With ice cream or without?
A heated battle has broken out among scientists: is our earth being bombarded with huge chunks of ice from space that are evaporating in the upper atmosphere, or are they all absurd pipe dreams and instrument errors? At a conference, both sides will present their latest arguments for or against this hypothesis. According to Louis Frank of the University of Iowa, about every 20 minutes, small comets made of water ice enter the atmosphere, where they evaporate in the mesosphere. Frank believes this is how Earth's oceans were filled and perhaps even how life was brought to our planet from outer space. One of the main indications for his idea are images of the upper layers of the air, which show black spots in UV light. There, according to Frank, the ice absorbs the radiation.
This was recently contradicted by George Parks of the University of Washington (see "The Battle for Ice," Spectrum Ticker, November 19, 1997). He'd found that the camera picked up dark spots even when there weren't any. This artifact appeared even in the lab.
Frank promptly countered on December 9th at the American Geophysical Union meeting. There, together with John B. Sigwarth, he presented new images made with the instruments on board the Polar satellite. This time the photos were taken at different distances from Earth, first at an altitude of five and then at eight Earth radii. The images from farther away showed about 80 percent fewer spots than those from lower orbit. A camera error, on the other hand, should always occur the same way, regardless of the position. Frank thinks the possibility that the dark spots are just black noise has been disproved.
But the skeptics attack his hypothesis in another way: calculations by Bashar Rizk and Alex J. Dessler of the Lunar and Planetary Laboratory at the University of Arizona have shown that ice lumps weighing 30 tons and about 10 meters in diameter, like Frank describes them vaporizing in the sky with a bright flare. Their brightness should be between the full moon and Venus, so that they could be seen with the naked eye even during the day. At dusk the sky should sparkle like a Christmas tree. Scientists are therefore wondering, “Where are they [the ice comets]? We should be able to see them."
Timothy D. Swindle and David A. Kring of the same institute are looking for signs of cosmic chunks of ice in the composition of our atmosphere. If the small comets formed in deep space, they should have contained about the same proportion of noble gases as are found in the sun and its planets. Bombarding the Earth with chunks of ice for billions of years should have brought corresponding amounts of the gases with them. Swindle and Kring calculated that the krypton and xenon content would then have to be 500 times higher than the current value, and the argon content should even be 30,000 times higher.
Finally, Jennifer A. Grier and Alfred S. McEwen from the Lunar and Planetary Laboratory present the results of their lunar crater count. According to the small comet hypothesis, the moon should experience an impact every minute. This results in 400,000 new craters every year that would be at least 50 meters in diameter and surrounded by a bright ring 150 meters in diameter. Grier and McEwen compared images taken by Apollo 17 in 1972 with images taken by the Clementine probe, which mapped the moon 22 years later. Overall, they evaluated an area of 52,000 square kilometers. All of the 3920 bright points counted were already present in 1972. According to Grier and McEwen, instead of the predicted 400,000 comets, at most 33 comet the moon each year.
As is so often the case in science, there are good reasons for and equally solid evidence against a hypothesis. In any case, the last word has not yet been spoken at this conference on the question of whether chunks of ice from outer space are haunting us or not.
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