The waters of Luna
UREY INSISTED THAT THE MOON ABOVE WAS ANYTHING BUT ‘BONE-DRY’
Harold Urey was right after all. Urey ( 1893-1981) was easily one of the greatest scientists of the 20th century; he was the discoverer of deuterium, led the isotope- separation effort for the Manhattan Project and founded the field of “cosmochemistry,” which is what we are talking about in this column.
Cosmochemistry is the study of why celestial bodies, including the one we’re on, have the mix of substances they do. This area of research is just at the outset of a long history, as is demonstrated by the fact that we still aren’t sure how the hell Earth ended up with so much water. The stuff ’s everywhere — it falls from the sky — but nobody’s too certain where exactly it came from originally.
When Urey began to jot down speculations about the chemical composition of the planets in the 1950s, he argued that exploration of the moon was the most immediate and affordable way to gather clues to questions like that.
Our moon, Urey said, was the “Rosetta Stone of the solar system.” Urey’s arguments pointed the U. S. military-industrial complex in the direction of the moon, and when photography missions began to return close-up photos of its surface in the 1960s, he was among the first scientists to see them.
But things got weird. Urey looked at lunar photographs and saw features he found easiest to explain by the prior presence of water on the moon. It couldn’t have sat on the surface for long, or in great quantities, since there are no rivers or streams apparent. But he wondered if lunar craters could at some point have been filled with water or ice, and the moon’s face is full of “sinuous rilles” (now there’s a band name just lying around waiting to be picked up) that must have resulted from the flow of some fluid. Urey was not, at first, convinced of the consensus that this fluid had to be lava.
Urey’s idle speculation did not go down well, involving him in exhausting, unpleasant controversies. If you know the history of science, you remember that when Mars was first photographed carefully from the Earth, people went off on all kinds of wild tangents, interpreting improbably straight lines as “canals” built by some sort of Martian civilization.
Urey seemed to some colleagues to have been caught in analogous tomfoolery.
And when the Apollo project finally sent men to the moon — although Urey’s personal offer to take a one- way trip was rejected — the samples they brought back suggested that the moon was “bone- dry.” The “bone- dry moon” became the overwhelmingly dominant hypothesis well into the 21st century, even though, as the pedants sometimes point out, bones are actually one-third water.
Well, last week brought the culmination of a scientific revolution: Earth’s moon turns out to be fairly moist after all. The evidence for this had built up slowly, and involved some backtracking. There was actually quite a lot of potential evidence for lunar hydration in the Apollo cornucopia of samples; it was just that anything pointing to water tended to be dismissed as resulting from terrestrial contamination, because moon dust interfered with the carefully designed seals on the astronauts’ sample- collection equipment, or it was chalked up to recent meteor impacts.
Meanwhile, in 1976, the Soviets physically drilled into the moon with an unmanned probe and brought back a core sample that was seemingly about one part water per thousand. With the golden age of lunar exploration over, this finding was largely neglected in the West. What, are you going to trust a Russian?
Urey died with his dream of a dewy moon in tatters, but a few fringe weirdos sensed that the possibility of lunar water had not been ruled out.
Around the turn of the century, a number of lunar exploration missions began to detect possible signs of water. One probe, NASA’S Clementine ( 1994), took radar readings of the lunar surface as part of an experiment that was actually cooked up, half-whimsically, after launch.
Clementine found doubtful evidence of plenty of water around the moon’s poles. Other probes equipped with spectroscopes detected signals that could have been water, but they were designed to read the wrong part of the spectrum, and couldn’t distinguish water from other possible molecules containing hydroxyl groups.
Funnily enough, this question has not been settled by a space probe, but by a Boeing 747 with a really good infrared camera — the Stratospheric Observatory for Infrared Astronomy ( SOFIA), a joint project of NASA and the German Aerospace Center.
SOFIA results published last week in Nature Astronomy reveal the presence of what can only be molecular water concentrated at the poles. The water is thought to lurk mostly in lunar “cold traps” — shadowed parts of the surface never exposed to sunlight, which would otherwise boil it off.
Or, to put it another way: “Near (the moon’s) poles there may be depressions on which the sun never shines, where condensed volatile substances might be present.” This is a perfect description of the cold traps whose existence is now strongly suspected. It comes, of course, from Harold Urey’s 1952 book, “The Planets: Their Origin and Development.”
Sometimes, if you want to know the future, the best approach is to dig out an old book.