“Lots Of Planets Have A North”

. . . but not all! Hyperion’s no planet of course, but its situation could apply as much to a planet as it does to this moon.

Hyperion is the next largish moon out from Saturn after the big one, and is in a way a pair with Mimas. Mimas is the smallest world in the system which is roughly spherical. By contrast, Hyperion is the largest object in the system which isn’t. It has quite a distinctive appearance besides this, in that its craters are oddly deep for their diameter, giving the impression of being like coral or pumice, or maybe chimneys or organ pipes, and in fact it is like pumice in that it’s unusually porous, so this may be more than coincidence. If it were a small object, say a decimetre or so across at its largest width, I can imagine holding it in my hand and finding it to be very light for its size. If I licked said object, I would expect it to try to suck my tongue in with capillary action. It just looks very odd. Kind of delicate and easily crushed.

In fact Hyperion is bloody huge! Not perhaps by the standards of spheroidal bodies elsewhere in the Solar System, but considered as an object in its own right. It’s 360 x 205 x 266 kilometres in size, and was the first known decidedly non-round moon, discovered in 1848 CE. Hence a box containing Hyperion would have a volume of nineteen and two-thirds million cubic kilometres. Its extreme ends are as far apart as Glasgow and Nottingham or Cork and Donegal. Not huge on a global scale by any means, but still massive. Enough to cast a shadow over most of Ireland. It’s the kind of size which would constitute a reasonable and fairly arduous road trip which you’d need a toilet break from. Also, that largest axis is actually quite close to the diameter of Mimas, which like any moon or planet is not perfectly round. The least diameter of that moon is only twenty kilometres greater than thisses greatest. Hyperion is so nearly round. It’s a runner-up in the sphericality stakes, and you can see that from its rather ovoid shape. Its gravity has proven to be enough to smooth it out but not quite enough to finish the job and make it round.

When I was a teenager I used to think of Hyperion as the largest possible size for a cylindrical space station. It’s special in that way because once an artificial object exceeds its dimensions, its design becomes at least somewhat constrained by the force of gravity to being made approximately spherical. A cylindrical space habitat could exist which was 360 kilometres in length and 205 kilometres in diameter, giving it a habitable internal surface the size of Laos, about which I thought I’d blogged at some point but apparently didn’t. Maybe I should. The surface area of Hyperion itself is rather imponderable because not only is it irregular but it also has many craters and is very porous. Nearly half of it is empty space, more or less, meaning that its real volume is quite a bit smaller than it seems. It doesn’t just look like a sponge. However, this is a common or even universal characteristic for small irregular bodies in the system and is also found with, for instance, Phobos and Deimos. Its shape also means that it has three times the gravity at its narrowest diameter than at its most elongated locations, although that gravity is still quite low regardless of whereabouts on the surface you are. It’s only 54% as dense as water, sharing that low density with Saturn itself and a number of other local moons. Like Saturn, it would float on water but unlike Saturn it would actually stand a chance of finding a body of water large enough to float on.

Getting back to the title, “lots of planets have a north”. That is, on the whole planets and moons in the Solar System, and presumably beyond, rotate around a single axis, wobbling only slightly over a long period of time compared to the length of their day. Most or all of the moons I’ve been into in any depth on here have captured rotation, where they always present one face to their planet but still have day and night because they orbit that planet without facing the Sun at all times. Titan, for example, has a day about two weeks long, but above its haze Saturn hangs in the same place in its sky at all times, or is invisible due to being below the horizon. The Sun, though, rises in the east and sets in the west like on most other planets, meaning that as you stand on the surface at the equinox with the setting Sun to your left, you are facing north. Titan, like many other places, has a north. However, the next “large” moon out from Saturn hasn’t. Every time the Sun rises and sets on Hyperion, it does so in a different place from the previous day, chaotically. Therefore, Hyperion has no north or south. There is no way, based on either magnetic polarity or rotation, that a map of this moon could be oriented, and it tumbles through its orbit with no simple pattern.

Hyperion occupies an intermediate position in moons’ relationships with their planets. Moons closer to Saturn, including Titan but all the others, have captured rotation. Of moons further away, Iapetus at least also has captured rotation. However, Phœbe, which is still further out, has its own rotation period. There seems to be a set of circumstances which leads certain bodies not to have compass directions. It isn’t clear what they are because Iapetus once again shows the same face to Saturn at all times. What, then, is going on with Hyperion’s rotation, and can these circumstances happen to planets? Are there planets without a “north” too?

One possibility for Hyperion’s peculiar shape is that it’s a chip off the old block, that is, a remnant of a much larger but shattered moon. It’s another of those bodies, like Vesta, with a large feature which almost makes it a vignette for it. In this case it’s a crater-like ellipse occupying one entire side of the moon, although it seems to have no name. It has a rim and a central peak like a conventional crater but is itself so heavily cratered it no longer really counts as one itself. Personally, I wonder if this impact was in some way connected to its formation, and that there was some kind of “proto-Hyperion” which was destroyed by that very impact, but I can’t work out the dynamics of such an event so maybe not. The moon does have a latitude and longitude system though, which is hard to understand because it doesn’t have an axis of rotation or a magnetic field. I’m guessing that an arbitrary feature was chosen, possibly the central peak of the area surrounded by Bond-Lassell Dorsum, which is the rim of the apparent large crater. The other features are labelled with latitude and longitude even though this has little meaning, so basically the compass directions have been chosen at random for the sake of convenience so far as I can tell.

The moon’s orbit has a fairly high eccentricity for a fairly large moon at 0.1, i.e. its distance from Saturn varies by about ten percent. It also orbits once every three weeks compared to Titan’s fortnight, meaning that Titan is likely to have a gravitational influence on it, and keeps its orbit from becoming more circular. Just as the probability that Enceladus would solidify and become a quiet moon is low, so is the probability that Hyperion would rotate conventionally. Even very slight influences on its movement push it into states where it won’t spin on an axis. I would expect this to be partly linked to its shape. The real oddity is not so much that it’s in this intermediate state as that the next large moon out, Iapetus, does still have captured rotation despite the increased distance from Saturn. Hyperion takes thirteen days to return approximately to its previous orientation, which is close to Titan’s period, but this may not be simply related.

As well as consisting mainly of water ice and empty space, the moon probably also contains frozen methane and dry ice. Being covered in a dark substance, it’s possible that heat from sunlight has caused some of this to evaporate and contribute to the porosity. Impacts on its surface probably crunch through to a considerable depth and throw débris free of the moon, hence the single central peak and dorsum, which suggests to me that they were formed when Hyperion was part of a larger moon. The reddish colour of the dark material possibly responsible for this heating is similar to that on Iapetus, which I will shortly cover. It’s also concentrated in the bottoms of the craters, so it isn’t immediately apparent that the moon averages as dark as it does.

The composition of the moon is likely to be the same all the way through due to its low gravity. If it formed part of a larger body in the past, it might be expected to show traces of stratification, but it’s also a rubble pile and very porous, so the chances are it would be jumbled up by that calamity in the same way as Cynthia probably formed from Earth’s disrupted outer layers, although it that case the stronger gravity would have sorted the fragments.

The name “Hyperion” is very popular and applied to many different things in the wider world. It’s the name of a series of SF novels by Dan Simmons, a classical record label and an investment company. The original Hyperion is, unsurprisingly, a titan in Greek mythology and the name literally means “the one that goes on high”, and is therefore associated with the Sun. One of the craters on the moon is named Helios. Keats abandoned a poem on the titan. There is a possibly projected tale that Hyperion was the first person to understand the movements of the Sun and Cynthia and their effects on the seasons. If there was such a person at any point, Hyperion would be an appropriate name for them.

That, then, is Hyperion. The next moon is one whose reputation precedes it and was noticed as having a very distinctive appearance long before any spacecraft visited it: Iapetus.