I was having a browse of some of the Cassini pictures the other day. The orbiter has returned some absolutely amazing images of the Saturnian moon system. My eye was particularly caught by Hyperion … but not because it’s pretty. In fact, it may possibly be the most ugly-looking thing in the Solar System (bar a few politicians).
(Image via Wikipedia, from Cassini’s September ’05 flyby.)
Hyperion is weird.
No, really, it’s weird.
First of all it has a massively eccentric orbit around Saturn. Secondly, despite being fairly large, the moon itself is appreciably egg-shaped (as you can clearly see in the above photo!). Its longest axis is 360 Km, its shortest is 205. Egg-shaped orbit, egg-shaped moon, apparently!
But there’s more. In terms of composition, Hyperion also appears to be a freak. Its mass is tiny. Despite being around 2/3rds the size of Mimas, it has only 15% of that other Saturnian moon’s mass. Probably this is partly why it’s so un-spherical; there’s just not enough surface gravity to flatten things out. Hyperion’s density is also rather low. In fact, some estimates claim that its insides are as much as 40% empty space.
Yup, that’s right, it appears to be partly-hollow. Presumably its interior must contain some absolutely massive caves.
Of the bits of it that actually are solid, current thinking holds that they’re mainly composed of water ice. This at least is fairly normal for Saturn’s moons. At Saturn’s distance from the Sun, things are cold enough for water to remain frozen pretty much indefinitely; it’s only once you get inside the asteroid belt that solar radiation would have been intense enough to evaporate off the early store of water. (That’s one of the reasons why there’s such a sharp compositional change between the rocky inner planets and the big, icy moons of the gas giants; basically we live inside the so-called ‘snow line’.)
However, in addition to its icy surface, Hyperion also appears to be covered in some sort of dark material. Its reflectivity is too low for pure ice; Hyperion reflects about 20-30% of incoming light. By comparison, snow reflects about 70-80%. What exactly this material is isn’t known, although there is a suggestion that its source might be another moon called Phoebe.
Another bit of compositional weirdness is the dark material that fills the bottom of Hyperion’s craters. This dark reddish substance appears to be some sort of hydrocarbon, perhaps similar to the stuff that gives Iapetus its dark hemisphere. But again, this material has never been directly-sampled, so its exact nature is uncertain.
There is one final point of high weirdness. Hyperion’s rotation is unstable. Its rotational axis flops around pretty much at random, almost as if the moon were severely drunk. This appears to be down to the conflicting tides it experiences from Saturn and Titan, and also due to the dramatic eccentricity of its orbit. (The tidal force falls off with the cube of the distance, not the square, and so is very sensitive even to small changes in orbital distance.)
Although possibly the ugliest moon in the solar system, Hyperion is an interesting object, and one which poses some interesting questions for models of planetary formation and structure. It also seems to be one of these things where we know just enough for it to be really tantalising, but not quite enough yet for any really concrete answers.