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.