From OttawaRasc

Worlds in Collision: Impact Structures on the Outer Moons

by Simon Hanmer

We’re all familiar with the classical impact structures on the Moon and the Rocky Planets. Through our telescopes, we can observe the geometries of impact craters and the giant Impact Basins on the Moon. Through books and the Internet, we’ve a pretty good idea what these features look like on Mercury and Mars. But what happens when a comet, an asteroid or a large meteor impacts the surface of an icy moon, such as those that orbit the Outer Planets? What do those impact structures look like, and how do they compare with those preserved on the Rocky Planets?

Consider the old battered surface of Callisto, one of Jupiter’s ice moons. Because Callisto is essentially cold and “dead”, the impact features that have accumulated since the beginning of the Solar System are still beautifully preserved.

Valhalla is a 4000 km wide multi-ring Impact Basin, superficially very much like the Orientale Basin on Earth’s Moon, or the flooded equivalent that we call the Imbrium Basin. However, Callisto is made of ice - and ice does not have the same strength characteristics as rock – so it’s not surprising to see significant differences. The central depression of Valhalla is shallow. The number of concentric ridges that surround the impact site is double the number for the best lunar examples. In addition, there are no rays of ejecta flung out from the impact site. Everything points to impact into something relatively soft - more of a “splat” than an explosive impact!

That would also explain the low relief of the concentric ridges and the shallow central cavity; the initial impact structure would have collapsed somewhat like melting ice cream. In addition to the ten or so principal concentric ridges or scarps, there are many minor low relief scarps between them, another feature we don’t see on the Moon or on Mercury.

Valhalla is an old Impact Basin, as indicated by the density of smaller impact craters that have scarred the concentric ridges of the Impact Basin itself. However, this is not the oldest part of the Callisto’s icy surface; other parts of the moon have even higher impact crater densities and so are even older. The impact craters are obvious and show some features similar to those of the Rocky Planets. For example, the larger craters have central peaks just like on Earth’s Moon.

But there’s something a bit odd about this battered icy surface; the walls of many craters look like they have been “eaten away”, have moved around, and have even slid down onto the crater floor. The ice of Callisto’s surface is apparentlycapable of crumbling and moving around like sand, which is strange behaviour for ice.

Europa is another of Jupiter’s ice moons, but one with a very young and active surface – as we can tell from the very low number of preserved impact craters. Europa is so active that it’s constantly resurfacing itself and eliminating older impact structures. Even from space it looks as smooth as a billiard ball, which is a reflection of the fact that Europa’s ice is too weak to support the weight of a significant mountain chain like the Himalayas on Earth, or the ring of mountains around the Mare Imbrium on Earth’s Moon. However, comets, asteroids and meteors do strike the surface of Europa, so what do the impact features look like in this relatively weak ice?

Pwyll crater, Cilix crater, the multi-ring impact structure Tyre, and Mannann’an all show the same very low topographic relief, very shallow crater floors, and very low crater walls. Again, this is a function of the inability of the relatively weak ice of the moon’s surface to resist the pressures generated by major differences in topography.

The multi-ring structure Tyre is only about 40 km across. On Earth’s Moon, this multi-ring geometry is only found in giant Impact Basins.

The accepted interpretation of these low relief impact features is that they formed in soft - perhaps relatively “warm” and relatively thin – ice.

The Pwyll impact crater is 25 km wide. In contrast to other impact structures on Europa, this one has an enormous set of ejecta rays thousands of km in length. Because the rays cut across all the cracks and ice ridges, Pwyll must be a very young feature. Why does this impact crater look like a classical lunar feature, such as Tycho on Earth’s Moon, when other impact features on Europa are so different? One possible answer is that this impact crater formed in relatively “cold” and relatively thick - and therefore stronger - ice than the Tyre multi-ring structure. The problem is that Pwyll shows no topographic relief whatsoever – just like Tyre.

This suggests that the ice was too soft to support a deep hole and high crater walls, all of which puts us back to square one in trying to explain the ejecta rays. Clearly some impact structures on Europa remain a bit of a mystery.

Impact structures on the surface of Jupiter’s ice moon Ganymede come in two distinct flavours – depending on whether they occur in the older dark terrain or the younger light-coloured terrain. In the dark terrain, the impact craters are more numerous, but they look like those of Europa; the larger ones are generally shallow with very little relief and show no ejecta rays at all. In the light terrain, all impact craters are well formed, with deep floors and high walls, and are surrounded by very obvious splashy white ejecta rays. In detailed images of the older dark terrain, there are three kinds of impact features.

First, closely-spaced, slightly curved grooves are the remnants of a giant multi-ring Impact Basin similar to Callisto’s Valhalla. Second, those grooves have been overprinted by large impact craters, which are therefore younger than the Impact Basin. Both the grooves and the larger impact craters have very subdued topographic relief; does this indicate impacts into relatively soft ice? Third, the large impact craters and the grooves are battered by smaller, younger impact craters that are well formed with deep floors, high crater walls and central peaks; in other words classical impact craters. These small craters look like they formed in relatively hard ice. As you know from shoveling your driveway, relatively “warm” ice is relatively soft, and relatively “cold” ice is relatively hard. Is it possible that Ganymede's impact structures provide evidence for the progressive cooling and stiffening of the surface ice with time?

Consider one of Ganymede’s named impact craters in soft ice: Neith, 160 km wide. Not only is Neith’s topographic relief very subdued, it’s inverted! Close to its centre is a ring of what looks like chaotic rubble, and in the centre - instead of a hole - there's a hill! The walls of the initial impact crater that formed in the soft ice collapsed, and by flowing in from all sides towards the centre the ice produced a dome at the impact site. This process of flow in soft material to fill in a crater is known as “Viscous Relaxation” because the pressure in the ice around the impact cavity is relaxed by viscous flow of the ice back into the hole. This is not something weird and unique to Ganymede; exactly the same thing can be seen in the impact crater Doh on Callisto.

In another part of Ganymede’s surface we find wide expanses of light coloured young ice with large blocks of older, darker ice floating in it. The impacts in the younger ice are very different from those in the dark terrain. Just like Pwyll on Europa, they are very splashy with extensive white ejecta rays. There are some exceptions where a splashy crater has formed in dark ice, but that simply tells us that the particular patch of dark ice is very thin and is underlain by strong, light coloured young ice. In detail, the impact craters in the light young ice on Ganymede are well formed with deep floors, high crater walls, central peaks, and well defined collapse structures preserved against crater walls.

The beautiful preservation of these classical impact crater features tells us that the young ice must’ve been strong, and presumably relatively “cold”. So can we explain all the differences between the subdued older and splashy younger impact craters on Ganymede in terms of the hardening of the moon’s surface ice as the temperature decreased with geological time? I’m afraid not!

Consider an impact crater chain that crosses the boundary between battered old ice and less battered younger ice. In a classic example of one such crater chain, formed by an impactor that had broken into separate bodies prior to impact, the part of the crater chain in the old ice shows no ejecta material, but abundant splashy light coloured ejecta is associated with the same crater chain in the younger ice. The craters look similar in both parts of the chain, so it’s reasonable to assume that the ice was equally strong in both the older and younger terrains at the time of impact. I have no idea how to explain the distribution of impact ejecta in this example.

Finally, what happens when a relatively large impactor hits a relatively small icy moon? Consider the crater Odysseus in Saturn’s relatively small ice moon Tethys, only ~500 km in diameter. Compared with the size of the moon, Odysseus is so large that its floor curves with the moon’s surface so, if you were standing at the foot of the crater wall, the other side of the crater would be out of sight beyond your immediate horizon. The relative sizes of moon and impactor were such that the moon only just held together, and it's probable that icy Tethys was soft enough to absorb the impact rather than shatter.

Now consider Miranda, the strangest Moon in the Outer Solar System, in orbit around Uranus. Its surface comprises huge angular blocks of a strange “banded” terrain with few impact craters, surrounded by a densely impacted terrain. The impact crater distribution would suggest that the banded terrain is younger, but the internal structure of one block of banded terrain appears to be folded like a chevron (or sergeant's stripes), and the fold appears to be truncated at the terrain boundary. Geologically this would suggest that the banded terrain was folded first, and then included in the surrounding terrane - in apparent contradiction to the sequence deduced from the impact cratering. The initial explanation for this moon was that – unlike Tethys - it did indeed explode and shatter when struck by a massive object, but that its exploded components didn’t separate by much and were able to accrete back together again to form a “Frankenstein” moon by gravitational attraction.

Planetary scientists have since realised that this simply won’t work, and are now suggesting that the bands in the banded terrain represent fractures at various angles that guided the eruption of ice volcanism that replaced the older ice - and that the large blocks of banded terrain are bounded by large geological faults that explain their abrupt boundaries. To date this is a pretty sketchy theory, but it’s better than the model of an exploded and reassembled moon to explain Miranda.