Triton Ices, Nitrogen and Propellant

The kit

Nitrogen, Methane, Carbon Monoxide, Carbon Dioxide, Water Ice and Tholins

Most of Pluto’s inventory, 15 years closer, and exposed rather than buried.

Voyager 2 identified nitrogen ice as the dominant surface material on Triton, with methane, carbon monoxide, carbon dioxide, water ice and tholins — the reddish organic polymers that ultraviolet light and charged particles make from simple carbon compounds. At 38 kelvin every one of them is a solid and stays one.

The one axis on which Triton clearly loses is the one that has been measured. New Horizons mapped Pluto’s nitrogen in July 2015. Nothing has mapped Triton’s.

Carbon dioxide is the item Pluto does not have, and it matters for anyone building rather than fueling — CO&sub2; is the feedstock for carbonate and carbon chemistry that CO and CH&sub4; do not supply on their own. Observed

The nitrogen gap

A Global Layer Between 100 Meters and 1 Kilometer Spans a Factor of 10

Voyager 2 mapped one hemisphere at low resolution in August 1989, and nothing has looked since.

Nothing distinguishes these three rows with present data. Voyager 2 imaged roughly one hemisphere, at resolution adequate for geology and inadequate for inventory, on a single pass on 25 August 1989. No mission to Triton is funded, planned or proposed on any agency roadmap.

Sublimating nitrogen ice from 38 kelvin costs about 275 kilojoules per kilogram, or 0.218 kWh/kg at 35 percent conversion efficiency — essentially the same as Pluto’s 0.217 at 40 K. The energy is not the question. The inventory is.

The atmosphere is not a resource

Triton’s Atmosphere Is a Science Object. The Surface Ice Is the Resource.

At 1.4 to 1.9 pascals it is roughly a hundred-thousandth of Titan’s.

Triton has an atmosphere, which is unusual for a body that size, and it is nitrogen in vapor equilibrium with the surface ice. It is also thin enough to be useless as an engineering tool. The surface pressure of 1.4 to 1.9 pascals gives a column mass of a few hundredths of a kilogram per square meter, against Titan’s total atmospheric mass of 9.24 × 10¹⁸ kilograms.

Nothing aerodynamic works. Descent and ascent are fully propulsive, at 1.45 km/s each way. There is no aerocapture at Triton itself — the aerocapture in the arrival budget happens at Neptune, not at the moon. There is no convective heat rejection of the kind that makes Titan the only outer-system surface where a radiator leaves the mass budget; waste heat at Triton goes out by radiator, into a 38 K sky that at least makes the radiator small.

The plumes are a different matter. Voyager 2 saw nitrogen geysers reaching kilometers above the surface, and whatever drives them — solid-state greenhouse warming of the nitrogen layer, or internal heat — is both the strongest scientific reason to go and a hazard to anything sited downwind of one. Observed

Propellant and water

Liquid Oxygen and Methane Are Available Without Synthesis

Both propellants are already on the ground as solids. Cracking them is where the energy goes.

Triton carries methane and water ice at the same surface, which means the two halves of a liquid-oxygen/methane propellant pair are both present without any Sabatier step or carbon-dioxide reduction of the kind Mars requires. Melt water, electrolyze it for oxygen, and take methane off the surface directly.

Electrolysis is an order of magnitude more expensive than liberating the water in the first place. The rule the belt analysis derived holds here with more force: ship water as water, crack it where the power is.

In the outer system the power is a reactor, so the rule becomes narrower — crack propellant only where a reactor already exists for another reason, which at Neptune means at the Triton installation and nowhere else. Every scoop sortie, every mass-driver shot and every ascent traces back to the same reactor, which is why the uncrewed baseline is a power-plant question before it is anything else. Modeled

Record

Provenance and Data Sources