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Rice University
Office of News & Media Relations

DATE: May 8, 2003
CONTACT: Jade Boyd
PHONE: (713) 348-6778
EMAIL: jadeboyd{at}rice.edu

METEORITES RAINED ON EARTH AFTER MASSIVE ASTEROID BREAKUP

Geologists Find Meteorites 100 Times More Common in Wake of Ancient
Asteroid Collision

Using fossil meteorites and ancient limestone unearthed throughout
southern Sweden, marine geologists at Rice University have discovered
that a colossal collision in the asteroid belt some 500 million years
ago led to intense meteorite strikes over the Earth's surface.

The research, which appears in this week's issue of Science magazine,
is based upon an analysis of fossil meteorites and limestone samples
from five Swedish quarries located as much as 310 miles (500 km.)
apart. The limestone formed from sea bottom sediments during a 2
million-year span about 480 million years ago, sealing the intact
meteorites, as well as trace minerals from disintegrated meteorites,
in a lithographic time capsule. 

"What we are doing is astronomy, but instead of looking up at the
stars, we are looking down into the Earth," said lead researcher
Birger Schmitz, who conducted his analysis during his tenure as the
Wiess Visiting Professor of Earth Science at Rice. Schmitz is
professor of marine geology at Göteborg University in Sweden.

Meteorite activity on earth is relatively uniform today, with an
average of about one meteorite per year falling every 7,700 square
miles (12,500 sq. km). The new study found a 100-fold increase in
meteorite activity during the period when the limestone was forming,
a level of activity that was present over the entire
150,000-square-mile (250,000 sq. km.) search area.

Some 20 percent of the meteorites landing on Earth today are remnants
of a very large asteroid that planetary scientists refer to as the
"L-chondrite parent body." This asteroid broke apart around 500
million years ago in what scientists believe is the largest collision
that occurred in late solar system history. 

Schmitz and his colleagues looked for unique extraterrestrial forms
of the mineral chromite that are found only in meteorites from the
L-chondrite breakup. They found that all the intact fossil meteorites
in the Swedish limestone came from the breakup. Moreover, they found
matching concentrations of silt and sand-sized grains of
extraterrestrial chromite in limestone from all five quarries,
indicating that meteorite activity following the breakup was
occurring at the same rate over the entire area.

The research helps explain why Schmitz and his colleagues at Göteborg
have been able to collect so many fossilized meteorites from a single
quarry near Kinnekulle, Sweden over the past decade. Fossil
meteorites embedded in stratified rock are extremely rare. Only 55
have ever been recovered, and Schmitz's group found 50 of those.

"It is true that we are lucky to be looking in just the right place -
a layer of lithified sediments that was forming on the sea floor
immediately after this massive collision," said Schmitz. "But on the
other hand, we would never have started looking there in the first
place if the quarry workers hadn't been finding the meteorites on a
regular, yet still rare, basis." 

Until Schmitz's group started working with the quarry crew, the
fossilized meteorites were discarded because they blemish the
finished limestone. Schmitz believes it's possible that similar
concentrations of fossilized meteorites and extraterrestrial chromite
grains are present worldwide in limestone that formed during the
period following the asteroid breakup. He recently got funding to
look for evidence of this in China, and he said there are South
American sites that are also favorable.

The research was sponsored by the National Geographic Society and the
Swedish Research Council.

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