THE GENTLE COSMIC RAIN

By Jeffrey Kluger

Hardly anybody took Louis Frank seriously when he first proposed, more than 10 years ago, that the Earth was being bombarded by cosmic snowballs at the rate of as many as 30 a minute. Part of the problem was how preposterous his theory sounded: every day, he suggested, tens of thousands of icy comets, each the size of a small house and containing 40 tons of water, were vaporizing in the upper atmosphere and raining down on Earth. It didn't help that the University of Iowa physicist happened to release his findings on April 1, 1986. "Newspapers," he recalls, "phoned to ask if this was an April Fool's joke."

Frank is unlikely to hear that kind of question again. Last week, at the American Geophysical Union's annual convention in Baltimore, Md., he backed up his theory with fresh evidence: satellite images that capture his cosmic hail in midflight. Suddenly it seems entirely possible that the source of much of the water on Earth--and even of life itself--might be Frank's "gentle cosmic rain."

Frank first began formulating his theories in the 1980s when he was analyzing satellite pictures and found the atmosphere in the images flyspecked with thousands of spots. The altitude of the flecks and the wavelength of light they absorbed led him to conclude that they were clouds of extraterrestrial water that had somehow been carried to Earth.

Few others were convinced, however, because of the graininess of the images. So Frank came to the Geophysical Union meeting last week armed with better photos from a new satellite, and this time his pictures carried the day. "We have a large population of objects that have not been detected before," he now says confidently. Extrapolating from the number of those objects he saw in his pictures, he estimated that as many as 43,000 of these celestial snowballs arrive on Earth every day.

How could so many comets stay hidden so long? For one thing, they are not true comets. Big-name comets like Hale-Bopp may measure 20 miles across--giants compared with Frank's 40-ft. pellets. The large comets are studded with rock and metal, while Frank's are almost all water.

It's a good thing for us that so much of what rains down on Earth amounts to cosmic bird shot. A collision with a full-scale comet would be a global calamity; the minicomets, by contrast, are remarkably fragile. Well before they hit the planet--between 600 miles and 15,000 miles up--they begin to disintegrate. Sunlight then breaks them down further, transforming them into ordinary clouds that produce ordinary rain. Over the course of 20,000 years, this cosmic sprinkling can add an inch of water to the planet; multiplied by the Earth's 4.5 billion-year history, that could help account for the oceans themselves.

But it is not just water the comets import. In order to remain intact in space, they must be held together by a supporting shell. Frank believes that shell is made of carbon, created as cosmic rays break down traces of methane. Carbon, of course, is a basic building block of biology. It may be, Frank says, that his comets carried the very stuff of life to Earth, helping give rise to all the planet's creatures. That may still sound preposterous, but this time nobody's saying April Fool.

--By Jeffrey Kluger