Callisto – Ancient World
A journey past the major (Galilean) moons of Jupiter from the inside out is also a journey from intense activity to virtual quiescence, and from full internal differentiation into metal, rock and volatiles to a much more homogeneous mix. So while Io is unremittingly active and fully differentiated, Callisto is the other end of the spectrum: it has an ancient surface and is mostly an ice/rock mix.
Unsurprisingly, the gas giant at the centre is the main cause: the closest moons get the most tidal heating and probably also the most energy input by meteorite impacts during the Late Heavy Bombardment due to gravitational focussing by Jupiter1. It may also be a product of the nature of the protojovian disk from which the moons coalesced: at greater distances from Jupiter the material in the disk was sparser and moons formed more slowly. Callisto is almost twice as far from Jupiter as Ganymede (1,880,000km vs. 1,070,000km) and is thought to have formed over a much longer period2. It didn’t get the quick burst of the energy of coalescence which could trigger full differentiation.
A cratered world
Callisto has a very old surface: no signs of volcanism, little tectonic fracturing and still displaying most of the impact craters which have built up over its history. It shows clearly just how important proximity to Jupiter can be: it has much the same composition and size as Ganymede, but its location far away from the gas giant means it has none of the complex surface breakup features seen on its neighbour.
The surface isn’t completely pristine, though: there are a lot fewer small impact craters than you’d expect, and many craters have incomplete rims as if parts of them have broken off. The surface is a mix of rock and ice, and the degradation is thought to be due to sublimation of the ice component and landsliding down of the leftover rock lag. This may set up a feedback loop where dark rock lag at the base of scarps warms the ice there and makes it sublimate away, undermining the slope and making it fail some more3.
All this goes to show that you don’t need eruptions from within to modify the surface of planet – while Voyager II images had suggested that there may be cryovolcanism here, better resolution Galileo images showed no proof of that and it’s more likely smooth areas were formed by degradation processes like these.
It’s not surprising that nothing is getting out to the surface from inside Callisto: modelling using the gravitational data from Gallileo suggests the moon has a thick ‘stagnant lid’ of ice and rock about 100km thick. There’s a lot of discussion about exactly what layers there are down there, but generally it’s thought though it’s about 50/50 rock and ice overall, it’s rockier with depth. That’s likely due to slow convection in a rock/ice mantle, with denser rock moving towards the centre and cleaner less dense ice accumulating at the top4. This convection would occur as it does in the earth’s rock mantle: over long timescales in the solid state with no melting necessarily occurring.
There is some liquid H2O in Callisto, though: Galileo’s magnetometer showed the moon causes perturbations in Jupiter’s magnetic field which are best explained by a liquid saltwater ocean beneath the surface5. That’s difficult to square with theories of a convecting ice mantle, which should keep temperatures too low for so much liquid. For that reason, it’s commonly suggested that there’s a significant amount of ammonia along with the liquid water, about 5%6. That acts as antifreeze: it lowers the freezing temperature of ice by almost 100K and would make it possible for a liquid ocean to not only exist, but be as much as 120km thick7.
Why go to Callisto?
Such an inactive moon may seem like an unlikely target for human exploration: the lack of differentiation would lead to less useful mineral deposits near the surface and the thickness of the icy lid makes discovering life there unlikely. But as a staging-post, it’s ideal. Unlike Jupiter’s other moons, it’s far enough out to suffer little irradiation from the gas giant so, apart from being very cold, it’s a fairly benign environment. Also, because it has such a stable surface, there are none of the geological hazards you’d encounter on, say, Io.
Looking to the future, we could use it as a base for exploring the other Galilean moons or for voyages further out into the solar system. Maybe there are some advantages to being the strong, silent type.
- Barr & Canup (2010) Nature Geoscience: DOI:10.1038/NGO746 [↩ ]
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- Kuskov & Kronrod (2005) Icarus: 177, 550-569 [↩ ]