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Wilde believes that the composition of the oceans may be the crucial clue to the problem of the excess volatiles. If the water was here from the beginning then it could have changed over time. When you start with a reservoir of water, it is like a fresh pool. But if you let it sit, it will become increasingly stagnant. It deteriorates because there is no refreshing of the waters. On the other hand, if you have a source of freshwater coming in continuously, then eventually the ocean should come into equilibrium with that source.<\/p>\n\n\n\n
In the case of the oceans, if you start with as much water as we have now then you would expect the salinity, for example, and the amount of dissolved materials in it to change over time. But the geological record of the past five hundred years indicates that the salinity of the oceans has been relatively constant. This conclusion is based on living marine fossils that have a restricted tolerance for salinity but have been around unhanged for about half a billion years. One example of such a living fossil is the brachiopod Lingula<\/em>, a small invertebrate creature with a brown, tongue shaped shell that lives in mud or sand and has a preference for quiet waters.<\/p>\n\n\n\n Wilde realises that the small comets may bring in more than enough water to fill the ocean basins over time but he has no problem accounting for any excess. Extra water may be used to put water into the earth’s mantle and grease the motions of the continental plates, all the while keeping the actual ocean volume relatively constant. “Some of the excess water might be one of the reasons for plate tectonics,” ventures Wilde. “You have to have this type of lubricant otherwise everything would lock up occasionally.” It would be “a startling geological revelation” he adds, if the composition of the small comets should mimic the composition of the oceans.<\/p>\n\n\n\n There is no evidence that this may be the case – at least for comets in general. Studies of Halley’s comet conducted by the European space agency\u2019s tiny Gioto spacecraft in 1986 indicate that the comet’s chemical fingerprint seems to match that of the earth’s oceans. In the planetary sciences, the relative amounts of the isotopes of hydrogen and oxygen and sulphur present in a sample can serve as a fingerprint of its origins. Isotopes are the members in the family of a given element such as hydrogen, for instance, which has an isotope called deuterium. Hydrogen differs from the heavy-duty deuterium atom in the weight of its nucleus.<\/p>\n\n\n\n