Weird Worlds: Fornax

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It has been said that "carbon chemistry is the chemistry of life, silicon chemistry is the chemistry of rocks". Of course, this is a severely oversimplified point of view, based on the limitations imposed on stable carbon chemistry by the high temperatures and ubiquity of water on Earth; many other biospheres incorporate silicon in a variety of ways. However, Fornax challenges this statement in another way: the chemistry of life and the chemistry of rocks are not necessarily entirely disjoint!

Fornax bears the highest-temperature biosphere currently known, and is likely near, if not at, the limits of what chemical biosystems can support. Fornax is in some ways a hybrid of our own solar system's Venus and Mercury--closer in size to Venus, and with a non-negligible atmosphere, but with an elliptical orbit even closer than Mercury's, with a 3:2 spin-orbit synchronization. Daytime surface temperatures reach 900C, but can drop as low as 400C at night. Due to these temperature variations, which would be exacerbated by a 1:1 tidal lock, the orbital eccentricity and consequent lack of 1:1 tidal locking are critical for the survival of the Fornaxian biosphere. While photosynthetic biospheres can exist on water-based "eyeball" worlds, so long as there is sufficient water for atmospheric or glacial processes to continually replenish the dayside supply, the much lower-abundance materials which serve as Fornax's biosolvents would be expected to entirely freeze out on any similar worlds which have a permanent dark side.

Fornaxian life is based on an ionic liquid solvent system consisting of a mixture of metal chlorides and fluorides. The major metallic species are iron, calcium, sodium, magnesium, potassium, and copper. The surface atmospheric pressure is 0.52 bars, consisting of 0.3 bars of sulfur dioxide, 0.2 bars of nitrogen, 0.01 bars of carbon dioxide, and less than 0.01 bars each of sulfur, argon, aluminum trichloride, iron dichloride, zinc dichloride, copper (I) chloride, and silicon tetrafluoride.

Biochemistry in this environment is based on halogenated aluminosilicate polymers with additional metal ion and metal oxide functional groups. With carbon integrated only as a relatively rare heteroatom, this constitutes the most thoroughly silicon-based, as well as hottest, biosphere yet known. While tetravalent silica is roughly equivalent to carbon in Earthling biochemistry, aluminum serves a role similar to nitrogen, with regular aluminum substitutions in silicate polymers introducing local negative charge concentrations and trivalent structures. Such aluminum substitutions, creating replicable surface charge patterns, are a key feature of the clay-substrate hypothesis for the origin of prebiotic chemistry on Earth; it is an intriguing possibility that Fornax may be an example of a world where autocatalytic aluminosilicates were able to make the jump to full independently-evolving genetic and autocatalytic biosystem directly, rather than simply providing a prebiotic template for the development of more typical replicator systems. Oxygen functions similarly to hydrogen in colder biosystems, saturating excess silicon and aluminum valences and forming weak intermolecular bonds.

Like carbon, nitrogen and phosphorus are also occasional heteroatoms in Fornaxian biochemistry. As on Earth, phosphorus circulates almost entirely in the form of phosphate ions and nitrogen is regularly fixed from the atmosphere and returned to gaseous form when biological materials decay. The fixing of nitrogen is, however, simpler than the equivalent process in Earthling biology, as they high ambient temperatures significantly reduce the activation energy hill that must be overcome to split N2 molecules.

The dominant cycle for producer-consumer energy metabolism on Fornax is still based fundamentally on oxygen; autotrophs split oxygen from silicate and alumina groups to form aluminate anions, halogen-substituted silicates, and desaturated siloxanes. However, free oxygen at Fornaxian temperatures would rapidly recombine with any newly created reduced biomolecules. Thus, as on Oxio and Brimstone, autotrophic organisms consume sulfur (in this case, purely in gaseous form) to bind oxygen and reduce its chemical activity. Biogenic production of sulfur dioxide on Fornax is similar to biogenic production of oxygen on Earth, resulting in the high proportion of SO2 in the atmosphere. Heterotrophs inhale sulfur dioxide to oxidize silicon and aluminum(and occasionally other metals), and exhale waste sulfur, while also regenerating soluble halogen ions in the process. Substituting oxygen bonds for fluorine bonds, however, is not energetically favorable, as the silicon-fluorine bond is among the strongest chemical single bonds known; this leads to Si-F bonds being preserved in catabolic metabolism, eventually resulting in the release of waste silicon tetrafluoride gas and the depletion of soluble fluoride ions. Autotrophic organisms must therefore periodically expend energy to capture atmospheric SiF4, partially incorporating the bound fluorines into new biomolecules and returning the rest to solution.

Fornaxian conditions are immediately lethal to humans, and no practical mechanisms for supporting long-term habitation are available. All close-range exploration must be done by robotic remotes. However, the lowered temperatures of the Fornaxian night have resulted in most organisms developing resistance to freezing. This makes offworld transport of biological specimens for laboratory study surprisingly straightforward.

Commentary

This world was inspired by Feinberg & Shapiro's highly underspecified "Thermia" proposal (Life Beyond Earth, 1980), in which they suggest that silicates could form biomolecules above 1000C. However, Petkowski, Bains (of "Many Chemistries Could Be Used to Build Living Systems" fame), & Seager (in "On the Potential of Silicon as a Building Block for Life") point out that silica doesn't look so good for forming biological structures when all of the oxygen-network bonds are as labile as hydrogen bonds in a 1000-degree melt, and Feinberg & Shapiro didn't really explain what kind of distinction (if any) they had in mind between solvent and partially-solvated structures.

However, many salts, which can form ionic solvents, and silica-containing minerals, melt into solution at well below 1000 degrees, and others are solid up to much higher temperatures. Since I have been doing rather a lot of thinking about Hal Clement's Sarrians (who live at a mere 500 degrees), slightly lowering the operating temperature, introducing an ionic solvent, and throwing in sulfur to modulate oxygen metabolism seemed like obvious compromises to bring Thermia to "life".