[LINK] "In defense of Callisto"

[rfmcdonald]

Callisto, outermost of second-largest of four Jupiter's planet-sized moons, has been neglected. Even though it's a huge complex world very nearly the size of Mercury, it's neglected in favour of more spectacular moons elsewhere in the Jupiter system--volcanic Io, Europa with its oceans and possible life, even near-twin Ganymede with its grooved terrain--or other planet's moons like Saturn's atmosphere-laden Titan. It's even neglected in science fiction. When Jupiter is stellified at the end of Arthur C. Clarke's 2010--I blogged about the likely consequences of the stellification in January here--it's Ganymede that gets warmed into habitability, while Callisto is still a boring wasteland. Callisto's marked as a dead world, hence an uninteresting one.

Douglas Muir has stepped up to Callisto's defense. It's interesting, too, with its own evolutionary processes and mysteries! (Is it for certain it doesn't have a subsurface ocean, though?)

First, let's get the painfully obvious stuff out of the way: yes, Callisto is a bit on the bland side. It doesn't have an atmosphere, like Titan. It doesn' have volcanoes, like Io, or geysers, like Enceladus. It probably doesn't have a huge internal ocean of water like Europa. It's not part of a "double planet" like Charon. It doesn't have a magnetic field, like Ganymede. It's just a large, icy moon with a heavily cratered surface. So, yes, you could say that Callisto is less interesting than some other places.
Except that you'd be stupid to say that, because Callisto is actually pretty damn interesting.

Let's start with the most common misconception, which is that Callisto is "geologically dead". We're told that its surface is "saturated" with craters, so that any new crater would obliterate one or more old ones. Craters, nothing but craters. Right?

Wrong. Much of Callisto's surface is -- wait for it -- eroded. Yes, it's full of craters, but there are vast regions where the craters have been degraded to the point where you can hardly recognize them. All that's left are smooth, undulating basins with lumps or spires in the middle.

What's causing the erosion? Well, take a moment to consider how odd Callisto actually is. It's a large icy world that's relatively warm -- daytime surface temperatures get up to around 160 Kelvin, or about -170 Fahrenheit, and can peak at another 10 K higher than that at noon on the equator. That's actually pretty toasty for an ice moon. The other moons of Jupiter are all 30 or 40 degrees cooler than that. Callisto is warmer because it's dark -- it has a really low albedo. (Why? We're not sure. One guess is that radiolysis has broken down organic compounds, leaving a sooty residue.) Whatever the reason, Callisto is the warmest large icy body in the Solar System.

So Callisto gets warm enough that water ice can sublime. That's very different from, say, someplace like Titan. At Titan's 95 K, water ice is completely inert, dead as granite. But at 160 K? Water can actually have a vapor pressure. A very tiny vapor pressure, to be sure. But over geological time, many millions of years, water ice will slowly sublime away into the vacuum. The sharp edges of craters will gradually blur and then slump. Much of the vapor is lost to space, but some condenses as bright, reflective frost. That's what we're seeing when we look at Callisto... mostly dark stuff, but with gleaming shiny bright bits. So if you could walk around the surface of Callisto, it wouldn't look much like Earth's Moon, all gravel and sharp edges. Instead, most features would be rounded and soft-edged.

[. . .]

You'll still see a lot of people saying that Callisto's surface is "old", "ancient", or even "pristine". No. Even at the macro level, all those big craters have been softened by erosion, and the composition of the surface has been dramatically changed by radiolysis and the movement of volatiles. It's like the difference between a bright new shiny penny, and one that's old, worn down, and tarnished. And at the micro level, the scale of a human walking around, Callisto's surface has been completely transformed. It's not old at all.

And it's probably still evolving. Callisto's surface is being shaped by subtle, slow processes -- sublimation, condensation, radiolysis -- working over geologic time. These things aren't flashy. But they get results, and they're just as interesting as the faster and more blatant processes taking place on Io or Titan.

Go, read the post and the comments.