Okay, I exaggerate…but not by nearly as much as you’d think. The ice giants of our solar system, Uranus and Neptune, are typically dismissed as habitats for mankind: there is solid surface chauvinism (which leads us to overlook Venus at 55 kilometers altitude or so despite it being far more Earth-like than Mars’s surface), but even aside from that, the likes of Uranus and Neptune are cold. How cold?
At the “1 bar level”, the altitude that corresponds to atmospheric pressure identical to Earth’s surface, Uranus averages 76 kelvin, Neptune 72 kelvin. In Fahrenheit terms that’s -323 and -330, respectively. For perspective, in energetic terms that’s closer to absolute zero than to the coldest reaches of Antarctica. Cold enough for nitrogen and oxygen to be liquid, not gaseous. You wouldn’t need a pressurized suit, but these aren’t exactly shirt-sleeve conditions either.
But that’s not the end of the story; remember, these worlds don’t have a solid surface, so atmosphere just continues deeper than the “1 bar level”, denser and…warmer. Planets, especially ones this massive, generate heat in their interiors; Neptune’s and Uranus’s cores are heated by gravitational compression and radiogenic decay just as Earth’s is, and they reach temperatures exceeding 9000 degrees (broadly comparable to Earth, actually). So one would imagine there must be a layer in between the “1 bar layer” and the core where the temperatures are more or less Earth-like. And indeed there is.
Both Uranus and Neptune have “clarified” atmospheres similar to Class III gas giants in the Sudarsky system, devoid of clouds in their upper reaches since at their temperature ranges no condensate is stable, which with their atmospheric composition leads to a blue color (for the same reasons the sky on Earth is blue; you effectively have an ocean of sky, i.e. gas, so if you look down you see blue, not just if you look up, as on a solid world like Earth). Uranus for reasons that remain unknown has thick haze in its upper atmosphere, whereas Neptune’s air is clearer, so the hues are distinct between the two bodies, but the colors are variations on a theme: blue.
But deeper down it’s warmer, and distinct decks of clouds at depth (where atmospheric attenuation makes them less visible to the human eye from orbit) appear. I include here a temperature versus pressure diagram for Uranus’s atmosphere, with the distinct cloud decks labeled. At the 1 bar level it’s clear, without clouds, bar the occasional wisp of methane, but as temperature warms up with depth you get first ammonia clouds (similar to the ones that appear higher up in Jupiter’s and Saturn’s atmospheres and give those planets their characteristic colors), then finally…water clouds.
Chart by Ruslik0 of Wikipedia. CC-BY-SA 3.0.
It might seem strange, but it’s right there in the name: ice giants have a lot of ice in them, “ice” of course being astronomical shorthand for volatile chemicals such as ammonia, methane, and water, which are found in far greater abundance in the outer solar system (beyond the “snow line”).
The graph is in millibars and kelvin and so might not be immediately obvious to the casual observer, but 300 kilometers below the “1 bar level”, the pressure is 200 bars, which is 200 times that found at Earth’s surface. Temperatures are at perhaps 330 kelvin, which equals 134 degrees Fahrenheit. Approaching pressure cooker conditions, but keep in mind the water cloud layer extends much higher up. At the 80 bar level or so temperatures are 250 kelvins, or around -10 Fahrenheit, with abundant water ice in those clouds.
80 bars is of course 80 times Earth’s pressure at sea level, so one would assume a person would need a pressure suit…but one would be mistaken. The deepest “saturation dive” (a dive underwater using special breathing mixes) ever successfully attempted in experiments was 700 meters underwater, which corresponds to a pressure of 71 bars. About the same. You could go down to this layer of hospitable temperature and familiar rains and snows with scuba-style equipment. On Uranus, a planet that orbits almost twenty times further from the sun than our own does. Bonkers already.
But where it gets really fancy is that special breathing mixes are required to withstand 71 bars of pressure. The human body mostly cares about whether it can be oxygenated; to that end, a “partial pressure” of oxygen similar to Earth sea level (0.21 bars) is typically maintained in high-pressure breathing mixes, with inert gases increasing with abundance to match the ambient pressure (typically underwater, but the same principle generalized to atmospheres on other planets as well). On our own planet nitrogen already serves as a rather inert “filler gas” (78% of our atmosphere is nitrogen and it’s rather unnecessary for metabolism; humans if anything breathe easier in a pure oxygen atmosphere of the correct partial pressure, even though total pressure is only a fifth as thick as real air at sea level), but as you ramp up the pressure of nitrogen in a breathing mix narcosis sets in at just a few bars. Indeed many gases are narcotic (xenon is so narcotic it’s been seriously proposed as an anaesthetic…). Helium is an exception, though; at no known concentration does it cause narcosis. So at higher pressures “heliox” is favored: a breathing mix containing 0.2 bars of oxygen along with the appropriate amount of helium (which could easily be 10 bars or more).
However, an ultimate limit is reached; by the time you reach 20 bars or so of pressure “high-pressure nervous syndrome” (HPNS) takes hold, leading to what’s sometimes dubbed “helium tremors” or only “tremors”. Those who have seen “The Abyss” (James Cameron’s masterpiece) are familiar with this phenomenon. It gets progressively worse at pressures beyond this range, to the point a man can’t function at all. And it doesn’t matter what kind of gas you use: it’s a direct physical effect of pressure on the human body. However, there is a little cheat, which is operationally dangerous but has been done experimentally: “hydreliox”.
As the name suggests, hydreliox breathing mixes combine, once again, a sea-level-esque partial pressure of oxygen with an appropriate quantity of helium and hydrogen. Hydrogen is a narcotic gas, unlike helium, but unlike nitrogen, for instance, it’s only weakly narcotic, and this helps to offset the effects of the tremors, enabling function at much greater depths than the 20 bar or so limit that ordinary heliox imposes.
Hydrogen also helps in this realm in as much as it’s extremely light compared to nitrogen, even helium, or indeed any other gas; hydrogen is fifteen times less dense than normal air at a given pressure (because hydrogen’s atomic weight is 1, compared to 16 for the most common isotope of oxygen). This matters, because at 71 bars your lungs are processing 71 times as many molecules with each breath; hydrogen at 71 bars though only weighs as much as 5 bars of normal air (i.e. the mass your lungs are processing is something like 5 times normal air; heavy, but workable). The ultimate limit, given further development of the same techniques only briefly experimented with to date, is likely substantially greater than 71 bars of pressure; getting carbon dioxide processed effectively is the next big hurdle beyond 70 bars, but the ultimate limit is likely imposed by the lungs’ ability to mechanically ventilate the sheer mass of gas, which with a predominately hydrogen/helium atmosphere would surely be reached by 200 bars. But something like 100 bars of primarily hydrogen and helium gas should still be breathable for humans (assuming enough oxygen was there).
Where it gets really interesting is that hydrogen and helium are exactly the components that gas-giant atmospheres tend to be composed of! Uranus’s atmosphere specifically is 83% hydrogen, 15% helium, and 2% methane. That’s not breathable for humans, but the point is the pressure just wouldn’t feel crushing until you got pretty deep down. Even all the way down to the 100 bar level the sensation would be “thick air”, not “being crushed underwater”…and the 100 bar level is where you’ll find chemistry and meteorology that would strike Earthlings as familiar.
How familiar? There are water clouds down there, which mean that you’ll see fluffy white clouds that resemble Earth’s, you’ll see rains, you’ll see snows, all of which you could drink. It’s chemically the same stuff as falls out of the sky on Earth. The temperature range is also Earth-like, ranging from a freezing winter’s day to a tropical sauna, depending on the depth. The pressure? At 80 bars or so, you’re talking single-digit multiples of the mass of the air pressing against your skin compared to Earth air. Thick, with resistance, but the tactile sensation would be air. The human body would withstand it without a problem, with gradual acclimatization. No pressure suit is required. You wouldn’t even need any thermal protection beyond the sort of outerwear you’d don on Earth.
Chemical protection? That’s not necessary either. The chemistry in an ice-giant atmosphere is shockingly human-friendly. Even the ammonia cloud layer, which one would think would be caustic, is primarily comprised of ammonia ices; the ammonia snowflakes and ice fogs that would hit the skin would sublime rapidly at that layer, with the quantity of ammonia contacting your skin being no worse than that found in an after-bite stick. The cleaning fluid you get from the grocery store is more irritating than what you’d encounter on Uranus or Neptune. Meanwhile, the chemistry at work in the water-cloud layer, where the rains are familiar, offers nothing to suggest that humans would be anything other than at home there. There’s simply no component that’s unfriendly to human life.
Water? We love water; we drink water. Helium gas? Inert. Hydrogen gas? Inert too, as far as the human body is concerned. Even methane is inert to our biochemistry; the only danger of high methane concentrations is displacement of oxygen, not anything due to the substance itself (the stuff itself doesn’t even have a smell; household natural gas, which is primarily methane, has a “rotten egg” odor added in just so you can detect it when it leaks!).
Indeed, all of the basic building blocks of life in chemical element terms, common to all life as we know it, the so called “CHON” elements (carbon, hydrogen, oxygen, and nitrogen) are all found in vastly greater abundance in the ice giants, as well as in the gas giants and in general in bodies beyond the “snow line”, than in our part of the solar system (which famously is rather volatile-depleted). Solar heating is minimal, but geothermal energy is abundant enough out there; tidal heating even keeps most of the icy moons liquid in their interiors, and the geothermal gradient of the gas giants’ interiors is enough to make the Earth’s look like a pipsqueak. So with all the elements needed and with energy gradients aplenty, one has to wonder if an ice giant would be a superior abode for life…
But aliens aside, the depths of Uranus and Neptune seem oddly…hospitable? Acclimate to the pressure and all you’d need would be a breathing mask to provide oxygen. And of course without a solid surface you’d need a floating habitat of some sort (Cloud City would be ideal but even a simple zeppelin could do in a pinch). But with those two components? You’re golden in what is perhaps the most Earth-like environment in the solar system. Yes, really.
Mars certainly harbors the most Earth-like surface, but broadening beyond surfaces, Venus’s cloudtops look attractive. Consider, though, that Venus at the habitable temperature and pressure range contains decks of clouds made of sulfuric acid. Far less human-friendly than the water clouds you’d encounter at depth in Uranus or Neptune.
Like Venus, Uranus and Neptune would offer only rarefied resources for a cloud city; certainly no ground exists to mine metals and so forth out of in quantity, but the atmosphere itself could provide for essentially all needs. Even better than Venus’s atmosphere. Venus offers carbon dioxide and nitrogen in abundance, which can be used to craft carbon fiber and various carbon and nitrogen based polymers; excellent building material. Uranus and Neptune, amusingly enough, offer the same portfolio of chemical elements to harvest out of the atmosphere and build out of as Venus: methane offers copious carbon, and ammonia offers nitrogen in quantity. Oxygen comes courtesy of water. Notice that unlike Venus, Uranus and Neptune’s CNO-bearing compounds are all hydrogenated; meaning that unlike Venus, which is extremely depleted in hydrogen (the most substantial remaining source is the sulfuric acid in the clouds), hydrogen is everywhere inside these giants.
All the advantages of Venus, and then some; no clouds of corrosive sulfuric acid, either — just compounds that humans on Earth would find familiar in the household or the terrestrial environment. Even the temperature regime is much friendlier, in as much as it’s energetically much easier to heat yourself and your environment up a bit than to cool down below the ambient temperature. Pressure? Not as friendly, but still viable for human breathing with supplemental oxygen.
Indeed, imagine the meteorology in these depths; several hundred miles below the highest cloudtops you would be in between cloud layers, most likely, so you wouldn’t see much, if any, open sky, if you did, per chance, the sun would still shine distinctly, but it would be a much more diffuse form of light. The sky even at 100 bars of pressure would still be bright, easily enough to see by, and the precipitation…imagine snowflakes identical to what we have on Earth, only falling in an ocean of sky, never reaching any ground…though blowing and drifting might be a more accurate description. These gas giants are famously windy, and the thick air down there would ensure that snow would fall only slowly, with many twists, turns, and eddies as gravity works its will. The net effect would probably feel remarkably similar to being inside a snow globe that was shaken up. Quite eerie.
It’s even possible you could see good old-fashioned thunderstorms down there, considering there’s more than enough of an energy gradient to drive convection; cumulonimbus clouds that would strike humans as familiar from Earth could be abundant down there, even in the water cloud layer. And yes, this would even include thundersnow.
About the only truly hostile aspect of life there would be, as on Venus, the need for oxygen masks, since the environment provides no free oxygen. Certainly not enough to breathe.
But consider that at 100 bars of pressure, you don’t need much oxygen by fraction for the mixture to be breathable. 0.2% of the total amounts to 0.2 bars of partial pressure, similar to Earth sea level! Even this much is not provided by the ambient environment of our ice giants, but consider oxygen is abundant in the atmosphere of these bodies; an ice giant in another solar system might see some of its oxygen stores liberated. Perhaps by local life that, as on Earth, oxygenates the atmosphere so severely that even the reducing effects of abundant hydrogen can’t sequester it fully before some fraction of it lingers in the air.
Or perhaps by cosmic radiation splitting water in the upper atmosphere into hydrogen and oxygen; such a planet around a bluer, brighter star than our sun may well undergo this process, and this would be particularly likely to occur in the more radiation-rich galactic core…which is also enriched with heavy elements that provide radiogenic decay, powering even more internal heating than we see in our own solar system (and meaning the water-cloud layer rises to altitudes with less pressure…indeed Neptune is already generating more heat than it receives from the sun, so similar mechanisms could easily take hold, producing shockingly habitable conditions deep down even in the total absence of sunlight, i.e. on a rogue planet in interstellar space).
It wouldn’t take much; 0.2% is hardly enough to breathe at 1 bar of total pressure, but at 100 bars? 0.2% would be enough partial pressure to breathe. And the rest of the atmosphere of a gas giant is hydrogen and helium, exactly the two gases the most friendly to high-pressure human breathing. 0.2% certainly feels plausible as the sort of persistent free oxygen fraction that even radiation-driven processes could produce, let alone local life that tips the atmosphere into disequilibrium. So natural hydreliox at high pressure in a layer of familiar rains and snows could be a common environment found throughout the universe, especially in higher-radiation regions. Heck, for all we know nearby A-type stars like Sirius, Vega, or Altair might host such bodies right now: utterly alien in their physics, yet totally habitable for man, if we only had the courage to go…
Or even to look. Ironically such environments, where the physics are utterly un-Earth-like yet the chemistry is familiar, and would plausibly combine on an actual planet, are familiar from classic science fiction and especially space opera. But after the fixation on the surface liquid water habitable zone took hold after the 1960s, such possibilities were quietly forgotten, both by actual scientists and by science-fiction authors. Mercifully, due to recent discoveries in the outer solar system as well as more knowledge of “extremophile” microbes on Earth, this is starting to ebb in recent years, but consider the approach here: look for the elemental building blocks, look for an energy gradient, and you just might find a hospitable environment for life, and quite possibly even for humans.
Not every world where you can walk out, take a deep breath, and taste the snowflakes is going to look like Earth. In fact, perhaps most won’t…there might be more Earth-like environments under the cloudtops of these ice giants than there are other Earths. Mind-bending, yet that’s what the physics and the chemistry almost suggests…