The Moon's so-called seas are dry, but our satellite may still host frozen lakes, and these could provide crucial water for astronauts on any future lunar bases.
Lunar volcanoes stopped erupting more than a billion years ago, but a team of scientists thinks they left us a precious legacy that could make establishing bases on the Moon far more practical. If the team is right, some craters contain thick sheets of ice that could save long-term Moon missions the immense costs of transporting water from Earth or extracting it from what are thought to be less concentrated deposits.
The case for volcanic origins of lunar ice deposits is set out in The Planetary Science Journal. So far it is based on models of lunar volcanism, and conditions since, rather than direct observations.
According to Colorado University Boulder graduate student Andrew Wilcoski and co-authors, the ancient Moon would have had water in its interior, just as the Earth's mantle does. Volcanoes would have spewed out this water during the eruptions, along with a lot of ash, lava, and other gasses. This would have created a thin, temporary atmosphere of water vapor and carbon monoxide. Some of this water would have settled on the Moon's surface and, if out of direct sunlight, turned to ice.
“We envision it as a frost on the Moon that built up over time," Wilcoski said in a statement. For a time the Moon's poles may even have glinted silver near the border between the day and night sides.
In places where the Sun gets high in the sky the ice would have long ago sublimed to gas and been lost. Near the poles, however, it would have a far better chance of survival at the bottom of craters that have not experienced sunlight since their formation. The Moon is not nearly as tilted compared to its orbital plane as the Earth, and so lacks strong seasons.
We know from meteorites and the Lunar Reconnaissance Orbiter that water ice exists at the lunar poles, but as the paper notes, “Its origin, abundance, and distribution are not well understood.” The widespread assumption is it was delivered by comets colliding with the Moon or the solar wind, in which case it may be limited to small amounts near the surface.
Wilcoski and team's work is more optimistic. The paper estimates 40 percent of the water vapor released by lunar volcanoes between 4 and 2 billion years ago settled at the poles, and in some places the ice would be hundreds of meters thick – enough to keep a lunar base supplied for a very long time.
"It's possible that 5 or 10 meters below the surface, you have big sheets of ice," said co-author Dr Paul Hayne.
If the authors are right, the deposits are not symmetric, with the south pole having almost twice the ice mass of the north. This reflects the fact the lunar south pole has larger and colder “cold traps” in which the ice could accumulate, based on a combination of depth and proximity to the pole.
Following recent evidence plants given water can grow in lunar soil, albeit with difficulty, the prospects for a lunar base capable of sustaining itself for substantial periods look good.
An intriguing aspect to the work is that the authors think the atmosphere created by large eruptions could have lasted for thousands of years before settling or escaping to space. The carbon monoxide would have made it poisonous to any visitors with human-like biology, however, and with an average of 22,000 years between eruptions, for most of the time the Moon would have been more like today.