HOW much water lies beneath the surface of the Earth? Recent measurements that pin down the properties of hot, wet rocks could help settle the dispute between scientists who think the deep layers of the Earth are dry and those who believe there is more water locked up in the mantle than there is in the oceans.
The types of rock that exist deep in the mantle between 410 and 660 kilometres down, where temperatures exceed 1000 掳C, can hold as much as 3 per cent of their weight in water. This has led some scientists to propose that there is a vast reservoir of water within the planet which could have replenished the surface oceans after the young Earth was bombarded by asteroids and blasted by a brighter sun.
However, the fact that minerals in the lab can hold water doesn鈥檛 mean they do so in the mantle, say critics. In rocks formed where the mantle pushes up through the Earth鈥檚 crust the amount of water is very small.
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But now it should be possible to look for the signature of water in the mantle itself, rather than relying on supposition or rocks that have uncertain histories. Joseph Smyth from the University of Colorado in Boulder has been working with colleagues to measure the velocity of earthquake waves in wet mantle rocks by simulating the high temperatures and pressures of the mantle in the lab. His latest results suggest that watery regions of mantle will leave a distinct signature in the seismic measurements collected after every major earthquake. This kind of information should be readily available 鈥� someone just has to start looking for it in earthquake records.
