Newly confirmed water on the moon could help sustain lunar astronauts and even propel missions to Mars, if harvesting it can be made practical. A microwave device being developed by NASA could do just that.

Three spacecraft – India's Chandrayaan-1 and NASA's Cassini and Deep Impact probes – have detected the absorption of infrared light at a wavelength that indicates the presence of either water or hydroxyl, a molecule made up of a hydrogen and an oxygen atom. All found the signature to be stronger at the poles than at lower latitudes.

Some of these molecules may be created continuously when solar wind protons – hydrogen ions – bind to oxygen atoms in the lunar soil. Comet impacts may also have brought water to the moon.

Water delivered by comets or generated by the solar wind could randomly diffuse over time into permanently shadowed craters at the lunar poles, which were recently measured to be colder than PlutoMovie Camera.

"Once it gets in there, it's not going to come out," says Carle Pieters of Brown University in Providence, Rhode Island, lead scientist for the NASA-built instrument that made the Chandrayaan-1 measurements.

'Railroad to space'

So far, the water does not appear to be very abundant – a baseball-field-sized swathe of lunar soil might yield only "a nice glass of water", Pieters told New Scientist.

But if it could be harvested, lunar astronauts could use it as drinking water and split it into oxygen and hydrogen to make rocket fuel for their return journeys. That would slash launch costs, since it would reduce the amount of fuel they would need to lug with them from Earth.

Rocket fuel produced on the moon might even help mount a human mission to Mars. Because of the moon's weaker gravity, it would take less energy to loft fuel into space for a Mars mission from the lunar surface than it would from Earth.

"It completely changes the spaceflight paradigm," says Paul Spudis of the Lunar and Planetary Institute in Houston, Texas. "It's like building a transcontinental railroad to space."

Cold plate

But how do you extract water that is likely locked up as small concentrations of ice in the lunar soil? Microwaves could provide the key, according to work by Edwin Ethridge of NASA's Marshall Space Flight Center and William Kaukler of the University of Alabama, both in Huntsville, who first demonstrated the technique in 2006.

They used an ordinary microwave oven to zap simulated lunar soil that had been cooled to moon-like temperatures of -150 °C.

Keeping the soil in a vacuum to simulate lunar conditions, they found that heating it to just -50 °C with microwaves made the water ice sublimate, or transform directly from solid to vapour. The vapour then diffused out from higher-pressure pores in the soil to the low-pressure vacuum above.

On the moon, the vapour could be collected by holding a cold metal plate above the soil. The rising water vapour would then condense as frost onto the cold plate and "you could scrape it off", Kaukler says.

Baking and processing dry lunar soil at high temperatures could also release oxygen and hydrogen for rocket fuel or other uses. But that would take about 100 times as much energy as extracting them from native lunar water, Spudis says: "Everything becomes easier and cheaper and quicker."