We have a pretty good idea of what attributes a planet would need to sustain life and one can imagine how life might develop on a planet, even in difficult environments.

I find it extremely difficult to imagine what a habitable natural satellite would most probably look like.

While it is pleasing to think of the forest moon of Endor or that of Avatar, those moons are pretty much planets as far as what the environment looks like.

Apparently, tidally locked moons might have a chance to regulate temperature more easily through plate tectonics, but I believe it also means only one side of the moon is directed towards the sun and the other always faces towards the primary. How would vegetation spread this way? If liquid water forms on one side, it most probably could not on the other side.

Would it be more likely to have tidally locked moon develop life or rather the contrary?

Would a habitable moon also possess poles the same way the earth does?

How likely is it that a moon possesses a stable atmosphere?

What would be the effect of gravity on climate and life?

Or the effect of the rotation around the primary?

What conditions on the orbit and on the primary would prevent extreme variations in temperature?

Could a habitable moon have seasons? Would those seasons most probably vary in length? Would their occurrences follow a foreseeable pattern?

I know this is a complex matter, which might be seen as "too broad", but I very specifically look for basic ground rules to create believable habitable moons. Any idea, insight, or link to a study would be appreciated.

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    Habitable by whom? Humans? Mammals? Giant insects? – Monica Cellio Oct 1 '14 at 1:40
  • Here is a post I wrote discussing a lot of these issues: planetplanet.net/2014/11/18/…. It's geared toward Pandora but most of this (apart from the Alpha Centauri system stuff) is relevant to the general case of habitable moons. – Sean Raymond Aug 2 '16 at 8:12
  • Consider Europa as being “more typical”. From space we only see the frozen lid, not forests or an Earth-like world. – JDługosz Aug 2 '16 at 10:52

Actually, when a moon is "tidally locked", this means that it always shows the same face toward its primary, in this case, the larger planet it is orbiting, not the sun. This means that its rotational period would be equal to its orbital period. It would have a day/night cycle either slightly longer or slightly shorter than the period of its orbit around its parent planet, depending on whether its orbit was retrograde or prograde.

However, any moon massive enough to have an atmosphere is probably large enough that it is not tidally locked, and its day may well be shorter than its orbital period.

For all intents and purposes, such a moon is pretty much like any other habitable planet, with the added complication that its larger/more massive primary planet would be quite likely to cause frequent eclipses.

Since the plane of orbit may not be in the plane of the primary's orbit, these eclipses may not occur every orbit, but may occur for several, then there could be a gap of several orbits before more eclipses occur.

Let me preface this by saying that there are two good books for starting places: What if the Moon Didn't Exist (especially comments related to construction and history) and The Grand Tour (which gives detailed information about many moons in the solar system and lots of artist's visualizations).

I believe that any moon that can support life would be markedly earth like. In order for the moon to support a stable atmosphere (presumably capable of developing civilization) it would need to be relatively large - Mars is the smallest body I'm familiar with that has such an atmosphere.

Given the fact that the Earth's moon is huge in proportion to the planet, there's a problem with the size of the planet such a moon would orbit - I believe it would have to be larger than Jupiter for hypothesized accretion formation methods to work. At that point we're looking more at a double-star system rather than star->planet->moon system.

Assuming we aren't worried about how the moon formed and we have proportion ratios similar to earth and its moon (about 100:1 mass, 4:1 diameter), there'd be a couple of characteristics of system:

A. The planet is roughly Uranus's mass and about midway between the size of Earth and Neptune in diameter (~25k km). It could either be a gas or terrestrial body (not aware of any in that range with good data). As other answers stated, this will cause massive tidal forces and make a tidal lock likely (strong tides have a tendency to moderate a body's rotation).

B. Remember that the moon rotates, although probably slowly. The moon's rotational period is going to be the same as the orbital period. The speed of the moon is limited because if it was going too fast it would be lost to space. I was surprised to discover here that the rotational period for a moon in the size range we want (c. 6000 km) is less than a week.

  1. How would vegetation spread this way? Assuming tidal heating and a thick enough atmosphere (and hopefully cloud cover to prevent heat loss), you could have water at night. In fact, you'd have moderate the temperatures somewhat or have vegetation that can take deep freezes. Don't forget that vegetation can crack or even explode if it gets cold enough. In any case, you'd probably have a vegetation band at least in the tropics.

  2. Would it be more likely to have tidally locked moon develop life or rather the contrary? Stronger tides would create huge seas, so it would be hard to build cities easily accessible by sea - remember tides occur twice a day, however long the days are. So while life could survive, you'd probably want it to happen at some distance from the coast. Or be strong swimmers.

  3. Would a habitable moon also possess poles the same way the earth does? Poles? At the axis? Yes. These would also be (probably) in the plane of solar system, so they'd be colder due to less direct light, although Uranus offers some interesting possibilities.

  4. How likely is it that a moon possess a stable atmosphere? If you're using it as a setting for life, I believe it's a requirement. The moon's other characteristics have to be built around the atmosphere.

  5. What would be the effect of gravity on climate and life? Structures (biological or otherwise) would have to be either short or lithe to withstand the tidal forces and frequent earthquakes. Climate will probably be fairly homogeneous compared with ours, since tidal stirring would moderate sea temps I assume. You'll also have much wider tidal zones and/or steeper cliffs with intense erosion due to quakes (do some reading on Io for ideas of tectonic activity). Vegetation near oceans will probably have a short life span or be able to detach and float.

  6. Or the effect of the rotation around the primary? In general, one side would be very alone in space and the other would always have a planet above it. This could create some interesting anthropology. You could have very different religions on the different sides, since on one side the sun would be less important than the planet and on the other side the sun would be the primary feature of the sky.

  7. What conditions on the orbit and on the primary would prevent extreme variations in temperature? Extreme variations in temperature would be best moderated by an atmosphere. The composition and feature of the atmosphere produce a greenhouse effect to protect the planet.

  8. Could a habitable moon have seasons? Would those seasons most probably vary in length? Would their occurences follow a forseeable pattern? I was originally thinking that there wouldn't be seasons, since the axis would probably be the same as the planet's and would be likely to be vertical, however it is possible that the whole system could be tipped. Then there would be seasons like we have on earth (or Uranus).

In general, I think the moon is going to end up rather earth-like to support life. It's hard to image a significantly different world that could have native life (in a non-augmented environment).

I worked in this answer from my rough ideas of likelihood of a particular system existing. However, the system is pretty unlikely, it might not be a stretch to come up with another system - like a binary double tidally-locked system capable of supporting life, like Pluto and Charon, where one or both bodies have a face that never receives direct light. But that's digressing.

  • Hi Josiah. Welcome to WorldBuilding and thank you for this thorough answer! Are this and this the books you mentioned in your first paragraph? – Monica Cellio Oct 3 '14 at 1:09
  • Yes Monica, those are the ones. Thanks for linking them - I'll have to remember to do that in future. – Josiah Oct 3 '14 at 1:20
  • You can edit those links into your post if you like. By the way, if you haven't already, you might want to check out our other questions tagged moons, planets, and astronomy -- sounds like you could offer some good answers on other questions too. :-) – Monica Cellio Oct 3 '14 at 1:24
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    I think once the moon gets tidal-locked, there would be no eathquakes or strong tides. These are produced only because planet's rotation and orbital period are different, or because of high eccentricity of the trajectory. Once eccentricity is zero and orbital period matches rotation period, there is almost no effect of tidal forces, even if the primary is very close. – Irigi Oct 9 '14 at 16:16
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    @Irigi - that's what made me think that the tides would be intense. Io, which is tidally locked, has substantial tidal effects; further reading points out that these are due to eccentricity rather than rotation. – Josiah Oct 14 '14 at 20:53

There are two major differences between a moon and a planet:

1) Distance to the sun rapidly changing

If we take the largest moons in our solar system as an example, a decent sized moon would have an orbital radius of 1-2 million kilometers. On one side of the planet to the other, in a matter of about 1-2 weeks, we could be talking about a moon being 4 million km closer to the sun than it was a week ago. The differential between the Earth's closest and furthest points from the sun is about 5 million km.

The thing is that the seasons are not based on distance from the sun, so I'm entirely unsure what effect this would produce. I imagine, at the very least, the weather would be slightly more chaotic than on Earth. With an axial tilt, you might end up with something akin to mini-seasons, though the effect might be very slight.

2) Much more powerful gravitational forces

If we again take our system's largest moons as an example, we would have a habitable moon with, at the very least, a gas giant affecting it instead of our own Moon. That's almost 26,000 moons worth of mass, which is equal, given the masses and distances of Jupiter and Callisto or Titan and Saturn, 17-19 times the gravitational exertion between the moon and gas giant than our planet and moon have over each other.

Thus, if we're talking about a truly habitable world that's fairly earth-like, the tidal forces we have on our planet would pale in comparison to the tidal forces a Callisto- or Titan-like moon would feel under the shadow of a Jupiter-like planet. This would result in possibilities like

  • Heavy and frequent quakes
  • Huge differences in "high tides" and "low tides" of oceans
  • "Tidal heating" which can result in heavy volcanic activity
  • Incredibly strong ocean currents

I am unaware if these changes would also result in stronger or stranger weather patterns, but I wouldn't be surprised.

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    4 million km is about 2.6% of Earth's orbital distance from the Sun. It's an interesting point I hadn't considered before but I don't think the changes would be enormous since as you say the Earth's orbital period also varies by that much and the difference in sun distance has less effect than seasonal variation does. – Tim B Sep 20 '14 at 19:10
  • But we're talking about an orbit that changes 4 million km in a single week, and back again. We're talking about the entire differential in the Earth's orbit that takes 6 months to achieve being accomplished in a week. That's not nothing. I would imagine that it would have effects, but that the effects wouldn't be huge. Ergo mini-seasons. Like having an especially cold week in summer, a couple of mild weeks, and an especially hot week. Which might create some strong weather patterns, especially when considering the tidal heating and strong currents. – Attackfarm Sep 20 '14 at 22:06
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    Earth-moon distance is only 384 thousand kms. Saturn-Titan distance is 1.2 million kms. Earth's orbit changes as much as 5m km. I dont think the smaller fluctuations of orbit would have as large an impact as you imply. Also consider that in the case of Titan that is a much smaller proportion of its distance to the sun. – Octopus Oct 24 '14 at 17:58
  • Earth's moon is irrelevant and not discussed. The largest orbital radius of a large moon in our solar system was close to 2 million km. That's a differential of 4 million. That's pretty close to the Earth's 5 million km in a much more rapid time frame. The "large" impact I implied was "weakly chaotic weather patterns". I stand by this guesstimate. Also, the distance between Titan and the Sun is irrelevant, as we are speaking of a fictional goldilocks-zone moon, using Titan was a reference for orbital radius and gravitational effects. – Attackfarm Nov 4 '14 at 9:22
  • Orbital distance will vary also by the variation of the gas giant's orbit. It won't be a perfect circle any more than Earth's orbit is. – Zan Lynx Jan 15 '15 at 0:03

A tidally locked moon like ours could have normal day/night sequences, just longer. Please note that unless there is when we known as a Moon eclipse, the near face of Moon can be fully iluminated by Sun (as it happens every Full Moon). So there are no reasons for differences between near side and far side. It is just that one side have a big (or not so big) planet always on sky, and the other side doesn't.

Ability to develop life seems not to depend on being a satellite or not, just on existence of liquid water, which depends on having enough gravity and being on the habitable zone of the star. If Jupiter were where Earth is, and Earth were that Jupiter's tidally locked satellite, it could still be our living sphere. Really, main difference would be only the day/night cycle not being 24h (with its consecuences on climate, vegetation and animal customs).

You have posed a lot of separate questions I think I have addressed all at once. If not, I'll be happy to update the answer as you wish via comments.

Just as extra data: Jupiter's biggest satellite, Ganymede, is a bit bigger than our Moon. So it may be that you need a superjupiter in order to have a credible Earth-sized satellite. Or just no, our Moon is actually an example of a supersatellite. Comparison Left: Moon and Ganymede. Rigth: Earth.

The thing that matters is what the planetary body is made of (size, gravity, atmosphere, liquid water), not what it is orbiting.

Whether the "planet" is orbiting a sun, a gas giant, a planet smaller than a gas giant, something like a binary star system or even a black hole doesn't matter.

All that matters is what conditions that results in on the surface. (A black hole for example is unlikely to result in a life bearing planet as you would need another source of light and heat but by itself would not prevent life if there was another body in the system providing the resources needed or the planet had a high source of internal thermal energy).

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