The moon is not exactly the shrivelled prune of a satellite we once thought. Beneath its dusty surface there is water – quite a lot of it. A recent analysis of lunar rocks reveals that they have the same concentration of water as the Earth's upper mantle, the layer of near-molten rock just beneath the crust. The findings leave traditional thinking about how the moon formed in deep water.
"For the last three years geochemists and planetary scientists have been circling around the possibility of water-rich domains in the lunar interior," says 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. "This study puts the question to rest. It's the first clear evidence."
In 2008, Alberto Saal of Brown University in Providence, Rhode Island, and his colleagues discovered small amounts of water – some 50 parts per million – inside tiny bubbles of volcanic glass collected during the Apollo 15 and 17 missions in the 1960s and 1970s.
These rocks formed around 3 billion years ago from the magma ejected by lunar volcanoes. But many scientists were sceptical of the result.
Now a new analysis of glass beads from the same sample by many of the same researchers 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. The researchers used a focused beam of caesium ions to dislodge molecules of water from the inclusions and measured the amount with a mass spectrometer.
That moon rocks can hold any water is a surprise because it contradicts the prevailing ideas about how the moon formed. Current thinking is that some 4 billion years ago, a Mars-sized object smashed into the proto-Earth, knocking off a chunk of planet that eventually formed into the moon. But such a violent collision should have vapourised any water inherent to the moon.
"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 Eiler 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."
In late 2009, NASA's Lunar Crater Observing and Sensing Satellite (LCROSS) crashed into a crater near the moon's south pole at 9000 kilometres per hour, sending at least 120 litres of water flying away from the impact site amongst the debris.
Scientists had assumed that these pockets of frozen water at the moon's poles came from external sources like meteors or comets. But 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
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