The Moon May Have Had Lots of Water in the Distant Past

And that raises its interest to astrobiologists.

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Dry as dust today, but maybe it wasn’t always that way.

Yanhao Lin and colleagues from the University of Amsterdam in the Netherlands claim in a recent paper that the early Moon was rather wet, containing at least 0.3 to 1.7 percent water at the time the liquid magma covering its surface crystallized, which happened shortly after the Moon formed. This could equate to water ocean depths of 0.5 to 3 kilometers, assuming that the water was retained for some period of time.

The Moon is thought to have originated from a gigantic impact 4.5 billion years ago, which explains its inclined orbit, low density, the high spin of the Earth-Moon system, and the small amount of water, gases, and light elements in general. Indeed, the most characteristic feature of lunar rocks has been how devoid of water they are, leading to the traditional view of the Moon as a dead, dry rock in space.

More recent work, however, together with the finding of clay (usually taken as evidence of contact with liquid water) within Apollo lunar samples, calls into question how dry the Moon really was immediately after its formation. Was there liquid water standing in some impact craters for extended time periods? Might there even have been a limited water cycle? If so, it certainly was short on a geological time scale, but how short exactly? These are incredibly difficult questions to answer, because any traces of water-induced landforms would be long erased by more than four billion years of pounding by solar wind, cosmic radiation, and micrometeorites.

Even without much water, the Moon is important to astrobiology. Large impact craters at the south pole contain much water ice, which can be retrieved for human consumption should we ever build a lunar base. There may be an even bigger treasure in lunar ice: We might find organic molecules dating back to the origin and early evolution of life on Earth. It is quite plausible that rock fragments were dislodged from Earth during meteorite impacts more than 3.5 billion years ago and later buried on the Moon. If so, the Moon is the only location where we might find these remnants, which could help solve the riddle of how life originated on Earth—a strong motivation indeed to visit the Moon again.

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