I'm currently heavily revising a story which is largely set on a habitable but not exactly hospitable moon of a gas giant orbiting on the outer edge (or just beyond) of the habitable zone around a Sun-like star. I should note that the moon is not naturally habitable - it was terraformed in the distant past, and its ecosystem is a very simple engineered one.

For plot purposes, I need the moon to support unaided human life but get very cold very quickly when it passes into the gas giant's shadow and ideally stay this way for a day or two by Earth's standards. I've tried to research what factors might allow this to happen - distance to primary, the primary's diameter, inclination, atmospheric density, size of the moon, ocean coverage, cloud layers and so on but I've not been able to find a consistent answer.

How can I design this moon so that it is habitable but will get cold enough quickly enough to pose a significant threat to human life every time it enters the primary's shadow?

There is supposed to be little in the way of inclination so the eclipses are fairly regular events.

  • $\begingroup$ How often do you want eclipses to happen? $\endgroup$ Mar 21, 2015 at 21:45
  • $\begingroup$ Maybe once a week as measured on Earth, but honestly I could accept anywhere between once every few days and once a month (as measured on Earth) as long as the scenario works. I know that's a vague answer but it's something I can write around as long as the freezing eclipse can work. $\endgroup$
    – MG1981
    Mar 21, 2015 at 21:53

4 Answers 4


Only the short-term weather is extreme, not the overall heat inertia in the rocks and oceans, but that is what bothers people; e.g. terrible cold weather.

On Earth, think of lake-effect blizzards off Lake Michigan for example. The people may initially anticipate a "cool night" dropping as much as we have precident for on Earthly examples. So they are not prepared when a chain of events causes a arctic front to move in along with infrastructre-destroying weather at the boundary of the front. We get Ice Storms here in Texas, for example.

So, look at the complexity and chaos of weather, not just the modest average lowering of temperature. Having night on both sides can mess up the normal heat transport cycles in the air.

There are also effects to change temperature by other effects. Los Angeles can have swimsuit weather at Chriatmas, because of the pressure change of air coming from the mountains. I can't think of what might cause the opposite off hand, but think about chain reactions and phase-change.


Since the moon is most likely tidally locked, there are actually multiple factors in play.

The side facing the gas giant will be the "heat pole", which will receive lots of incoming energy from the gas giant in the form of infrared radiation. the side facing away will be the cold pole, and will radiate energy to space when facing away from the sun.

If there is a radiation field the moon passes through, there will be a "radiation pole" 90 degrees from the hot pole, facing in the direction of orbit, and a "sheltered pole" 90 degrees opposite to the hot pole in the trailing direction.

Mapping ecosystems can be thought of as painting rings on the moon, with one set going from hot to cold pole, and one set going from radiation pole to sheltered pole. The moon will have a sort of checkerboard pattern where the various bands intersect.


If the parent planet is at the outer edge of the star's habitable zone then when the moon passed behind the planet the temperature will conceivably drop to life threatening levels. You only have to look at how cold it can get on Mars which can fall to -73°C Source which is certainly life threatening. While this is largely the function of Mars's thin atmosphere - an atmosphere somewhat thinner than Earth's but one that allows humans to live would still produce potentially life threatening cold nights/eclipses.

The orbital period of the moon shouldn't be a problem. The orbits of the Galilean moons of Jupiter range from hours (for Io) to 7 days for Ganymede. So you can put your moon at whatever distance gives you the "correct" period of "night".

There will probably be some warming due to the interaction with the magnetic field of the planet - look at the moons of Jupiter again for examples here - but you can easily say that this isn't enough to keep the moon warm enough.


Ring Shadow

Consider a gas giant like Saturn:


But stick it on its side like Uranus. Now the rings will "extend" the eclipse, by blocking a larger portion of the light (the moon will orbit in the same plane as the gas giant's orbit, and not its rotation).

This gives you a longer eclipse with the same size planet, increasing the cooling and giving you lower temperatures.

It's extremely unlikely that this situation would form naturally, but since it's terraformed you can say that they also did some orbit adjustment.


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