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Recently, we took a look at the difference between the gas giants like Saturn and Jupiter and the ice giants like Neptune and Uranus. Today, we are going to look at something else icy: icy moons. We already passed by an icy moon in the ice giant video, Triton, Neptune’s largest moon. Triton is tough to spot, as a large telescope is required to get a good view of anything as far away as Neptune, but it is quite bright. It is just one of a few icy moons in our solar system.

So what makes a moon an icy moon? Just having some ice isn’t enough, even our Moon has some ice on it. The ice on our Moon is locked away in craters at the North and South Pole, while the rest of our natural satellite is ice free. The Moon is only able to hold on to the ice it has thanks to the it’s lack of axial tilt and the depth of the craters. Here on Earth, our axial tilt or obliquity means that there are times of the year when the Sun doesn’t set for the poles, and other times when it doesn’t rise. This doesn’t happen with the Moon, with both poles seeing the Sun low in the sky all year round. The low angle of the sunlight means that craters which are deep enough never have sunlight reach the bottom. Any water that lands in these craters, from things like comets, will be frozen, and stuck that way. Frozen water won’t float away form the Moon, but water that has been melted will. Water on the Moon is under such low pressure, it doesn’t take much heat to boil it, and the direct light of the Sun on the Moon can reach scorching temperatures during the day. For any part of the Moon away from the poles, the Sun will get high enough in the sky to hit almost any part of the ground, vaporizing any ice it reaches. You need a lot more ice than just a bit trapped in craters to count as an icy moon.

When we talk about icy moons we’re talking about things like Jupiter’s moons Ganymede, Europa and Callisto, Saturn’s moon Enceladus and the aforementioned Triton orbiting around Neptune. All of these moons appear to be encased in a shell of ice, completely, their whole surface appears to be ice. Starting with Ganymede, the largest moon of Jupiter and the largest moon in the solar system, bigger than the planet Mercury. Looking at it’s surface, it’s not as pale as some of the other icy moons. Some parts of the moon are much darker, but this is thought to be due to impacts from the moons formation leaving behind clays and other rocky material. Much of the surface is ice, just rather dirty ice. Much of the paler region is covered in grooves and ridges, similar to fault lines. These cracks and faults may be signs of the ice shifting in the past, possibly allowing new ice to form on the surface leading to a paler colour, though the exact cause and mechanism are not truly understood. Along with the pale colour, although Ganymede does have craters, it isn’t covered in them. Some of it’s craters are interrupted or broken up by the grooves and ridges on its surface, while others have faded away to mere outlines. This breakdown of the craters is an indication that something different is happening with these icy moons, compared to rocky ones like ours. Here on Earth we have plate tectonics, where portions of the Earths solid outer layer, or lithosphere, are broken up into separate continental and oceanic plates. As these plates float over the liquid mantel underneath, they collide, sometimes causing one plate to get subsumed or slip under another plate when they crash together. This subduction essentially pushes the sinking plate into the mantle where it melts. This melted liquid rock from the mantle can erupt out of volcanoes and be deposited on the surface. This leads to a cycle over time where solid material is broken down and destroyed in one place, at the subduction zones, while new material flows out of volcanoes and at separating or divergent plate boundaries. This leads to old surface features, like craters, disappearing. We can add to that things like rain, wind, all the normal erosion that we get here on Earth, but these icy moons don’t seem to have any atmosphere of much significance, so things like wind and rain working on the surface aren’t going to be a factor. What does seem to be a factor is water underneath this ice. This water may push its way to the surface, where it may float off into space or get caught by the moons gravity and fall back to the surface as ice or snow. This is cryovolcanism, and it isn’t thought to be a major feature on Ganymede, more so on other moons. Just like the Earths broken solid surface floating on liquid rock, icy moons may have liquid water under their surface, with different parts of the surface moving separately. This could cause different bits of ice to crash together, possibly slipping under others, essentially refreshing the surface in a similar way to how our tectonic plates work. This is thought to be a more likely explanation of the grooves and ridges on Ganymede, as well some of the other Moons. Despite being made of ice, the solid ice surface of these moons is sometimes called a lithosphere just like the Earths solid surface, though cryosphere might be more accurate.

Still orbiting Jupiter, if we look at Europa we will see some similarities and some differences. Europa is certainly paler, it looks quite white, as you might expect from a ball of ice. It also has some tracks or lines crisscrossing its surface, similar to the grooves and ridges on Ganymede. Europa does seem to feature cryovolcanism, with what appears to be plumes of water spraying out of cracks in the surface having been imaged by the Hubble Space Telescope. Callisto has a similar pale greyish white colour, but it is darker than Europa in most places, broken up by paler white spots. Most of Callisto has craters, but generally only larger ones. Smaller craters are almost absent, and many old craters appear to be smoothed out. Although Callisto doesn’t show signs of cracks and faults, it does have some signs of cryovolcanism, though this may have only occurred in the past. These are the icy moons, the ones that orbit Jupiter at least, and they are the best studied right now. Not only do all of them appear to be completely encased in ice, almost all of them show signs of having some liquid water as well.

All of these moons do seem to have solid cores as well, they are balls of rock surround by ice, with the possibility of some water in between the two. However, if we look at these moons, they’re way out by Jupiter, and so they should be quite cold. Water can stay liquid below it’s normal freezing point if it contains antifreeze, including ammonia, but many of these moons have average surface temperatures lower than -100 degrees Celsius. For them to retain liquid water, something must be keeping the water warm enough. One possibility is that there is underlying volcanism in some of these moons, big ones like Ganymede in particular. Mars is a planet even larger than Mercury and Ganymede which has lost its volcanic activity, being too small to retain heat from its formation. Of course, a liquid rock mantle wouldn’t be necessary, just rock that was hot enough to melt ice. even still, this heat would gradually dissipate into space, unless it was maintained by factors such as nuclear decay. Even in that case, nuclear decay doesn’t last forever, and isotopes will eventually decay to a stable, non-radioactive state. The chance of internal heat being great enough and retained long enough is even less likely with smaller moons, like Enceladus orbiting Saturn. Enceladus is far smaller than Ganymede and even our own Moon, but it is one of the icy moons known to have cryovolcanism, with plumes of water being imaged and even sampled by passing space probes. These moons, at least some of them, are melting ice and retaining liquid water, but they don’t seem to be generating the necessary heat internally. Instead, they are getting it from the planet that they orbit.

A good example of the process at hand is actually a very non-icy moon, Io, the smallest of the Galilean moons orbiting Jupiter. Io has always reminded me a little of a pizza. It is covered in red, yellow and orange areas, scattered with small darker spots, many of which are close to circular. It has that kind of reddy-orange colour of tomato sauce and cheese and the darker regions could be black olives or pepperoni, whatever you prefer on your pizza. In reality, these dark spots are volcanoes, and the surface of Io is covered in lava and sulfur deposits. Being so small, there is little chance that Io has retained enough heat all on its own to be this volcanic. Instead, Io is volcanically active thanks to tides. In the same way that the Moon and the Sun pull water around the Earth, the gravity of Jupiter pulls rock inside Io. The massive tidal forces supplied by the giant planet cause the rock within Io to stretch and bend. The stresses caused by these tidal forces generate heat and that heat turns the rock liquid. This is how Io retains it’s incredible amount of volcanic activity. Io is so volcanic that its volcanoes erupting and new material melting back into its mantle causes the moon to essentially turn inside out. It doesn’t fully invert itself, as it is believed to be differentiated, with a distinct core and mantle, meaning that material from the core is likely stuck there. In the video, I said Io turns itself inside out every few years, but this is really just how long it takes for most surface features to be refreshed, actually covering the whole surface with new rock takes substantially longer.

Triton orbiting Neptune is less well studied than the other moons, being so far away. It look similar in many ways to Enceladus or Europa, being quite pale and shiny, but it is larger than Enceladus, a little smaller than any of the four Galilean moons of Jupiter. While it is less studied, it does seem to be an icy moon of some sort, being way out by Neptune and not orbiting super close to Neptune, it may not have the geothermal heating necessary to generate a subsurface liquid ocean, but it has been witnessed undergoing what is likely cryovolcanism. Cryovolcanism is when the liquid water underneath the icy shell erupts into space. It doesn’t quite work the same as normal volcanism, which is mostly caused by a build up of pressure. Rather, the vacuum of space is, as you know, a vacuum, so the pressure is incredibly low. If water from a subsurface ocean manages to reach up through cracks in the ice, the low-pressure of space essentially causes it to vaporize, or boil off, immediately, shooting a plume of freezing water, often turning into ice, into space. The water isn’t getting pushed out from underneath, it’s getting pulled out by the vacuum of space. On Triton, there is a chance that water ice under a shell of solid nitrogen is melting, causing an increase in pressure and bursting through, which would be a different phenomenon, but it is unsure as of yet.

Given the ample supply of liquid water, particularly on Enceladus and Europa, the icy moons of our solar system seem to be some of the best locations to look for the possibility of life. The moons that are orbiting around Jupiter, Ganymede, Europa and Callisto, those are just past the Asteroid Belt after Mars. Here on Earth we have meteors that come from Mars. If something big crashes into Mars and blows off little chunks of rock, some of those chunks of rock can fall down here to the Earth. Due to this, there is a chance that if we ever do in that life exists on Mars, it could be related to life here on Earth. Panspermia is the theory that life can spread through space and that the origins of life on Earth may actually come from another planet. This works both ways so it could equally be applied to Mars, if there’s life on Mars, that life might have gotten there thanks to a comet crashing into the Earth and sending seeded material with Earth bacteria to the Red Planet. Jupiter’s moons are a little further away, but not as far away is Saturn’s moons. Furthermore, Jupiter has an incredibly powerful magnetic field, and this isn’t always the best for life. Ionising radiation, including some of the stuff coming from the Sun, can cause a lot of difficulty for the formation of life. This ionising radiation can hang around in a planets magnetic field, as it does in the Earths Van Allen Belts. These belts are regions where high energy particles have become trapped, requiring missions to the Moon to be highly protected and shielded against radiation. Jupiter’s Van Allen belts are huge, reaching out as far as many of its moons.

Way out by Saturn, the little moon Enceladus is way less likely to have been affected by things happening in the inner solar system, so although life that pops up on the moons around Jupiter is still unlikely to be related to us, it’s even less likely if we find life in one of the moons orbiting around Saturn. Unfortunately, right now Jupiter and its icy moons are the target, with the JUpiter ICy moon Explorer or JUICE, a space probe already on its way to Jupiter, with a second, the Europa Clipper, planned to be launched in a couple of years. Jupiter’s icy moons, particularly Europa, are getting a lot of attention right now, but just a little bit further away, that moon orbiting Saturn, Enceladus, might be even better target in the future and hopefully that’s something we can pull off in our lifetimes. Jupiter’s Icy moon Explorer should get there around 2031, the Europa Clipper in the 2030’s as well, so there should be a couple of probes going around Jupiter in the next decade. Hopefully it won’t take much more than a decade more to get something around Enceladus, though there are no concrete plans for such a mission currently. If you want to get updates on these kinds of things, one way to help is to subscribe to this website or my YouTube channel, where I will discuss these things as they happen. Of course, there will be plenty of other things to discuss between now and the 2030’s, so I hope you come back for some of those posts as well.