Mega-Moon

No, King John did not sign the Magna Carta here. Buddy Holly is not alive and well here. Christmas has never been celebrated here. Nor is this in Surrey. That’s Runnymede. Incidentally, Buddy Holly doesn’t live there either, although Christmas has definitely been celebrated in it. However, this is Ganymede, the largest moon in the Solar System and therefore the largest Galilean. It’s larger than Mercury and Pluto, but smaller than Mars. That said, it’s only 45% of Mercury’s mass.

To explain the rest, ‘1066 And All That’ claims that King John signed the Magna Carta on Ganymede. This opens up the possibility of a weirdly transposed version of English or British history where all the stuff that went on in our Middle Ages could be copied and used to tell the tale of a British version of the entire Solar System, perhaps with Jupiter as its capital. That makes me wonder where Scotland is. Ganymede also turns up in the rather startlingly entitled ‘Buddy Holly Is Alive And Well On Ganymede’, a novel which recounts the tale of one Oliver Vale, conceived at the moment of Buddy Holly’s death, who thirty years later finds that all the TV stations in the world have their signal hijacked by a broadcast of a rather bemused Buddy Holly on Ganymede who knows nothing of his situation except that there’s a TV camera pointing at him and a sign next to it reading

For assistance, contact
Oliver Vale, 10146 Southwest 163rd Street, Topeka, Kansas, U.S.A.

It’s actually quite a good book.

As for Xmas, Isaac Asimov wrote a 1940 CE story called ‘Christmas On Ganymede’ about a mining company on said moon whose employees hear about Christmas and proceed to go on strike until visited by Santa in his flying sleigh pulled by reindeer. This version of Ganymede is much denser than the real one and has an almost-breathable atmosphere containing oxygen. I suspect Asimov already knew Ganymede wasn’t like that.

I’ve always called it “Ganymeed”, but it’s supposed to be pronounced “Ganymeedee”, which is how Buddy Holly says it in the book so it must be true, but I think both are acceptable pronunciations. Ganymede, or Ganymedes, himself, was a Trojan prince abducted by Zeus to serve as a cup-bearer to the Olympians. This means Ganymede was Zeus’s sexual partner in a pederastic setting, so the situation is mixed. On the one hand, we have a moon acknowledging homosexuality, but on the other current values place him as a victim in the same way as Io and Europa are of Zeus’s insatiable lust. He’s the basis of Aquarius, but the constellation Crater has nothing to do with either. I don’t know why Ganymede was the name given to the largest moon and I’m now wondering if Kepler or Marius, who named them in 1614, was secretly gay.

The next largest moon is Saturn’s Titan, which is also larger than Mercury. This makes it the ninth largest known object in the system. It’s the tenth largest by mass, again just ahead of Titan and giving it a larger surface area than Eurasia by quite a margin, and slightly larger than the Atlantic. It also contains an internal ocean with more water than exists on Earth. It takes four times as long to orbit Jupiter as Io does, and twice as long as Europa, so once again it’s in orbital harmony with other Galileans. It’s also the most massive moon, which puts it in a slightly odd position as its surface gravity is lower than Io’s or Europa’s, because it continues the trend of decreasing density with distance from Jupiter. It gets closest to Europa, at 400 000 kilometres, just over the distance between Earth and Cynthia. Callisto is somewhat apart from the others. Io and Europa taken together are less massive, but the imbalance between a single large moon and several or many smaller ones whose combined mass is less doesn’t apply in the Jovian system. In a way, Ganymede is the Jupiter of Jovian moons. It also, perhaps surprisingly, has the lowest escape velocity of all the Galileans, meaning that it won’t be able to hold onto anything like a proper atmosphere, or even the kind of atmosphere the inner two have. Like those though, it orbits within the radiation belts. Until the outer planets and moons were more thoroughly explored in the 1980s and more recently, it wasn’t clear out of the three moons of Ganymede, Titan and Triton which one was the biggest.

The moon was big enough for large Earth-bound telescopes to make out at least one of its surface features, Galileo Regio. The rather vaguely named regiones are Galileo, Marius, Perrine, Barnard and Nicholson, and are the dark patches. They also have sulci across them, of which there are over a dozen. Unlike the two inner Galileans, Ganymede’s surface has not been extensively reworked due to tidal forces and it therefore has a fair number of craters, though not as many as somewhere like Mercury. It’s the brightest of all the moons in our sky other than Cynthia, although it’s dimmer per unit area than Europa, because it’s also the largest. To some extent it resembles Cynthia, as the darker regiones are like the maria (seas) and there are also craters, but the broad sulci are not found on the lunar surface. Due to the surface being largely ice and at this temperature being softer than rock as we know it, it isn’t as craggy either, although it’s not as smooth as Europa. The gravity being lower might contribute to this. The maximum elevation is found among the sulci, which reach about seven hundred metres above the surface.

Galileo Regio is the size of Antarctica. It covers a third of the hemisphere facing away from Jupiter. Putting this into perspective, this means that as far as Earth is concerned, our continents and oceans would mainly be visible from Jupiter with a good telescope, although Australia might not be, and Jupiter is almost our neighbour in cosmic terms. All we’d be able to do from that distance is discern that the continents and oceans existed and were differently-coloured from each other. The most distinctive feature of the moon, and let’s once again affirm that by a more recent view than the 2006 IAU definition Ganymede is also a planet as much as Pluto is, is its grooved surface and the stripe-like features where they’re bundled together. These lighter sulci are newer than the dark regiones. It and Earth are the only known bodies which have lateral faulting, that is, where a fracture in the ground leads to the surfaces sliding along the fault rather than subsiding or rising. These sulci divide the terrain into polygonal blocks, the regiones, up to a thousand kilometres across. Although the moon is not currently active and drifting doesn’t occur, it has done in the past and this arrangement of plates separated by lateral fault zones is similar to Earth’s continental plates, making Ganymede the only other world in the system which has this kind of arrangement. Not even Venus, which is geologically quite like Earth in many ways, has this feature.

The crust is somewhat weak and can’t stand heavy weights upon it, and is underlaid by a much deeper layer of water. This leads to “drowned” looking craters which look quite similar to the ones on the lunar surface which became flooded with lava in æons past, but unlike them their origins are not associated with flows of liquid but mere collapse into the surface due to the weakness of the material. Most craters are on the regiones because they’re older. About half of the bulk of the planet (let’s call a spade a spade now we’re allowed to again) is ice and half is rock, although it isn’t clear how this is distributed. It does have a very large rocky core under the deep oceans, and also its own magnetic field, practically guaranteeing an iron-rich centre like Earth’s. Although one way of looking at the interior is as a frozen-over deep ocean of salt water over an ocean bed, it could equally well be described as a planet where ice and water replace our rocks and magma, with a mantle of water rather than molten rock. However, because the gravity is so low there, the pressure at the bottom of the ocean/mantle wouldn’t be excessive. As well as ice, there is clay mixed in with the crust, and there may also be ammonia ice. There’s also more dry ice at the poles, and as with several of the other moons the leading and trailing hemispheres of the planet have different surface compositions, probably because of Io again as the trailing side has more frozen sulphur dioxide. I have to admit that I don’t understand why these moons have deposits from Io on the trailing hemisphere rather than the leading one because it seems to me that they’d be entering a cloud of the stuff, which would then land on the “front” of the moon.

The crust is eight hundred kilometres deep and contains the kind of ice we’re familiar with here along with, as I’ve said, clay, but may also contain bubbles of the same kind of oxygen as we have in our atmosphere. Above it is, for the same reasons as on Europa, an extremely thin atmosphere of oxygen and ozone, and I’m guessing the ozone is formed by Jovian radiation in the same way as an electrical spark forms ozone here. This is a small fraction of a nanobar in pressure. Deep in the crust are large clusters of rock, which might either be piles at the bottom due to its inability to support their weight or embedded in the crust due to its ability to support it! There is then what may be a further hundred kilometres of water, ten times deeper than our Marianas Trench. This is salty, as can be seen by the way aurora behaves on the planet, influenced by the magnetic behaviour of the brine. The fact that there is so much water seems appropriate for a planet named after Zeus’s water-carrier. Ganymede is the only moon in the system with a magnetic field.

Beneath the ocean lies a layer which makes the existence of life as we know it unlikely. Although the lower gravity reduces the pressure, the ocean is so deep that it manages to compress the water back into ice, but of a different kind than we would come across here: tetragonal ice. There is more than one kind of tetragonal ice, and this one is referred to as “ice VI”. The ice we encounter close at hand on Earth’s surface is hexagonal, as can be seen for example in hexagonally-symmetrical snowflakes and the hexagonal columns which form in frost and elsewhere. Ganymede’s lower layer of ice is heavier than water and more akin to the normal behaviour of freezing materials than our ice, because it contracts on freezing. Its crystals consist of elongated cuboids composed each of ten water molecules. Depending on the pressure, its melting point can be as high as 82°C or as low as 0.16°C and it’s 31% denser than water. This kind of ice turns up in the interior of some icy moons. In Ganymede’s case it seems to rule out the existence of thermal vents which could provide energy for life, as it probably does elsewhere in the Universe in many ocean planets, because it forms a thick layer on top of the rocky surface below it which volcanism wouldn’t be able to penetrate.

The structure of the ocean may not be that simple though. The ocean may in fact be arranged in four layers separated by shells of ice of different kinds. Beneath the “ordinary” ice crust on the outside, there may be a relatively shallow ocean on top of a layer of ice III snow. Ice III has a similar crystal structure to ice IV, but because this is snow it would consist of non-hexagonal flakes, perhaps more like needles than hexagons or six-pointed stars. This could be floating on top of a second, deeper ocean, below which is a layer of ice V. This is tens to hundreds of kilometres deep and is monoclinic in structure – that is, two of its axes of symmetry are at 90° but the third is slanted. Examples of monoclinic minerals include gypsum (blackboard chalk), jade and some feldspars (which can be enormous crystals the size of buses found in caves). Then there’s a final layer of water followed by the aforementioned ice VI base.

Below the rocks is a liquid outer core consisting of a mixture of iron pyrites (fools’ gold – this is a sulphide of iron) and iron, and the final solid inner core is made of iron.

A few other bits and pieces. The radiation on the surface is sufficiently weak to be fatal to unprotected humans after a few weeks, but it still wouldn’t be a good idea to go there. There are ray craters like those we see on Cynthia, such as Tycho, which may have light or dark rays depending on where the impact occurred. The “drowned” craters are described as “palimpsests”, after the faint remnants of writing seen in old documents which have been over-written later. Nobody understands why there is a strong magnetic field.

For such a large moon I find it a little disappointing that Ganymede isn’t better known. It feels like there’s either a lack of information on the place or that it’s overshadowed by its more exciting neighbours. Io has the hyperactive volcanism, Europa the possibility of life. Ganymede has if anything a greater right to be thought of as a planet than any other moon in this solar system, being larger than Mercury, and might be expected to be either more interesting or a better-studied place but it definitely comes across as more placid. Also, for its size it’s surprisingly light. This lack of knowledge is likely to change in the next few years when the European Jupiter Icy Moons Explorer (JUICE) is launched to investigate Europa, Ganymede and Callisto, excluding the non-icy Io. This will ultimately orbit Ganymede for around two years before being crashed into its surface. There’s a lot of that, isn’t there?

Next stop Callisto.

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.

Amalthea The Obscure

I have an almost irresistible urge to obscurity, and I’m probably not alone in this. With respect to European languages, I found Finnish the most interesting because it was unlike the others, although it’s very distantly related to Hungarian (about as closely as English is to Farsi). With respect to animals, I find the minor phyla fascinating, such as the Loricifera. Laos is an interesting country to me because it never seems to crop up anywhere in news reports and so on. Likewise, among the moons of Jupiter there are many obscurities, not the least of which is the tiny Leda, discovered from Earth and yet thought to be only eight kilometres in diameter, now revised up to twenty-one but still only about the size of Rutland in cross-sectional area. Jupiter is now known to have six dozen and seven moons, and Saturn six dozen and ten. If the orbits of the outermost of these moons formed visible ovals, both systems would be clearly visible as discs in our night sky, and both systems are about fifty million kilometres in diameter. This means that the furthest moons can approach each other by about eight percent of the minimum distance between the two planets, and the space can be thought of to some extent to consist of these two satellite systems separated by a still considerable distance but still dominated by them. Both of them are like mini-solar systems in themselves, with four of Jupiter’s moons large enough to be thought of as planets, and six of Saturn’s large enough to be spherical. Being larger, these are of course the less obscure moons, and although they interest me, I still feel the urge to think about others.

Amalthea is the fifth largest moon of Jupiter and the fifth to be discovered, in 1892. It’s therefore the largest Jovian moon which isn’t round. It stays within the orbit of Io, the violently volcanic innermost Galilean world, and as such appears to be covered in the sulphur compounds spewed out by that and is said to be the reddest world in the Solar System. I’d take that with a pinch of salt because I’ve heard that about several other bodies, but it’s definitely redder than Mars. The above painting shows the space probe Galileo passing Amalthea and indicates how close it is to Jupiter. Two satellites are closer to Jupiter, and much smaller, but Amalthea was the last one in the Solar System to be discovered by someone looking through a telescope as opposed to a photograph or set of photographs taken through telescopes, or by space probes. It’s less than fifty thousand kilometres above the cloud tops, meaning that if it were much closer it would be ripped apart to form a ring, and in fact the two closer moons are only able to exist because they’re so small. Amalthea already contributes to the “gossamer ring” of Jupiter.

From the surface of Amalthea, Jupiter would stretch a quarter of the way across the sky. It would be more than eight thousand times the size of the Sun or our own satellite in our sky. As such, it’s been suggested as a possible popular tourist destination, because it would have a truly astounding view of its primary. However, since it’s so small its surface gravity is extremely low at a five hundredth of ours. A sixty-four kilo person would weigh only a hundred and twenty-eight grammes on the surface on average. Being irregular, this average would vary a lot. Its diameters are 250 × 146 × 128 km, and those are just the extremes, making its largest dimension about half the size north to south of England and about as far as London to Exeter. For a long time it was only really known from this photo taken by one of the Voyager spacecraft in 1979:

Two types of features are known from its surface: craters and faculæ. The latter are bright patches and include Ida and Lyctos. The craters are called Pan and Gæa. It could also be thought of as having several extremely high mountains on its surface, so high in fact that the gravity at the top of the highest is only a quarter of that at the bottom of the moon’s lowest point. Contrasting that with Earth, someone standing at the North Pole, which is probably the point on this planet’s solid surface closest to the centre, ignoring the sea bed, would only be 1% heavier than at the peak of Mount Chimborazo, the highest mountain in Ecuador, which is of course taller than Mount Everest measured from the centre of the planet.

Amalthea is kind of named after a goat, Ἀμάλθεια, whose name for some reason is spelt with an extra I(ota). Ἀμάλθεια was the goat who suckled Zeus as a baby, and became an obsolete constellation around Capella in our sky called Capra, as in goat. Nothing to do with Capricorn(us) except possibly that the reason Capricorn isn’t called “Capra” is that there used to be another constellation of that name.

In terms of surface area, it’s about the same size as England, although it’s very difficult to work out due to its irregular shape. Its density is only 89% that of water, which suggests that it, like many other small bodies in the Solar System, is a “rubble pile” rather than a completely solid object all the way through, or can be thought of as riddled with a network of caves and voids making up most of the moon’s bulk. It’s also somewhat redder at the front than the back because of the accumulation of matter from Io, and like Io it gives out more heat than it receives from the Sun, which in its case is probably due to the fact that Jupiter has an internal heat source and being bombarded with charged particles. It’s also icy, which means it can’t have started off where it is now as it would’ve been melted by Jupiter. The large satellites decrease in density with distance because of this influence: Io is the driest moon in the Solar System whereas Callisto is mainly made of ice, so Amalthea must have fallen into its orbit or been captured as a former asteroid after Jupiter had cooled down a bit. It orbits Jupiter once every twelve hours.

Amalthea As A Sci-Fi Location

I should reiterate first of all that although I love the idea of human beings settling the worlds of the Solar System and beyond, I don’t believe it will ever happen for statistical reasons which I’ve mentioned many times on here. Even so, they probably bear repeating. Suppose only a hundred worlds were colonised by humans and ended up with an average of a hundred million people living on each with a life span of a hundred years. That’s already more people than have ever lived, meaning that in such a scenario there’d be a better chance of being alive on one of those worlds than on Earth before it happened. For this reason, it seems unlikely that we will ever even visit Mars, because once that happened there would surely be no turning back. Consequently this is in the realm of fantasy, sad though that is.

Amalthea is a spectacular place because of the view of Jupiter it affords. Other than that, it seems to be a convenient place to base mining operations for the Jovian atmosphere. It’s been suggested that the Dædalus Project use Jupiter as a source for its fusion propulsion. In the 1970s, the British Interplanetary Society designed a space probe for Barnard’s Star which would take about six decades to get there, flexible enough to be aimed at other star systems such as Alpha Centauri, which would’ve taken only forty-odd years. This means that if it had been built and launched just after it had been designed, it could have reached it by now. The idea was to travel to Jupiter, mine helium-3 and hydrogen from the atmosphere and fuse them to create thrust to accelerate the spacecraft to 7% of the speed of light, and then travel to nearby stars, using their magnetospheres to brake the craft and possibly replicating it when it got there. If this had gone ahead, Amalthea would’ve been a good base to undertake the mining operations from, being very close to Jupiter. What I don’t know is whether Jupiter’s radiation belts extend as far in as that moon. This is not quite science fiction, and is an example of the kind of plans made in the 1970s regarding space. At the same time, there was a plan for a Mars mission from 1979-81 and the Stanford Torus Project to build a centrifugal wheel in space a mile across which would serve as a permanent habitat for space colonists. None of this happened even though the plans for all of them were at an advanced stage, and my degree of disappointment is so great here that I no longer believe that anything like that will ever happen. Hence “Amalthea As A Sci-Fi Location”.

Jupiter has a powerful magnetic field which traps ionising radiation within belts like the Van Allen belts around our own planet, through which the Apollo astronauts had to travel quickly through a particularly thin region in order to avoid the danger. Our own natural satellite orbits far outside these belts, but at least three of Jupiter’s largest moons orbit bang in the middle of them, and they’re more powerful by far than our own. They can be detected from here because of the radio waves they emit. The largest moon, Ganymede, has its own magnetosphere within Jupiter’s. Io, the first large moon, interacts intensely with Jupiter and has a toroidal electrical current tube running between it and Jupiter, half on either side of the moon and planet, called the Io Flux Tube. There’s an aurora-like discharge at 65° north and south latitudes where this tube touches down on Jupiter. The radio signals from Jupiter coincide with where Io is at the time. Amalthea is nowhere near as massive as Io, and only manages to create a void in its orbit within the magnetosphere. However, because it’s elongated and has such a low gravity, dust particles can easily leave its surface and form a ring in the vicinity of that orbit as well as the other rings Jupiter has.

Arthur C Clarke originally had the 2001 monolith sited on Amalthea. He also explored what would happen if an astronaut were to escape the moon’s gravity accidentally, which where it faces away from Jupiter can be as low as a metre per second, but can be as high as ninety metres per second in other places on the surface. Piers Anthony imagined Amalthea as the Solar System equivalent of the Bahamas, and in fact I can get on board with this. I can easily imagine Amalthea as a bit like Monaco, a moon whose wealth is completely out of proportion to its size. It has the helium-3 mining industry and is also a resort, and I kind of imagine it’s covered in holiday makers and casinos. Getting back to Arthur C Clarke, the moon is used as a radiation shield in ‘A Meeting With Medusa’. I don’t know that this would work. Possibly on the trailing end of the moon?

In geographical terms (amaltheographical?), Amalthea would be anything but a microstate. Considering its surface alone, its area is about that of England’s and it’s also as heavily cratered as the lunar highlands and contains caverns. In a way it resembles a pumice stone. Taking its interior into account, it has many times its surface area in a way which is, however, currently difficult to calculate. It’s also high in water ice, so there’s no shortage of water. The two big problems are the low gravity and the intense radiation, although the latter is a possible energy source. Possibly settlements on or in Amalthea would have to create artificial gravity by constantly rotating. But it would be worth it. Gaining a corner in the helium-3 market would allow it to provide much of the fusion power for the Solar System and interstellar missions, and the latter could be crucial for the continued survival of complex life in the Universe if it turns out that there is no other advanced life nearby in this Galaxy. This could make the stakes very high indeed. At the same time, the sightseeing opportunities would encourage tourism, assuming that becomes economically feasible. Hence there are two different industries available on a moon which has ready-made living space inside it: tourism and mining.

The temperature, however, is pretty low. It varies between -108 and -153°C, which is considerably colder than the lowest temperatures on this planet, near the South Pole in the middle of winter. Although the Sun would be accordingly dimmer, it probably wouldn’t be noticeable even though it’s only a twenty-fifth as bright as it appears from here because of the way human vision adapts to light. It isn’t a sunny place, not really, although it is in a way because there’s no atmosphere or cloud. There is, however, something like a six-hour eclipse of the Sun by Jupiter, which would be lit by Jovian thunderstorms and auroræ, followed by six hours of daylight and a view of the Jovian surface from very close up. The Great Red Spot alone would be much larger than the Sun in our sky.

Call me bitter, because I am, but we will never see any of this in person. Nor will our descendants, and the reasons for this are basically that we are too up our own behinds to pursue an optimistic vision of the future. This is entirely within human capacity but is guaranteed never to happen because if it had, we would probably not have been born at a time before it already had. I find this immensely depressing. How about you?