Weird Worlds: Cannonball

Introduction to Xenobiology | Rust >

Cannonball is a world with an enormous metallic core and very thin silicate mantle and crust--like a giant version of Mercury, orbiting a K-class orange dwarf star.

Large quantities of water can't exist on this world, because it is destroyed on geological timescales by interaction with iron. Neither can free oxygen--it doesn't matter how much of it might be produced by life, because there's always more iron to suck it up.

So, how does biology exist here at all?

Life on Cannonball originated in primordial oceans of iron and nickel carbonyl (Fe(CO)5 and Ni(CO)4) under an atmosphere or carbon dioxide, carbon monoxide, hydrogen sulfide, and nitrogen. Rather than pure carbon backbone chains, Cannonball's major biological macromolecules are based on organometallic iron coordination polymers carbon monoxide, cyanide, and thioformaldehyde subunits. Unlike Earth life, where water is the solvent and carbon dioxide the main source of building material, on Cannonball, Fe(CO)5 is both the solvent, the largest component of structural material, and a central component of energy metabolism! (Other metal carbonyls are also employed for a variety of biochemical roles; for example, dititanium carbonyls, which can carry 10, 11, or 12 CO groups, serve a similar role to hemoglobin and ATP in Earthling biochemistry, shuttling CO groups around the body and allowing rapid, reversible energy storage.)

Early life on Cannonball had access to large quantities of energy by breaking down the metal carbonyls that it lived in to extract carbon monoxide and perform the decomposition reaction reaction 2CO -> C + CO2. On its own, this reaction results in the "sooting out" of elemental crystalline carbon, which is effectively lost to the biosphere until being recycled by geological processes. Consuming atmospheric hydrogen sulfide, however, provides the raw materials for constructing thioformaldehyde monomers which retain the liberated carbon. The structural requirements for thioformaldehyde, however, do not exceed the amount of carbon produced by basic energy metabolism.

This basic energy production process, therefore, eventually resulted in an "inverse oxygen crisis"; rather than introducing a new poison into the environment (like elemental oxygen), Cannonball chemoautotrophs remove a critical resource from the environment--the liquid they live in! The steady retreat of lake shores as the lakes were converted into organometallic material, crystalline carbon, and CO2 resulted in early pressure to colonize dry land, where the next major revolution occurred: Cannonball "plants" learned to capture energy from sunlight in order to consume CO2 from the air and break down complex organometallic molecules in the soil in order regenerate carbon monoxide and hydrogen sulfide, with hydrogen sulfide being released as a waste product and carbon monoxide used to manufacture new iron carbonyl.

In its modern equilibrium state, Cannonball has become a dry desert world, with all of the primordial liquid having been consumed by early life.  Autotrophs take in CO2 from the air and iron, sulfur, carbon, and hydrogen from the soil and produce new "water" (iron carbonyl) and release hydrogen sulfide; while heterotrophs (fungus and animals-equivalents) feed on plants (or each other) to acquire all of their liquid, breathe in hydrogen sulfide, and breathe out carbon dioxide and carbon sulfides. The equilibrium life-processed atmosphere is a mix of inert nitrogen, carbon dioxide, hydrogen sulfide, and traces of hydrogen cyanide, escaped carbonyl vapor, CO, CS2, and CSO. Like Earthlings deprived of oxygen, multicellular Cannonball animals deprived of hydrogen sulfide end up dying--but not because they can't produce energy! They still can, but only at the cost of accumulating waste carbon crystals in their cells, which eventually causes irreparable damage analogous to freezing an Earthling and damaging their cells with ice crystals. It is thus essential that multicellular organisms over-produce carbon-sulfur compounds relative to their structural needs, which are excreted as waste.

As on Earth, fixation of atmospheric nitrogen is handled primarily by specialized microbes which live in symbiotic relationships with photo-autotrophs.

The structure of Cannonball organisms is dictated by the properties of their biosolvent to a much greater degree than that of water-based life is. The lack of environmental sources for Fe(CO)5, and the energy cost to autotrophs of manufacturing it, mean that all photoautotrophs on Cannonball face similar challenges to desert plants on Earth, and are subject to convergent evolution in forms to minimize surface area, retain liquid against a high vapor pressure, and protect themselves from herbivores. The same, of course, is true of Cannonballs "animals", which are subject to similar constraints are Earth's desert animals in terms of their need to conserve scarce supplies of liquid.

Unlike the water used by Earthling organisms, however, iron carbonyl introduces additional unique constraints. In particular, it is neither colorless in the visible spectrum, nor completely stable against photodegradation. Thus, no organs--like leaves or eyes--rely on transmission of light through a liquid medium. Photosynthetic surfaces are not obviously distinctive on casual inspection, but they are universally opaque, and microscopic analysis reveals that light-capturing molecular complexes are directly embedded in the outermost cellular membranes, rather than being contained in internal organelles. Eyes on Cannonball are most similar to the infrared-sensing organs of Earthling pit vipers. The most advanced eyes found in Cannonball organisms are arranged like reflecting telescopes, with a pinhole aperture projecting light onto a reflective back surface, which focuses light onto a retina at the front of the eye.

With the early destruction of the seas and conversion of Cannonball into a desert world, multicellular life could not develop in ways parallel to Earth. Rather, multicellular life had to develop directly on land. The evolution of mobile heterotrophs ("animals") in particular parallels the development of Earthling dictyostelid cellular slime molds. These organisms pull of the neat trick of being able to exist in both unicellular and multicellular modes. When food is plentiful, they exist as individual single-celled amoebae, which can move around on micro-scale surfaces by extending pseudopodia. However, when food becomes scarce, they aggregate into a multicellular pseudoplasmodium, which can be macroscopic in size (up to 4mm long) and moves in response to heat, light, and humidity in order to seek out a more suitable environment, engulfing bacteria and fungi along the way. Larger pseudoplasmodia are capable of moving more quickly, and with more efficiency in terms of slime production (due to lower surface to volume ratio); given a sufficient population in the absence of pre-existing animals, intraspecific competition for speed and resource efficiency, allowing some colonies to travel farther and faster to beat out others for access to new food sources. A similar organism became the common ancestor of all macroscopic animal life on Cannonball; in the absence of pre-existing animal life to compete with produced evolutionary pressure towards larger pseudoplasmodium sizes to support faster and longer-range movement, eventually producing a class of organisms like a minimally-differentiated slug with no distinct mouth, gut, or limbs, which feed by engulfing their food and directly absorbing prey cells through the skin.

Deposition of hematite and iron sulfide scales (similar to those of Earthling volcano snail) for light protection when moving about on the surface was another early adaptation shared by nearly all Cannonball animals. These biochemical pathways were duplicated for structural use in the development of both endoskeletons and exoskeletons in various animal lineages. Incorporation of other metals has produced a wide variety of biological steels and steel composites serving a wide range of structural needs in the Cannonball ecology.

Iron pentacarbonyl has a liquid range of -21C to 103C at standard pressure. Cannonball organisms thus exist in a very similar temperature range to that of Earthlings, and contact can thus be handled with little more than an atmospheric isolation suit. Just as carbon monoxide and iron carbonyls are highly toxic to humans, however, oxygen is equally toxic to Cannonball natives (carbonyls burn quite easily), so two-way isolation is essential. High UV exposure is also considerably more dangerous to them than it is to us; excessive exposure to blue or UV spectrum light does not merely result in burns and genetic damage; rather, it results in decomposition of carbonyl fluids, causing organisms to crystallize into a messy mixture of Fe2(CO)9, powdered iron, and carbon (plus elemental sulfur and other impurities), while exploding from the release of CO and CO2 gasses, along with traces of water and formaldehyde.

This world was inspired by a prompt from Stephen Gillett's World-Building (<- Amazon Affiliate link).