How can Ganymede have an Earth-like gravity without us having realized it?

Question

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.)

Answer 1

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.)

Answer 2

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).

http://solarviews.com/eng/ganymede.htm

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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?

Answer 7

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.

Addendum:
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.

Answer 8

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.

Answer 9

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.

Answer 10

Magnetizm

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.

Answer 11

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.