View of the Moon.
Until the last decade, scientists thought that the Moon was arid, with any water existing mainly as pockets of ice in permanently shaded craters near the poles.
That research is based on observations gathered by NASA's Lunar Reconnaissance Orbiter, which has been studying our planet's close companion since 2009.
As for where water molecules on the lunar surface come from, scientists hypothesize that the source is hydrogen ions in the solar wind.
More recently, they identified surface water in sparse populations of molecules bound to the lunar soil, or regolith.
These small batches of water vary in amount and location and are dependant on the time of the day. Water is more common at higher latitudes according to NASA, but the molecules move around the surface of the moon as it heats.
Water molecules remain tightly bound to the regolith until surface temperatures peak near lunar noon. The researchers analysed the daytime variations of ultraviolet light reflected by the lunar surface from 2009 to 2016. "Lunar water can potentially be used by humans to make fuel or to use for radiation shielding or thermal management; if these materials do not need to be launched from Earth, that makes these future missions more affordable", said Amanda Hendrix, the lead author of the study, Express.co.uk reports. I'm excited about these latest results because the amount of water interpreted here is consistent with what lab measurements indicate is possible.
"This is an important new result about lunar water, a hot topic as our nation's space program returns to a focus on lunar exploration", Kurt Retherford, a co-author on the new research and the principal investigator of the LAMP instrument at Southwest Research Institute in Texas, said in a NASA statement.
But this was not the case and, instead, NASA said water on the Moon builds up over time rather than "raining down directly from the solar wind".
Water on other planetary bodies could be a valuable resource not just for human explorers to drink, but also to serve as fuel for future robotic exploration, since water can be split to form rocket fuel, saving missions from having to carry that fuel from Earth.
John Keller, NASA's deputy project scientist at for LRO, said: "This result is an important step in advancing the water story on the Moon and is a result of years of accumulated data from the LRO mission".
NASA is leading a sustainable return to the Moon with commercial and worldwide partners to expand human presence in space and bring back new knowledge and opportunities.