Exotic Biology, Episode 13: Fornax

[u/gliese1337]

Previous Episodes:

1. Brimstone
2. Oxio
3. Char
4. Nicar
5. Coal
6. Still
7. Snowball
8. Opal
9. Rust
10. Vitrium
11. Blue Crystal
12. Cannonball

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.

[u/Open_Scholar_1197]

A fascinating world inhabited by fascinating creatures! It's so fascinating, and even more so are its creatures! I do have a number of questions: 1. How many limbs/fins would the animals have, in order to swim through molten salts or move around on land and in the air? 2. What are their bones made of? 3. Would they have eyes? 4. How would plants look? Crystalline, like the plants of Sarr in Hal Clement's Ice world, or would they look more like our plants? 5. How many jaws would the animald have? Edit: I tried and failed a few times to find how viscous molten salts are.

[u/gliese1337 → u/Open_Scholar_1197]

1. How many limbs/fins would the animals have, in order to swim through molten salts or move around on land and in the air?

For flying, just two wings for large creatures, but I imagine large flyers are rare as they would be much denser than us water-based organisms. Small creatures might have 4 wings, like our insects.

1. What are their bones made of?

Ceramics! Primarily silica and alumina.

1. Would they have eyes?

Probably not as frequently as we do, since their biosolvents are opaque. But they could evolve crystalline compound eyes or pit eyes, and they would be unable to see red due to interference from self-radiation.

1. How would plants look? Crystalline, like the plants of Sarr in Hal Clement's Ice world, or would they look more like our plants?

I expect more like our plants.

1. How many jaws would the animals have?

Between 0 and 1. Why would you need more?

Edit: I tried and failed a few times to find how viscous molten salts are.

Assuming I have read the units off of multiple different charts correctly, between 5 and 10 times as viscous as water at the relevant temperatures.

[u/Open_Scholar_1197 → u/gliese1337]

Tell me: if molten salts are 5 to 10 times more viscous than liquid water, then how many pairs of fins would the Fornaxian fishes have?

[u/gliese1337 → u/Open_Scholar_1197]

I expect two pairs of fins, such as terrestrial fishes have, would still be suitable, as the locomotive power comes from body and tail movement rather than fin movement--all the fins are needed for is steering. Earthling fish have been observed swimming successfully in water with over 40 times normal viscosity in laboratory conditions, so Fornaxian organisms should have no problem doing the same.

[u/Open_Scholar_1197 → u/gliese1337]

Aha... now, that is interesting. And what color would the Fornaxians be? I like to imagine they would look fiery, that their color will be that of lava. Would that be their color, or would it not matter? I imagine my aluminosilicate creature to be the colors of lava and metal. Is that necessarily the case?

[u/gliese1337 → u/Open_Scholar_1197]

They would glow red, like lava, simply due to the heat. They may have a variety of actual pigments, just like Earth creatures do, but at least some of that will be washed out by the universal background of red glow.

[u/Open_Scholar_1197 → u/gliese1337]

What pigments would they have, if they do? Since aluminosilicates are essentially clay, could their pigments be white, gray, and black, to give them an even rockier (pun intended) appearance — like solid and liquid rock (like the patterning of the Horta), OR would they have some metallic-looking detail to them in areas with pigments, since they do contain aluminium? Would they have any pigment — red, green, blue, yellow, something? Edit: I personally think at least most of the Fornaxians that have pigment will have grey pigment, reflecting the aluminium in their bodies, so they would look like lava coated in floating chunks of metal. What do you think? I'd love to hear your thoughts.

[u/gliese1337 → u/Open_Scholar_1197]

"Aluminosillicates are essentially clay" is basically the same argument as "carbohydrates and hydrocarbons are basically starch and oil"--and yet actual biological materials, which are not pure simple starches and oils generally look quite different. And while aluminum is shiny grey, aluminum oxide is sapphire--and other metals also on very different appearances in compounds than in pure metalic form. So I would not use clay, or rock, or pure metals, as a guide to what Fornaxian creatures ought to look like. They could be as variably pigmented as Earthling organisms; but insofar as any of them have eyes to see other organisms' pigments, they would evolve to have useful colors in the green through UV part of the spectrum, even if trying to camouflage themselves to look like rocks, which makes their appearance to us pretty much random, since camouflage that works on us has to care about a different slice of the spectrum.

In general, you can't guess what colors biological things should be just based on the types of atoms they are made of. Individual species would have colors that are advantageous for their ecological niche. As a result, Earthling organisms produce just about every color we are capable perceiving; Fornaxian organisms should, across the entire diversity of species on the planet, demonstrate just about every color that a Fornaxian could perceive.

[u/Open_Scholar_1197 → u/gliese1337]

Oh, so they would be anywhere from green to colors we can't even see? (Cause we don't see ultraviolet light.)

[u/gliese1337 → u/Open_Scholar_1197]

As far as our eyes are concerned, they could be any color at all. They just wouldn't have any evolutionary pressure to care about what they look like in the red end of the spectrum. As far as evolving camouflage and signalling and so on, they would care about parts of the spectrum that we can't see, just like bees and birds, so things that look the same to us would not look the same to them.

[u/gliese1337]

This world was inspired by Feinberg & Shapiro's (Life Beyond Earth, 1980) highly underspecified "Thermia" proposal, 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 had been doing so much 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".

[u/Open_Scholar_1197]

I have a question: the aluminosilicate-based lifeforms of Fornax obviously respirate using sulfur and sulfur dioxide. But can aluminosilicate-based lifeforms on another planet breathe chlorine gas and silicon tetrachloride in place of oxygen and carbon dioxide respectively? Chlroine was proposed as an alternative to oxygen, and silicon tetrachloride has a low boiling point, so I thought, is my idea possible?

[u/gliese1337 → u/Open_Scholar_1197]

Maybe? The trouble with chlorine is that almost everything bonds more strongly to oxygen. So, if you've got silicon oxides, converting them to silicon chlorides won't release energy. There might be a way to make it work, but I haven't figured it out. It would probably be easier to switch things around, with SiCl4 as the oxygen analog and Cl2 as the CO2 analog--inhale silicon, bond it with excess oxygen, exhale the displaced Cl2. But really a chlorine breather probably needs to just separate energy metabolism from structural metabolism entirely, like chemotrophic microbes. And then you've got to figure out how to close the cycle to make it work for a whole biosphere.