Weird Worlds: Brimstone

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Brimstone is a small sulfur-rich world, similar to a slightly larger and warmer version of Oxio, or a much larger and hotter version of our solar system's Io. Like both Oxio and Io, Brimstone is a moon of a gas giant, which allows tidal heating to compensate for the lack of primordial internal heat to maintain tectonic activity and transport sulfur from the core to the surface despite being small enough to have lost most of its low-mass volatiles. Having formed in a hotter environment, Brimstone had a lower primordial water fraction than Oxio, leading to a less oxidized modern environment more chemically similar to Io, with large quantities of elemental sulfur and a 0.66-bar atmosphere composed primarily of sulfur dioxide (0.3 bars), carbon dioxide (0.25 bars), and nitrogen (0.1 bars), with traces of carbon disulfide, carbonyl sulfide, a variety of carbon tetrahalides, argon, xenon, and sulfur vapor. The average surface temperature is 132 degrees Celsius. Sulfur is a close analog on Brimstone for the function of water on Earth, as it is both the primary biosolvent and the driver of weather systems, with clouds and rain of liquid sulfur. Weather systems are generally less violent on Brimstone than on Earth due to the lower heat of vaporization (approximately one quarter that of water) and lower vapor pressures of sulfur, resulting in less potential rainfall and enlarged desert regions. However, sulfur vapor also undergoes much more significant, non-ideal alterations in density with changes in pressure, as the equilibrium of molecular structures shifts between heavier and lighter sulfur ring structures.

The liquid phase consists almost entirely S8 rings, with just under 7% concentration of lighter species. The small concentration of S2 and S4 structures is, however, critical to Brimstone biology, as they are much more easily transported across membranes than the large S8 rings, and provide much more convenient feedstocks for sulfur-involved reactions. On long time scales (compared to typical reaction speeds), S2 concentrations will naturally re-equilibrate as S2 molecules are removed from solution by pumping or consumption in anabolic reactions, but several highly conserved enzyme complexes exist specifically to cleave S8 rings into S4 and S2 groups to feed into other reactions. Large S8 rings are, however, sometimes used directly in the synthesis of large polysulfides. While sulfur-sulfur bonds lend rigidity and insolubility to many Earthling protein complexes, sulfur chains induce improved solubility in the Brimstone biosystem, and carbon-carbon double bonds serve an analogous structural purpose at Brimstone's higher temperatures (similar to Vitrium biology).

Sulfur is a nonpolar solvent, and as such dissolves several small hydrocarbon species. However, it is also a weak Lewis acid (electron acceptor), and so preferentially dissolves Lewis bases (electron donors), and is not a lipophile. Additionally, while some bioavaiilable hydrogen is retained in the form sulfuric acid and hydrogen halides, hydrocarbons are also quite rare, as on Oxio, being substituted with halogen-rich equivalents. Fluorine, chlorine, bromine, and iodine are all highly active in Brimstone biology, as organohalogen groups exhibit higher solubilities with increasing halogen atomic number; functional groups with different halogen terminations are thus selected to precisely control the solvent activity of different macromolecules. Bilayer membranes are composed of long-tail fluorocarbons (essentially, teflon) with solvent-facing nucleophillic thiol heads. With sulfur being both aprotic and non-polar, Brimstone biology cannot rely on ion pumping for energy management purposes, and so relies exclusively on intramolecular electron transport, as on Blue Crystal (a world which is otherwise quite different in nearly every way!)

As on Oxio, energy metabolism is primarily oxygen-based, though mediated by sulfur oxide molecules. Photosynthesis is weakly oxygenic, as there are several non-oxygenated atmospheric carbon sources available (namely, carbon disulfide and the carbon halides), with all oxygen liberated from CO2 being re-bound in sulfur dioxide. As the formation of both carbon disulfide and carbonyl sulfide are endothermic (being produced as as hormone molecules and by lighting and UV-light activated atmospheric reactions), sulfur is a complete bystander to the respiration process, serving only as a vehicle for oxygen. Catabolic reactions of SO2 with typical energy-rich biomolecules (such as fluorolipids) produces CO2, carbon tetrahalides, and elemental sulfur as byproducts. In fact, all atmospheric halides are biogenic in origin, resulting from the breakdown of organohalogen molecules whose halogen content was originally organically fixed from geological sources.

Unlike water, ammonia, nitrogen, and sulfuric acid, but like the iron carbonyl used on Cannonball, molten sulfur is not a transparent fluid. Thus, photo-active structures, such as retinas and photosynthetic pigments, cannot be deeply embedded inside the fluid. Eye and leaf structures thus mirror those on Cannonball and Rust, and the photopic zone of oceans, lakes, and rivers is limited to the upper few millimeters, and most marine creatures completely lack eyes (similar to the situation on Vitrium, though for different underlying reasons). Unlike Cannonball's iron carbonyl and Rust's hydrogen peroxide, however, elemental sulfur is not subject to photodegradation, so protective pigments are not required. The default color of most organisms is therefore a orange-yellow to red, based on the color of the molten sulfur solvent itself.

In low concentrations, water vapor and elemental oxygen are minor irritants to Brimstone life. Exposures to oxygen levels suitable for human life, however, generally results in spontaneous combustion. The Brimstone atmosphere is highly toxic to humans, and temperatures are rapidly lethal. In-person exploration of the planetary surface is possible in a positive-pressure refrigerated environment suit, but is generally considered impractical and discouraged due to the potential damage to the native lifeforms. Refrigerated habitats may be built on the surface, but most exploration must be conducted via robotic remotes. However, the relatively low temperatures and pressures required to support Brimstone life make offworld transport of biological specimens relatively straightforward. Frozen specimens (below 112C) may be safely transported in human-compatible environments, as long as they are contained to prevent contamination from toxic off-gassing.