Almost all of the moon's water comes from the action of particles in the solar wind and not from the shattered remnants of water-rich comets and meteorites, as was once believed. So says a fresh analysis of lunar rock samples from two American moonshots.
The Apollo missions painted a picture of the moon as a bone-dry desert. So astronomers were surprised when three probes in 2009 showed that a lot of water is locked up in minerals in the soil.
There are three possible sources of that water. Comets and meteorites were a popular idea for a while. Another good candidate is the solar wind – the plasma that streams from the sun's upper atmosphere and smashes high-energy protons into the moon's surface. Cosmic rays from outside the solar system can also inject ions into lunar rocks, causing chemical changes that create water.
"All these mechanisms seem to work, but we still didn't know which is the dominant source of lunar water," says NASA scientist Alan Stern, who was not involved in the new work.
To find out the likeliest source of the water, Alice Stephant and Francois Robert at the National Museum of Natural History in Paris, France, and colleagues measured the ratio of hydrogen and deuterium in soil samples from the Apollo 16 and Apollo 17 missions. They ran the samples through a type of mass spectrometer that not only detects which isotopes are present but how deep down they are in a surface sample.
Because there tends to be more deuterium further from the sun, each possible source of lunar water should give a different ratio. Comets and meteorites have distinctive proportions, while protons from the solar wind or cosmic rays would each have different ratios.
They concluded that nearly all the water in the first 200 nanometres of the lunar surface comes from the solar wind. In fact, the contribution from comets and meteorites is "negligible", the team says.
The finding confirms suspicions raised by a different experiment in 2012 that also suggested space rocks were not the source of water on the surface of the moon.
"We confirm that result," says Stephant. "Water-rich meteorite and comet impacts do not bring important amounts of water to the surface of the moon."
Alberto Saal at Brown University in Providence, Rhode Island, is pleased with the result. "I think the idea that most of the water in the surface of the moon comes from solar wind implantations is most likely correct," he says.
Interestingly, the team found that cosmic rays were at work deeper down below the surface – meaning at least some of the moon's water comes from outside the solar system.