What is this strange object? A space eyeball? The ‘Death Star’ from Star Wars? It’s actually a lump of solid ice named Mimas, a small moon of Saturn, less than 300 km in diameter, and the ‘iris’ of the eye is in fact a huge impact crater, named after the astronomer Herschel. The crater’s diameter is almost a third of that of its host so it’s amazing that the whole moon managed to survive such a massive impact. However, the shock waves must have shaken it to its core, as evidenced by fractures on the opposite side of the moon to the crater.
Note how the moon is totally covered with craters of various sizes, with newer ones overlapping older ones in a dense pattern. The Herschel crater itself is deformed by many smaller craters, showing that it the moon continued to be bombarded by space debris long after this major impact. In other words, craters can give us important clues about the relative age of surface features – the greater the crater density, the older the surface or any particular feature. As the surface of Mimas is covered in craters, it clearly has not been disturbed by anything else for a very long time.
Enceladus, another of Saturn’s icy moons is very different. Here you can see a clear contrast in cratering density between the right-hand and left-hand sides of the image. The right is heavily cratered, whereas the left has very few craters and instead has a smoother, rather wrinkled appearance. The lack of craters here indicates that the surface is relatively young – it would originally have had as many craters as the right-hand side but they have been smoothed out or replaced in some way by smoother ice, and there has not been enough time since this process for much new cratering to occur.
As you may have read elsewhere (including in this blog), Enceladus is actually geologically active at its South Pole, spewing out water vapour from large cracks in its crust, and it’s thought that cryo-volcanism, whereby water from a sub-surface ocean reaches the surface and freezes, is responsible for the resurfacing that we can see on the left of this image.
Equally, there are very few impact craters on Earth, because our planet also has a very young surface – it’s constantly being recycled by plate tectonics.
As well as telling us about the age of the surface of a planet or moon, craters may have other stories to tell. For example, on Mars and the Moon they can create rather special environments. If they are deep enough, some parts of a crater floor can remain shaded from sunlight, keeping temperatures very low and hence preserving ice that would otherwise melt in sunlight.
Here you can see the ice-filled Korolov crater near the north pole of Mars – the crater is 82 km across and 2 km deep so that’s quite a lot of ice! And patches of water ice have also been confirmed in numerous craters at both the south and north poles of the Moon – potentially a very valuable resource for future exploration of our nearest neighbour.
https://solarsystem.nasa.gov/moons/saturn-moons/mimas/in-depth/
https://www.space.com/35956-saturn-moon-enceladus-craters-cassini-photo.html
https://blogs.nasa.gov/pluto/2015/10/13/the-impact-of-craters/
https://www.nasa.gov/feature/ames/ice-confirmed-at-the-moon-s-poles
https://www.esa.int/Our_Activities/Space_Science/Mars_Express/Mars_Express_gets_festive_A_winter_wonderland_on_Mars
Image Credits:
1. NASA/JPL-Caltech/Space Science Institute (Cassini mission)
2. NASA/JPL-Caltech/Space Science Institute (Cassini mission)
3. ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO (Mars Express mission)