The Tunnel
Ammonia as a life solvent, radiative cooling, and contact engineering
Spoiler warning: this page discusses the novel's plot up to this chapter. It doesn't quote passages from the book.
The Eridians build a xenonite tunnel between both ships. Grace deduces they live in high-pressure, high-temperature ammonia, confirming life can use solvents other than water. Physical first contact is about to happen.
Ammonia as a life solvent
Grace deduces the Eridians live at high temperature and pressure. Ammonia is their "water."
The science behind it
Ammonia (NH₃) is liquid between -77°C and -33°C at atmospheric pressure. But under high pressure, its boiling point rises: at 29 atmospheres it boils at about 65°C. Above 132.4°C (its critical temperature) it can't be liquid at any pressure.
As a solvent, NH₃ can dissolve many compounds. It forms hydrogen bonds (like water, but weaker). Theoretically, it could support complex biochemical reactions.
Life needs a solvent where molecules move, interact, and react. Water is excellent, but not the only option. Other candidates: liquid methane (Titan), sulfuric acid (Venus), ammonia.
The Stefan-Boltzmann law (P = σAT⁴) lets you calculate how long an object takes to cool in vacuum. Higher temperature = faster radiation. If the cylinder was still hot after 40 minutes in vacuum, it must have started out very hot. Exactly how hot depends on its mass, surface area and material, which is what you can explore with the simulator.
Key terms
Try it yourself
How to use the simulator
In a vacuum there's no air to carry heat away, so a hot object can only cool by radiating, following the Stefan-Boltzmann law (P = σ·A·T⁴). Because power scales with the fourth power of temperature, a very hot object cools quickly at first and ever more slowly afterwards. The model uses a 2 kg metal cylinder with 0.1 m² of surface.
Controls
- Initial temperature: 50 to 500°C.
- Time in space: 1 to 120 minutes.
Things to try
- Fix the time at 40 minutes and try different starting temperatures. See how close the final results end up: initial differences shrink a lot over time.
- Compare the first 5 minutes with the last 5 of a 120-minute run: the drop is much faster at the start.
- Remember that the result depends on the object's mass, area and material, which the model fixes. A heavier object, or one with less surface, would stay hot much longer.
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Open “Radiative cooling in a vacuum” on its own page, with an explanation and a worked example →
The xenonite tunnel: engineering first contact
Eridians build a xenonite tunnel and glue it to the Hail Mary's aluminum hull with alien adhesive. Each side has its own atmosphere.
The science behind it
Bonding two completely different materials (alien xenonite + human aluminum) without knowing the other's exact composition implies an extraordinarily versatile adhesive.
The tunnel has a dividing wall supporting the pressure difference: 1 atm of oxygen (Grace's side) vs 29 atm of ammonia (Eridian side). Catastrophic failure would kill both.
Wall knocking ("knock, knock, knock") is the most primitive communication technology possible: requires no language, no electronics, just transmits "I'm here and I'm intelligent."
Key terms
Real science vs fiction: could ammonia support life?
This isn't something the novel made up: astrobiologists have suggested ammonia as an alternative to water for decades. It shares several of water's key traits. It's a polar molecule, it forms hydrogen bonds, and it dissolves many salts and organic compounds. It even dissolves alkali metals like sodium, which water can't do without reacting violently.
It has drawbacks too. Its dielectric constant (about 22 at its boiling point, against roughly 80 for water) is much lower, so it's worse at pulling charges apart and many salts dissolve less readily. At normal pressure it's only liquid over a narrow window, from −78°C to −33°C. That's a range of 44°C, compared with water's 100°C.
Pressure widens that window, but only up to a limit no amount of pressure can push past: the critical point. For ammonia it sits at 132.4°C and about 113 atmospheres. Above that temperature there's no longer any difference between liquid and gas, and ammonia becomes a supercritical fluid. Water, by contrast, stays liquid up to 374°C under enough pressure; at 29 atmospheres it remains liquid up to about 232°C.
That has an interesting consequence. If the Eridian environment is around 210°C, as chapters 14 and 15 describe, its ammonia atmosphere can't be liquid: it would be a dense gas or a supercritical fluid. Under those conditions the most plausible liquid solvent for cell chemistry would be water, not ammonia.
There are real clues in our own solar system. The Cassini probe detected ammonia in the plumes of Enceladus, and Titan's subsurface ocean is thought to be water mixed with ammonia, which acts as an antifreeze. Mixtures like these are now among the favourite places to look for life beyond Earth.
Key terms
Worked example: how thick does the tunnel wall need to be?
The Eridian side of the tunnel is at 29 atmospheres, with the vacuum of space outside. One atmosphere is 101,325 pascals, so the wall holds back about 2.94 million pascals (2.94 MPa). That's nearly 300 tonnes of force on every square metre.
For a thin-walled tube, the stress the material carries around its circumference (the hoop stress) follows a simple formula: σ = P·r / t, where P is the pressure, r the radius of the tube and t the wall thickness.
Take a tunnel 1 metre across (r = 0.5 m) with a 5 mm aluminium wall. The stress comes to σ = 2.94 MPa × 0.5 m / 0.005 m ≈ 294 MPa. A typical structural aluminium alloy (6061-T6) starts to deform permanently at around 275 MPa, so the wall would be right at its limit with no safety margin at all.
Engineers usually insist on a safety factor of at least 2, which means an aluminium wall would have to be over 1 cm thick. The divider in the middle has it tough as well: a disc 1 m across with a 28-atmosphere difference carries about 2.2 million newtons, the weight of some 225 tonnes. That's why the novel needs a material like xenonite, far stronger than any human alloy.