Saturn’s third largest moon was already known to be peculiar long before any spacecraft reached it. This was because its brightness seemed to vary so much. It was noted that it was six times brighter when east of Saturn than when west of it, and it emerged that the moon had one black hemisphere and one white one, in 1671 by Cassini when he discovered it. It’s the outermost of the four Sidera Lodoicea. After it the only substantial moon is Phœbe.
It’s also Saturn’s third largest moon, but is considerably less dense than Rhea or Titan at only 9% greater than water. You might also notice that in those two photos there is a triangular projection at the equator. This is the profile of the ring that goes around the world at that latitude. And of course there is a large crater in the light hemisphere with a central peak and a smaller one near the south pole. The moon as a whole, like most apparently spherical worlds in the system, is in fact slightly wider across the equator than between the poles even ignoring the ridge, with a deviation of 4% from perfect sphericality compared to Earth’s 0.3%
Although I’ve said the hemisphere is black, and it’s certainly very dark, but in fact practically nothing is truly black of course, and in fact it’s a very dark red. A fairly evident question to ask is whether it’s a light moon with a dark coating, a dark moon with a light coating or just made up of two different substances. Its symmetry suggests very much that the latter is not so and that the chances are dark matter (not that kind) was deposited on the leading hemisphere from somewhere else. In particular it can be seen to coat the rims of craters at the edge of the hemisphere on one side and the floors on the other. However, there is also a ring of material on the trailing hemisphere, which is harder to explain with the hypothesis that it’s being deposited from an extraneous source. It’s been suggested that it’s from Phœbe, but that’s a very small moon and never comes closer than seven million kilometres to Iapetus. The composition is also very different. There seem to be no craters at all on the dark side at first, but in fact this is due to the colour making it difficult to see them. The situation, in fact, is rather similar to that on Cynthia, with all the maria on the inboard hemisphere but a few patches on the outboard. In the case of Earth’s moon, the material of the maria was extruded from the interior, kind of – they’re lava fields. However, on Iapetus the distinction is between leading and trailing hemispheres rather than the one facing Saturn and the one facing away, which is why it’s so noticeable from Earth. The colour of the material is closest to that of Callisto. The arrangement actually is similar to the Yin-Yang symbol:
The dark side of the moon is something like ten times darker than the bright side. It’s also organic and contains nitriles, similar to some of the compounds found in Titan’s atmosphere. The spectrum is similar to that of the outer belt D-type asteroids, some of which may originate in the Kuiper Belt around Pluto and beyond. It’s a few decametres thick in some areas, possibly more elsewhere. It kind of reminds me of some kind of bacterial culture growing across the moon like it’s a petri dish but I wouldn’t suggest for a moment that it’s actually alive. What may have happened is that water ice may be slowly evaporating from the surface, leaving behind a residue. The temperature range on Iapetus is the widest in the Saturn system because of its unusual orbit. The moon is more tilted than the other large moons and takes seventy-nine days to orbit, which means its day also lasts that long, leading to daytime temperatures warmer than anywhere else and night time temperatures colder. During the day, the temperature gets as high as -144°C in the dark area and -160°C in the light. It seems, then, that nobody really knows what causes it.
This is a closeup of the equatorial ridge, which amounts to a mountain range. This is up to twenty kilometres high, in other words higher than Everest, twenty across and 1 300 kilometres long, and is thought to be a collapsed ring. Iapetus is thought to have had a ring like Saturn’s, though much less spectacular of course, which ended up falling onto the surface many millions of years ago. It isn’t the only ridge on a Saturnian moon. Pan, Atlas and possibly Daphnis also have them, but all of these are very small and close to the rings. It means, basically, that there is a tendency for moons circling Saturn to be ringed, though not just with the discrete particles which may be present around Rhea but with equatorial ridges which are the remnants of crashed rings. These have never been found outside Saturn’s system and may result from ring material from Saturn itself landing on the surface, or it may be that it had its own moon which broke up and dropped to the surface in bits. There are a few more things to say about the ridge on Iapetus. One is that it’s only on the dark side, but there are partial rings so that may still make sense. Another is that it may or may not have roots. Earth’s mountains extend deep into the crust because they would otherwise not be balanced – they’re floating on the mantle and therefore are a bit like icebergs except that only half of them is below the surface. It isn’t clear whether it’s also true of this ridge. It’s also notable that Iapetus has a weird orbit with a high tilt and a lot of space between it and Hyperion, which would enable it to build up a large undisturbed ring system. It would be possible to see the curvature of the moon from on top of the highest parts of the ridge.
Although it isn’t dramatic, Iapetus has a more tilted orbit than the other large moons of Saturn at about fifteen degrees to Saturn’s equator. This makes it the only moon with a proper view of the planet’s rings, but unfortunately because it’s also so far out, Saturn and its rings are quite small in the sky, so you can’t win really. It seems like this pattern would be repeated throughout the Universe. Ringed planets are likely to have close moons whose view of the rings is poor because they’re edge on, and further moons which aren’t but in whose skies their planets will be relatively small and unspectacular. However, this does raise the question of where the ring material came from, since Iapetus is nowhere near the rings and doesn’t orbit in the same plane of even quite a sparse ring. However, there is a very sparse ring around the orbit of Phœbe which extends into the orbit of Iapetus, and if kicked up from Phœbe will be orbiting in the opposite direction from Iapetus, whose orbit is normal in that respect. This means Iapetus would encounter the material coming much faster the other way. Moons with such tilted orbits are usually small and lumpy, and it’s worth comparing Iapetus with Neptune’s moon Triton. Triton also has a highly tilted orbit but moves backwards compared to most other moons and has a lot in common with Pluto, so it’s probably a captured planet rather than a moon which formed in Neptune’s vicinity. The same does not appear to be true of Iapetus because it’s relatively small and orbits in the same direction as the other large moons. It’s tempting to imagine that the same mishap caused the ring to collapse onto the surface, gave it a dark and light side and caused its orbit to tilt, but what that was, if it happened at all, who knows?
Landslides on Iapetus are common and tend to move slowly over long distances. This is not unusual and also occurs on Mars, Venus and elsewhere. These are called sturzstroms and may be partly extended by the particles skating across the ice, and they may in any case be quite finely-grained. It’s like the material is a liquid spilling out and flooding over a flat plain, and it comes to mind that this is what happened with the lunar maria, so the comparison with Iapetus might extend quite far. The height they fall from is a twentieth or less of the distance they end up travelling, and they happen in smaller craters. They leave behind gaps in the rim walls, where the rim is still there but the slope leading up to it has slid down, leaving a steep edge on the inner side. This even happens on the equatorial ridge, except for the “inner side” bit because it hasn’t got one. They’re the largest landslides in the Solar System on any icy body. I have to admit to being rather puzzled about this right now because Iapetus seems like quite a quiet, uneventful place. Whatever the cause, what seems to be happening is that the friction among the particles and between them and the base heats them up and softens them, which lubricates the movement somewhat. The shape of these landslides is similar to our own oceans’.
The moon is only one-fifth rock and is likely to be solid all the way through. There often seems to be a situation with icy moons (and possibly also the likes of Pluto) where the density is likely to increase somewhat as non-icy meteorites impact them, and it’s complicated to consider the question of whether this happens faster with smaller moons than big ones. On the one hand, each meteorite makes a bigger difference to a small body, but on the other it’s a smaller target. This might also apply to the relative size of craters. The surface of Iapetus is mainly water and dry ice, iron and iron oxide. Presumably this is true of the lighter hemisphere and the layer below the dark red matter on the dark side. The size of particles on the light side create Rayleigh scattering like blue eyes and skies, and this results in a multicoloured surface, although it’s quite unsaturated. In other words, the light side glitters like diamond. This could also mean there’s a little Cherenkov radiation, which is where subatomic particles exceeding the local speed of light in a medium stimulate the emission of blue-white light like a sonic boom. There is also more dry ice the further into the trailing hemisphere you go. On the dark side, there is nanoparticle hæmatite and iron. I find this somewhat odd because it seems to mean there are two different kinds of red material on that side, so does this mean there is a tendency for solid matter to be red? It kind of makes sense that there is in the case of mixtures, because a variety of different compounds are more likely to be different colours and mixing them together leads to the “brown splodge” effect you get with paints and inks. Brown, of course, is somewhat red. However, this raises the question of why mixtures tend towards the red end of the spectrum. All that said, the fact remains that there are two types of red substance on the dark side, only one of which is a mixture. Whatever it is, all the dark material in Saturn’s system, including the darker rings, seems to be made of the same stuff as it’s all spectrally similar.
The prominent basin (or crater) in the southern hemisphere on the light side is called Engelier. It’s five hundred kilometres wide. However, there’s an even larger one on the dark side called Turgis, almost half of the moon’s diameter at 480 kilometres. This is odd because that makes Turgis one of the largest craters in the Solar System but it’s far from the most prominent, probably because of the dark material over it. The crater is a site of landslides, also found in the smaller superimposed Malun crater. Turgis interrupts the ridge, which only occurs on the dark side, and there’s a small stretch on the other side of the crater. Engelier is “on top of” a faintly outlined crater called Gerin.
The light side is divided into two “terræ”: Roncevaux in the north and Saragossa in the south. The entire moon has about the same surface area as Australia, so if each “land” is half the light hemisphere they will have a surface area about three-quarters the size of Kalaalit Nunaat (Greenland), Earth’s largest island. They also share with that island the feature of being covered in white water ice. Saragossa’s most distinctive feature is Engelier. Both of them cross their respective poles, so again they resemble the terrestrial island in being partly polar. The division between Saragossa and Roncevaux could be thought of as where the ridge would be.
I’d like to finish by addressing a topic I probably should’ve started with: the name Iapetus. I pronounce it /i’apətəs/, but recognise that most Anglophones say it with a long, diphthong I at the start: “eye-A-pettus”. Arthur C Clarke’s ‘2001 A Space Odyssey’ originally envisaged the second monument to be situated there, but spelt it “Japetus”. This is an older spelling, dating from the time before I and J were separate letters. Iapetus is also, like Tethys, the name of a prehistoric ocean on Earth, circumpolar in the Southern Hemisphere separating landmasses which are today on opposite sides of the Atlantic and is therefore named, like the Atlantic Ocean, after a titan. The Iapetus Ocean was present from the late Precambrian until the Silurian.
Iapetus the titan seems to be linked to Japheth the son of Noah. The name means “piercer” and his sons were seen as the ancestors of humanity, with their worst qualities exemplifying those of human beings, so there seems to be a definite link between the myth as recorded in Torah and in Greek myth.
Next time (but one): Phœbe – moon with a fiddly letter in its name.
A Box Of Chocolates 