I have a world idea (and definitely not the only one with such an idea in which the main location is a moon of a gas giant, that has life like on Earth.

What are the effects of the presence of the gas giant in "global" scale?

  • 1
    $\begingroup$ It's pretty simple: Huge tidal forces and unpredictable day/night cycles. Though you can solve those problems by having the moon orbit very far away and have their orbits be perpendicular to the sun. (this would be very rare and would need incredible circumstances) $\endgroup$
    – Bloc97
    Oct 24, 2016 at 1:59
  • $\begingroup$ If the moon were tidally locked to the gas giant there would be a lot of problems... At least for life as we know it on Earth. $\endgroup$
    – Bloc97
    Oct 24, 2016 at 3:43
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    $\begingroup$ Possible duplicate of Habitable moon of a gas giant: working out the sizes and distances $\endgroup$
    – Mołot
    Oct 24, 2016 at 5:11
  • $\begingroup$ @Bloc97 All major moons in the solar system are tidally locked to their mother planet. That gives them stable day-night cycles in the range of hours to a few days with gas giants. The side facing the planet has it's dayly eclipse, but a lot of light from the planet during the night. If one large moon were so far out from its planet that it had not been tidally locked, the orbit would be perfectly stable anyway. $\endgroup$
    – Karl
    Oct 24, 2016 at 17:18
  • $\begingroup$ @Karl I don't think a jovian planet emit light in the high energy spectrum, I thought they mostly emitted radio and infrared? $\endgroup$
    – Bloc97
    Oct 24, 2016 at 17:32

2 Answers 2


The moon has a “hot” and “cold” pole (closest and farthest from the primary respectively), but the local climate is affected by the rapid day/night cycle due to orbit around the primary, preventing temperature and atmospheric stratification around the hot and cold poles.

In addition to the heat from the sun and primary, the moon is also affected by tidal disturbances caused by resonating orbits with the other moons. If is the largest of the moons (nearly the size of the Earth), the extra energy from the tidal flexing drives a powerful tectonic engine. Earthquakes and volcanic eruptions regularly rock the moon, while cycles of mountain building have thrown up impressive ranges along the edges of continents. In addition to the tidal energy, the moon is immersed in the huge magnetosphere of the primary and a powerful electric current is generated by the passage of the moon through the field. Finally, the moon is in the radiation field of the primary, so the moon is showered in ionizing radiation, although the molten core generates a magnetic field that provides some protection.

The hydrological cycle is complex, with the dominant effect being the flow of heat from the hot pole (facing the primary) to the cold pole (facing away from the primary), tending to drive water from one hemisphere to the other, which is modified by the rapid day/night cycle caused by rotation about the primary. This prevents stratification and while the cold pole is colder on average, liquid water exists and rapidly evaporates during the local day. The heat engine caused by tidal flexing also ensures water remains liquid, and the average temperature is somewhat higher than the Earth, leading to a hot, humid climate. The cold hemisphere has more water in the form of large seas and numerous lakes, while the hot hemisphere is dryer. An unusual effect is plate tectonics becomes a large part of the hydrological cycle, with water being subducted under various plates and being released in volcanic eruptions centuries later. Since the moon lies inside a Jovian system, comets strike the planet at regular intervals, bringing more water to replace outgassing.


The hot pole itself is mostly scrubland, and the vegetation changes character as you move from the hot pole to the cold pole, reflecting the exposure to the primary rather than the local Sun. The leading edge of the planet (in the direction of the orbit around the primary) is also scrubland due to the effects of powerful magnetic, electrical and radiation fields impacting on the moon’s surface, while the trailing edge is much lusher due to the greater levels of protection. Mapping ecological boundaries is somewhat like looking at a ball with two sets of stripes painted on at 90 degrees to each other, leaving a checkerboard of habitats defined by the amount of energy received relative to the hot pole and radiation pole.

Once you have "mapped out" the various ecological zones and placed them in a geological context (where are the oceans and continents, for example), the rest of the world building should become clear.

  • $\begingroup$ I would add that when the moon passes behind the jovian planet, it would be night all around the planet. This creates interesting day/night cycles that can be reflected on intelligent species' traditions and how the fauna/flora copes with long nights. $\endgroup$
    – Bloc97
    Oct 24, 2016 at 3:30
  • $\begingroup$ Also, newly formed craters on the planet would be less likely (as meteors would mostly fall into the gas giant), so combined with the overreactive tectonics, you would have unique geographic features on the moon not found on normal planets. $\endgroup$
    – Bloc97
    Oct 24, 2016 at 3:31
  • $\begingroup$ Meteors: you have to balance the reduction due to a large solid angle being blocked by the primary (<50% reduction) with the fact that the primary will attract more stuff from all directions. $\endgroup$
    – JDługosz
    Oct 24, 2016 at 5:03
  • $\begingroup$ Highly sesimically active worlds, as an Earthlike moon would be, will be worlds with rich biospheres. This is due to recycling and replacement of trace elements and nutrients on the moon's surface. Earth is fairly seismically active, this explains, partially, our biosphere, the gas giant moon would be more so. Hence a rich and complex biosphere can be expected. $\endgroup$
    – a4android
    Oct 24, 2016 at 8:45
  • $\begingroup$ How much actual heat would a moon receive from the gas giant onto the side facing it? $\endgroup$
    – Tim B
    Oct 24, 2016 at 14:54

If the earth-like moon is large enough to hold an atmosphere (and is warm enough in average), it will likely be totally fine for life.

It will have kicked out or swallowed all moons (hence it got so big) in close vincinity in the first few million years of its existence, so the orbit is stable.

If it's too close to the planet, it gets sterilised every day, OK. No life.

If it is close, but outside of the radiation belt, it will be tidally locked, have a rotation period in the range of a day, fine. There is a daily solar eclipse that lasts for a few hours or so, but the IR radiation from the planet smooths that out. The outer half of the moon never sees the planet anyway. The magnetic field of the planet shields the atmosphere perfectly.

If it is further out, but still tidally locked, the day-night cyle is longer, that might be bad for life developing.

If it is even further out it can be not tidally locked, super. The moon should perhaps have his own magnetic field, which is no problem if it's earth sized. If it has a rotation period less than 48 hours or so, it's fine for life.


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