Scientists have been speculating on how minute water reached the Moon ever since its discovery over the past decade by orbiting spacecraft.
There are a number of possibilities ranging from comet or meteor impacts, through to trapped volcanic gases during the Moon's formation four and half billion years ago.
Reporting in the journal Nature Geoscience, Dr Yang Liu from the University of Tennessee and colleagues, say hydrogen transported in the solar wind and combining with oxygen in the lunar soil is the likely cause.
Liu and colleagues reached their conclusion after examining samples of lunar soil brought back by the Apollo 11, 16 and 17 missions.
Lunar soils or regolith, contain tiny glassy grains called agglutinates, which are produced by micro meteoroids as they hit the Moon's surface, causing the soil to suddenly heat up and cool rapidly.
Liu and colleagues found agglutinates contain between 200 and 300 parts per million of water and water molecule segments called hydroxyls.
They then used spectroscopy to identify the ratios of hydrogen to deuterium (heavy hydrogen) to determine if they originated from asteroid or comet impact, from lunar volcanic eruptions, or from the solar wind.
Solar windThey found the ratio of hydrogen to deuterium to be extremely low, most closely matching the ratio found in solar wind.
Liu and colleagues say their findings may explain the origins of a significant amount of lunar water.
"Our findings imply that ice in (lunar) polar cold traps could contain hydrogen atoms ultimately derived from the solar wind," the researchers say.
"We suggest that a similar mechanism may contribute to hydroxyl on the surfaces of other airless terrestrial bodies where the solar wind directly interacts with the surface, such as Mercury and the asteroid Vesta.
As the solar system began to form 4.56 billion years ago, water condensed out of the primordial gas as ices in the colder outer solar system.
Scientists think it was later transported to the inner solar system by comets and water bearing asteroids or by ancient carbonaceous condrite meteors which date back to the solar systems earliest epoch.
Case closedDr Trevor Ireland from the Australian National University, says the findings aren't surprising.
"The lack of deuterium is the key. The Sun hasn't got any deuterium because it's a great nuclear fuel for fusing helium," says Ireland.
"Because the solar wind lacks deuterium, the corresponding deuterium depletion in the hydroxyl becomes the smoking gun."
He sas the findings show how the solar wind can react with the lunar surface to produce water.
"People had thought about that before, but nobody had a way of proving it until this work," says Ireland.