March 14, 2000

Analysis of Lunar Debris Offers Clues to Life on Earth
By JOHN NOBLE WILFORD

In popular lore, the Apollo 14 astronauts may be best remembered for things they were not sent to the Moon in 1971 to do. Alan Shepard drove a golf ball in the low lunar gravity to a distance Tiger Woods would die for. Edgar Mitchell on his own conducted what he said were extrasensory perception experiments.

Yet the astronauts kept their eyes on the prize and took time to collect plenty of rocks and soil and bring them back in airtight containers. One gram of that soil -- or regolith, as lunar dirt is called -- has only now been put to a rigorous analysis, which has revealed what may be a surprising new chapter in the history of the solar system.

The findings have even inspired speculation about another possible extraterrestrial influence on the course of life on Earth. Could an upsurge of meteorite bombardment have contributed to the emergence of complex lifeforms with diverse anatomical designs? The speculation is already drawing the fire of critics.

By dating minute glass beads in lunar soil, melted debris from the heat of asteroid and comet impacts, scientists at the University of California at Berkeley and the Berkeley Geochronology Center have made two discoveries, one expected and the other a big surprise.

The ages of many beads, the size of tiny peas or smaller, confirmed the heavy meteorite bombardment of the Moon from 4 billion to 3.5 billion years ago. This was a violent epoch in the young solar system, which formed 4.5 billion years ago. Impacts of asteroids and comets gouged out many of the larger lunar craters and basins that remain virtually unchanged on the waterless, airless Moon.

How the Moon Might Tell Earth's Story
On the Moon debris from impacts of comets or meteors is not disturbed by wind or water. So it creates a record of lunar bombardment. Scientists who have analyzed this debris say it suggests a period of heavy impact activity occurred about the same time complex life forms emerged on Earth.


USGS/NASA

Edgar Mitchell on a moonwalk during the Apollo 14 mission.
The scientists had not expected to find so many of the beads with ages less than 400 million years old. After a relatively quiet period of 3 billion years, they said, the inner solar system appeared to erupt with another meteorite blitz, which continues today. The Earth, being larger than the Moon, would presumably have suffered even more impacts, though erosion and shifting crustal plates would have erased most of the traces.

Describing the findings in the current issue of the journal Science, the Berkeley researchers concluded that the age data from 155 glass beads "show that the crater production rate has not been constant over the last 3 billion years, as had been generally assumed." The data, they said, also provided evidence for a striking increase in the cratering rate over the last 400 million years.

The research was conducted by Timothy S. Culler, a graduate student at the University of California; his faculty adviser, Dr. Richard A. Muller, a physicist, and Dr. Paul R. Penne, director of the Berkeley Geochronology Center and specialist in dating technology. The ages of the impact-formed beads were determined by a method of dating the decay of radioactive potassium to argon.

Other scientists said they had confidence in the analytical techniques used to date the beads. Dr. Graham Ryder of the Lunar and Planetary Institute in Houston said the Berkeley researchers "persuasively argue that there has been a substantial increase in impacting in the Earth-Moon system" in recent times. Some geologists were concerned, however, that the soil sample, coming from near the surface, might not be representative of impact frequencies.

"If the sample is biased, then you could be drawing a totally erroneous conclusion," Dr. Paul D. Spudis, a geologist at the Lunar and Planetary Institute, said in an interview.

Dr. Renne said in response: "We understand those things well. There really is no uncertainty about the chronology we've put together."

But the sharpest criticism by geologists and paleontologists was aimed at provocative remarks by the Berkeley scientists in the last paragraph of their report. The data, they said, "show an increase in the cratering rate roughly coincident with the 'Cambrian explosion' of complex life on Earth."

While conceding that the bead ages were not specific enough to support claims of a relationship to terrestrial life, the scientists said "the correlation is permissible evidence of a causal relation." Their last sentence read: "It is possible that increased debris influx had a net stimulating effect on biotic diversity, despite the occasional occurrence of large impacts with destructive consequences."

A planetary geologist who spoke on condition of anonymity said the speculation "really came out of left field." For one thing, the Cambrian explosion, the sudden rise of more diverse organisms as preserved most famously in the Burgess shale of British Columbia, started more than 500 million years ago. Nor is it clear, paleontologists said, what life forms in what ecological niches were destroyed by impacts at this time to clear the way for a new epoch.

In an interview, Dr. Muller said he was not arguing that there was a connection, "just noting that the impact increase happens to coincide with the explosion of life."

The scientists said the more recent period of bombardment came from relatively small objects, which have left few young large craters on the Moon. Their dating was not specific enough, they said, to detect any upsurge in impacts at the time the dinosaurs perished, presumably by an asteroid or comet impact 65 million years ago.

Dr. Muller had the dinosaur extinction in mind when he proposed the current research. In the 1980's, he proposed a controversial theory that the Sun has an unseen companion star, which he called Nemesis, that orbits the Sun every 26 million years and periodically knocks comets out of their orbits, sending them hurtling through the inner solar system. This might, he said, explain the apparent 26-million-year cycles of widespread extinctions, including the one that doomed the dinosaurs and cleared the way for mammals.

Looking for evidence of increased impacts coinciding with those extinctions, Dr. Muller had Mr. Culler examine glass beads collected by Apollo 14 and other lunar missions to see if they reflected spurts of objects crashing into the Moon.

In a statement issued by the University of California, Dr. Muller said: "The 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."

Writing an accompanying article in Science, Dr. Ryder of the Lunar and Planetary Institute said there was now "some fairly hard evidence for the earlier tentative conclusion that cratering in the Earth-Moon system has not been constant over the past 3 billion years."

In that case, he concluded, there may be "considerable uncertainties in the absolute dating of other planetary surfaces, such as Mars." Geologists have usually estimated these ages on the abundance of craters left there by the steady bombardment of asteroids and comets over time.