“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.

The Saturnian System

(this is effectively a poster, if you want to download it, but it uses a lot of black ink).

Saturn and its moons are the second example of a mini-solar system within the big one. For thousands of years, Saturn was thought to be the outer limit of the Solar System, and has its own associations because of that, but for today I want to concentrate on the whole system of Saturn, with moons, rings and magnetosphere all included, rather than the planet itself.

Saturn has a prodigious number of moons, the count sometimes exceeding Jupiter’s. This is because of the Titius-Bode series. As you go further out, the orbits of the planets get more widely separated, meaning that a planet of the same mass has a longer gravitational reach over its surroundings. Saturn is of course considerably less massive than Jupiter, but its Hill Sphere, the region where its gravity is dominant, is bigger than Jupiter’s, at 1025 radii compared to Jupiter’s 687. Working this out in kilometres, Jupiter’s has a diameter of 96 million kilometres and Saturn’s is 119 million. Against this is the fact that the system is less cluttered out by Saturn than it is near Jupiter, with the asteroid belt being near the larger planet. Saturn has eighty-three moons not including the ones which form part of the rings, compared to Jupiter’s eighty. There was a point when Saturn’s moon count far exceeded Jupiter’s, but this seems to be over. The Hill Spheres are nowhere near each other and there is no competition between the two in this way. Unlike the magnetospheres.

When Voyager 2 was on its way to Saturn, it encountered Jupiter’s magnetotail in February 1981, which may indicate that the tail is forked. It did so again in May that year by which time it was nine-tenths of the way there, or around eighty million kilometres from Saturn. Saturn can even be within Jupiter’s magnetotail at times. As far as Saturn’s magnetosphere is concerned, all its moons out to Titan orbit entirely within it. Titan itself is very close to the edge and passes in and out of it, spending about a fifth of its time within. It’s surrounded by a doughnut of hydrogen extending inwards to Rhea, which is the second-largest moon. The bow shock is somewhat further out and extends north and south of the planet for at least thirty radii. Sunward it extends for almost two million kilometres. This means that of the large moons, only Iapetus and Phoebe orbit outside it entirely. As well as the neutral hydrogen torus around the orbit of Titan, there’s an inner torus of rarefied plasma of ionised hydrogen and oxygen, which effectively means protons and oxygen ions, whose outer diameter is about 400 000 kilometres. At the edge of this torus the temperature is over 400 million degrees C, but it should be born in mind that Earth’s thermosphere is 2 500°C and the Sun’s atmosphere is over a million Kelvin, which is hot but didn’t destroy the probe recently sent there. Temperature really represents the average kinetic energy of the particles and not heat. In a sauna, the air temperature can be over 100°C but the effect on the human body is nowhere near as harsh as boiling water for this reason.

Titan comprises 96% of the mass of all Saturn’s moons put together. This seems actually to be more typical than Jupiter with its four large moons, as similar mass distributions are found among the moons of Uranus and Neptune. The whole system has a kind of quietness and serenity to it, at least from afar. Some of the moons are active, but there’s nothing like the hot volcanism found on Io. All the moons are substantially icy. Saturn’s moons are unique in that some of them have trojans – moons which share their orbits but are sixty degrees behind or ahead of the larger moons. Saturn in general has quite a cluttered and ice-strewn neighbourhood in connection with its rings, and this seems to be part of this aspect of it. This means that the exact number of moons can never be determined because the size of bodies orbiting it goes all the way down, fairly evenly, to miscroscopic grains of ice and dust. In a way, all that can be said is that Titan is the biggest by far, being about the same size as Ganymede.

The five large inner moons, Mimas, Enceladus, Tethys, Dione and Rhea, all participate in the magnetosphere, absorbing protons, as do the particles making up the very sparse E ring. I’ll talk about the rings in detail when I get to Saturn itself, but another unique feature of Saturn’s system is the interaction between the particularly substantial rings and the magnetosphere. The other giant planets have much less substantial rings and therefore less significant interactions. Electrons are absorbed by the main rings, and below the main rings towards Saturn is the least radioactive region of the entire Solar System outside of large bodies and their atmospheres because the rings act as a radiation shield. There is, however, nothing as strong as the plasma tunnels and torus around Io, which influences radio transmissions from Jupiter.

Radio signals from Saturn are weaker than the ones from Jupiter in a broad range from twenty kilohertz to one megahertz, so listening to long or medium wave radio stations there would be right out. Like Jupiter’s System III, which is the common rotation of the interior of the planet with its magnetosphere, Saturn has its own System III, lasting ten hours, 34 minutes and two dozen seconds. There is nothing as strong as Io’s influence, but there is a relatively mild variation corresponding to the time taken for Dione to orbit, 2.7 days. This could be coincidence. When Saturn passes close to Jupiter’s magnetotail, the radio transmissions become undetectable but it isn’t clear whether they cease because of it or are just overwhelmed by Jovian radio noise.

The moons have fairly regularly spaced orbits out to Rhea, although there are some smaller moons which either share orbits with larger moons or regularly swap over. Titan, though, is over twice as far from Saturn as Rhea, then Hyperion is relatively close to Titan, Iapetus over twice as far from Saturn as Hyperion, and finally Phœbe is much further out and orbits backwards compared to the others and the majority of other worlds in the Solar System. This suggests that Phœbe is a captured asteroid. Surprisingly, although it was discovered in 1898, no moons further out were found until the twenty-first century despite the fact that the planet was visited several times by spacecraft. However, almost four dozen moons have now been found which orbit backwards. More than two dozen moons have yet to receive names because there are just so many of them. Even the most distant moon is well within Saturn’s Hill sphere, so it’s still possible that there are more. There’s also a cluster of moons, including shepherd moons and coörbitals, near the rings and possibly even within them, but it should be borne in mind that there’s a judgement call here regarding how big a ring particle is before it counts as a moon or moonlet.

Saturn, and therefore its system to some extent, is tilted 27° with respect to its orbit. This also tilts some of the moons but others are already at odd angles and it’s fairly meaningless to regard them as influenced by this tilt. For Dermott’s Law, mentioned in connection with the Galileans a couple of days ago, T=0.462 days and C=1.59.

I’m going to end on a personal note. I don’t remember Kepler’s third law of planetary motion very clearly, so I always use Saturn to work it out. Saturn is about ten AU from the Sun, i.e. ten times Earth’s distance. The cube of this is a thousand, and that’s square root is thirty. Saturn takes thirty years to orbit the Sun once, hence the Saturn Return of astrology, meaning that the cube of the semimajor axis (average distance from the Sun) of a planet is directly proportional to the square of its sidereal period (“year”).

Next time I’ll be looking at Saturn itself, including its rings, the famous hexagon and the unexpected connection with a certain comedian.