Lecture #35: Saturn and its Moons

I. Saturn -- similar to Jupiter, but smaller, a bit cooler, and less dense -- Saturn's density is so low it would float in water -- Saturn's surface temperature is much higher than would be expected from solar heating alone. It has no Greenhouse effect, so it must be due to internal heating. Radioactive decay isn't enough by itself either. It must be our old friend gravity! So Saturn is still collapsing, as small drops of liquid hydrogen sink deep in its interior. -- Saturn, like Jupiter, rotates rapidly, and its core contains large amounts of liquid hydrogen which acts like a metal and conducts currents. The presence of currents and rapid rotation give both Saturn and Jupiter their very strong magnetic fields -- Saturn also has very clear rings, which are less rocky and more icy than Jupiter's rings II. Saturn's Rings -- Saturn's rings form inside the Roche limit, which is the distance from Saturn where tidal forces from Saturn begin to tear apart large Moons. -- smaller moons experience weaker tidal stretching, so the internal strength of the rock can hold them together -- such smaller moons inside the ring region help "shepherd" the rings, and prevent them from becoming chaotic -- larger moons outside the rings, such as Mimas, create gaps in the rings along special orbits that resonate with the orbital period of the moon. This means that particles in the gap return to exactly the same orientation with respect to Mimas every few revolutions, and this yields a coherent force that eventually clears them out of the gap. -- the largest gap is called the Cassini division, which occurs around the orbit that would go around twice every time Mimas goes around once. The satellite now headed to Saturn is called Cassini in his honor (no relation to Oleg Cassini the fashion designer!) -- the densest region of the rings would have enough material that a spacewalking astronaut could "swim" through it by imparting backward momentum to the ring particles! III. Saturn's Moons -- Saturn's moons are smaller and even icier than the Galilean moons of Jupiter. The iciness of the moons probably explains why Saturn's rings are also icier than Jupiter's. -- Small moons do not have to form with the planet, many may be captured comets or captured Centaur asteroids (which are basically comets too). -- Saturn has an interesting pair of moons in orbits that are even closer together than the width of the moons. But they don't collide, their gravitational attraction makes them actually swap orbits as they pass! -- other orbital phenomena include small moons corralled at the Lagrangian points of larger moons, just like the way Jupiter corralls the Trojan asteroids. Note the orbit is around the center-of-mass, not really the central body, an effect we have not been including. -- Saturn also has a very interesting moon called Enceladus, which shows signs of liquid water much like Europa. It could be active for the same reason as Europa, thought to be weak tidal heating due to occasional deviations in Enceladus' orbit. In support of this idea is the fact that Enceladus is nearly in resonance with the moon Dione, and this could cause orbital perturbations. -- Saturn's largest moon is Titan, which has a nitrogen atmosphere quite similar to that of Earth, except there is of course no oxygen. Instead, there is a lot of vapor from volatile hydrocarbon ices like methane. The rain there is like frozen gasoline! And there may be gooey oceans of organic molecules like a thick sea of natural gas, concealed by the smoggy orange atmosphere. Since you can't see stars there, and it has no magnetic field, it would be very hard to navigate those oceans!