If you read or listened to the news in October about the discovery of “water” on the Moon, you might be wondering what all the fuss is about. Before we get into these latest discoveries, let’s clarify a few things. 

First, if you look up the definition of water in most dictionaries it will state that this is the name given to the liquid state of H2O — this is not what was found on the Moon. Rather, scientists have discovered further evidence for frozen H2O, or water ice, and what we term molecular water, which is locked up in lunar materials — we’ll return to this later.

Second, a bit of history. Unlike Mars, where we see abundant evidence for the presence of past water (e.g., valley networks), and water ice locked up in the polar ice caps at the present-day, the general view of the Moon is that it has always been, and remains, virtually bone dry. Indeed, this was the view based on the initial studies of samples collected by the Apollo astronauts in the late 1960s and early 1970s, and also by the simple fact that H2O, in any form, will rapidly break down when exposed to sunlight on the lunar surface. During the remainder of the 20th century, a series of studies using data from spacecraft sent to the Moon, Earth-based observatories, and theoretical calculations, suggested that water ice might be present, but the studies were often contradictory or inconclusive.

Our understanding of water on the Moon has, however, changed dramatically during the first two decades of the 21st century. It had actually been theorized in the early 1960s, but a series of missions and other scientific studies have confirmed that there are places on the Moon that essentially never see the Sun — mostly in meteorite impact craters, which formed steep-sided depressions, close to the lunar poles. These are referred to as permanently shadowed regions or cold traps, and the temperatures in these areas have been measured to be as low as minus 238 oC! The idea is that any water, however miniscule in abundance, would be trapped in these cold traps and slowly accumulate over billions of years. This theory received a significant boost in 2008 and 2009 when India’s Chandrayaan-1 spacecraft NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS), respectively, conducted impact experiments on the Moon (essentially deliberately targeting a small mass at the lunar surface) and then analysed the plume of debris that was excavated. Both experiments detected water lending strong support for the presence of water ice in these PSRs. 

Another important piece of evidence came in 2011 when scientists re-analysed the famous “orange soil” collected during the Apollo mission in 1972. Using modern laboratory techniques not available in the 1970s, these scientists showed that these glasses, produced by volcanoes on the Moon more than three billion years ago, contained a substantial amount of water locked up in their structure.