I’m thinking about the antromorphic principle, and would apply that: if you see a lush forest, it’s because it took millions of years of evolution to get plants adapted to all that radiation and darkness. After all, on Earth, bacteria have adapted to boiling water of 100 C (hot springs) as well as -40 C and below in the Artic; animals live deep in the water with hundreds of pounds of pressure and no light; plants live on high mountains with very thin atmosphere, little oxygen and harsh UV radiation; some life forms use not photosynthesis, but use the chemical heat from underwater vents in the ocean for their life energy.
So, just because it looks like an Earth jungle doesn’t mean it hasn’t super-duper strong radiation-shielding on it’s cells (on Earth, most insect shells are surprisingly radiation-resistant) plus some advanced damage-repair (we have some of that in Earth organisms, but don’t need it as desperatly.
Maybe they have some hibernating cycle for the times of darkness, like Earth plants do in winter, when there is little sun. We don’t know how it works, but it’s there, so it obviously works.
Outside-universe explanation: well trees look pretty!
Colibri – excellent refs. The duration of eclipses is shorter than I’d imagined, and Titan is farther out than I;'d realized.(But note that some of the satellites are eclipsed regularly – Callisto is mentioned because it’s unusual in that regard) I still maintain that an immersion of hours in total shadow is going to have big implications for the planet’s life and weather patterns, if not as severe as I’d imagined.
Of course, as shown on screen, Polyphemus, Endor and Yavin (forgot that one) are an awful lot closerr to their primaries than the Galilean moons, but we’ll chalk that up to cinematic coolness. I’m still doubting those Earth-like forests and jungles.
This isn’t so even in our own solar system. Uranus, with an axial inclination of about 98 degrees, rotates more or less on its side compared to the other planets. Its moons orbit in approximately the plane of its axial tilt, so they orbit nearly at right angles to the other planets and satellites of the solar system. Because of this, eclipses of the moons are also rare, happening only about every 42 years during Uranus’s equinoxes.
ETA: We have some pretty bizarre planets and satellites even in our very limited sample in the Solar System. Two of eight planets (Venus and Uranus) rotate retrograde to the others, and there are some odd satellite orbits as well.
Another thought that hasn’t been brought up, wouldn’t being a sizable moon of a Jovian planet increase the chances of biosphere disrupting impactor collisions (dinosaur-slayer level). I’ve read that Jupiter gravity keeps the inner solar system “clean”, and we here on Earth have a nice 65 million year lucky streak going. While this influence seems to works out well for us, I’m not sure if Europa and friends the get the same benefit being in Jupiter’s gravity well.
Not sure if this has been linked, but the Solar System Simulator lets you see what Jupiter looks like from the surface of its Big 4 moons.
Clarke had Europa supporting very primitive life even now, and becoming even more verdant in 2010: Odyssey Two and its sequels after Jupiter ignited (thanks to the Monoliths) to become the ministar Lucifer.
And for those saying that such a moon would have to have its own magnetic field, who says the fictional examples don’t? Pandora, certainly, has some pretty funky magnetic phenomena going on: That’s what causes the mountains (which have high concentrations of superconducting unobtainium ore) to levitate, and what scrambles Earthly electronics in that region.
Don’t underestimate the ability of life-forms to adapt to high radiation levels. Deinococcus radiodurans is a bacterium that can thrive after a 5,000 Gray dose of radiation (equal to about 500,000 rem of gamma radiation, 5-10 Gy will kill a human), and further up the complexity ladder tardigrades can survive similar doses. Keep in mind that nowhere on earth will an organism be subjected to anywhere near these levels of ionizing radiation - it just happens that the adaptations they have to survive desiccation also seem to work for radiation exposure. If organisms on earth can evolve such radiation tolerance basically by accident, organisms evolving on a hypothetical moon-planet under constant selective pressure should be able to do just fine.
Why do we keep discussing radiation exposure? Even if the planet was smack in the middle of a gas giant’s radiation belt, even if it didn’t have it’s own magnetic field to protect it, has everyone forgotten about the atmosphere this moon presumably has? UNLIKE totally unprotected Ganymede?
It’s own atmosphere, and its own magnetic field; as I understand it, it is presently believed that without a magnetic field the solar wind & radiation belts will blow away most of the atmosphere and you’ll end up with something like Mars. So radiation isn’t likely to be a problem.
As far as Alan Dean Foster, the book generally brought up in relation to Avatar is Midworld, which centers on a jungle world, with a collective world mind, and with inhabitants who live symbiotically with it in places called Home Trees. And they are threatened by an offworld base that threatens the tree that holds their ancestors in order to extract a special substance. So there’s a fair amount of similarities, although there are quite a few differences as well.
The discussion about the scientific plausibility or implausiblity aside, movie directors don’t care about the science; they care about the story, the characters, and the effects. If it sounds good or looks good then that is enough reason, even if it isn’t fully exploited.
Personally I’d trace the inspiration to the fact that several of our solar systems’s gas giant moons are being talked up as possible locations of life, albeit microbiologic if so. Not lush rain forests but perhaps living off of geothermal heat sources perhaps induced by gravitional shifts. See here for example. Other than Mars these moons are the next best bets locally; small step to speculate a full ecosystem evolving from there … for a creative writer type anyway.
One fictional example of a Earthlike moon of a gas giant is a story by Poul Anderson. I’ve forgotten the name of the story, but it involved an alien Sir-Francis-Drake-like sea captain and his crew. (As far as I can remember, there are no humans in the story.) This may be the earliest science fictional example of an inhabited moon of a non-solar system gas giant, although I wouldn’t be surprised if there were earlier examples.
The moon they’re on is tidally locked (as are virtually all natural satellites) and the country the captain is from is on the farside of the moon (facing away from the planet). So this captain and crew are circumnavigating their world for the first time and are awestruck when they get to the nearside and have this big planet perpetually hanging in the sky. Don’t remember too much else about the story.
As far as eclipses go, since the moon will be tidally locked, the life on it will have to be evolved to live on a world which gets very long nights. Much longer than any eclipse. And half the moon will not get any eclipses at all, since it will be night on that side during the eclipses. I don’t see the eclipses as being a problem.
That sounds an awful lot like the first book of Robert J. Sawyer’s Quintaglio Ascension trilogy, Far Seer, where one of the sentient mini-tyrannosaurs living on a gas giant moon goes on a pilgrimage to the other side of the moon to see “the Face of God” and realizes that it’s a planet they’re orbiting.
Regarding tidally locking: why would that make the nights long? Because of the orbital period around the planet? Wouldn’t that depend on how far the moon orbits from the planet?
In his novel Nemesis, Isaac Asimov pictures that situation.
Nemesis is a red dwarf, so a planet must be very close to it in order to receive enough energy to support life.
But a planet so close to its primary must be tidally locked, with eternal day in one side and eternal night in the other.
So, Asimov made the planet a gas giant, with a big moon orbiting it. In the moon there were the conditions to support life.
