Ganymede

When I was younger, my favorite author--hands down--was Robert A. Heinlein. One of the "Big Three" science fiction writers (along with Arthur C. Clarke and Isaac Asimov), most of Heinlein's early work was geared toward teenagers, telling rousing adventure stories in space. Later, his writing got more adult-oriented, more overtly politically charged, and to be completely honest, worse. But I still enjoy picking up one of the books I read and reread as a kid and reading through it in an afternoon, largely because Heinlein put a lot of effort into writing "hard" scifi that was as grounded as possible in actual science.

One of my favorite books of his was called Farmer in the Sky, and tells the story of a family which emigrates to the Galilean moon of Ganymede in order to participate in its terraforming and escape a crushingly overcrowded Earth (frequent themes in Heinlein's writing). The book devotes a great deal of time to explaining (in a way that, at least to me, managed to be consistently interesting) how the colonists would go about seeding volcanic dust and crushed rock with organic material in order to convert it into arable soil, as well as how pumping oxygen and greenhouse gases into the moon's atmosphere could give it breathable--and warm enough--air. Later, near the climax of the book, a syzygy of Ganymede, Europa, Io, and Jupiter* causes enormous tidal strains that cause a catastrophic earthquake (moonquake? Ganymedequake?).

Ganymede, as seen by the Galileo probe (Source)

Despite being a pretty smart guy, Heinlein was an author, not a scientist, and he got a few things wrong. But I don't mind, because looking into the science behind the book led me to some pretty interesting discoveries.

First off, that pseudo-syzygy: Heinlein goes to great pains in the book to emphasize how infrequent such an event would be, given the orbits of the three moons involved. But what he didn't realize is that Ganymede, Europa, and Io lining up in front of Jupiter wouldn't just be infrequent--it's actually impossible. The three inner Galilean moons are actually in a 4:2:1 orbital resonance, where it takes Europa twice as long to orbit as it does Io, and it takes Ganymede four times as long. As a result, the three can actually never line up all at once. If you don't believe me, try watching this great little animation for a while:

(Source)

As I looked into things further, I started to wonder how plausible growing anything on Ganymede would really be (assuming an amenable atmosphere and range of temperatures). The book is very aware of how far away Jupiter is--a large section of it is devoted entirely to the trip there--but once the characters arrive on Ganymede, this is never really mentioned again. However, plants require sun to grow, and there's a whole lot less sunlight illuminating Ganymede as there is powering photosynthesis on Earth. So to figure it out, I decided to bust out the old inverse-square law (to his credit, my first exposure to that term actually did come from reading Heinlein) and figure out exactly how much energy is reaching Ganymede.\[F=\frac{L_{sun}}{4 \pi D_G^2}\] We know the luminosity of the sun pretty well as about $4 \times 10^{33}$ erg/sec, and Ganymede is on average the same distance from the sun as Jupiter--around $8 \times 10^{11}$ meters. This gives us a flux of about $5 \times 10^8$ ergs per second per square meter. What does that mean? Well, an astronomical unit away from the Sun, the Earth is pulling in a flux of about $1.5 \times 10^{10}$ ergs per second per square meter. To be fair, many plants can grow in the shade on Earth, where they may be receiving 1%-20% of the solar energy they would receive in full sunlight. Ganymede, receiving 3% of the flux that Earth gets, would only be able to host the very hardiest of these--true agriculture would be impossible. Wheat, a vital crop, struggles to grow even in very mild shade. To drive the point home, here's a great illustration of how the Sun would look from various planets in the solar system. 

Hard to imagine a dot 1/5 the diameter of the Sun on Earth keeping any plants alive. (Source)

One last mistake of Heinlein's can be forgiven, since the information wasn't available at the time. However, his characterization of Ganymede as a dusty, dormant-volcanic ball was pretty far off the mark. As it turns out, the moon's surface is extremely rich with water ice. It does have an iron core, and its convection makes Ganymede the only moon in our solar system known to have a magnetosphere. However, salts on the surface suggest that 200 kilometers below lies a vast saltwater ocean--in fact, after growing up fascinated by Europa's subterranean (subeuropean?) oceans and the possibility of life there, I was surprised to find that Ganymede and Callisto are both thought to harbor deep oceans as well (though the conditions are much less favorable for life on the outer moons, which are not heated by tidal forces to the same degree as Europa). Volcanism on Ganymede is virtually nonexistent.

So what prompted me to start pondering Ganymede? Well, a few days ago JPL and the USGS put out a pretty remarkable map of the moon using data collected by the Pioneer and Voyager missions along with the Galileo probe. I'm including an image here, but its worth following through to the source and really examining it, because it's incredibly detailed. The color, obviously, is false, but it represents an incredibly chaotic landscape, with jarring divisions between old and new terrain shaped by three distinct phases of the moon's geologic evolution, from a stretch of frequent impacts to one of tectonic upheaval into a final period of relative calm.

Ganymede (you should really click through to the Source)

Doesn't look like there are any farms down there.

*Actually, this technically isn't a syzygy--a syzygy is defined as a straight-line alignment of exactly three celestial bodies (I imagine because the term was coined to describe the conditions that result in a solar eclipse on Earth). There isn't a word that I'm aware of to describe the straight-line alignment of four celestial bodies, so I went with syzygy anyway. Also because it's just a fun word (and a good-but-not-great episode of The X-Files--wow, this is turning into a nerdy post).