orbit and takes it's rightful place as a major moon.

That's my ideas on the history of Titan and Iapetus. Anybody's welcome to comment if you care to.

Thanks :)

Michael C. Emmert