As soon as I saw the first pictures of Enceladus from the Voyagers, I realised it was special. For almost a decade, Enceladus had just been another name on the list of Saturn’s moons to me. The most interesting moons in the system up until then had probably been Titan, Iapetus and Phoebe, for reasons which will emerge when I get to them. One thing which was known far in advance of any spacecraft approaching it was that Enceladus is the brightest world in the Solar System. In order to explain how bright it is, the distinction between bolometric and geometric albedo needs to be made clear.
Albedo is the proportion of light reflected back from a surface, and is measured in two different ways, giving two different figures. One is bolometric albedo, also known as Bond albedo. This is the fraction of power in the total light, visible or otherwise, reflected back into space. In the case of Enceladus this is 81%. Geometric albedo is something else, by which I mean it’s a little odd. It’s the ratio of the brightness of a surface seen from the light source illuminating it to an idealised flat surface which reflects back with uniform brightness all over its surface. For some reason I don’t understand, Enceladus is so bright by this measure that its geometric albedo is actually greater than one! It’s 1.38. This is the visual geometric albedo, which only takes the ratio of visible light reflected back into consideration, so there are various ways in which it could be brighter. For instance, a fluorescent surface could have a figure greater than one. Enceladus is of course not fluorescent or luminous, but it does actually have such a brightness. This is because the surface of the moon reflects light back directly to its source without scattering, so whereas a piece of white paper might bounce light off to the sides, Enceladus doesn’t. It’s as if you’re standing on the night side shining a torch straight down at the surface.
One result of this extreme reflection of light is that Enceladus is unusually cold for a Saturnian moon. It hardly turns any of the light reaching its surface into heat and also reflects heat, so its surface temperature is only -189°C. Even so, it has a liquid water interior, but a 60:40 water ice:rock ratio like several other of the moons. This is what makes it so interesting. I first realised this was so during the Voyager mission, and thought to myself that this would make it suitable for life. However, this would require an energy source and more complex chemicals than just water. The place reminds me a little of Europa, but it’s a lot cleaner. The surface is mainly white, unsurprisingly, with pale aqua streaks on it. It’s a lot smaller than Europa though, having a diameter of only 504 kilometres. This gives it a surface gravity only one percent of Earth’s because of its much lower density. This is what led me to write a story about a couple who honeymooned on Enceladus, having got married on Titan, but I didn’t finish it and it didn’t come together. There’s something very “honeymoony” about the place to me, being so white and pure like a wedding dress, and also very floaty, floatier by far than Saturn. In my story there was a hotel with a glass wall stretching all the way down, populated by life from the moon’s ocean, with which it was continuous. I don’t really feel I can discuss the place without mentioning the possibility of life. Of all the possible habitats for “life as we know it”, Enceladus seems to be the most neglected.
As I’ve said, there needs to be an energy source for life to exist. This is less an assumption than a law of physics. In this moon’s case, the highly reflective surface rules out sunlight, but the interior is nonetheless liquid so energy must be coming from somewhere. The interiors of planets and moons are often heated either because they haven’t cooled down yet or because of radioactivity. Enceladus is both too small and too “watery” for this to be so here, but what does seem to be happening is similar to the Galileans in the Jovian system, notably Io and Europa, and again, more like the latter than the former. However, it isn’t clear where this is coming from. It’s undoubtedly there because of the geysers, or volcanoes depending on how you think of eruptions of water from the surface, in the southern “tiger stripes” region. This can’t be happening without a heat source. One possible explanation is the largish moon two orbits out from it, Dione, although it’s also been suggested that it’s due to Janus. Resonances between various moons in Saturn’s system need to take into account the fact that Enceladus is heated but Mimas apparently isn’t.
Whatever the cause, the churning interior of the moon has a major effect on its surface. The terrain has been divided into six different types. The cratered areas are the oldest and are of two types. They differ from Mimas and Tethys in not having any relatively large impact basins but there are lots of smaller craters between ten and thirty kilometres in diameter. The difference between the two areas is that one has well-preserved craters and the other shows signs of collapse, with lower rims and smoother central peaks, which suggests that only the latter has undergone heating since they formed. Although both areas are cratered, it’s only on a par with the least-cratered parts of the other Saturnian moons such as the smooth plains of Dione’s leading hemisphere. Three other types of landscape are intermediate between the cratered and craterless kinds, with both grooves and craters, and the final type only has grooves.
These grooves can also be thought of as valleys and ridges, and they indicate that the crust moves and are possibly formed by water seeping out from inside. However, unlike Europa, which has what look like (but presumably aren’t) scratches on its surface, Enceladus looks clean except for a bluish tinge to some of them. It’s the smallest active moon known.
The moon has a plume, as seen above. This was only discovered by Cassini, since the Voyager spacecraft only approached to about ninety thousand kilometres whereas Cassini got within 175 kilometres, mainly because even the first approach of 1 200 showed something weird going on. It was discovered that the moon deflects Saturn’s magnetic field but only at the south pole, where it had been night time when the Voyagers took a look. It turned out to be the newest terrain on the moon and to be strewn with house-sized blocks of ice. There was a relatively dense cloud of water vapour over it too. Moreover, it was found that the surface temperature at the south pole was around -163°C. Like Saturn itself, Enceladus is warmer at the south pole than at the equator. All the jets of ice are from the tiger stripes. They also contribute to a very tenuous ring around the orbit of the moon referred to as the E Ring, and I’m not sure if it’s called that because of Enceladus beginning with an E or it was just allocated that letter because it was the fifth ring to be discovered. It couldn’t be seen easily from here if at all.
Cassini was flown through the plumes and found them to be mainly ice with some ammonia, methane and carbon dioxide and monoxide, all under one percent. Later on, amines were discovered. These are organic compounds which have an NH2 group at one end, which includes the amino acids from which protein is made, and includes some other important biochemical compounds such as neurotransmitters, hormones and some alkaloids. It’s always “some alkaloids” incidentally, as it’s a family resemblance definition. Hence there are geysers on Enceladus which spew out chemicals associated with life as found on this planet, which could be evidence of life there. Then again, maybe not.
The tiger stripes themselves are four sulci which bend at the side facing away from Saturn and branch on the side facing it. They’re five hundred metres deep and about 130 kilometres long, and are called Cairo, Baghdad, Alexandria and Damascus. Along with the other sulci, they’re named after cities mentioned in ‘The Arabian Nights’. Almost the whole surface of the moon is covered in a substance resembling snow, although it isn’t clear that it’s made of snowflakes and it probably isn’t. The ice around the tiger stripes is different, being larger crystals and absorbing red light, which is what gives them their faint turquoise appearance. They contain dry ice and organic material, which sounds to me like it stains them that colour, and it occurs to me that blue-green algæ are also that colour. The fact that the crystals are larger and unlike the surface ice grains elsewhere also means they can only be a maximum of a few thousand years old, as otherwise the magnetic field of Saturn would’ve converted them into the other form. Presumably they’re being constantly replenished by the geysers.
Whether or not there is life in or under the geysers, it’s probable that the extremophile organisms living in some unusual environments on Earth, such as geothermal vents, and these organisms do produce methane, which is found in the emissions from these geysers on Enceladus. They also contain sodium, chlorine and carbonate ions, indicating that the water is salty and contains washing soda. The presence of ammonia within the plumes means that the mixture of compounds could be liquid at as low as -103°C, but even without it the internal heating is sufficient to cause them. The presence of the ocean can only be guaranteed in the south pole region, and it may be around thirty-five kilometres deep, which is favourable to life as it means geothermal vents would be relatively close to the surface and not covered in compressed ice.
The situation where a mixture of ammonia and water freezes at a lower temperature than either, which is one possibility here, is known as a eutectic mixture. This also occurs with salt water, which freezes at -21.3°C and is used to thaw ice on roads. In the case of the liquid here, not only might it be a mixture of the two, but also salt is involved. Ignoring the salt, a 36% concentration of ammonia would be sufficient for being liquid at this temperature. However, if this is true, I’d expect there to be more ammonia in the E Ring, which there isn’t, and the water ice content of the geysers is something like 99%, so I don’t really get why they think this would be a solution. Maybe the salt makes a big difference.
The total volume of Enceladus is about a twentieth that of the World Ocean, and it isn’t entirely water, so it isn’t one of those moons with more water than Earth for once. Its diameter is such that it would almost cover the North Sea, and its surface area is slightly smaller than Mozambique and somewhat larger than Turkey.
Compared to Mimas, Enceladus is very active, but being closer to Jupiter the “death star moon” should experience more tidal forces and therefore activity than the “honeymoon” (yes, I know it doesn’t really work. Just play along), but in fact it’s very much the other way round. This is all the more mysterious because Enceladus has a more circular orbit than Mimas, which ought to render it less active due to less variation in tides. It’s also odd that the south pole is hotter than the rest of the moon. Although it makes sense that Enceladus is partly heated due to its orbital residence with Dione, Mimas has the same relationship with Tethys. It looks like it must have started off very liquid at an early stage and stayed that way. Theoretically, Enceladus could be a quiet moon like Mimas, but it has two stable states whereas Mimas only has one, so even if Mimas started off active, it would’ve frozen through, whereas Enceladus would not. Unlike Mimas, Enceladus is constantly losing mass and this could have led to subsidence under the south pole. If Enceladus is the same age as most of the objects in the Solar System, it would’ve started off 30% more massive. However, it’s also been suggested that the entire Saturnian satellite system only formed during the Cretaceous because the orbital dynamics of the moons is not stable enough to have lasted since the formation of Saturn. Against this is the modelling done of the system which seems to show that the ocean has been there since Precambrian times, although only back into the Cryogenian, when Earth itself seems to have been frozen over, although that’s long enough for life to appear there.
One final issue: Because Enceladus is active and internally heated, it’s possible that it would be suitable for a human settlement, even maybe a honeymoon hotel.
That’s it for Enceladus then. I’m not sure what I’ll post next, but after that I’ll be talking about Tethys. I’m conscious of ignoring the global situation rather heavily and can’t decide whether I should persist in doing this or not.
A Box Of Chocolates 