I mean Callisto, the fourth major moon of Jupiter. Yeah, this is a nerd post.
So I'm a regular reader of the Planetary Society's blog. If you're interested in astronomy and space exploration, it's full of good stuff.
But just the other day, in an article about Pluto's atmosphere -- which, yes, Pluto has an atmosphere; told you this stuff was interesting -- I came across this sentence:
It's just more data to support the expectation that New Horizons will, when it finally gets to Pluto, find something that's not a Callisto-like boring old dead-for-billions-of-years world but instead a place with a changeable, varying surface and a fascinating but complicated history.
Geez. Poor Callisto.
This isn't anything new. Discussion of Saturn's moon Titan -- the only moon with a real atmosphere -- often converge on the question of whether Titan has volcanic activity or not. And the no-volcanism models use the term "Callisto with weather". Typically in an extremely cruel and denigratory way. "It's like Callisto," they snicker, "boring old dead-for-billions-of-years Callisto! Except with weather. Like that helps."
I think it's time to say a word -- just a word or two -- in defense of poor old Callisto.
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.
Here's another consequence: from a distance, Callisto looks heavily cratered. But if you get up close, it doesn't. At scales of a kilometer or less, you don't see a lot of small craters. They've mostly been eroded away.
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.
And this is just what we know from a handful of flybys with Galileo. Callisto is big. It's the the third largest moon in the Solar System -- comfortably larger than Earth's Moon, and just a whisker smaller than the planet Mercury. (The three big moons, Titan and Ganymede and Callisto, are in a class by themselves; there's a big jump in size between them and #4, Io.) So there's room for all sorts of surprises.
Callisto also holds the answers to some interesting questions. For instance: when we look at the Moon, many of the craters that we see were created all at once, in the Late Heavy Bombardment. Did the LHB hit Callisto too? Or was it purely a phenomenon of the Inner Solar System? If the latter, then does Callisto show a smooth age distribution of craters? Or might there be some other, different event that scarred much of its surface?
So, Callisto is an excellent moon -- active, interesting, and well worthy of closer examination. But here's another thing. There are seven large moons in the Solar System: the four Galilean moons of Jupiter, Earth's Moon, Saturn's Titan, and Neptune's Triton. (After that there's a big gap, and the next largest moons are pretty dinky.) Seven isn't a very big number, but there's reason to think that the other moons are all, in their different ways, kinda strange.
The other large moons of Jupiter are all locked in an orbital resonance with each other: Europa's orbital period is exactly twice that of Io, and Ganymede's is exactly twice Europa's. This results in tidal heating -- in Io's case, enough to drive a lot of volcanoes. And they're all constantly blasted by particles from Jupiter's radiation belts. It's thought that this is why Ganymede doesn't have an atmosphere. (Even though it's a bit bigger than Titan, which does.) It also means that radiolysis plays a big part in their surface chemistry -- much bigger than on Callisto.
Meanwhile, Titan is weird because it has an atmosphere, clouds, and rain. Earth's Moon is weird because it's rocky and hot. And Triton is weird because it's not a real moon at all, but a captured plutoid in a retrograde orbit.
Looked at this way, Callisto isn't "boring" so much as it's "normal". It's a perfectly straightforward moon. It's, like, the control. We don't really know what sorts of small worlds exist out there beyond out Solar System, but I'd venture a guess that there are a lot of places like Callisto.