Solvents of Life
Why water and ammonia can support life, and why they don't mix
Spoiler warning: this page discusses the novel's plot up to this chapter. It doesn't quote passages from the book.
Grace and Rocky can't breathe the same air, touch the same materials, or share the same space without a hermetic seal. It's not by choice: their life chemistries are mutually destructive. Water and ammonia are the solvents of two civilizations that must collaborate without touching.
Why water is the perfect solvent for Earth life
Grace can't breathe ammonia. Rocky would die on contact with water. The xenonite seal in the tunnel is the only thing separating them from mutual death.
The science behind it
A biological solvent must meet several requirements: be liquid over a useful temperature range, be polar (to dissolve charged salts and proteins), form hydrogen bonds (to structure large molecules like DNA), and be abundant in the universe.
Water meets all these criteria almost perfectly. Its structure of two hydrogen atoms and one oxygen creates a polar molecule with a positive face and a negative face. This allows it to surround and dissolve practically any electrically charged molecule.
Water's hydrogen bonds structure DNA (the two strands are held together by H-bonds), proteins (proper protein folding depends on H-bonds with water), and cell membranes (phospholipids organize into bilayers to hide their hydrophobic parts).
Without water, biochemistry as we know it falls apart. Every enzymatic reaction, every cellular signal, every DNA replication occurs in an aqueous environment.
Key terms
Try it yourself
How to use the simulator
All the biochemistry we know happens in water, but life on other worlds might use a different liquid. The comparator sets water against two commonly proposed alternatives, ammonia and methanol, on the properties that matter most for life: the temperatures over which they stay liquid, how well they dissolve things, whether they form hydrogen bonds and how common they are in the universe.
Controls
- Solvent buttons: pick water, ammonia or methanol to see its profile.
- Property buttons: choose a property to compare all three on it.
Things to try
- Compare liquid ranges at 1 atm: water stays liquid across 100°C, ammonia across only about 44°C, and at very low temperatures.
- Look at polarity and hydrogen bonding. They're what let a solvent dissolve salts and build structures like membranes.
- Remember that pressure shifts the boiling point but barely moves the melting point. Higher pressure widens the liquid range only at the top.
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Open “Life solvent comparator” on its own page, with an explanation and a worked example →
Ammonia: the Eridians' water
Rocky breathes ammonia at 29 atmospheres and 210°C. For him, ammonia is as natural as water for Grace. His entire biochemistry is built on a different solvent.
The science behind it
Ammonia (NH₃) has surprisingly similar properties to water: it's polar, forms hydrogen bonds, and can act as a solvent for salts and organic molecules. At 1 atmosphere, ammonia is a gas (boils at -33°C), but at high pressure its boiling point rises considerably.
At 29 atmospheres (the pressure in Rocky's environment), ammonia only stays liquid up to about 65°C, and above 132.4°C (its critical temperature) it can't be liquid at any pressure. At the 210°C of Rocky's environment it would be a supercritical fluid, dense but with no liquid surface. This is a bit of licence on the novel's part: under those conditions the most plausible liquid solvent would be water, which stays liquid at 29 atmospheres up to about 232°C.
Ammonia-based life would require a completely different biochemistry from Earth's. Chemical bonds, enzymes, cell membranes — everything would be optimized to operate in an ammonia environment rather than water. It's not an "inferior" version of life, but an equally valid alternative solution to the same problem: how to build and maintain complex molecular structures.
The fact that both Earth life (water-based) and Eridian life (ammonia-based) developed similar complexity suggests that complex biochemistry can emerge in different solvents, as long as they have appropriate physical-chemical properties.
Key terms
Chemical incompatibility: why they can't touch
Grace and Rocky build the xenonite tunnel precisely because any direct mixing would be catastrophic. Grace's air is poison to Rocky and vice versa.
The science behind it
Ammonia and water don't just differ as solvents — they react with each other. Ammonia is a base (accepts protons); water acts as a weak acid (donates protons). The reaction NH₃ + H₂O → NH₄⁺ + OH⁻ releases energy and drastically alters pH.
For an ammonia-based organism, contact with water would be like bathing in strong acid: its entire biochemistry is tuned for a very specific pH range and conditions. The presence of water would break the bonds maintaining its proteins and cell membranes.
Likewise, ammonia at the concentrations and pressures of Rocky's environment would immediately destroy Grace's proteins. The pH inside her cells would change radically; enzymes would denature and stop functioning within seconds.
The xenonite solution is not just engineering: it's the only physical possibility for two mutually incompatible life forms to collaborate. The strongest material in the known universe, holding the border between two chemically opposite worlds.