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Astronomers have seen the first firm evidence that there are deposits of icy water on the surface of the moon 鈥� and they may have been laying there for the past 3 billion years.

The ice is nestled within craters at both poles of the moon that are permanently in shadow and bitterly cold at 鈭�163 掳C. The presence of accessible water on the surface of the moon could prove useful if we ever establish a permanent base there.

Ultraviolet scans in 2008 and 2009 using spectrometers aboard India鈥檚 first lunar orbiter, Chandrayaan-1, had hinted the moons polar shadowlands might harbour water, but the signals were ambiguous. They could equally have come from isolated ions sometimes found in water, such as hydroxyl, not water itself.

Now, separate spectroscopic evidence collected by the orbiter during the same period has helped settle the matter. It also collected near-infrared emission data from the surface of the moon, and three specific signals from shadowy regions of both poles uniquely match those of water frost here on Earth.

鈥淥urs is the first direct and definitive evidence for surface-exposed water ice in the lunar polar regions,鈥� says Shuai Li of the Hawaii Institute of Geophysics and Planetology in Honolulu, who led the analysis.

The ice only appeared in 3.5 per cent of the shadow lands, but in the areas where it did appear, it covered 30 per cent of the surface, says Li.

Li thinks the ice has been sitting there for a long time鈥攁s much as 3 billion years. That鈥檚 when water stopped diffusing to the surface from within the moon, and when asteroids and meteorites rich in water stopped battering the moon鈥檚 surface.

Most of the water present at that time would eventually have mixed back into the surface or evaporated into space from the heat of the Sun, but not in the shadow lands. 鈥淪o only those old and thick deposits can last longer,鈥� says Li.

鈥淭heir detection of water ice in the lunar polar shadowed regions is quite convincing,鈥� says Carle Pieters of Brown University in Providence, Rhode Island. 鈥淭hey have proven the existence of water ice on the surface in parts of the shadowed polar areas, a major and exciting step.鈥�