05/21/1998 07:00:00

Posthumous Paper by Gene Shoemaker Details Evidence of Comet Shower That Pummeled Earth 36 Million Years Ago

PASADENA—Geochemical evidence from a rock quarry in northern Italy indicates that a shower of comets hit Earth about 36 million years ago.

The findings not only account for the huge craters at Popagai in Siberia and at Chesapeake Bay in Maryland, but posit that they were but a tiny fraction of the comets active during a period of two or three million years during the late Eocene period. The work provides indirect evidence that a gravitational perturbation of the Oort comet cloud outside the orbit of Pluto was responsible for sending a wave of comets swarming toward the center of the solar system.

In a paper published today in the journal Science, a group from the California Institute of Technology, the U.S. Geological Survey Flagstaff office, and the Coldigioco Geological Observatory in Italy, report their evidence of a very large increase in the amount of extraterrestrial dust hitting Earth in the late Eocene period. The writers include the husband-and-wife team of Gene and Carolyn Shoemaker. Gene Shoemaker died in a car crash last year while the research was in progress.

According to lead author Ken Farley, a geochemist at Caltech, the contribution of Shoemaker was especially crucial in the breakthrough.

"Basically, Gene saw my earlier work and recognized it as a new way to test an important question: are large impact craters on Earth produced by collisions with comets or asteroids," Farley says.

"He suggested we study a quarry near Massignano, Italy, where seafloor deposits record debris related to the large impact events 36 million years ago. He said that if there had been a comet shower, the technique I've been working on might show it clearly in these sediments."

Carolyn Shoemaker said that she and her husband went to Italy last year to perform field work in support of the paper.

"Gene was pretty excited about the work Ken was doing," she said. "He was glad Ken was taking it on. It's exciting work, and it's a rather new type of work."

The matter involved detecting the helium isotope known as 3He, which is rare on Earth but common in extraterrestrial materials. 3He is very abundant in the sun, and some of it is ejected from the sun as solar wind throughout the solar system. The helium is easily picked up and carried along by extraterrestrial objects such as asteroids and comets and their associated dust particles.

Thus, arrival of extraterrestrial matter on Earth's surface can be detected by measuring its associated 3He. And even this material is unlikely to include large objects like asteroids and comets. Because these heavy, solid objects fall into the atmosphere with a high velocity, they melt or vaporize, giving their helium up to the atmosphere. This 3He never falls below very high altitudes, and soon reenters space.

But tiny particles entering the atmosphere are another story. These particles can pass through the atmosphere at low temperatures, and so retain helium. These particles accumulate on the seafloor, and seafloor sediments provide an archive of these particles going back hundreds of millions of years.

Elevated levels of 3He would suggest an unusually dusty inner solar system, possibly because of enhanced abundances of active comets. Such an elevated abundance of comets might arise when a passing star or other gravity anomaly kicks a huge number of comets from the Oort cloud into elliptical, sun-approaching orbits.

When Farley took Shoemaker's suggestion and traveled to the Italian quarry, he discovered that there was indeed an elevated flux of 3He-laced materials in a sedimentary layer some 50 feet beneath the surface. Because this region of Italy was submerged in water until about 10 million years ago, the comet impacts and microscopic debris had accumulated on the ocean bed, and this debris was preserved because dying organisms had cooperatively covered the debris over the eons.

The depth of the sedimentary layer suggested to the researchers that the 3He had been deposited about 36 million years ago. This corresponds to the dating of the craters at Popagai and Chesapeake Bay.

More precisely, the 3He measurements show enhanced solar system dustiness associated with the impacts 36 million years ago, but with the dustiness beginning 0.5 million years before the impacts and continuing for about 1.5 million years after. The conclusion is that there were a large number of Earth-crossing comets and much dust from their tails for a period of about 2.5 million years.

In addition to Gene and Carolyn Shoemaker and Ken Farley, the paper was cowritten by Alessandro Montanari, who holds joint appointments at the Coldigioco Geological Observatory in Apiro, Italy, and the School of Mines in Paris.

Written by Robert Tindol