Habitable Moons

[victory143]

I'm just wondering, If you had a Super-earth, could you get a moon big enough to hold an atmosphere and support life to orbit it? Like Laythe.

That would be so awesome if you had a Earth-like moon orbiting a planet

[KvickFlygarn87]

Laythe couldn't support life. Too much dang radiation, and since it is as far from Kerbol as it is, the "water" has two possibilities:

1. It's not water, but something else, ammonia or ammonium for example.

2. Tidal warming. This'd result in cryovolcanoes IIRC.

[Ralathon]

Why would you need a super earth for that? A gas giant has moons too.

Anyway, the main issue lies in the gravity well. To hold an atmosphere in the goldilocks zone that retains water you need a pretty high escape velocity. Too low and you bleed your oceans into space. This is the main barrier for a habitable moon. The biggest moons in the solar system are only about 1/40th the mass of earth. Mars is probably close to the lower limit for a habitable planet (seems to have had liquid water at some point) and it is still about 5 times heavier than the biggest moon.

So we need to get heavy moons. If the moon has to coevolve with the primary you need a pretty heavy parent. A big moon requires a lot of stuff, so you need a big gravity well to scoop up the materials. So you're looking at a jupiter mass object. Furthermore, gas giants don't like to form close to their parent stars. So you need them to migrate towards the goldilocks zone after formation. If the moon has to be captured you'll have some rather crazy capture dynamics. You'll probably end up with a Earth-Theia impact scenario.

In other words, big habitable moons are going to be very rare and you'll predominantly find them around gas giants.

Such a big moon is going to experience some serious tidal forces. If it isn't tidally locked you'll get massive tidal bulges. If it is tidally locked you'll have a day/night cycle roughly equal to the orbital period. This means long days with big heat differences, so you'll see big storms. In either case I don't recommend living near a shoreline.

If you orbit a gas giant you'll likely see some very pretty auroras. If your magnetic field is weak enough they'll be visible all over the surface.

All in all they can probably exist and be a decent place to live. But it is going to be very rare to find one. You need a rather unique set of circumstances to make one.

[jwenting]

or you'd need a star in the final stage of its evolution that has gotten a lot hotter than it originally was, moving the habitable zone out towards that gas giant.

[Ralathon]

or you'd need a star in the final stage of its evolution that has gotten a lot hotter than it originally was, moving the habitable zone out towards that gas giant.

Yea, but that phase tends to last rather short. So if we use the sun as star it'd be 10 billion years of Titan style and then 20 million years of habitability. That's not enough for life to evolve and generate a breathable atmosphere. Not to mention it probably spends most of those 20 million years getting rid of the volatiles in the atmosphere.

Maybe it would work if you had a binary system with the second star regulating the energy output of the first. Star swells thanks to helium fusion, surface gravity decreases, second star starts to scoop up matter from the primary star, reduces mass of the primary star, star slows down fusion. Still, this is a rather unlikely scenario. It'd be quite a sight though. A red giant in the sky slowly getting eaten by a white dwarf.

[K^2]

Gas giant can be problematic. All four Galilean moons are tidally locked to Jupiter. And this is going to be the situation for any large moon that has formed near a gas giant. That means you can't rely on moon's rotation for day-night cycle. Instead, it has to be the moon's orbit around the planet that provides the day-night cycle. A cycle significantly longer than Earth's is going to cause very extreme temperature variations, so it has to be on the order of couple of days. This would put Io in the golden zone for habitable moon if Jupiter was in Sun's habitable zone and Io was a bit larger. But Io experiences all sorts of problems due to its proximity to Jupiter. An Earth-like planet in such a low orbit would be losing too much of its atmosphere and having too much geological activity for any complex life to be plausible.

The other possibility is having the moon orbit far enough from the parent planet to prevent tidal locking. Large moons aren't supposed to form so far out, so it'd have to be a capture. But a capture from the wrong part of the system would mean that the moon missed out on the young Earth stage of development crucial for generating sufficient quantities of organic molecules needed for life to start. So it needs to be a capture from habitable zone. But we have a gas giant in a habitable zone of this star, so not much else. Now we are looking at a capture from another star system, a planet that got knocked out of its cozy habitable zone orbit, remained frozen in the void, and became alive as a captured moon of a gas giant in a habitable zone of another star.

I'm sure in all of cosmos such things happen. But I wouldn't count on finding one of these.

On the other hand, if we take parent planet to be a super-Earth, this whole thing becomes more plausible. An impact event in early formation can result in an Earth-sized moon that's far enough from the super-Earth to prevent tidal locking. And it would allow normal geological evolution of the planet, allowing for subsequent biological evolution if the conditions are right.

So yeah, I'd go with a moon of a super-Earth if I was looking for a habitable moon.

[crazyewok]

Well we may have one in our own solar system. All eyes on on Europa at the moment. Of course it not habbital to humans.

[jwenting]

Yea, but that phase tends to last rather short. So if we use the sun as star it'd be 10 billion years of Titan style and then 20 million years of habitability. That's not enough for life to evolve and generate a breathable atmosphere. Not to mention it probably spends most of those 20 million years getting rid of the volatiles in the atmosphere.

didn't say it'd last long enough for life to evolve, let alone a space faring civilisation

But it might serve as a colony for a species that evolved elsewhere, maybe closer in on a planet that became uninhabitable by the heating up of that same star.

[mdatspace]

I concur with Ralathon and K^2. A habitable earth-like moon seems to be the result of certain circumstances that are probably not very common.

[victory143]

Ok guys thanks.

[Themohawkninja]

Major issue though. If you had a terrestrial moon large enough to have an atmospheric pressure of near one ATM, it would either have to be in a binary system with Earth, or Earth would have to be so large that you couldn't physically put a rocket into orbit with current technology.

[Armchair Rocket Scientist]

All right. According to this, a moon would need about 0.25 earth masses to retain water vapor at Earth's temperature. This can be taken as a MINIMUM for habitability. There are other factors, such as stellar wind and photodissociation ripping away atmospheres, small worlds not being able to maintain magnetic fields, etc.

Now, there are three main ways moons form: accretion (moon forms around planet just like planets form around a star), capture (moon forms somewhere else and is capture into orbit around the planet), and giant impact (giant impact throws lots of material into orbit, which accretes into moons).

The only moons in our solar system formed by accretion are those of the gas giants, and all are tiny compared to their parents. Jupiter is 5000 times the combined mass of its satellites. Saturn: 4000. Uranus: 9500. The captured moons are even smaller: with the exception of Triton, none are large enough to be spherical, and Triton is still 5000 times less massive than Neptune. For an accretion-formed moon to meet the 0.25 Earth limit for habitability, its parent would therefore have to be at least 3 times the mass of Jupiter

Moons formed by giant impacts can be larger than any other moons. Earth's moon is about 1/80th its parent's mass, and Charon is an enormous 11% of Pluto's mass. In addition, many dwarf planets beyond Neptune feature smaller impact-formed moons, with Pluto having at least four. "Moons" could be even larger than Charon compared to their parent, but such a system would most likely be considered a binary planet rather than a planet and a moon. I'm not sure if it would even be possible for a giant impact to form moons around a gas giant, but a rocky super-earth could potentially have a habitable companion.

[Guest Brody_Peffley]

All right. According to this, a moon would need about 0.25 earth masses to retain water vapor at Earth's temperature. This can be taken as a MINIMUM for habitability. There are other factors, such as stellar wind and photodissociation ripping away atmospheres, small worlds not being able to maintain magnetic fields, etc.

Now, there are three main ways moons form: accretion (moon forms around planet just like planets form around a star), capture (moon forms somewhere else and is capture into orbit around the planet), and giant impact (giant impact throws lots of material into orbit, which accretes into moons).

The only moons in our solar system formed by accretion are those of the gas giants, and all are tiny compared to their parents. Jupiter is 5000 times the combined mass of its satellites. Saturn: 4000. Uranus: 9500. The captured moons are even smaller: with the exception of Triton, none are large enough to be spherical, and Triton is still 5000 times less massive than Neptune. For an accretion-formed moon to meet the 0.25 Earth limit for habitability, its parent would therefore have to be at least 3 times the mass of Jupiter

Moons formed by giant impacts can be larger than any other moons. Earth's moon is about 1/80th its parent's mass, and Charon is an enormous 11% of Pluto's mass. In addition, many dwarf planets beyond Neptune feature smaller impact-formed moons, with Pluto having at least four. "Moons" could be even larger than Charon compared to their parent, but such a system would most likely be considered a binary planet rather than a planet and a moon. I'm not sure if it would even be possible for a giant impact to form moons around a gas giant, but a rocky super-earth could potentially have a habitable companion.

Well Titan has 1 atmphosphere. It also has water mixed with ammonia. And scientist have said that life could evolve in methane lakes and ammonia water. So it can be interesting when the first people go to titan and land and maybe find life. But I have bets on europa and mars though

[Guest Brody_Peffley]

Well Titan has 1 atmphosphere. It also has water mixed with ammonia. And scientist have said that life could evolve in methane lakes and ammonia water. So it can be interesting when the first people go to titan and land and maybe find life. But I have bets on europa and mars though

Sorry but since titan has such a think atmosphere. I believe we can colonize ganymede in the near future. I hope when I get enough money, I'll start my own space program to search and explore planets and moons.

[Armchair Rocket Scientist]

Well Titan has 1 atmphosphere. It also has water mixed with ammonia. And scientist have said that life could evolve in methane lakes and ammonia water. So it can be interesting when the first people go to titan and land and maybe find life. But I have bets on europa and mars though

True. Subsurface oceans seem to be very common on icy moons. In fact, I would guess that Europa-like habitable worlds are many times more abundant in the universe than Earthlike ones. However, OP specifically asked about a moon big enough to hold an atmosphere.

If you take into account the possibility of life emerging in thalassogens other than water, the minimum size for habitability gets quite a bit smaller - at lower temperatures, gas particles move slower, so a smaller object can retain an atmosphere.

[iZakky]

Laythe couldn't support life. Too much dang radiation, and since it is as far from Kerbol as it is, the "water" has two possibilities:

1. It's not water, but something else, ammonia or ammonium for example.

2. Tidal warming. This'd result in cryovolcanoes IIRC.

dude take it easy on the kid LOOL

[crubs]

I was also wondering. If the parent body was orbiting in the habitable zone, it could in theory hold multiple habitable moons. Can you imagine the implications for the development of space travel of a civilization that lived in a solar system with multiple habitable worlds?

[peadar1987]

Laythe couldn't support life. Too much dang radiation, and since it is as far from Kerbol as it is, the "water" has two possibilities:

1. It's not water, but something else, ammonia or ammonium for example.

2. Tidal warming. This'd result in cryovolcanoes IIRC.

Well, your other points stand, but the radiation wouldn't actually be a problem at sea level. See my post here: http://forum.kerbalspaceprogram.com/threads/57915-Would-Laythe-really-be-habitable-Redux?p=779513&viewfull=1#post779513 (I put a lot of work into it, so I spam it every chance I get!)

[Armchair Rocket Scientist]

I was also wondering. If the parent body was orbiting in the habitable zone, it could in theory hold multiple habitable moons. Can you imagine the implications for the development of space travel of a civilization that lived in a solar system with multiple habitable worlds?

There's some hardcore necromancy in here, but it is actually continuing the thread so maybe the mods will let us keep it open.

The short answer is "yes, but it's unlikely." As I said in my previous posts, gas giants really need to be at least three Jupiter masses to support a moon that could sustain Earthlike surface conditions. A 13 jupiter mass object at the boundary between a planet and a brown dwarf would probably have at most 1 earth mass worth of moons. This means that in theory it could have three habitable moons, but in reality often you'd get one habitable-sized moon and a bunch of moons too small to support life.

Another problem is the moons would have a limited habitable zone around the gas giant.

For instance, young gas giants are extremely hot; the hypothetical planet Tyche was predicted to have a surface temperature of 200 K, or hotter than Jupiter, despite being in the Oort Cloud and receiving virtually no energy from the sun. That's a 4 MJ planet at 5 billion years old. A 10+ MJ planet could very well put out enough radiation in the first few hundred million years of its life to boil away the atmospheres of close-in moons. Brown dwarfs would be even worse; in addition to their greater mass, they'd get an extra burst of initial heat from deuterium fusion. You also may end up with a goldilocks zone of distance from the planet where the tidal heating and radiation levels are suitable for a moon to retain a habitable temperature and atmosphere. Finally, remember that just because a planet is of earthlike size and composition and located in the habitable zone it isn't necessarily actually habitable.

However, it would be quite possible for a planetary system to have multiple habitable worlds. This article: http://www.newscientist.com/article/dn25653-ultimate-solar-system-could-contain-60-earths.html#.U_UT3GOHOhQ

claims that a single star could support 36, but this is unfortunately bogus for a variety of reasons, namely that as mentioned five habitable moons around a single gas giant ain't happening, and if you packed four superjovian planets into a habitable zone they'd probably destabilize each other's trojan points, if not the planets themselves. Nevertheless, you could probably fit a total of four habitable moons into a system if you were very lucky, and maybe a couple trojan planets. With much smaller gas giants, too small to have habitable moons, you could probably still fit a few planets at the trojan points.

[magnemoe]

or you'd need a star in the final stage of its evolution that has gotten a lot hotter than it originally was, moving the habitable zone out towards that gas giant.

Other solar systems have gas giants in other orbits than far from the sun, some are very close, some chance you get some of them in the goldilock zone.

Radiation from gas giants, Jupiter has lots of radiation, Saturn has little. Tidal stress and higher chance for asteroid / comet impact might be more of a problem,

Tidal stress is likely to be an problem if you have lots of large moons. If you only have one tidal locked one its not an issue but gas giant tend to have many.

My guess is that gas giant moons with earth like condition is rarer than planets but not very rare.

One fun fact is that the gas giant help hold on to the atmosphere so a Mars sized moon might keep both an dense atmosphere and an active core because of tidal forces.

Another idea is double planets, the real joker is two earth like planets in thigh orbit.

[SargeRho]

There are I think 64 known gas giants in the circumstellar habitable zone. That's 64 possibilities of multiple habitable worlds.

[K^2]

There are I think 64 known gas giants in the circumstellar habitable zone. That's 64 possibilities of multiple habitable worlds.

I've pointed out problems with gas giant as a parent planet on the first page.

[magnemoe]

I've pointed out problems with gas giant as a parent planet on the first page.

And the problem is having many large moons, a 2-12 days long day will be unpleasant for us but would hardly make life impossible.

Radiation might also be an major problem but depend on gas giant itself and the orbit.

[FleshJeb]

Another idea is double planets, the real joker is two earth like planets in thigh orbit.

Have you read Rocheworld?

http://en.wikipedia.org/wiki/Rocheworld

[magnemoe]

Have you read Rocheworld?

http://en.wikipedia.org/wiki/Rocheworld

Yes, part of my idea but this would also work for less extreme variations.

Duna and Ike is a double planet after most definitions, had they been larger both could have life.