A new analysis of the volcanic glass beads, collected during the Apollo 15 and 17 missions in the 1960s and 1970s, by Alberto Saal and many of the same researchers from Brown University in Providence, Rhode Island, who analyzed it in 2008, has revealed that the rocks actually contain at least 10 times more water, between 615 and 1410 parts per million. These levels are comparable to the 500 to 1000 parts per million of water in the Earth's upper mantle.
This time around, Erik Hauri of the Carnegie Institution of Washington analysed a different part of the rocks, called melt inclusions. These are globules of once-molten rock trapped inside crystals in the rock. The crystals protected the molten globules – and the water inside – from the huge changes in pressure as the rocks were ejected from the moon's interior, which would otherwise suck out any water and gas.
Brown University undergraduate Thomas Weinreich searched through thousands of glass pebbles from a sample of titanium-rich "orange soil", discovered by astronaut Harrison Schmitt during the Apollo 17 mission, to find 10 that contained melt inclusions.
"If the whole moon has the amount of water equivalent to what we analysed, then the giant impact theory is in a little trouble," says Saal. "How the water got there is a question people need to work on."
But John Eiler of the California Institute of Technology in Pasadena, who has studied water on the moon but was not involved in the new study, cautions that the new discovery does not mean that the moon is swollen with water like a ripe melon. "These glass fragments are quite exotic in their composition," he says. "They don't represent the majority of the interior. It doesn't tell you the moon as a whole is water rich, only that there is evidence of water-rich domains."
Hauri's new study raises the possibility that volcanic eruptions early in the moon's history may have created a water-rich cloud – a transient atmosphere – that rained down and froze in the moon's darkest and coolest patches.
The next step, says Saal, is to measure the hydrogen isotopic composition of the ice caps and compare it with the isotopic composition of the volcanic-glass pebbles. If they are roughly the same, it would support the idea that the ice came from volcanic eruptions. If they are different, a more plausible explanation is that the water came from comet and meteorite impacts.
Journal reference: Science DOI: 10.1126/science.1204626