"We traced most of these events to known meteoroid streams, but the really surprising part is that we also found evidence of four meteoroid streams that were previously undiscovered," said Mehdi Benna from NASA's Goddard Space Flight Centre in Greenbelt, Maryland, and the lead author of the study published in the journal Nature Geosciences.
"The water being lost is likely ancient, either dating back to the formation of the Moon or deposited early in its history," Benna added.
The findings will help scientists understand the history of lunar water -- a potential resource for sustaining long-term operations on the Moon and human exploration of deep space.
Earlier models had predicted that meteoroid impacts could release water from the Moon as a vapour, but scientists hadn't yet observed the phenomenon.
Now, the team has found dozens of these events in data collected by NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) -- a robotic mission that orbited the Moon to gather detailed information about the structure and composition of the thin lunar atmosphere.
The newly identified meteoroid streams, observed by LADEE, occurred on January 9, April 2, April 5 and April 9, 2014.
"The Moon doesn't have significant amounts of H2O or OH in its atmosphere most of the time," said Richard Elphic, the LADEE project scientist.
"But when the Moon passed through one of these meteoroid streams, enough vapour was ejected for us to detect it. And then, when the event was over, the H2O or OH went away," Elphic said.
To release water, the meteoroids had to penetrate at least three inches below the surface.
Because the material on the lunar surface is fluffy, even a meteoroid that's a fraction of an inch across can penetrate far enough to release a puff of vapour.
With each impact, a small shock wave fans out and ejects water from the surrounding area.
"When a stream of meteoroids rains down on the lunar surface, the liberated water will enter the exosphere and spread through it. About two-thirds of that vapor escapes into space, but about one-third lands back on the surface of the Moon," said the NASA-led study.
These findings could help explain the deposits of ice in cold traps in the dark reaches of craters near the poles.
The team ruled out the possibility that all of the water detected came from the meteoroids themselves.
"We know that some of the water must be coming from the Moon, because the mass of water being released is greater than the water mass within the meteoroids coming in," said Dana Hurley of the Johns Hopkins University Applied Physics Laboratory.