I know I’ve done a lot of elaborate worldbuilding for my “barely breathable” world orbiting Sirius B…but all of a sudden I’ve had an inspiration, which involves flipping the script to something less Martian and decidedly more spectacular…and alien.
I began with the basic concept of a liquid ammonia ocean; ammonia is a common volatile compound beyond the “frost line”, abundant everywhere in the outer solar system (ammonia ices are what give Jupiter its highest clouds, for instance, that we see in our telescopes). At Earth-like pressure, ammonia freezes at -108 degrees Fahrenheit and boils at -28 degrees Fahrenheit, a comparable liquid range to water, just shifted down the temperature scale. Naturally this has long stimulated interest in ammonia as a solvent in alien biochemistry.
But a straight ammonia ocean seems a bit boring, so I’ve pondered for a while: what if life got started in an ammonia ocean, but converted the compound to a “higher” nitrogen-carbon molecule? Large-scale modification of geochemistry by biology has been known to happen: Earth’s atmosphere was originally carbon-dioxide-rich, but was gradually converted by photosynthetic metabolism to the oxygen-rich atmosphere we know and love today. The same could happen to an ocean.
In my idea, the compound in question that biology produces from ammonia would be hydrazine; instead of ammonia’s NH3, hydrazine’s formulation is N2H4. Its formation from ammonia is not energetically favored in terms of the raw chemistry, but it could easily be part of a larger metabolism that releases hydrazine as a waste product, potentially to store later, as a fuel source. Hardly far-fetched: in real life hydrazine is widely used as a rocket fuel.
As its use as rocket fuel suggests, it’s not the most stable compound: it’s prone to ignition in an oxygen atmosphere. But in a reducing, rather than oxidizing, environment, it wouldn’t be a problem. The usual way to accomplish this would be a nitrogen- or carbon-dioxide-dominated atmosphere, but beyond the frost line, if we’re dealing with an outer-solar-system-type body, especially if it’s large in mass, we could suppose a hydrogen-dominated atmosphere, a la the gas giants, but with our planet still possessing a solid surface. Say 50 bars or so of hydrogen: enough to be thick, but with a recognizably terrestrial surface as opposed to the more exotic boundaries at work in the gas giants’ interiors.
One obstacle to converting an ocean of ammonia to hydrazine is that hydrazine’s melting point is all the way up at 35 degrees Fahrenheit, its boiling point 237 degrees Fahrenheit. Sound familiar? It should, because these numbers are almost identical to water! If we’re dealing with a planet cold enough to have a liquid ammonia ocean, then as it’s converted into hydrazine the ocean will freeze solid. Oops.
But there might be an escape hatch: ammonia, while abundant, tends to occur in the same cosmic environments where water abounds too, so realistically an “ammonia ocean” would be highly intermixed with our familiar water. Ammonia and water in combination form a “eutectic”, and can remain liquid at temperatures as low as -149 degrees Fahrenheit, the maximum freezing point depression being reached with a mixture of 1/3 ammonia with 2/3 water. For this reason, ammonia is widely used as an “antifreeze” domestically; and it’s precisely because of this antifreeze property that liquid or slushy oceans consisting of water-ammonia mixtures are suspected to be widespread across the outer solar system.
Hydrazine? It has similar properties as ammonia in this regard. The required mixture is somewhat different — you’re talking 45% hydrazine or so and 55% water — but hydrazine and water also form a “eutectic”, which can remain liquid in temperatures as cold as -112 degrees Fahrenheit. Thus, a water-ammonia ocean whose native life converted the ammonia inventory to hydrazine could remain liquid over geological time, given the proper conditions, while hydrazine is rather exotic chemically, is geophysically not fantastic, and would serve as a potent energy source when paired with any oxygen that might be liberated.
Which could be substantial. Water itself contains a large inventory of oxygen (the O in H2O…); biological processes could liberate it, but in a system like Sirius or frankly any ultraviolet-radiation-rich environment, you may well see regular liberation of oxygen from water. Free oxygen and free hydrogen is not a stable mixture: in a hydrogen-dominated atmosphere, any free oxygen will react with the ambient hydrogen to form water, but the truth is this process only occurs at a certain rate, which in a colder environment than Earth is going to be substantially slower than what we’re accustomed to. Consider also that the oxygen production rate is going to be higher than what we’re accustomed to, owing to the more UV-rich solar spectrum in the Sirius system.
With a high enough rate of oxygen being liberated, a trace fraction of free oxygen will persist in the atmosphere as it is produced and cycled through. The effect is that local life will have oxygen available to react with the hydrazine in the oceans, a potent source of energy for their metabolism. Fancy effects like periodic ignition of the air will also be prevalent, which sounds scary but Earth degasses flammable methane at a certain rate, and the resulting will-o’-the-wisp is widely viewed as striking and pretty rather than frightening.
The really fancy aspect of a small oxygen fraction is that at a high enough ambient pressure, the air could easily be human-breathable. At 50 bars, primarily hydrogen (realistically also helium, but whatever), even a free oxygen fraction of 0.2% yields a partial pressure of 0.1 bars, the same as Earth’s air at 18,000 feet in elevation. You can breathe that amount of oxygen forever if you need to.
50 bars is of course 50 times the pressure at Earth sea level, which sounds crushing, but hydrogen’s trick is it’s a very light gas. 50 bars of the stuff adds up to a density of 4 kilograms per cubic meter, compared to 1 kilogram per cubic meter for Earth air. It’d be heavy, but human lungs can adapt to moving that much mass. That much has been experimentally demonstrated in deep-diving experiments that used “hydreliox” mixtures: the high-pressure nervous syndrome (“helium tremors”) that is inevitable at such high pressure is actually offset by the weakly narcotic effects of hydrogen gas in the breathing mix, thus enabling natural breathing even at extreme pressure. The results suggest that 100 bars or even beyond might be achievable in deep diving.
Anyway, hydrogen has its peculiarities, but it’s fairly “normal” as far as atmospheric gases go: the stuff is transparent and the sky would, to the naked eye, appear more or less Earth-like. At 1 bar it would appear blue, the same as on our planet, though from the surface of a planet under 50 bars of atmosphere the sky would be much brighter owing to the denser gas, so it may well appear white, with objects like the sun having their spectrum reddened due to scattering, even though their shapes etc would still be quite clear even through the thick atmosphere (it’s heavy air, but hydrogen is very transparent).
With a mixture of hydrazine and water serving as the ocean, and 50 bars of hydrogen with a breathable trace of oxygen serving as atmosphere, what would the experience of standing on this planet be like? One would imagine that the familiar meteorology of rains and snows would occur, but with a hydrazine-water mixture as opposed to “straight” water…but you might be wrong. From a eutectic mixture of water and hydrazine, water is strongly favored to evaporate. Thus the moisture that does waft into the atmosphere would be overwhelmingly water…which would, at the low temperatures we’re discussing (-100 Fahrenheit or so), form ice crystals. Even in the warmer tropics, it would be far below the freezing point of straight water. This is a world where the oceans are predominately liquid but where any moisture in the atmosphere — the entire air column, worldwide — is snow and ice.
Any continents that the planetary geology makes rise up out of the sea would be rapidly subjected to heavy snowfall, and would glaciate. This is a world where it’s oceans and ice sheets…and nothing else, bar outcrops of bare rock the glacial ice flows around. In those areas you might see rocky, even gravel-y beaches backing out onto sheer rock faces, the ocean waves crashing ashore. If you were there, the air would feel distinctly heavy, the ocean might have a peculiar absence of smell or perhaps would faintly smell “clean”, but it might even pass for a scene from one of Earth’s more remote polar regions…until a snow squall comes in, and the 50 bars of atmosphere betrays itself by snowflakes that aggregate into immense masses and drift in the wind almost as if underwater, falling only very slowly, yet with every wind gust feeling like it’d knock you down off your feet, forceful, even if not fast.
If I go with this idea for my world orbiting Sirius B, my previous idea of Devils-Tower-like volcanic plugs dominating the geology is still workable, it just would all be covered by glacial ice that would flow rapidly down sheer cliffs, which of course would generate some extreme katabatic winds, a la Antarctica.
All the ice crystals dominating the atmosphere’s cloud cover and broader open sky means that halos and arcs would be extremely common. Sirius B and Sirius A would both usually be accompanied by halos and sundogs, as well as various arcs that could stretch all the way across the sky, similar to what’s seen in Earth’s most extreme polar regions today, only here it would be omnipresent, despite an ocean that would usually be open water.
Resurrecting a previous idea that I abandoned because my previous world concept had to be too close to Sirius B to make it work, in terms of the orbital dynamics, I’m envisioning that our planet here might be a planetary-mass moon of a gas giant world, orbiting closely, meaning that an orb of a planet would dominate the sky as seen from half of its surface. If we take inspiration from Uranus, and make the gas-giant primary exhibit an extreme axial tilt, i.e. it’s tilted on its side, i.e. it possesses high obliquity, then there’s a possibility of a truly extreme seasonal cycle, not to mention alien solar geometry: for half the year the sun would be above the horizon, similar to our world’s midnight sun but much more extreme…and then the other year the sun would be below the horizon, and it would be dark, bar the light received from Sirius A as well as the primary’s “planetshine”.
In order for all this to work, our planet would have to be located relatively far from Sirius B, and in a region of space where its equilibrium temperature would be fairly cold. But that’s okay: Sirius A approaches to within 8.2 AU of Sirius B, and Sirius A has 25 times our sun’s luminosity. Even without any contribution from Sirius B at all, with an albedo of 0.4 (modestly more reflective than Earth, owing to all the ice sheets), that periastron distance implies an equilibrium temperature of -111 degrees Fahrenheit. At apastron (maximum separation), Sirius A is 32 AU away, implying an equilibrium temperature of -285 degrees Fahrenheit. Frigid, but keep in mind at 1 AU Sirius B, despite having only 2.5% as much luminosity as our sun, is still putting out enough light to keep equilibrium temperatures at -282 Fahrenheit, and that’s without Sirius A’s contribution.
Keep in mind that hydrogen is a very weak greenhouse gas, so there’s not going to be all that strong of a greenhouse effect here; nevertheless, at 50 bars there’s enough various effects here from pressure alone to warm the planet’s surface pretty significantly. Also keep in mind as a moon there’s tidal flexing going on from libration alone, which heats the interior. And, as a young world, there’s a lot of geothermal heat emanating from its interior: radioisotope concentrations are sufficient to support the formation of natural nuclear fission reactors (yes, really; those are a thing), so there will be various hot spots under the sea surface where plumes of warmer water drive evaporation, perhaps serving as convection hot spots for the atmosphere above, even as sources for atmospheric rivers that transport moisture (remember: in the form of snowfall) far poleward as the seasons change.
A theme in worldbuilding my Sirius system is that there are so many heat sources you almost have a hard time keeping the planet cool enough to do anything really interesting!
At equilibrium temperature, when Sirius A is at its furthest remove, and assuming 1 AU of distance from Sirius B, we’re dealing with a total amount of illumination that’s 5% of Earth. That sounds dim but the suns would both still look very bright and the scene would very much be daylight when they were up above the horizon. In temperature terms the number my calculator spits out is -251 Fahrenheit. At closest approach, Sirius A shines at 39% as bright as the sun as seen from Earth, so the total luminosity rises to 42% or so. The number my calculator spits out is -105 Fahrenheit for equilibrium temperature.
So it can be done, you just need to keep the planet very far away from Sirius B; Sirius A approaches only to 8 AU, so an orbit around 1 AU from Sirius B, an eighth of Sirius A’s closest approach, should be stable over geological time. It all works. And at 1 AU from Sirius B, you’re not dealing with tidal locking or any of the usual effects from being close to a dim star (Earth is 1 AU from our sun, and we’re not tidally locked despite being a much older planet; we might be tidally locked if our sun was much more massive, but Sirius B is (oddly enough) roughly the same mass as our sun, it’s just dimmer (owing to it being a white dwarf; nuclear fusion has ceased, thus making it technically a stellar remnant rather than a star, per se)).
With regard to the temperatures, keep in mind I’m assuming an albedo of 0.4. If global temperatures are really -100 Fahrenheit or colder, then the colder half of the planet will easily freeze over, despite the eutectic nature of the ocean mix, thus perhaps making the surface more reflective. With an albedo of 0.6 (more reflective, comparable to some older glacial ices on Earth), that -104 Fahrenheit at high summer drops to -139. If we’re dealing with high obliquity, though, then different hemispheres of the planet will be continuously illuminated; in the bright hemisphere temperatures might be much milder than the global average would suggest, though with such a thick atmosphere the gradient might be mostly smoothed out via vigorous storm activity, rather than building up in the form of a huge ongoing temperature difference from pole to pole.
And those storms could be vigorous! If the ocean water is still warm enough to stay open, and if the continents are covered in ice sheets and are very high in altitude, then you’re dealing with ocean temperatures around -80 Celsius, when temperatures aloft coming from the nearby ice sheet could easily be -130 Celsius or colder, which is the same recipe as what produces “polar lows” on Earth, only far more explosive. On Earth they’re sometimes known as “arctic hurricanes”, but here these lows could blossom into true analogues of our major hurricanes, with Category 5 wind gusts and perfectly clear, calm, annular “eyes”, their central pressures being very low compared to the surrounding region of atmosphere. Convection could bloom from the ocean, and keep in mind hydrogen has a much larger “scale height” than our atmosphere’s gases do; especially combined with the higher pressure, clouds could easily grow 100 miles high above the surface before topping out, meaning these eyes, surrounded by intense eyewalls of snow and ice, thunder and lightning, could be far more intimidating than anything we see on Earth. Yet here it could be the signature weather system, aside from the broader “rivers” of atmospheric moisture that might interact with these smaller but more intense systems in complex fashion.
Even the seas themselves might have a certain apocalyptic quality to them: already the ocean is a eutectic mixture of hydrazine and water; the glaciers flowing into it are primarily water, rather than hydrazine, meaning that a complex density and freezing-point boundary at the mouths of the glaciers would be in play. Close to the glaciers the surface layer might be mostly water, which would freeze, but as it mixes with hydrazine further offshore it would melt, the deeper layers potentially being more hydrazine-rich, the fluid-dynamic behavior being very intense as the two layers mix.
We see similar processes at work on Earth at saltwater-freshwater interfaces, but here the gradients involved are much more extreme. You could even see analogues of Earth’s “haloclines”, seen in certain underwater caves, where two layers of fluid of different densities form a boundary, like a shimmering surface of liquid within the liquid itself. And like the more usual ocean-atmosphere boundary, waves can form in this liquid; they can intermix, upwell, downwell, and circulate. On a world like this with such dynamic, violent weather and a eutectic mixture of an ocean, I imagine that “internal waves” could become truly extreme in intensity compared to anything seen on Earth, and easily visible underwater as, in certain areas, a shimmering maelstrom of a surface would present itself well under the “actual” surface of the ocean. Utterly terrifying for human explorers, but potentially a rich environment for native forms of life.
As in my previous concept for “Cerberus”, my planet orbiting Sirius B, the age of the system suggests that the native forms of life would be simple…but that doesn’t mean they’d be microscopic. Sailor’s eyeballs, caulerpa, and other forms of algae in Earth’s seas grow large enough to hold in your hand, and yet they’re just one cell, full of many nuclei that interchange fluid across a mostly evacuated volume, sharing one membrane, which can be branching and complex in shape, to collect sunlight (as in the case of Earth algae) or even in principle to suspension-feed nutrients, chemicals for chemosynthesis, and various biological material from the ocean currents (as would be more likely the case on this planet).
One idea I had was even that the vigorous storms and thick atmosphere would favor selection for wind dispersal, as fragments of branching unicellular growths (that could nevertheless reach sizes that dwarf a man, especially in colonial form; think something more like a coral reef, but far simpler in anatomy) leave the ocean and are lofted by air currents across great distances. Over time, bubbles of ambient hydrogen inside the envelope, especially if heated, including via radioactive elements that would still be abundant and energetic on a planet like this, could power some peppy buoyancy when time comes to reproduce.
These seeds could float through the air through these snowstorms for weeks, months, years, spherical volumes resembling balloons but unmistakably biological in origin on close examination, potentially being quite abundant in the skies of this planet, yet they would not be metabolically active nor even achieve true powered flight. Heck, they wouldn’t even have any senses beyond the basics of sensing ocean water, the appropriate nutrients being present, and anchoring (via a holdfast, a la some cnidaria in Earth’s oceans) into the seabed once they splash down. But between these forms floating in the air, the mature stage where they’re branching like some cnidarian counterpart of a forest all around the seabed, and the branches that wash up on the beach or come back down to earth rather than splashing down into the sea, the world would be unmistakably living, despite having no macroscopic complex life in the form of animals and the like.
The life in question could even be naturally “glowy”, bioluminescent, not as a complex or derived signaling mechanism, but rather as a simple byproduct of its metabolism, light being produced as a waste product (which all known life does to some extent; the human body itself generates “ultraweak photon emission” as a result of metabolism…it’s been speculated that primitive life-forms might have been much less “efficient” in this regard and might have been much more “glowy”, for want of a better term). Pigmentation could vary or, as in Earth plants’ signature green, it could be primarily one distinct color. Red would be very distinct and very classic sci-fi (going back at least to the Martian “red weed” of H.G. Wells’s imagination), but really with an architecture like this almost any coloration would be plausible. Consider that hydrazine in quantity is toxic to known Earth life, so although the basic architecture of carbon-based chemistry in water would be viable, the details of the metabolism and even the architecture of the native life here would be very different from the life we know from Earth.
Nevertheless, the hard science checks out on every point here: a world where you could take off your spacesuit, and even take off your breathing mask and take the ambient air into your lungs on the beach, your ears hearing the full power of a wave crashing ashore, a rumble of roaring thunder, your face feeling the soft brush of falling snow…perhaps the world nearest to Earth in my fictional universe where you can stand on solid ground and breathe, but still not remotely Earth-like in almost any other way. Totally on-brand for my science-fiction-cum-space-opera setting, and a direction I think I’ll find far more inspiring than my previous concept for what lies in the realm of the brightest star in our night sky…