New data from Chandrayaan-1 has revealed how the moon “produces its own water.” Much like a big sponge, it absorbs charged particles emitted by the sun, which then interact with oxygen on the lunar surface to produce water.

A scientific instrument on Chandrayaan-1 — the Sub keV Atom Reflecting Analyser or SARA — made this discovery that was published in the latest edition of the Planetary and Space Science journal.

According to European Space Agency (ESA) scientists, hydrogen nuclei from solar winds are absorbed by the lunar regolith (a loose collection of irregular dust grains making up the moon’s surface). An interaction between the hydrogen nuclei and oxygen present in the dust grains are expected to produce hydroxyls and water.

SARA, developed by the ESA and the Indian Space Research Organisation, was designed to study the moon’s surface composition and solar wind-surface interactions. Recently, another instrument on the Indian spacecraft, the Moon Mineralogy Mapper — an imaging spectrometer developed by the U.S. National Aeronautics and Space Administration — first found water molecules on the lunar surface.

Not all nuclei absorbed

SARA’s results also highlight a mystery: not every hydrogen nucleus is absorbed. One out of every five rebounds into space, combining to form an atom of hydrogen. “We didn’t expect to see this at all,” said Stas Barabash of the Swedish Institute of Space Physics, who is the European Principal Investigator for SARA.

Hydrogen shoots off at speeds of around 200 km per second and escapes without being deflected by the moon’s weak gravity, the team found.

This knowledge provides timely advice for scientists who are readying ESA’s BepiColombo mission to mercury. The spacecraft will carry two instruments similar to SARA and may find that the innermost planet is reflecting more hydrogen than the moon because the solar wind is more concentrated closer to the sun.

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