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.

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.