---
title: "Hypothetical types of biochemistry"
chunk: 5/7
source: "https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:51:22.917946+00:00"
instance: "kb-cron"
---

=== Methane and other hydrocarbons ===
Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. It has a cosmic abundance comparable with ammonia. Hydrocarbons could act as a solvent over a wide range of temperatures but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.
There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. A life-form which solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.
Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present. The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.

==== Azotosome ====
A hypothetical cell membrane termed an azotosome, able to function in liquid methane in Titan conditions was computer-modelled in an article published in February 2015. Composed of acrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water. An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere. Later studies questioned whether acrylonitrile would be able to self-assemble into azotosomes. However, in 2025 a new mechanism was proposed by scientists Christian Mayer and Conor Nixon to overcome the previous barriers to self-assembly of azotosomes in liquid methane, based on 'splashing' of a methane lake surface film by a hydrocarbon raindrop.

=== Hydrogen fluoride ===
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At atmospheric pressure, its melting point is 189.15 K (−84.00 °C), and its boiling point is 292.69 K (19.54 °C); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbour molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath and Carl Sagan.
HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it. Like water and ammonia, liquid hydrogen fluoride supports an acid–base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF.
However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.

=== Hydrogen sulfide ===
Hydrogen sulfide is the closest chemical analog to water, but is less polar and is a weaker inorganic solvent. Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there. On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live on sulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.