I recently watched a special on cave divers in the Yucatan Peninsula and part way through that show the divers encountered a feature called a Halocline, a meeting of two different water masses of differing salinity. It looked like a mirror, or a surface in the middle of the water. This sparked an idea for a planet:

enter image description here

Welcome to Sepharad

This planet features an "ocean" of a yet to be determined liquid that is about 100ft thick and suspended above a denser and warmer atmosphere below it.

My questions are:

What liquid should be used and how dense would that atmosphere have to be to float that liquid?

Is there a better way to get the desired effect (like a mirror or a surface in the sky) without floating a liquid?

You may change details of the setup (pressure, liquid depth ect.) if those details lend you different results.

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    $\begingroup$ The "atmosphere" would need to be denser than the liquid, for starters. For example, if you want a layer of liquid water floating on something then that something must have a density higher than 1 g/cm³; for comparison, liquid air, with a density of 0.87 g/cm³ is less dense than water. $\endgroup$
    – AlexP
    Commented Nov 28, 2017 at 0:11
  • $\begingroup$ We need a very high air density (at least 500 atm), and eventually the gas will escape from "lower" to "upper" atmosphere, but this setup with "liquid above" is interesting and worth asking on physics.stackexchange.com $\endgroup$
    – Alexander
    Commented Nov 28, 2017 at 0:43
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    $\begingroup$ Is the entire planet covered in both liquids? It sounds like you are basically describing a water world with a denser liquid underneath and a less dense liquid above it. $\endgroup$ Commented Nov 28, 2017 at 0:43
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    $\begingroup$ Related to Is it possible to have a Gas heavier than a liquid? $\endgroup$
    – Alexander
    Commented Nov 28, 2017 at 0:46
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    $\begingroup$ Instead of gas, look at supercritical fluid. There have been some discussions in the past on that… see my answer here for an intro that applies to this question. The gas-like and liquid-like properties can vary with conditions based on altitude. $\endgroup$
    – JDługosz
    Commented Nov 28, 2017 at 4:11

5 Answers 5


There are several pairs of liquids and gasses where the gas is more dense, and thus the liquid will float on it... for a while at least, as in many cases one will dissolve into the other, which ruins the effect!

There is only one pair that I know of in which the gas will not dissolve into the liquid, and in which this effect has in fact been practically demonstrated (and in a YouTube video, no less! Can You Float a Liquid on a Gas?): Supercritical xenon, and liquid NaK alloy. (Technically, supercritical xenon isn't actually a gas, but close enough!) This works because NaK is a super light metal with a very low melting point, xenon is a really heavy gas, xenon doesn't dissolve in or react with NaK, and you can increase the density of a supercritical fluids way past normal liquid densities by increasing the pressure.

This pair exists at extremely high pressures, but normal human-comfortable temperatures. And liquid NaK is, in fact, mirror-shiny! Unfortunately, everything else about it is not particularly human-friendly. You could probably dissolve enough oxygen in the xenon to be breathable, but xenon is a powerful anaesthetic--any humans trying to breathe this atmosphere, even if they were acclimatized to the pressure, would rapidly fall unconscious. Also, the oxygen would react with the NaK, which would screw everything up in short order. You probably want a strongly reducing atmosphere, with hydrogen mixed with the xenon, to maintain the mirror layer. That could be breathable to some form of life, but not humans! Water vapor would also react with the NaK layer, so you'll need an alternative, non-oxygen-containing, biosolvent. (I thought for a moment a mixture of supercritical xenon and supercritical CO2 as a biosolvent might work, but that would probably react with the potassium. So I dunno... unless you can come up with a reducing, non-oxygen containing solvent that's liquid between -13C and 785C, this might just have to be a lifeless planet after all.)

You also have the problem of where the necessary planetary quantities of xenon, sodium, and potassium came from... eh, Magratheans did it!

  • 5
    $\begingroup$ Supercritical xenon and liquid NaK... I'll have to remember that. This isn't the first liquid-floating-on-gas question I've seen on WorldBuilding. That Cody'sLab video also shows that a water-ethanol mixture can also float on supercritical xenon without dissolving too much xenon and becoming too dense or turning into a clathrate and freezing. 150-proof alcohol is much more amenable to having an oxygenated atmosphere under it than NaK is. Plus, it's transparent, so you could actually get sunlight reaching the ground through it. $\endgroup$ Commented Nov 28, 2017 at 4:26
  • $\begingroup$ @SomeoneElse37 It would still be in chemical disequilibrium, though, so you'd need an explanation for how and why oxygen & ethanol are being continuously regenerated. That would allow you to use a reducing atmosphere with water as a solvent, though--the natives would just have to be hydrogen breathers. $\endgroup$ Commented Nov 28, 2017 at 5:26
  • $\begingroup$ The mirror effect was rather caused by the light hitting the boundary between the two layers at a wide angle (close to horizontal). If it were narrower, the light would refract. Now, about the liquid floating in gas: This is different than a pressurized tank: Surface pressure is the highest, stratosphere has the lowest pressure, because pressure is the weight of the atmosphere above you. So, the liquid would float, but not up the atmosphere. Instead, it will float somewhere in the middle layers of the atmosphere. $\endgroup$ Commented Nov 28, 2017 at 9:54
  • $\begingroup$ My deepwater xenon habitat scheme: halfbakery.com/idea/Xenon_20Breathers#1134277206 My math says you could have a bubble of xenon at 430 meters undersea. The anaesthetic effect can be overcome with directed evolution etc. I was thinking fish would fall into your habitat but actually they should be able to swim right on through. $\endgroup$
    – Willk
    Commented Nov 28, 2017 at 16:44
  • $\begingroup$ @will: If Xenon is soluble, then why would it sink to the ocean floor at those pressures, and if it were not, then how would it get there in the first place? Do you think deep sea vents provide Xenon at a rate faster than its dissolving rate? $\endgroup$ Commented Nov 28, 2017 at 19:23

Let me build on @AlexP's comment, because he's completely right. You can't have liquid water floating above any atmosphere no matter the pressure because LOX is lighter than water. The water would fall and the LOX rise (and then probably boil into a gas).

But let's explore the idea in a little different way...

Because the idea is super cool.

Water vapor gets up there in the form of clouds, which precipitate rain. Might it be possible to have a dense but breathable atmosphere below that permits a very high concentration of water vapor above? And can that water vapor be dense enough to permit, for lack of a better example, fish?

For the moment I'm going to completely ignore the fact that fish would fall through water vapor for exactly the same reason AlexP described. I'm also going to ignore what happens to the poor suckers when they happen to swim out the bottom of the pond....

Now, full disclosure, I am not by any stretch of the imagination a meteorologist or climatologist. I am officially pulling this random string of barely related thoughts out of not particularlly dense air. You've been warned.... (And if you're tempted to downvote just because my science is off by miles, remember that I'm having a lot more fun than you are. Thbppttt!)

So, you don't tell us who's living on your world, so let's assume some humans have adapted to the climate. According to Wiki the earth is 78% Nitrogen, 20% Oxygen, 1% Argon, 0.04% CO2, some stuff we don't care about, water vapor to between .001% and 5%, and 0.000179% Methane. That's 99.0412% of our atmosphere excluding the stuff I'm not going to worry about and assuming low water content.

It's the Nitrogen that's really killing us. That stuff is thin. So we need to reduce it to something like 45%. It's gonna smell and it's going to be honking hot, but we need to vaporize water anyway, right? Let's up the Methane to 2.5%, the Oxygen to 45%, the Argon to 3%, the CO2 just a smidge to 0.05% and up our water vapor to the range of 2.5% - 7.5%. Take those elements I'm not caring about and up them appropriately to fill in the gap.

BTW, you need to remember that honking hot comment...

OK, we have an atmosphere that would make Venus proud and hey, humans might actually grow accustomed to sucking it in. It'll be a bit too much like living in your 7th grade gym locker room... but we're an adaptable species.

Now... here's a guess that will likely send the far more science-oriented among us screaming into the night... right after they found me in a dark alley and beat the crap out of me. But...

We need a lot of "light" water vapor. That suggests bonding it to something that will help it float.

  • H3O2 is more dense than water, but it has a lower specific gravity by volume, which suggests it might "float" in the atmosphere as vapor more easily than H2O. It's also slick (low coefficient of friction), which would give a large amount of it a somewhat glassene appearance and it would feel/be slippery (good for the fish!).

  • Now, let's mix that with just a bit of Ammonia. NOT A LOT! Just enough to lower the density a bit.

  • Finally, let's mix in some alcohol! (YAY! Happy fish!) A quick Google search lists ethanol as the preferred molecule. Based on the fact that my more libatious friends in high school actually lived through high school, I'm going to go out on a limb and guess humanity could withstand 25% ethanol in the water-vapor-brew.

  • And something altogether magical that keeps those three elements bonded together without weighing them down like iron. I'm going to utterly ignore that, too. Thbppttt!

OK, I'm guessing again, but let's say 74% H3O2, 25% ethanol, and 1% ammonia. (And let's hope the beating I get doesn't break any bones.)

So, what I sincerely believe (that's a really strong word, here. Maybe "wish" would be better!) is that we have a breathable atmosphere with a lucious, thick water vapor layer above it that just might be thick enough (thanks to the heat!) for a fish (well... a flying fish, anyway) to live in. The critters that evolve to love this layer will fly-swim in it, but are very unlikely to ever exit it as their flying ability won't lift them in the thin upper atmosphere and their swim ability won't save them when they plummet to the ground in the lower atmosphere. Hydrogen sacks would help tremendously. And my guess is that from space the planet would look like a cloud-covered mirror. If you live on the surface, don't expect to see the sun anytime soon.

And it will be HOT! HOT HOT HOT! So you might not end up with an hospitable planet.... That would be a weakness in the storyline, overcoming what might be debilitating heat... But just maybe.... The critters of such a planet would be COOL! (no actual pun intended. ... well, maybe a little one.)

My muse behind this was the atmosphere of Titan and the layered atmospheres of Jupiter and Saturn. Which are nothing at all like I described.

  • 3
    $\begingroup$ Clouds are not water vapor! They're suspensions of droplets of liquid (or in some cases frozen) water in air, Actual gaseous vapor in air is clear as ... air. $\endgroup$ Commented Nov 28, 2017 at 5:39
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    $\begingroup$ Also, what does "more dense ... but has a lower specific gravity by volume" even mean? Usually "more dense" means more mass per volume, in direct contradiction of the second part of this. $\endgroup$ Commented Nov 28, 2017 at 5:41
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    $\begingroup$ Also also, it does not look like H3O2 is actually a thing. A google search turns up a bazilion hoaxy/cranky homeopathy 2.0 sites, and some more reliable-looking chemical investigations of a H3O2- anion which supposedly appears in small quantities in ordinary water, as a temporary combination of ordinary H2O and OH- anions. $\endgroup$ Commented Nov 28, 2017 at 5:58
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    $\begingroup$ @HenningMakholm, did you miss the part where I said, "And if you're tempted to downvote just because my science is off by miles, remember that I'm having a lot more fun than you are. Thbppttt!" There are a small handful of people who sit in the peanut galleries of this site and challenge the science fiction believing that, somehow, the world is a better place by doing so. Please try not to be one of those people. If you're happy with Logan's answer (a GREAT answer, and probably the only one that will be fully supported by science), then upvote it. $\endgroup$
    – JBH
    Commented Nov 28, 2017 at 6:11
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    $\begingroup$ If you feel that my answer is in no way useful to the OP, who is writing a work of fiction, then by all means downvote it. If you can provide a better answer, then please, I beg of you on my hands and knees, provide it! Because there are plenty of complainers on this site, but few people who really enjoy answering ... often because of the complainers. $\endgroup$
    – JBH
    Commented Nov 28, 2017 at 6:11

Water is slightly diamagnetic, which means it will repel a magnetic field of either polarity. One well known example of this is the levitating frog experiment, which used a 16 tesla magnetic field:

So if your planet had strong enough magnetic field, it would repel water. Earth's magnetic field varies in strength in different parts of the globe, so a localized strong field could allow water to exist on ground level elsewhere.

Of course this leaves a lot of things open about the source of the magnetic field and the stability of the situation. For comparison, neutron stars have magnetic fields of 1011 teslas, so there do exist natural processes that cause very strong fields.


Liquid over gas seems like it's a pretty hard slog.

Gas over gas: The densest gas I can find is per-fluoro-butane C4F10 with a molecular weight of 238, so at stand temp and pressure it's about 7 times as dense as air. (Yes, UCl6 is denser, but has side effects)

Keeping them unmixed would be an issue, although they probably would establish a gradient.

Would make it interesting for air ships. And if you go too deep you drown.

Liquid on liquid. This has the advantage that you can get immiscible liquids. Probably want water for the top layer. Move up to perfluoro-pentane in the above, and you get a liquid that boils at 28 C Eruptions of smothering gas when the ocean gets too warm.

Currents and waves when you have two liquids of similar density are fascinating.


I'll answer the second part of this question because yes, there are absolutely easier ways to do this than with a floating ocean. The phenomenon that causes that mirror-like effect is Total Internal Reflection. This happens when light reaches a boundary between two substances with different refraction indices. When this happens, the light can either pass through or be reflected, and is often a combination of the two. This behavior is governed by Snell's law: $$n_1 * sin(\theta_1) = n_2 * sin(\theta_2)$$ where $n_1$ and $n_2$ are the refractive indices of the materials and $\theta$ is the angles of incidence. To obtain total internal reflection, the outgoing angle $\theta_2$ must be 90 degrees or greater. The diagram below sums this up nicely, from Josell7 at Wikipedia:

enter image description here

Of course, Josell is working with water an air, a pretty typical physics example, but the theory is still the same. Our goal is to alter $n_1$ and $n_2$ to obtain as high a returned angle as possible, thus increasing the angles which have total internal refraction.

Before I get into worldbuilding options, I want to point out a couple places where this already happens on Earth. Total internal reflection is not a rare phenomenon, and anyone who's looked at the surface while underwater in a pool can tell you it happens. The place I want to highlight in particular is mirages. These are instances where a layer of hot, non-dense air near the surface is viewed from above and at a great distance. When the light moves moves from the sky to your eyes, it comes in a such a shallow angle that the small difference in density (which is generally a proxy for refractive index) is enough to cause it to reflect off the layer of hot air and back to our eyes. To us, this appears as a small patch of sky on the ground in the distance, which is why they're often mistaken for water!

Unlike when trying to suspend a liquid above a gas, density is working with us here. We want to have a very dense gas near the ground and a light gas directly above it. Our best bet for the light gas is helium, because its refractive index is 1.000035. Ironically, this is a counterexample to my comment above, where I said that more dense things generally have a higher refractive index, as hydrogen gas has an RI of 1.000132.

If we simply add a layer of helium gas to our current Earth, we can solve for the critical angle with this equation, derived from Snell's law above: $$\theta_{crit} = arcsin(\frac{n_2}{n_1})$$ which in our case is $$\theta_{crit} = arcsin(\frac{1.000035}{1.000292}) = 88.7$$ so capping Earth with a layer of helium would make the two degrees above the horizon totally reflective. Cool! Let's do better.

Other gases with a high refractive index include nasty things like benzene and chloroform. However, those gases only get us up to about 85 degrees. The densest non-reactive gas known gets us no further. So gases on gases will be very hard to do.

The best solution is the one already present in nature- make your creatures live in an ocean, and the critical angle jumps to ~45 degrees, which is about as high as you can get.

Update from the people over at Chemistry:

There doesn't seem to be a fundamental limit to the refractive index, but it depends a lot on temperature and pressure. One answer mentioned a refractive index as high as 1.029 for a gas, which raises our critical angle to 75 degrees- on par with an idealized halocline!d

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    $\begingroup$ Probably worth noting that helium actually escapes earth's atmosphere physics.stackexchange.com/questions/78586/… $\endgroup$
    – bendl
    Commented Nov 28, 2017 at 18:53
  • $\begingroup$ If you're worried about the shortage (which happens on geologic time scales), have a planet with a lot of alpha decay in the core (uranium 238, for example) which is the source of all the helium. That way the helium would have a source as well as the solar wind sink. $\endgroup$
    – Dubukay
    Commented Nov 28, 2017 at 19:13
  • $\begingroup$ I was referring to the geological time scale, by the time anything managed to evolve, most of the helium would be gone. That being said the massive amount of alpha decay is a very good idea $\endgroup$
    – bendl
    Commented Nov 28, 2017 at 19:15
  • $\begingroup$ Unfortunately, there's no way to keep that distinct layer of gas; all gasses are miscible with each other, and the boundary would rapidly diffuse away, no matter how much alpha decay you have replenishing the supply of helium. $\endgroup$ Commented Nov 28, 2017 at 20:28
  • $\begingroup$ How does something like a halocline fare in the internal refraction department? $\endgroup$ Commented Nov 29, 2017 at 8:21

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