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.
A Box Of Chocolates 