University of Iowa physicist Louis A. Frank and his former graduate student announced last May in a NASA news release and at an American Geophysical Union news briefing that images from their Visible Imaging System on the Polar spacecraft show Earth is showered by a steady stream of water-packed objects, small comets that bombard our planet at a rate of between five and 30 per minute. They published the results in the Oct. 1 Geophysical Research Letters. If true, the discovery would force a rethinking of the origins of the oceans, terrestrial life and the formation of the solar system.

In five independent studies to be published Dec. 15, scientists -- including three teams from The University of Arizona -- conclude that theoretical calculations and observational evidence rule out the small-comet hypothesis.

If the small-comet theory were correct, the Earth's sky would be a continual display of bright clouds and flashes, according to calculations by Bashar Rizk and Alex J. Dessler of the UA Lunar and Planetary Laboratory. If 30,000 small comets bombard Earth daily, as the theory says, constant meteor-like displays would be visible even during the day.

The expanding cloud of tiny ice particles that small-comet theory suggests is created when a 30-ton, 40-foot-diameter comet breaks up high in the atmosphere would have a brightness somewhere between that of Venus and the full moon, Rizk and Dessler calculate. (Dessler in the late 1980s published a review of several earlier scientific studies that rigorously tested small-comet theory.)

If, as small-comet theory says, a small comet strikes Earth about every three seconds, it would be visible for at least a minute to the naked eye, readily seen by anyone looking up, Rizk and Dessler add. "Where are they? We should see them," the LPL researchers puzzle.

"A whole-Earth flux of 20 comets per minute implies the sudden appearance of at least two bright patches of light every five minutes," they report in GRL. "The two-hour periods after sunset and before sunrise ought to produce the most spectacular sightings -- intermittent punctuations of bright, rapidly moving points of light." Twilight would be much more exciting in Tucson, Cairo, Sydney, Capetown and other communities, say Rizk and Dessler. Citizens of Fairbanks, Montreal, Moscow and Stockholm would be treated to near all-night meteor shows, they add.

Small-comet theory requires that the bombarding comets were formed in very cold regions far the from sun, Timothy D. Swindle and David A. Kring of the LPL note in their paper. Comets that formed far out in space condensed from the same dust and gas that accreted into planets, trapping "noble" gases in the same ratio as the sun and the rest of the solar system. Noble gases are inert, or non-reactive gases, not easily removed from the atmosphere. They include argon, krypton, xenon, as well as the more common nitrogen, helium and neon.

Swindle and Kring analyzed how much noble gas the small comets would have delivered to Earth's atmosphere over the lifetime of the solar system. "We know that if the Earth's atmosphere were bombarded according to small planet theory, it would have a dramatically different composition," Kring said in an interview.

At the current rate of supposed small comet bombardment, Earth should have 500 times as much krypton and xenon and 30,000 times as much argon in its atmosphere, Swindle and Kring calculate. Put another way, all the kypton and xenon in Earth's atmosphere would have been delivered by small comets in 10 million years. All the argon present would have been added in 100,000 years. The scenario for Mars' atmosphere is an even more enigmatic: Small comets would have delivered the known martian inventory of krypton and xenon in 500 years and the known inventory of argon in about 60,000 years.

Either the rate of supposed small-comet bombardment is today 30,000 times greater than it has been over the 4.5-billion-year lifetime of the solar system, or the comets formed much nearer the sun, about the distance of Jupiter, for the theory to fit the observed noble gas inventory, Swindle and Kring conclude. Comets can be greatly depleted in noble gases if they form closer to the sun, near Jupiter. "The problem with that idea is that it is completely inconsistent with several other physical conditions that Frank's team require to explain other features of their hypothesis," Kring said in the interview.

If the small-comet hypothesis is right, a small comet hits the moon at a rate of almost one per minute, Jennifer A. Grier and Alfred S. McEwen of the UA Lunar and Planetary Laboratory report in their paper. That is, scientists should see evidence of 400,000 comet hits on the moon annually.

Even a small, low-density comet would excavate a crater at least 50 meters in diameter and spread bright ejecta over an area of at least 150 meters in diameter, Grier and McEwen calculate. (The lunar surface darkens over time; the underlying, unexposed soil is lighter in color.)

Grier and McEwen compared Apollo 17 images taken in late 1972 to Clementine images taken 22 years later for a 52,000 square-kilometer area of the moon, which is about half the size of Kentucky and more than one-tenth of one percent of the lunar surface. Any crater and bright spot seen in the 1994 Clementine images but not visible in the 1972 Apollo photos might record a small impactor hit.

Each of the 3,920 bright spots seen over the study area in 1994 by the Clementine spacecraft was also recorded by Apollo. If the small-comet theory were correct, Clementine imaging should have discovered more than 10,000 new bright impact spots over this area.

Grier and McEwen calculate from the spacecraft observations an upper limit of 33 impacts a year for the entire moon, not 400,000 hits per year as expected according to the small-comet hypothesis. Small comets with properties hypothesized by Frank's team are probably more than a billion times less abundant than predicted, Grier and McEwen further conclude.

In another research article published in the Dec. 15 Geophysical Review Letters, a team of researchers who use a Polar spacecraft camera similar to the Frank team's Visible Imaging System also report seeing the dark pixels, or black points, that Frank interprets as evidence for small comets.

This team, however, concludes the dark pixels are an inherent camera feature, or "noise" rather than real features.

A fifth paper in the journal suggests meteorites as small as 50 centimeters in diameter create plumes of atmospheric gas that Frank and his team interpret as small comets. The large-scale analogy to this phenomenon is the Comet Shoemaker-Levy impacts on Jupiter, they report.