[meteorite-list] Angrites hot under the colar

From: Pete Pete <rsvp321_at_meteoritecentral.com>
Date: Fri, 31 Oct 2008 10:15:27 -0400
Message-ID: <BAY141-W44C90151EF16E8F932FE3F8200_at_phx.gbl>

Hi, Darren and List,

You would think then, that there would be more angrite meteorites found, and chondrules in meteorites would be more rare...?


> From: cynapse at charter.net
> To: meteorite-list at meteoritecentral.com
> Date: Fri, 31 Oct 2008 09:30:39 -0500
> Subject: [meteorite-list] Angrites hot under the colar
> 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
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Received on Fri 31 Oct 2008 10:15:27 AM PDT

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