[meteorite-list] Angrites hot under the colar

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Fri, 31 Oct 2008 09:30:39 -0500
Message-ID: <uf5mg4th3kfqd51jad7ruajp325otom939_at_4ax.com>

http://www.space.com/scienceastronomy/081030-planet-formation.html

Ancient Meteorites Give Clues to Planet Formation
By SPACE.com staff

posted: 30 October 2008
02:04 pm ET

Meteorites that are among the oldest rocks ever found have provided new clues
about the conditions that existed at the beginning of the solar system, solving
a longstanding mystery and overturning some accepted ideas about the way planets
form.

The ancient meteorites, called angrites, still contain magnetic records about
the very early history of planets, like disk drives salvaged from an ancient
computer, new research by MIT planetary scientist Benjamin P. Weiss indicates.

The results of the study, which was by a grant from the National Science
Foundation's Instrumentation and Facilities Program, are detailed in the Oct. 31
issue of the journal Science.

The analysis showed that surprisingly, during the formation of the solar system,
when dust and rubble in a disk around the sun collided and stuck together to
form ever-larger rocks and eventually the planets we know today, even objects
much smaller than planets ? just 100 miles (160 kilometers) across or so ? were
large enough to melt almost completely.

This total melting of the planet-forming chunks of rock, called planetesimals,
caused their constituents to separate out, with lighter materials including
silicates floating to the surface and eventually forming a crust, while heavier
iron-rich material sank down to the core, where it began swirling around to
produce a magnetic dynamo. The researchers were able to study traces of the
magnetic fields produced by that dynamo, now recorded in the meteorites that
fell to Earth.

"The magnetism in meteorites has been a longstanding mystery," Weiss said, and
the realization that such small bodies could have melted and formed magnetic
dynamos is a major step toward solving that riddle.

Until relatively recently, it was commonly thought that the planetesimals ?
similar to the asteroids seen in the solar system today ? that came together to
build planets were "just homogenous, unmelted rocky material, with no
large-scale structure," Weiss said. "Now we're realizing that many of the things
that were forming planets were mini-planets themselves, with crusts and mantles
and cores."

That could change theorists' picture of how the planets themselves took shape.

If the smaller bodies were already molten as they slammed together to build up
larger planet-sized bodies, that could "significantly change our understanding"
of the processes that took place in the early years of the nascent planets, as
their internal structures were forming, Weiss said. This could have implications
for how different minerals are distributed in the Earth's crust, mantle and core
today, for example.

"Events happened surprisingly fast at the beginning of the solar system," Weiss
said. Some of the angrite meteorites in this study formed just 3 million years
after the birth of the solar system itself, 4,568 million years ago, and show
signs that their parent body had a magnetic field that was 20 to 40 percent as
strong as Earth's today.

"We are used to thinking of dynamo magnetic fields in rocky bodies as uncommon
phenomena today," Weiss said. "But it may be that short-lived planetesimal
dynamos were widespread in the early solar system."



http://sciencenow.sciencemag.org/cgi/content/full/2008/1030/2

First Planets Lived Fast and Died Young

By Phil Berardelli
ScienceNOW Daily News
30 October 2008
Surprising findings from some of the oldest known meteorites suggest that our
solar system was once chock-full of miniature planets, complete with metallic
cores and rocky crusts. The findings could force a rethinking of how the solar
system and its constituent bodies evolved.

Some 4.568 billion years ago, our sun and solar system condensed out of a
primordial cloud of dust and gas. Within about 3 million years, small, rocky
objects called planetesimals were circulating in the nascent solar system.
Fragments of these planetesimals remain today as meteorites called achondrites,
which scientists have pored over for clues to how planets formed. Oddly, the
meteorites are magnetic, which is strange because the planetesimals were
supposed to be just large agglomerations of rubble.

A U.S.-Canadian team took a new approach to the problem, testing samples of
three well-preserved achondrite meteorites with an extremely sensitive
magnetometer. What they discovered stunned them: The meteorites showed evidence
of ancient magnetic fields similar to those of rocks formed on Earth within the
planet's magnetic field. In other words, the team reports this week in Science,
the 4.565-billion-year-old meteorites once were part of bodies that were either
big enough or hot enough to produce central, molten, metallic cores.

"The meteorites, therefore, are essentially magnetic recording tapes," says
planetary scientist and lead author Benjamin Weiss of the Massachusetts
Institute of Technology in Cambridge. The magnetic fields that they recorded
were probably generated by molten metal swirling around inside the planet's core
like a giant, rotating dynamo, as happens on Earth. Although most asteroids now
are rocky through and through, the new findings suggest that back at the
beginning of the solar system even planetesimals could melt at their cores and
retain a rocky crust. These bodies could be as small as 160 kilometers in
diameter, the research suggests. The planetesimals, which eventually merged to
form the rocky planets, were more planetlike than previously thought, with cores
that must have formed and melted within just a few million years of the
formation of the solar system, Weiss says.

The paper "makes a good case," says planetary scientist David Stevenson of the
California Institute of Technology in Pasadena. Although dynamos are thought to
require large planetary cores, it's possible that the



for those with access to _Science_ full text:

http://www.sciencemag.org/cgi/content/abstract/322/5902/713


and here's a related PDF:

http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2143.pdf
Received on Fri 31 Oct 2008 10:30:39 AM PDT