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.