A computer model of the satellite's ancient volcanic activity has shown how ejected water has been frozen to the surface for thousands of years, forming layers of ice many metres below the poles.
Water has been proven to exist on the Moon. But its origin, quantity and distribution remain the subject of much research. According to one hypothesis, some of this water could be of volcanic origin.
Using a computer model, scientists at the University of California at Boulder (USA) have plotted the dynamics of volcanic gases after an eruption: how they enveloped the satellite, flew into space and settled on the surface. The article was published in The Planetary Science Journal.
Planetary scientists calculate that the Moon was a fairly hot place at the beginning of its existence. Tens of thousands of eruptions occurred on the satellite between four and two billion years ago. Lava flowed in rivers, spreading over vast areas. The dark spots on the moon's surface - called seas, lakes and marshes - are the same lava that froze in the lowlands. But scientists searching for lunar water are interested in another eruption product: volcanic gases.
A few years ago, experts at the Lunar and Planetary Institute in Houston calculated from the volume of lunar basalt that the gases ejected formed clouds of carbon dioxide and water vapour - a rarefied temporary atmosphere. The analysis showed that, at the peak of volcanic activity, this atmosphere was one and a half times denser than the current atmosphere of Mars.
The authors of the new study decided to test whether these clouds could have frosted the surface. Scientists estimate that the Moon erupted on average every 22,000 years to form an atmosphere during that period. A computer model of the dynamics of these volcanic eruptions showed that much of the gas and steam escaped into space but about 41% of the water was deposited on the satellite's surface.
"The atmosphere had been thinning for about 1000 years - enough time for ice to form," commented Andrew Wilcoski of the Laboratory of Atmospheric and Space Physics at the University of California at Boulder. - In our mind, [this precipitation] was like frost that had been accumulating for centuries."
There was probably so much ice that, from Earth, you could see a thin, shiny band on the moon at the boundary of day and night. And thick ice caps covered the poles of our planet's satellite. The model estimates that about 8.2 quadrillion litres of water could have been deposited on the moon over the entire eruption period.
"There are probably thick layers of ice five to ten metres below the surface," says study co-author Paul Hayne, who has long studied water on the moon.
Volcanic activity is not the only potential source of water on the Moon. Asteroids and comets may have brought a significant fraction of the water. Water could also have been generated by solar winds. A study of the amount, distribution and composition of lunar ice will help confirm or refute these hypotheses.