Small meteorite impacts release water from the Moon, reports an article published online this week in Nature Geoscience. This research suggests that a very small amount of water is pervasive in lunar subsoil, and preserved from early in the Moon’s history.

A decade ago, trace amounts of water were found on the surface of the Moon, and not only in polar ice deposits as previously thought. Researchers have attributed the origin water to solar wind and meteorites. However, the source and extent of these water traces have been debated.

Mehdi Benna and colleagues present detections of anomalously high and episodic amounts of water in the lunar atmosphere by an instrument aboard NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE). The LADEE orbited the Moon from October 2013 to April 2014.

The authors find that most of these detections coincide with 29 meteor streams during the study period. By studying the amount of water released by meteor streams of different sizes, the authors determine that the uppermost 8 cm of lunar soil is dehydrated. Below this, they calculate that water is uniformly present at concentrations up to about 0.05%.

The authors estimate that meteorite impacts on the Moon cause the loss of as much as 200 tonnes of water per year. They also suggest that the subsurface water that is being released has been retained since the Moon formed, or soon after.

These findings may lay the groundwork for future investigation into the origin and fate of water on the Moon.