Moonday: Hyperion to a Saturn

So excellent a king; that was, to this/Hyperion to a satyr…
Hamlet, Act I, Scene 2

Saturn’s strange moon Hyperion, one of the largest irregularly-shaped objects in the Solar System.

Hyperion is a small moon of Saturn: much smaller than the previous moons we’ve looked at on this blog. It’s also nowhere close to being spherical, as you can see from the photo (taken by Cassini, natch). However, it’s possibly my favorite Solar System moon simply because it’s so strange: everything about it invites further exploration, from its composition to its crazy rotation.

Hyperion compared to Earth and the Moon.

First, let’s start with size: Hyperion is one of the largest irregularly-shaped objects in the Solar System, but it’s still not large. The longest axis is about 360 kilometers (224 miles), which is about 3% of Earth’s diameter. In a size comparison between Earth, the Moon, and Hyperion, the Saturnian satellite barely shows up at all. However, the weirdness is just beginning: Hyperion’s mass is also small compared to its size, giving it a density about half that of liquid water. Its composition is mostly ice, which is fairly typical for Saturnian moons (like Enceladus), but the density is too low even for that. After all, most solids are more dense than water (ice being a rare exception), so Hyperion must have a lot of empty space. It not only looks spongy, it must be spongy somehow.

Understanding its composition is complicated by an apparent layer of darker material laid over the ice. Enceladus is one of the more reflective objects in the Solar System, but Hyperion is has a much lower albedo, indicating that some detritus (possibly from another moon Phoebe) is dusting it. A lot of this dark material has collected in the bottom of the craters, which don’t resemble impact basins found elsewhere. Cassini scientists think now that impacts compressed the spongy surface material, just like you can pack snow onto a snowball, whereas impacts on more solid moons send off shock waves and make shallower craters. Perhaps Hyperion itself is a fragment, the memento of an impact from the Solar System’s violent past.

Animation of Hyperion’s chaotic rotation, generated from still images. Click for more information.

Again unlike the Moon, Io, Europa, and Enceladus, Hyperion is not tidally-locked to Saturn. The combination of Saturn’s gravity, the pull from other moons orbiting Saturn, and Hyperion’s weird shape create a wild chaotic rotation: instead of presenting one face to the planet, it tumbles end-over-end, but in an unpredictable way. (This is the essence of chaos in physics: though every process involved is straightforward, but the combination leads to complexity.) If you could somehow stand on the surface of Hyperion, Saturn and the Sun would rise in different places every “day”, and at different times.

Though it may not be habitable in any sense, though it may not have oceans or volcanoes or an atmosphere, I love this icy spongy potato of a moon. We study our Solar System not just for the predictable and the sake of finding sameness, but to catalog its weirdness as well, in hopes that in learning its varieties we can know its full history. Besides, what harm is there in studying things for themselves, and wondering at the beautiful variety our universe holds?

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