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I've begun the laborious (but fun!) process of putting another world together.

Known constraints include:

  • Habitable, (super-)Earth-sized, tidally-locked moon to a gas giant; the giant, where visible, is reasonably large in the sky to be rather spectacular a sight;
  • All land forms a single supercontinent, exact size to be determined;
  • The "day length" on the moon (i.e. its orbital period w.r.t. the gas giant) is about 40 hours long; year length currently undefined;
  • We may assume a moderate axial tilt for the gas giant (20-something degrees), and the moon may orbit around the giant's equatorial plane, or any similar plane that would generate "some tilt" with respect to the sun. The moon's own axial tilt is undefined at this point, but likely small; at any rate, the point is that some semblance of seasons should be in place;
  • The moon's climate and precise geography is still up in the air, as I'd like to get the astronomy out of the way first;
  • Low volcanic activity on the moon. After all, it needs to be a good place to live (and we can also ignore any radiation from the gas giant, save for spectacular auroras);
  • Other "fixed", but presumably less relevant details: The star is a "blue straggler" by virtue of me wanting it to be blue (NB: This is likely completely nonsensical, ignore it); the giant may have other moons so that they may occasionally be seen by the inhabitants of our main moon.

I've not figured out how much illumination the moon will get from the sun yet, because I've got a more pressing question: What would the temperature "under" the gas giant side (the place where the giant would be seen 90° up in the sky) be, where the tidal bulge is, ignoring the moon's own climate? Will it tend towards being hot, or will it tend towards being cold? One can find contradictory information about this, as this answer suggests a large, freezing (or frozen) ocean at that spot; this answer, ignoring poles, suggests that the "under-giant" region may be hot instead, as the giant might be "hot". Does it depend on the giant? Does it even matter at all, or can one go with whatever he wants? I'd certainly appreciate some concrete information on this particular question, as I was unable to find a solid, logical line of reasoning to follow about this one.

I'm personally rather in favour of the side "towards" the giant (or "under", if you're on the moon) being hot rather than cold, but it'd bug me to simply assume this without clear understanding of how and why that would be (and whether it's even possible or likely).

Being "science-based" is important to me (no wizards allowed), but I'd always put the super-hard minutiae away in favour of a "plausible" solution that fits with the concept at hand. In other words, don't worry about maths, solar size and distance, or the time needed for the moon to form in comparison to the sun's lifespan.

NB: It's also highly likely that some of the assumptions I've made earlier in this post are wrong; after all, I've only been reading up on tidal locking since yesterday. If that is the case, corrections will be greatly appreciated.

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  • $\begingroup$ "tidally locked" "The "day length" on the moon is about 40 hours long" an immediate problem with terminology here, there's no day as such on tidally locked body, I understand it orbits a gas giant not the star, so I presume you mean the period it's not in the shadow of this gas giant (when the sun isn't eclipsed by the planet it orbits)? no idea for the proper term for that myself but you might want to detail that point to avoid misunderstandings. $\endgroup$
    – Pelinore
    Commented Feb 7, 2019 at 13:13
  • $\begingroup$ That's why I put "day" in quotes. I'm well-aware that in a "locked" moon, the length of the "day" is equal to its orbital period around the gas giant (as explained in this answer, for an example). I considered the implication to be obvious, but I'll amend the question for clarity. $\endgroup$
    – Lumos
    Commented Feb 7, 2019 at 13:41
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    $\begingroup$ Pelinore is wrong about the moon not having a day. A day is aperiod between successive star rises, or between successive star sets, or between successive midnights, etc. If the moon has an orbital period of 40 hours any spot on the moon's surface will have daylight followed by night darkeness lasting for a total of 40 hours. Of course different places on the moon will be in daylight while others are in night. $\endgroup$
    – M. A. Golding
    Commented Feb 7, 2019 at 22:17

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I think that AtmosphericPrisonEscape's answer may be a bit misleading in some respects, as my comments say. For example, there are many tidally locked moons of gas giants in our solar system but only Io is a volcanic hell.

There have been many other questions about possible Earth sized habitable moons of gas giant planets.

I have answered a number of those questions.

Here is a link to a previous question and my answer to that question includes a link to a question and answer which has links to previous questions and answers.

What are the day and night fluctuations for a moon orbiting a planet the size of Jupiter?1

The article "Exomoon Habitability Constrained by Illumination and Tidal heating" by Rene Heller and Roy Barnes Astrobiology, January 2013, discusses factors affecting the habitability of exomoons.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3549631/2

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  • $\begingroup$ Thanks for the clarification about the moons, I was unaware of that. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Feb 8, 2019 at 0:50
  • $\begingroup$ Thanks. One thing I'm seeing from multiple sources is that the moon is likely to orbit around the equator of the giant, ergo the area on the moon that falls "under" the giant would be on the moon's equator, correct? If that's the case, and given the "relatively Earth-like" axis tilt, wouldn't it be logical if the area "under" the giant tends towards heat rather than cold? $\endgroup$
    – Lumos
    Commented Feb 8, 2019 at 12:35
  • $\begingroup$ @Lumos. The moon is probably a few thousand kilos or miles in diameter and possibly a hundred times that in distance from the planet. Thus the moon's equator is not likely to be hotter due to being very slightly closer to any infrared radiation emitted by the planet. The axial tilt of the moon compared to the star will be the same as the axial tilt of the planet, which in turn can be anything from 0 degrees to 90 degrees. The part of the moon where the star is overhead will be hot for the same reason the subsolar point on Earth is, the angle of the star's rays. $\endgroup$
    – M. A. Golding
    Commented Feb 8, 2019 at 17:27
  • $\begingroup$ @M.A.Golding: Cheers. So, if I'm understanding this correctly, the terrain "closest" to the gas giant is up for being determined through the regular means, and will not strongly depend on the presence of the gas giant in any way. $\endgroup$
    – Lumos
    Commented Feb 9, 2019 at 7:15
  • $\begingroup$ @Lumos The Moon will probably be slightly deformed by the tides. There used to be a theory that the Moon was deformed and the high side faced earth and the far side was lower and might have an atmosphere. But it is now known that the slightly lower side faces the Earth. It is quite possible your moon might might be a few miles longer in the direction facing toward and from the planet than in the side facing at right angles to the planet, but if the moon is large enough to be habitable that will be a very slight deformation. $\endgroup$
    – M. A. Golding
    Commented Feb 12, 2019 at 19:16
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Well, if you want to get the astronomy out of the way first, there are more severe problems than if it's hot or cold on your moon, or which colour the gas giant is:

Habitable, (super-)Earth-sized, tidally-locked moon to a gas giant; the giant, where visible, is reasonably large in the sky to be rather spectacular a sight;

That's possible.

All land forms a single supercontinent, exact size to be determined;

Your planet is going to have increased volcanic activity, and if there is liquid water, plate tectonics should be possible.

The "day length" on the moon (i.e. its orbital period w.r.t. the gas giant) is about 40 hours long; year length currently undefined;

There is no year. A tidally locked planet must have its orbital spin vector parallel to its orbital spin vector. Thus, only days exist on your planet. Of course the insolation it recieves would vary together with the gas giant parameters.

We may assume a moderate axial tilt for the gas giant (20-something degrees), and the moon may orbit around the giant's equatorial plane, or any similar plane that would generate "some tilt" with respect to the sun. The moon's own axial tilt is undefined at this point, but likely small; at any rate, the point is that some semblance of seasons should be in place;

Well, the tidal bulge from the gas giant's rotation will force the moons orbital spin to be parallel to the gas giant spin (or said differently, it will reduce its orbital inclination to 0 w.r.t. the gas giants equatorial plane). So in the end you must have all three vectors parallel to each other, moon spin, moon orbit angular momentum and gas giant spin.

Low volcanic activity on the moon. After all, it needs to be a good place to live (and we can also ignore any radiation from the gas giant, save for spectacular auroras);

I guess that's alright after all. If you check the moons of Saturn, you will see that the moons out to 20 planetary radii are easily tidally locked (our moon is even 60 planetary radii away from its host). The story with Io is in fact that the Laplace-resonance with the other Galilean Moons pumps eccentricity into its orbit, which keeps its internal heat dissipation at a strong rate.

Other "fixed", but presumably less relevant details: The star is a "blue straggler" by virtue of me wanting it to be blue; the giant may have other moons so that they may occasionally be seen by the inhabitants of our main moon.

Blue straggler? Did you look that up? The most compelling mechanism for making Blue Stragglers is presently the Roche-Lobe-Overflow in a close-encounter binary system as a result of high star density in globular clusters. No good environment to keep stable planetary orbits. Not to mention that the lifetime of the blue straggler is drastically reduced after it became blue, so there's probably not enough time to evolve complex life.

And whether it is going to be hot or cold is a parameter depending on the distance of the gas giant to the star, the star's luminosity, inclination of the moons orbit around the gas giant and the moons atmosphere. But you can probably work out a simple equilibrium temperature for your gas giant. The equilibrium temperature of the moon will then be that of the gas giant plusminus some daily variation given by the atmosphere, where the variation will be zero for a very dense atmosphere. In this way you could construct something halfway believable.

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  • $\begingroup$ Thanks for the reply! May I address some points for clarification? Comments are far too short, so I'm afraid this must be broken up into sections. "> You want to be tidally locked to a gas giant? Well..." Interesting. I was under the impression that tidal locking produces some volcanic activity, not necessarily as much as Io's. I'm not averse to limited volcanic activity on the moon, but ultimately this is something I'm not afraid to sweep under the rug if I have to. Still, sounds like some solid reading about volcanoes is in order. $\endgroup$
    – Lumos
    Commented Feb 7, 2019 at 17:22
  • $\begingroup$ "> There is no year." Perhaps I used the term incorrectly with regards to a moon orbiting a planet. By "year" I meant the moon's orbital period around the sun, i.e. what "year" means in the context of Earth. Are you saying that my use of the word was improper, or that for some inexplicable reason the moon wouldn't be affected by its parent's motion around the sun? (Or something else which I've missed?) In the same paragraph you also say that the planet's "orbital spin vector must be parallel to its orbital spin vector", and it sounds like another vector was intended for one of those. :) $\endgroup$
    – Lumos
    Commented Feb 7, 2019 at 17:22
  • $\begingroup$ "> Blue straggler? Did you even look that up?" To be fair, the honest answer here would be NOPE! Definitely not to the level that I'd hear of a "Roche lobe", or its overflow. I merely wanted a blue star. The superficial research I did suggested that habitable planets around blue stars would have too long orbital periods, and I'd like my moon to have a "year" (orbital period around the sun) relatively close to ours. Blue stragglers seemed like a good excuse to achieve that. The star being blue carries special significance to me (even though it's otherwise unimportant). $\endgroup$
    – Lumos
    Commented Feb 7, 2019 at 17:23
  • $\begingroup$ @Lumos how do you think "Blue stragglers" are different from regular blue stars in terms of luminosity? $\endgroup$
    – Alexander
    Commented Feb 7, 2019 at 20:16
  • $\begingroup$ Actually AtomosphericPrisonEscape is wrong about a tidally locked moon of a gas giant being a volcanic hell like Io because there are about 100 other tidally locked moons of gas giants that aren't volcanic hells like Io. It depends on the mass of the gas giant planet, and the distance of the moon's orbit, etc. $\endgroup$
    – M. A. Golding
    Commented Feb 7, 2019 at 22:10

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