Analysis of data from Chandrayaan-1's SARA instrument confirms how water is likely being generated on the Moon, with some surprising results.

Last month, data from Chandrayaan-1's Moon Mineralogy Mapper, Deep Impact, Cassini and LRO all provided evidence for water not only existing on the Moon, but also being created there. Now, further evidence from Chandrayaan-1's SARA (Sub-keV Atom Reflecting Analyzer) instrument confirms the solar wind as the key player in generating water.

The solar wind – a stream of charged particles – bombards the lunar surface, the top layer of which is a loose collection of grains, collectively known as the regolith. Incoming particles from the solar wind should be trapped in the spaces between the grains and absorbed. Protons landing on the lunar surface are expected to interact with the oxygen in the regolith to produce hydroxyl – one oxygen and one hydrogen atom – and water.

The signature for these molecules was recently found and the SARA results confirm that solar hydrogen nuclei are indeed being absorbed by the lunar regolith. But there is a curiosity in the results: roughly one in every five protons rebounds into space. In the process, the proton joins with an electron to become an atom of hydrogen.

“We didn’t expect to see this at all,” says Stas Barabash, Swedish Institute of Space Physics, and the European Principal Investigator for SARA.

Furthermore, the hydrogen is shooting off into space at speeds of around 200 kilometres per second, without being deflected by the Moon's weak gravity, or, since hydrogen is also electrically neutral, by magnetic fields in space. Since these atoms travel in straight lines it should be possible to trace them back to their points of origin to create a new type of surface map.

Whilst the Moon does not generate a global magnetic field, some lunar rocks are magnetized. Barabash and his team are currently making images, to look for such ‘magnetic anomalies’ in lunar rocks. These generate magnetic bubbles that deflect incoming protons away into surrounding regions making magnetic rocks appear dark in a hydrogen image.

Like the Moon, Mercury also suffers from the direct hits of solar wind particles, and the SARA team expects that a similar phenomenon may occur here too, but that the innermost planet is likely reflecting more hydrogen than the Moon because the solar wind is more concentrated closer to the Sun.

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