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Glass holds grains of truth about Earth's meteor history
Microscopic glass grains gathered from the moon almost 30 years ago are providing clues about the history of meteor activity on Earth and possibly the origin of life on the planet.
While scientists are aware of high meteor activity in our solar system 3 to 4 billion years ago, new data gleaned from lunar dust indicates another era of high activity beginning about 400 million years ago and continuing today. This recent period coincides with a time of important biological events on Earth, including the evolution of more complex organisms and intelligent life.
The Earth itself does not offer up a lot of information about meteor activity beyond the past few hundred million years because erosion, plate tectonics and other geologic processes have erased most evidence of impact craters.
The moon, on the other hand, has no atmosphere or tectonic activity, and its meteor impact history is therefore well preserved on the surface. Craters resulting from meteoric activity many hundreds of millions of years ago are even visible from Earth.
Since Earth and the moon are next to each other, their impact records are generally considered to be the same.
"If you study one, you are also studying the other," said Richard Muller, a professor of physics at the University of California at Berkeley and research physicist at Lawrence Berkeley National Laboratory.
Muller and a team of scientists from Berkeley and LBL turned their attention to moon dust samples brought back by Apollo 14 astronauts in 1971 for information about meteor activity on Earth.
The group received permission to analyze one gram of moon debris from the mission, which was carried out by NASA.
Muller figured that by dating spherules, or tiny glass beads, found in the sample, he and his colleagues could get a good idea of meteor activity.
When a meteor hits the surface of an object like the moon, it melts the rock and sends heated particles flying outward in all directions. As this material falls back to the surface, it cools, creating spherules.
Muller postulated that these microscopic glass droplets are dispersed far and wide.
"You could have sat anywhere (on the moon) and been hit by these little glass beads," said Muller. The moon debris contained information not just from the impact crater in which it was gathered, but about impacts from all over the lunar orb.
The scientists separated 155 spherules from the many grains in the sample. Chemical analysis bore out Muller's theory by showing that the 155 beads came from 146 different craters around the moon.
The team then used radioisotope-dating techniques to determine the age of the glass beads. As expected, they found evidence of high meteor activity between 3 and 4 billion years ago and a significant slowdown in impacts around the 3 billion-year mark.
To their surprise, they also noted that about 400 million years ago the number of impacts increased again to levels similar to those of about 3.2 billion years ago.
"Essentially during this time the majority of evolution that humans consider important evolved," said Muller. "Most remarkable was the evolution toward complexity," he said.
The role of catastrophic events such as the impact of an asteroid on the evolution of life is not completely understood, Muller said. But, he suggests, "episodic catastrophes give a new opportunity to the next group."
This new evidence shows how "space stuff has a bearing on the history of life on Earth," said Muller. Until the impact theory for the extinction of dinosaurs was raised, the relationship between outer space and Earth has been largely ignored. "There has been a paradigm shift," he said.
What initiated the new spate of meteoric activity 400 million years ago remains a question.
"It is not easy to explain why this happened," said Muller.
Copyright 1999, Environmental News Network, All Rights Reserved
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