Space dust may rain destruction on Earth.
This week, as Hollywood dramatizes the instantaneous Armageddon brought on by a comet impact, two scientists look to extraterrestrial dust to explain extended bouts of extinctions. Stephen J. Kortenkamp of the Carnegie Institution of Washington (D.C.) and Stanley F. Dermott of the University of Florida in Gainesville describe their proposal in the May 8 Science.
The issue of how interplanetary dust affects Earth gained prominence after a pair of scientists hypothesized that waves of space particles triggered the last 10 ice ages (SN: 10/4/97, p. 220). That possibility was raised by Richard A. Muller of the University of California Berkeley and Gordon J. MacDonald of the International Institute for Applied Systems Analysis in Laxenburg, Austria.
In Muller and MacDonald's model, the amount of dust reaching Earth rises and falls as the plane of Earth's orbit tilts up and down through the plane of the solar system. This orbital bobbing, they surmised, would cause the planet to pass through a thin band of dust every 100,000 years--a period that matches the ice age cycle. Every 100,000 years or so, the planet's temperature plummets, then rises in short thaws like the one that has graced the climate for the last 10,000 years.
Kortenkamp and Dermott tested this hypothesis by creating a model that computes the path of dust leaving the asteroid belt and falling toward the sun. Such debris accounts for perhaps three-quarters of the interplanetary dust that reaches Earth; the rest comes from comets, say the researchers.
The model calculations for the last 1.2 million years show that dust amounts do not vary with the tilt of Earth's orbit, a contradiction of Muller and MacDonald's proposal. Instead, say Kortenkamp and Dermott, the shape of the orbit alters dust accumulation.
Over a period of 100,000 years, Earth's orbit gradually shifts from a nearly perfect circle to a slight oval and back again. In the circular orbit, the planet travels more slowly through the dust cloud and therefore sweeps up two to three times as much debris as it does in the oval orbit, according to the model. Kortenkamp compares this to a vacuum cleaner's picking up more dirt when pushed slowly over a carpet.
Records of extraterrestrial dust gleaned from seafloor sediments confirm the model results in part, says Kenneth A. Farley of the California Institute of Technology in Pasadena. Like the model, the sediments show dust increasing by a factor of 3 every 100,000 years. The problem lies in the timing. The sediments contain more dust when the model predicts that accumulations should be declining. "Something is really peculiar here," says Farley.
The answer may be that the model does not examine all of the asteroidal dust falling to Earth. Dermott is now studying the sizable fraction that gets caught temporarily in orbits just outside Earth and is later knocked loose.
Climate researchers are skeptical that a tripling of dust accumulation triggered the ice ages. Kortenkamp and Dermott leave that question aside but speculate that the amount of dust could rise to more than 300 times modern values following major collisions in the asteroid belt. Persisting for a million years or more, these dust storms could disrupt climate and bring about long periods of extinctions. Such collisions would also eject larger chunks, which could wallop Earth in the wake of the dust waves.
Farley remains skeptical, however, because sea sediment records going back to the days of the dinosaurs show no dramatic dust surges. "If such an event has occurred, it has not occurred in the last 65.5 million years."
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|Title Annotation:||research indicates interplanetary dust may have caused ice ages|
|Article Type:||Brief Article|
|Date:||May 9, 1998|
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