Old rocks drown dry Moon theory.

Abstract

Apollo 12 A basalt sample from this landing site has water with an unexpected proportion of a heavy hydrogen isotope, raising questions about the origin of the Moon's inner water. Larry Taylor always said he’d eat his shorts if water was ever found on the Moon. He never expected his own research to bring that pledge back to haunt him. The petrologist, based at the University of Tennessee in Knoxville, was just 32 years old at the first Lunar and Planetary Science Conference in 1970, where his colleagues described their analyses of Moon rocks collected the previous year during the Apollo 11 mission. Taylor saw only pure metallic iron in the samples — a sign that there wasn’t any water around to rust the iron. This and other results that year led to the party line: the Moon is, and always was, bone dry. Forty years on, at the same annual conference near Houston, Texas, Taylor and his colleagues announced that they have been wrong all along. At the meeting last week, three groups presented evidence that certain crystals in the volcanic rocks collected by Apollo astronauts contain as much as several thousand parts per million of water. These findings go much deeper than the glimpses of frozen water on the Moon’s surface — discoveries that were made recently by India’s Chandrayaan-1 and NASA’s Lunar Reconnaissance Orbiter spacecraft (see Nature doi:10.1038/ news.2009.931; 2009). The new studies of the Apollo samples provide hints of what lurks within the Moon. The results suggest that the lunar interior has always held some water — challenging theorists to change their thinking about how the Moon formed during a fiery impact, and how the once-molten body cooled. The work also hints that comets have played a more important part in delivering water to the Moon than researchers had previously thought. “This is revolutionary,” says Linda ElkinsTanton, a lunar scientist at the Massachusetts Institute of Technology in Cambridge. As for Taylor, the one-time water sceptic has eaten his words — and more. In January, colleagues gave him a chocolate cake, iced white with pink polka dots to represent his boxer shorts. The first clues to a wet lunar interior were published in 2008, says Taylor, after researchers found traces of water buried inside beads of volcanic glass found in Apollo samples (A. E. Saal et al. Nature 454, 192–195; 2008). It took advances in ion mass spectrometers to detect such tiny amounts of water. It also required people willing to challenge 40 years of conventional wisdom and endure the chuckles of disbelieving colleagues. Although the volcanic beads offered evidence for inner water, those rocks also had limitations. The beads formed in violent, ‘fire-fountain’ eruptions that significantly altered their chemistry, making them uncertain proxies for rocks inside the Moon. The new detections of water came from a different source — tiny apatite crystals found within dark basalt from the Moon’s maria, or ‘seas’. The crystals, which took shape within the vast fields of flowing lava that once filled the maria, contain much higher amounts of water than the glass beads. Because the basalt came from quieter eruptions than the dramatic fire fountains that formed the glass, Taylor says, the chemistry of these rocks can more easily be used to calculate the water content of the original magma body from which they came, deep in the Moon’s mantle. The three groups at the meeting reached different conclusions about the past water content of the Moon, but all three suggest that the lunar interior could have contained tens of thousands of times more water than previously thought, although still orders of magnitude less than Earth. A Moon with so much moisture would have been a more active place. Water lowers the melting point of mantle rock and makes it easier for magma to form. It even allows for the possibility of convection of rock inside the Moon, something long discounted. This would have helped the Moon to cool more quickly than researchers previously thought, and could explain some puzzling aspects of lunar geology. Samples collected during Apollo missions suggest a wet interior, raising questions about lunar origins. Old rocks drown dry Moon theory

DOI: 10.1038/464150a

Cite this paper

@article{Hand2010OldRD, title={Old rocks drown dry Moon theory.}, author={Eric Hand}, journal={Nature}, year={2010}, volume={464 7286}, pages={150-1} }