Two movies – The Return of the Jedi and Avatar – feature not merely life on the moon of a gas giant, but lush jungle life thriving on such a moon. This strikes me as
a.) extremely picturesque
b.) highly unlikely
I have to admit that life on the moon of a gas giant would seem to me to be either very different from life on a straight planet – we’re talking either creatures adapted to very different environments, or else people living under domes or underground, Farmer in the Sky notwithstanding. In our system, at least, the gas giants are too far from the sun, and there’s radiation emitted from them. Add to this the fact that the moon passes behind the gas giant frequently and for long periods, cutting off what little light there is. I just find it very hard to accept that this is a friendly environment for Earth-type life, let alone luxuriant jungles.
So why do they do this? If it’s for the cinematic potential, neither movie is explouiting this to anything like its potential. Although I’ve read a lot of SF, I don’t recall this situation being a major feature in any important story (or SF artwork, like Chesley Bonestell’s). Is there some seminal work, like a novel, influential short story, or some NASA report suggesting this as a real possibility?This seems very far removed from the aforementioned Farmer in the cSky, or Clarke’s 2010.
or is it all highly romanticized response to all the talk about Jupiter’s moons and the possibility of life there? (The early 21st century version of Inhabited Mars and Venus)
Yeah, I thought about the Flinx books, but, despite Alan Dean Foster’s position as King of the Movie Tie-Ins, I don’t see him as a major influence in film. Besides, I think Cameron obliquely slighted Foster’s novelization of his film in his intro to Orson Scott Card’s novel of The Abyss. He wouldn’t be going to Foster for inspiration.
There’s no reason at all the moon of a gas giant couldn’t have an Earthlike environment, including forests. Several of the moons of the gas giants of the solar system are larger than some of the planets; there is no reason one couldn’t be as large as Earth. Just because the gas giants of our system are far from the Sun doesn’t mean they would be in other systems; many of the large exoplanets that have been discovered are quite close to their stars. And the orbital period of many moons is a matter of days; also, they orbit sufficiently far from their planet that they would not be blocked from receiving sunlight very often. So most of the assumptions in the OP that would make an Earthlike moon unlikely are unwarranted.
The planet (moon) Jinx is from Larry Niven’s Known Space series. It’s shaped like an Easter egg, kind of. I agree it has nothing to do with Avatar, of course.
I didn’t say they were requirements, but based on the examples we have (our own solar system, plus a few extra-solar ones), gas giants tend to be far out. The radiation issue is still there. And i’m not convinced that orbiting a gas giant – with what is still significant blockage from the sun, not to mention large swings in distance from it – won’t have significant effects.
Actually, it looks like our system is an outlier in this regard, and lots of other systems have giants (we assume gas giants) much closer in - there’s selection bias at work, sure, but we can still safely say that our particular “small rocks then big gasbags then snowballs” layout is not a hard rule.
Unless the moon has a magnetic field of its own or is very far from its planet, it would probably be blasted by lethal radiation from the planet’s magnetosphere. According to Wikipedia, Ganymede gets 8 rem of radiation every 24 hours on the surface, and that’s at 5 AU out, so it would probably be a lot more if it were closer to the sun. Looking at wiki again, that level of radiation would kill you in a matter of months. After one month, you’d have accumulated 240 rem, which has a 35% fatality rate, and it just keeps getting worse after that.
On the one hand, Jupiter’s magnetic field is the strongest in the solar system, so a different planet might throw less radiation at its moons. On the other hand, any planet close enough to its star to support life would be getting a lot more charged particles than Jupiter, and I wouldn’t want to spend all my time in Earth’s Van Allen belts either.
So, if you want life on a gas giant moon, it has to be either very resistant to radiation, or shielded by rock or the moon’s own magnetic field. I don’t think forests hiding inside rock would be very impressive.
Radiation wouldn’t be a problem per se if the moon had a shielding atmosphere like Earth’s. What concerns me is whether a planet/moon could retain an atmosphere under those conditions. Titan has a thick atmosphere but it’s a lot colder than Earth and Saturn’s magnetosphere isn’t as fierce as Jupiter’s. Also, wouldn’t tidal locking make the days extremely long, leading to greater temperature fluctuations?
Thanks, Peaton, for supporting what i said about radiation.
I also seriously think you folks are underestimating the effect of the “monthly” eclipse by ythe gas giant. Every month, when you go behind the giant for only a few days you are completely cut off from the sun. It’s not like an eclipse on earth, or like night time. In both those cases at least half the planet is still getting direct sunlight (and, even so, the temperature drops on the night side. Even in the umbra of an eclipse on earth there’s a marked temperature drop). So it’s going to get cold for those “few days”. Really cold. any life-forms are going to have to be adapted to regular extreme temperature swings. I suspect this will also lead to furious winds and extreme weather.
Instead of gentle and lush jungles, I’m thinking it’ll be more like New England, cubed. If you don’t like the weather, wait a minute.
But we’re working off of instruments biased towards finding massive, tightly orbiting planets. Kepler may improve that view but right now we’re working off of skewed data.
As for a magnetic field, what would the resulting impact be of a earth like dynamo within a Saturn or Jupiter magentic field? I’d imagine there’d be some awesome night time light shows.
Well if the planet is like Jupiter I’d imagine there’d be some adaptation that would try and harvest the IR given off by a gas giant. - though really that’s be a very thin gruel in comparison to “real” sunlight
More to the point, we have only just started developing the technology to locate planets orbiting other stars. We’ve only directly seen a very few, the most separated; others have been detected by the wobble they cause in the star around which they orbit or by changes in the star’s spectrum.
This is really important technology and I hope we can find more.
I seriously think you are overestimating the frequency and duration of eclipses. Are the moons of a gas giant completely eclipsed each month, and for “days at a time”? Earth’s moon is only eclipsed a couple times a year, and those are often just partial. The duration is only a matter of hours, not days. As I mentioned, many of the gas giant moons have an orbital period of a few days, not almost a month like Earth’s moon, which means they would spend even less time in the planet’s shadow.
The frequency of eclipses will also depend on the inclination of the moon’s orbit to the planet. The orbit of Titan, for example, only crosses Saturn’s line of sight to the Sun twice during Saturn’s 28-year orbital period, so eclipses only take place every 14 years. Titan orbits 1.2 million km out, three times farther out than the Moon; its orbital period is 16 days. So at a guess Titan is going to be eclipsed for only a few hours within a 28 year period.
It should be pretty easy to find a combination of distance, orbital period, and inclination so that the moon of a gas giant would only experience rare, short, eclipses.
My impression from reading a few astronomy type books was that it was at least as likely if not more so that habitable worlds could be located on moons of gas giants - simply because there is a good chance there are more moons out there. Judging from our solar system gas giants are obviously very capable of ‘collecting’ and holding many moons in orbit whereas the sun itself only holds a limited number of planets within a 1-2 AU orbit - the rest seem to be lost to gravity fluctuations.
If Jupiter had been located in Earth’s orbit - we would have many large enough moons where life could begin. If some of those bodies had developed with their own magnetic fields (and why couldn’t they?) then the odds increase.
Jupiter and most gas giants are BIG – it’s likely that the moons are eclipsed every “month” It’s not like the Earth-Moon system where the moon barely covers the sun and eclipses only happen when you’re lined up at the Line of Nodes. Gas giants are big enough to eclipse their moons at every revolution unless they’re far enough out. Galileo watched his “Medician stars” eclipse every few days.
I’m sure you could find patholigical systems where eclipses weren’t the order of the day (In fact, if the moons are to be habitable, the farther out they are, the better), but the way solar systems are made, all bodies not captured are likely to be in the same ecliptic plane, with all satellites in the same plane. I think it’s a fair bet that any satellites to a gas giant not extremely far away will be regularly eclipsed.
As a follow up to my last post, Ganymede has an orbital radius of about 1 million km, and an orbital period of about 7 days. This gives an orbital circumference of a bit over 6 million km, and an orbital speed of about 37,000 km/hr.
Jupiter’s diameter is about 142,000 km. I’m not going to try to calculate the width of its shadow at the distance of Ganymede, but obviously the umbra is going to be much narrower than Jupiter’s diameter. Ganymede is clearly going to spend no more than a few hours in Jupiter’s shadow every seven days at best.
This article (pdf) gives eclipse periods of the Galilean satellites of Jupiter, which last from about 2 to 5 hours depending on the satellite. Callisto goes for periods of three years without an eclipse.
In what sense of "regularly’? As I mentioned, Titan is eclipsed only once every 14 years, and I would not regard its orbit or relationship to its planet as being “pathological.”