From OttawaRasc

Cold as Ice: Ice Moons of Jupiter

by Simon Hanmer

In a series of presentations on the Outer Moons of the Solar System, we’ll be looking at the effects of fracturing, volcanism and impacts. Prior to that, we should have a sense of what the surfaces of the Ice Moons actually look like in general. Note that the "ice" of the outer Solar System is really a mix of water ice and ammonia - cleaning fluid - which softens the ice quite a bit. If it was just water ice at the extremely low temperatures of the outer Solar System – minus 270 to minus 170 degrees C – "ice" would be simply too hard and too stiff to do half the things I’m going to tell you that ice really does

Because Europa probably has a subterranean salty ocean under the outer ice, many planetary scientists draw parallels with the ice caps on Earth - especially where Earth’s ice is underlain by ocean water. How realistic is this comparison? This is an important question because, if the comparison is good, then we have a natural laboratory for Europa right here where we can access it.

Consider the Arctic Ice off the North end of Greenland. Land ice and sea ice behave quite differently. The land ice is apparently quite solid and featureless, but the sea ice is fractured and broken into large chunks - (icebergs) - that are well defined and separated from each other by a matrix of thin ice. Locally, small isolated areas of open water that have not linked up to form channels. Elsewhere off North Greenland, solid sea ice passes progressively southward through a transition into icebergs surrounded by thin ice and lots of open water.

Cracks in the solid sea ice are about the same scale as the icebergs that are calved from it, which tells us that the cracks in the ice do not grow in length beyond their original size. This will be an important observation to keep in mind when we look at the Outer Moons

Now let’s move out to Jupiter and take a look at the Ice Moon Europa. The low density of impact craters on Europa tells us that its icy surface is very young and very active. Young ice has effectively erased all evidence of the Great Bombardment of about 4 Billion years ago. Because of the orbital mechanics of Europa in Jupiter’s powerful gravity field, the Moon is flexed like a tennis ball. This generates relatively straight cracks - or fractures - in the surface ice, that can be over 1500 Km long. These long fractures are characteristic of Europa, but so is the “Chaotic Terrain” where the long fractures are distinctly absent. The ice that makes up Europa’s fractures is different to the ice that the fractures cut across.

This tells us that the fractures are actually narrow zones where ice with impurities – probably mostly carbon – was injected from within the Moon into the long cracks.

So what does the ice that the fractures are cutting across look like close-up? Well - more fractures!! In fact it’s difficult to find a part of Europa’s surface that is not fractured. Some are discrete simple ridges. Others are more complex and have a central valley down the middle – they're called triple bands or tribands. Some ridges clearly cross-cut others, from which we can work out a time sequence – or a history of fracturing and injection of new ice. In addition to the ridges, there are broad zones with complicated shapes and lateral branches, all filled with new ice that itself has an internal structure.

Chaotic Terrain cuts – and therefore is younger than – both the broad zones and the long ridges. What exactly is Chaotic terrain? It comprises ground with a rubbly surface, very different from the ice surrounding it. It either cuts - or sits on top of - all the structure in the surrounding ice, so it’s commonly the youngest thing in a given Europan neighbourhood. In effect, Chaotic Terrain is a gigantic ice eruption – in other words the product of ice volcanism.

How deep do these surface features go in Europa’s ice? Complex networks of ridges and tribands that cross-cut and offset each other can be interspersed with ice blisters, technically referred to as “Freckles”, which are in fact volcanoes made of ice that rose from within the Moon. The ridges and freckles avoid each other, as though the one is controlling the distribution of the other. Either the ridges deep walls of strong ice that guide the rise of the ice volcanoes, or the ice volcanoes are deep pipes of strong ice that influenced the propagation of the cracks that give rise to the ice ridges.

This pattern of ridges and ice volcanoes is also developed on a very large scale. The largest ice volcanoes on Europa are Thera and Thrace. The latter rises above the Moon’s surface as you might expect, but the floor of the former sits below the average surface level of its surroundings - in effect it's a hole!

Now, how does all this compare with Earth’s icecaps? Some planetologists have identified “icebergs” in terrains of blocky ice on Europa that they compare to those on our planet. The boundaries of these ice blocks truncate ridges and tribands and disrupt ice volcanoes. However, these blocks are not icebergs! They are not separated from one another by a matrix of thin ice, and there are no dilated cracks. Instead, the blocks are in a matrix that has destroyed the ridge structure - that is still visible in the blocks - and the blocks have been shuffled and misoriented with respect to each other. Commonly, the blocky terrain is then cut by very late, very long fractures.

I don't think that blocky ice is an analogue of Polar Sea Ice at all! It is a very coarse variety of Chaotic Terrain. In other words - this is the product of Ice Volcanism, something we don’t see on Earth – and certainly not at this scale.

If Europa is the type example of a young and active Ice Moon surface - its sister Callisto is just the opposite. Callisto’s surface is densely scarred with evidence for major meteoric bombardment, and therefore must be ancient. The question is – of course – did it look like Europa in its once active youth? I don’t know, but that active youth would not have been very long lived because - unlike Europa - Callisto is not differentiated into a core, a mantle and a crust. which tells us that it wasn’t warm enough internally to be active for very long after its formation.

The average surface of Callisto is covered with impact craters, but there’s something a bit odd about this battered icy. The walls of the craters look like they have been “eaten away” – they are nothing like those of the Rocky Planets. Locally, the crater walls seem to have moved around, even slid down into the impact crater pit. In other cases, craters have been partially flooded, but by what? The bottom line is that the ice of Callisto’s surface is capable of crumbling and moving around like sand, which is pretty strange behaviour for ice.f strange behaviour, there is evidence that the strength of Callisto’s icy surface has changed with time. Older craters that are battered by younger ones tend to be less well defined than the younger craters. This suggests that the older craters formed when the surface ice was too soft and weak to support the pressure difference created by the topographic relief between a deep crater floor and high crater walls, so the craters collapsed – rather like melting ice cream. The younger craters formed when the same ice was stronger – so they’re well formed and preserved. The simplest way to account for this is if Callisto cooled from cold to even colder early in its history, thereby allowing the ice to get stronger with time.

Finally, Saturn's Moon Iapetus provides us with a spectacular example of mechanical ice behaviour. A giant rubbly landslide of crumbly ice has half-filled a crater that sits within an even larger crater. The cliff that represents the scar the landslide left behind is 15 Km high! I have to keep reminding myself that this is ice we are looking at, behaving just like rocks do on the Rocky Planets.

In summary, the ice surfaces of the Outer Moons are nothing at all like Earth's Polar Sea Ice. On Europa – the classical Ice Moon – the surface ice features are a mix of fracturing and ice volcanism that is nothing like what we see on Earth. Why the difference? Probably because Europa’s surface ice sits on a substrate of relatively “warm” - perhaps “slushy” – ice, while Earth’s ice sits directly on water. However, there are those who think that Europa contains a subterranean ocean, but in this model the Europan ice is much thicker than Earth’s Sea Ice. Either way ... there's a major difference between the ice of the Outer Moons and Earth's ice caps.