Meteoroid Bombardment Of Moon Has Intensified In Past 500 Million Years, Coinciding With Blossoming Of Life On Earth

By dating minute glass beads thrown out by impacts over the millennia, scientists at the University of California, Berkeley, and the Berkeley Geochronology Center have not only confirmed expected intense meteor activity 4 to 3.5 billion years ago, when the large lunar seas or maria were formed, but have discovered another peak of activity that began 500 million years ago and continues today.

The tapering off of the first peak of activity, which probably included many large comets and asteroids, coincides with the earliest know evidence of life on Earth. The second and ongoing peak, which from the evidence seems to have been mostly smaller debris, began around the time of the great explosion of life known as the Cambrian.

"The first life on Earth arose just after this real crescendo around 3.5 billion years ago," said Paul R. Renne, adjunct professor of geology and geophysics at UC Berkeley and director of the Berkeley Geochronology Center. "Maybe life began on Earth many times, but the meteors only stopped wiping it out about 3 billion years ago."

The more recent and ongoing activity is even more intriguing.

"It's not surprising that the impacts tapered off about 3 billion years ago. The solar system was just getting cleaned up, primarily by Jupiter and the Sun," said Richard A. Muller, a professor of physics at UC Berkeley and a research physicist at Lawrence Berkeley National Laboratory. "What is surprising is the reversion from a benign to a violent solar system about 500 million years ago.

"This work opens up a new field that tells us something about the history of our solar system that was totally unanticipated. Until now we did not realize how peculiar the past 500 million years has been."

UC Berkeley graduate student Timothy S. Culler, along with Renne, Muller and Timothy A. Becker, laboratory manager at the Berkeley Geochronology Center, report their findings in the March 10 issue of the journal Science.

Though all the Berkeley researchers agree on the new impact chronology for the moon, they have their own ideas about its implications.

Renne, for example, leans toward the theory that interstellar dust seeded the Earth with organic molecules, from water to amino acids, that were incorporated into life on Earth during the past 500 million years.

"Life already here would suddenly have a new stimulus, a greater need to evolve quickly and more raw material to do it," Renne said. "Impacts would have to be really, really big and really, really frequent to be deleterious to life on Earth, and it's clear that the flux over the past 500 million years has been relatively small objects. We don't see a lot of young large craters on the moon. We've come to accept the idea that impacts are strictly bad news for life on Earth, but now that's not so clear."

Culler, the graduate student who originated the project under the supervision of Muller and Renne, sees the intense meteor activity as evidence that large meteor impacts played a major role in the evolution and extinction of life.

"It shows that large impacts may have been more frequent in the last 500 million years, creating more extinctions, like the comet or asteroid that wiped out the dinosaurs 65 million years ago, " Culler said. "Even a number of smaller impacts can have a disastrous effect on the atmosphere and cause mass extinctions."

Muller too emphasizes the role impacts have played in the history of life on Earth. It's not surprising that the recent intense period of meteor activity coincides with the rapid radiation of life on Earth, he said.

"We're only beginning to realize the role played by catastrophe in the evolution of life," he said. "When it comes to survival of the fittest, it's not only the ability to compete with other species that counts, but also the ability to survive occasional catastrophe. That requires complexity and flexibility."

Muller has proposed several controversial theories about the solar system, including that the sun has an unseen companion star, one he calls Nemesis, that orbits the sun every 26 million years and periodically knocks comets out of their orbits, sending them hurtling toward the inner solar system. He also has proposed that periodic climate changes are the result of the Earth's orbit periodically tilting up out of the orbital plane of the planets and intersecting a cloud of dust, debris and meteoroids.

The current research was suggested by Muller in 1991, in part as a way to determine whether the moon's impact record shows evidence of a 26 million-year cycle. Muller hit upon the idea of argon-40/argon-39 dating of lunar spherules as a way to get a more precise chronology of the intensity of bombardment of the moon and, by implication, the Earth.

"I realized that we didn't have to go to the individual craters in order to determine their age, because the craters sent samples to us," Muller said. "We could obtain samples of hundreds of different craters from just one location, without having the expense of going back to the moon. This idea is likely to open up a completely new round of lunar analysis."

Spherules are mostly basaltic glass, Culler said, created when a meteor hits the surface and generates intense heat that melts the rock and splatters it outward. As droplets of molten rock fall back to the surface they quickly cool to form a glass, much like obsidian.

Culler, Becker and Renne analyzed 155 beads from one gram of lunar soil picked up in 1971 by Apollo 14 from the Fra Mauro formation - a lunar highland bordering Mare Imbrium. The mineral composition of each bead was determined with a microprobe before it was laser melted and the argon gas captured for isotopic analysis.

Contrary to assumptions, they found that the cratering rate on the moon has not been constant over its history. Approximately twice as many impacts occurred between 4 and 3 billion years ago as occurred between 2 and 1/2 billion years ago. About 500 million years ago the intensity of impacts increased nearly to what it was at the peak of activity 3.2 billion years ago.

Though the dating method was not sensitive enough to reveal a 26 million-year cycle in the impact record, "these findings fit in nicely with the Nemesis theory," Muller said. "I think most of the debris came from perturbations in the outer solar system by Nemesis."

For the future, Renne says, it is "critical to launch new lunar sampling missions targeted to areas rich in potassium," in order to confirm the results and probe further back into the moon's history.

The project was funded by the Ann and Gordon Getty Foundation, through the Berkeley Geochronology Center and Richard Muller. NASA provided the lunar samples.