Polyphemus and Pandora: what evocative names!
In giving these new worlds names from classical mythology, their discoverers followed a tradition that dates back to 1781, when the British astronomer Sir William Herschel discovered the solar system’s seventh planet, the first found beyond the wandering bodies visible to the naked eye that had been known since before humans were humans. Eventually the new planet was named Uranus, in Greek mythology the personification of heaven and the son and husband of Gaia, the Earth goddess—though Herschel had hoped to name it Georgium Sidus, in honour of King George III: a planet called George!
In myth, Polyphemus, with a name meaning “very famous,” was a Cyclops, a cannibalistic one-eyed giant encountered by Odysseus in Homer’s Odyssey. It seems an apt name for a giant world dominated by a single glaring-eye storm. And to the Greeks, Pandora, whose name means “giver of all,” was the first woman. Out of curiosity she opened up the famous “Pandora’s Box” (actually a jar), thus releasing all the evils of mankind, leaving only Hope inside the box as consolation. Certainly it seems appropriate that a world as fecund as Pandora should be given the name of the Greek Eve.
But if you were to follow Jake Sully down the ramp off the Valkyrie, it’s probably not the moon’s name you’d be thinking of in your first moments on Pandora, but its low gravity.
Colonel Quaritch is suspicious of Pandora’s low gravity. He obsessively pumps iron to avoid being made “soft” as a result.
Pandora’s gravity is about eighty per cent of Earth’s. Its diameter is three-quarters of Earth’s, and its mass about half; in size it’s a world somewhere between Earth and Mars, which has around one-third Earth’s gravity.
But Pandora’s air is thicker, about twenty per cent denser than Earth’s. You might wonder how a low-gravity world can hold on to a thick atmosphere, as Pandora evidently does. On any world air molecules can be heated to “escape velocity” and just fly off into space, like tiny spacecraft. A battering by the solar wind, charged particles from the sun, adds to that leakage as well. The lower the gravity, the more air will escape to space. Thus our moon with around a sixth Earth’s gravity is all but airless.
But gravity isn’t the only factor when it comes to a world keeping its air. Titan has around the same gravity as the moon, but, as we saw in Chapter 7, its atmosphere is more massive than Earth’s. This is because it is so cold out there at the orbit of Saturn; Titan’s air molecules move much more slowly, on average, and fewer escape. On the other hand Venus, only a tad smaller than Earth, has a much more massive atmosphere than the Earth because it’s too hot; all the heavy carbon dioxide that’s locked up in the rocks on Earth is baked out into the air on Venus.
Leakage of an atmosphere can be surprisingly slow too. It’s thought that an Earthlike atmosphere somehow delivered to one-third-gravity Mars (perhaps as part of a “terraforming” project, making Mars into a second Earth) would take around ten million years to leak away. That’s a slow enough process for a civilisation to manage an artificial atmosphere if it had to (recall the air machines on Burroughs’ Barsoom). There are natural inputs to a planet’s air too, from outgassing via volcanoes, and impacts from comets. And there are other special factors. Pandora orbits between radiation belts surrounding its primary Polyphemus, which deflect the charged-particle wind from the sun. This is evidently a complex question; a world’s lower gravity does not imply it must have thinner air.
But what effect would a different gravity have on living things?
Galileo was able to figure out the basic physics of gravity and bodies back in 1638: “It would be impossible to fashion skeletons for men, horses or other animals which could exist and carry out their functions [proportionally] when such animals were increased to immense weight…”
This work was the origin of the famous “square-cube law.” If you double the size of an animal, its cross-section goes up as the square of the size—four times—but its volume, and therefore its mass, goes up as the cube—eight times. This basic rule is central to “biomechanics,” the discipline of how living things are put together mechanically. This means that you couldn’t just double the size of an elephant in some genetic-engineering brainstorm and expect it to function; its four-times-thicker muscles wouldn’t be able to raise its eight-times-greater weight.
Ah, but what if you transported said elephant to a lower-gravity world, like Pandora?
There’s a difference between mass and weight. Mass is a resistance to motion. You would have the same mass even in zero gravity, in space. You get weight in a gravity field. Weight is mass multiplied by the acceleration due to gravity, which is approximately ten metres per second per second on Earth. Weight is the load you have to carry around. In space you would still have mass, but no weight.
We all have a maximum weight we can bear, given the strength of our bones and muscles. But in a lower gravity field, you could carry around more mass: higher mass times lower gravity comes out to the same weight. How much more mass depends on the weakness of the gravity.
Humans have complicated geometries, so let’s simplify things. There’s an old joke about the farmer who’s having trouble with his milk production, and he calls in theoretical physicists from the local university to help. After weeks of intensive study, back comes the report which begins: “Consider a spherical cow…” (Well, it made me laugh.) The point is, to figure out basic principles, scientists will often make simplified models of the real world to make the calculations easier, even if the models are somewhat unlike the real thing.
And in that spirit, consider a cylindrical biologist.
Here’s Dr. Grace Augustine, standing tall on Earth, probably giving some RDA desk jockey a hard time. She could be represented by a pillar a bit less than two metres tall, say twenty centimetres diameter. The pressure she’s exerting on the bones holding her up is her weight divided by her cross-sectional area.
Now let’s suppose we stretch her up by twenty-five per cent, without making her any wider. She’ll still be shorter than the average Na’vi, at around three metres. Her mass has gone up twenty-five per cent, and so has her weight, but her cross-section hasn’t changed. So the pressure on her bones is up twenty-five per cent too.
That could be a problem, if we kept stretching her. Grace’s bones can support only a certain maximum weight, because beyond that the pressure would overcome the binding energy of her bones’ molecules; the bones would splinter and Grace would fall. So, in a given gravity field, and with bones of a given strength, there is a limit on Grace’s height—and indeed her mass—unless you thicken up her bones like an elephant’s.
But now let’s whisk Tall-Grace to Pandora. The gravity here is eighty per cent of Earth’s. And so, though her mass is unchanged, her weight (twenty-five per cent more mass times eighty per cent gravity) is the same as Short-Grace’s back on Earth, because the lower gravity has cancelled out the extra height. And thus the pressure on Tall-Grace’s bones is as low as it was for Small-Grace back on Earth, and she feels no discomfort.
There are plenty of subtleties beyond this simple argument. Even given their height the Na’vi look remarkably slender—narrower bones mean higher pressure—but, as we’ll see in Chapter 25, their bones are strengthened by a naturally occurring carbon fibre.
And we should remember that the Na’vi didn’t have to be as tall as they are. No animal has to grow as large as the laws of physics allow it to. The Na’vi’s apparent close relative, the prolemuris, is no more than a metre and a half tall, just as on Earth our hominid ancestors were all chimp-sized until the emergence of Homo erectus, about as tall as us, a couple of million years ago. The Na’vi are as tall as they are because something in their evolutionary history made it right for them to be so. However their height does illustrate that a human body form that would be impossibly tall and slender on Earth can work on Pandora.
What are the limits? How big could a land animal grow on Pandora?
Pandoran beasts are big. Even the direhorse is larger than any horse on Earth. The heaviest living land animal on Earth is the African elephant; a bull can stand some four metres tall at the shoulder. The heaviest animal of all was the brachiosaurus which died out some hundred and thirty million years ago, and stood around seven metres tall at the shoulder. The largest land animal we see on Pandora in Avatar is probably the hammer-head titanothere at maybe six metres tall—like an elephant scaled up in Pandora’s gravity field. Perhaps greater beasts roam in parts of Pandora yet unexplored.
Pandora’s low gravity would help you fly, especially with the aid of that thick air. On Titan, the air is so thick and the gravity so low that a human could fly by the power of her own muscles, flapping artificial wings. So we could have predicted big flying animals on Pandora.
Earth’s largest flying creature was the winged reptile Pteranodon ingens, which flew over Kansas some eighty million years ago, with a wingspan of around nine metres. On Pandora a mountain banshee exceeds that at around twelve metres wingspan, and a leonopteryx would dwarf it, with a wingspan of thirty metres. The size a flying creature could reach depends on other factors than gravity, such as the density of the air and the oxygen content—the more oxygen, the more energy you have available to keep you aloft.
On Earth you have to look to the sea for the real monsters in size. The blue whale is thought to be the heaviest animal ever to have existed, weighing in at some hundred and ninety tonnes (compared to around five tonnes for an African elephant). If we visit Pandora’s oceans in the future, there will be monsters, I have no doubt.
And what of the tremendous trees of Pandora?
On Earth, the basic physical constraint on tree height is the need for the tree to be able to lift water to its uppermost leaves. The tallest known tree on Earth is a sequoia in northern California, at a hundred and sixteen metres tall. The theory says that a tree could possibly reach as much as a hundred and thirty metres—and there have been historical accounts of trees a hundred and twenty metres tall. By comparison Hometree on Pandora is some three hundred metres tall, nearly three times the size of that big old sequoia. This is more than the simple gravity scaling might suggest, but Hometree evidently has a different architecture from a sequoia, with pillar-like multiple trunks, themselves as sturdy as sequoias, enclosing a large internal hollow.
Pandora’s low gravity would enable some wistful architectural designs: impossibly long arches, impossibly slender columns. We don’t see any native architecture on Pandora; with the hometrees available for habitation I suppose building is unnecessary. And the humans at Hell’s Gate show no imagination in their own functional building schemes. Maybe the Stone Arches are a glimpse of what would be possible.
But in fact the Stone Arches seem to be a product of the single most remarkable physical phenomenon on Pandora: its unobtanium, and the magnetic fields with which it is associated. And if you followed Jake Sully to Pandora you would very quickly learn that unobtanium is the reason you, and RDA, are here.