Liquid water exposed to the vacuum of space simultaneously begins to boil and freeze, a strange-sounding combination driven by the near-total absence of atmospheric pressure. But that dramatic moment is only the beginning of the story. Depending on where in space the water ends up and what forces act on it, water can shatter into its atomic parts under ultraviolet light, form exotic types of ice on interstellar dust grains, hide for hundreds of millions of years in permanently shadowed lunar craters, or float as wobbling spheres aboard a spacecraft. The fate of water in space is less a single answer than a catalog of possibilities shaped by pressure, temperature, radiation, and gravity.
What Vacuum Does to Liquid Water
On Earth, atmospheric pressure keeps water liquid at familiar temperatures. Remove that pressure and water enters a physical tug-of-war. At the near-zero pressure of open space, water’s boiling point drops below its freezing point, which means the liquid tries to boil and freeze at the same time. The surface of the water vigorously vaporizes, and the rapid evaporation pulls heat away from the remaining liquid so quickly that it chills into ice. A small amount of exposed water in a vacuum will puff into vapor and ice crystals within seconds.
This is not a hypothetical scenario limited to thought experiments. Astronauts have seen it firsthand when water escapes through a valve or is deliberately released outside a spacecraft. The vapor expands rapidly into a cloud of fine ice particles that drift apart and slowly sublimate, meaning they transition directly from solid to gas without ever becoming liquid again. In the deep cold of shadowed space, where temperatures can drop below minus 150 degrees Celsius, those ice particles can persist for a long time. In direct sunlight, they sublimate faster because solar heating drives the molecules off the surface.
Floating Spheres and Shape-Shifting Droplets
Inside a pressurized spacecraft, where air pressure keeps water from boiling, the absence of gravity creates its own spectacle. Without the downward pull that flattens water into puddles and pools on Earth, surface tension dominates. Water naturally pulls itself into spheres, the shape with the least surface area for a given volume. Aboard the International Space Station, free-floating water balls are a familiar sight, wobbling and oscillating when nudged.
These floating droplets do more than entertain. Researchers have used the microgravity environment to study how water droplets change shape under different forces, observing behaviors that gravity masks on Earth’s surface.1Physics. Water Droplets Shape-Shift on the ISS The practical relevance is real: understanding how fluids move without gravity matters for designing everything from fuel tanks to life-support plumbing. In microgravity, bubbles do not rise and heavy liquids do not sink, so engineers cannot rely on buoyancy to separate air from water or vapor from liquid. Instead, spacecraft water-processing systems use spinning centrifugal separators and carefully designed capillary geometries to handle the physics.2Space Habitation. Toward sustainable living in space: A review of environmental control and life support system technologies
How Ultraviolet Light Tears Water Apart
Sunlight in space is unfiltered by any atmosphere, and the far-ultraviolet portion of the spectrum is energetic enough to rip water molecules apart. This process, called photodissociation, splits a water molecule into fragments. The dominant pathway produces a hydrogen atom and a hydroxyl radical. A less common pathway, occurring about a third as often, produces molecular hydrogen and an oxygen atom.3The Journal of Chemical Physics. Vacuum Ultraviolet Photochemistry. III. Primary Processes in the Vacuum Ultraviolet Photolysis of Water and Ammonia Once broken apart, those fragments scatter and rarely recombine. Hydrogen atoms, being the lightest element, escape easily from any body without strong gravity.
This is a major reason why small, airless bodies struggle to hold onto water over geological time. The Moon, asteroids, and even Mars lose water partly because solar ultraviolet radiation steadily dismantles exposed water molecules. On Mars, the thin atmosphere provides only modest protection, and the hydrogen produced by photodissociation escapes to space, slowly drying the planet. Any water that survives long-term in these environments does so by hiding: buried under soil, locked inside minerals, or trapped in places that never see sunlight.
Exotic Ice on Interstellar Dust
Far from any star, in the cold dense clouds of gas and dust that fill parts of our galaxy, water forms molecule by molecule on the surfaces of tiny dust grains. At temperatures around 10 Kelvin (roughly minus 263 degrees Celsius), oxygen atoms landing on a grain surface meet hydrogen atoms that are slowly hopping across it. The hydrogenation is efficient: oxygen converts rapidly to hydroxyl and then to water, building up thin icy mantles on the grains.4Astronomy & Astrophysics. Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms Laboratory experiments reproducing these conditions have confirmed that the reaction proceeds with high efficiency, forming water molecules about half the time an oxygen atom encounters a hydrogen atom on the grain surface.4Astronomy & Astrophysics. Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms
Simulations of this grain-surface chemistry show that at 10 Kelvin, the atoms are mostly immobile except for lightweight hydrogen, which acts as the key reactant. Each time a hydroxyl radical forms and stays on the grain, a nearby hydrogen atom finds it and completes the water molecule. The result is that oxygen converts quickly into water ice, making it the dominant species coating the grain.5Astronomy & Astrophysics. Water formation on bare grains: When the chemistry on dust impacts interstellar gas
The ice that forms this way is not the crystalline ice you pull from your freezer. It is amorphous, meaning its molecules are arranged in a disordered, glass-like structure rather than a neat crystal lattice. This amorphous solid water has an internal structure where molecules bond to their neighbors in roughly tetrahedral arrangements, but the overall pattern is chaotic. As the ice warms, interesting things happen: molecules rearrange into more stable configurations, and pores open up within the ice. At around 60 Kelvin, pores can reach radii of a few angstroms. At higher temperatures, these pores grow and connect to each other, reaching radii of roughly 15 angstroms before the ice eventually transitions to a crystalline form.6Astronomy & Astrophysics. Pore evolution in interstellar ice analogues Those pores matter because they can trap other molecules, including organic compounds relevant to the chemistry of life.
Water Hiding on the Moon
The Moon has no atmosphere to speak of, and most of its surface bakes under direct sunlight at temperatures that would vaporize any exposed ice within hours. Yet the Moon is not entirely dry. Near the lunar poles, inside deep craters whose floors never receive sunlight, temperatures stay cold enough for water ice to persist. These permanently shadowed regions have been a focus of exploration for decades, and multiple lines of evidence suggest that ice is indeed present in at least some of them.
High-resolution mapping of one such area, Scott-E crater, found that surface roughness in regions where models predict thermally stable ice is significantly lower than in adjacent areas where ice is not expected. The correspondence suggests that processes like preferential cold-trapping in depressions or changes in surface texture driven by ice-rich soil could explain the pattern.7Geophysical Research Letters. Geomorphic Evidence for the Presence of Ice Deposits in the Permanently Shadowed Regions of Scott‐E Crater on the Moon The ice does not necessarily sit as clean sheets on the surface. It may be mixed into the top layer of lunar soil, or buried under a thin blanket of debris.
How long these ice deposits have been accumulating is an open question, and the answer turns out to be shorter than many people assume. The Moon’s axis has shifted over geological time, and the permanently shadowed regions that exist today appeared and grew after that axial shift. Researchers studying the history of the Moon’s polar shadows found that the area in Cabeus Crater where NASA’s LCROSS mission detected water vapor became continuously shadowed only about 900 million years ago, meaning cold-trapping of ice there is a relatively recent phenomenon in the Moon’s four-and-a-half-billion-year history.8PubMed Central. Past extent of lunar permanently shadowed areas That finding implies that estimates of how much ice the Moon has trapped need to be revised downward, because the traps simply have not been open as long as once thought.
Even within permanently shadowed regions, ice is not safe from disturbance. Impact simulations show that craters larger than about 200 meters in diameter can excavate and redistribute shallow ice, sometimes allowing it to resettle in colder spots but sometimes exposing it to loss. Smaller impacts and the steady rain of micrometeorites churn the top layer of soil, mixing any ice vertically through the regolith.9npj Space Exploration. Impacts into the lunar permanently shadowed regions For future missions hoping to mine lunar water, understanding this churning process is critical: the ice may be there, but it is probably patchy, mixed with soil, and distributed unevenly.
Why Mars Cannot Hold Liquid Water on Its Surface
Mars has fascinated water-hunters for generations, but its current surface is a hostile place for liquid water. The atmospheric pressure on Mars averages about 600 pascals, less than one percent of Earth’s sea-level pressure. At that pressure, liquid water is thermodynamically unstable: it will either freeze or sublimate rather than persist as a liquid.10PubMed Central. Stability of the Liquid Water Phase on Mars: A Thermodynamic Analysis Considering Martian Atmospheric Conditions and Perchlorate Brine Solutions Even if temperatures briefly climb above freezing in some equatorial locations, the pressure is simply too low for a puddle to form and stay.
That does not mean Mars is waterless. Vast quantities of water ice exist at the Martian poles and buried under regolith at mid-latitudes. Laboratory experiments simulating Martian conditions found that even a thin layer of soil dramatically slows the sublimation of buried ice, dropping the rate by an order of magnitude with just 50 millimeters of overlying material. Extrapolating those measurements, a one-meter-thick layer of ice buried under a meter of Martian-like soil at 235 Kelvin could persist for roughly 800 years.11Geophysical Research Letters. Sublimation rate of ice under simulated Mars conditions and the effect of layers of mock regolith JSC Mars‐1 Over geological timescales that is not long, but it shows how burial can preserve ice in an environment that would otherwise destroy it quickly. Deeper deposits, insulated by thicker layers, can survive far longer. Mars almost certainly has enormous underground ice reserves accumulated over billions of years, making it a prime target for future exploration.
Water Locked Inside Rocks
Not all water in space exists as ice or vapor. A substantial amount is chemically bound inside minerals, locked into the crystal structure of rocks rather than sitting in pores or on surfaces. Certain types of meteorites, known as carbonaceous chondrites, are especially water-rich. Analysis of a particular class of these meteorites found indigenous water contents ranging from about 2 to 10.5 percent by weight, with an average of around 7 percent.12Meteoritics & Planetary Science. The water content of CM carbonaceous chondrite falls and finds, and their susceptibility to terrestrial contamination That water is incorporated into minerals called phyllosilicates, essentially clay-like materials that formed when rock reacted with liquid water early in the solar system’s history. The water is so tightly bound that it shows minimal exchange with Earth’s environment even after the meteorite has sat on the ground for extended periods.
These measurements of meteorites are consistent with what spacecraft have found when visiting their parent asteroids. The OSIRIS-REx mission to asteroid Bennu and Japan’s Hayabusa2 mission to Ryugu both confirmed that C-complex asteroids are hydrated, carrying water locked into their minerals at levels matching the meteorite data. This has practical implications: if humans ever need water in deep space, asteroids could serve as supply depots. Heating the rock releases the bound water, and there is a lot of rock out there. The water would need processing before anyone could drink it, but the raw material is abundant.
Recycling Every Drop Aboard Spacecraft
Launching water from Earth’s surface into orbit costs thousands of dollars per kilogram, which gives space agencies strong motivation to recycle every molecule. The International Space Station’s water recovery system, delivered by NASA, includes a Water Processor Assembly and a Urine Processor Assembly working in tandem.13SAE Technical Paper Series. Status of the Regenerative ECLSS Water Recovery System Together, they collect humidity from the cabin air, wastewater from hygiene, and distilled urine, processing it all back into drinkable water. Current ISS operations achieve roughly 90 percent total water recovery. The Urine Processor alone recovers about 70 to 85 percent of urine water, and a newer Brine Processor Assembly pushes that figure to approximately 98 percent by extracting water from the concentrated leftover brine.2Space Habitation. Toward sustainable living in space: A review of environmental control and life support system technologies
Keeping that recycled water safe to drink presents its own challenges. Microbial growth in water storage and distribution lines is a persistent concern, and spacecraft systems use silver ions as a residual biocide to suppress contamination.14ResearchGate. Disinfection of Spacecraft Potable Water Systems by Passivation with Ionic Silver Unlike the chlorine used in municipal water systems on Earth, silver does not produce strong tastes or odors, and it remains effective at low concentrations over long periods. For missions to Mars, where resupply is impossible, the target is closing the water loop to 98 percent or better, essentially losing almost nothing.2Space Habitation. Toward sustainable living in space: A review of environmental control and life support system technologies
Water as a Radiation Shield
Water has a property that makes it unexpectedly useful for protecting astronauts from space radiation: it is rich in hydrogen, and hydrogen is one of the best elements for absorbing and scattering the high-energy particles that permeate space. On Earth, the atmosphere and magnetic field handle this shielding. In deep space, crews are exposed to a steady bath of galactic cosmic rays and the occasional intense burst from a solar particle event.
Engineers have explored using the water already aboard a spacecraft as part of the radiation shield. A prototype water-filled garment, designed to be worn during solar particle events, was tested aboard the ISS. The concept is elegant: water that the crew will eventually drink or use for other purposes is temporarily routed into a wearable vest, providing emergency shielding without adding dead weight to the spacecraft. Testing found the garment offered good shielding potential while remaining comfortable enough to wear, and the water was recycled afterward without waste.15PubMed Central. A water-filled garment to protect astronauts during interplanetary missions tested on board the ISS The idea of multi-purposing water, using it as shielding when radiation spikes and as drinking water otherwise, reflects a broader philosophy in spacecraft design where every kilogram needs to earn its place several times over.
Cometary Ice and What Sublimation Looks Like Up Close
Comets are perhaps the most dramatic examples of what happens to water in space. A comet’s nucleus is a mixture of ice and dust, often described loosely as a dirty snowball. When a comet’s orbit brings it closer to the Sun, solar heating causes the ice to sublimate, skipping the liquid phase entirely and turning straight into vapor. That vapor drags dust particles along with it, forming the coma (the fuzzy envelope around the nucleus) and eventually the tail that stretches millions of kilometers behind.
Laboratory experiments have recreated this process under controlled vacuum conditions, using mixtures of ice and dust cooled to simulate cometary surfaces. These experiments show that as the ice sublimates, the dust left behind can form a lag deposit, a dry crust that insulates the remaining ice underneath and slows further sublimation.16A&A. Sublimation of ice-dust mixtures in cooled vacuum environments to reproduce cometary morphologies This self-armoring effect helps explain why comets can survive many trips around the Sun rather than evaporating entirely on their first close approach. The interplay between sublimation and dust mantling creates the pits, cliffs, and layered terrains that spacecraft like Rosetta observed up close on Comet 67P.
The ice inside comets is not pure water. It contains trapped gases like carbon dioxide, carbon monoxide, methanol, and ammonia, all frozen together in the amorphous ice matrix described earlier. As the comet warms and that amorphous ice begins to crystallize, the trapped gases are suddenly released, sometimes causing outbursts of activity even when the comet is still far from the Sun. Comets, in this sense, are time capsules: their ice preserves a record of the chemical environment in the outer solar system billions of years ago, and watching how that ice behaves as it warms tells researchers about conditions that existed before Earth formed.