SAN FRANCISCO--The provocative idea that thousands of house-sized comets disintegrate daily in Earth's atmosphere came under heavy fire by skeptics here at the fall meeting of the American Geophysical Union. But the theory's main proponent countered with new evidence that satellite images of the comets are real. And as the two sides locked horns, an alternate proposal surfaced: The claimed traces of comets may instead be vapor trails from small meteoroids.
More than a decade ago, space physicist Louis Frank of the University of Iowa in Iowa City proposed that countless icy comets break up high above the planet, perhaps supplying the bulk of Earth's water over the millennia. Recent images of dark "holes" in ultraviolet (UV) satellite images of the atmosphere have bolstered the idea. However, a statistical analysis of instrument noise in the satellite cameras, conducted by University of Washington, Seattle, geophysicist George Parks, suggested that the spots are artifacts--the equivalent of TV "snow" (Science, 14 November, p. 1217 ).
Today Frank responded with a new analysis of the satellite images. The number of atmospheric holes in the images drops by about 80% when the satellite is farther from Earth, says Frank. That's just what one would expect from real impacts, he says; instrument noise would show the same pattern at low or high altitudes. "This is very powerful evidence that the holes are real," Frank adds.
But now, an armada of planetary scientists from the University of Arizona, Tucson, has posited three new arguments that the comets cannot exist. Their studies, to appear on 15 December in Geophysical Research Letters, imply that the comets would spark frequent bright flashes above Earth, create bright impact craters on the moon, and throw off the balance of the noble gases argon, krypton, and xenon in our atmosphere. None of these effects, the scientists say, is evident. Frank counters that not enough is known about the comets to make those predictions.
There could be another explanation for the holes--if they are real. Supercomputer simulations by physicist Mark Boslough of Sandia National Laboratories in Albuquerque, New Mexico, show that meteoroids as small as 50 centimeters across would expel energetic, oxygen-poor plumes behind them as they burn up. Such plumes would appear on UV satellite images as the type of dark spot that Frank sees. However, to account for thousands of impacts per day, the meteoroids would need to be about the size of tennis balls--too small to produce plumes that a satellite could detect.