Habitability in Avatar -- greenhouse effect and disallowed orbits

[tracer]

Gods, I feel like such a geek for this, but here goes:

According to the Avatar Wiki page on Pandora, the the gas giant the planet's in orbit around (named "Poluphemus", apparently) is itself in orbit around Alpha Centauri A.

The planet's atmosphere is 18% CO₂ with a total pressure of 0.9 atmospheres at sea level. That much CO₂ would cause a substantial greenhouse effect, warming the planet substantially -- yet the surface temperature is in the range where liquid water can exist. This, coupled with the fact that Alpha Centauri A is 1.5 times as bright as our Sun, means that Pandora/Polyphemus would have to orbit Alpha Centauri A substantially farther away than Earth orbits the Sun.

BUT, Alpha Centauri is a binary star system (trinary if you count Proxima), and at periastron, Alpha Centauri B is only 11.4 A.U. away from Alpha Centauri A. This means ANY object orbiting Alpha Centauri A can't be more than about 2.8 A.U. away from it, or else the perturbations from Alpha Centauri B would throw it out of the star system.

With as much off a greenhouse effect as an 18% CO₂ nearly-Earth-pressure atmosphere would have, would 2.8 A.U. even be within the liquid water zone?

[IsaacKuo]

I don't know how to do the greenhouse effect calculation, but the brightness is easy enough. At a distance of, say, 2.5A.U., the insolation would be only 24% of Earth's.

[tracer]

True, I shoulda done the inverse-square calculation myself.

Receiving only 24% of the light that the Earth does should more than offset the greenhouse effect from even that much CO₂.

Hey! Now we know how to fix Global Warming. Just move the Earth farther away from the sun! (Gotta watch out that you don't get too close to the orbit of Mars, though.)

[Gillianren]

Gods, I feel like such a geek for this, but here goes:

According to the Avatar Wiki page on Pandora, the the gas giant the planet's in orbit around (named "Poluphemus", apparently) is itself in orbit around Alpha Centauri A.

Ooo, my turn to be a geek!

Polyyphemus would make something approaching sense. He was the Cyclops Odysseus blinded in the Odyssey. I can even make an incredibly contrived mythological explanation for a connection between him and Pandora, thought it would be incredibly contrived.

[ABR.]

Polyyphemus....

My sincerest apologies, Gillianren, but I was drawn to this misspelling as a moth is drawn to the flame.

[Swift]

Hey! Now we know how to fix Global Warming. Just move the Earth farther away from the sun! (Gotta watch out that you don't get too close to the orbit of Mars, though.)

IIRC, in Larry Niven's "Known Space" science fiction, the Puppeteers did exactly that to their home world.

[HenrikOlsen]

Not global warming in the sense we use it, but their power use had become so large that they had to move the planet in order to get rid of the excess heat.

[brother_unknown]

Here, I thought Pandora was a satellite of a giant planet. Only the planet (or binary twin never rotates) and the minor moons never move either. Then there's the problem of the tides and the total lack of rain even though those waterfalls just keep on falling. maybe they are recycling?

[IsaacKuo]

Here, I thought Pandora was a satellite of a giant planet. Only the planet (or binary twin never rotates) and the minor moons never move either.

Pandora is a moon of a giant planet. The planet would not appear to move in the sky, assuming the moon is tide-locked (this is to be expected). The other moons would generally move around, but this movement may or may not be obvious on the time scale of the story.

Then there's the problem of the tides and the total lack of rain even though those waterfalls just keep on falling. maybe they are recycling?

What problem with tides? And it's not clear that there was no rain during the time frame of the story.

[Gigabyte]

It was raining in several scenes.

[tracer]

Pandora is a moon of a giant planet. The planet would not appear to move in the sky, assuming the moon is tide-locked (this is to be expected).

This would also mean that the side of Pandora that the Na'vi were on (the side facing the gas-giant Polyphemus) would experience night when facing the daylit side of Polyphemus. Assuming Polyphemus has a decent albedo, the night sky should be lit up well enough to read by!

It would also mean that, for a few hours during the day, the sun (Alpha Centauri A) should be eclipsed BEHIND Polyphemus, giving them a much darker midday "night" than they would have during the real night. Unless Pandora's orbit around Polyphemus was HIGHLY inclined.*


Then again, since the shots of Pandora from space show a planet covered mostly in oceans (like the Earth), there may be enough liquid sloshing around on the surface to prevent tidelocking with Polyphemus. In which case its rotational period may be separate from its orbital period.

Did they say in the movie how long a day/night cycle lasts on Pandora?





*) I know this because a couple decades ago, I wrote a science fiction/fantasy story set on a world orbiting a gas giant: http://www.rogermwilcox.name/stories/rylon.html.

[IsaacKuo]

This would also mean that the side of Pandora that the Na'vi were on (the side facing the gas-giant Polyphemus) would experience night when facing the daylit side of Polyphemus. Assuming Polyphemus has a decent albedo, the night sky should be lit up well enough to read by!

Generally, yes. I had also calculated things for a story set on such a moon, with a ~24 hour orbit/day. The size of the gas giant (Mu Arae d) would be large enough that night-time lighting would not be strictly necessary.

It would also mean that, for a few hours during the day, the sun (Alpha Centauri A) should be eclipsed BEHIND Polyphemus, giving them a much darker midday "night" than they would have during the real night. Unless Pandora's orbit around Polyphemus was HIGHLY inclined.*

Pandora's orbit could indeed be highly inclined. An inclination of, say, 10 degrees should do it. Of course, there would still be two times during the year with daily eclipses.

For my calculations, I didn't get such a long eclipse time--maybe up to 2 hours, depending on the time of year. Of course, this all depends on the specific size and mass of the gas giant, the orbital distance, and so on.

Then again, since the shots of Pandora from space show a planet covered mostly in oceans (like the Earth), there may be enough liquid sloshing around on the surface to prevent tidelocking with Polyphemus.

Liquid sloshing around would accelerate the process of tide-locking, rather than retard it.

[clint]

Liquid sloshing around would accelerate the process of tide-locking, rather than retard it.

Wouldn't that make a relatively moderate/stable climate highly unlikely?
I remember clear skies, not any wind to speak of, moderate temperatures day and night - oh, and lots of huge vegetation

[IsaacKuo]

Wouldn't that make a relatively moderate/stable climate highly unlikely?

I'm not sure I understand the question. Here on Earth, we have liquid sloshing around. Since Earth is not tide-locked to either the Sun or the Moon, we have tidal flows of ocean water.

Pandora is presumably tide-locked, so it would not have tides due to the planet. It would still have solar tides, although these would be pretty weak under the assumptions in this discussion thread (planetary orbit at much greater than 1A.U.)

[clint]

I'm not sure I understand the question. Here on Earth, we have liquid sloshing around. Since Earth is not tide-locked to either the Sun or the Moon, we have tidal flows of ocean water.

Pandora is presumably tide-locked, so it would not have tides due to the planet. It would still have solar tides, although these would be pretty weak under the assumptions in this discussion thread (planetary orbit at much greater than 1A.U.)

Ok, my mistake: re-reading my post and the thread I just realized that somehow I was automatically thinking of tidally locked explanets
in thight orbits around their stars and thus with extreme temperature differences between day and night sides.

Not the case here, since Pandora is tidally locked to its parent planet, but not to its star.

[Inclusa]

What if we have Pandora as our moon instead of our very moon?

[chrlzs]

It was raining in several scenes.

Er.. which scene/s? I don't recall any.

[PlutonianEmpire]

Here, I thought Pandora was a satellite of a giant planet. Only the planet (or binary twin never rotates) and the minor moons never move either. Then there's the problem of the tides and the total lack of rain even though those waterfalls just keep on falling. maybe they are recycling?

The waterfalls on the surface are fine. And since it rained periodically (some scenes have rain in them -- I remember at least three), they shouldn't go away anytime soon, since waterfalls are always parts of rivers, and rivers always have a source body of water (a lake somewhere).

But.

The waterfalls coming out of the "floating mountains" just screams bad science. A waterfall needs a river, and a river needs a lake to originate from, which most of the time are permanent, especially since they're on the surface. But on those floating mountains, the source lakes would be rapidly depleted, depending on the water's flow rate and how much it rains, and the waterfalls wouldn't last long. And yet, the movie depicts them quite literally as perpetual.

Pandora is a moon of a giant planet. The planet would not appear to move in the sky, assuming the moon is tide-locked (this is to be expected). The other moons would generally move around, but this movement may or may not be obvious on the time scale of the story.

What problem with tides? And it's not clear that there was no rain during the time frame of the story.

The time frame was at least three (Earth) months, according to what one of the characters implied, and I think some scenes showed the actual times and dates. And as stated previously, there was rain in some of the scenes.

Pandora's orbit could indeed be highly inclined. An inclination of, say, 10 degrees should do it. Of course, there would still be two times during the year with daily eclipses.

The Avater Wiki stated the moon's axial tilt was about 29 degrees, so assuming a tide-locked rotation, Pandora's orbital inclination would, of course, be about 29 degrees.

For my calculations, I didn't get such a long eclipse time--maybe up to 2 hours, depending on the time of year. Of course, this all depends on the specific size and mass of the gas giant, the orbital distance, and so on.

The same Wiki also states that Polyphemus is "smaller than Saturn, but with a higher mass." We have an example of that right here at home: Neptune is a little bit smaller than Uranus, but has a higher mass.

I remember clear skies, not any wind to speak of, moderate temperatures day and night - oh, and lots of huge vegetation

I'm guessing they were close to the equator. And as stated previously, the time scale was about 3 months. Plenty of time for rain, even if it was offscreen.

I'm not sure I understand the question. Here on Earth, we have liquid sloshing around. Since Earth is not tide-locked to either the Sun or the Moon, we have tidal flows of ocean water.

Pandora is presumably tide-locked, so it would not have tides due to the planet. It would still have solar tides, although these would be pretty weak under the assumptions in this discussion thread (planetary orbit at much greater than 1A.U.)

Don't forget the other moons. It is stated somewhere in the canon info published on the wiki and the book that Polyphemus has other large moons. They might play a role in Pandora's tides, depending on the scale of the planet-moon system. It is stated there are 14 moons in all, which I guess might be sorta accurate, since being in a Sol-like sun's habitable zone would result in a smaller hill sphere radius than at a distance comparable to Jupiter, which will probably limit the number of moons a planet has.

What if we have Pandora as our moon instead of our very moon?

That's opening up a whole new can of worms right there!

[JustAFriend]

IIRC, in Larry Niven's "Known Space" science fiction, the Puppeteers did exactly that to their home world.

They did it in the global warming episode of "Futurama", too....

[whimsyfree]

18% CO2 is a lot. I think they over-egged it. 4% would be quite enought to make it unbreathable. If Pandora approaches Earth mass then it seems over-massive to be a moon, unless it were captured. In that case a large swathe of Polyyphemus orbitals would be cleared. If it's much smaller, how did it retain its atmosphere? Then again it's apparently made of magic anti-gravity rocks or something so planetary physics as we know it need not apply. :-/

[swampyankee]

IIRC, in Larry Niven's "Known Space" science fiction, the Puppeteers did exactly that to their home world.

In one of his books, A World out of Time, Earth was moved for some reason or another. iirc, it was because of global warming, but I can't remember why. I didn't particularly dislike the book, but I never thought highly enough of it to reread it.

[publiusr]

If you can swallow the floating mountains and the unobtanium, who cares about the orbits? Just say that any excess heat light is somehow absorbed by the element--and its presence is why the moon isn't phase locked as normal matter would be due to its effects.

[Van Rijn]

In one of his books, A World out of Time, Earth was moved for some reason or another. iirc, it was because of global warming, but I can't remember why.

This is a little spoilerish, so for anyone intending to read the book I'll give a


*SPOILERS*


warning.


There was an interstellar war with humans that had colonized other star systems. Humans from the colonies built a giant fusion pulse rocket and used it to fling a gas giant into the sun (in the story, I believe this was Persephone - a distant gas giant supposedly found orbiting the sun some time in the future). When the planet hit the sun it temporarily increased the sun's luminosity. In response the solar system's civilization built its own pulse rocket and used it on Uranus or Neptune (can't remember which), which they used to gravity drag the Earth further from the sun. There's more to it than that, but I'll leave it there.

I didn't particularly dislike the book, but I never thought highly enough of it to reread it.

I liked the story a lot because there were just so many grand ideas. However, I've since had concerns about the feasibility of a lot of the ideas presented, so I'm not as impressed with it as I was originally.

[PlutonianEmpire]

18% CO2 is a lot. I think they over-egged it. 4% would be quite enought to make it unbreathable. If Pandora approaches Earth mass then it seems over-massive to be a moon, unless it were captured. In that case a large swathe of Polyyphemus orbitals would be cleared. If it's much smaller, how did it retain its atmosphere? Then again it's apparently made of magic anti-gravity rocks or something so planetary physics as we know it need not apply. :-/

The Diameter, according to the book and the wiki, is 11447 kilometers (meaning a radius of 5723.5 kilometers), with a mass of 0.72 Earth masses.

It is quite irritating they don't use more digits past the decimal points.

And, it is already quite known that the atmosphere already is unbreathable for humans. The natives, of course, can breathe in the atmosphere. Of course, I don't know if it is even possible for natives to evolve to breathe such high CO2 concentrations.

As for being over-massive, I don't know if we truly know enough yet to make a determination one way or the other. But I could be wrong. I recall hearing some groups are doing higher-precision analysis of Kepler data to find Earth-size exomoons, although I doubt they'll find any.

[whimsyfree]

The Diameter, according to the book and the wiki, is 11447 kilometers (meaning a radius of 5723.5 kilometers), with a mass of 0.72 Earth masses.

The book and wiki aren't the film. I assume the latter is something fans made up, and the former something that was made up for fans.

It is quite irritating they don't use more digits past the decimal points.

I'll give you some: 0.721402805. My made up numbers are as good as any you'll find on the interwebs, I assure you!

I don't know if it is even possible for natives to evolve to breathe such high CO2 concentrations.

I don't see why not. I'm assuming there's some oxygen to go along with it.

As for being over-massive, I don't know if we truly know enough yet to make a determination one way or the other. But I could be wrong. I recall hearing some groups are doing higher-precision analysis of Kepler data to find Earth-size exomoons, although I doubt they'll find any.

Papers I've read on the formation of moons from disks around gas giants suggest the moons should be much smaller than the gas giant (up to about 10^-4 the mass). Apparently Pandora is much more massive than that, and has a bunch of similar mass siblings. It makes the sky more sci-fi, I guess, and formation theories are just theories anyway. Actually large co-moons would probably be bad because their gravity would induce excess tidal dissipation in Pandora, leading to massive vulcanism (see Io).

[publiusr]

One way or another, Pandora is an engineered world. Existing only in CGI for real, one could say the fictional world was made by some super entity that took lfe forms from all over and made them work by introducing anti-gravity elements that in trace amounts allow large creatures and slightly different gravity while not losing a lot of atmosphere. Here there is no need for the internet, just that tree. No need for fuel burning aircraft or ground mobility, there are animals easily ridden for that. We can't ride on flying animals--we have to have engines.

It is if some Q level entity wanted a race of creatures that had all the modern conveniences, but in a greener, animal based form not available to us here.

[whimsyfree]

One way or another, Pandora is an engineered world. Existing only in CGI for real, one could say the fictional world was made by some super entity that took lfe forms from all over and made them work by introducing anti-gravity elements that in trace amounts allow large creatures and slightly different gravity while not losing a lot of atmosphere. Here there is no need for the internet, just that tree. No need for fuel burning aircraft or ground mobility, there are animals easily ridden for that. We can't ride on flying animals--we have to have engines.

It is if some Q level entity wanted a race of creatures that had all the modern conveniences, but in a greener, animal based form not available to us here.

"Q level entity"? is that what they're calling hollywood execs now?

What you describe is (science) fantasy, not science fiction. I'm only interested in the scientific aspects of the story and ignore fantasy elements (such as the anti-gravity element and planetary consciousness). Trying to argue with fans who are convinced it's all possible (if not real) is an unproductive exercise in my experience.

0.72 M_E is not too small to hold an atmosphere, but rideable flying animals would require gravity that was a small fraction of Earth's or a much thicker atmosphere, which are inconsistent with Pandora's stated properties. They seem shy about its orbit, but

Because of its high axial tilt (29°), Pandora exhibits considerable annual variation in the day-to-night ratio. In addition, its elliptical orbit produces seasonal temperature variations and a range in daytime illumination of about ten percent.

seems to confuse Pandora's orbit with Polyphemus'. If anyone can be bothered they can get a handle on the Polyphemus' apastron and periastron from

At the closest point in its orbit, ACB is about 2,300 times as bright as Earth's full moon; at its furthest point, it is still one hundred and seventy times as bright.

and thence Pandora's semimajor axis (its eccentricity should be negligible) from

Polyphemus occasionally eclipses ACB at night for about one hundred minutes, but the light reflected by the planet still keeps the night from being dark.

(The last bit makes me think Pandora's rotation is not synchronous, which is very unlikely).

As to the life,

Due to Pandora's lower gravity, most creatures on Pandora are hexapods (six-legged)

doesn't make any sense at all. Lower gravity reduces the need for support and increases the rewards to a hopping, bounding or leaping gait.

[aquitaine]

0.72 ME is not too small to hold an atmosphere, but rideable flying animals would require gravity that was a small fraction of Earth's or a much thicker atmosphere, which are inconsistent with Pandora's stated properties. They seem shy about its orbit, but

Just as a point of reference, despite having an atmosphere thicker than ours Titan has a mass of only 0.0225 Earth masses, despite it's radius being 0.4 Earth's.


If anyone asks I was only in it for the Sfx.....

[whimsyfree]

Just as a point of reference, despite having an atmosphere thicker than ours Titan has a mass of only 0.0225 Earth masses, despite it's radius being 0.4 Earth's.

Of what import do you consider that "point of reference"?

[Van Rijn]

0.72 M_E is not too small to hold an atmosphere, but rideable flying animals would require gravity that was a small fraction of Earth's or a much thicker atmosphere, which are inconsistent with Pandora's stated properties.

People can fly on ultralight airplanes on Earth, so I don't see a physical reason why there would need to be even lower gravity or thicker atmosphere than is assumed for Pandora for there to be flying animals that could be ridden there. These are not Earth species, they are stated to be built differently (carbon fiber bones, for instance), and though it's not discussed in the movie, the flying animals seem to have air intakes on their wings similar to one of Poul Anderson's fictional flying species (in Anderson's stories, it allows them to have a very high energy metabolism).