Scientists have discovered that the most likely source of water on Moon is the constant stream of charged particles from the Sun known as the solar wind.
The findings by researchers from the University of Michigan imply that ice inside permanently shadowed polar craters on the Moon, sometimes called cold traps, could contain hydrogen atoms ultimately derived from the solar wind.
Theoretical models of lunar water stability dating to the late 1970s suggest that hydrogen ions (protons) from the solar wind can combine with oxygen on the Moon's surface to form water and related compounds called hydroxyls, which consist of one atom of hydrogen and one of oxygen and are known as OH.
Researchers present infrared spectroscopy and mass spectrometry analyses of Apollo samples that reveal the presence of significant amounts of hydroxyl inside glasses formed in the lunar regolith by micrometeorite impacts.
"We found that the 'water' component, the hydroxyl, in the lunar regolith is mostly from solar wind implantation of protons, which locally combined with oxygen to form hydroxyls that moved into the interior of glasses by impact melting," said Youxue Zhang, Professor of Geological Sciences.
"Our work shows that the 'water' component, the hydroxyl, is widespread in lunar materials, although not in the form of ice or liquid water that can easily be used in a future manned lunar base," Zhang said in a statement.
"This also means that water likely exists on Mercury and on asteroids such as Vesta or Eros further within our solar system," Yang Liu of U-T, the first author of the paper said.
"These planetary bodies have very different environments, but all have the potential to produce water," she said.
Over the last five years, spacecraft observations and new lab measurements of Apollo lunar samples have overturned the long-held belief that the moon is bone-dry.
In 2009, NASA's Lunar Crater Observation and Sensing satellite slammed into a permanently shadowed lunar crater and ejected a plume of material that was surprisingly rich in water ice.
Water and related compounds have also been detected in the lunar regolith, the layer of fine powder and rock fragments that coats the lunar surface.
However, the origin of lunar surface water has remained unclear.
The study findings are published in the journal 'Nature Geoscience'.