Cosmic rain

Published: Monday 30 June 1997

The seas are supposed to be the result of condensation of water vapour originating from the Earth itself. Now, there is evidence that the water may have a cometary source: the Earth's outer surface is being constantly showered with gigantic snowballs weighing 20-40 tonnes at a,6te of close to 43,000 per day. The snowballs disintegrate and evaporate harmlessly in the outer atmos phere, some 1,000-24,000 kin above the Earth's surface, eventually com ing down as rain. Calculations show that this water could account for the oceans over billions of years.

The evidence comes from the National Aeronautics and Space Administration's orbiting space- craft, Polar. The data was interpreted by a research team led by Louis A Frank of the University of Iowa, US. Frank had first suggested this bombardment I I years ago. At that time, it had been dismissed by most scientists as ridiculous. The expla- nation was required to account for dark holes in the Earth's atmos- phere photographed by Polar. According to the explanation, the holes were a result of absorption of solar radiation by the snowballs when they hit the atmosphere and evaporated. Scientists agree that the evidence will have to be confirmed since it has wide ramifications. For in-stance, the concept of the hydro- logical cycle will need rethinking.

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