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I have an idea: can a moon float on the atmosphere of a gas giant, just like a piece of wood floats on the water? It is based on the following situation:

  1. The density of the atmosphere increases when getting deeper to the core.

  2. The moon spins inwards to the planet slowly because of some damping such as affected by other moons or molecular cloud.

  3. The rotational speed of atmosphere is nearly the same as the orbital speed of moon so that it would not collide with the atmosphere violently.

Is that kind of situation possible?

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    $\begingroup$ If it was within the atmosphere it wouldn't be a moon. $\endgroup$ – sphennings May 18 '17 at 2:27
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    $\begingroup$ @apaul34208 You mean “fall in” not sucked in. Are you saying that the principle of boyancy does not apply when gravity becomee strong enough? $\endgroup$ – JDługosz May 18 '17 at 2:40
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    $\begingroup$ I'm not being pedantic, but pointing out that the mental model is wrong. Imagine standing on the rim of a deep well. If your balance and traction are sufficient, you stay there. If not, you fall. The deepness doesn’t suck at you. $\endgroup$ – JDługosz May 18 '17 at 5:34
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    $\begingroup$ "There's no such thing as gravity. The Earth sucks." $\endgroup$ – Dave Sherohman May 18 '17 at 7:38
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    $\begingroup$ @RonJohn It's not an Astronomy question because it is about a hypothetical scenario. $\endgroup$ – a CVn May 18 '17 at 13:22

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No. Tidal forces would break it up at Roche’s Limit, so a moon cannot get this close. That’s what caused the rings of Saturn, some 140 million years ago.

No. The orbital speed is many, many times the rotation of the planet. A planet spinning so fast that the equator was at orbital speed would be flying apart.

No. A rock would also be crushed by the pressure, so you would not have the gas (or gas and ice) be denser than rock at some depth. When you get heat and pressure, things become more mixed. So a rock will dissolve in the gas at sufficient depth, for any rock that falls in.

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    $\begingroup$ Atmospheric drag is covered by the second point, even though not explicitly. Maybe worth rephrasing it? $\endgroup$ – L.Dutch May 18 '17 at 5:27
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    $\begingroup$ @Raditz_35 I wouldn't call anything that survives for only a few seconds a "moon", but rather a "meteor" or a "really bad day" (mostly depending on size and velocity). $\endgroup$ – a CVn May 18 '17 at 13:23
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    $\begingroup$ @MichaelKjörling Maybe you wouldn't, but maybe the author would, maybe it is just long enough for him to work with $\endgroup$ – Raditz_35 May 18 '17 at 13:25
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    $\begingroup$ I was referringnto the process of deorbiting noted in the OP: «the moon spins inwards to the planet slowly because of some damping such as affected by other moons or molecular cloud» $\endgroup$ – JDługosz May 18 '17 at 16:06
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    $\begingroup$ @Raditz_35 if the angle is steeper than nearly tangent as the orbit crosses the Roche limit, the moon will be shreaded, just the same. An an analogy, Roche Distance == grating board. What’s the optimum angle for pushing the cabbage through so it passes intact? $\endgroup$ – JDługosz May 19 '17 at 15:12
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An artificial, floating "moon" could work. Of course since it isn't in orbit, it's not a moon. Then again, we don't have a word for such an object, so calling it moon wouldn't be that far off...

It could have been constructed out of some kind of aerogel-like material in vacuum, so it would essentially be a solid, rigid foam filled with vacuum bubbles. It would need to be strong enough to withstand the pressure, non-permeating enough so gas does not leak in through the walls enough to matter, and with low enough density so it floats. If vacuum seems implausible, it could be filled with helium, and still be light enough to float on top of Methane/CO2/Nitrogen/Oxygen atmosphere.

Perhaps it was a construction project which got abandoned before completed Perhaps it was stripped of all technology when the ancients left it. Or perhaps it still has some ancient technology on/in it if that fits your story. If the point of your story isn't how the "moon" got there, then you can just not cover that. Or you can describe its structure, but let the reader to draw the conclusion that it must be artificial.

So it is just a solid, stable, floating object, big enough to withstand erosion for millions of years, which got there somehow. As the top side eroded and accumulated dust (eroded stuff from bottom side will just fall off), it would slowly get denser and slowly sink lower. It would also occasionally (every few millenia or whatever) turn upside down, as top side gets heavier in this process. Then the loose stuff on the top side would drop down, and it would rise in the atmosphere. So it would not be a good place for any life which couldn't anchor itself, as there's this cataclysm every few millenia...

It could also be hollow or have a vast cave network. Just note that it would not be a balloon then, the hollow inside would be filled with normal atmosphere and not provide lift, all the lift needs to be provided by the solid "aerogel" material.

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  • $\begingroup$ You have to use vacuum. The outer layers (and the only habitable ones) are mostly hydrogen. $\endgroup$ – Joshua May 19 '17 at 1:39
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    $\begingroup$ A vacuum filled aerogel would have to be made from a phenomenally strong material, stronger than any known. $\endgroup$ – Level River St May 19 '17 at 19:08
  • $\begingroup$ @LevelRiverSt Well, creating a smalk moon-sized object out of non-porous aerogel would be a feat incomprehensible to current human tech. Without doing any math, I suspect a nanomaterial (graphene/nanotubes like) mesh would easily withstand the pressure. It's kinda solid, rigid material after all. $\endgroup$ – hyde May 19 '17 at 20:23
  • $\begingroup$ Withstanding pressure on the inside is easy. Withstanding pressure on the outside is hard. Rigidity is important. Perfect symmetry helps, but once the vacuum vessel is distorted, it implodes catastrophically youtube.com/watch?v=Zz95_VvTxZM . that tank car weighs many tons yet contains only a few tens of kg of air. $\endgroup$ – Level River St May 19 '17 at 20:47
  • $\begingroup$ @LevelRiverSt In this case it's not a single vacuum vessel. It is "many" (understatement) small vessels (cell size probably micrometers). So comparison to single big tank is probably not relevent. $\endgroup$ – hyde May 19 '17 at 21:51
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No

Obligatory xkcd What-if

Nope! Jupiter's pressure, density, and temperature curves are different from ours. At the point in Jupiter's atmosphere where the density is high enough for a submarine to float, the pressure is high enough to crush the submarine,[1] and the temperature is high enough to melt it.[2]

1Which makes it more dense.
2Which makes it harder to drive.

In brief: what looks like a fluid surface to the naked eye does not have any buoyancy to speak of. You need to get quite deep below this surface in order to achieve any kind of buoyancy. And when you reach that point, the pressure and temperature will quickly reduce your moon to lava which will fall towards the center of the planet and join its core.

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  • $\begingroup$ That argument just shoes that a floating object in Jupiter needs to be less dense than a submarine - that is, a lot dense than water - and probably more resistant. This way it could float higher in where pressure and temperature are less extreme. $\endgroup$ – Pere May 18 '17 at 20:45
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    $\begingroup$ @Pete If ever I find a moon made out of helium birthday party balloons I will be sure to keep your comment in mind. $\endgroup$ – MichaelK May 19 '17 at 6:54
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    $\begingroup$ @MichaelK even helium balloons wouldn't cut it, since gas giants are mostly made of hydrogen, which is less dense than helium at any given pressure. Come to think of it, hydrogen balloons wouldn't work either, since they'd be at higher pressure than their surroundings, which are also made of hydrogen. It would have to be made of aerogel or vacuum encased in very hard, light spheres, or some other such fancy thing. $\endgroup$ – Nathaniel May 19 '17 at 7:50
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    $\begingroup$ @Nathaniel You could use a hot hydrogen balloon, but that's very far from a "moon" $\endgroup$ – Rick May 19 '17 at 12:30
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Quoting my answer on Physics.SE:

No, a shared atmosphere between body and moon is not possible.

For a natural satellite to remain, the orbit must be very stable, because those satellites exist for billions of years. Even the tiniest bit of atmosphere (a few molecules) would cause a tiny drag. However, drag adds up, so over a long time period, even a heavy object (such as the moon) would be dragged down due to drag and ultimately collide with the body it is rotating around.

(...)

One more way to look at it: if a satellite would have enough gravitational pull to pull up an observer in a balloon, it would certainly pull up the atmosphere; therefore the satellite would be in the atmosphere, which is impossible. Therefore, a a satellite can never have enough gravitational pull to pull up an observer inside the atmosphere.

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An idea similar hyde's, but with a natural origin: If there is life on the planet, this could be a huge decomposing carcass. Hydrogen-producing bacteria are digesting the tissues within the tough scaffolding, causing the carcass to bloat, and become much like the aerogel described in the other answer. The more bloated it gets the higher it rises in the atmosphere.

It could possibly also work without life. Alkaline vents on Earth build up a mineral structure that is full of cavities. If the structure is delicate enough, and the cavities are filled with hydrogen, pieces could break off and float up to the sky! The size of the pieces would be sufficiently random, that once in a while you could see moon-sized mountains rising up.

The depths of gas giants are so alien to me that I have no idea how much of this is actually possible. As a reader, I would be okay with this. Especially if it happens outside of our solar system.

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Decades ago, astronomers thought that something similar to that might be possible. They believed that the Great Red Spot on Jupiter might be some kind of giant solid object floating in the atmosphere of Jupiter.

But for decades it has been known that the Great Red Spot and similar things on Jupiter and other gas giant planets are giant permanent storms in their atmospheres.

And other answers make it seem very improbable for solid objects to exist at the levels and pressures needed to float in the Jovian atmosphere.

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I think that it is possible, or at least not too much of a stretch. As others have pointed out it probably wouldn't be a moon (at least not one made of moon materials or structure). However there are several things that could affect it.

The Magnus Effect - If the 'moon' were a ball of something rotating counter to direction of motion at a high enough rotational speed it would gain lift and theoretically buoyancy. This is the most feasible explanation I can see. See https://en.wikipedia.org/wiki/Magnus_effect

A skipping moon - The orbit of the moon could be such that it occasionally contacted the atmosphere. However, when this situation is experienced by orbital objects, it tends to be unstable and results in massive losses of orbital energy. This would also depend on the shape and speed of the moon as well as it's angle of entry to the atmosphere. Another possibility with this would be a moon that hangs low enough that it periodically dips into the atmosphere but is sucked away in just in time by a very close and much more massive body that moves nearby (but further out). Here's some more info on atmospheric skipping https://en.wikipedia.org/wiki/Skip_reentry

Saturn has some very interesting effects of the moons that orbit among its rings. Some moons will actually drag on the rings causing gravity ripples or wakes. And Saturn's F Ring is twisted because of it's nearby interactions with shepherd moons.

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No - You cannot have a floating moon.

Yes - you can have a balloon, but not on a gas giant.

Gas giants have atmospheres rich in hydrogen. As hydrogen is the lightest gas, there is no other gas that will float in it. Forget about balloons with vacuum inside - there is no known material strong enough that it won't collapse yet light enough to float (in air, let alone in hydrogen.)

Venus, however, has thick atmosphere of carbon dioxide. Balloons filled with gas lighter than carbon dioxide will float. Even better, you can fill a balloon with hydrogen without risk of fire, because hydrogen doesn't burn in carbon dioxide.

The thickness of venus's atmosphere leads to high pressure, which in turn leads to high density. Once you get to pressures of 10atm (ten times earth's atmospheric pressure) which occurs above the surface, the lifting force, for a balloon of the same volume, is ten times what it would be at 1atm. This means your balloon can be much smaller. Unfortunately the temperature is already over 200C by that pressure, which is OK for equipment but not for humans.

The temperatures and pressures at venus's surface are enormous. Therefore it is widely proposed that a human colony on venus would be a floating city, staying airborne by means of a balloon. At about 50km above the surface of Venus, the pressure equals 1atm (standard earth atmosphere) and the temperature is around 75C, conditions at which a floating human colony might be viable (provided adequate cooling is possible.)

Google will find a richness of references (too many to include here)

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  • $\begingroup$ The inside of the baloon is under the same pressure. So, the gas inside is 10× denser and heavier for the same volume. Is the boyancy really 10× greater? You might want to include a link. $\endgroup$ – JDługosz May 19 '17 at 19:42
  • $\begingroup$ But what does that have to do with the question or scenareo in the OP? $\endgroup$ – JDługosz May 19 '17 at 19:44
  • $\begingroup$ @JDługosz using the ideal gas model, volume of a gas depends only on the number of gas molecules, not on their type. CO2 has molecular mass of 44 and Hydrogen has molecular mass of 2. At normal Earth conditions, a 24 litre balloon would contain 2g of hydrogen and displace 44g of CO2, giving us 44-2=42g of lift. Bear in mind that the pressure inside and outside a balloon are almost identical. So at ten times normal earth pressure and the same temperature, our 24 litre balloon would contain 20g of hydrogen and it would displace 440g of CO2 giving us 440-20=420g of lift, exactly 10 times as much. $\endgroup$ – Level River St May 19 '17 at 20:15
  • $\begingroup$ The OP wants something solid floating in the atmosphere of a planet. What I mention is the closest OP can realistically get if OP wants to follow hard science. The idea of floating exploration vessels and / or colonies on venus is well known and might form the basis of a "scientifically authentic" story. $\endgroup$ – Level River St May 19 '17 at 20:19
  • $\begingroup$ I made an edit to make that clear. If you took a single balloon and tried it, then raised the air pressure by 10×, that (same) baloon would shrink and it displaces the same number of molecules, leaving the lift unchanged. $\endgroup$ – JDługosz May 19 '17 at 20:22
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Anything in the atmosphere is a balloon, not a moon, no matter how big it is.

Given that restriction I see a way to get something like what you're after (note, though, that it will have no surface gravity!)

Life arose on the gas giant. This was problematic as it had a tendency to fall and get squashed/roasted. Fast-living single-cell organisms could survive due to turbulence (many are thrown down and die, some are thrown up and survive) but as you start getting bigger this becomes a non-viable strategy. Non-microscopic life is limited to wings or buoyancy as a route to survival.

Buoyancy can be subdivided into lighter-than-air and hot-air approaches.

Thus we have three means of survival, two of which take a fair amount of energy to sustain. This makes lighter-than-air a very desirable approach if you can pull it off. On a terrestrial planet you could split water and fill a gas bag with hydrogen, but a gas giant is mostly hydrogen. Thus you need something lighter than hydrogen and there's only one possible candidate: nothing.

Thus we have a critter that grows hollow, evacuated crystals within it's body. Now, consider a coral reef--a structure built up of the dead bodies of the ancestors of the creatures living on it's surface.

Your "moon" is a vast "coral" reef floating in the atmosphere.

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Meissner effect is a candidate.

I've seen this in a movie called The Veil so, you could do it, too. But, I'll try to suggest a sciency way.

The orbital velocity around the gas giant would have to be more than tens of kilometers per second. The friction forces with the atmosphere at that speed would cause a lot of heating, frying everything close enough and eventually slowing down the asteroid or moon until it crashes.

So, you must do something with the gravitational interaction between the moon and the planet. To weaken that, you need to have a repulsive force that acts only between the two celestial bodies, but, somehow, does not interact strongly with other celestial bodies or with the people on the moon.

My suggestion is to have the solid iron core of the gas giant magnetized, while the frozen core of the moon is a high temperature superconductor. A good candidate is the hydrogen sulfide which would be compressed at ungodly pressures such as those expected at the core of a celestial body. Hydrogen sulfide is a high temperature superconductor at those pressures and temperatures below 203K (as discussed here). Due to the Meissner effect, the superconducting moon core will repel the magnetized planet core, and everything would be awesome except cell phones functioning.

Needless to say, such a system has a very low probability to exist in nature, so, unless the universe is infinite, it must have been made by aliens.

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