Tuesday, February 16, 2010

A Pandoric Column: A Science-Based Look at Avatar

Above: A stolen picture of a pretty stroll on Pandora.

I'm not into writing movie reviews, and I don't intend to change this with Avatar, James Cameron's latest installment of revolutionary cinema. Suffice it to say, I thought the movie's visuals were stunning while the predictable story sucked. The combination of these two factors averaged out to an enjoyable experience that lost a lot on second viewing.

But Avatar offers entertainment beyond a three-hour brain drain in a theater made too-dark through bulky 3D glasses. It gives me a chance to do the kind of supernerd mental exercise that's kept Star Trek fans drooling for four decades. One thing almost universally praised about Avatar, after all, is the phenomenal world it depicts. And Pandora is a beautiful place. The question my inner geek keeps asking, though, is whether such a place could actually exist. Does James Cameron's paradise follow the rules of science? Could there really be a world like Pandora floating out there somewhere?

Just the Facts

We don't know much about Pandora. All we have is what we can infer from the images on the screen and a few bits of expository dialogue.

So what we know is this: Pandora is one of several large moons of a gas giant planet orbiting a yellow star similar to the sun. The gas giant appears to be blue when observed from space, closer in color to Neptune than Jupiter, suggesting a lot of atmospheric methane. In order for Pandora to be warm enough to support life, though, its primary must be much closer to its sun than Neptune is to ours, and it must receive a lot more solar radiation. One result of this is that the primary's atmosphere is more chaotic, with more violent storms and eddy currents than have been observed on Neptune.

Pandora itself has less mass than Earth, which translates to the slightly lower gravity mentioned in passing by Colonel Quaritch. Whether this owes to a difference in the moon's size or its density is not explained. Pandora's gravity is strong enough, however, to maintain a dense atmosphere, which is why animals walking on the moon's surface don't explode and surface water doesn't boil away. The movie never tells us the exact composition of this atmosphere, though we know the air is poisonous to humans.

Pandora has evolved a lush and diverse ecosystem with pervasive examples of large mega flora and mega fauna. This suggests, based on the rate of evolution suggested by Earth's example, that the moon can't be much younger than Earth's four-and-a-half billion years. It took four billion years for the first trees to evolve on Earth, after all, and Pandora has some really big trees. The moon also has a stable climate. Most of the territory seen appears to exist in a tropical or subtropical region--though oddly enough we never see rain. The water cycle is active, though, as we see a number of high-volume rivers and streams, as well as a few waterfalls in unreasonable locations.

The waterfalls aren't the only odd aspect of this moon. In one region, Pandora displays magnetic anomalies that make technological navigation useless. This same region has a number of strange rock formations, including a naturally-occurring structure of some generic gray rock formed into giant rings balanced on their edges. And it has mountains that float.

Pandoric Geophysics

I must admit, my assumption while watching the film was that the entire notion of Pandora was nothing more than the fantasy of a science-deprived mind. And I found support for this assumption when I considered Pandora's orbit. At first glance, there seemed to be something very, very wrong with Pandora's orbit.

We don't know exactly how long a day on Pandora is, but we do know that it's not too far off from the 24-hour days of Earth. Humans maintain the same circadian rhythm they've evolved on Earth. They wake up at sunrise and sleep at sunset, so a Pandoran day must be similar in length to an Earth day.

But Pandora isn't a planet. It's a moon orbiting a gas giant. And moons orbiting gas giants tend to be tidally locked, meaning they keep the same side facing their primary at all points in their orbit. The fact that Pandora's primary seems always to occupy the same patch of sky relative to, say, Home Tree suggests Pandora also is tidally locked. The day-night cycle, then, matches the amount of time it takes the moon to complete an orbit of its planet. And 24 hours seems way too fast for any real Pandora to orbit its planet. In order for Pandora to have an orbital period of 24 hours, I'd think it would either have to be moving at an immense speed, or it would have to orbit at an extremely low altitude. There's no way. At the very least, a Pandora with a 24-hour day would long ago have fallen inside its primary's Roche limit and shattered.

Fortunately, there are complicated mathematical equations that will tell you this sort of thing, and these equations are easy to find on the internet. So let us consider that first point. Just what would be the Roche limit of Pandora's primary?

For those unfamiliar with the concept, a planet's Roche limit is the distance from that planet a satellite can orbit without tidal forces shattering it to pieces and turning it into a pretty set of rings. The assumption is that Saturn's rings exist because some satellite ignored the Roche limit and paid the price. Neptune's moon, Triton, is on the edge of Neptune's Roche limit, needing nothing more than a slight push to fall over the edge. So in a real universe, would Pandora be doomed to a similar outcome?

To figure this, we have to know the mass of both Pandora and its primary. We don't know either, but let us make a guess. Say Pandora's mass roughly matches that of Earth. (Quaritch says it's lower, but from observation, things fall on Pandora at about the same rate as they fall on Earth. The difference is nominal.) Let us also say that the mass of Pandora's primary matches that of Jupiter. It could be much larger, but we don't know this. Plug everything into the equation, and what do you get? The Roche limit for Pandora's primary falls well within the radius of the primary itself. In essence, Pandora would have to orbit within its primary to be in trouble.

Okay, so Pandora isn't doomed to become a set of rings. But what about the speed of its orbit? How close would Pandora have to orbit to have a 24-hour day? Plug everything into that equation, and you get a distance of 180,000 miles from the primary's center. If the primary is as large as Jupiter, then Pandora would orbit some 138,000 miles above the atmosphere, 40,000 miles farther than our own moon orbits above us.

So, the upshot: it's possible. Pandora could exist.

As a side note, though, consider the other moons you see in Pandora's sky. All moons of any planet will exist in the same orbital plane, meaning they'll occupy the same piece of Pandora's sky. These other moons seem to be located wherever they'd be most aesthetically pleasing. So while Pandora could exist, these other moons probably could not.

What About the Air?

The composition of Pandora's atmosphere is never explained. We just know that humans can't breathe it. The air doesn't kill instantly, though; it's not as if some caustic agent eats away at the lungs. Humans recover quickly from exposure, eliminating a number of harmful gases such as chlorine from the list of possibilities. It's more that humans suffocate in the air without masks supplying oxygen. This suggests the problem with Pandora's atmosphere is more a lack of oxygen than an abundance of anything else.

This is unreasonable, though, for the simple reason that things burn on Pandora. Things burn quite well, in fact; a significant portion of the predictable plot depends on this fact. And when things burn, they burn beneath a sky the same color blue as that produced by the scattering of light by oxygen molecules in Earth's atmosphere. And they burn while animals as large as Earth's dinosaurs run or fly at energetic speeds, thanks to their ability to efficiently metabolize foods and produce energy. It's unlikely that anything other than oxygen would allow that kind of metabolism, just as it's unlikely that anything other than oxygen would produce a blue sky or allow a fire hot enough to burn a giant tree. It's clear, therefore, that a significant portion of Pandora's atmosphere must be oxygen.

Of course, that's not all it must be. Oxygen only makes up 20% of Earth's atmosphere, after all, so I assume a similar level for Pandora. In fact, any greater proportion of oxygen would allow fires to burn too well. But what about the other 80% of the atmosphere? On Earth, the dominant gas is nitrogen, but what about Pandora?

My chemistry isn't strong enough to come up with an informed answer. My first guess would be a carbon-oxygen compound, carbon monoxide or carbon dioxide, assuming either could exist in great quantity with pure O2. This would explain the difficulty humans have breathing the air. It also would be a step toward explaining the shear size of Pandora's plant life. It's reasonable to think that a warm world with an 80% carbon dioxide atmosphere would have enough carbon floating around to produce a thousand-foot tree. The problem with that theory, though, is that carbon monoxide and carbon dioxide suppress fires, so burning would involve a fight between oxygen and carbon.

But Floating Mountains? I Mean, Come on! Really?

Okay, so this is the hard part. How do you explain a mountain, a collection of rock no different than any rock on Earth, that floats? Yeah. This one's harder to explain, but there is a possibility.

It starts with the horribly-named material called unobtainium, the unimaginably valuable substance that justifies the expense to which humans have gone in order to colonize Pandora. Like anything else on Pandora, we have no idea what unobtainium is. Despite its -ium suffix, we know unobtainium isn't an element, as all the elements in the real world with an atomic number lower than 115 have been discovered and given better names. Anything new with a higher atomic number isn't likely to occur in nature, and if it did, it would be radioactive. Unobtainium must be a mineral then. It probably should have been named unobtainite.

All we know about unobtainium, we get from a clunky expository speech from our resident corporate stooge, who tells us the substance occurs in great abundance beneath Pandora's surface. And it floats. The corporate stooge keeps a sample floating over a little plastic circle on his desk.

Now, I'm not going to simply say the stuff contains some sort of antigravity particle that allows it to ignore Pandora's mass or something, as that's Star Trek science. I will theorize, though, that unobtainium may have some electromagnetic property that, if placed in certain environments, will repel matter. Maybe the little plastic circle on the corporate stooge's desk is also an electromagnet, so the interaction lets the unobtainium drift at whim.

Meanwhile, we know that Pandora orbits a gas giant. We know from observing gas giants in our own solar system that they can generate strong magnetic fields. Jupiter provides one example. Jupiter also provides the example of Io, the shallowest of its large moons. Io orbits close enough that it interacts with Jupiter's magnetic field, creating the Io flux tube, a stream of magnetic energy between the moon and planet. An enormous amount of electromagnetic energy passes constantly between Io and Jupiter.

Perhaps there is a similar flux tube between Pandora and its primary. Perhaps this electromagnetic energy interacts with unobtainium in Pandoric rock at the focal point of this interaction, allowing the rock to negate the downward force of Pandora's gravity so that mountains can float. It also could explain the giant rocks formed into rings, as these rocks may have formed from some crystalization process under the electromagnetic influence.

Another theory is that tidal forces acted upon Pandora from its primary could negate gravity in a location directly facing the primary, especially since we know that Pandora must have a shallow orbit. But we see near the end of the film that while the rocks here float, the bodies of Nav'i killed in battle fall like rocks. A gravitational effect would be reflected in everything in this location, including the slaughtered warriors and the pretty waterfalls.

And that's one thing about the floating mountains that not even Star Trek science can explain. Vast streams of water pour from several of these mountains. It's difficult to estimate just how much, as we don't know the width, depth, or velocity of the streams as they fall. These streams do look similar in size to known waterfalls on Earth, though. One example would be Yosemite Falls, which has an estimated flow of 300 cubic feet, or 2400 gallons, per second.

But any basin that feeds a waterfall at that volume would have to receive at least that much water in precipitation. The basins that feed these falls are tiny, nothing more than large, isolated rocks the size of a football field or two at most. Imagine the rain a football field would have to receive to feed a constant waterfall. There is no vast area where water collects. For a basin this small, inflow would have to look pretty much like outflow. Rain would have to fall on the mountains in an unreasonable waterfall as constant as the falls they produce. Yet we never see any precipitation here at all. The waterfalls, therefore, are nothing more than pretty pictures.

And About Those Glowing Leaves?

Finally, we have the biology of Pandora. Could evolution give us a Pandoric world, with its biolumenescence and its massive trees and its little spinny bugs that glow in the dark? In all honestly, I haven't the slightest idea.

I really don't know what's required for bioluminescence, for instance. I do know it results from a chemical reaction, and that like any form of light generation, it consumes some amount of energy. To occur in the amounts it occurs on Pandora, it would have to use a lot of energy. Nature doesn't necessarily mind using a lot of energy--and as previously discussed, Pandora may have a lot of energy available--but there usually has to be a reason. On Earth, bioluminescence has at least one of four purposes: it exists to attract mates or prey, to repel predators, to communicate, or to illuminate dark places. Most of Earth's bioluminescence, in fact, occurs in places where there isn't much light, such as near the bottom of deep oceans. It's not particularly dark on Pandora, though, even at night. So why would so much of Pandora glow? Did evolution on Pandora simply hit some bioluminescent arms race?

Pandora's life offers other puzzles. While we see a diverse collection of animals, they tend to follow certain themes. They all are brightly colored, which would be unexpected in an environment with so many predators. On Earth, natural selection tends to eliminate animals that stand out. The only true examples of brightly-colored creatures tend to be isolated, such as birds on tropical islands without predators.

This is only one example in which Pandora's animals move counter to what would be evolutionarily reasonable. A number of large animals, for instance, have six legs. It takes a certain amount of energy to move a leg, though, and more legs simply means more required energy. Evolution tends to move toward efficiency, though. This is why Earth's mammals have moved toward the minimum number of legs required. Humans get by with two.

Pandora's animals also are much larger than animals on Earth. The Nav'i, Pandora's resident humanoids, are more than twice as tall as humans. Certain hippopotamus-like creatures come in at brontosaurus sizes. A evil cat-like predator is at least as big as an Earth hippopotamus, yet it moves with incredible strength and speed. And it moves at these speeds through giant bushes with giant leaves at the base of thousand-foot trees.

Much of this Pandoric tendency toward the outsized could be explained with the lower gravity, which reduces energy requirements. This alone wouldn't be enough, though, as we've already seen the gravity isn't that much lower. It's possible the high-carbon atmosphere allows for the giant plant-life, and that the giant plant life in turn allows for giant animals. There may simply be much more energy available on Pandora.

I think the biggest problem presented by Pandora's life forms, though, is their almost Trekian similarity to animals we know. Pandora has its giant, six-legged feline that acts like a panther. It has its barking doglike creatures that look like Baskervillian hounds. And it has its people, blue, bipedal humanoids with opposable thumbs and binocular vision that utilizes giant eyes that look a lot like our own. The Nav'i, in fact, look like they could be our more-perfect cousins, with hair that grows in the same places and mouths that can be used to communicate with the same movement of tongues. Nav'i women have obvious mammary glands in the same location as humans. And it is strongly implied that Nav'i sex is simply a version of what people do. In short, it's all too similar to what we have on Earth. So while it may be possible that such a similar place could exist, it seems extremely unlikely.

Of course, Star Trek's more extreme fanboys and fangirls have a number of built-in explanations for why the only obvious difference between Humans and Vulcans is the shape of the ear, and it's possible that James Cameron has a similar explanation for Pandora. And if he doesn't, it's a sure thing that eventually, somebody like me will come up with one. For now, though, I think the simplest explanation is the best:

It's Only a Movie

And that, I think, is the rub with Pandora. James Cameron has created a fantasy world, a more perfect and unspoiled Earth. He has created the perfect answer for what seems so lacking to so many in this world we've nearly destroyed. Science fiction has always done this sort of thing, and while some of it may depend more on the science and some of it more on the fiction, the audience has long showed its willingness to embrace or ignore both, just as it's demonstrated a willingness to ignore quality of story. It won't matter to most whether Pandora can exist, because to them, when they're sitting in that darkened theater, it already does.


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