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?
A Box Of Chocolates 