The Moon's watery past

While most astronomy textbooks since the 1960s report that the Moon is bone dry, there is some recent evidence that may force future revisions. Last year, a team of researchers led by Alberto Saal of Brown University and Erik Hauri of the Carnegie Institution reexamined small, green volcanic glass beads collected by Apollo astronauts back in 1971 and 1972.

Using a new, advanced microprobe technique, the researchers were astonished to discover that the extraterrestrial glass beads contained as much as 46 parts per million of lunar water. That's far more water than anyone ever imagined could be locked up inside "bone dry" moonrocks. When Saal and Hauri extrapolated the calculations back in time, they found that the Moon's interior may have held as much as 750 ppm of water.

"This suggests the very intriguing possibility that the Moon's interior might have contained just as much water as the Earth's mantle," Hauri told reporters last year.

Clues left by the glass beads have cast a shadow on the current, popular explanation for lunar origin, the so-called Big Whack Theory.

According to this theory, the early Earth was smacked by a Mars-sized protoplanet 4.5 billion years ago; the event spewed a vast ring of molten crust into space that quickly coalesced, in orbit, to form the Moon.

And here's the rub: a Big Whack impact would have vaporized volatile materials - especially water. Yet the glass-bead game played by Saal and Hauri reveals that lots more water was left behind after the Moon formed.

The discovery of "hidden" water in lunar glass - and recent studies showing even more water locked up inside the Moon as ice - calls to mind a discarded theory proposed by the late J.J. Gilvarry back in the 1950s.

Gilvarry was an astrophysicist with the National Academy of Sciences and the RAND Corporation. He calculated that the Moon, with a heritage clearly linked to Earth, would have outgassed enough water to fill seas to a depth of 2 km (1.2 miles). The idea of a primordial wet Moon was popular in the early 20th century. But at the dawn of the Space Age, Gilvarry was one of the few wet-Moon holdouts.

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