SwRI-French scientists discover potential link between iron meteoritesand the original "building blocks" that formed the Earth (Forwarded)

Southwest Research Institute
PO Drawer 28510
San Antonio, TX 78228-0510

For more information, contact:
Deb Schmid, Communications Department
(210) 522-2254

February 10, 2006

SwRI-French scientists discover potential link between iron meteorites
and the original "building blocks" that formed the Earth

Boulder, Colorado -- Iron meteorites are probably the surviving
fragments of the long-lost asteroid-like bodies that formed the Earth
and other nearby rocky planets, according to researchers from Southwest
Research Institute (SwRI) and Observatorie de la Cote d'Azur in Nice,
France. Their findings are described in the Feb.16 issue of Nature.

Iron meteorites, which are composed of iron and nickel alloys, represent
some of the earliest material formed in the solar system, with most
coming from the cores of small asteroids. According to Dr. William
Bottke, an SwRI research scientist and leader of the joint U.S.-French
team, iron-meteorite parent bodies probably emerged from the same disk
of planetary debris that produced the Earth and other inner solar system
planets.

"Small bodies that form quickly in the inner solar system end up melting
and differentiating from the decay of short-lived radioactive elements,"
explains Bottke. "Iron meteorites came from the molten material that
sinks to the center of these objects, cools and solidifies."

For these meteorites to arrive on Earth, they must have been extracted
from their parent bodies and kept around for billions of years. The
team's computer simulations found that any asteroids managing to avoid
being gobbled up by the planets were quickly demolished by impacts. Each
breakup, however, produces millions of fragments, many in the form of
iron meteorites. These leftovers were scattered across the solar system
by gravitational interactions with protoplanetary bodies, with some
reaching the relative safety of the asteroid belt. Over billions of
years, a few of the survivors escaped their captivity in the asteroid
belt and were delivered to Earth.

"This means that certain iron meteorites may tell us what the precursor
material for the primordial Earth was like, while also helping us unlock
several fundamental questions about the Earth's origins," says Bottke.
"There's also the possibility that larger versions of this material may
still be hiding among the asteroids. The hunt for them is on."

A new way to look at iron meteorites

A potential problem in using meteorites to understand the formation of
Earth and other terrestrial planets -- Mercury, Venus and Mars -- is
that most come from the distant asteroid belt. This population of
interplanetary bodies, ranging from tiny pebbles to Texas-sized objects,
is located between the orbits of Mars and Jupiter about 140 million
miles from Earth.

Most members of the asteroid belt are assumed to have formed there, so
the vast majority of meteorite samples tell us about formation events in
that region, not those that took place near Earth. Meteorite
compositions are so diverse, however, that it is difficult to reconcile
that all came from this one, fairly narrow region of space.

"While tens of thousands of stony meteorites have been collected, most
can be traced back to perhaps a few tens of parent asteroids," says Dr.
Alessandro Morbidelli of the Observatorie de la Cote d'Azur. "What is
strange is that the iron meteorites, despite their smaller numbers,
represent almost two-thirds of all of the unique parent asteroids
sampled to date."

To explain this discrepancy, the team tracked the origin and evolution
of iron-meteorite parent bodies using several computer models. They
found that while many iron meteorites are likely residing in the
asteroid belt today, their precursors probably did not form there.
Instead, the simulations indicate that the precursors of most iron
meteorites formed in the terrestrial planet region.

To investigate this hypothesis, the researchers first examined the
constraints provided by the meteorites themselves. Iron meteorites are
unusual in that most come from the disrupted cores of small melted
(differentiated) asteroids that formed very early in solar system
history. These are precisely the kinds of bodies that computer models
predict should have formed near Earth.

"It is hard to produce small differentiated bodies in the asteroid belt
without also melting lots of large asteroids," explains Dr. Robert
Grimm, assistant director of the SwRI Space Studies Department. "These
events would produce a number of telltale signs that would be easily
detected by observers."

Using computer simulations, the team then tracked how a disk of
asteroid-like bodies interacting with a host of protoplanetary objects
in the terrestrial planet region might evolve. Simulations showed that
some of these asteroid-like bodies could have scattered far enough to
take up residence in the asteroid belt.

"While the amount of material reaching the asteroid belt was limited,
much of it was placed in regions likely to produce meteorites," says
SwRI Research Scientist Dr. David Nesvorný. En route to the asteroid
belt, the parent bodies of the iron meteorites were repeatedly bashed by
other bodies, allowing core fragments from numerous bodies to escape.

"This could explain the many differences seen among iron meteorites,"
says Dr. David O'Brien of the Observatorie de la Cote d'Azur.

NASA's Origins of Solar Systems and Planetary Geology and Geophysics
programs funded the research of the SwRI investigators. The paper, "Iron
Meteorites as Remnants of Planetesimals from the Terrestrial Planet
Region," by Bottke, Nesvorný, Grimm, Morbidelli, and O'Brien, appears in
the Feb. 16 issue of Nature.