Weird Worlds: Vitrium

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Vitrium is a small, warm world--a half-way point between Venus and Mars.

A weak magnetic field and strong solar radiation have stripped it of most of its primordial volatiles, and particularly depleted it of hydrogen, leaving the surface heavily oxidized--but unlike Mars, and like Venus, Vitrium had enough sulfur to retain a small amount of hydrogen locked up in sulfuric acid, with a small amount (approximately 1.7%) of dissolved water. Many planetologists believe that Vitrium in fact passed through a Venusian stage in its early history, before losing enough atmosphere to cool down to the point that seas could form (with present-day surface temperatures ranging between 50 and 150C, comfortably in the middle of sulfuric acid's liquid range); others, however, think it more likely that the planet simply started out with a relatively low carbon fraction, such that early heating and hydrogen loss were driven primarily by a water, rather than CO2, atmosphere.

As on Venus, sulfuric acid forms clouds on Vitrium, as well as rain, and pools into numerous small seas. Sulfur trioxide also forms thin clouds in the upper atmosphere, but temperatures are too high to support liquid sulfur trioxide anywhere on the surface. The relatively small amount of surface liquid means that Vitrium has no globe-spanning oceans or well-defined global sea level, with most sea basins being entirely disconnected from each other, and having been disconnected for as long as the world was cool enough for seas to form. Nevertheless, lifeforms in each basin are broadly similar, indicating that the current biological system originated originated once and thence colonized the rest of the world.
As on Blue Crystal, life on Vitrium makes extensive use of silicon as a scaffolding element for biomolecules--but unlike Blue Crystal life, Vitrium's biology almost entirely avoids unstable Si-Si bonds, instead relying on strong silicon-oxygen bonds to form organosilicone, or siloxane, polymers, with Si-C and C-C bonds used in various functional groups (in particular, C-C bridges are frequently employed analogously to disulfide bridges used in Earthling biology, or peroxide bridges in  Blue Crystal biology; despite the much greater bioavailability of sulfur on Vitrium, C-C bonds are sometimes required for their greater stability in Vitrium's higher temperature conditions), and fluorine is introduced as a much more common heteroatom than is found in either Earthling or Blue Crystal biology. In fact, the variety of silicone chemistry that is stabilized in a sulfuric acid solvent system seems to exactly balance what is lost to strictly organic chemistry in the same environment, and relatively stiff fluorinated polymers serve to simplify many aspects of membrane structure and enzyme function by introducing a third "fluorophilic" phase, in addition to hydrophilic and lipophilic chemical phases, for controlling chemical segregation.

This, of course, leads us to one of the great paradoxes of Vitrium biology: while on most life-bearing worlds, the base oxidation states of common biomolecules seem correlated to the prevailing geochemistry, thus minimizing anabolic energy expenditure and maximizing structural stability, with the exception of its primary solvent (H2SO4) and fluorolipids, Vitrium's biology seems shockingly under-oxidized. This is a direct result of sulfuric acid's destabilization of much of the carbon-oxygen chemistry that occurs in Earthling biology via the dehydration mechanism. Any biomolecule or close complex containing large quantities of both oxygen and hydrogen is subject to spontaneous degradation to draw more water into solution.

Sulfuric acid also destabilizes many halide salts, which has resulted in significant quantities of hydrochloric and hydrofluoric acid dissolved in the seas as well, although in lower proportions than water. This of course explains the higher availability of fluorine to Vitrium's ecology, but it is also essential to silicon fixation. While inorganic carbon is readily available in gaseous form as carbon dioxide, just as on Earth, inorganic silicon is typically trapped in solid silicate form; and while silica is usually much more susceptible at attack in alkaline environments (a fact which is exploited by the Blue Crystal biosphere to fix inorganic silicon), it is also specifically vulnerable to hydrofluoric acid. A critical base layer of Vitrium's global ecology thus relies on actively concentrating hydrofluoric acid to dissolve silica out of rocks and sediment in order to make it available to biological processes.

All vascularized life on Vitrium also concentrates hydrofluoric acid in its tissues to some extent to improve handling of silica (an adaptation that seems to have developed independently multiple times in different multicellular lineages), but, just as on Blue Crystal, oxidation of food back to crystalline or amorphous silica happens only in microorganisms which can easily eject the resulting crystals from their cells, except where silica deposition is used for structural purposes (e.g., shell-building, where opaline silica fills a similar role as calcium carbonate, which does not exist in Vitrium's chemical environment, does on Earth). Siloxane food molecules are typically broken down into silanol (H3Si(OH)) units which are then repackaged into disiloxane gas (the silicon analog of dimethyl ether) for elimination with the energy-producing release of water.

Some atmospheric and oceanic disiloxane is taken up by autotrophs as a pre-fixed silicon source, but just like carbon being recycled into carbon dioxide, and nitrogen being recycled back into N2 gas by denitrifying organisms (a niche which exists on Vitrium just as it does on Earth), organic silicon is eventually recycled back into granular silica by "desilifying" organisms, which also consume atmospheric sulfur trioxide to reconstitute sulfuric acid from the water produced by oxidizing hydrogens. Meanwhile, both carbon and silicon fixation by autotrophs release oxygen and sulfur trioxide into the atmosphere after splitting hydrogens from sulfuric acid.

Like water and LN2, clean sulfuric acid is a visually clear liquid, and not susceptible to easy photodegradation. However, it is also a much stronger ionic solvent than water, and, as a result, very little sulfuric acid on Vitrium appears actually transparent, with the seas and typical bodily fluids being generally brown in color. At an ecological level, this means the photic zone of Vitrium's seas is only a few millimeters to a few centimeters thick. Photosynthesis occurs strictly at the surface, and deep-sea creatures, which rely on marine snow for nutrition, are universally eyeless. Among land organisms, both compound and liquid-filled camera-like eyes exist, but these depend on active filtration to produce ocular fluids that are, in fact, transparent.

The modern, largely biogenic, atmosphere is composed primarily of nitrogen, with large quantities of CO2, SO3, and free oxygen, and trace quantities of disiloxane and hydrofluoric, hydrochloric, and sulfuric acid vapor.

Direct interaction with Vitrium organisms is impractical; Vitrium biology is violently toxic to Earthlings, and vice versa, with Vitrium biofluids causing corrosion and exothermic dehydration of Earthling cells, and exposure to high concentrations of water vapor or damp surfaces causing osmotic lysis and strong heating of Vitrium tissues (similar to the reactions of Rust native lifeforms). Even with active cooling and chemically-resistant environment suits, human presence in Vitrium environments is strongly discouraged, with investigation being done through robotic remotes. Off-world transport of Vitrium specimens is, however, relatively straightforward, once remote manipulation is accounted for; Vitrium organisms operate in similar pressures to Earthlings, and many can be cooled to enter a state a state of reversible suspended animation, as long as temperature is maintained above 10C to avoid freezing.