In defense of Callisto

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.

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.

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. 

 

 

 

 

Comments

[Russell Arben Fox]

Doug, this was a totally fascinating read; a great way to start out my Monday morning. Born (yet frustrated) intellectual critic that I am, I couldn't help but think while reading it that this post is exactly the sort of thing which drives the vision of some of the true internet enthusiasts out there: the idea that what the world wide web gives us is the opportunity for everyone's expertise to contribute to everyone else's overall knowledge. You have some real awareness here, and now you have a platform from which to share it.

One actual question about Castillo though: exactly what kind of "organic compounds" could that moon have possessed which could have been broken down by radiolysis? I await your learned response.

[Doug M.]

It's not a learned response so much as a hastily researched one. But: organic compounds are everywhere. After all, carbon is a pretty common element, and tolerably reactive even at low temperatures. Combine it with hydrogen, and you easily get methane and ethane. Combine it with oxygen, you get carbon dioxide and carbon monixide. Combine it with nitrogen, you get nitriles and other cyanide compounds. These, in turn, will react with each other pretty readily to form a wide range of simple organic compounds.

In the particular case of Callisto, Galileo found spectra of CO2, sulfur, cyanide, and an odd spectrum that could mean the presence of "tholins", the miscellaneous organic compounds that form the "smog" in the atmosphere of Titan.

A 2008 paper gave this analysis:

"A significant effect of ion bombardment of ices is the creation of new species through radiolysis... Ionizing radiation not only forms more complex molecules, it can efficiently decompose organics. At low doses, CO and CO2 are the principal decomposition products of a number of organics and carbonates... A carbon cycle may be occurring on Callisto, with CO2, carbonates and carbon suboxides as principal end-products. The overall dark gray visible appearance of Callisto is consistent with laboratory measurements of carbonization of organics through radiation. Surfaces made up of organics exposed to high amounts of radiation (e.g., inner solar system, or older surfaces in the outer solar system) get darker and blacker (or grayer), whereas organic-containing surfaces exposed to less radiation are redder."

-- This last is why a lot of Kuiper Belt objects have a reddish color; that seems to be the default color of more-or-less primordial organics.

Notice that over time, you have different effects feeding into each other. Radiolysis breaks primordial "red" organic compounds down into darker sooty stuff. This in turn lowers the moon's albedo, causing it to get warmer. This increases sublimation, changing the topography as discussed above. Sublimation also releases water vapor and carbon dioxide into Callisto's very thin and tenuous atmosphere. These in turn may get ionized and cracked by radiation; the hydrogen would be lost to space quickly, but the carbon and oxygen may linger long enough to react with the surface.

There are probably several feedback loops at work here, gradually evolving over geologic time.

There's still a lot we don't know. And won't know for a while to come. The next probe to the Jupiter system -- JUNO -- should launch later this year, but JUNO will be looking at Jupiter itself rather than the moons. Nobdy's planning to visit Callisto again until 2025 at the earliest, and that's if EJSM / Laplace gets fully funded, which at the moment is not looking like a good bet. "Sometime between 2025 and 2030", would be my guess.

Planetary science is awesome! but you need to be very patient.

Doug M.

[Bill Higgins-- Beam Jockey]

Callisto would get some sympathy from Mercury, which is often dismissed as a "boring old dead-for-billions-of-years world."

[Andy Nathan]

Great Post! I really liked your description of Callisto. I have read a lot about Titan, but not Callisto. Thanks!

[Air Hexagonal]

Your post is my first read this Monday morning. I don't know a lot about space thing, but Callisto remind me about something enemy name :D *no offence

Callisto will thank you for defensing it from "boring old dead-for-billions-of-years world."

I'll read about Callisto soon after this.