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Sunscreen 4 billion years ago.

Sunscreen 4 billion years ago

Scientists have often wondered what kind of molecule shielded the first living cells from a sizzling death due to ultraviolet solar radiation. A group of atmospheric researchers is proposing the newest solution to this problem: sulfur.

In today's world, ozone molecules (O.sub.3) in the stratosphere absorb the dangerous ultraviolet light. This ozone zomes from oxygen molecules (O.sub.2) lower in the atmosphere that drift up and are broken down by sunlight. But most scientists believe oxygen did not start accumulating in the atmosphere until relatively recently, about 2.5 billion years ago, when photosynthetic algae arrived on the scene. Since the first living organisms are thought to have developed some 3.5 billion to 4 billion years ago, that leaves about 1 billion years of life without atmospheric ultraviolet protection.

In ozone's absence, ring molecules of sulfur atoms, particularly S.sub.8, might have done the job of absorbing ultraviolet light, theorize James Kasting from Pennsylvania State University in University Park and his colleagues. Sulfur was definitely available, with volcanoes pouring out some 10.sup.12 tons of it each year. Yet no one knows whether the sulfur disappeared into the oceans or remained in the atmosphere. The sulfur hypothesis requires that Earth's surface temperatures remained above 45[deg.]C (113[deg.]F), or else the sulfur molecules would condense out of the atmosphere.

While Kasting's group suggests sulfur as a screen, many other scientists believe there was no need for an ultraviolet blocker in the primordial atmosphere, since some primitive life forms seem to be well adapted to high levels of ultraviolet.
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Title Annotation:sulfur may have screened ultraviolet light
Author:Monastersky, Richard
Publication:Science News
Date:Dec 24, 1988
Words:271
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