Stephen Baxter - The Science of Avatar

So with a cross-section of ten centimetres squared, and if Pandora’s gravity is eighty per cent of Earth’s, the pressure required to hold up the lump is (weight divided by area) about sixteen hundred pascals.

The standard formula for magnetic pressure (easy to find in any physics text) tells us that the pressure exerted by a magnetic field is proportional to the square of the field strength. And the standard unit of magnetic field strength, or “flux density,” is the tesla (T)—named after Nikola Tesla, a Serbian-American inventor once played by David Bowie, in the 2006 movie The Prestige . (A tesla is equivalent to ten thousand gauss, in other units.)

It turns out that to get a pressure of thirteen hundred pascals you need a magnetic field strength of around sixty mT (milli-teslas—each a thousandth of a tesla). How strong is this? Well, it’s several hundred times the strength of Earth’s magnetic field at ground level (which is only about a ten-thousandth of a tesla; a tesla is actually a pretty large amount). It’s stronger than a toy fridge magnet, at a few milli-teslas, but weaker than the coil gap in a loudspeaker, which might be about a tesla. So it’s certainly plausible that a lump like Selfridge’s desk ornament could be lifted by a magnet of everyday household use.

It seems remarkable that even a toy magnet is so much stronger than Earth’s magnetic field—and, if you use one to pick up a pin, you will witness its magnetism overcoming the gravity pull of an entire planet . But you have to think of magnetic field strength as a kind of density; there’s an enormous amount of energy stored in Earth’s field, which works globally—even if locally, on a very small scale, it is much weaker than the fridge magnet. And on larger scales, whereas electrical and magnetic forces can attract or repel (think of positive and negative charges, north and south poles) gravity only ever attracts. So electromagnetic forces can be strong on short scales but cancel out on larger scales, whereas the attractive force of gravity just piles up and up. That’s why the structure of your body is dominated by electromagnetic forces, but the structure of the universe, such as the orbits of planets and the spiral forms of galaxies, is determined by gravity, not electromagnetism.

Selfridge’s toy is one thing. What about the Hallelujah Mountains?

Just as I imagined a biologist as a cylinder ( Chapter 14), now imagine a mountain as a cube, a hundred metres on a side (many of the mountains are a lot larger), with the density of rock. This is a lot more mass than the desk ornament—around two million tonnes—and the pressure needed to keep it up is much greater too, at around one million, six hundred thousand pascals. And the magnetic field strength we need is greater too, around a couple of teslas.

A couple of teslas might not sound much. It’s well within what modern human technology can produce—the big magnetic resonance imaging systems in hospitals can run up to fields of several teslas, on a small scale.

But this is several thousand times Earth’s field strength. It’s stronger than the magnetic field around Jupiter. It’s stronger even than the sun’s field at the location of a solar flare, an event powerful enough to batter the Earth across more than a hundred million kilometres with enough charged particles to crash power grids. But there are stronger magnetic fields in nature; the field at the surface of a neutron star, a compressed supernova remnant of the kind that created unobtanium in the first place, can run to hundreds of millions of teslas. (Robert L. Forward’s novel Dragon’s Egg (1980) and my own Flux (1993) showed life forms shaped by this bizarre environment.)

In the movie Avatar we see visual evidence of strong magnetic fields of a poetic sort. The Stone Arches that congregate over areas of strong flux, such as the Tree of Souls, are reminiscent of “solar prominences,” areas of intense magnetic activity on the surface of the sun where glowing plasma is lifted along flux lines to form tremendous arches—some big enough to straddle the Earth. The Arches are in fact a relic of Pandora’s magnetic fields. During the region’s formation flux loops shaped the rock when it was still molten, and held it there until it cooled and hardened. As a result arch formations can be used to locate unobtanium deposits, and act as warnings for pilots of aircraft of the presence of hazardous magnetic fields.

Regarding the flying mountains, even if you had the field strength, there are also questions of stability. If you experiment with fridge magnets you’ll find that supporting an object by repulsion isn’t so easy, as the object will slide off to one side or another, or flip over so that unlike poles are drawn together. With a superconducting body the effect is different, as the floating body is cushioned by the magnetic field excluded from its interior. Maglev experiments have shown that for stability you need the supporting field to be stronger at its periphery than at its centre, to keep the floating object in place. On Pandora, how could such a shaped field come about in nature? Perhaps there was some kind of feedback effect between the magnetic fields in the floating rocks and the still-molten ground, when the mountains were formed. Or, some researchers in the Avatar universe have speculated, the Hallelujahs could represent a balance achieved by a kind of consciousness, just as Eywa is integral to the balance of the ecology… Even so the Hallelujahs aren’t entirely stable, however. They have been known to collide, hence the Na’vi name for them of “Thundering Rocks”: tktktk .

Certainly Pandora’s intense magnetic fields will add to the hazards of a very hazardous world.

Colonel Quaritch likes to welcome newcomers to Pandora with a scary depiction of its dangerous life forms, the plants, the animals, the natives, all of which, according to him, want nothing more than to kill humans.

But Pandora would be a ferociously hazardous place even without any life forms at all.

Pandora is a volcanic world. And it’s that way because of where it orbits.

Consider the moons of Jupiter. Of the four largest moons, discovered by Galileo—Io, Europa, Ganymede and Callisto—closest-in Io is some six Jupiter radiuses from the giant planet’s centre, while furthest-out Callisto is about twenty-six radiuses out. Io has had its orbit tweaked into an ellipse by its neighbours, Europa and Ganymede. As a result Jupiter and the neighbouring moons together raise ferocious tides on Io—and not of water, as our moon raises tides in Earth’s oceans, but of rock. The whole moon is flexed and squeezed, an effect that heats Io from within, just as a rubber ball gets hot if you knead it in your grip. It’s just the same for Pandora, which too orbits a gas giant and has sister moons, so we must expect it to suffer similar tidal flexing.

Because of the heat injected by gravitational kneading, Io is the most volcanic world known. Its calderas spew out a hundred times as much lava as from all of Earth’s volcanoes—and that from a surface area just one-twelfth the size. The whole surface is riddled with sulphurous pits, lava pools and magma-spewing fissures. In NASA spacecraft images Io looks like nothing so much as a vast plate of pizza. This is unusual for a small world. Smaller planets lose their inner heat more quickly, and tectonic activity generally seizes up; that’s the case on the moon and even on Mars. Not on Io—and not on Pandora.

Clearly Pandora is not such an active world as this. But it is an arena of much more intense tectonic activity than Earth: a world of fractured continents, of volcanoes and earthquakes, of hot springs and geysers, and with its air polluted by carbon dioxide, hydrogen sulphide and other volcanic products.