Earth-Like Gas Giant Moons

January 12, 2009 11:11 PM

What would be the conditions required on the moon of a gas giant to support Earth life?

Let's say you have a moon of a gas giant in some fictional star system much like our own. If a spaceship from Earth came there, what would the parameters be if the settlers wanted to touch down and build some cities without being under domes?

My thoughts are that it would need to be a small gas giant, and the moon would have to orbit very close to it (since moons of gas giants seem to inevitably experience tidal lock, and you need to have days that are relatively in sync with Earth days, right?) but this would mean that only half the planet would receive sunlight and be habitable-- thereby making the rest into a featureless icescape, with a permanently crepuscular region between the two? Can I solve this by giving the moon a strong ecliptic tilt? Would this put the moon too close to form in the first place or too close to maintain its shape? Would it bathe the surface in too much radiation for life to exist? Would tidal forces turn it into a barren volcanic wasteland? If it had an orbit at something of an angle to the gas giant's orbit, would the problems be mostly resolved?

In addition to the parameters of its physical location, what kinds of unusual astronomical phenomena could inhabitants expect to see on the surface, if any?

I'm a Latin major who has only a casual and mostly science-fictional interest in astrophysics, and I only want a relatively vague idea. If you know of a book or article that basically answers this question, please point me to it. Google turned up some things but I want more and more complete answers, since of course this is kind of a bizarre subject and I doubt much concrete research has been done on it.

posted by Electrius to science & nature (17 answers total)

This page has some orbital periods of moons of Jupiter, which don't seem to be that long. Io is 1.8 days,

The Roche limit is what determines whether a moon is too close to hold together.

Back when they thought Mercury was tide-locked to the Sun, there used to be some SF imagining a small habitable zone on the borders. However, I'm not sure a tide-locked body could sustain an atmosphere: if the atmosphere freezes on the dark side, it would rush over from the light side. Volcanic heating might help that though.
posted by TheophileEscargot at 11:25 PM on January 12, 2009

You don't want it too close, otherwise the tides will form Io-like volcanoes. Additional large satellites are probably out too, as it is thought to be when Ganymede and Europa are opposing Jupiter relative to Io that the strongest tides are felt and hence the largest, most atmosphere-poisoning sulfur and sulfur dioxide plumes are created.

Speaking of atmosphere, how is this moon going to hang on to any? A domeless world needs an atmosphere, and the Jovian moons are too small.
posted by ikkyu2 at 12:43 AM on January 13, 2009

Terraforming the moons of Jupiter is a standard trope in science fiction, but the more we learn about the Jovian system, the less likely it seem. One big problem is radiation. Jupiter has a massively powerful magnetic field, and a combination of that with ionized gas ejected by Io means that the radiation level at all of the four major Jovian moons would be lethal to life out in the open in fairly short order. High energy particles striking the atmosphere of the terraformed moon would produce X-rays and gamma rays. (That happens here, too, but it doesn't happen at a high enough rate to represet a threat to life.)

To protect life on that moon from that radiation, either everything would have to be inside domes (which I gather isn't what you have in mind) or the moon itself would have to have a very hefty magnetic field, and the atmosphere would have to be extremely thick, probably a lot thicker than Earth's atmosphere.

Or else you'd have to wrap the entire thing in a forcefield, or solve the radiation problem with some other form of unobtainium.
posted by Chocolate Pickle at 12:58 AM on January 13, 2009 [1 favorite]

I'm no expert, but there seem to be some misconceptions in the question.

First, if a moon is tide-locked to a planet, that is not the same as being tide-locked to the sun. It would still receive sunlight over most of its body throughout its day (except when the planet eclipses the sun). Note that the "dark side of the moon" isn't really always dark; it just always faces away from earth.

I'm not sure why the length of the day would be particularly important to life. Perhaps someone can throw in some supporting ideas. It seems to me that, while many earth life-forms are adapted to a 24-hour day, it's not a necessary condition for life. For example, many species live so deep in the ocean, they barely receive sunlight at all. Others survive north of the arctic circle, where it can be night for weeks at a time.

The book The Seven Mysteries of Life would actually go a long way to answering many of your questions. It's a long, dense read, though!
posted by knave at 1:44 AM on January 13, 2009 [1 favorite]

Temperature, gravity, food availability (which is really just other Earth life being able to live there too), air quality (ditto, to a slightly lesser extent), and radiation from the mother planet and from space all have to be within fairly narrow ranges. That's not to say a moon couldn't have life, but it would be adapted to survive there, not on Earth, and the reverse would also be true. The less complex the organism, the more likely it is to be able to survive; this is why viruses and bacteria are so tough, there is less to go wrong with them.

Astronomically speaking a gas giant that is located closely enough to its star that it has sufficient heat to support Earth-like life nearby may also not be able to maintain its atmosphere, hence would become a ball of rock like our Earth and other inner planets. Life probably has better chances of evolving on the gas giant itself than on the surrounding moons: there's more space, more chemicals, more of a mix of those chemicals, and more energy in the pre-ecosystem to spark it off.

Honestly Earth-like life on other planets is probably a fantasy in the same category as, say, Barsoom or Perelandra. When we knew nothing about the conditions of Mars and Venus, authors were free to speculate. When we knew nothing about the conditions of other solar systems--and we still know very little--authors were freer to speculate believably than they are now. There may be nothing out there that remotely resembles us.

This doesn't ruin SF. I recommend to you Peter Watts's novel Blindsight, and Charles Stross's essay The High Frontier Redux.
posted by aeschenkarnos at 4:07 AM on January 13, 2009 [2 favorites]

...or the moon itself would have to have a very hefty magnetic field...

Perhaps that could be explained as a result of a large internal mass of molten metal under flux due to the gravitational effect of the gas giant.
posted by odinsdream at 6:57 AM on January 13, 2009

You want to be careful of radiation. Too close, and the giant's radiation belts may fry the moon. That's one of the problems with life on Jupiter's Galilean satellites.
posted by Guy_Inamonkeysuit at 6:58 AM on January 13, 2009

odinsdream, intense gravitational energy has a pretty dramatic effect on, say, Io. You wouldn't want to live there...
posted by Guy_Inamonkeysuit at 7:01 AM on January 13, 2009

For reference: Titan, in orbit around Saturn, has quite a dense atomsphere -- 1.5bar at the surface. Combine this with the gravity (.19 earth) and you could strap on wings and fly. Just don't inhale -- while the atmopshere is 96% Nitrogen, the propane isn't good for you, and the hydrogen cyanide is anti-good for you.

The core thing to human habitability: Can liquid water exist on the surface? If so, you're looking at a world that has an average temp in the 0--100C range, and that has a significant amount of oxygen. That's most of the battle right there. Gravity should be failry high (.5 to 1.5g) but if you have a planet with liquid water and near 1G gravity, you either have an earthlike planet or one that can be made into one easily (more easily that Mars or Venus)
posted by eriko at 7:14 AM on January 13, 2009

Others. Yes, radiation is an issue (the Jupiter radiation belts are very much a problem here) but a real atmosphere will help some. However, to keep such an atmosphere, you need to deflect some of that radiation, thus, you need a magnetosphere, which implies "iron core."

Distance also helps. Too close, Roche limit, no planet. Too far, unstable orbit, something will pull it out of orbit and into a stellar orbit.

Stellar Insolation is harder. Our gas giants are too far out to make liquid water easy.
posted by eriko at 7:18 AM on January 13, 2009

Sorry, the question was a little unclear. I'm assuming an Earth- or Mars- sized satellite orbiting the gas giant, so it would be large enough to maintain a nitrogen-oxygen atmosphere and oceans. I'm not looking for something where life could necessarily evolve on its own— it could be an unstable situation— but something that lasts at least a few million years. My question was really whether or not such a body could orbit a gas giant, and if so, how would the orbit of the body and the gas giant work?
posted by Electrius at 7:21 AM on January 13, 2009

This guy from NASA thinks it's plausible. No details, though. Some more speculation here, and some fascinating stuff about "tidal heating" here.
posted by EarBucket at 8:08 AM on January 13, 2009

This might, and thats a stretch, be feasible for ameobas and other small animals, but complex large animals live at the top of the food chain. I seriously doubt any scenario will let you move an entire ecosystem to another planet enough to support large animals. Small things like the acidity in the water or the salt level could spell doom for the entire ecosystem. For instance we cant even move established and hardy species to different Earth ecosystems most of the time because of the differences in environment. Heck, we have a lot of trouble keeping animals healthy and alive in artificial environments designed for them like zoos.

I think the only animals that would survive would be genetically engineered ones or ones that are kept alive through technological means and human intervention. Youre not getting a whole self-supporting ecosystem in there even if you solve the larger issues of heat, toxicity, terraforming, and radiation.

if the settlers wanted to touch down and build some cities without being under domes

Without animals and plants the settlers will die. Usually science fiction writers solve this problem by throwing in more fantastic technology like magical food makers and magical water cleansers. I think if you want a realistic view of a self-sufficient colony then you should go more towards a large terraform project and genetically engineered humans, plants, and animals.
posted by damn dirty ape at 8:46 AM on January 13, 2009

All that being said, Ringworld and Pern were both good stories despite being very hand-wavey with all the orbital mechanics and whatnot.

If you're writing fiction just assume you're going to be granted license to a point. If you give even somewhat plausible explanations for technical issues, even if they end up not being realistically viable, you can get on with the story in most cases without breaking the suspension of disbelief.
posted by odinsdream at 9:05 AM on January 13, 2009

It's not just being in the liquid-water zone. It's also necessary to be near enough to the star so that enough light arrives to support photosynthesis at a reasonable rate.

That's why terraformng even Mars would be problematic. The average solar flux on Mars (energy per square meter) is 43% of Earth. At Jupiter it's just 4%.

Most terrestial plants will die if they only get 4% of normal sunlight (i.e. spend all their time in deep shade). There are plants which have evolved to live in the shade (some orchids, for example) but you can't create a viable ecosystem out of just those. And no plant which contributes significantly to human nutrition can grow in such a light-poor environment.
posted by Chocolate Pickle at 1:39 PM on January 13, 2009

I think any orbit other than in the plane of the solar system would be highly unlikely to occur.

Regarding magnetic fields: whereas Jupiter's is very strong (too strong as stated above), Saturn's is quite weak, so a strong magnetic field isn't inevitable for a gas giant. You do want some magnetic field, though, to protect from solar and cosmic radiation. Mars's very weak field is a big problem for potential human habitation there. Protection of humans from radiation in transit to Mars is also an issue.

Aside: Earth's magnetic field is just the right strength, a la Goldilocks. It keeps out most--but not all--of the solar and cosmic radiation. Enough gets in to drive a certain amount of DNA change (there are terrestrial sources of radiation too). This is a driving force of evolution. Presumably, there never have been humans here without that radiation. Someone once asked in AskMeFi, regarding cosmic radiation, "Why aren't we all mutants?". We are all mutants.
posted by neuron at 10:35 PM on January 13, 2009 [1 favorite]

And no plant which contributes significantly to human nutrition can grow in such a light-poor environment.
They're technically not a plant, but mushrooms can.

On your other point: viable ecosystems are extremely intertwined. Many life forms on Earth survive under suboptimal or harsh conditions (eg, your orchids in their shade) only because other lifeforms pick up the slack; in the orchids' case, by dying and providing rich, damp soil.
posted by aeschenkarnos at 11:36 PM on January 13, 2009