Ice in the coldest places in the solar system

Craters where the Sun never shines

Earth's spin axis is tilted 23.4°, which gives us seasons. The Moon's axis is tilted only about 1.5° to its path around the Sun. So near the lunar poles the Sun always skims the horizon, and the floors of deep craters stay in shadow all year round, for billions of years.

These permanently shadowed regions (PSRs) are among the coldest places measured anywhere in the solar system. NASA's Lunar Reconnaissance Orbiter (LRO) has measured temperatures below about −240 °C (around 30 kelvin) on some crater floors near the poles, colder than the surface of Pluto.

At those temperatures, water molecules that wander in, from comet and asteroid impacts, volcanic gases or the solar wind reacting with the soil, freeze and stay put. The craters are cold traps.

Famous examples near the south pole include Shackleton, Cabeus, Haworth and Faustini. Near the north pole, Hermite holds the record for the coldest temperature measured on the Moon.

How we found water on the Moon

When Apollo samples came back bone dry in the 1970s, most scientists concluded the Moon had no water at all. That changed slowly, clue by clue:

• 1994, Clementine: radar signals bounced off the south pole hinted at ice, though the result was debated.
• 1998, Lunar Prospector: found extra hydrogen concentrated at both poles, a strong sign of water ice in the soil.
• 2008, Chandrayaan-1: India's first Moon mission sent an impact probe down near the south pole and carried NASA's Moon Mineralogy Mapper (M3).
• 2009, M3 results: M3 detected the infrared fingerprint of water and hydroxyl (OH) molecules across wide areas of the surface, strongest towards the poles. NASA's Cassini and Deep Impact spacecraft confirmed it.
• 9 October 2009, LCROSS: NASA deliberately crashed a spent 2-tonne rocket stage into the shadowed crater Cabeus. A shepherding spacecraft flew through the plume and measured water: roughly 155 kg of water vapour and ice, about 5.6% of the material by mass.
• 2018: a new analysis of M3 data showed exposed water ice on the surface inside permanently shadowed craters at both poles.
• 2020, SOFIA: NASA's airborne telescope detected molecular water even in sunlit soil around Clavius crater, at about 100 to 400 parts per million: roughly a 350 ml bottle's worth in a cubic metre of soil.
• 2023: Chinese scientists found water stored inside tiny glass beads in Chang'e 5 samples, a reservoir that the solar wind can keep topping up.

Why lunar ice matters

Water is heavy and expensive to launch from Earth: every kilogram sent to the Moon costs a fortune. Finding it there changes what is possible:

• Drinking water and water for growing food.
• Oxygen to breathe, split from water with electricity from solar panels.
• Rocket fuel: water can be split into hydrogen and oxygen, the same propellants many rockets burn. A lunar "filling station" could make deep-space missions cheaper.
• Radiation shielding: water is good at blocking radiation around habitats.
• Science: the ice is a frozen record of where the inner solar system's water came from, layer by layer over billions of years.

That is why the lunar south pole is the target for NASA's Artemis landings, India's Chandrayaan-3, China's Chang'e 7 and several private landers. See Future on the Moon for how it might be mined.

What we still don't know

• How much ice is there, and in what form? It could be thick, pure sheets, or a few percent of frost mixed through the soil like grains of sugar in sand. That makes a huge difference for mining.
• How deep does it go? Orbiters only sense the top metre or so. Only drills and rovers on the ground can tell.
• Recent attempts were bumpy. NASA cancelled and then revived the VIPER rover; it is now planned to fly on a Blue Origin lander. Intuitive Machines' IM-2 lander carried an ice drill to the south pole in March 2025 but landed on its side. NASA's small Lunar Trailblazer orbiter, built to map the water, lost contact soon after launch in 2025.
• Who can use it? The Artemis Accords and the 1967 Outer Space Treaty say no nation can own the Moon, but rules for using its resources are still being worked out.

To work in the dark craters, NASA built ShadowCam, a super-sensitive camera on Korea's Danuri orbiter. Since 2023 it has been photographing the inside of permanently shadowed craters using the faint light reflected off nearby crater walls.