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

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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 Nesvorny. 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, Nesvorny, Grimm, Morbidelli, and O'Brien, appears in
the Feb. 16 issue of Nature.