In The Martian, stranded astronaut Mark Watney makes water by obtaining hydrogen from leftover hydrazine rocket propellant and reacting it with oxygen. The underlying chemistry is real, but doing it inside a crew habitat would be extraordinarily hazardous—and a practical Mars mission would almost certainly choose a safer water supply.
The stakes are immediate. Watney has some drinking water, but he needs far more to grow potatoes in a makeshift farm. Transporting each extra kilogram of water from Earth is expensive, and Mars offers no convenient faucet. His solution illustrates both the power and the limits of scientific improvisation.
Why Watney Needed More Than His Drinking Water
People need water to drink, prepare food and maintain hygiene. Crops need it too, often in much greater total amounts than the astronaut intends to consume directly. Watney's potato plan therefore turns a small life-support problem into a much larger agricultural one.
He cannot simply scoop liquid water from the ground outside. Under typical surface conditions, Mars is cold and its atmospheric pressure is less than one percent of Earth's at sea level. Exposed liquid water is unstable: it tends to freeze, evaporate or both. Yet Mars is not water-free. Spacecraft have identified substantial deposits of water ice, and NASA's Phoenix lander confirmed ice in shallow northern soil in 2008.
In the story, however, the immediate resource Watney can exploit is not a deposit of buried ice. It is leftover hydrazine associated with a spacecraft. This detail matters because the water is manufactured from onboard chemicals, rather than extracted from Martian soil.
How Hydrazine Can Become Water
Hydrazine, with the formula N2H4, contains hydrogen and nitrogen. Historically, spacecraft thrusters have used catalysts containing iridium to break down hydrazine and release hot gases. In Watney's simplified chemical picture, the useful part is the hydrogen, which can combine with oxygen to produce water.
Two idealized reactions show the idea:
N2H4 → N2 + 2H2
2H2 + O2 → 2H2O
These equations account for the atoms, but they do not describe an uncomplicated real-world device. NASA technical literature explains that catalytic hydrazine decomposition can produce ammonia as well as nitrogen and hydrogen. Actual yields and gas composition depend on operating conditions. Any water intended for drinking or plants would also require reliable collection and purification.
There is a useful limit to the chemistry. In the ideal case, one kilogram of pure hydrazine could yield about 1.12 kilograms of water if sufficient oxygen were supplied and all its hydrogen ended up in water. Roughly one kilogram of oxygen would be consumed. This is a mass-balance illustration, not a prediction of Watney's actual production or a usable engineering recipe.
The oxygen cannot be ignored: he must obtain it from a finite supply or generate it using the habitat's systems. Chemistry does not create matter from nothing. It rearranges material already available, which is why accounting for every gas is so important.
Why the Film's Water Experiment Is So Dangerous
The spectacular risk is not that forming water is impossible. It is that hydrogen and oxygen can react violently, especially when their proportions and ignition conditions are uncontrolled. Hydrazine itself is highly toxic, and its decomposition releases considerable heat. Ammonia or residual propellant could introduce additional hazards in a confined living space.
In The Martian, Watney's near-disaster dramatizes the danger of losing track of the gases in the habitat. This is a credible general lesson, even though the film compresses equipment design, atmospheric monitoring and contamination control into a few tense scenes. A pressurized habitat is also a life-support system: a fire or uncontrolled reaction threatens much more than one experiment.
Professional spacecraft engineering deals with such risks through isolation, monitoring, automated safeguards and extensive testing. An improvised reaction beside a sleeping or farming area would not meet that standard. That distinction is the answer to the movie's realism question: the chemistry is plausible; the survival strategy is extremely unsafe.
How Real Mars Astronauts Might Obtain Water
A future crew would likely combine stored reserves, rigorous recycling and extraction of local resources. The most promising local source is water ice. NASA's Subsurface Water Ice Mapping project has identified regions of likely shallow ice, helping scientists assess where astronauts might land. Mining, melting and purifying ice would still require energy, machinery and maintenance, but it would avoid relying on toxic propellant as a water feedstock.
NASA has already demonstrated the value of recycling in space. In 2023, the International Space Station's environmental control and life-support system demonstrated an overall water recovery rate of approximately 98 percent with help from a brine processor. That does not mean a Mars habitat can run forever without make-up water; it shows how dramatically a closed-loop system can reduce resupply needs.
Another technology addresses a related resource. The MOXIE experiment aboard NASA's Perseverance rover extracted oxygen from Mars's carbon-dioxide-rich atmosphere. Across its completed Mars operations, it produced 122 grams of oxygen and reached 12 grams per hour at its most efficient. This was a technology demonstration, not a full-scale crew oxygen plant or a water-making machine. Still, it helps establish that useful materials can be manufactured locally.
Ice extraction, atmospheric oxygen production and recycling solve different pieces of the same logistical puzzle. Together, they offer a more credible design path than emergency water chemistry carried out in a living room.
What The Martian Gets Right—and What It Simplifies
The story gets something fundamental right: survival on Mars depends on understanding resources as flows of matter and energy. Hydrogen must come from somewhere. Oxygen must come from somewhere. Water vapor must be captured, and recovered water must be made safe. A clever idea is only the first step; a reliable life-support process needs controls and redundancy.
What the story simplifies is equally important. The clean textbook decomposition of hydrazine is an idealization. Real catalyst chemistry produces mixed gases, and neither hydrogen handling nor purification is a casual task. The film also represents a specific fictional emergency—not NASA's recommended method for supplying a Mars farm.
So, would Watney's water-making trick work? As a matter of chemical possibility, yes: hydrogen from a suitable feedstock can react with oxygen to form water. As a practical way for a lone astronaut to irrigate potatoes inside a habitat, it is a dangerously fragile solution. The more realistic lesson of The Martian is not how to make an explosive shortcut work, but why future explorers will need robust recycling systems and dependable access to Martian ice.
Sources & Further Reading
- Louisiana State University — Chemistry in The Martian
- NASA Technical Reports Server — Hydrazine Thrusters
- NASA Science — Mars Phoenix
- NASA Science — SWIM Subsurface Water Ice Map
- NASA — MOXIE Completes Mars Mission
- NASA — ISS Water Recovery Milestone
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