Oceans often come to the forefront when imagining the worlds that may be out there in the wider universe, whether as an exercise in science, speculation, or creating works of fiction. In these exercises liquid water oceans predominate, because our own planet’s surface is dominated by them, and all known life uses water; thus in the search for another Earth, which would arguably be the greatest discovery of all time, liquid water is an essential condition. Worldbuilding for fiction usually involves Earth-like life living on Earth-like planets, thus also necessitating liquid water.
Liquid water, however, is not the only substance that forms oceans (a “thalassogen”). In our own solar system Titan, a planetary-mass moon of Saturn, has hydrocarbon seas comparable in size to the Great Lakes. Even if alternative biochemistries to the water-carbon paradigm of Earth life prove to be incapable of sustaining life, planets with what we might call “exotic oceans” make an intriguing setting for scientific speculation or, much more fruitfully, worldbuilding. In this post I aim to present possibilities for oceans that are often overlooked by worldbuilders.
Exotic Water Oceans
One of the possibilities closest to home is to have a liquid water ocean but discard the surface part, instead locating the liquid water ocean underground, usually under an ice cap, kept liquid by heat from the core of the planet and gravitational tides. This is obviously inspired by real-life scientific findings of moons in the outer solar system harboring oceans. Europa was in the late 20th century considered the strongest candidate, but in the early 21st century Enceladus stole its thunder. The wildest discovery yet is that Pluto very likely has a subsurface water ocean. There are less definite indicators that subsurface water oceans may be quite common in the planetary-mass moons in the outer solar system, and possibly in the Kuiper Belt and Oort cloud too. If many (if not most) of these worlds have underground oceans, it would mean that all but a tiny fraction of our solar system’s ocean volume is in the outer as opposed to the inner system, a possibility that to many would have seemed crazy only a few decades ago.
If minerals mix upward from the core of these worlds into the oceans and hydrothermal vent activity takes place at the ocean floor, then such worlds would be good candidates for life to develop. Such life would be similar to that found in hydrothermal vents and deep-sea environments on Earth, which is the very environment where many scientists believe Earth’s life originated.
As the ToughSF blog points out in the truly excellent post over there about colonizing the Uranus system, any moon that has liquid oceans reaching down to the core would have very easy access to heavy minerals compared to anywhere else in an outer-solar-system environment, thus becoming a strong candidate for colonization. There would be boreholes melted down through the ice to enable ingress and egress to the underground ocean, submarines and buoyant transports for personnel, equipment, and raw materials, chambers carved out from the ice next to the columns for habitation and other needs (such as repairing vehicles), mining operations with big submersible vehicles on the ocean floor, and a launch apparatus of some sort on the surface. The entire setup would make an excellent science fiction setting, striking me as something like a more exotic version of The Abyss, yet to my knowledge such a setting is hardly ever used.
Even better, this setting can be spiced up significantly. ToughSF blog suggests advances in spaceflight may render imports from the inner solar system cheaper than digging new tunnels in the Uranian moons, thus leading these rather large-scale operations to be abandoned. Abandonment, of course, opens up a lot of space for telling interesting stories, perhaps especially if it is only partially abandoned. The presence of life, especially macroscopic plant and animal life, in the deep sea would be a fascinating addition to such a setting, opening up a compelling motive for scientific researchers and their facilities to move in. Over time this could turn into a full-fledged research center, which could even evolve into a university of some sort over time. The presence of sea-monster-like animals perhaps resembling Earth’s giant squid is an even cooler possibility, as is the presence of intelligent life. These possibilities, again, are hardly ever used as far as I am aware.
As rarely used as subsurface oceans on ice planets are as settings, at least this setting does occasionally show up to a greater extent than almost any other exotic water ocean at this point in time. One possibility that seems to be showing up more often recently, driven mostly by recent scientific discoveries (in this case outside the solar system), is retaining the surface liquid water ocean but bringing in an exotic pressure or temperature. “Supercritical water” is the most exotic variant of this sort of setting, and it sets up a gradient on a planet where the gas in the atmosphere smoothly transitions to a denser and more liquid form without a definable surface. This is because the water is past its critical point, 705 degrees Fahrenheit under 217 atmospheres of pressure. This obviously is very different from the conditions we see on Earth, and the pressure is the reason the water can stay liquid at such great temperatures.
Oceans Beyond Water
We think of water’s liquid range as between 32 and 212 degrees Fahrenheit, and this is true at one atmosphere of pressure. Under ten atmospheres, however, 32 to 356 degrees is the liquid range; under fifty atmospheres it expands to 32 to 509 degrees, and so forth. Greater atmospheric pressure helps to keep a variety of substances liquid across a wide range of temperatures. Carbon dioxide, for instance, cannot be liquid at one atmosphere of pressure (hence why it’s used as “dry ice”), but past about 5 atmospheres of pressure at around -68 degrees Fahrenheit it becomes liquid. Under 50 atmospheres of pressure, carbon dioxide boils at around 60 degrees Fahrenheit and freezes at -69 degrees Fahrenheit, a liquid range about as wide as water at one atmosphere of pressure. Carbon dioxide is cosmically abundant, particularly in inner-solar-system environments, and would make an excellent thalassogen, so much so that Venus (famously thick with carbon dioxide in its atmosphere) is speculated to have had oceans of supercritical carbon dioxide on its surface at some point in the past.
Other cosmically abundant substances include ammonia, which at one atmosphere of pressure has a liquid range between -107 and -28 degrees Fahrenheit, temperatures commonly found in outer-solar-system-type environments. At around ten atmospheres of pressure, however, the liquid range extends up to room temperature, and at 100 atmospheres up to around 200 degrees Fahrenheit! Ammonia oceans might be quite common in the universe, though one obstacle may be that in the icy planets it’s usually abundant in water is abundant also; for this reason it may be more realistic to have it mix in with water oceans acting as an antifreeze keeping water liquid at cold temperatures, as is speculated to be the case in many of the oceans of the outer solar system.
Of course, “outer” is somewhat relative; further out than a Jupiter-like distance both water and ammonia are, at least on planetary surfaces, safely frozen and have the consistency of rock. Oxygen is cosmically abundant, and has a liquid range of -297 to -360 degrees Fahrenheit under one atmosphere of pressure, which is rather wide. Under 5 atmospheres of pressure the boiling point rises to -263 degrees Fahrenheit. Methane, another cold thalassogen that is cosmically abundant, is liquid between -296 and -258 degrees Fahrenheit under one atmosphere of pressure; under 5 atmospheres the boiling point creeps up to -216 degrees Fahrenheit, yielding a water-like liquid range. In these cryogenic regimes, nitrogen may also serve as a thalassogen, being liquid between -346 and -320 degrees under one atmosphere of pressure. This is a quite narrow range, but again at a mere 5 atmospheres of pressure it expands upward to -290 degrees Fahrenheit.
In even colder environments, oceans of hydrogen may exist, hydrogen being liquid from -434 to -423 degrees Fahrenheit at one atmosphere, expanding up to -411 degrees under 5 atmospheres. Helium is liquid in still lower temperatures, the coldest of all chemical elements: under -452 degrees Fahrenheit and extending down to absolute zero (-459 degrees Fahrenheit) at one atmosphere of pressure. Below around -456 degrees Fahrenheit, it becomes a superfluid, a phase of matter notable for having zero viscosity, crawling up and over containers, flowing through pores, and finding its own level. That would make for a really exotic ocean.
Hydrogen and helium are found in supercritical phase in Jupiter and Saturn, and presumably other gas giants of their class too, achieving liquid-like densities deep within their atmospheres. This is a slightly more explored setting, usually in the context of gas giant life, than some of the other possibilities mentioned.
Looking toward the opposite end of the spectrum, hot as opposed to cold worlds, we find sulfur, an abundant element in inner-system environments, has a liquid range between 235 and 832 degrees Fahrenheit, rather large, together implying liquid sulfur oceans may be commonplace. Further up, we have by far the most common hot thalassogen seen in science fiction: lava, or liquid rock. Rocks of course have various melting and boiling points, but liquid rock on Earth usually ranges between 1100 and 2200 degrees Fahrenheit. The boiling point of quartz, one of the most common minerals, is 4046 degrees Fahrenheit, so any lava ocean would be below that range. Some minerals, of course, are much tougher, but lava oceans seem unlikely to exist beyond that mid-four-digit temperature range. Molten rock and associated substances represents the hottest thalassogen that I know of.
All of these thalassogens as far as I know would appear similar to water: clear with a bluish tint if you look through a large volume, with a few exceptions. Lava, obviously, looks nothing like water, and liquid sulfur is particularly interesting, grading from a pale-yellow fluid at low temperatures to a dark red viscous fluid at higher temperatures to a less-viscous black fluid at still higher temperatures. Much of the inspiration for thalassogens, including the word itself, come from this section of Robert Freitas’s Xenology. The most exotic thalassogen of all that I will include in this post, and one that I have virtually never seen even mentioned, I have saved for last.
The Most Exotic Ocean of All: Radon
Worldbuilders, for all their imagination, seem to give very little consideration to the possibilities of radioactivity, radioactive decay, and nuclear reactions in general. One user on the worldbuilding subreddit, however, brought up the intriguing possibility of a radon planet a few years ago. Radon, as many of us know, is a radioactive element predominately produced through the decay of uranium, and aside from this process occurs in nature only in minute quantities. Thus it is very unlikely that radon could dominate a planet’s or atmosphere’s composition in a realistic scenario. However, a planet that formed in a region of space highly enriched in heavy elements, perhaps through local supernova activity, could have a high quantity of uranium in its crust, far higher than our own planet. This uranium would naturally produce a very large quantity of radon as a decay product, enough for it to be a significant portion of the atmosphere, though the atmosphere of any good-size terrestrial planet would likely be dominated by nitrogen or carbon dioxide.
Where this gets really interesting is that, although radon is often thought of as just a gas (as that is its phase at standard temperature and pressure), it is actually liquid from -100 to -79 degrees Fahrenheit under one atmosphere of pressure. This is a narrow range, but increasing the pressure should widen it (unfortunately I don’t have a source readily available to give exact numbers). Liquid radon, otherwise clear and colorless, glows because of the energy from radioactivity; the color of the glow has been described as blue-green, blue, or lilac. A whole ocean of the stuff at night would be an incredible sight, and wouldn’t look out of place in any science-fiction or fantasy setting (the glowing blue liquid “energy” in 1982’s Tron, for example, is similar in appearance). Such glows in speculative fiction settings are typically achieved by bioluminescence, but radon presents another possibility that I have never seen explored.
Even better, solid radon glows yellow at higher temperatures and orange-red at lower temperatures, so the snow and ice produced by the radon equivalent of the water cycle would also have a fantastic quality to it. There is also, given the radioactivity, the theoretical possibility of it being used as an energy source of some sort, either by colonists, or more speculatively, by indigenous life. With enough uranium in the crust, there would be enough new radon produced to overwhelm the high decay rate, leading to an equilibrium of radon oceans, assuming the planet’s surface was at the correct (rather cold) temperature. For a more or less pure liquid radon ocean other thalassogens such as water or ammonia would need to be absent, otherwise the radon would dissolve into the water. We may suppose the planet is very poor in ices, perhaps due to having formed close in to its sun before migrating outward, or accept the radon as just a substantial component of a water and/or ammonia ocean. This may make it easier for indigenous life to arise, though it would tamp down the exotic factor for colonists (for instance, the radon snow would be diluted by water or ammonia snow).
Whatever other challenges they may face, colonists on these “uranium planets” would have an abundance of fissile materials and thus nuclear energy sources, the export market for which would likely be the reason the place was colonized to begin with, excepting purely scientific ventures. Such a planet could easily acquire great strategic and economic importance, with immense possibilities for intrigue and plots long after its original discovery. Between this and the cooler and more fantastic characteristics, it would make an excellent setting for a science fiction or even fantasy universe, and this makes radon not only the most exotic thalassogen explored in this post but also the most original and interesting for worldbuilding.