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.
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