WE THOUGHT it was dry as a bone, but now it seems the moon鈥檚 parched surface has water hidden beneath it.
Alberto Saal of Brown University in Providence, Rhode Island, and colleagues re-examined lunar volcanic rocks collected in the 1970s during the Apollo 15 and 17 missions. They found up to 50 parts per million of water trapped in tiny spheres of volcanic glass.
That鈥檚 not much compared with the 500 to 1000 ppm of water in Earth鈥檚 mantle. Yet when the lava erupted some 3 billion years ago it was exceedingly hot 鈥� perhaps up to 7000 掳C 鈥� so most of the water initially present would have diffused out of the magma, leaving only a small amount by the time it cooled.
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To find out how much water was in the magma, Saal鈥檚 team measured its concentration 鈥� as well as volatile chemicals such as chlorine 鈥� at the core of the glass spherules and compared them with levels at the outer edge to work out how fast each was lost.
They concluded the moon鈥檚 mantle has between 260 and 700 ppm of water. 鈥淭his is very surprising, because for 40 years people have studied lunar rocks and no one found any water,鈥� says Saal. 鈥淲e got lucky.鈥�
Saal cautions that future crewed missions will not be able to wring any water out of the rocks. There is, however, a slim chance that the vapour has accumulated in ice somewhere on the moon鈥檚 surface.
