Weird Worlds: Rust

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Rust is a near-copy of Mars, only slightly larger (with a surface gravity of approximately 0.5g), more water (mostly in the form of ice), and a thicker CO2-dominated atmosphere, resulting in less extreme temperature variations between the tropics and poles. Like Cannonball, Rust is a desert world, but the differing properties of the local biosolvent produce significantly different pressures on Rust's native life.

Earlier in its history, Rust was much more Earthlike, and it was in this environment that life of Rust originated. As such, Rust biochemistry is in many ways not unusual from an Earthling perspective, following the typical template of carbon-based, water-solvent life. Examples of this primitive type of life can still be found in high-salinity subterranean reservoirs, but as the world desiccated, cooled, and oxidized, surface life developed a number of biochemical innovations to survive in the new environment, which is rapidly fatal to most Earthling organisms.

The most significant of these adaptations are the biogenic production of hydrogen peroxide and perchlorate ions, each of which serves multiple biochemical purposes. Both of these chemicals are strongly hygroscopic, and sodium perchlorate is particularly concentrated in the surface tissues of multicellular autotrophic organisms (but not heterotrophs), permitting efficient scavenging of scarce water vapor from the atmosphere to replenish intracellular fluids. The need to scavenge small quantities of water vapor from the air, as well as carbon dioxide, combined with the low vapor pressure of the salty water/peroxide solution that is the basis of intracellular fluids for surface life, leads to evolutionary pressures to develop large surface area leaf structures despite the global desert environment.

Both chemicals, hydrogen peroxide and sodium perchlorate, also serve as antifreezes and glassification agents, allowing metabolic processes to continue in liquid down to temperatures between -55 and -70 Celsius, and for organisms to survive freezing solid at lower temperatures. This glassification ability is critical for nearly the entire biosphere on a annual basis, when CO2 snow in the polar highlands during northern-hemisphere winter reduces atmospheric pressures and temperatures across the globe, shutting down all surface biological activity for several months until northern summer restores the atmosphere. And in fact, intracellular eutectic solutions are typically over 60% hydrogen peroxide, suggesting that it is in fact hydrogen peroxide, rather than water, that should be identified as the primary biosolvent of Rust life forms. These high concentrations of hydrogen peroxide not only permit survival at low temperatures, but actually restrict these organisms to survival at low temperatures, such that the high oxidative reactivity of peroxide can be controlled; most Rust organisms will auto-oxidize at temperatures approaching 0C, and those which can survive higher temperatures do so by pre-emptively desiccating themselves and entering a cryptobiotic tun state. Additional stabilizing compounds, primarily pyrophosphates and aromatic amides, are also present in high concentrations to control peroxide activity.

All surface organisms also make extensive use of pigmentation molecules to protect their intracellular fluids from photolysis. Similar to life on Cannonball, eyes on Rust are restricted in structure to pin-hole and reflective structures, like pit vipers' IR sensing organs, avoiding the need for light to transit a liquid-filled structure where it might induce photolysis of peroxide. Also like Cannonball, photosynthetic organisms are unable to use internal organelles to harvest photons, with photoelectric pigments analogous to opsins and bacteriochlorophylls directly embedded in the outer membranes of surface cells. The evolution of animal-analog organisms on Rust also parallels the evolutionary history of Cannonball, tracing back to dictyostelid-like slugs; skeletal structures are much more similar to what we find on Earth, however, being composed of mineralized nitrogenated complex carbohydrates.

In addition to acting as an endogenous biosolvent, hydrogen peroxide also fills the function of the primary metabolic oxidizer, analogous to oxygen in the Earthling biospheres. Like Earthling plants, Rust autotrophs use water as a hydrogen source for synthesizing carbohydrates, but, rather than producing gaseous oxygen to release into the atmosphere, instead use the excess oxygen to convert additional water into hydrogen peroxide. While excess oxygen is occasionally released into the environment, leading to a trace amount detectable in Rust's atmosphere, its primary biological sink is in re-uptake to produce additional peroxide, rather than direct oxidation of food.

Thus, rather than breathing in oxygen from the air, Rust's heterotrophs eat or drink hydrogen peroxide to replenish their fluids and provide oxidative capacity. In famine conditions, heterotrophs can directly decompose hydrogen peroxide and release free oxygen to produce energy, much like Cannonball's lifeforms get most of their energy from decomposition of their own endogenous biosolvent, but oxidation of food molecules to produce CO2 is of course more normal and and considerably more efficient; due to the ubiquity of oxidative potential, anaerobic respiration is completely absent in the surface biosphere, and surface life seems to completely lack genes for either anaerobic respiration or fermentation. Both existing forms of energy production (aerobic respiration and peroxide decomposition), however, produce water as a byproduct, which must be eliminated to maintain the proportion of hydrogen peroxide in the body and keep intracellular fluids liquid. This is why, unlike autotrophs, heterotrophs are not observed to concentrate perchlorates in their outer layers--all of their water is either acquired in food or generated endogenously, and attracting excess is in fact dangerous for them. The resulting water-rich ices which form when heterotroph waste products freeze are strong fertilizers for the growth of autotrophic organisms, with root absorption of water allowing much faster growth than relying solely on atmospheric scavenging.

While the Rust environment is generally inimical to Earthling life, Earthling-suitable environments tend to be much more immediately lethal to Rust life forms, due both to the higher temperatures and the high availability of water vapor in our atmosphere. Surface-adapted Rust life in fact has a very violent reaction to exposure to liquid water, with osmotic pressure destroying their cells and the resulting reduced concentration of stabilizer molecules resulting in the rapid decomposition of hydrogen peroxide. This results in rising temperatures, which speeds the reaction and typically leads to rapid auto-oxidation. Even when it is possible to interact directly with Rust life given arctic clothing and an oxygen mask, it is thus strongly recommended that human researchers never touch any large life form directly, to minimize the risk to the native life form of being burned to ash, and to the human of being splashed with boiling water and hydrogen peroxide!

Glassified and tun-state organisms, however, are considerably more resistant to water, and can also survive vacuum conditions and increased radiation exposure. This makes it remarkably easy to transport Rust native organisms for study off-world.

The inspiration for this world came from the 2007 papers A possible biogenic origin for hydrogen peroxide on Mars and The hydrogen peroxide-water hypothesis for life on Mars and the problem of detection.