Snowballs from space? I don't think so, planetary scientists are saying after a blizzard of news reports several weeks ago announcing that mini-comets were showering tons of water on our planet every day in the form of house-sized snowballs.
The reports were based on a presentation by University of Iowa astronomer Louis Frank at the American Geophysical Union meeting last month, and an accompanying NASA press release, which said that "the Earth's upper atmosphere is being sprayed by a steady stream of water-bearing objects. . . ."
The presentation conjured up visions of these extra-terrestrial ice balls as perhaps a critical source of the very moisture that helped start life on Earth. But in the afterglow of such images, planetary scientists are mounting a massive, albeit informal, campaign casting doubt on Frank's theory.
One of their main complaints is that the cosmic snowball research hadn't been reviewed yet by other scientists, and Frank's previous papers on interplanetary comets had been rejected by prestigious journals.
Even though researchers didn't have the data in front of them, many felt the finding just wasn't plausible. The reasoning behind it "just doesn't hold water," planetary scientist Donald Yeomans, of the Jet Propulsion Laboratory in Pasadena, said in an Internet chat.
Other researchers chatting about the announcement on an electronic bulletin board dedicated to so-called "near Earth objects" weren't so restrained.
If the theory was correct, "You should be able to see these things with a pair of binoculars in the evening sky at the rate of one every few minutes," wrote JPL planetary scientist Alan Harris. "So [the] hypothesis is wacko."
A member of the U.S. Air Force Space Command said he saw the story in his local paper. "I'm surprised to learn this theory is being seriously discussed," he said.
A University of Hawaii scientist lamented his loss of innocence. "The new mini-comet affair has made me realize a scary fact," he wrote. "I no longer believe official NASA press releases."
To be sure, controversy is an essential ingredient of science, which is less an accumulation of solid fact than a running argument. And it's not unusual for theories or evidence to be shot down.
"Pioneering research has to go through this," said planetary scientist William Bottke of Caltech. "Someone puts out an idea and the rest of the scientific community debates whether it's valid."
Moreover, there's no doubt that Frank saw something. Along with his announcement at the American Geophysical Union on May 28th, he released satellite images of bright streaks thousands of miles above the Earth that appear to be trails of broken-up water molecules. In addition, he has confirmed the existence of unexplained dark spots in satellite images, which Frank says are the incoming comets.
But while other researchers now agree that the spots themselves are real, they question Frank's interpretation that the spots are house-sized snowballs smacking into Earth's atmosphere at the rate of thousands per day, bringing enough water to fill all the world's oceans.
"He may have found something, but it's not what he thinks it is," Bottke said.
Most upsetting to scientists is that they "haven't seen the data," Bottke said. "It was put out in a NASA press release, not a peer-reviewed paper."
Frank, meanwhile, sticks by his theory, and has said his ideas will ultimately be vindicated by further observations. He relies on several lines of argument.
First, ultraviolet images taken from NASA's new Polar satellite contain dark spots, or "atmospheric holes" as Frank calls them, which no one can explain. Frank contends they are clouds of water, leftover wet spots from comets that disintegrated in Earth's upper atmosphere.
Second, Frank sees bright streaks or "wakes" of water fragments more than a thousand miles above the Earth's surface, exactly where such comet remains might be expected to linger.
No one disagrees about the spots or the streaks. They simply don't believe Frank's interpretation that all that water is pouring onto Earth.
For one thing, the inner solar system is very dry, researchers point out. If all these wet snowballs are soaking our immediate neighborhood in space, where's the water on Venus? Or Mars? Or the moon? Or interplanetary space?
Even the stratosphere high above the Earth is known to be extremely dry. "The water level in the stratosphere is well accounted for and an influx from these 'comets' would be greatly noticed," argued a scientist from NASA's Ames Research Center in Mountain View, Calif. "How did these [water] molecules completely bypass the stratosphere?"
More puzzling, the mini-comets have left no trace of craters on the moon.
"At the rate [Frank's] talking about, the lunar surface should be covered with very fresh . . . craters and the Apollo astronauts would have certainly noticed an [influx of comets or water] of this magnitude," Yeomans said.