Volcanoes as water suppliers: As a reconstruction suggests, as yet unrecognized layers of water ice could be hiding under the surface of the moon. Accordingly, primeval volcanic eruptions on the Earth’s satellite released so much water vapor and other gases that atmospheres were created temporarily. Enough water ice froze out of these to form layers of ice up to 400 meters thick – relics of which may still be preserved under the regolith to this day.
It is now clear that there is water even on the moon. The precious water is bound in the minerals of the moon rocks, but also as water ice in the shady, cold craters of the polar regions. However, where this lunar water ice came from and how much it is is still a matter of dispute. Possible sources are asteroid and comet impacts, but also the plasma tail of the earth or the solar wind.
Volcanic eruptions as water suppliers?
But there is another potential source of water on the moon: the lunar volcanoes. The extensive, dark basalt surfaces of the Mondmare and numerous lava caves are evidence that the moon was volcanically active until around two billion years ago. Samples from the Apollo missions also suggest that the lunar magma also contained water. Accordingly, larger amounts of water vapor could have been released during the eruptions.
Andrew Wilcoski and his colleagues from the University of Colorado in Boulder have now investigated how much water vapor this was and where it went. Their hypothesis: After a lunar volcanic eruption, part of the released water vapor could have condensed as ice on the cold moon surface. This frozen water ice could then have accumulated as a thick layer of ice in the polar cold traps – if the deposition outweighed the water loss through sublimation and outgassing into space.
The moon had an atmosphere – but only part of the time
Wilcoski and his colleagues investigated whether this scenario is realistic and what it could mean for the water ice deposits that still exist on the moon today, using a physical simulation that took into account, among other things, the water content of lunar magma, condensation and sublimation, and lunar temperatures. The researchers calculated the number and size of the lunar eruptions on the basis of previous studies, which assume around 100 cubic kilometers of lava per eruption and around ten million cubic kilometers in total.
The reconstruction showed: On average, a lunar volcanic eruption released around 1.2 billion tons of water vapor along with other gases – enough to give the moon a thin atmosphere, at least temporarily. “The lifespan of such an atmosphere was around 2,500 years,” the researchers report. During this time, some of the gases escaped into space, but around 40 percent of the water vapor froze out and was deposited as frost.
Frost layer once visible from Earth
The frost condensing out of the volcanic gases occurred everywhere where the lunar surface was cold enough: at the poles, but also on the night side of the moon. “Because of the strong daily fluctuations in lunar temperatures, the frost could have formed on the night side at all latitudes,” report Wilcoski and his colleagues. The layer of ice grew during the night and reached its greatest thickness just before dusk.
This lunar “rime” may even have been thick and extensive enough to be visible from Earth. “It would be seen as a bright layer stretching from the morning terminator across the night side,” the researchers describe the sight. White ice caps would also have been visible at the poles.
Thick layers of ice at the lunar poles
But the bottom line is that while most of the nocturnal frost deposits evaporated again during the following lunar day, this was different in the permanently cold polar regions of the moon: temperatures there were low enough to hold the water ice during the lunar day. According to the reconstruction, an ice layer up to 410 meters thick could have formed over time in the regions beyond the north and south 80th parallel.
The amount of water ice accumulated there is considerable: According to the researchers’ calculations, three billion tons of water ice could have been deposited at the North Pole, and even more than five billion tons at the South Pole. Although some of these primeval ice reserves have sublimated to this day, most of this water ice may have survived. The team put these deposits at up to 3.5 billion tons at the lunar South Pole and 1.65 billion tons at the North Pole.
Available to date?
But where is this ice? “The fact that these massive ice deposits are not visible on the Moon today means they must be buried or mixed with regolith,” Wilcoski and his colleagues say. Above all, the impacts of micrometeorites and meteorites over the course of millions of years mixed up the upper meters of the lunar subsoil in such a way that the ice was gradually covered.
“It is therefore quite possible that thick layers of ice are hidden five to ten meters below the surface of the moon,” says co-author Paul Hayne. In his view, robotic excavations could therefore be worthwhile in future missions to the lunar south pole. “We should really start looking for that,” Hayne said. Because for future astronauts and lunar colonies, this water ice would be a welcome and necessary resource. (The Planetary Science Journal, 2022; doi: 10.3847/PSJ/ac649c)
Quelle: University of Colorado at Boulder