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.