Scientists on Sunday said they had found water molecules in samples of lunar soil, and their unusual signature points to the Sun as the indirect source.
Samples returned to Earth by the Apollo missions carry molecules of water and a precursor of water called hydroxyl, according to their study, published in the journal Nature Geoscience.
Researchers led by Yang Liu at the University of Tennessee theorise that the molecules developed from a reaction between hydrogen ions in the solar wind -- the blast of particles from the Sun -- and a loose surface soil called regolith.
The Sun was formed around 4.5 billion years ago from a cloud of gas, a reaction in which all the deuterium in the nebula reacted with hydrogen to form helium.
As a result, unlike all other objects in the Solar System, the Sun is deuterium-less. Sure enough, the samples were tellingly poor in deuterium, the investigators found.
The tests used infrared spectroscopy to get a chemical signature of regolith grains from the Apollo 11, 16 and 17 missions. Two samples came from plains locations, and one from the lunar highlands.
Although the molecules are dissolved within the grains and do not exist as liquid water, the findings powerfully boost the notion that the Moon is not the utterly arid place it was once thought to be, say the authors.
Since 2009, when NASA found water crystals in a deep crater near the Moon's southern pole, evidence has suggested that the Moon was once a pretty moist place and may still have frozen water at depth.
Until now, the source of water in the inner Solar System, the region extending to the asteroid belt between Mars and Jupiter, is believed to be comets and other water-rich rocks which whack into planets and other bodies.
So if the study is right, hydrogen from the solar wind could be a second, hitherto unimagined source.
The solar wind whacks into the lunar surface at 1.6 million kms (a million miles) per hour, penetrating the lunar soil to a depth of up to 100 nanometres (100 billionths of a metre), according to some calculations.
The impact is so brutal that the Moon's mass diminishes by around million tonnes per hour, a figure that however is tiny when compared to the size of our satellite.
Hydroxyl is a bond between one hydrogen and one oxygen atom, while water (H20) comprises two hydrogen atoms and one oxygen atom.
How the solar hydrogen combines with oxygen in the regolith grains to make the molecules is unclear.
But the phenomenon could occur in other places in the inner Solar System, the authors suggest.
"A similar mechanism may contribute to hydroxyl on the surfaces of other airless terrestrial bodies where the solar wind directly interacts with the surface," says the study.
Examples of this could be Mercury, the rocky planet that is nearest the Sun, and Vesta, the second biggest (and the brightest) object in the asteroid belt between Mars and Jupiter.
On Earth, we are shielded from the solar wind thanks to the atmosphere and the planet's magnetic field.