Mimas is special. In fact, I hope all worlds described in this series are special, but to me, Mimas is special. I’m no fan of ‘Star Wars’, so I note in passing that it looks a bit like the Death Star but may not go too much further in commenting on that. Or I might.
The Death Star is 160 kilometres in diameter, and apparently (I know little of the franchise) was replaced by another one 200 kilometres across. If that’s so, the scale on the above picture is approximately correct because Mimas has a diameter of 396 kilometres. That’s slightly smaller than mainland Scotland, although obviously its surface area is greater.
Do I need to say it’s a moon of Saturn?
Mimas is a kind of landmark in the Solar System, and in fact in terms of size generally. It’s the smallest body which has achieved approximately spherical shape by means of hydrostatic equilibrium. Most or all bodies smaller than Mimas are far from being round, and most or all bodies bigger than it are round. However, this isn’t inevitable, for two reasons. One is that objects smaller than Mimas can still just happen to be round, and considering the huge number of smaller objects there are, I wouldn’t be surprised if some of them just happened to turn out to be round by chance. It’s also possible that a larger body than Mimas could turn out to be irregular in shape due either to having lower density or being made of stronger materials. Vesta, for instance, is larger than Mimas but isn’t anywhere near as round. Taking this the other way, there can also be smaller but denser or weaker bodies which are round for that reason. An extreme example would be a neutron star, which would be only ten kilometres in diameter but would be so perfectly round it would act as a mirror. Another factor which might or might not come into consideration is surface tension. If an object made of liquid water were able to hold together in a vacuum, the chances are it could be a tiny fraction of the size of Mimas and still be spheroidal. Hence I can ask a question I don’t know the answer to: is the surface tension of molten lava sufficient to make a body spherical at a much smaller size than Mimas?
A few bits of maths can be done with this moon before actually considering anything else about it other than its rough shape and size. It has a surface area of 493 650 km2, slightly smaller than Spain, although the climate is somewhat cooler and there’s no rain at all, on the plain or otherwise. Its volume is 32.6 million cubic kilometres, which makes it less than a five hundredth the volume of Cynthia or five times the volume of all the water on this planet. And it is in fact substantially made of water ice, at a cold enough temperature that it will have contracted from the volume it would’ve been at freezing point.
Before the Voyager probes, nobody had any idea that Mimas looked like the Death Star, and since they got there in 1980 CE and ‘Star Wars’ started in 1977, there couldn’t have been any conscious inspiration, but it does make me wonder if these things sometimes happen in other ways, but I imagine this is not the kind of thing which comes up much in astronomy. Pioneer 11, with its poor camera, had approached the 400 km satellite to within 104 263 kilometres, too distant to pick out any details, even the absolutely bloody massive one of the enormous crater Herschel which is the first thing anyone notices about it. However, Voyager 1 didn’t get much closer than Pioneer 11 at 88 400 kilometres, and I don’t know about now but at the time only about half the moon was seen.
Herschel is a third the diameter of Mimas itself, with walls five kilometres high and a central peak six kilometres in height and twenty by thirty kilometres across. To scale on Earth, it would be wider than Canada. It’s centred on the equator, which makes me wonder if that’s significant. None of the other craters on the surface are more than fifty kilometres across. There’s also a distinctive distribution of craters, where more than an entire hemisphere only has small craters less than twenty kilometres in diameter and the other hemisphere, which of course includes Herschel, has larger craters. There are also valleys.
The name can be pronounced either “My mass”, which is what I say or “Me mass”, which is closer to the classical pronunciation. The way I pronounce the names of astronomical bodies reflects a time before I knew much about the way Greek and Latin were spoken and therefore I often say them as if they were English words. The adjective, a little surprisingly, is “Mimantean”, like “Atlas” and “Atlantean.” Mimas the mythological figure was the son of Gaia and born from the blood of Uranus’s castration. I’m not quite sure how they could be both.
Herschel is of course mainly flat, meaning that the horizon from anywhere on its surface is further away than the horizon at eye level on Earth. From the central peak it would be even further. This probably means that of any spheroidal body in the system, the central peak of Mimas is the record-breaking location for seeing the maximum portion of any world at something like a twentieth of the moon’s surface. From the rim, it’s possible to see all the way across the crater, a distance of up to thirty kilometres, but also, because the rim is raised five kilometres above the “geoid”, it’s possible to see the rim from more than twenty kilometres away. To some extent it’s mysterious that the moon managed to hold together at all from the impact which formed the crater. Although there are much larger impact basins elsewhere, only the one on Mimas has a practically flat surface because of the small size and low gravity of the moon.
The way Mimas moves suggests that it contains a liquid ocean, but some scientists consider this unlikely because the moon is so small one would normally expect it to be frozen solid, so it isn’t known for sure if this is so. It’s more likely that the reason is either that the core is not spherical, not at the centre, or that HersUnlike many other places, Mimas has no ray craters. These are craters whose rims have lines radiating out from them such as Tycho, as can be seen clearly on Cynthia. This is thought to be due to the extreme brightness of the surface, which reflects 96% of the light falling upon it, thought to be due to it being covered in the kind of frost found on Earth. However, there are chasms around ten kilometres wide, around one to two kilometres deep and up to ninety kilometres long. I would imagine these are cracks caused by the impact, and on another Saturnian moon, which I will cover in future, it’s thought that an impact broke the entire moon apart and it fell back together again. I would expect the same to have happened to Mimas, although its lower gravity might have stopped this from happening.
Mimas is at least sixty percent water ice. What isn’t is probably due to impact by non-icy meteorites becoming embedded and gradually sinking into the interior.
It has exactly half the orbital period of the more distant moon Tethys and orbits twice for every three orbits of the moon Pandora, which is a shepherd moon. It’s also responsible for the Cassini Division.
The only other thing I can think of is that the map of the moon’s surface temperature looks like Pacman.
Because I just spent two posts not talking about the Solar System, for reasons I hope make sense, tomorrow’s post will be about one of the most interesting moons of all: Enceladus.
A Box Of Chocolates 