Imagine a small primitive humanoid civilization that developed independently in caves under the surface of Ganymede. We can assume there's enough light that filters through the crystalline surface to support life, and that there's enough air trapped in these caves for them to breathe.

But let's say these people also happen have a gravity that's slightly greater than Earth's. How could that be the case? And why wouldn't Earth's astronomers have discovered that before now?

Also, are there any other significant factors that would make it difficult for Earth-like life to thrive? Things that would be harder to hand-wave away?

(The SF here is about as hard as cotton candy, so answers don't need to be completely realistic. I'd just like to avoid directly contradicting known observations any more than I need to.)

  • $\begingroup$ Welcome to Worldbuilding. Please take the tour and visit the help center. Can you add a tag explaining which kind of answer are you looking for? Science based or magic? $\endgroup$ – L.Dutch - Reinstate Monica Sep 18 '18 at 14:31
  • $\begingroup$ Thanks. I added the science-based tag, and I'm checking out the tour right now. $\endgroup$ – Admiral Jota Sep 18 '18 at 14:33
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    $\begingroup$ Are there aliens or Q involved? Gravity is linked to mass, and both govern orbital characteristics, so unless there is some external force at play, gravity is set for Ganymede. Also, did life evolve there, or was it seeded? Because humans are not inevitable as a product of evolution... $\endgroup$ – bukwyrm Sep 18 '18 at 14:47
  • $\begingroup$ I'm pretty flexible on the ultimate origins of life there. I could happily go with an "ancient aliens seeded both Earth and Ganymede billions of years ago" theory if that makes things easier. And I'd be fine with suggesting those ancient aliens used some unknown advanced technology or "impossible" materials to intentionally craft an ideal environment there. $\endgroup$ – Admiral Jota Sep 18 '18 at 14:52
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    $\begingroup$ I think you are left with magic (or technology sufficiently advanced to be indistinguishable). $\endgroup$ – bukwyrm Sep 19 '18 at 4:57

11 Answers 11


Don't change the mass - change the density.

(Soft science ahead - all hands brace for impact!)

One thing you probably shouldn't do is change Ganymede's mass. That would change its orbit (and its influence on the other moons) in unavoidable and easily observable ways. You'd have to do some elaborate hand-waving to make Ganymede appear to be its apparent mass while having a very different actual mass.

To have a solution from changing the density will still require some hand-waving, but maybe it's allowable in a "cotton-candy-scifi" universe...you can be the judge of that!

To attain earth-like gravity in your caves, we would have to: 1) make Ganymede's core unnaturally dense and its mantle unnaturally light, and 2) place your caves much closer to the core. The handwaving required to make this happen is two-fold:

Firstly, to actually concentrate Ganymede's mass this much in the core, you could not use any naturally occurring material in the known universe. Materials made of conventional elements are too light, and electron- or neutron-degenerate matter would not remain compressed under earthlike gravity--it would explode. So...probably the best soft-sci-fi solution (without invoking artificial gravity generators) is that Ganymede's core contains degenerate matter which for some reason can't decompress. (Is it special matter? Is it in a fluke, naturally occurring statis field? Handwave!) Similarly, you'll need to handwave a material to compose Ganymede's mantle that is extremely light and somehow looks to our telescopes like a salty ocean. (See https://en.wikipedia.org/wiki/Ganymede_(moon)#Composition ) Which bring us to our next point...

We will need to handwave some of our observations of Ganymede's physical appearance and its moment of inertia factor ( https://en.wikipedia.org/wiki/Moment_of_inertia_factor ). To be honest, I don't think there will be any self-consistent and elegant way to explain away all of the observations we've made of it. But at the very least, try to have a reason for why Ganymede's surface is or appears to be made up of water ice and silicate rock, and why it appears to have a subsurface salty ocean and an iron-rich core.

(To tackle the surface, I would offer this...our extremely light mantle-material is somehow also fairly tough and rigid, and the silicate rock of the surface is mostly layers of dust/fragments from meteor impacts.)

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    $\begingroup$ Thanks -- this looks very promising! Good food for thought about the exotic materials and physical appearance. (I'll upvote you as soon as I have enough rep to do so.) $\endgroup$ – Admiral Jota Sep 18 '18 at 15:31
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    $\begingroup$ Yes, I've been thinking about it a bit more since I wrote my answer and a black hole which the people live near was the only thing I could come up with that is even slightly viable - and even that has enough problems to put it firmly in the realm of cotton candy science) $\endgroup$ – Tim B Sep 18 '18 at 16:09
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    $\begingroup$ Yeah, I hadn't forgotten black holes. They would require a different kind of handwaving, is all (why doesn't the rest of the moon fall into it?). $\endgroup$ – Qami Sep 18 '18 at 17:37
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    $\begingroup$ @KeithMorrison : that would be true of distances outside of the body's initial surface. If we take a point inside the sun, however--say somewhere at half the sun's radius--then that point would feel a higher gravitational pull if all the sun's mass were compacted to within half the sun's diameter. $\endgroup$ – Qami Sep 18 '18 at 21:19
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    $\begingroup$ I made some quick calculations on this. The "real radius" of Ganymede would have to be rGanymede = sqrt(massGanymede/massEarth)*rEarth or about 1000 km (instead of 2634 km), which doesn't sound too bad. But density would then have to be about 35000 kg/m^3, more than any normal material on earth. $\endgroup$ – Dubu Sep 19 '18 at 11:19

I'm sorry, but it's impossible.

Ganymede has 2.4% of Earth's mass. That mass is what generates gravity.

If it had more gravity then it would distort the orbits of the other moons and we would know about it. We know the mass of every substantial body in the solar system (and in fact some of them were detected because they were distorting the orbits of things we did know about and we were able to go look in the right place).


You need to come up with a way to achieve your goals that does not involve gravity as we know it. For example clawed feet to grasp the ice, magnetic boots, or even just bouncing around in ice tunnels are all possible.

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    $\begingroup$ Mass combines with radius to make gravity! If it were a lot smaller, it could have an Earth-like gravity. That's why the "variable density" idea is more workable: if there were a much more dense (i.e., small-radius) core and the "people" were near it, G would be much higher. $\endgroup$ – Jeffiekins Sep 18 '18 at 20:32
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    $\begingroup$ It might be impossible, but not for the reason you mention. See "what if? little planet" $\endgroup$ – Eric Duminil Sep 19 '18 at 7:02
  • $\begingroup$ @EricDuminil: That's for an asteroid with 2m diameter. Ganymede is way bigger than that. $\endgroup$ – nikie Sep 19 '18 at 9:41
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    $\begingroup$ @user151841: If you replace Ganymede with a soccer ball of Ganymede's mass, nothing at all would change for Jupiter or the other moons. The only thing that would change would be the gravity at Ganymede's surface. The same thing happens (i.e. nothing) if the sun becomes a black hole. $\endgroup$ – Eric Duminil Sep 19 '18 at 19:51
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    $\begingroup$ @EricDuminil Well that illuminates the other half of the problem. If Ganymede were the size of a soccer ball, but had the same mass, we wouldn't known it was there, and we would have had to figure out the mystery of the "missing moon" of Jupiter. We would have concluded there was a very dense, very small moon that had to be there, because of its gravitational effects on the orbits. However, if it's large enough that we can see it, we know it's there, and from seeing it, we can understand its orbit, and deduce its mass. There's really no way around it. We know it's there one way or the other. $\endgroup$ – user151841 Sep 24 '18 at 0:34

Let's back track and figure out how we know the mass/gravity on Ganymede. (Longer read here).

First off, we need to calculate the radius of the Earth. This has been known to a relatively high degree of accuracy for a very long time. Then we need to measure what Earth's 'gravitational pull', or mass, is, by using an object of a known mass. With this in hand, we can actually calculate the mass of the sun knowing its distance to Earth (again, science has proven this).

From here we can measure the mass of any planet in our solar system with relative ease. With Jupiter's mass now known, we can actually watch Ganymede and calculate its mass as well.

At any point, if there was an error (and rest assured, there isn't one large enough to accomplish what you request), it would affect our measurements of everything down that linked chain. So in your case, we'd have to have grossly mismeasured either Jupiter's orbital movements, or Ganymede's (or likely both to get the increase in mass you need).

Suffice it to say, this is highly unlikely.

On to your other question, check out the amount of radiation on Ganymede. At 8 rem a day, it is definitely going to be wreaking havoc on your earth-like life over time.

  • $\begingroup$ I think there's a mistake in that paper you cite.. You can only get the mass of the sun as described therein. To get the mass of a planet, it needs to have a moon (the method allows you to get the mass of a central body - not those orbiting it). So the mass of Ganymede is estimated - not calculated. Unless it's determined by perturbations of other moons' orbits, but that's hardly trivial. $\endgroup$ – Oscar Bravo Sep 19 '18 at 14:56

If the caves is rotating very quickly, the inhabitants would experience something they perceive as gravity while inside the cave. Upon stepping outside the cave they would become almost weightless.

Imagine the inside Ganymede there is a sphere that rotates much faster than the planet itself. Why? You'll need a reason, like some other inhabitants wanted an amusement ride, but got bored and left, or something smacked into Ganymede just right. Between Ganymede's surface and the sphere maybe there's a layer of something liquidy, with very little friction. Inside that is a rapidly spinning sphere, or at least an annulus (donut). The inhabitants inside there would believe there was gravity outward toward the surface. Getting to them might require some kind of special arrangement, but if that arrangement is airtight, then your air will stay in place too.

  • $\begingroup$ I like the giant gravitron idea. Perhaps some massive, ancient generation ship crashed, embedding itself into the surface, simultaneously introducing life and an extreme spin. $\endgroup$ – Wazoople Sep 18 '18 at 20:29
  • $\begingroup$ @Wazoople Or maybe Ganymede is the ship, and some ring material has glommed on over time... $\endgroup$ – jimm101 Sep 19 '18 at 13:22

Possibly there is some semi-scientific or magical form of gravity generators that generate gravity (as in many space operas like Star Trek and Star Wars). Gravity generators are used to provide artificial gravity in space ships in many space operas.

And perhaps somebody placed such gravity generators beneath the floors of sealed air filled caverns under the surface of Ganymede. The light in those caverns may also be artificial. If the caverns are sealed and air tight the air will be kept in by the caverns, and the artificial gravity wouldn't be needed to retain atmosphere, but might be necessary to provide gravity for the health of the human population.

In fact it is considered possible that there could be lifeforms in liquid oceans beneath the ice covered surfaces of Ganymede and other moons in the outer solar system. So what you are proposing is vaguely similar to that speculation, except that you propose small air-filled caverns in the ice instead of a world wide ocean beneath the ice.

The combined effect of those gravity generators should increase Ganymede's overall gravity and make it seem a bit more massive than it actually is. But if those gravity generators are beneath only a tiny fraction of the Ganymedean surface the total effect may be very slight.

And when space probes are put in orbit around Ganymede they may detect the effects of those gravity generators, just as the first lunar satellites detected mass concentrations (mascons) in the moon.

https://en.wikipedia.org/wiki/Mass_concentration_(astronomy) 1

And possibly analysis of the strange gravity readings may prove that they can't be the result of Ganymedean mascons but must be caused by generated gravity.


They can live inside a spinning centrifuge. By controlling the speed (and tilting the floor) it can generate any level of gravity-like acceleration needed, from Ganymede to Earth or higher. The centrifuge was left there by a previous, more advanced civilization that also left all their other life-support systems. Is Jupiter's intense radiation a problem for them?

  • $\begingroup$ Welcome to Worldbuilding! Your answer is good, but you should remove the last sentence. If you have a question for the OP, it should be asked in a comment. I know you can't comment yet, and that's always a problem for new users, but there are a ton of questions that don't need any extra clarification. For the time being, go ahead and skip any questions that you can't answer without more info until you get enough reputation to comment. Good luck! $\endgroup$ – John Locke Sep 19 '18 at 18:48

Ganymede itself is packed with high-density materials such as tungsten and uranium deposits resulting in an earth-like overall mass easily 50 times as huge as it should be. Artificial Superheavy elements beyond anything ever manufactured in a lab.

The Surface of Ganymede is covered in a thick layer of Cavorite Dust, resulting in its unusually high density being almost wholly cancelled out, what gravity/Mass-Effect that filters through the Cavorite is only a couple percent of its natural strength.

Within the caves, gravity is unaffected and the inhabitants experience earth-normal conditions.

If you want the surface itself to have earth-normal gravity, you could handwave that the Cavorite attenuates the effects of gravity so that it falls off very rapidly, eg: over a matter of meters. Shortening the length of the gravity waves to something you could measure on a yard-stick. Meaning you can walk around as normal, but throw a ball high into the air and it won't be coming down again.

Having gone away and looked up material densities, I realised that the required density in order for Ganymede to be literally 5000 times as massive as it appears is well beyond tungsten or uranium or even Osmium or Hassium.
You need a material with a density of 779,634,464,751.96 kg/m^3 to do it.

I have corrected my answer accordingly.

  • $\begingroup$ I was out by 10 in my density calculation and thought that you'd need an artificial super-dense element... Bit disappointed you only need tungsten. $\endgroup$ – Oscar Bravo Sep 21 '18 at 12:38
  • $\begingroup$ Disclaimer. I have no idea whether a core of tungsten and uranium deposits would be sufficient to produce an earth-like mass! But checking vs Iron by molar weight would probably be a useful comparison. $\endgroup$ – Ruadhan Sep 21 '18 at 14:23
  • $\begingroup$ Just went and did some figuring. Iron is 7850kg/m^3, while Tungsten is 19600kg/m^3. So technically speaking it's somewhere between 2 and 3 times as dense. You'd need something 5 times more dense than tungsten to achieve earthlike mass with Ganymede. Uranium is less dense than Tungsten at 18900kg/m^3. So yes. you'd probably need an artificial super-dense element. Good luck manufacturing one that isn't a ridiculously short-lived radioactive element. $\endgroup$ – Ruadhan Sep 21 '18 at 14:28
  • $\begingroup$ Osmium is 22590kg/m^3, still not viable for this, and Hassium (the densest material ever made in a lab) is a little denser at 22610kg/m^3. $\endgroup$ – Ruadhan Sep 21 '18 at 14:34
  • $\begingroup$ I think you were right the first time... To get the same gravitational acceleration (at the surface) as Earth (9.81$m/s^2$), but keep the same radius, Ganymede (1.5$m/s^2$) needs only to get about 7 times heavier ($a \propto m$), therefore 7 times more dense. Its current density is about 2$g/cm^3$ so that takes us to about 15$g/cm^3$. Which is easily attainable with normal matter. $\endgroup$ – Oscar Bravo Sep 24 '18 at 6:09

I think your only 'realistic' solution is a gravity generator with very limited range. If Ganymede's actual gravitational attraction was larger than it should be, it would affect its orbit, and the orbit of anything else that got near it, which would have been detected from afar by astronomers.

A gravity generator (presumably built and then abandoned by some ancient species) that only reached a very short distance above the surface, so as the keep atmosphere and inhabitants firmly rooted, but not far enough to affect orbital characteristics should fill the bill. Naturally, a real gravitational field would not act that way, but since you're inventing a gravity generator that generates artificial gravity, you're entirely free to make that artificial gravity behave in a non-standard manner.

  • $\begingroup$ Gravity is generated by mass and has an infinite range. How can you circumvent this from a scientific point of view? $\endgroup$ – L.Dutch - Reinstate Monica Sep 19 '18 at 16:05
  • $\begingroup$ @L.Dutch The poster is positing an artificial generator of gravitational field that doesn't need mass and has a short range. No need to actually invent it since this is WorldBuilding and not Physics. $\endgroup$ – Oscar Bravo Sep 21 '18 at 11:57
  • $\begingroup$ Topologically speaking, you can view gravity as an indentation in space-time (ie: the old rubber-sheet demo) Generally the distortion is across a wide area and the indentation is very shallow, so if you wanted to have a close-ranged gravity field you'd need to essentially "scrunch up" spacetime to do it. like gripping part of the sheet and pulling it together so that it hangs loose in the middle. If I knew how to actually implement that, I'd have won a nobel prize :P $\endgroup$ – Ruadhan Sep 24 '18 at 8:05

Some answers are a bit misleading - especially those quoting the Scientific American article. You can only get the mass of the primary object from simple orbital mechanics. So you can't get Ganymede's mass simply from observing the radius and period of its orbit around Jupiter (it's a pretty good way to get Jupiter's mass - but that's not the point). Any object at the radius of Ganymede would orbit Jupiter in the same period - regardless of its mass.

For a sphere of given size, the gravitational field at the surface is depends on the density so that:

$$ \rho = \frac{3g}{4\pi G r} $$

So if you want Earth-gravity on a planet the size of Ganymede, you'd need to make it out of material with a density of about $15\space g/cm^3$.

This is pretty dense - about three times Earth's density. However, if Ganymede is mostly made of some very dense elements like Tungsten or Uranium (as mentioned by @Ruadhan) it would work.

  • $\begingroup$ I'm afraid it really wouldn't work. By my reckoning, you'd need a material at least 7 factors of 10 denser than the densest material ever manufactured or found in order to make ganymede remotely as heavy as earth. but if we assume Aliens created an artificial planetoid out of stable super-heavy isotopes and coated it in more normal materials, that'd do it. Tungsten and Uranium are pretty piddly when compared to the requirements. $\endgroup$ – Ruadhan Sep 21 '18 at 14:45
  • $\begingroup$ See note above - I made a mistake the first time, but I think this calculation is correct. A small, dense planet can easily have an Earth-like gravitational field. It might be quite unlikely, but normal metals are plenty dense (as you said in your first answer!) $\endgroup$ – Oscar Bravo Sep 24 '18 at 6:15


Replace gravity with magnetizm. Denizens of your caves know only metal. No wood, no furs, no plastics. They wear steel, build from steel, and their food is.. complicated. Beneath the caves there is a powerfull source of magnetism - ancient spaceship or natural phenomenon. (it explains why they have so much iron to start with)

So, all metallic objects are pushed down, and since people have nothing else, it works exactly like gravity. Except that people almost fly up if they are nude - but you can use that in your story too.

  • $\begingroup$ I like this idea, ganymede being packed with rare-earths isn't beyond the realm of possibility either. $\endgroup$ – Ruadhan Sep 24 '18 at 8:06

Ganymede is not a naturally formed moon; it is an alien spaceship that abducted people in the [insert] age who then defeated their captors and lived inside the spaceship, which then drifted until it was captured by Jupiter. The spaceship has of course artificial gravity and is built around a reactor in the core, but the reactor is on stand-by mode, only supplying the people living there with oxygen, water, etc. needed for their survival. They farm the alien and Earth plants the aliens gathered for study and maybe have some domestic animals, too, also originally gathered by the aliens.


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