The Safest Satellite

Calista Flockhart sticks in my mind. She used to play Ally McBeal, and most remarkably she’s the absolute double of a friend of mine who lived in Yorkshire when it was on. I used to commute to Leeds at the time and stay over at her house. But there are three other reasons Ms Flockhart has come to my attention intermittently. She was the “single female lawyer” of Futurama fame, impersonated by Turanga Leela with a stick-on googly eye. She was rather thin, and that used to worry me although I suppose I mustn’t “skinny-shame”. And finally, her name was similar to that of Jupiter’s outermost large moon, Callisto, illustrated above.

I’m sure the actor wasn’t named after the moon, but ultimately the Greek nymph. Once again, Kallisto, or more correctly Καλλιστώ, was one of Zeus’s “conquests”. I can’t help but think that some kind of “me too” moment should’ve come to pass in Olympus at some point, and I’m not really joking. Being a religious figure, Zeus was I imagine seen as a rôle model by many a Greek male, and seems to have spent most of his time raping and sexually harrassing people. Then again, maybe this was around anyway and merely served as an expression of that behaviour. However, just as Ganymede was Zeus’s homosexual lover, so was Kallisto, even though she was female. Zeus transformed himself into the likeness of Artemis to seduce her, meaning that they were lesbian lovers. So we have the two hetero moons and the two gay moons, which in a way is neat. Kallisto’s other claim to fame is that while pregnant she was thrown into the sky to escape the anger of the real Artemis, which stretched her tail and changed her into Ursa Major. ‘Αρκας, their son, became Ursa Minor.

Kallisto means “most beautiful”. When I learnt this, I suddenly realised that the Greek ending for the superlative, -ιστος, was cognate with its English and Germanic equivalent “-est”, although I don’t think you can do much with καλλος in that way. Anyway, I thought it was neat.

Callisto the moon is beautiful if you like that sort of thing. It’s somewhat similar to Ganymede but has an older surface, is a little smaller and is somewhat apart from the other Galileans, taking more than sixteen days to orbit, and therefore having a day more than two weeks long. Due to its separation, it doesn’t undergo the tidal stresses and strains of the others and therefore hasn’t had its surface remodelled at all since it formed. It’s both the most heavily cratered known body in the system and, at least when the Voyager probes visited, the least dense. It continues the trend of reducing density found among the Galileans. It’s also unique among them in orbiting outside the radiation belts, although it’s still within a fairly strong magnetic field. This is what makes it the “safest satellite”. Unlike the others, if humans ever went there landing on Callisto would be basically the same job as the Apollo astronauts did, and if anything there’d be less radiation because it’s five times as far from the Sun, although perhaps Jovian cosmic rays would still be a hazard.

It’s slightly smaller than Mercury, by about twenty kilometres, but still larger than Cynthia and Pluto. By mass, it’s the twelfth largest world in the system, being somewhat more massive than Cynthia and Io. It has the lowest gravity of the Galileans at around an eighth of Earth’s. There are so many craters that it’s hard for any more to fit on. Any new craters would probably overlap with old ones. This has happened because the surface froze before the Late Heavy Bombardment, so it retains a record of how violent the early Solar System was. Extremely, it seems. This also suggests strongly that Jupiter was almost like a second Sun at the time, although by Callisto’s distance, 1 882 700 kilometres away, it was well-frozen. However, an important influence on the inner moons is the tidal tugs on each other, which don’t affect Callisto, so that heating effect is absent. Nonetheless, Io’s density and complete absence of water does seem to indicate it was pretty hot that close.

The place nowadays all seems to be all about peace and serenity, which considering the onslaught it clearly received thousands of millions of years ago and the scars it still bears is pretty ironic. All the other moons have got something going on, Io most of all but the others show signs of activity fairly recently. Callisto doesn’t. It lacks anything like the regiones and sulci of Ganymede or the smooth surface of Europa, which implies that the latter underwent some melting after most of its meteorites hit. Of the four therefore, Callisto has the oldest surface. Nothing ever happens there, at least on the surface.

However, that doesn’t mean it’s boring! There are two gigantic impact basins, Valhalla and Asgard, the former of which is three hundred and sixty kilometres across at the centre and is surrounded, like Asgard, with rings, in this case up to eighteen hundred kilometres from the centre. It is in fact the largest impact basin in the system, comparable in appearance to Mare Orientale on Cynthia and Caloris Basin on Mercury. If the centre of Valhalla was in Glasgow, the outermost ring would cross Lithuania, southern Spain and Kalaalit Nunaat (Greenland), and this is on a moon with less than a seventh our planet’s diameter. On the moon itself it stretches across almost a quarter of the way round its world. The central crater is a palimpsest, a type of crater also common on Ganymede which has been partly eroded over time in one way or another. I personally imagine the cause in this case is that the impactor melted the surface, considering it’s mainly made of ice, but I don’t know what the experts think. The ringed area around it has outward facing slopes with steep escarpments, and although those sound like waves emanating from the impact they’re probably grabens – downward fractured areas like the equatorial rings on Vesta. Further out still, at the edge of the area, the rings are more vaguely defined and consist of troughs.

The other impact basin, Asgard, is a “mere” sixteen hundred kilometres in diameter, making it the size of Greenland/Kalaalit Nunaat. The centres of the two basins are about nine thousand kilometres apart. At its centre is the crater Doh, which has a large raised area at the centre. A third ringed structure is superimposed on it, called Utgard, which is slightly smaller than Adlinda, the third largest. There are also faculæ, which are frosty-looking spots dotted about, of which only one, Kol, seems to be named. The features on Callisto are named after mythological beings and items in Nordic and Inuit folklore.

The presence of the ringed basins on Callisto would be expected to lead to distinctive features on their antipodes, because the shape of the moon would focus the shockwaves on the other side as they travelled across the surface, but I haven’t heard that this is so, even though there are good-quality images of that side.

Considering the number of craters on Callisto, it’s unsurprising that there are also catenæ. These are chains of craters caused by objects breaking up before they reach the surface, which happens due to their size and also when they’re rubble piles, which many small objects are. There are at least eight of these. They occur elsewhere in the system, but are bound to be more common on this moon due to the extreme nature of the cratering. I first learnt the word “catena”, meaning chain, from this context, and eventually noticed the Castilian word «cadena». It may be worth answering the question at this point of why craters tend to be circular. After all, don’t they strike the surface of a body at various angles? If a hard projectile is thrown at a soft surface, it would only produce a round dent if it was perpendicular. The reason craters are circular is that it isn’t the mechanical impact of the object that causes the dent, but the heat and explosion of the energy release, so craters of this kind are more like bomb craters than the kind induced by a pebble hitting some mud. The catena above, Gomul, is actually within the rings of Valhalla.

Ganymede may have a complex interior consisting of alternating shells of ice of various kinds separated by water, and the similarity between the two moons might lead one to expect Callisto to have the same, but this doesn’t appear to be so. Instead, it probably looks like this:

As mentioned in the post about Ganymede, hexagonal ice is the kind we’re likely to encounter on Earth’s surface. The ocean is hundred and fifty to two hundred kilometres deep and since the moon is not geologically active, it has no thermal vents supplying it with energy. In any case, the ice is so thick there’s no chance of penetration. The rock portion at the centre is also even proportionately much smaller than Ganymede’s and there seems to be no magnetic field either. The interior also differs from Ganymede’s in containing a layer of ice VII. Surprisingly, ice VII is actually present on Earth inside diamonds. It can only form with a combination of high pressure and low temperature, so it proved to be a surprise that it was present on Earth, but on Callisto it’s to be expected. It’s fifty percent denser than our own ice and has a cubic crystal habit. This doesn’t mean it has cube-shaped crystals, but that the axes of symmetry are equal and at right angles to each other. Diamonds also have cubic symmetry, so in a way ice VII is like diamond, and it’s also extremely hard, being about as tough as quartz. Its melting point is always at least 82°C and can be above 400, so in many ways this is not like the ice we’re familiar with at all. The moon also gets steadily rockier towards the centre. The lack of activity means there is no magnetic field, which would be generated by currents in metallic liquid. This also means that unlike Ganymede there is no aurora, but there probably wouldn’t be anyway because it’s too far from the radiation belt.

There is an atmosphere, although it’s unsurprisingly extremely thin. It consists of carbon dioxide, and it’s a little surprising even that’s there because left alone it would leave the moon within a hundred hours due to its low escape velocity. It’s thought that there is dry ice slowly subliming from the surface, which also contributes to the smoothing out of the features seen, for instance, in the lower and gentler crater peaks. Ther’s also atomic hydrogen, which stretches higher up from the leading hemisphere.

The question arises here of whether Callisto is actually just a moon, unlike the other Galileans. The recent rival definition of planet requires it to be geologically active, and this is certainly true of Io in particular but also Europa and Ganymede. Callisto, however, is only active in that carbon dioxide seems to be gradually evaporating from its surface and it lacks any apparent internal or surface activity. Nothing much seems to have happened on its ancient surface for over four æons apart from the occasional meteor or comet strike: most of the craters are very old. Therefore, although I doubt anyone has ever considered the question, the body isn’t really a planet, but just a moon. In fact it may even be the largest moon that isn’t also a planet.

Out of all the bodies in the system, strangely Callisto may be one of the most hospitable to humans for exploration and settlement. The level of radiation on the surface is not only relatively low compared to the other Galileans, but actually lower than most of the inner planets and bodies in the asteroid belt except for Earth. This is because it’s over five times further out. It’s also more accessible than more distant moons, and is also fairly large. It’s larger than Cynthia and almost the same size as Mercury. Consequently, it has been considered as a potential target for astronaut visitation. As just mentioned, it’s extremely geologically stable, and there’s an ample source of water on the moon. It could also serve as a base for activities on the other Galileans and Jupiter, which is a good source of fuel for interstellar travel. In fact the moon itself provides this in the form of water ice, which could also be used as a source of oxygen for breathing. The interior, having water in liquid form, is also likely to be warm enough for habitation at some level. NASA carried out an investigation into the possibility in 2003 called HOPE – the Human Outer Planets Exploration – and suggested that it would be possible to reach Callisto by 2040. Of course this won’t happen but it’s nice to dream. I remember noticing that Nigel Calder included Callisto as a major power base in a simulation of Solar System power politics in his 1978 TV series ‘Spaceships Of The Mind’, although I’m surprised enough was known about it that far back to suggest such a situation.

Callisto doesn’t seem to crop up much in science fiction, possibly because not much happens there, but an exception is Asimov’s ‘The Callistan Menace’. This is a story about the mystery of astronauts attempting to visit the moon but never returning. I’m not going to spoil it, but its depiction of the place is quite inaccurate as it’s given a substantial atmosphere even though the author knew it couldn’t have one even back then. It’s also a bit unusual in referring to it as Callisto at a time when usual practice was to number the moons – Callisto is “Jupiter IV”.

Right, that’s it for Callisto. I’m not sure what to do next because Jupiter has something like eighty more moons but the Jovian system has already been covered. I might talk about the Galileans as a group, or I might move on to Saturn.

Planet Cueball

The first time I saw images of Jupiter’s moon Europa, it reminded me, for some reason, of a softball. I realise it looks a lot more like a cue ball than that, and I can’t explain why I got that association rather than the other. Because I was thinking of a relatively pristine object, it always makes me feel that it’s a bit worn out, scuffed, dirty and in particular scratched, and it makes me feel like I’ve got dusty hands like I’ve just picked up a mucky ball in dry but dirty conditions, as prevailed in our sports hall at school. I may be wrong about this, but my impression of Kent generally is that it’s rather dustier and sandier than the English Midlands, and that does make sense given its slightly warmer, drier climate. Over the channel it seems to become slightly more so, but I don’t know because it doesn’t seem like the difference is that big. The average annual temperature in Canterbury is 11°C and precipitation is 728 mm. Compare this to a place I don’t live (because I don’t want to doxx myself) but do live fairly near, Oakham is slightly drier at 716 mm precipitation annually and slightly cooler at 9.8°C, so in fact it seems not to be true.

But this post is not about the climate of East Kent but if anything, the climate of Jupiter’s moon Europa. Europa is in some ways very Earth-like in a way no other planet (see here for why I’m calling it that) is. It’s the smallest Galilean at 3 126 kilometres in diameter, which makes it slightly smaller than Cynthia. There are of course more than six dozen still smaller Jovian moons and if we could see Europa from the distance we see the lunar surface from, it would look about the same size, but would be four and a half times brighter and lacks the shadows our satellite has due to its flatter relief.

The “accident” of its naming opens it up to comparisons to the pretend continent with a similar name, and it’s also worth explaining why it has the same name, so let’s start with that. Europa the mythical, or possibly historical, figure was King Minos of Crete’s wife. There have been attempts to connect the name to the Akkadian word for “west”, ‘ereb, and that’s quite neat because it then allows Asia to be connected to a word for “east” and Afrika to a word for “south” (I think), but it may not work. It might also mean “wide face”, which is how it sounds in Greek. As usual for these stories, Zeus abducted or raped Europa, and this time he was in the form of a bull hiding in her father’s herds. This was commemorated as the constellation Taurus. The association with Europe is therefore somewhat surprising, but the way it worked was that it was initially applied to cis Balkan Thrace by the Greeks, then became the name of a Roman province including that area, which was then used to supplant the division which had emerged between the eastern and western Roman Empire. I have to say this explanation really feels like it has a lot missing from it. The element Europium is named after it, and just in passing I want to say that Europe is a fake continent. It’s actually just Eurasia’s biggest peninsula, and from that rejection, Asia is also a misleading name. There’s just Eurasia. That said, I regard myself as Northwestern European, while recognising that this doesn’t refer to my origins in a part of a continent but just as from that part of that peninsula. (This may be enlightening). This is the convoluted route whereby Europa came to refer to two such different things.

The surface of the roughly Cynthia-sized Europa is three times the size of the terrestrial region at thirty million square kilometres. This makes the planet’s surface twice the size of Antarctica. Another way of thinking of this is that Europa’s surface is equal in area to the combined area of Antarctica and the Arctic Ocean. We kind of have our own Europa right here, as well as our own Europe, but the Europa orbiting Jupiter is colder even than the South Pole in midwinter, at least on the solid surface, at a temperature of -160°C. The temperature at the equator varies daily between -141 and -187°C. The poles are actually warmer than the equator at night, and the north pole is warmer than the south at those times. This range of temperature happens to be the one (below freezing) where the properties of water ice change most.

Europa is very bright, having a surface of water ice, although it doesn’t reflect as much light as Enceladus as its surface is “dirtier”. Compared to the other Galileans, it’s composed much more like the inner planets, being mainly silicate rock with an iron core. The chief difference is that its surface is solid water ice with an ocean of salt water underneath. Back in a period referred to as the Cryogenian, Earth was in a somewhat similar state with a crust of ice covering a salty ocean over silicate rock and an iron core of course, although Earth is much larger than Europa and it had continents and oceans underneath the ice, unlike the moon, which is probably more homogenous. This was 700 million years ago, and is sometimes thought to have stimulated evolution enough to trigger the Cambrian Explosion.

It’s difficult to talk about Europa without talking about the possibility of life, so I’m going to break my self-imposed rule here and do that. It wasn’t initially clear whether the ice was simply frozen solid or covered a water ocean, but the latter appears to be so. Salt water can be detected by space probes because of its ions, which being charged behaves differently in terms of magnetism than fresh water. The surface, though mainly water ice, is also covered in sulphates and there is some sulphuric acid, but these may well be from Io’s volcanism. Like most moons, Europa faces the planet it orbits at all times, giving it a leading and a trailing hemisphere, and the sulphates, which include Epsom salts, and sulphuric acid are mainly deposited on the latter, indicating that it doesn’t come from the ocean but from Io, or it would be evenly distributed. The leading hemisphere, by contrast, has sodium chloride on its surface. This would lower the freezing point of the water, making it more likely that “life as we know it” could exist there. There is a “found footage” film, ‘Europa Report’, which takes pains with accuracy and depicts complex multicellular life in the ocean, and ‘2010’ also shows complex life there. The main difficulty as I see it is that although the situation isn’t as bad as on Io, the radiation belts are still significant, but I presume the ice provides shielding. As well as the other constituents, there’s dry ice and frozen hydrogen peroxide, the latter of which is thought to be formed by the radiation.

If there is life, it’s likely to derive its energy from deep-sea vents, as also happens on Earth, and like Io, the energy for this volcanism comes from the flexing of the crust and planet from tidal forces of Jupiter and the other Galileans. This is thought to be responsible for the cracks on the surface. Also like Io, Europa’s surface is almost devoid of craters, strongly suggesting that it was liquid more recently than Ganymede and particularly Callisto, the two outer Galileans. When the Voyagers visited, the encounter was relatively distant and the moon wasn’t mapped in as much detail as the others, so the knowledge and research done into the moon lagged behind that on the others. Three types of feature were identified: lineæ, which are the “cracks”, flexūs and maculæ. It was from “macula” used in this naming that I first learnt the Latin word for spot, as in “immaculate”. None of the features are very high or low and the surface is unusually smooth. There are currently forty-five named lineæ, formed when cracks appear in the surface and material seeps up from the interior to fill them, which then freezes. Salt is highest in the lineæ.

Europa takes three days and thirteen hours (plus a bit) to orbit Jupiter. Like most other moons its day lasts as long as its orbit. This period is significant because it’s almost exactly twice Io’s. Roughly every three and a half days, Io and Europa are within a quarter of a million kilometres of each other, making them larger than Cynthia in each other’s skies and this causes them to pull on each other, raising tides in their surfaces and elsewhere and heating each other independently of solar radiation. Perhaps surprisingly, although Europa is the least massive moon of the four Galileans, it has the second highest gravity at 0.134 g, somewhat lower than Cynthia’s. The next moon out, Ganymede, also the largest moon in the Solar System but I’ll come to that later, again has almost exactly double Europa’s period. The Darian calendar, originally designed for Mars, has been adapted for use with the Galileans.

The surface is covered in icy regolith, substantially broken down by the radiation, with grains about the same size as snowflakes, though presumably not so regularly formed. This means it would be possible to ski on Europa, although there are no real slopes. Also the radiation would quickly kill you unless you had really good shielding on your ski suit. Maybe one day. Incidentally, radiation shielding doesn’t have to consist of lead or some other heavy metal, and synthetics work quite well. That said, I don’t know how powerful the radiation is there. It’s weaker than on Io though, and unlike Io, Europa doesn’t have the flux tube. However, although it was long considered quiescent, it does have cryovolcanism. There are domes on its surface which may have volcanic origins and of course it seems to have actual volcanism, or rather volcanism like Earth’s, in the form of deep sea vents. The cracks in the surface, which rapidly freeze over, expose water which evaporates into the atmosphere like steam. And yes, it has an atmosphere, though even thinner than Io’s, but unlike Io’s the main constituent is oxygen. This is generated by the radiation splitting the steam and Europa’s gravity being insufficient to hang onto the hydrogen.

Finally, the Galileo probe was deliberately pushed into Jupiter’s atmosphere to destroy it because of its own discovery of a salt ocean on Europa, to protect any potential life which might exist there.

That’s Europa then. Next: Ganymede.

The Liver Of The Solar System

We’re only here because of Jupiter. That statement is true for a couple of reasons, but it’s no exaggeration.

In spite of appearance and activity, I am of course a herbalist of twenty-three years standing, and you don’t get to be a herbalist without treating the liver. This is absolutely not homeedandherbs, which is effectively moribund, but Jupiter is our liver as a star system. The liver has many functions, but one important one is to detoxify and store toxins. Liver herbs are also associated with Jupiter in the melothesic system. Jupiter the planet performs a similar rôle in absorbing and “detoxifying” asteroids and comets which would otherwise pelt the inner planets, by attracting them to it and often literally absorbing them.

In July 1994 CE, Comet Shoemaker-Levy 9 impacted Jupiter, leaving temporary “bruises” like this one. Jupiter is in any case a big target, making it prima facie more than thirteen hundred times as likely to be hit by débris than Earth even leaving aside its greater gravity and larger Hill Sphere. Considering that, Jupiter becomes more than six thousand times as likely a target.

Jupiter is also responsible for Earth’s formation in the first place. Jupiter’s year lasts 11.86 times as long as ours. This means there’s a potential orbit just within ours whose objects orbit the Sun once every 361 days. This is so close to ours that Earth actually dips into it for a short time each December. Jupiter cleared that orbit of dust and rocks 4 600 million years ago, just marginally to one side because its orbit was exactly twelve times as long and almost every dozen years the protoplanets there were slightly tugged by its gravity. This led to a crowded ring of matter which was to become Earth. Hence in a second sense we are only here because of Jupiter. There isn’t anything special about Earth in that respect either, although the orbits of the other planets don’t work out as exactly. Uranus is close though – its year is close to seven times as long as Jupiter’s. I haven’t checked this out but presume that the ratios are something like 2 in 7 or something less obvious. Earth is the largest terrestrial planet though, and has the most straightforward ratio. It should also be borne in mind that Earth’s gravitational pull may have done the same thing and that the orbits of some of the planets may not be fixed. I haven’t worked all of this out yet.

I may have quoted this too many times, but the Solar System has been described as consisting of “the Sun, Jupiter and assorted débris”. This is a little misleading as Jupiter only has a mass of about a thousandth that of the Sun and all the rest of the matter in the system taken together has a mass of forty percent of Jupiter’s, which is not negligible. In terms of size, there’s a star, a planet about a tenth of the star’s diameter, and the rest. As mentioned yesterday, the Jovian moon system is like a mini-star system in itself and the magnetosphere reaches out almost to Saturn, making it bigger than the entire inner Solar System. From here, Jupiter is usually just slightly too small to make out its disc, but is easy to spot as a very bright star in the night sky, which can sometimes cast visible shadows. Venus is the only other body orbiting the Sun alone which can do that.

Jupiter gives the impression of turbulent and frantic behaviour, like a boiling pot of multicoloured paint. Olaf Stapledon compared it to streaky bacon, although that doesn’t do justice to the colours. Probably in the absence of the opportunity to find out much else about it, Jupiter’s stripes have been meticulously labelled, thus:

The general shape of the planet can be seen in this diagram: Jupiter is notably flattened at the poles and bulges at the edges. This is also true of Earth but in our case the planet is more or less rigid except for the atmosphere and ocean, so it’s only three permille wider at the Equator than at the poles, something I discussed in ‘For The High Jump‘. Jupiter, being largely fluid, is six percent wider at the equator, which is two-thirds the diameter of Earth itself. Saturn is even more squashed. It’s like someone sat on it. Geddit?

Being fluid, the planet doesn’t rotate as a single object but consists of System I, System II and System III, meaning it doesn’t have a single fixed day. In fact no planet has but that’s another story to do with fixed stars versus the Sun. System I is the equatorial rotation up to 9° latitude, System II the polar, actually everything further from the equator than that and is five minutes slower and System III the rotation of the extremely active radio signals from the planet. Additionally there’s the Great Equatorial Current, which is faster at nine hours, fifty minutes and 34.6 seconds, according to an estimate made in 1897. This is over twelve kilometres a second, compared to Earth’s equatorial velocity of 463 metres per second. This is the kind of frenetic and torrid environment Jupiter is. The whole planet takes a bit under ten hours to rotate. It also does so practically upright. There are no seasons. The Jovian year lasts 94 425 days according to the equatorial current rotation, but this is not a definitive figure because Jupiter doesn’t have one definitive day. This differential rotation also means there’s a lot of turbulence in the atmosphere between different latitudes, because they’re rotating at different velocities.

The problem of conveying longitude encountered with the Sun, that of attempting to find a fixed point on an essentially fluid surface, is also present here. No less than six systems exist for doing this. They’re significant because of comparing observations made by the various different space probes sent there since 1973. There is a second similar problem with Jupiter: where’s the surface? This and the other issue are characteristic of gas giants. The problem here is that you might say Earth’s surface is the bit we stand on, especially if we’re Jesus, but on Jupiter there just is nothing to stand on and although at some point there is liquid and solid in the interior, conditions there are so extreme that there’s about as much point considering it the surface as the core of the Sun. Most people go for the visible cloud tops, but sometimes you can see further down into the atmosphere than that.

The belts are dark, the zones light. Zone is actually the Greek word for “belt”. There are diverse variations within the belts and zones, but before I get there I should mention the elephant in the room, the Great Red Spot. This is a large oval 22° south of the equator, varying in width and drifts westward, which is a pity as if it didn’t it could be used as a marker for longitude. It also oscillates north and south by around 1 800 kilometres over a cycle of almost ninety days. First observed in 1664 by Robert Hooke, famous for his microscopy, the GRS may or may not be a persistent feature. It actually isn’t permanent. For instance, it disappeared completely in about 1980. It also fluctuates in size somewhat, but has recently been 16 350 kilometres east-west. Its nature and the reason for its colour are still unknown. The earliest idea was that it was a giant active volcano, which was at the time when Jupiter was thought to be largely a solid body. I don’t understand why they thought it was, though, because its density is easy to measure given the movements of the Galilean satellites and it clearly was not a massive lump of rock.

After the volcano theory was rejected, it was suggested that it was a large ovoid object floating in the atmosphere and bobbing up and down, because it appears to change in colour and size. Better resolution and lenses seem to have led to the realisation that it was some kind of anticyclone, being in the southern hemisphere, but this isn’t really an explanation because its persistence, size, location and colour are all puzzling. Two suggestions are that it’s a Taylor Column and a Soliton (‘Star Trek’ fans may have heard of that). A Taylor Column occurs when a rotating fluid meets an obstruction. Drag then forms a cylindrical structure. This would clearly require some kind of body floating in the atmosphere, or possibly in the liquid below it, and moreover an extremely large one considering the enormous strength of the Jovian gravitational field. A soliton is a wave packet which stays bunched together as it moves and is able to collide with other waves without losing its form. It’s hypothetically possible that solitons made of gravity waves (or possibly gravitational waves) could be used to achieve warp drives, but this isn’t relevant to the Great Red Spot, which is a fluid phenomenon, although I imagine that’s why it cropped up in ‘Star Trek’ (TNG – ‘New Ground’). It was first knowingly observed in a Scottish canal in 1834. They’re a bit like sonic booms. Solitons are generated in front of fast moving vessels in canals or rivers because of the horizontal and vertical restriction in the water. They’re like wakes moving ahead of an object instead of behind it because they have nowhere else to go. Once again, the idea of the flow being restricted is a little strange because it suggests the presence of solid obstructions, but maybe it’s more to do with the currents or turbulence being particularly markèd at those points.

There is another fascinating and mysterious aspect to the Great Red Spot which I don’t think has ever been explained. It occurs at the same latitude as several other phenomena on other planets. Olympus Mons on Mars seems to be caused by a hot spot in the planet’s mantle and is 20° north of the equator, and Hawaiʻi, caused by a similar hot spot, is also 20° north of the Equator, although in the latter case this is obscured by the movement of the Pacific Plate. Mars also seems to have drifted because the possible remnants of former moons which impacted its surface are no longer at its equator, which they should be given its current moons’ locations. Also, both of these phenomena are north of the equators rather than south of it. I’ve seen a diagram attempting to explain this by inscribing a tetrahedron one of whose vertices was at a pole, but I don’t know how relevant that is. Neptune has also had a dark spot 23° north of its equator but this may not be the Great Dark Spot as discovered by Voyager. It’s difficult to know if this is cherry-picking.

The other mystery about the GRS is its colour, which varies. Nobody knows what causes this. I find that somewhat surprising because I’d expect its spectrum to reveal its composition, but apparently it doesn’t. One suggestion is that it’s due to tholins generated by the action of solar ultraviolet light on acetylene and ammonium hydrosulphide. Another factor may be the greater altitude of the area. It is of course something like twice the size of Earth’s surface area. I don’t know if anyone has tried to correlate its changes with the activity of the Sun. It’s also colder than its surroundings, which is to be expected considering it’s higher up. It’s also extremely noisy, to the extent that as the sound from it travels up into the upper atmosphere it gets converted to heat and the region above the spot is 1 330°C. This may not be as spectacular as it sounds though, because temperature and heat are different. Low Earth Orbit, for example, technically has a very high temperature but it’s still freezing up there in the shadows.

There are other more transient spots, probably hurricanes, in the atmosphere, but weather systems in general are much longer-lasting in Jupiter’s atmosphere than ours because there’s no friction from a solid surface and also little variation due to the absence of land and liquid regions. Also, because the planet is so much bigger, so are the storms and other winds. Hurricanes often last decades. This raises the question of what weather would be like on a water world. If the figures relating to Jupiter’s axial tilt and surface are fed into a climate model for Earth, the result is a banded arrangement with persistent hurricanes, suggesting that conditions on such a planet, which might otherwise be habitable, could be quite hostile, and the weather conditions at particular latitudes would effectively constitute the climate because they’d be so stable.

Hydrogen and helium make up the bulk of the planet’s atmosphere, and therefore also the bulk of the planet itself, in similar proportions to the Universe in general and also the Sun. It managed to hang on to them because it’s colder on the outside and has such a high escape velocity. In 1939, the South Temperate Zone suffered a disturbance leading to the formation of a single white oval from four merging predecessors in 2000, which started to turn red in 2005. This time scale gives a good indication of how stable the weather is there. It’s also fascinating how Jupiter’s sheer size gives it a known history stretching back into Stuart times, which isn’t true of other planets except for Cynthia and Earth. Features on Mars were well-recognised but the occurrence of storms wasn’t observed until the nineteenth century, and Venus is just blank. This also underlines how dynamic the planet is compared to most others. The Jovian troposphere is somewhat like ours in terms of physical structure, with a falling temperature and pressure with height extending through clouds and leading up to a reversal and gradual increase of temperature marking the lower boundary of the stratosphere, then a mesosphere and thermosphere where the temperature is technically very high, but the chemical composition of the atmosphere is very different. It’s 90% hydrogen, 4.5% helium and has a significant amount of deuterium in it, though well under one percent. This compares to the one in six thousand atoms of hydrogen in Earth’s water. Deuterium also shows up in the compounds replacing the more abundant hydrogen. Methane, ammonia, water vapour, acetylene and phosphine are all present, as is carbon monoxide, but the really surprising constituent, though only present at less than one part per million, is the rather strangely named germane. Germane is like methane but has germanium instead of carbon in its molecules. Like many of the constituents of the Jovian atmosphere, germane would spontaneously ignite in our own. I don’t understand why there’s germane there. Germanium is not a particularly common element and its silicon analogue silane might be expected to be more widespread but it isn’t there. Germane is also denser than methane or silane, so its presence in detectable layers of the atmosphere is peculiar. I don’t think it’s found on any other planet. Incidentally, the presence of phosphine may not be a clue for life existing on Jupiter because the planet’s chemistry is not like that of Venus and conditions are very different. Here, it’s probably formed under high pressure much further down and churned up by convection currents. Methane is no surprise, and the carbon monoxide is probably the result of oxygen being relatively scarce in the original part of the solar nebula from which the planet formed.

You know that bit in ‘Fly Me To The Moon’? “Let me see what spring is like on Jupiter and Mars”? Well, whereas there are seasons on Mars, there are none on Jupiter, so it ain’t gonna happen. This is because Jupiter’s axial tilt is only 3°, so it basically has no seasons, although the butterfly effect might come into play. I suspect this is for two reasons. Firstly, Jupiter is the original planet in this system, so it probably determined the positions of the orbits, and secondly it’s so massive nothing could knock it off-kilter, so it ends up with a tiny tilt. In fact it’s surprising it tilts at all. If it did have seasons, each would last almost three years. Some people draw a link between the traditional Chinese cycle of twelve animals and the Western Zodiac because the planet spents around a year in each sign from our perspective. It should be pointed out that the strict 30° division of the ecliptic used in Western astrology doesn’t correspond to the actual portions of the zodiacal constellations in the ecliptic, and as is practically common knowledge nowadays, Ophiuchus is also in the circle and is ignored for astrological purposes. In the astronomical zodiac, Jupiter is currently in Aquarius but I don’t know how closely this corresponds to the astrological ephemeris, and it’s about to be the Year Of The Tiger. The orbit around the Sun is thrice more eccentric than ours at almost five percent, so there is a little variation in how much radiation and therefore heat Jupiter gets from the Sun.

However, Jupiter actually generates twice as much radiation as it receives, so there’s another reason it has no seasons: it’s actually warm itself, or in fact hot. This is because it’s still hot from the formation of the Solar System, since it has more than a thousand times the volume of Earth but only about 130 times our surface area, and possibly because it’s still contracting, although the contraction may be caused by the cooling rather than the other way round. At the core, the temperature is 20 000 K or higher, more than three times as hot as the Sun’s photosphere and almost as hot as solar flares, with an internal pressure of forty-five million times that of our atmosphere. There are two rival theories about the centre of the planet. One holds that there is no core in the sense of a solid rocky globe, but the planet just gets denser and denser towards the centre, and the other, more popular theory posits the existence of an Earth-sized rocky core. Somewhat away from the centre is a deep layer of liquid metallic hydrogen. Under very high pressures, various gases, such as oxygen and xenon, become metals. This may constitute up to 80% of Jupiter’s radius, and is responsible for generating the enormous magnetic field. The pressure here is a “mere” three million atmospheres and the temperature 11 000 K, so it’s still hotter than the Sun’s surface. Above this layer is molecular liquid hydrogen, twenty-five thousand kilometres below the clouds. The temperature finally drops below that of the Sun three thousand kilometres below the “surface”, where the pressure is ninety kilobars. A thousand kilometres down, the hydrogen becomes gaseous and the temperature is only around 2 000 K, then it falls to -143°C at the cloud tops. The magnetic field generated by the metallic hydrogen is about ten times the strength of Earth’s at this level, but it’s at an angle of almost 11° to the axis of rotation. All of this pressure stuff is exacerbated by the fact that Jupiter’s gravity is over two and a half times ours.

Jupiter has jet streams like Earth’s, but because of the coloured clouds, the white ones being mainly frozen ammonia, they’re more vividly colour-coded than ours. A jet stream is a relatively narrow, fast, horizontally undulating air current moving east to west and drifting north and south assuming the planet spins prograde. They’re formed by the Sun heating the atmosphere. There are four such streams on Earth, two subtropical and two polar. On Jupiter they’re driven by internal heating. Moving through the latitudes, there are alternating regions of faster and slower east-west winds, each of which is a jet stream, even though models show fewer jet streams on larger planets. Each stream is also “rolling”, in that it is a kind of horizontal whirlwind separated from its neighbours north and south.

Zones have more ammonia than belts, hence their paler appearance – they’re cloudier. The belt clouds are lower and thinner, and belts are warmer than zones. This makes sense if you think of ammonia condensing or freezing out of the atmosphere. I get the impression on looking at pictures of Jupiter that the belts look lower and possibly have shadows cast upon them by the clouds in the zones. Air seems to be warmed and rises in the zones, causing clouds to form as it expands and cools. In the belts, it sinks, becoming warmer and losing its clouds. The air flow generally tends to “stay in lane”. It doesn’t deviate in latitude much except within its belt or zone. At the poles, there are large circles in which not much seems to be happening. These caps can extend further towards the equator or less so, and the northernmost two bands after the north polar region can become incorporated into them temporarily. This extends to the NNTB (North North Temperate Belt), which can fade entirely, as it did in 1924. Consequently the NTZ varies in width. South of that, the NTB often has dark spots on its southern edge. The North Tropical Zone, NTrZ, is where the System I movement of the atmosphere comes uncoupled from the more polar System II. This leads me to ponder whether the planet consists of a series of nested hollow cylinders, such that the temperate regions north and south are in fact continuous but hidden under the more equatorial regions. They wouldn’t be homogenous in properties of course because the conditions deeper in the atmosphere are bound to be very different. Also, the liquid hydrogen ocean is not that far beneath the cloud tops.

The largest region on Jupiter is the EZ, or Equatorial Zone, with an area about an eighth that of the whole planet. That’s eight thousand million square kilometres, making it the largest visible feature in the entire Solar System. It’s something like six or seven times the entire surface area of all the inner planets taken together. While I’m at it, Earth mapped onto Jupiter would be the size of India on a map of Earth. There are many features in the EZ compared to most of the rest of the planet. For instance, it often shows plumes from its northern edge projecting southwest. A narrow belt appears occasionally at the equator itself. The southern side includes a “dent” where the Great Red Spot begins. The GRS itself is a feature dominating the South Tropical Zone, and this raises the question of why it’s in the southern hemisphere without any corresponding feature in the north. then again, the bands are not symmetrical either side of the equator either. The only thing I can think of right now is the very slight tilt of the planet combined with its greater orbital eccentricity creates slightly different conditions in the northern and southern hemispheres.

The planet emits decametric radio waves. This is of the order of thirty megahertz but they peak at seven to eight megahertz, so it’s close to the analogue VHF band used for FM radio on Earth, though the frequency is slightly lower. There are amateur radio projects monitoring Jupiter’s radio transmissions, which were discovered in 1955. Since they’re stronger in some parts of the planet than others, they provide fixed points enabling longitude and a “true” rotation period to be determined, but they aren’t associated with any visible features. They’re also polarised, like visible light passing through the plastic in front of a flatscreen monitor – they vibrate only at a fixed angle. This is due to Jupiter’s magnetic field and the charged particles moving within it. One of the moons, Io, influences the radio transmissions but I’ll talk about that more when I get to it. The decametric transmissions occur in short bursts sporadically. They last between a few minutes and several hours.

There are also decimetric waves, and these are continual and don’t have peaks at particular frequencies within that wave band, which is in the UHF range now used by mobile phones and previously by analogue PAL TV. They’re differently polarised and emitted from the volume around Jupiter. They’re synchrotron radiation, caused by charged particles moving in curves somewhat like the centrifugal effect, and show there are electrons moving almost at the speed of light. From Earth’s perspective this radiation fluctuates up and down according to whether we’re facing the planet’s magnetic equator or not.

This image is a painting made for Carl Sagan’s 1980 TV series ‘Cosmos’ and will be removed on request. As well as providing a fairly accurate image of what the planet looks like at cloud top level, it also illustrates Sagan’s speculations regarding life there. Although I am restraining myself from commenting on life elsewhere in the Solar System, the image without the organisms is still interesting. There are diffuse crystals of ammonia in the blue sky creating a halo around the rather smaller-looking Sun. A vortex can be seen towering over the scene to the left, with a bank of white clouds to the right, and there are a number of smaller vortices visible. Then there are long, almost straight clouds winding off into the distance, and of course on Jupiter the horizon would be several times further away than on Earth at around fifteen kilometres, although the clouds make it difficult to judge. Sagan proposed three ecological niches of organisms. There are “sinkers”, aerial phytoplankton which survive by photosynthesis and gradually sink into the depths of the planet, reproducing as they go until conditions kill them, “floaters”, somewhat jellyfish-like and balloon-like floating herbivores several kilometres across who can be seen in this image, and “hunters”, one of which can be seen at bottom right, who have a kind of retro, 1930s quality to them but look a little like Art Deco biplanes with round heads and sharp projections at the front. Asimov and Arthur C Clarke both believed that Jupiter was actually even more suitable for life than Earth, although the former’s belief was based on an earlier model of the planet which posited a vast, deep ocean beneath the clouds.

Right, so that’s Jupiter. I’ll probably do Io next.

The Jovian System

I started this series of posts with a survey of our Solar System itself and I’m going to do the same with Jupiter and its moons. When Steve suggested this project, he also suggested working outward from the Sun. The problems with doing this become very evident once one gets to Jupiter, although they were already there with the asteroid belt.

Just with the asteroid belt, I mentioned that although the average distances from the Sun can be organised into bands according to their ratio to Jupiter’s “year” (the official name is “sidereal period”), this isn’t evident at any one moment because many of their orbits are markèdly elliptical and an asteroid in, say, the Hilda group near the outer edge of the belt may well approach the Sun at its closest at the average distance of a Flora asteroid near the inner. Vesta and Ceres seem to approach each other to within four million kilometres, and this will sometimes happen, but lines drawn between each closest approach (perihelion) and the Sun are different lines and the tilt of their orbits also differs, so it isn’t like the system is a flat surface with all the orbits in a plane with their ellipses lined up precisely, or even approximately.

When it comes to Jupiter, a separate problem begins to become evident. All four of the gas giants have extensive satellite systems, and these moons orbit at various distances from the planets, and therefore from the Sun. A moon which is closest to the Sun at one time will be the furthest from it at another, and some of them even regularly swap orbits. It’s actually worth considering this in detail because of what it illustrates about the nature of the systems in general. It’s not much of an exaggeration to say that each of the four planets and their moons is like a mini-solar system in its own right. Perhaps unexpectedly, the system with the most moons is Saturn’s, not Jupiter’s, even though Jupiter is larger, more massive and closer to the asteroid belt. However, for today I’ll mainly be considering the Jovian system rather than the others.

Just before I get going on that, there are “rogue planets”, which in a sense are technically not planets at all, wandering through interstellar space independently of specific stars. These may well have their own satellite systems, and are in a sense “failed stars” because they’re too small to shine, but may even so be several times Jupiter’s mass. Jupiter is therefore in a sense almost our second local “solar” system. Incidentally, there seems to be a gap between the largest planets and the smallest stars, in that the former are much less massive than the other, and there’s also a gap in the sizes of the two types of body because planets tend not to get much bigger than Jupiter in diameter. Above that point, the gravity increases and compresses the substance of the planet more, although there are also examples of planets so close to their suns and therefore hot that they become “puffy planets” which are far larger but also much less dense than Jupiter.

I’ll start with Sinope. Sinope is the most distant moon of Jupiter, and has a surprisingly long astronomical history. It was discovered in 1914 and although it’s quite small, no moon has since been discovered which orbits further out, in spite of today’s space telescopes, the several space missions sent to and through the Jovian system and the discovery of other moons which have turned out to be much smaller, so it was quite an achievement to do that over a century ago. Sinope orbits an average of 24 371 650 kilometres from Jupiter, which is a figure more precisely known today than before. Its eccentricity is, however, considerable, at 0.3366550, meaning that its maximum distance from Jupiter is around 32 576 000 kilometres, which is only a sixth greater than the gap between the orbits of Venus and Mercury at its own aphelion (greatest distance from the Sun). The diameter of that orbit is therefore almost 49 million kilometres, which is comparable to the distances between the orbits of all the inner planets.

Sinope is important because it can be thought of as marking some kind of outer limit to the Jovian system. If we could see that orbit in the night sky it would look larger than the Sun to us. Since it’s further away from us, this means the Jovian system is also literally much larger than the Sun. Sinope takes over two years to orbit Jupiter. There is a large asteroid in the belt named Hilda, whose diameter is 170 kilometres and has an aphelion of 678 million kilometres. Sinope, assuming it to be orbiting in the same plane, takes on average 24 371 650 kilometres off Jupiter’s distance from the Sun, meaning it will be somewhere around 38 million kilometres from Hilda, perhaps less (or perhaps more). Hence Jupiter’s outer moons are actually not that far from the outer asteroid belt. On the other side, Sinope adds the same distance to Jupiter’s orbit and Saturn’s outermost known moon can be taken into consideration, taking it out to 841 million kilometres from the Sun, and Saturn’s apparent counterpart, the as-yet unnamed S2004 S26, approaches the Sun to within 1326 million kilometres, leaving a gap of just under 485 million kilometres. The gap between the two systems is quite small.

Incidentally, another moon, Pasiphaë, is slightly further in but also more eccentric than Sinope, so it can sometimes get even further out.

The magnetosphere also needs to be taken into consideration. Jupiter has a strong magnetic field which starts to interact with the Sun far in front of the position of the planet itself, and also trails behind it in a tail longer than the sunward side. This amounts to eighty radii of the planet to the bow shock, which is the surface where the speed of the solar wind suddenly drops in response to Jupiter’s magnetic field, and is named after the wave in front of the bow of a ship. The bow shock also extends “above” and “below” Jupiter’s orbit by about the same distance, making it the biggest “bump” in the system. The shock is located about six million miles inward of the planet, which is within the satellite system. However, the magnetotail is another matter. The bow shock is actually compressed by the solar wind, so the magnetotail is much, much larger. The entire magnetosphere is somewhat similar to a teardrop shape viewed in cross section perpendicular to the orbit, and the magnetotail is a gradually tapering part away from the Sun. Magnetotails generally are much larger than the magnetic objects associated with them and in Jupiter’s is around 489 million kilometres long, which is almost as far as Saturn and also means that the outermost moons of that planet actually pass through Jupiter’s magnetotail at times, and that the magnetospheres probably touch sometimes. Strictly speaking, magnetic fields have infinite range but after a while it gets silly.

Like Earth’s Van Allen belts of Apollo mission fame, further in towards the planet Jupiter traps charged particles, which are unfortunately where three of the four largest moons orbit. There is also a plasma tunnel, but this will be made clearer on a later date.

Jupiter has eighty moons. Sixty are less than ten kilometres across. I tend to think of both Jupiter and Saturn as like archipelagos of islands with a few large islands and multitudes of smaller ones. In Jupiter’s case, the moons are grouped into orbital zones with large gaps between them. I’ve already talked about Amalthea, one of the inner moons, and I’m not planning to plod through a massive long list of mostly tiny, boring and very similar moons, but they’re collectively of interest and the way they’re grouped is also significant.

The Galileans are the “big four”. Each of them is practically a planet in its own right, and they were also the first moons to be discovered orbiting another planet, by Galileo in 1609. Another astronomer, Marius, found them just one day later and he’s responsible for the names. These are also the first celestial bodies to be given names in written history. However, the Chinese astronomer 甘德 discovered either Ganymede or Callisto in 364 BCE, because they are bright enough to be visible to the naked eye of someone with good vision. All of them are brighter than Vega from here. The Galileans form an important rung on the ladder of establishing the scale of the system and Kepler’s laws of planetary motion. When they’re relatively nearby, that is, when Earth and Jupiter are on the same side of the Sun, it’s fairly easy to look through a telescope and time their movements, as in, the points when they’re furthest from Jupiter, when they pass behind and in front of the planet and emerge on the other sides, a total of two dozen events. Their relative distances can be measured using this observation because of their maximum visual distance from Jupiter, and this enables it to be observed that, like the planets with the Sun, the cube of their average distance is directly proportional to the square of the time taken to orbit, Kepler’s third law of planetary motion. Then, when Jupiter is on the other side of the Sun from us, there’s a delay in these observations of up to almost exactly a thousand seconds, which enables the width of our orbit to be calculated if one knows the speed of light. This in turn enables the scale of the orbits all observable planets in the Solar System to be calculated, and the difference between the periods of Jupiter’s Galilean moons and a hypothetical planet orbiting an object the mass of the Sun enables the mass of Jupiter compared to the Sun to be worked out as well. Working out the speed of light itself is a somewhat different problem. I’ve tried to do this but was stymied by fog. You need a clear day, a hill, a cogwheel, a mirror and a distant telescope.

The moons are organised into six groups. There are the inner moons, which include Amalthea, the Galileans Io, Europa, Ganymede and Callisto, and the Himalia, Ananke, Carme and Pasiphaë groups. These occur in bunches of orbits, but before I get to that I want to point out something else which is rarely mentioned: they changed the names of many of the moons in 1975. When I was a small child, before Pioneer 10 and 11 had been sent there, the names of the moons were completely different. This would’ve been in about 1972. By the early 1980s, the names of the outer moons had completely changed. The previous names were as follows:

• VI – Hestia
• X – Demeter
• VII – Hera
• XII – Adrastea
• XI – Pan
• VIII – Poseidon
• IX – Hades

The corresponding names now, in order, are: Himalia, Lysithea, Elara, Ananke, Carme, Pasiphaë and Sinope. Many more moons have been discovered since then. It’s all the more confusing because one of the inner moons is now named Adrastea. The scheme I was familiar with was apparently the 1955 proposal, which was used after a phase during which they were simply referred to by their Roman numerals, listed in order of discovery. There were also proposals in 1962 and 1973, and once again Adrastea is used, this time to refer to Himalia. The current names are the 1975 IAU version, and there is also Carl Sagan’s 1976 version. Nowadays, the moon names ending in E are retrograde – they orbit in the opposite direction from the majority of bodies in the Solar System – and prograde moons have names ending in A. There was also a tendency to choose names from the lovers of Zeus or Jupiter in Greek or Roman mythology, of which there are a very large number, so the supply was clearly considered almost inexhaustible. The view was also taken that irregular moons shouldn’t be named at all but just left with Roman numerals. Now that eighty moons are known, I suspect they’ve finally run out of lovers. The question arises in my mind of why there are no homosexual lovers since homophobia didn’t exist in the Greco-Roman world before the arrival of Christianity, but I think this is because one of the reasons Jupiter and Zeus had so many is so they could serve as the origin story for various beings seen as a mixture of the qualities of the two parents. There’s also an inconsistent tendency for the moons to be given names across the systems which start with the same letter, such as Hestia and Himalia, and Poseidon, Persephone and Pasiphaë. Up until the 1970s, there seemed little point in naming them since at that time they were simply rocks spinning round Jupiter without much being known about them, although Isaac Asimov does refer to them in his ‘Lucky Starr And The Moons Of Jupiter’, though by the numerals rather than the name.

The inner satellites are all small, but Amalthea is the biggest satellite after the Galileans. Himalia is only slightly smaller although it isn’t an inner satellite. I’ve never really got used to using the newer names by the way. There are four small inner moons. Metis is actually technically too close to hold together, which is appropriate since it’s named after a titaness who turned herself into a fly and was eaten by Zeus. Incidentally, if I’d written the sequel to ‘Replicas’ it would’ve included a planet called Metis as an important plot point, but sadly it was not to be. The real Metis is on the brink of being devoured by Jupiter and is also only ten kilometres across. The three other moons were discovered via the Voyager probes in 1979 and not named for quite some time after. The spacing of their orbits is similar in scale to that of the Galileans. Amalthea may have associated moonlets but they’re not confirmed, the “flashes” only having been detected once.

After the Galileans there’s a big gap, and to some extent Jupiter’s system reflects the shape of the Solar System here in that there are four smaller inner moons like the four smaller inner planets followed by four much larger moons like the gas giants, but unlike the Solar System Ganymede, the largest moon of all, and in fact the largest moon in the entire Solar System, is the third large body rather than the first, and there doesn’t seem to be anything corresponding to the asteroid belt. The pattern of distribution of moon sizes may be a guide to how other star systems form and the Galilean orbits are in harmony with each other. Callisto is somewhat separated from the others, making it easier to spot and reflecting something like the Bode-Titius Series with the spacing of the planets. However, after Callisto comes a big gap. There is one small moon, Themisto, discovered in 1975, orbiting about halfway across that gap, but wasn’t observed for long enough for its orbit to be established. It was lost for a quarter of a century, and none of the probes investigated it. It’s fairly common for small Solar System bodies to be lost and later found again.

The next bunch, of seven moons, includes the incredible Leda, which is absolutely tiny for a moon discovered and confirmed from Earth observations in the 1970s. It’s turned out to be somewhat bigger than originally thought, and was discovered by the extremely prolific “discoverer” Charles Kowal who also observed Themisto, in 1974. Kowal also discovered the centaur Chiron. This set of moons is tilted at 30° to the inner group and has more elliptical orbits, all of which line up with each other. These are between eleven and thirteen million kilometres from Jupiter.

There is then another gap, within which orbit Carpo and Valetudo, closer to the third group rather than orbiting in isolation like Themisto. Unlike the outer group, however, they orbit in the same direction as the inner moons. Valetudo is only one kilometre in diameter, like several other moons, making it joint smallest, although there will presumably be some differences in size. It’s also currently the smallest named moon. I don’t know if they’re going to bother naming the others of this size, but the asteroid Adonis was named and is only five hundred metres across, although it’s also a potentially hazardous asteroid so that may be why it got one.

The outermost group orbits backwards compared to the others and in fact compared to most other bodies in the Solar System, which generally orbit clockwise viewed from the South. Hence they all have names ending in E: Carme, Ananke, Pasiphaë and Sinopë, which apparently is supposed to have a diæresis over the E. Incidentally there’s a village in this county called Sinope and also a town in Turkey, probably named after the nymph in the latter case, and no longer spelt that way. By the time you get to the outermost group, the orbits are considerably perturbed by the Sun. There’s a concept called the Hill Sphere, which is the sphere within which a body’s gravitational influence is stronger than any others, generally a planet and its star. Jupiter’s is fifty-five million kilometres in diameter, so the outermost group of moons are close to its edge. The ellipses of their orbits are also lined up, but currently at close to right angles to the middle group.

Although Jupiter’s Hill Sphere is not as large as Neptune’s, which is the furthest known large planet from the Sun and so has more elbow room despite its much smaller mass, Jupiter is more likely to sweep bodies up into its. This is because it only takes a dozen years to orbit the Sun compared to Neptune’s more than a gross, and is doing so much faster and in a more crowded region of the system.

The Solar System has jokingly been described as consisting of the Sun, Jupiter and assorted débris. Jupiter, although far less massive than the Sun, has around two and a half times the mass of all the other known bodies in the system put together.

There are many more things to say about Jupiter and its moons, but these will be about the planet and the bodies themselves, so for now I’m going to knock this on the head and publish it.