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Lunar Glass Beads Offer Clues to Craters

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Michael D. Wheeler

Comets and asteroids have bombarded the moon and Earth over the last 3 billion years, leaving craters in their wake. But the frequency of these bombardments -- and whether periodic or episodic variations have occurred -- remains a mystery. With a better understanding of how and why these craters were formed, scientists theorize that they can learn more of the history of the solar system and assess some of the influences of these impacts on Earth.

To that end, researchers at the University of California have refined a novel technique for determining lunar cratering rates. Using soil samples taken from the Apollo 14 space mission, they used a 6-W argon-ion laser from Coherent Inc. to heat lunar spherules, or glass beads, found in the soil. The spherules, abundant in all lunar soil samples gathered on the Apollo missions, were formed as a result of lunar impacts and volcanic eruptions. Based on the composition of the spherules and the presence of melt splashes and coatings, it is possible to determine whether the beads formed as a result of eruption or impact.

The dating method melts the soil and measures the decay of radioactive potassium to argon. After the glass solidifies, new argon caused by the decay of potassium is trapped inside the beads. The argon laser heats the beads, and a sensitive mass spectrometer analyzes the gas.

"The Ar-ion laser wavelength is absorbed extremely efficiently by the iron-rich glasses we had," explained Paul R. Renne, a member of the research team. "The laser is a vintage 1985 Coherent 6-W laser. It heats the samples cleanly and progressively until they remelt and release all gas trapped within."

Renne said the study was the result of a theory of his colleague Richard A. Muller that lunar impacts were periodic every 30 million years. "He recognized that these little glass beads represent a record of impacts on the lunar surface, so this was a good way to test the theory. As it turns out, we didn't really test the theory because we can't date these tiny objects precisely enough -- but we did discover interesting things."

The data the group collected showed that the amount of glasses formed in the past 400 million years greatly exceed that from the previous 3 billion years. They also suggested a substantial decrease in the lunar cratering rate from 3.5 billion years ago to 400 million years ago. Results of the study appeared in the March 10 issue of Science.

Photonics Spectra
Aug 2000
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