[meteorite-list] New Study: Some Asteroids are Like Onions

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:10:01 2004
Message-ID: <200304022246.OAA08414_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/onion_asteroids_030402.html

New Study: Some Asteroids are Like Onions
By Robert Roy Britt
space.com
02 April 2003

A new study of several meteorites collected on Earth and thought to have
come from the same large asteroid reveal the structure of the parent space
rock to have been something like an onion, with layer upon layer of
differing structure.

The asteroid, long ago destroyed in a collision, was once hot enough to have
a molten core and cooled from the outside inward, the research shows,
confirming a long-held expectation that had eluded supporting research.

Asteroids are leftovers of planet formation. While some rocks got together
to build planets about 4.5 billion years ago, a bunch never achieved as
much. Most of this debris now orbits the Sun in the so-called asteroid belt,
between Mars and Jupiter. Collisions in the belt have been frequent through
time, and some of the resulting smaller chunks make their way to Earth,
where they fall as meteorites.

Scientists see these meteorites as a collective window to planet formation
and the evolution of the early solar system.

Researchers already suspected that the initial asteroids, sometimes called
planetesimals because they were like precursors to planets, were heated
internally by the decay of a short-lived aluminum isotope that was common in
the early solar system. The middles of some asteroids would have melted.

The new work, led by Mario Trieloff of the University of Heidelberg,
Germany, examined crystals in several meteorites known as H-group
chondrites, all of which were presumed to have come from the same parent
asteroid. Some of the crystals were damaged by spontaneous fission generated
long ago by decaying plutonium. The damage was healed by high temperatures
-- like those occurring in the center of an asteroid -- but remain in
meteorites that were cooler, presumably from outer layers of the asteroid.

This allowed Trieloff's team to create a temperature map of the original
asteroid. The map confirms a suspected layered composition, the so-called
onion model.

"This cooling behavior is in perfect agreement with what we expect, if an
asteroid is heated by an internal heat source that comes from the rocks
itself," Trieloff said. The research will be detailed in the April 3 issue
of the journal Nature.

Scientists have sought confirmation of the onion model for decades, but it
was lacking. Perhaps, some thought, the H-chondrites came from many
different asteroids, instead of one.

John Wood of the Harvard-Smithsonian Center analyzed the research for
Nature.

Trieloff and his colleagues "now dispel these doubts," Wood writes, by
showing that the chondrites "can be fitted into a straightforward model of a
planetesimal with a radius of about 100 kilometers [62 miles] and an
onion-shell structure, which was internally heated - and cooled over about
100 million years."

The study looked at just one sort of asteroid, however, and it does not
represent the structures of asteroids in general. Trieloff points out that
there are about 10 different major chondrite classes in meteorites, material
that represents "at least 50 originally different asteroids."

Meteorites in other chondrite classes typically do not show the diversity of
types that would indicate the extensive layering of a parent object as found
in the new study. Among these are so-called carbonaceous chondrites.

Trieloff told SPACE.com that the parent bodies of these other chondrites
might either have formed a few million years later, after the aluminum
isotope had already decayed, and so never had a chance to grow so hot. Or
the parent asteroids might simply have been smaller (size affects heat,
too).

Yet another class of chondrites (called L and LL) represent asteroids that
probably did have a layered structure but might have broken apart before
cooling down, thereby eliminating the sort of evidence that the new study
looked for, Trieloff said.
Received on Wed 02 Apr 2003 05:46:21 PM PST