[meteorite-list] Iron Meteorites May Be Solar System Boomerangs

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Feb 16 12:43:02 2006
Message-ID: <200602161741.k1GHfIZ18195_at_zagami.jpl.nasa.gov>


Iron meteorites may be solar system boomerangs
Maggie McKee
New Scientist
16 February 2006

Iron meteorites thought to have originated in the asteroid belt beyond
Mars may actually have formed near Earth, a new study reports. The work
may resolve a mystery over why only a few asteroids appear to have
melted in the past and could offer researchers insights into the
composition of the Earth's interior.

Iron meteorites are made up of iron and nickel alloys and comprise about
6% of all catalogued space rocks on Earth. Like their "stony" meteorite
counterparts, they are thought to have fallen to Earth after being
nudged out of the asteroid belt between Mars and Jupiter.

But unlike their stony cousins, iron meteorites are thought to have come
from asteroids that were once so hot they had melted, and
"differentiated" into layers. In this process, heavy elements like iron
would have sunk to the asteroids' centres - the source of the iron
meteorites - and lightweight, lava-like basalts would have solidified on
their surfaces.

Studies show that the known iron meteorites come from about 80 different
parent asteroids, while the thousands of known stony meteorites broke
off from just 40 or so parent bodies. That suggests astronomers should
see many "differentiated" asteroids in the asteroid belt today, says
William Bottke of the Southwest Research Institute in Boulder, Colorado, US.
Curious problems

But observations have turned up few basalt-covered asteroids, and
asteroid "families" - fragments of larger parent bodies that broke up
after a collision - show no signs of differentiation, says Bottke. "The
asteroid belt seems to be telling us something very different than the
meteorite record," Bottke told New Scientist. "It raises a lot of
curious problems."

So Bottke and colleagues approached the problem by considering how
asteroids become differentiated. Asteroids are thought to melt from the
heat produced by the radioactive decay of isotope aluminium-26. This
isotope has a half-life of only 700,000 years, so melting should have
occurred only on those asteroids that took shape shortly after the solar
system formed, about 4.6 billion years ago.

Gravity causes objects closer to the Sun to move around it more quickly.
That means that dust clumps should have collided more often near the Sun
to form rocky bodies, and that those bodies should have melted. "If you
expect melting to happen anywhere, it's going to take place closer to
the Sun than farther away," says Bottke.

Prodigal sons return

Bottke and his colleagues ran computer models of small bodies forming
near the Sun and discovered that interactions with the rocky inner
planets could have caused these bodies to break up. The interactions
also caused a small fraction of them - about 1% of those that formed
near Earth, and about 10% of those born near Mars - to be scattered
outward into the asteroid belt.

There, they would have remained in stable orbits for several billion
years until gravitational interactions with other objects and radiation
from the Sun nudged some onto paths towards Earth, where they fell as
iron meteorites.

The scenario may explain why so few objects in the asteroid belt appear
differentiated, and why recent studies of iron meteorites show they are
1 million to 2 million years older than stony meteorites, Bottke says.
It may also reveal the composition of our planet's primordial building
blocks, he adds: "Iron meteorites may tell us what the precursor
material for the primordial Earth was like."

Journal reference: Nature (vol 439, p 821)
Received on Thu 16 Feb 2006 12:41:17 PM PST

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