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- http: //planetary.org/articlearchive/headlines/1998/headln-052198.html
Comets Pummeled Earth 36 Million Years Ago
Posthumous Paper by Astronomer Gene Shoemaker
Details Evidence of Cataclysmic Comet Shower
May 20, 1998
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.
Shoemaker's Legacy of Discovery
In a paper published today in the journal Science,
a group from the California Institute of
Technology, the United States 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, well known for their work detecting
comets and asteroids. Gene Shoemaker died in a car
crash last year while this 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 sea-floor 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.
Tracking an Ancient Disaster
In geologic samples, the researchers detected a
helium isotope known as 3He, which is rare on
Earth but common in extraterrestrial materials.
This isotope is 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 sea
floor, and sea floor 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 a
flurry 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.
Discovery in Italy
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.