Weird Worlds: Blue Crystal

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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.