'waterworld' May Have Been Moon's Past

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By Louis Varricchio, M.Sc.

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 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.

Gilvarry’s calculations regarding the behavior of water and atmospheric gases on the Moon were made with great care—they were based upon Sir James Jeans’ calculations of the amount of time it takes a planet’s atmosphere to escape into space.

Gilvarry demonstrated that our natural satellite could have supported an Earthlike atmosphere and surface water for millions of years—that’s short by Earth’s atmospheric and hydrospheric timeline, but still long enough to be interesting.

When astronomers today say the Moon is too small to have supported either an atmosphere or liquid water, they may need to review the math. According to Gilvarry’s data, our Moon, a dwarf planet of sorts, could have retained an appreciable atmosphere long enough for it to act as a “lid” on surface water.

An intriguing footnote: Gilvarry employed Baldwin’s curve of crater depth-diameter values alongside U.S. Atomic Energy Commission crater data gleaned from the “Ivy Mike” fusion-device test in the Marshall Islands in 1952. Why? Well, while most of the Moon’s highland craters fit Baldwin’s depth-diameter curve for impact craters formed on land, the shallow slope lunar maria  (the so-called “lunar seas”) matched Baldwin’s depth-diameter curve of a typical impact crater formed in deep water. According to Gilvarry, the maria profiles looked a lot like the underwater crater that marks ground zero of the “Ivy Mike” shot. (The explosion vaporized the Marshall Islands atoll of Elugelab and left behind a 6,240 feet wide submarine crater.)

Gilvarry might have relished the irony of demonstrating that Galileo’s misnamed “lunar seas” may actually have been formed in ancient lunar seas. 

All that aside, there’s no hard proof that the ancient Moon had an extensive hydrosphere as Gilvarry proposed. Yet, Saal’s and Hauri’s water-rich Apollo glass beads may prompt young, openminded scientists to revisit a long lost lunar theory of the 1950s that went out of fashion alongside Detroit’s auto mobile tailfins.

Louis Varricchio, M.Sc., is a former NASA science writer. He is a member of the NASA/JPL Solar System Ambassador program and the Association of Lunar and Planetary Observers.

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