Planetary science: Detection of molecular water and cold ‘water traps’ on the Moon

The unambiguous detection of molecular water (H₂O) on the Moon and areas on the Moon where water could be trapped stably are reported in two papers published this week in Nature Astronomy. These findings may have implications for future missions to the Moon.

Previous research has reported signs of hydration on the lunar surface, particularly around the south pole. However, these detections are based on a spectral signature, at 3 µm, that cannot discriminate between H₂O and hydroxyl bound in minerals.

In one paper, Casey Honniball and colleagues analysed data from the Stratospheric Observatory for Infrared Astronomy (SOFIA) airborne telescope that observed the Moon at 6 µm. At this wavelength, they were able to detect a spectral signature of H₂O that is not shared with other hydroxyl compounds. They found that water is present at high southern latitudes in abundances around 100 to 400 parts per millionH₂O. The authors suggest that the detected water is probably stored in glass or between grains on the lunar surface that protect it from the harsh environment.

In another study, Paul Hayne and colleagues examined the distribution of permanently shadowed areas — known as cold traps — where water could be captured and remain permanently. The authors assessed a whole range of possible sizes for cold traps, down to 1 centimetre in diameter. They found that small scale ‘micro’ cold traps are hundreds to thousands of times more numerous than larger cold traps, and they can be found at both poles. The authors suggest that approximately 40,000 m2 of the lunar surface has the capacity to trap water.

These findings indicate that water is efficiently produced or delivered on the Moon by various processes, and is likely to be stored in the Moon’s cold traps at both polar regions. The presence of water may have implications for future lunar missions targeting and accessing these potential ice reservoirs.