Although a world’s seas may be where its life is born, a world’s air also holds great meaning for mankind. In this post we will branch off from exploring exotic oceans to exploring exotic atmospheres, especially oxygen-rich ones on worlds harboring exotic oceans. Worldbuilders, for all the attention they pay to oceans, also pay attention to the atmospheres of planets, and for very good reason: man, after all, breathes air rather than water, and life on a planet’s landmasses won’t be very Earth-like without Earth-like air. For this reason fantasy worldbuilders, in particular, tend not to stray very far from the “78% nitrogen, 21% oxygen, 1 atmosphere of pressure” template set by Earth, since that is what is needed to support the various near-human races that tend to be prevalent in such settings. Science fiction planets also tend to hug the near-Earth range in order to support near-human and humanoid species (not to mention human colonists, who play a huge role in many science fiction stories), but since science-fictional settings (especially space opera) often feature many planets there is an opportunity, usually taken, to balance out all the Earth twins with more exotic planets.
Often these exotic planets are completely uninhabitable, or the site of truly alien biochemistry: any of the exotic oceans beyond water from my previous post on the subject would fit this category if they hosted life. There is a third category of planets, however, that has been getting more attention recently, likely stimulated by exoplanetary discoveries: planets that are more exotic than an Earth twin but retain a water ocean and oxygen atmosphere. In my post on exotic oceans I touched on two examples of this: subsurface oceans on ice planets and supercritical water oceans on ocean planets. These are examples of exotic water oceans. Another type of ocean that is somewhat less alien is an ocean so deep that it covers the entire surface of the planet. This is a common class of planet seen in science fiction, perhaps the best-known example being Kamino from Star Wars.
If the oceans were shallow enough to keep the water at the bottom from being compressed into ice, the process of minerals mixing up from the core would be more or less identical to Earth’s, thus providing no obstacle to Earth-like life arising. Ice at the bottom makes it considerably more alien and obstructs minerals from mixing up, though they may be able to transfer through the ice anyway if the ice caps on planets in our own solar system are any indication. 39 miles is the maximum depth that water remains liquid at temperatures around freezing. Any planet with ice caps will have near-freezing deep water since the coldest water sinks; all of Earth’s deep water is around freezing, for instance, so only a planet substantially hotter will have warmer deep water. Anyway, as long as the ocean is less than 39 miles deep ice won’t be a problem for a planet with Earth-like climate; for comparison Earth’s deepest point, the Marianas Trench, is under 7 miles deep, so this is quite deep indeed. Employing hotter temperatures enables water to remain liquid up to around 100 000 atmospheres of pressure at around 650 degrees Fahrenheit temperature, implying a depth of at least 600 miles of liquid. That’s really deep, and still (though just barely) short of the supercritical range, enabling the ocean to still have a distinct surface. Of course the atmosphere would have to have high pressure to prevent the water from boiling at such temperatures.
So even with Earth-like climate suitable for human colonists oceans can easily be much deeper than Earth’s without issue even for indigenous life; hothouse ocean worlds may have oceans so deep they make Earth’s look like a puddle. Of course, this post is about atmospheres, not oceans, so what sort of atmosphere would such a planet have?
From Ocean World to Oxygen World
For water-rich worlds water vapor would be an important constituent of any early atmosphere, and water just so happens to be a potent greenhouse gas. Thus at least in their early histories these planets may be quite hot, for closer-in planets perhaps leading to a runaway greenhouse effect similar to what is thought to have happened to Venus. This fascinating study from 2015 posits that water worlds in inner-solar-system environments may commonly enter such a phase in their evolution, with the heat causing extreme water loss, equal to several Earth oceans’ worth. This is indeed what almost certainly happened to Venus, but water worlds with their ultra-deep oceans have a lot more water to spare. This water as it is lifted up into the atmosphere would undergo photolysis, breaking down into hydrogen and oxygen, the hydrogen escaping into space and the oxygen being retained by the planet’s gravity. This would cause a conversion of most of the atmospheric water vapor and former ocean mass into atmospheric free oxygen, possibly adding up to several thousand bars of pure oxygen. All without any life having to be involved in its creation.
The 2015 study brings this up in an astrobiological context, pointing out that these oxygen atmospheres could easily be false positives for the presence of life. The study also points out that the photolysis process could lead to complete desiccation of a water world, ending up with a desert planet with a near-100% oxygen atmosphere under up to several thousand Earth atmospheres of pressure. Such an environment is rarely (if ever) explored in science fiction, yet in real life it may be a more common type of planet than anyone suspected. It would be a fascinating place for the development of indigenous life and/or for a human colony. The surface would contain minerals and plenty of breathable air (merely needing to be depressurized), and the climate could easily be rather pleasant under the right circumstances.
It is worth noting, however, that above 50 atmospheres of pressure (and a temperature of -180 Fahrenheit, easily met on any Earth-like world) oxygen becomes a supercritical fluid, with no boundary between liquid and gaseous phases. Several thousand atmospheres of oxygen will be more like being underwater than anything we would call air, arguably turning into a non-water exotic ocean. I cannot find any information in a cursory search on how suitable supercritical oxygen would be as a solvent in alternative biochemistries; even for conventional biochemistries, such as microbes subsisting off of atmospheric water vapor after the desiccation of its ancestral ocean (as is suspected to have occurred on Venus) it would be a fascinating atmosphere for life to evolve in.
Of course, aside from complete desiccation a partial desiccation is completely plausible, with temperatures decreasing along with the amount of water vapor converted into oxygen, eventually reaching an equilibrium where the boil-off stops before the ocean disappears. This would be rather convenient for planets that have so much water hot ice forms between the ocean bottom and the underlying rock; the desiccation process may remove enough water to permit mixing of minerals from the core but leave enough for it to be a deep ocean, now with a thick 98% or more oxygen atmosphere the planet didn’t have earlier. Now there is an environment much more interesting than your average ocean planet.
Breath of Life
For any native life forms that arise, making the leap to aerobic respiration, as Earth life did, would have a far larger impact than it did on Earth. In the Paleozoic era on Earth higher oxygen levels are thought to have supported much larger creatures than currently exist, particularly insect life. This was when the partial pressure of oxygen rose from 0.2 to 0.3 atmospheres or so, which is nothing compared to the dozens, hundreds, or even thousands of atmospheres on these worlds. This is because the energy available per volume of breath is that much greater. Perhaps the best baseline for how much bigger creatures would get is Meganeura (Paleozoic dragonflies) versus modern dragonflies; modern wingspans range up to 7 inches relative to 30 inches for their ancient counterparts. This implies wingspans could grow very large on our “oxygen worlds”. Of course due to the square-cube law a linear extrapolation cannot be made. Nevertheless it is reasonable to suppose that, for example, 50 atmospheres of oxygen could support, all other things being equal, creatures 250 times heavier (50 divided by Earth’s 0.2 atmospheres of oxygen (the other 0.8 being nitrogen)). This implies 6 times greater length, width, and depth for any given creature.
Similarly, 100 atmospheres (500 times Earth) implies 8 times greater dimensions, and 1000 atmospheres implies 17 times greater dimensions. In such exotic atmospheres, though, where the density is more similar to water, it is questionable whether the native creatures would resemble air-breathing creatures on Earth rather than their water-borne counterparts. A few dozen atmospheres worth of oxygen would I believe be the optimum for giving animals, especially flying ones, more pep without making the environment too similar to being underwater. The flying creatures of such a world would, even the low-altitude ones, still be more similar to birds than, say, squid.
From a worldbuilding perspective, keeping the pressure increase relatively modest improves the diversity of the setting, since these oxygen planets with global water oceans would still have plenty of life resembling Earth’s aquatic life in the actual water. However, for oxygen worlds that are completely desiccated and don’t have oceans the low altitudes of an atmosphere as thick as water would provide an intriguing substitute for an ocean, at least as far as the morphology of the life goes. This assumes that the Earth-like life in such settings can obtain sufficient water to be able to evolve a vibrant ecosystem. If not, then such worlds would be rather barren, unless life were introduced from elsewhere and evolved or was otherwise genetically engineered; this may make an interesting plot for human colonization in a science fiction story, either with the humans doing the introducing or a precursor species having done the same in the past.
One thing that would be seen in any animal life in such an atmosphere is flight; with only several Earth atmospheres worth of pressure much more buoyancy and lift can be attained, thus reducing the cost of powered flight. Under dozens of atmospheres of pressure we can expect flying animals to be completely dominant outside the water, even on desert or mixed land-ocean oxygen worlds. On oxygen worlds with global oceans, of course the only animals will be aquatic and airborne, as there is no land, excepting the ocean planets cold enough to have ice caps (which would function as land). Biological lighter-than-air flight, or gasbag lifeforms, would be more plausible in this sort of environment than any other, since the volume of gas required to achieve sufficient buoyancy is minimized. Plants, in particular, may employ this approach, as it requires less energy than powered flight. This may lead to floating structures in the lower atmosphere resembling coral reefs; if trees evolve that grow on top of these air reefs they would provide a forest or land environment on an ocean planet. This in my view is hard science fiction’s closest equivalent to the floating islands in the sky commonly seen in fantasy settings.
Another common trope, or at least one that is asked about often in worldbuilding forums, is the airborne animal that never lands. The alpine swift is the closest animal to this on Earth, being able to stay airborne for up to seven months at a time, only truly needing to land to reproduce, which it does by laying eggs on mountains. An ocean planet’s equivalent to the alpine swift, especially one that has much more abundant airborne food sources, would probably never land. Egg-laying in water, an inherently less secure place than a steep mountain on Earth, would likely be replaced by live birth on the wing over time, obviating the need to land.
In the ocean, the advent of air breathing in an exotic oxygen atmosphere would give air-breathing aquatic animals vastly more energy than water-breathing animals, thus air-breathers would dominate such a planet’s oceans to a far greater extent than they do on Earth.
This would combine to create a fascinating ecosystem on a fascinating planet, especially the oceanic version of an oxygen world (which in my view is much more interesting than the desert version), but there are other things a worldbuilder needs to consider. In particular, what are the prospects for human colonization of such a world?
Cities in the Clouds
The pressure on the surface, whether ocean or desert, would be far too high for human habitation, at least by the “shirt-sleeve environment” standard. Oxygen toxicity alone would be lethal, so surface habitation would require pressure suits similar to those used by underwater divers on Earth. Recreational divers don’t normally go beyond 130 feet (5 atmospheres of pressure), and technical divers don’t usually go beyond 350 feet (12 atmospheres), so in mild instances of these sort of exotic atmospheres regular SCUBA-type equipment could be used. The world record for SCUBA diving is around 1000 feet down, which is 31 atmospheres, so perhaps SCUBA-style equipment could be extended further into more typical instances of these oxygen worlds. Atmospheric diving suits are the ocean’s answer to spacesuits, and existing suits can be used down to 2000 feet quite easily, which is 62 atmospheres worth of pressure. For a research outpost or some such any oxygen world with 60 atmospheres or less of pressure would present no obstacle to permanent habitation of the surface whatsoever even using 2020-vintage off-the-shelf technology. Further development could easily extend this pressure range much higher.
Research bases are likely to appear in any colonization effort of such a world, but the surface is far from the most likely place for a real colony to be sited. This is because atmospheric pressure decreases gradually with increasing altitude; thus for any planet with high atmospheric pressure there is a region higher up in the atmosphere with around one Earth atmosphere of pressure. In the case of Venus this is at an altitude of around 55 kilometers, which conveniently also has Earth-like temperatures, to the extent it has been described as the most Earth-like environment in our solar system outside of Earth, and a promising site for future colonization. Jupiter, Saturn, Uranus, and Neptune also have such regions, though at very cold temperatures; also conveniently, all these planets except Jupiter have Earth-like surface gravity. That’s a total of four planets in our own solar system where a spacesuit isn’t required, only a breathing mask and (aside from Venus) warm clothing.
Similarly to these planets, oxygen planets with thick atmospheres will have a region of Earth-like pressure, only in their case the atmosphere will also be breathable! For a planet with 50 atmospheres of surface pressure and a pressure gradient like Earth’s the region of the most breathable air would be around 20 miles up, where there is 0.2 atmospheres worth of pressure (with a full oxygen atmosphere this is the same partial pressure as Earth). Atmospheres with higher pressure will have this region at higher altitude, and vice versa. This region will in any colonization effort host floating habitats (in larger instances these may be true “cloud cities”) where people can live in the same open-air shirt-sleeve conditions they are accustomed to on Earth. Local life would extend up to these pressure ranges, though with the dimensions of the flying creatures shrunk from multiples of Earth size down to similar to Earth size due to the lower oxygen, excepting creatures visiting from down below where they took a deep breath. This region would undoubtedly host the vast majority of any human colonist population.
The sky these denizens of the clouds would be surrounded by in most directions would generally be blue, just like it is on Earth; even under a red dwarf sun the air at this altitude is so thin (0.2 atmospheres) that the blue color would still be rather Earth-like. Under an Earth-like sun it would be a vivid dark blue similar to what we see at high altitudes. On the surface, the pressure of this exotic atmosphere would create a more alien skyscape; thicker air causes a brighter sky, under these pressures being sufficient to color sky milk-white instead of blue. Sunlight would be redshifted so that an Earth-like sun would appear reddish even at high noon, and a deep crimson red at every sunset. Cooler suns would appear even redder. Green flashes would be redshifted to perhaps yellow flashes for Sun-like stars, grading to orange or even red flashes for red dwarf stars. Rainbows would be correspondingly redshifted as well, appearing much like they do well into sunsets on Earth, again even under a Sun-like star.
Twilight would be longer on the surface of such a world given the thicker air (sun reflecting more off the air at higher altitudes where it hasn’t set yet) and brighter sky. Airglow at night would also be brighter. The aurora on such a world would be dominated by oxygen, thus red at high altitudes and green at low altitudes would be the predominant color, with yellow being more common than on Earth. The thicker atmosphere may lead to stronger aurora.
One of the reasons I mention red dwarf suns so much here is that this class of planets, according to the 2015 study, is particularly likely to exist around red dwarf stars, because these stars are more likely to be very luminous early in their life cycle, thus making runaway greenhouse effects much more likely to occur early in a planet’s history than they are around larger stars. This introduces the likelihood that most of these sort of planets that are out there may be tidally locked to their parent star, with one side being in eternal daylight and another side in eternal night. The best current scientific thinking suggests, however, that with an atmosphere as thick as these worlds have heat will be distributed so efficiently that tidal locking won’t adversely affect habitability. It does, however, add another alien element to these worlds. Worldbuilders that don’t wish to deal with tidal locking for whatever reason may, of course, easily set their oxygen world in a system with a larger sun without affecting plausibility.
Yet another alien element that could be introduced, though stretching plausibility much more than anything heretofore discussed, is blending this concept with the uranium planet mentioned in my previous post on the subject. After all, there is no reason to believe a planet whose rocky core is highly enriched in heavy elements couldn’t easily acquire a thick envelope of water during its formation, and undergo the same evolution as the water worlds we talked about earlier. The radon-enriched water ocean mentioned in that post would apply here, perhaps causing the massive ocean to faintly glow with a blue to lilac color. If the water ocean were sufficiently enriched with dissolved uranium from the core, then like the waterless uranium worlds it would make an excellent source of fissile materials, and a very dynamic setting for all manner of plots and intrigue. There is even an outside chance local life could evolve to use the radon or uranium as a power source, though exploring that is a post for another time.
In this exploration we have seen that the possibilities for exotic atmospheres are quite diverse and fascinating even if we restrict ourselves to oxygen. Oxygen has the key advantage in any story involving humans or Earth-like alien life that it is breathable by humans and that having a thick atmosphere of oxygen guarantees that some layer of the atmosphere will be a shirt-sleeve environment for humans. All manner of aerial settings open up, including cloud cities, zeppelins, airplanes, hot air balloons, airborne reefs, floating forests, gasbag lifeforms, and giant flying creatures, even including deep-diving and high-pressure settings usually restricted to water, while still retaining great options for diving deep into actual water. As a setting for any speculative fiction story exotic oxygen atmospheres have a lot of potential, which I for one hope skilled writers will use in the future to enliven the genre and the imaginations of readers.