The substructure, known as a hydroxyl, consists of one atom of hydrogen and one of oxygen, or OH, rather than two of hydrogen and one of oxygen, or H2O, the researchers from the University of Tennessee at Knoxville, University of Michigan and California Institute of Technology reported in the journal Nature Geoscience.
Scientists have known for about five years that the traditional view that the moon was bone dry was incorrect.
Spacecraft observations and new lab measurements of Apollo lunar samples found icy drops of water on the lunar surface.
NASA's robotic Lunar Crater Observation and Sensing Satellite crashed into a permanently shadowed southern lunar crater at 6,200 mph in 2009. The crash -- equivalent to detonating 2 tons of TNT -- sent a plume of material into the sky that scientists discovered was rich in water ice.
But they weren't sure how the water got there. It could have been indigenous or been a result of other sources, such as collisions from water-bearing comets.
The researchers concluded in the Nature Geoscience study that positively charged subatomic hydrogen proton particles coming from the solar wind -- a stream of charged particles ejected from the sun's upper atmosphere -- appear to have combined with oxygen on the moon's surface to form the OH hydroxyls.
The result is an "unanticipated, abundant reservoir" of OH and water in the dust, soil, broken rock and other loose material on the lunar surface, the authors wrote in the study, published online Sunday.
"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," Michigan geological sciences Professor Youxue Zhang said.
"This also means that water likely exists on Mercury and on asteroids such as Vesta or Eros further within our solar system," lead author Yang Liu from Tennessee said. "These planetary bodies have very different environments, but all have the potential to produce water."
Vesta is the second most-massive asteroid in the solar system's asteroid belt between the orbits of Mars and Jupiter. Eros is a near-Earth, Mars-crosser asteroid that the NEAR Shoemaker probe observed in a 1998 flyby and in 2000 photographs before landing on it in 2001.
Earth would receive the solar wind too, but its atmosphere and magnetic field deflect it. The moon has no such protection.