HAS THE EARTH SPRUNG A LEAK?

Geochemist Emily Pope of the Natural History Museum of Denmark was taken by surprise when she and her team examined the cliffs of Greenland and found remnants of prior oceans. The serpentine rocks, which are 3.8 billion years old, were formed on the bed of a primeval sea. But what happened to the water that had once covered this land? When they examined the serpentinite, researchers made an amazing discovery: The water 3.8 billion years ago was far lighter than that of today’s oceans because it contained considerably less deuterium. Deuterium is an especially heavy form of hydrogen, which, with one proton and one neutron in its nucleus, is twice as heavy as the ordinary element. It tends to remain in seawater even after evaporation. Therefore a higher concentration of deuterium provides clues as to how much water the oceans used to contain. According to these calculations, about one- quarter of Earth’s water has disappeared over the past 4 billion years in the form of lighter hydrogen that evaporated. Emily Pope and her team found two sources of this loss: Some disappeared when the continents were created, and the rest dissipated into space because the early atmosphere contained far less oxygen that could have combined with hydrogen to form water. Today hydrogen that evaporates returns to the Earth in the form of rain. Nevertheless the loss continues. Some 100,000 tons of hydrogen vanish into space each year. But not to worry: The oceans will not dry out. The water loss causes the level of the oceans to drop by only a tiny fraction of an inch per year.