New analysis of rock samples collected off the moon’s surface five decades ago shows how water in lunar minerals was generated by the bombardment of particles from solar wind -- and not by meteorite or comet impacts. The findings were published in Proceedings of the National Academy of Sciences this week. 

The Apollo missions of the 1970s painted a bone-dry picture of our moon, New Scientist explains, but back in 2009, researchers discovered hints of water clinging to lunar soil across the moon’s surface. The first detection was made by India's Chandrayaan-1 probe, and then data from NASA’s Cassini orbiter and Deep Impact spacecraft helped verify the likely presence of small amounts of water and hydroxyl molecules on the moon. 

To find the source of the water, Alice Stephant and François Robert from the Muséum National d'Histoire Naturelle in Paris measured the ratio of hydrogen and deuterium in lunar soil samples collected during the Apollo 16 and Apollo 17 missions. Water in the solar system naturally contains tiny amounts of heavy hydrogen or deuterium, Chemistry World explains, so measuring the proportions of the two isotopes could help researchers identify the origin of the water. 

The possible sources they investigated include water-rich meteorites, comet impacts, and solar wind -- the plasma stream of energized protons and electrons flowing out from the sun. Since the amount of deuterium quantities present depend on distances to the sun and the actions of cosmic rays, each source would give a different D/H ratio.

They found that most of the water from the surface of the soil grains comes from solar wind. “We see that there is higher water content when the ratio is lower,” Stephant says, which the duo interpreted as a signature of solar wind implantation. Protons from the solar wind combined with oxygen on the moon to generate water. Meanwhile, the contribution from meteorites and comets is negligible; The grains contained, on average, only about 15 percent water from these other sources. 

Image: Apollo 17 Gallery, MSFC History Office, NASA