Weird Worlds: Cronus

 < Fornax | Introduction to Xenobiology

Cronus is a cold world only slightly larger than Earth. The average surface temperature is approximately -150C, similar to Blue Crystal, with an atmosphere composed of 4 bars nitrogen, 2 bars of methane, 0.5 bars helium, 0.3 bars hydrogen, 0.1 bars of argon, 0.1 bars of neon, and traces of more complex hydrocarbons. At these pressures, methane and ethane are liquid on the surface, and methane powers a weather cycle just like water on Earth. Like our own solar system's Titan, Cronus features an orange blanket of high-altitude tholin smog produced by photochemistry. Unlike Blue Crystal, the surface pressure is not high enough to liquify nitrogen.

Also like Titan, the solid surface of Cronus is composed primarily of water ice, with about 10% admixture of ammonia ice (precise ratios varying by region). Surface geology is complex, with rocks composed of a wide variety of hydrated minerals. Unlike Titan, the ice crust is relatively thin, and underlain by a silicate crust and mantle. It is not known if a continuous water/ammonia ocean is present between the water and silicate layers, but silicate volcanism produces hot water/ammonia pockets with large loads of dissolved minerals, driving cryovolcanism on the surface which builds mountains and replenishes surface supplies of heavier elements.

Life on Cronus is based on a mixed methane/ethane solvent, with cells based on azotosomes--bilayer membranes composed of relatively small molecules with polar nitrile heads in the interior and short hydrocarbon tails interacting with the methane/ethane mixture. Unlike Earthling lipid-based vesicles, Cronus's azotosomes fundamentally depend on a mixture of different nitrile molecules for their stability; the largest single component in acrylonitrile, but while flexible pure acrylonitrile vesicles can be constructed, the lowest-energy state is a crystalline solid. Much like water and ammonia in combination form a eutectic mixture with a freezing point far below that of either pure substance, inclusion of additional nitrile molecules produces a "eutectic" membrane structure which strongly resists crystallization.

Due to the thinness of azotosome membranes, they do not contain complex membrane-embedded structures. Instead, equivalent macromolecules are attached to the inner and out surfaces of the membranes, giving Cronian cells a nearly-universal extremely rough texture. This also helps to account for the low solubilities of most materials in cryogenic methane, as the rough surfaces improve capture and adsorption of any rare solutes that a cell may encounter, and a large amount of chemistry in fact occurs in the exterior cellular environment. It is theorized that Cronian protolife may have originated as autocatalytic patterns on rough crystalline sheets, which later evolved to produce mixed-species eutectic sheets that could fold into independent vesicles. In its modern state, however, having developed cellular interiors, Cronian life does rely on the compartmentalization of membrane-bound vesicles to retain useful molecules at much higher concentrations than would be available otherwise, thus vastly improving control and reaction efficiencies, and to store genetic material. As on Blue Crystal and other worlds with aprotic solvents, electrochemistry is accomplished through electron conduction and intramolecular charge separation.

Cronian biochemistry is unusually sparse in its elemental repertoire. While trace heavier elements are available from weathering of aqueous rocks, none are strongly soluble. Thus, Cronian life relies almost exclusively on carbon, hydrogen, and nitrogen to build itself, with occasional inclusions of oxygen. Silicon is virtually unknown; although silanes would be useful functional molecules, the lack of surface-exposed silicate features means that, unlike on Blue Crystal (an even colder world), any silanes that might be geologically produced are hydrolyzed water-magmas long before they would be available to the surface biosphere--and silica grains are completely inaccessible, except as inert grains upon which microorganisms might grow. The limited elemental repertoire has the consequence that Cronian functional molecules are, on average, much larger than their Earthling equivalents. Conveniently, this serves to increase cellular surface roughness! Even so, life is sparse in the fluid column of Cronus's seas, with even photosynthetic life concentrating heavily on the seafloors, where rare low-solubility materials settle out. As on Blue Crystal, water is used for structural purposes by Cronian life, serving equivalent functions to silica, calcium carbonate, and hydroxyapatite for building cell walls and skeletons.

Photosynthesis is predictably hydrogenic, based on consumption of liquid methane and ethane and atmospheric nitrogen to produce complex nitro-organic molecules. Respiration is, conversely, hydrogen-breathing, consuming complex hydrocarbons and atmospheric hydrogen to regenerate methane and ethane. Large amounts of energy, however, can be stored in azides and polyazoles; large, nitrogen-rich molecules, which are much more stable in the cryogenic conditions of Cronus than they are in Earth standard conditions, serve functions very roughly equivalent to fats as compact energy stores, though the high reactivity of azide groups is also very useful in a wide variety of common metabolic cycles. As a result, Cronian biomass usually poses a serious detonation risk if warmed to Earth-standard temperatures, and conventional laboratory study of that subset of Cronian biomolecules which remain stable in human-compatible conditions should be done only after isolation and purification is accomplished in cryogenic conditions.

Free oxygen is poisonous to Cronian lifeforms, but, due to the low temperatures, not acutely so. Similarly, the low temperatures are the only serious risk to human life. Surface exploration is thus possible with suitably insulated and heated suits. Offworld transport of Cronian lifeforms requires maintenance of at least 3 bars of pressure, and temperatures between -140 and -180 C. Below 180C, most Cronian life will be killed by methane crystal formation; above -140, most lifeforms will have died, and there is attendant risk of gas explosion.

Weird Worlds: Fornax

< Brimstone | Introduction to Xenobiology | Cronus >

It has been said that "carbon chemistry is the chemistry of life, silicon chemistry is the chemistry of rocks". Of course, this is a severely oversimplified point of view, based on the limitations imposed on stable carbon chemistry by the high temperatures and ubiquity of water on Earth; many other biospheres incorporate silicon in a variety of ways. However, Fornax challenges this statement in another way: the chemistry of life and the chemistry of rocks are not necessarily entirely disjoint!

Fornax bears the highest-temperature biosphere currently known, and is likely near, if not at, the limits of what chemical biosystems can support. Fornax is in some ways a hybrid of our own solar system's Venus and Mercury--closer in size to Venus, and with a non-negligible atmosphere, but with an elliptical orbit even closer than Mercury's, with a 3:2 spin-orbit synchronization. Daytime surface temperatures reach 900C, but can drop as low as 400C at night. Due to these temperature variations, which would be exacerbated by a 1:1 tidal lock, the orbital eccentricity and consequent lack of 1:1 tidal locking are critical for the survival of the Fornaxian biosphere. While photosynthetic biospheres can exist on water-based "eyeball" worlds, so long as there is sufficient water for atmospheric or glacial processes to continually replenish the dayside supply, the much lower-abundance materials which serve as Fornax's biosolvents would be expected to entirely freeze out on any similar worlds which have a permanent dark side.

Fornaxian life is based on an ionic liquid solvent system consisting of a mixture of metal chlorides and fluorides. The major metallic species are iron, calcium, sodium, magnesium, potassium, and copper. The surface atmospheric pressure is 0.52 bars, consisting of 0.3 bars of sulfur dioxide, 0.2 bars of nitrogen, 0.01 bars of carbon dioxide, and less than 0.01 bars each of sulfur, argon, aluminum trichloride, iron dichloride, zinc dichloride, copper (I) chloride, and silicon tetrafluoride.

Biochemistry in this environment is based on halogenated aluminosilicate polymers with additional metal ion and metal oxide functional groups. With carbon integrated only as a relatively rare heteroatom, this constitutes the most thoroughly silicon-based, as well as hottest, biosphere yet known. While tetravalent silica is roughly equivalent to carbon in Earthling biochemistry, aluminum serves a role similar to nitrogen, with regular aluminum substitutions in silicate polymers introducing local negative charge concentrations and trivalent structures. Such aluminum substitutions, creating replicable surface charge patterns, are a key feature of the clay-substrate hypothesis for the origin of prebiotic chemistry on Earth; it is an intriguing possibility that Fornax may be an example of a world where autocatalytic aluminosilicates were able to make the jump to full independently-evolving genetic and autocatalytic biosystem directly, rather than simply providing a prebiotic template for the development of more typical replicator systems. Oxygen functions similarly to hydrogen in colder biosystems, saturating excess silicon and aluminum valences and forming weak intermolecular bonds.

Like carbon, nitrogen and phosphorus are also occasional heteroatoms in Fornaxian biochemistry. As on Earth, phosphorus circulates almost entirely in the form of phosphate ions and nitrogen is regularly fixed from the atmosphere and returned to gaseous form when biological materials decay. The fixing of nitrogen is, however, simpler than the equivalent process in Earthling biology, as they high ambient temperatures significantly reduce the activation energy hill that must be overcome to split N2 molecules.

The dominant cycle for producer-consumer energy metabolism on Fornax is still based fundamentally on oxygen; autotrophs split oxygen from silicate and alumina groups to form aluminate anions, halogen-substituted silicates, and desaturated siloxanes. However, free oxygen at Fornaxian temperatures would rapidly recombine with any newly created reduced biomolecules. Thus, as on Oxio and Brimstone, autotrophic organisms consume sulfur (in this case, purely in gaseous form) to bind oxygen and reduce its chemical activity. Biogenic production of sulfur dioxide on Fornax is similar to biogenic production of oxygen on Earth, resulting in the high proportion of SO2 in the atmosphere. Heterotrophs inhale sulfur dioxide to oxidize silicon and aluminum(and occasionally other metals), and exhale waste sulfur, while also regenerating soluble halogen ions in the process. Substituting oxygen bonds for fluorine bonds, however, is not energetically favorable, as the silicon-fluorine bond is among the strongest chemical single bonds known; this leads to Si-F bonds being preserved in catabolic metabolism, eventually resulting in the release of waste silicon tetrafluoride gas and the depletion of soluble fluoride ions. Autotrophic organisms must therefore periodically expend energy to capture atmospheric SiF4, partially incorporating the bound fluorines into new biomolecules and returning the rest to solution.

Fornaxian conditions are immediately lethal to humans, and no practical mechanisms for supporting long-term habitation are available. All close-range exploration must be done by robotic remotes. However, the lowered temperatures of the Fornaxian night have resulted in most organisms developing resistance to freezing. This makes offworld transport of biological specimens for laboratory study surprisingly straightforward.

Commentary

This world was inspired by Feinberg & Shapiro's highly underspecified "Thermia" proposal (Life Beyond Earth, 1980), in which they suggest that silicates could form biomolecules above 1000C. However, Petkowski, Bains (of "Many Chemistries Could Be Used to Build Living Systems" fame), & Seager (in "On the Potential of Silicon as a Building Block for Life") point out that silica doesn't look so good for forming biological structures when all of the oxygen-network bonds are as labile as hydrogen bonds in a 1000-degree melt, and Feinberg & Shapiro didn't really explain what kind of distinction (if any) they had in mind between solvent and partially-solvated structures.

However, many salts, which can form ionic solvents, and silica-containing minerals, melt into solution at well below 1000 degrees, and others are solid up to much higher temperatures. Since I have been doing rather a lot of thinking about Hal Clement's Sarrians (who live at a mere 500 degrees), slightly lowering the operating temperature, introducing an ionic solvent, and throwing in sulfur to modulate oxygen metabolism seemed like obvious compromises to bring Thermia to "life".

Weird Worlds: Brimstone

< Oxio | Introduction to Xenobiology | Fornax >

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.

Weird Worlds: Oxio

< Nicar | Introduction to Xenobiology | Brimstone >

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.

Weird Worlds: Nicar

< Snowball | Introduction to Xenobiology | Oxio >

Nicar is a carbon world, like Coal, formed from a protoplanetary disk with more carbon than oxygen; as on Coal and Cannonball, water is geologically unstable and the chemical environment is strongly reducing. The atmosphere is approximately 2 bars of nitrogen, with traces of ethane and more complex hydrocarbons as well as smaller amounts of methane and ammonia. The average global temperature is approximately -40C.

Unlike Coal, Nicar is not a super-Earth. It could be considered a carbonaceous analog of Mars--small enough that it loses hydrogen to space easily, and ammonia is easily photolyzed. Life originated on Nicar in ammonia seas under a methane/ammonia atmosphere and developed an initially hydrogen-breathing ecology, like on Coal--but, the accelerated release of hydrogen by photosynthetic life also accelerated the loss of hydrogen to space and the steady destruction of Nicar's primordial atmosphere and seas.

If it were an oxygen world, like Mars, Nicar would have inevitably become sterile like Mars as its oceans evaporated. However, while the ammonia ocean was shrinking, dissociation of methane simultaneously produced a growing layer of hydrocarbons--principally ethane, propane, butane, and pentane--with propane and butane condensing under a growing atmosphere of nitrogen to form a hydrocarbon cap over the remaining liquid ammonia, protecting it from further evaporation. Nicar thus has two oceanic layers of entirely dissimilar materials, with a colloidal mixing zone in between.

Life in the lower ocean continues to use essentially the same biochemical pathways as on Coal, although ammonia-consuming respiration is nearly universal due to extremely low hydrogen concentrations, which limits organisms' size dependent on pressure at depth. Near the surface, photosynthesizers still rely on hydrogenic pathways, although access to more complex feedstocks like butane and methylamine result in reduced bulk hydrogen output compared to a true hydrogen-breathing worlds. Nearly all released hydrogen, however, is quickly recaptured in either the upper ammonia layers or the lower hydrocarbon layers and used to hydrogenate unsaturated hydrocarbons, crack long-chain hydrocarbons, or regenerate dissolve nitrogen gas into new ammonia, all of which are energy-releasing reactions.

When these hydrogen-consuming reactions are accounted for, the net chemical equilibrium between producers and consumers in the Nicar biosphere is a mixture of the following three major equations:

6 C3H8 (propane) + 2 NH3 + 8 N2 <=> 3 C6H18N6
8 C4H10 (butane) + 4 NH3 + 16 N2 <=> 6 C6H18N6
6 CH3NH2 (methylamine) + 2 N2 <=> C6H18N6 + 4 NH3

along with several other more minor synthesis pathways.

When summarized this way, we can see that the energy metabolism on Nicar is not, in fact, ultimately centered around hydrogen; hydrogen is shuffled between hydrocarbons and ammonia and amine groups during the process, but fundamentally, energy is stored and structure built by incorporating nitrogen into the biosphere, and energy is released by freeing nitrogen. This is the exact reverse of oxygen metabolism, and serves as an excellent example of why "nitrogen breathers" do not exist.

The colloidal and hydrocarbon zones provide an additional source of interest for xenobiologists. Life in the hydrocarbon zone still uses ammonia as its intracellular solvent, but has adapted to the external environment by abandoning bilayer cell membranes, which are dissolved in the hydrocarbon phase, and instead developing single-layer inverse micelles. Multicellular colonies in this region appear to have only a single double-layer membrane separating intracellular environments, as the lipophilic tails of their membrane exteriors interlock with each other.

The lack of ammonia evaporation from the capped oceans also complicates life on land. While there is weather, with hydrocarbon clouds and hydrocarbon rain, the liquid that rains down on land is not directly usable as biosolvent. Thus, for all practical purposes, land biomes on Nicar are all deserts, similar to Rust and Cannonball, in which all available biosolvent must be manufactured by the biosphere itself. (Nevertheless, Nicar as a whole is not formally classified as a desert world, as it still has ammonia seas which support a large fraction of the biosphere.) Terrestrial ammonosynthesis is done by photosynthetic autotrophs which capture light hydrocarbon compounds and free nitrogen from the air, split hydrogen from hydrocarbons to form longer hydrocarbon chains (some of which are incorporated as components of fatty carboxamidines, and some of which are released into the environment, ultimately flowing into the upper ocean), and use the liberated hydrogen to fix nitrogen into ammonia. This fluid can then be eaten or drunk by heterotrophs, who again exclusively use ammonia respiration to produce energy and return nitrogen to the air. Nicar is thus one of very few worlds, like Rust, to have produced animal-analogs which have no need to breathe!

Human contact with Nicar organisms is possible with the use of a drysuit and oxygen mask. Earthling and Nicar biologies are, however, mutually corrosive to each other, so chemical isolation procedures must be strictly observed. The relative safety of human interaction, along with the relatively low pressures and clement temperature ranges suited to Nicar lifeforms make off-world transport of specimens for further study relatively straightforward.

Weird Worlds: Snowball

< Coal | Introduction to Xenobiology | Nicar >

Snowball is another cold super-Earth, but still below the mass limits, and above the temperature limits, where it could hold free hydrogen in its atmosphere. The average global temperature is approximately -60 C, and the world has extensive basins of dirty water ice.

The surface pressure is approximately 4 bars, composed primarily of nitrogen, with about 1/6th of an atmosphere of oxygen and traces of carbon dioxide, water vapor, and ammonia. There is no precipitation, except for occasional snowfalls following volcanic activity, but high-altitude water and CO2 clouds occasionally form, and winds can produce short-term dust and electrical storms.

The native life forms of Snowball employ a 35% ammonia-water eutectic solution as their biosolvent. Snowball's biochemistry is otherwise largely unremarkable for a system based on water-ammonia solution (refer to Still and Coal biological overviews for details), but this makes Snowball the only known example of an oxygen-breathing ammonious biosphere.

Many worlds, especially oxygen-bearing worlds, at some point pass through a biogeochemical crisis which radically restructures their native life. The most well-known of these is, of course, Earth's own Oxygen Crisis, brought about by cyanobacteria, which constituted a mass extinction event for the anaerobic biosphere but paved the way for high-energy oxygen metabolism. Snowball managed to come to its unique present state through a sequence of two such biogeochemical crises.

Early in its history, Snowball was a fairly normal aqueous ammonia world, with extensive oceans and a reducing atmosphere, with large quantities of ammonia and methane keeping the temperatures higher (although still cold by terrestrial standards), and primitive life was largely hydrogen-breathing. As hydrogen was lost to space in the world's first billion years, however, supplies of atmospheric methane were depleted and replaced with less hydrogen-rich carbon species, such as ethane, ethylene, hydrogen cyanide, and formaldehyde, autotrophs were forced to adapt to these new carbon sources and gradually transition from hydrogenic photosynthesis to hydrogen-consuming photosynthesis. In its first stages, this relied on splitting ammonia as a hydrogen source (a relatively simple exaptation of cellular machinery that already split hydrogen off of ammonia to form C-N bonds, upregulated and adapted to discard excess nitrogen), releasing free nitrogen into the atmosphere. Much like hydrogen sulfide-based photosynthesis by Earthling sulfur bacteria, this is much cheaper than splitting water, and ammonia was far more readily available across Snowball than hydrogen sulfide is on Earth, allowing this process to dominate the biosphere. Nitrogen-breathing, however, is not energetically favorable, so no mechanism was available to close the cycle. Thus, similar to Cannonball's Carbonyl Crisis, which transformed it into a desert world, Snowball went through a "Nitrogen Crisis"--a crisis not due to the presence of excessive nitrogen, but the depletion of ammonia which formed a large portion of the oceans and native organisms' own bodily fluids. This also led to raising the freezing point of the oceans while the average temperature of the planet continued to trend downwards with the loss of greenhouse gasses, converting Snowball as well into a desert world.

For several million years, Snowball life survived by actively concentrating environmental ammonia above ambient levels. This system was, however, always doomed to fail when environmental ammonia levels fell too low, and it became untenable for autotrophs to destroy their own body fluids. This resulted in a slow die-off and contraction of biosphere mass until two additional evolutionary breakthroughs occurred: first, the ability to split water to produce hydrogen for photosynthesis, and the ability to actively fix atmospheric nitrogen back into ammonia--which relied on the pre-existence of water as a new hydrogen source. While this may seem like an incredibly unlikely sequence of events, the first step was actually not as complex as it might seem; because oxygen was not available from atmospheric sources (CO2 being essentially nonexistent up to this point in the planets history, except for a short-lived quantity produced by volcanic activity) the ability to split water in order to obtain oxygen to incorporate into biomolecules was already ancient, much like the previously-exapted machinery for splitting ammonia. All that was necessary was for some organism to alter the regulation of these two pathways to produce excess oxygen instead of excess nitrogen, allowing it to persist in for longer in isolated puddles, lakes, and tide-pools than organisms which destroyed their own ammonia as such systems dried up. Initially, the resulting freed oxygen would be used up oxidizing the various oceanic and atmospheric carbon species, requiring a continued shift in autotrophic anabolism to use CO2 as another new carbon source. Nitrogen fixation was the much larger evolutionary step, but once oxygenic hydrogen production was in place, in the presence of atmospheric nitrogen, it was both energetically favorable and strongly incentivized as it would allow the lucky organism to survive outside the dwindling oceans and colonize the vast expanses of water-rich ice without relying on increasingly scarce ammonia rainfall. As on Earth, additional oxygen was consumed in the oxidation of oceanic iron, precipitating oxidized iron ore deposits. After that point, oxygen finally began to fill the atmosphere. Note that none of these developments immediately halted the contraction of the seas--in fact, they accelerated it. Without any immediate pressure to change, marine organisms continued to destroy ammonia, even as terrestrial organisms synthesized more for their own use--but at a much lower rate. Additionally, the production of carbon dioxide resulted in the formation of ammonia carbamate in chemical equilibrium, and free oxygen would react with unprotected environmental ammonia as well, producing more nitrogen and water.

The ensuing Oxygen Crisis was thus the cause of another major mass extinction, but also the ultimate salvation of the Snowball biosphere, as it allowed the exchange of water and nitrogen for oxygen and ammonia. Active nitrogen fixation seems to have evolved only once, with the successful microorganism undergoing adaptive radiation to not only colonize the ice-bound land and outcompete any other potential nitrogen-fixing competitors, but also to form partnerships with every other surviving lineage, such that all contemporary complex life on Snowball is a deeply intertwined symbiotic relationship, rather like those seen between Earthling plants and nitrogen-fixing bacteria, or between fungi and algae to form lichens, with some host organism providing energy and nutrients to ammonogenic microbes integrated into its tissues in exchange for regulation of ammonia levels. As oxygen tolerance, at a minimum, was a necessity for survival on Snowball, aerobic respiration on the other hand has independently arisen several times, leading to a variety of modern, high-energy, oxygen-breathing creatures. Oxygen tolerance is more complex for Snowball's organisms than it is for Earthlings (although the energy cost of oxygen tolerance even for us should not be understated; it seems a low cost only because we are accustomed to it!), since their basic biosolvent is slowly destroyed in the presence of oxygen and must be constantly maintained--but, much like the inhabitants of Blue Crystal and Rust, they are assisted by operation at low temperatures, which reduces spontaneous reaction rates for oxidation.

Of course, 65% of Snowball's biological fluids are still composed of dissolved water, which is also no longer available in liquid form! While, as previously stated, Snowball did once have liquid seas, water is now effectively a mineral nutrient. Some creatures are able to consume water ice directly, rather like Earthling animals enjoying a salt-lick, but most heterotrophs acquire water by eating other creatures, just like any other macronutrient. Autotrophs employ a mix of strategies for water acquisition; in many places, especially over frozen seas, whose geology is dominated by water ice, it can be readily dissolved out of the ground by root structures. Elsewhere, however, many organisms, both autotrophs and heterotrophs, actively scavenge it from the trace amounts found in the atmosphere, much like the peroxide-producing autotrophs of Rust.

Atmospheric ammonia and carbon dioxide still exist in chemical equilibrium with solid carbamate on the surface, which sublimes and redeposits based on weather conditions and local temperatures and concentrations of each gas. Carbamate is highly soluble, and also exists in the circulatory and intracellular fluids of essentially all Snowball lifeforms. This is actually rather convenient, as it makes atmospheric carbon capture much simpler for autotrophs, and eliminates the need for special carbon-dioxide-transport molecules in complex heterotrophs. Just as the gills of Earthling creatures double not only as gas exchange surfaces but also ion-exchange surfaces, all forms of respiratory structures on Snowball (lungs, gills, spiracle trachea, etc.) actively concentrate carbamate ions to shift the gas equilibrium and pump CO2 out of the organism. 

Human contact with Snowball organisms is possible with the use of a drysuit and warm clothing; oxygen is available from the atmosphere, but filter masks to remove trace ammonia are required. The relative safety of human interaction, along with the relatively low pressures suited to Coal lifeforms make off-world transport of specimens for further study relatively straightforward, although active refrigeration is usually required.

Weird Worlds: Coal

< Still | Introduction to Xenobiology | Snowball >

Like Still, Coal is a cool super-Earth. Unlike Still, Coal is a carbon world, formed from a protoplanetary disk with a higher proportion of carbon than oxygen. As a result, as on the iron-rich Cannonball, water is not geologically stable, as it reacts with carbon and nitrogen compounds to form carbon monoxide, methane, and urea.

The surface pressure is approximately 3 bars of 74% nitrogen, 16% hydrogen, 5% methane, and 5% ammonia, with ammonia clouds and precipitation, and traces of hydrogen cyanide, neon, argon, and xenon. Average global surface temperatures are around -40C. The oceans are anhydrous ammonia, with large quantities of dissolved salts, methylamide, formamide, hydrogen cyanide, urea, and nitro-silicon compounds. The large amount of methane produces a slight green tinge to the sky.

As on our own Solar system's Titan, photochemistry in the upper atmosphere produces a haze of complex hydrocarbons and carbon-nitrogen compounds (tholins). These compounds themselves and chemical energy obtained through hydrogenation are significant inputs to the surface ecosystem. Unlike Titan, however, ammonia rain regularly washes out the haze in the lower atmosphere, and native biology on Coal efficiently scavenges tholins reaching the surface, so large standing concentrations of hydrocarbons are rare.

In contrast to mixed ammonia-water worlds, life on Coal is able to optimize specifically for ammonia chemistry, and uses oxygen as a relatively rare heteroatom. Silane and it's reaction products with ammonia (the most common of which is silylamine), which is produced by volcanic activity along with its carbon-analog methane, also provides a source of bioavailable silicon which is also incorporated as an occasional heteroatom, as the Si-H bonds are considerably easier to break in favor of Si-C bonds than the Si-O bonds found in silica are--though it is not used to the same extent as on Blue Crystal or Vitrium, or even Opal, where silicon is a major structural element. The majority of crustal silicon on Coal is locked up in silicon carbide, rather silicates, which vastly reduces bioavailability compared to what might otherwise be predicted for a world with a strong alkaline solvent. In fact, it is the very same feature which permits the existence of the anhydrous ammonia solvent--namely, the high carbon fraction--which is also responsible for the unavailability of silicon!

In a strange parallel to nitrogen metabolism on Earth, the primary environmental source of oxygen for autotrophs, and disposal method of excess oxygen for heterotrophs, is urea. Fatty acids are substituted by carboxamidines (with ammonophilic -C=(NH2)-(NH) groups terminating hydrocarbon tails), with average tail lengths being shorter than those used in warm water biochemistries. Structural analogs of sugars and starches are fully nitrogenated, with =NH imidogen groups replacing nearly all uses of oxygen, and -NH2 amide groups replacing hydroxides, in water-based biochemistries. Amino-sugar synthesis and catabolism proceeds according to the large-scale equation

6 CH4 + 6 NH3 <=> C6N6H18 + 12 H2

However, the primary energy storage molecule in the Coal biosphere is not this glucose-analog, but acetylenamine, a more soluble derivative of acetylene which shares functions split between sugars and ATP in water-based biochemistries, and which can be hydrogenated to ethyleneamine (or further to methane and methylamine) to release large amounts of energy.

Although ammonia, like water, is a protonating solvent, proton pumps are a relatively rare energy-management mechanism on Coal. Instead, Coal lifeforms universally exploit ammonia's electron-solvating properties to store excited electrons directly and shuttle them across membranes and along electron-transport molecules to set up and exploit electrostatic gradients. This produces a consistent blue tinge (the color of low-density solvated electrons) to all native Coal cells.

As on Still, the most common respiratory pigments for complex heterotrophs are iridium-based, giving their circulatory fluids a bright yellow color, and facultative ammonia consumption is used to facilitate hydrogenation at depth or in other hydrogen-poor environments.

Human contact with Coal organisms is possible with the use of a drysuit and oxygen mask. Earthling and Coal biologies are, however, mutually corrosive to each other, so chemical isolation procedures must be strictly observed. The relative safety of human interaction, along with the relatively low pressures and clement temperature ranges suited to Coal lifeforms make off-world transport of specimens for further study relatively straightforward.

Weird Worlds: Still

< Opal | Introduction to Xenobiology | Still >

Still is a cool super-Earth with a mixture of water and ammonia oceans. The surface pressure is approximately 5 bars, with 3.5 bars of nitrogen, an average of 1 bar of ammonia vapor, 0.5 bars of hydrogen, and trace quantities of methane and water vapor. The average global temperature is approximately -30C--just above the boiling point of ammonia at 1 standard atmosphere, hence the large atmospheric partial pressure--but varies between daytime highs of just over 0C at the equator and -50C at the poles.

Life in mixed water and ammonia solution faces numerous chemical challenges--it must select from the set of potential macromolecules which are stable against reaction with both media. Nevertheless, the cosmic abundance of both water and ammonia, and their effects on increasing each other's liquid ranges, means such systems are common among hydrogen-rich terrestrial worlds.

The functional equivalents of sugars, and other reactive biomolecules, in Still biology are all strongly, but not entirely, nitrogenated, with amine an amidogen groups replacing =O and hydroxide groups. The basic glucose analog has the formula C6H15ON5, and forms nitrogenous starches through linking primary amide groups to form secondary amines with the release of ammonia. As is typical for hydrogen-rich worlds, methane provides the primary inorganic carbon source. Hydrogen-exchanging photosynthesis and respiration proceed according to the equilibrium

6 CH4 + 5 NH3 + H2O <=> C6H15ON5 + 13 H2

The detailed sub-reactions are about as unexpected as they are for oxygenic photosynthesis on Earth. Despite the suggestive elemental ratios, photosynthesis on Earth does not proceed by splitting oxygen from CO2 and combining it with water, but by splitting water and using the hydrogen to attack CO2. Similarly, photosynthesis on Still proceeds by splitting hydrogen off of ammonia (a considerably less energy-intensive process than splitting water) and using reactive amine and imidogen groups to attack water and methane. The genetic molecule is also highly analogous to Earthling DNA, using nitrogenous sugar backbones with phosphate linkages providing the repeating anionic charge structure to maintain solvation and proper pairing structure.

Cell membranes are slightly different from the water-solvated standard, as fatty acids are unstable in the low-pH ammonious environment. Instead, cell membranes on Still are formed from surfactant soaps. Protein structures are broadly similar to those of water-solvated life, but the ability of ammonia to dissolve a larger range of metals than pure water increases the usage of a variety of metal ions in Still biology for catalytic purposes. However, energy metabolism still makes extensive use of proton pumps to establish electrochemical gradients. Notably, large-scale pumping of solvated electrons, such as is seen in the rarer ammonia-dominated biochemistries, is completely absent here, as electron solutions are destabilized by the water component.

On a larger, ecological scale, hydrogenic photosynthesis is less energetically demanding than oxygenic photosynthesis. This is assisted by the fact that Still autotrophs do not waste any energy on accommodating oxidative stress (notably, through photorespiration), and the lower temperatures make energy capture more efficient in general (similar to, but to a lesser degree than, photosynthesis on Blue Crystal). Hydrogenation, conversely, releases less energy than aerobic respiration--but not so much less as to make complex animal-like life impossible. Still heterotrophs do have to consume more food than equivalent Earth animals, but not in direct proportion to the lower energy budget; they are also assisted by the greater efficiencies allowed by operation in lower-temperature, non-oxidizing environments.

Still heterotrophs are also not subject to "anhydronia" in the same way that Earthling aerobic organisms are subject to injury and death by hypoxia. Smaller organisms, with sufficient surface area for rapid gas exchange, can survive indefinitely by employing the lower-energy pathway of stripping hydrogen from ammonia to hydrogenate food molecules and produce methane and water, releasing free nitrogen. Larger organisms are constrained by their ability to reject the resulting nitrogen gas fast enough to avoid producing tissue-damaging bubbles, but can still survive on a lower concentration of hydrogen than might be naively assumed by direct analogy to Earth life. The is especially convenient for aquatic organisms as the solubility of hydrogen in water, or water-ammonia solution, is lower than that of oxygen. Even more conveniently, nitrogen remains solvated at higher concentrations are greater depths, making ammonia-consuming hydrogenation more feasible in exactly the ocean environments (far from the surface, with lower solvated hydrogen concentrations) where it is most useful. Many complex terrestrial organisms use an iridium-centered molecular complex to bind hydrogen analogous to the use of hemoglobin and hemocyanin to reversibly bind oxygen on Earth, giving their circulatory fluids a bright yellow color, which decomposes to produce green and blue pigments when exposed to light; this, however, is less useful for marine organisms which have limited access to hydrogen in the first place. A wide variety of microbial species survives by catalyzing the direct recombination of atmospheric hydrogen and nitrogen released from the oceans more quickly than it would occur by spontaneous reaction, thus stabilizing global ammonia levels.

Silica is considerably more soluble in aqueous ammonia than in the relatively pure water that forms Earth's hydrosphere, and is thus considerably more bioavailable. Complex silicon chemistry is still inhibited by the large water fraction, which is still highly reactive with polysilanes, but biologically-deposited crystalline and amorphous silica is much more common as a structural substance for Still organisms than it is on Earth, often being incorporated into biocomposites to form opal-like shells (such as occurs on Vitrium) and bones. Some natural organosilicon compounds are known, but organosilicon chemistry does not rise to the level of prevalence found on, e.g., Opal, which presents much greater evolutionary pressures favoring its use.

The planet gets its name, and its unique character, from the temperature differences between equator and poles, which span a range over which ammonia's vapor pressure varies widely. Near the equator, ammonia evaporates far more readily than at the poles, reducing the concentration of ammonia on the ocean surface and in rivers and lakes, while leaving it more enriched in the temperate and polar regions--in effect, the entire ocean-atmosphere system is a gigantic distillation apparatus! However, with the exception of extremophilic microbes which can evolve to adjust their biochemistry to varying water/ammonia fractions, the majority of macroscopic Still life has settled on a single homeostatically-maintained solvent mixture. This results in an ecological division analogous to that between saltwater and freshwater fish on Earth, but divided by latitude and ocean depth rather by ocean vs. river/lake environments.

Human contact with Still organisms is possible with the use of a drysuit and oxygen mask. Earthling and Still biologies are, however, mutually corrosive to each other, so chemical isolation procedures must be strictly observed. The relative safety of human interaction, along with the relatively low pressures and clement temperature ranges suited to Still lifeforms make off-world transport of specimens for further study relatively straightforward.

This world was broadly inspired by Hal Clement's Still River.

Weird Worlds: Opal

< Vitrium | Introduction to Xenobiology | Still >

Opal is dominated by an ocean and atmosphere composed primarily of carbon dioxide. The total surface atmospheric pressure is approximately 73 bars, 70 of which are provided by carbon dioxide gas (the remainder being primarily oxygen and nitrogen). This thick blanket of greenhouse gasses is balanced, however, by Opal's distance from its star, which would leave an Earthlike world permanently frozen over. This thick atmosphere also efficiently distributes heat across the globe, resulting in balmy temperatures of 30 to 40 C worldwide and year-round.

That does not, however, mean that Opal is without weather! Slight variations in insolation over the year due to Opal's slightly eccentric orbit result in sea levels rising and falling as large portions of the atmosphere condense and then re-evaporate, with the annual fall storms providing the vast majority of liquid delivered to inland regions. Additionally, just as Earth's surface conditions happen to straddle the liquid-solid-gas triple point of water, Opal's surface conditions happen to straddle the liquid-gas-supercritical fluid boundary of CO2. Thus, the oceanic regions of Opal are frequently subject to waves of critical opalescence--which is what gives the planet its name!

Unsurprisingly, biochemistry on Opal is primarily based on the liquid CO2 solvent system, and opalescence storms thus pose a significant hazard to organisms living on or near the surface, as they can cause internal cavitation and radical short-term changes to the solvation properties of intracellular fluids. Like the biology of Rust, Opal's biochemical systems are in many ways similar to Earthling life, even using many identical macromolecules which are soluble in CO2; in fact, the relatively high compatibility of CO2 as a solvent with Earthling biomolecules has resulted in extensive industrial use of liquid and supercritical CO2 on Earth for organic synthesis and purification. There are, however, several key differences, mainly owing to the non-polar and aprotic nature of CO2.

The typical lipophilic tails seen in, for example, the phospholipids that form Earthling cell membranes have low affinity for CO2 despite being non-polar. Instead, Opal life makes use of a combination of oxygenated polymers with CO2-philic carbonyl groups and short polydimethylsiloxane neck sections with lipid tails to act as surfactants for interfacing lipophilic and CO2-philic phases, carboxylic acids heads joined to siloxane tails forming monolayers to mediate between CO2-phillic and hydrophillic (polar) phases, and normal fatty acid monolayers to interface between hydrophillic and lipophillic phases. The existence of a third separate CO2-phillic phase means that Opaline life's bilayer membranes cannot be easily identified with Earthling (or Rust, or Vitrium) style vesicles or the reverse-vesicles more typical of non-polar solvents. The equivalent of phospholipid membranes have CO2-phillic heads extended into the bulk medium, which may protecting either membrane-internal acid-based polar ends between the layers, or lipophilic alkane tails; these two membrane types are intermixed to achieve different structural properties, much as the fluorophillic phase is exploited in Vitrium biochemistry. Some simpler classes of organisms rely on the integration of salts to stabilize the interiors of their polar membranes, (a structure which is still seen in some organelle membranes in eukaryotic organisms, providing evidence for an endosymbiotic origin) but eukaryotic organisms on Opal have developed more complex hydrophillic units based on sugar esters which greatly reduces their reliance on salts which are insoluble in the CO2 medium. Additionally, rather than the highly charged and hydrophillic DNA used by Earthling life, Opaline life operates on Peptide Nucleic Acid, or PNA. The alternate membrane structures and replacement of the genetic molecule backbone mean that Opaline life has a vastly reduced reliance on phosphorus as a limiting nutrient compared to Earthing life. It is also worth noting that fluorocarbons, used extensively in Vitrium's biochemistry, also exhibit significant CO2-philic properties--but unlike Vitrium, Opal's environment does not make fluorine sufficiently bioavailable, so it remains exceptionally rarely used despite its functional advantages, parallel to Earthling biology.

Unlike on Blue Crystal and Vitrium, silicon is still decidedly a heteroatom in Opal's collection of common biopolymers, with carbon acting as the primary polymer backbone atom in Opaline biochemistry. Silicon is approximately as bioavailable on Opal, in the form of silicic acid weathered out of silicate rocks by carbonic acid, as it is on Earth; but, the solvation properties of siloxanes in Opal's environment provided the evolutionary pressure to develop efficient methods of breaking Si-O bonds and forming Si-C bonds, which have only been achieved by artificial directed evolution on Earth. Coincidentally, this ability is also used by many of Opal's creatures to construct opaline shells, just as on Vitrium, standing in for the calcium carbonate that is such a common structural material on Earth. This fact, however, was not discovered until well after the planet's initial discovery and naming.

The presence of carbonic acid of course implies the presence of water--and indeed, water is a common component of Opaline biochemistry. Photosynthesis operates almost exactly as on Earth, but with the roles of CO2 and water reversed--CO2 is the readily-available solvent while water, serving as the hydrogen source, is pulled from the atmosphere. And the aqueous vacuole--a reverse-micelle containing a supply of water in which aqueous chemical reactions can be performed--is a nearly-universal organelle in Opaline cells. Aside from the various aqueous reactions that it permits, the vacuole is central to most cells' energy metabolism, as it is both the source and sink of protons which allow Opaline life to make use of proton-pump mechanisms just like Earthling and Vitrium life--a class of molecular machinery that is unavailable to most biochemical systems operating in aprotic solvents (such as those of Cannonball or Blue Crystal). The astute student may well then ask: why did Opaline life not evolve to use water as its primary medium, rather than CO2?

The answer to this is in fact incredibly simple: density. Liquid CO2 is approximately 10% heavier than liquid water in similar conditions. Thus, excess water that cannot be solvated in the CO2 seas floats, keeping it out of contact with the solid substrates from which essential nutrients and catalytic substrates are acquired. This is the source of another of Opal's unique weather phenomena: rainpuddles. When the ocean or atmosphere become super-saturated with water, it does not condense into small drops with can fall out of suspension; rather, drops accumulate into large "puddles", which can "rain" upwards and drift around near the ocean surface for quite some time before being broken up and re-dissolved. The inverse phenomenon can occasionally be observed on Earth at sufficient depths, with excess CO2 that cannot be solvated in water forming puddles on the ocean floor. The near-critical nature of the overlying atmosphere, with density quite similar to that of the liquid oceans, also means that these rainpuddles can sometimes be found drifting through the air, presenting a navigation hazard to low-flying creatures. This should also highlight the ease of flight on Opal; it is hardly more difficult than swimming. The colonization of the sky was thus a more direct step in Opal's evolutionary history than the colonization of the land! And, since water, oxygen (produced by photosynthesis and filtered by lungs) and nitrogen (also easily filtered out of the ocean or air) are all lifting agents in Opal's thick CO2 atmosphere, aerostatic and hovering flight are far more common, and employed by creatures with a far wider range of morphologies, than are seen on Earth.

Despite their high-pressure environment, Opaline organisms are actually relatively easy to transport for study off-world. While most cannot remain active significantly below 30C and 70 atmospheres, due to extremely precise tuning of proteins to operate in that extremely stable temperature range, their fluids do not actually freeze above -50C, at which point pressures can be brought down to a mere 5.1 atmospheres with no structural damage to the organism, significantly reducing the structural and mass requirements for transport vessels. The vast majority of Opaline organisms enter a state of extreme torpor or suspended animation under these conditions, exhibiting no significant metabolic activity or need for life support functions, and will return to full function with few or no long-term ill effects upon being re-pressurized and brought back up to normal temperatures. Direct contact with Opaline organisms is generally safe, requiring only a drysuit and oxygen mask to enter their native environment for brief periods. Long term interaction, however, requires active cooling garments to prevent heatstroke in human researchers, and standard decompression protocols when exiting the Opaline environment.

Weird Worlds: Vitrium

 < Blue Crystal | Introduction to Xenobiology | Opal >

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.

Weird Worlds: Blue Crystal

< Rust | Introduction to Xenobiology | Vitrium >

Blue Crystal is a cold, high-albedo world world, slightly smaller than Earth, orbiting an F-class dwarf star. Despite orbiting a hotter star than our own Sun, its wide orbit means that Blue Crystal receives only 1/14th as much sunlight, and has an average surface temperature of -150C. The entire surface is covered in crystal-clear oceans of liquid nitrogen and tinged blue by dissolved liquid oxygen, with trace amounts of dissolved water and carbon dioxide, under an atmosphere of cold nitrogen gas. The solid surface is largely covered in "minerals" composed of dirty water and CO2 ices, with the occasional siliceous mountain peaks poking through.

Those occasional siliceous peaks and periodic volcanic eruptions, along with the UV flux from its sun, are the key to life on Blue Crystal, as they are sources from which silanes--silicon-based analogs of hydrocarbons--dissolve into the liquid nitrogen sea.

Hydrosilicons are unstable in Earthlike conditions, but that makes them perfect for the Blue Crystal--while carbon-based organic chemistry reactions take far too long to support life in cryogenic conditions, silanes have just the reactivity needed to keep operating, without decomposing in excessive heat, and they dissolve relatively well--as well as anything does in a cryosolvent--in LN2.

Despite the cold temperatures and dim light, the Blue Crystal still supports a thriving, high-energy ecosystem. How? Well, the cold temperatures--only about half as hot as Earth--make most metabolic reactions more thermodynamically efficient. Heterotrophs make more efficient use of their food than Earthlings, and photosynthesis is about 5 times more efficient at capturing solar energy than on Earth. Additionally, the accessibility of d-orbitals, which enables higher coordination numbers for silicon compounds than for carbon, enables lower-energy pathways for fundamental biocatalyzed reactions like hydrolysis and ammonolysis. Thus, the total power available to the biosphere is about 1/3 as much as on Earth, rather than the 1/14th that might be naively predicted from simple insolation--and it needs less power to accomplish the same results as well.

Blue Crystal autotrophs operate surprisingly similarly to Earth plants, using CO2 as a carbon source and water as a hydrogen source--except they also chemically attack silica (SiO2) via alkaline hydroxyl and amide groups for an inorganic silicon source, using much more of it than carbon; and they release liquid, rather than gaseous, oxygen (not that it makes much difference when the oxygen simply mixes freely with the nitrogen sea, just as oxygen gas mixes freely with Earth's primarily-nitrogen atmosphere). Also unlike Earth life, the bulk biosolvent doesn't participate in much chemistry at all! LN2 is an inert bystander to the major metabolic reactions.

While LN2 makes up the majority of intracellular fluids and the forms the basis for extracellular blood, lymph, and sap analogs, it is not the only, nor even the most important, biochemically active solvent in Blue Crystal biology. While LN2 will dissolve hydrosilicons, thus leading to hydrosilicon polymers forming the major basis of Blue Crystal life, in objective terms it is not a very good solvent; that is why Blue Crystal looks the way it does, with its eponymous crystal-clear seas--aside from liquid oxygen, there just isn't that much stuff dissolved in the oceans to absorb or scatter light. Thus, just like Earth life uses fats to store certain molecules that are not water-soluble but are fat-soluble, Blue Crystal life uses mixtures of a variety of simple hydrocarbons and carbohydrates as alternative cryosolvents--and in fact, just as life on Earth is sometimes theorized to have originated around volcanic vents, it is thought that life on the Blue Crystal may have originated in cryovolcanic regions with higher temperatures (between -80C and -100C) where carbohydrates like ethanol are still liquid, before adapting to the larger LN2-dominated ocean environment. Blue Crystal lifeforms have been observed to use various mixtures of methanol, ethanediol, glycerol, acetone, methoxyethanol, and dimethylformamide as organelle-specific auxiliary solvents, which represent the majority of carbon usage by these organisms. None of these chemicals remain liquid in pure form at typical Blue Crystal temperatures, but eutectic mixtures of them do. This endogenous production of secondary organic solvents to augment LN2 chemistry has led some xenobiologists to formally classify Blue Crystal as a desert world, but due to the critical function of LN2 in Blue Crystal biology, this categorization is widely disputed. 

Cellular membranes on Blue Crystal are formed primarily from azotosilanes, with weakly polar CN nitrile groups accumulating on the interior of a bilayer, and non-polar polysilane tails extending into the LN2 solvent--effectively the inverse of Earthling phospholipid bilayers, and similar to the azotosomes found on methane worlds. Organic solvent vesicles, on the other hand, are bounded by monolayers mediating between the weakly polar interior and non-polar exterior.

Simple microbial heterotrophs on Blue Crystal come in two varieties: anaerobic, and aerobic, just like on Earth. Anaerobic microbes get energy from simply decomposing silanes entirely, producing crystals of pure silicon and releasing hydrogen (and the occasional bit of silica, CO2, and water, when breaking down more complex molecules). There's nothing like Earthling methanogens, because, unlike methane, SiH4 takes energy to produce, rather than releasing energy when produced. Aerobic microbes breathe in liquid oxygen and release dissolved water and silica crystals.

Multicellular creatures, on the other hand, can't afford to pollute their cells with hard-to-excrete solid silica; as an Cannonball, the development of complex multicellular life was contingent on evolving means of eliminating solid crystalline metabolic wastes. Multicellular organisms on Blue Crystal get almost all of their energy by simply desaturating silanes, producing water and disilyne (H2Si2, along with other low-saturation hydrosilicons) as waste products, with disilyne serving as an additional source of slightly-more-available "organic" silicon for autotrophs, and as a food source for crystal-bearing microbes. While waste water is also solid in bulk form at Blue Crystal temperatures, its greater reactivity compared to silica allows it to be solvated and transported by specialized carrier molecules, much like oxygen and CO2 are bound for transport by hemoglobin and hemocyanin in Earthling animals. Note that there is no direct analog of hemoglobin as an oxygen carrier; such molecules are unnecessary in Blue Crystal life, as liquid oxygen is fully miscible with LN2.

In addition to the increased d-orbital coordination options afforded to silicon compared to carbon, silicon as a polymer backbone element also allows for greater electron delocalization through sp3 hybridization. This is an important feature in Blue Crystal biology which accommodates for the aprotic nature of Blue Crystal's biosolvents, which precludes the use of the proton gradients that are so ubiquitous in Earthling life as a central feature of energy transfer. Charge-separation chemistry in Blue Crystal life is thus accomplish intramolecularly, using large semiconductive polysilanes to shift electrons between reaction centers. Similarly, the function of ATP in Blue Crystal energy metabolism is filled by disilene (which is also an extremely common substrate molecule for synthesis reactions and intermediary product of fermentation), with energy stored and released through conformation alterations rather than bond formation and breakage.

While the details of the complete anabolic and catabolic ecological cycles are rather complex (just as they are on Earth), the reactions of the major energy-storage cycles, analogous to the glucose cycle on Earth, are as follows:

Silanologenic Photosynthesis: 2 SiO2 + 4 H2O -> 2 H3Si(OH) + O2
Silylsilanogenic Photosynthesis: 4 SiO2 + 6 H2O -> 2 H6Si2 + 7 O2
Silanol Fermentation: 2 H3Si(OH) -> H4Si2 + 2 H2O
Partial Disilene Respiration: 2 H4Si2 + O2 -> 2 H2Si2 + 2 H2O
Partial Disilane Respiration: 2 H6Si2 + 3 O2 -> 2 H2Si2 + 6 H2O
Disilyne Respiration (exclusive to microbial organisms): 2 H2Si2 + 3 O2 -> 2 SiO2 + 2 H2O

Additionally, while CO2 is the primary source of inorganic carbon for Blue Crystal autotrophs (used to produce endogenous organic solvents and carbon heteroatoms for complex silicon polymers), just like Earth, it represents very little of the total free oxygen that they release, compared to the breakdown of silica and water. This is because much of the oxygen split from silica, and nearly all of the oxygen derived from CO2, is incorporated as heteroatoms in biomolecules. Overall, Blue Crystal life uses far more oxygen, and peroxo / peroxide groups in particular, than Earth life does. Just like the stability of silanes, this comes down to the temperatures at which Blue Crystal life operates. Peroxide groups are highly reactive in Earthlike conditions--but the disulfide bridges that are common in Earthling proteins are overly rigid and unreactive in Blue Crystal conditions!

In addition to its role in Blue Crystal metabolic cycles, water is also a significant structural molecule on Blue Crystal, employed at all scales as a skeletal material. Diatom-like microbes form shells from pure water crystals; many autotrophs and sessile heterotrophs (analogs of Earthling fungi) augment their cell walls with water ice networks, analogously to the incorporation of amorphous silica into the cell walls of Earthling plants; and bulk water ice, used as a matrix for a composite material with silicon biopolymers, functions similarly to calcium carbonate and hydroxyapatite in Earthling biology to form tough, macroscopic shells and bones, with properties similar to pykrete.

Obviously these structure cannot exist under Earthlike conditions; Blue Crystal bones would literally melt. However, they are far from the most significant limiting factor when it comes to studying Blue Crystal life. As previously stated, most components of Blue Crystal biology are extremely unstable under Earthlike conditions, and Blue Crystal cells will begin to decompose and auto-oxidize well below the freezing point of water. Transport off-world requires constant refrigeration and maintenance of pressures in excess of 30 atmospheres. Direct human contact with the cryogenic organisms of Blue Crystal is infeasible under conditions that remain non-lethal for both parties, so most research must be conducted in-situ by robotic remotes.

The inspiration for this one originally came from from Jack Chalker's Wellworld series (<- Amazon affiliate link), in which one of the mysterious northern hemisphere environments is the "ocean of oxygen". Basic chemical information was derived from the paper Many Chemistries Could Be Used to Build Living Systems by William Baines. The description of azotosilane membranes is an extrapolation from the computational chemistry work from Can polarity-inverted membranes self-assemble on Titan?, based on the observation that silanes generally have lower freezing points than alkanes, and so should be less susceptible to falling into the solid crystalline state rather than the liquid crystal membrane state.

Weird Worlds: Rust

< Cannonball | Introduction to Xenobiology | Blue Crystal >

Rust is a near-copy of Mars, only slightly larger (with a surface gravity of approximately 0.5g), more water (mostly in the form of ice), and a thicker CO2-dominated atmosphere, resulting in less extreme temperature variations between the tropics and poles. Like Cannonball, Rust is a desert world, but the differing properties of the local biosolvent produce significantly different pressures on Rust's native life.

Earlier in its history, Rust was much more Earthlike, and it was in this environment that life of Rust originated. As such, Rust biochemistry is in many ways not unusual from an Earthling perspective, following the typical template of carbon-based, water-solvent life. Examples of this primitive type of life can still be found in high-salinity subterranean reservoirs, but as the world desiccated, cooled, and oxidized, surface life developed a number of biochemical innovations to survive in the new environment, which is rapidly fatal to most Earthling organisms.

The most significant of these adaptations are the biogenic production of hydrogen peroxide and perchlorate ions, each of which serves multiple biochemical purposes. Both of these chemicals are strongly hygroscopic, and sodium perchlorate is particularly concentrated in the surface tissues of multicellular autotrophic organisms (but not heterotrophs), permitting efficient scavenging of scarce water vapor from the atmosphere to replenish intracellular fluids. The need to scavenge small quantities of water vapor from the air, as well as carbon dioxide, combined with the low vapor pressure of the salty water/peroxide solution that is the basis of intracellular fluids for surface life, leads to evolutionary pressures to develop large surface area leaf structures despite the global desert environment.

Both chemicals, hydrogen peroxide and sodium perchlorate, also serve as antifreezes and glassification agents, allowing metabolic processes to continue in liquid down to temperatures between -55 and -70 Celsius, and for organisms to survive freezing solid at lower temperatures. This glassification ability is critical for nearly the entire biosphere on a annual basis, when CO2 snow in the polar highlands during northern-hemisphere winter reduces atmospheric pressures and temperatures across the globe, shutting down all surface biological activity for several months until northern summer restores the atmosphere. And in fact, intracellular eutectic solutions are typically over 60% hydrogen peroxide, suggesting that it is in fact hydrogen peroxide, rather than water, that should be identified as the primary biosolvent of Rust life forms. These high concentrations of hydrogen peroxide not only permit survival at low temperatures, but actually restrict these organisms to survival at low temperatures, such that the high oxidative reactivity of peroxide can be controlled; most Rust organisms will auto-oxidize at temperatures approaching 0C, and those which can survive higher temperatures do so by pre-emptively desiccating themselves and entering a cryptobiotic tun state. Additional stabilizing compounds, primarily pyrophosphates and aromatic amides, are also present in high concentrations to control peroxide activity.

All surface organisms also make extensive use of pigmentation molecules to protect their intracellular fluids from photolysis. Similar to life on Cannonball, eyes on Rust are restricted in structure to pin-hole and reflective structures, like pit vipers' IR sensing organs, avoiding the need for light to transit a liquid-filled structure where it might induce photolysis of peroxide. Also like Cannonball, photosynthetic organisms are unable to use internal organelles to harvest photons, with photoelectric pigments analogous to opsins and bacteriochlorophylls directly embedded in the outer membranes of surface cells. The evolution of animal-analog organisms on Rust also parallels the evolutionary history of Cannonball, tracing back to dictyostelid-like slugs; skeletal structures are much more similar to what we find on Earth, however, being composed of mineralized nitrogenated complex carbohydrates.

In addition to acting as an endogenous biosolvent, hydrogen peroxide also fills the function of the primary metabolic oxidizer, analogous to oxygen in the Earthling biospheres. Like Earthling plants, Rust autotrophs use water as a hydrogen source for synthesizing carbohydrates, but, rather than producing gaseous oxygen to release into the atmosphere, instead use the excess oxygen to convert additional water into hydrogen peroxide. While excess oxygen is occasionally released into the environment, leading to a trace amount detectable in Rust's atmosphere, its primary biological sink is in re-uptake to produce additional peroxide, rather than direct oxidation of food.

Thus, rather than breathing in oxygen from the air, Rust's heterotrophs eat or drink hydrogen peroxide to replenish their fluids and provide oxidative capacity. In famine conditions, heterotrophs can directly decompose hydrogen peroxide and release free oxygen to produce energy, much like Cannonball's lifeforms get most of their energy from decomposition of their own endogenous biosolvent, but oxidation of food molecules to produce CO2 is of course more normal and and considerably more efficient; due to the ubiquity of oxidative potential, anaerobic respiration is completely absent in the surface biosphere, and surface life seems to completely lack genes for either anaerobic respiration or fermentation. Both existing forms of energy production (aerobic respiration and peroxide decomposition), however, produce water as a byproduct, which must be eliminated to maintain the proportion of hydrogen peroxide in the body and keep intracellular fluids liquid. This is why, unlike autotrophs, heterotrophs are not observed to concentrate perchlorates in their outer layers--all of their water is either acquired in food or generated endogenously, and attracting excess is in fact dangerous for them. The resulting water-rich ices which form when heterotroph waste products freeze are strong fertilizers for the growth of autotrophic organisms, with root absorption of water allowing much faster growth than relying solely on atmospheric scavenging.

While the Rust environment is generally inimical to Earthling life, Earthling-suitable environments tend to be much more immediately lethal to Rust life forms, due both to the higher temperatures and the high availability of water vapor in our atmosphere. Surface-adapted Rust life in fact has a very violent reaction to exposure to liquid water, with osmotic pressure destroying their cells and the resulting reduced concentration of stabilizer molecules resulting in the rapid decomposition of hydrogen peroxide. This results in rising temperatures, which speeds the reaction and typically leads to rapid auto-oxidation. Even when it is possible to interact directly with Rust life given arctic clothing and an oxygen mask, it is thus strongly recommended that human researchers never touch any large life form directly, to minimize the risk to the native life form of being burned to ash, and to the human of being splashed with boiling water and hydrogen peroxide!

Glassified and tun-state organisms, however, are considerably more resistant to water, and can also survive vacuum conditions and increased radiation exposure. This makes it remarkably easy to transport Rust native organisms for study off-world.

The inspiration for this world came from the 2007 papers A possible biogenic origin for hydrogen peroxide on Mars and The hydrogen peroxide-water hypothesis for life on Mars and the problem of detection.

Introduction to Xenobiology

Back in November of 2021, I had plans for NaNoWriMo.

I did not fulfill my plans for NaNoWriMo.

Instead, I started writing a series of fictional-non-fiction articles, like The Transgalatic Guide to Solar System M-17 or Wayne Barlowe's Expedition, detailing the exotic biochemistries of alien worlds. I had aimed to get through 12 such articles; I actually managed 9. Now, I am revising those originals and working on adding more to the series, to be posted here on my blog.

Chronological Listing

Each article is mostly self-contained, but later articles may refer back to earlier ones with interesting parallels or contrasts.

1. Cannonball
2. Rust
3. Blue Crystal
4. Vitrium
5. Opal
6. Still
7. Coal
8. Snowball
9. Nicar
10. Oxio
11. Brimstone
12. Fornax
13. Cronus

Categories

Desert Worlds

    Desert worlds may have large bodies of liquid, on or under the surface, but are characterized by a lack of environmentally-accessible liquid or supercritical biosolvent. Most desert worlds feature organisms which can produce their own biosolvent internally from environmental materials, but some may rely on melting or condensing it when the necessary molecule is accessible, but environmental conditions do not permit its existence in liquid form.

1. Cannonball
2. Rust
3. Snowball

Cryogenic Worlds

    Cryogenic worlds are those on which the native lifeforms exist in temperatures at which pure liquid water cannot exist, universally employing antifreeze mixtures or non-water solvents.

1. Rust
2. Blue Crystal
3. Still
4. Coal
5. Snowball
6. Nicar
7. Cronus

Silicon Worlds

    Silicon worlds are those on which the native biochemistry makes extensive use of silicon as a polymeric backbone atom. This might popularly be termed "silicon-based life"; however, most silicon-using biospheres also make extensive use of carbon chemistry, or highly heterogenous polymer bases, making any strict boundaries between "silicon-based" and "carbon-based" life very unclear. Silicon worlds are a notable category mainly in contrast to "aqueous worlds", rather than "carbon-based biology worlds" ("carbon world" having an entirely unrelated technical definition), as it is the presence of reactive liquid water, rather than competition from carbon chemistry, which limits the possibilities for complex silicon chemistry on such worlds.

1. Blue Crystal
2. Vitrium
3. Fornax

Sulfur Worlds

    Sulfur worlds are those on which large quantities of sulfur have been concentrated on the surface, resulting it its integration into solvent molecules (H2SO4, SO2, or elemental sulfur) and an expanded role in biochemistry.

1. Vitrium
2. Oxio
3. Brimstone

Non-Polar Solvents

    Biological systems relying exclusively on a single solvent system are extremely rare; just as Earthling life, which relies on polar water, nevertheless employs many molecules which are soluble only in the non-polar lipophillic phase, and accumulates non-polar fats and oils in vesicles, so do biosystems based on non-polar solvents frequently rely on polar chemistry. Nevertheless, the biosystems of the following worlds rely primarily on non-polar solvents, just as Earthlings rely primarily on water.

1. Cannonball
2. Blue Crystal
3. Opal
4. Brimstone
5. Cronus