Weird Worlds: Oxio

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Oxio is a small world, slightly larger than Mercury, orbiting a super-Jovian planet. Despite its small size, which leads to rapid loss of internal heat, tidal heating of the upper mantle provides energy to support continuing tectonic and volcanic activity. Due to its small size, Oxio is unable to retain water, and is only marginally large enough to retain gaseous nitrogen. Combined with extensive volcanic activity, this has led to the loss of more common volatiles and concentration of heavier sulfur compounds on the surface, much like Io in our own solar system. The atmosphere is composed primarily of sulfur dioxide, with trace amounts of nitrogen, carbon dioxide, phosgene (carbonyl chloride), carbonyl fluoride, argon, and xenon. The average atmospheric pressure is approximately 1.5 bars, but this varies considerably with temperature as sulfur dioxide evaporates or rains out. Surface temperatures average just under 0 Celsius.

Sulfur dioxide also forms salty oceans on Oxio, and acts as the biosolvent. SO2 is a polar solvent, like water, ammonia, and sulfuric acid, but it is aprotic, and does not support electron solvation like ammonia. Oxionic life instead produces charge gradients via a combination of intramolecular electron conduction, as occurs on Blue Crystal, pumping of sodium, chloride, and fluoride ions. The salt content of the oceans, however, is critical to Oxionic life for more than just supplying electrolytes for energy transfer and signaling. Secondly, salts improve the solvent properties of SO2, forming associations with many different macromolecules to improve their solubility. Thirdly, while small quantities of hydrogen are biovailable in the form of dissolved hydrochloric, hydrofluoric, and sulfuric acid, it is a relatively rare trace nutrient, with carbon-chlorine bonds, carbon-fluorine bonds, and polar nitrile groups replacing most of the functions played by hydrogen and hydroxide groups in water and ammonia-based chemistries. While some autotrophic organisms rely entirely on capturing atmospheric carbonyl halides to construct halogenated organics, several classes of microbes retain ancient chlorinase and fluorinase enzymes which convert halogen cations and organic anions into halogenated organics, forming new carbon-halogen bonds.

Photosynthesis is oxygenic, with oxygen sourced from carbon dioxide, carbonyl halides, and sulfur dioxide. Oxygen is not released as gas into the atmosphere, however; some freed oxygen is re-used to form sulfate ions, but the majority is converted into solid sulfur trioxide. Single-celled autotrophs generally eject the resulting crystals, contributing to the formation of sulfur trioxide sands, but complex multicellular autotrophs simply store the crystals as they grow, partially re-using them as a stronger source of oxidative power than the liquid sulfur dioxide.

Since sulfur dioxide is itself an oxidizer, most single-celled organisms simply use their own biosolvent directly as an oxidizer for aerobic respiration, producing carbon dioxide, carbonyl halides, small quantities of water, and elemental sulfur as waste products. Eventually, excreted sulfur will react with trioxide sands to regenerate new sulfur dioxide, but of course there are specialized chemosynthetic organisms which acquire energy by catalyzing this process.

The ubiquity of oxidative power in the Oxionic biosphere (wherever there is liquid to support life, there is also oxidizer) means that anaerobic respiration and fermentation are almost entirely unknown on Oxio. Additionally, there is very little pressure to use alternative oxidizers, like phosphates or nitrates. Even on Earth, phosphate reduction is an exceptionally rare metabolic strategy; phosphate is a rare but critical nutrient, being necessary for forming membranes and genetic molecules and in energy transfer, so it is almost never advantageous to waste it on energy production. All of that is also true for phopshate on Oxio, as well as for nitrate; no evidence of either phosphate or nitrate breathing organisms has yet been found. The Oxionic nitrogen cycle is thus very similar to the Oxionix and Earthling phosphate cycles, with nitrogen remaining in bound forms as it cycles through the ecosystem. A small ecological influx of new nitrogen and phosphorus are provided by weathering of phosphate and ammonium-bearing minerals.

Animal-analogs, complex multicellular heterotrophs, on Oxio rely on sulfur trioxide to support their high-energy metabolisms. Conveniently, eating plant-analogous complex autotrophs provides that source of oxidizer, stored as crystals in plant-analog cells, in the same package with other food molecules. Animal-analogs can also sometimes be seen eating trioxide-rich sands, similar to Earthling animals seeking out mineral salt-licks, although for very different underlying reasons. (Incidentally, terrestrial creatures on Oxio will also seek out salt-licks, for the same reasons as Earthling animals.) Trioxides are converted by the digestive system into dioxides and sulfate ions for internal transport. Oxio is thus yet another world which has produced animal-analogs which have no need to breathe; however, they do still require a pressurized atmosphere to prevent their bodily fluids from boiling!  When oxidizer supplies are low, animal-analogs can engage in dioxide respiration, producing elemental sulfur as waste, as a functional equivalent to anaerobic respiration. Just as humans cannot survive long without oxygen, however, there are strict limits on how long Oxionic animal-analogs can survive without a refreshed supply of sulfates (via ingestion of trioxides) to clean up intracellular sulfur waste.

The atmosphere of Oxio is extremely and acutely toxic to humans, and water and gaseous oxygen are similarly toxic and structurally damaging to Oxionic life. However, a simple drysuit, oxygen mask, and warm clothing are all that are required for human presence on the surface. Additionally, the relatively low pressures and clement temperature ranges suited to Oxionic life makes offworld transport of biological specimens fairly straightforward.