[meteorite-list] Magnetic fields of tetrataenite particles in pallasites shed light on earth's magnetic core

From: J Sinclair <john_at_meteoritecentral.com>
Date: Thu, 22 Jan 2015 12:02:58 -0500
Message-ID: <CAAeS-uv7+-YsUrx5wEoLdd+5dpbRpDSbvF7VQ0w+9XcoA5VCQQ_at_mail.gmail.com>

That's an excellent article for better a understanding of the
pallasites plus reference to pallasites we all know - Esquel, Imilac
and Brenham.



Here is the editor's summary from Nature.

Shortly after the birth of the Solar System, small planetary bodies
became hot enough to segregate into a liquid metal core surrounded by
rocky mantle. As the core cooled and froze, swirling motions of liquid
metal, driven by the expulsion of sulphur from the growing inner core,
generated a magnetic field. A class of meteorites known as pallasites
preserves this phase of Solar System history as in the form of
gem-quality crystals of the silicate mineral olivine embedded in a
metallic matrix of iron?nickel alloy. James Bryson et al. use
high-resolution magnetic imaging of the iron?nickel matrix of the
Imilac and Esquel pallasite meteorites to derive a time-series record
of magnetic activity on the pallasite parent body, encoded within
nanoscale intergrowths of iron-rich and nickel-rich phases. This
record captures the dying moments of the magnetic field generated as
the liquid core solidified, providing evidence for a long-lasting
magnetic dynamo driven by compositional convection.

On Wed, Jan 21, 2015 at 8:26 PM, Robin Whittle via Meteorite-list
<meteorite-list at meteoritecentral.com> wrote:
> Here is a write-up of some interesting research.
> - Robin
> http://phys.org/news/2015-01-death-dynamo-hard-space.html
> The researchers' magnetic measurements, supported by computer
> simulations, demonstrate that the magnetic fields of these
> asteroids were created by compositional, rather than thermal,
> convection - meaning that the field was long-lasting, intense and
> widespread. The results change our perspective on the way magnetic
> fields were generated during the early life of the solar system.
> These meteorites came from asteroids formed in the first few
> million years after the formation of the Solar System. At that
> time, planetary bodies were heated by radioactive decay to
> temperatures hot enough to cause them to melt and segregate into a
> liquid metal core surrounded by a rocky mantle. As their cores
> cooled and began to freeze, the swirling motions of liquid metal,
> driven by the expulsion of sulphur from the growing inner core,
> generated a magnetic field, just as the Earth does today.
> "It's funny that we study other bodies in order to learn more
> about the Earth," said Bryson. "Since asteroids are much smaller
> than the Earth, they cooled much more quickly, so these processes
> occur on shorter timescales, enabling us to study the whole
> process of core solidification."
> Scientists now think that the Earth's core only began to freeze
> relatively recently in geological terms, maybe less than a
> billion years ago. How this freezing has affected the Earth's
> magnetic field is not known. "In our meteorites we've been able to
> capture both the beginning and the end of core freezing, which
> will help us understand how these processes affected the Earth in
> the past and provide a possible glimpse of what might happen in
> the future," said Harrison.
> However, the Earth's core is freezing rather slowly. The solid
> inner core is getting bigger, and eventually the liquid outer core
> will disappear, killing the Earth's magnetic field, which protects
> us from the Sun's radiation. "There's no need to panic just yet,
> however," said Harrison. "The core won't completely freeze for
> billions of years, and chances are, the Sun will get us first."
> The article itself is behind a paywall:
> http://www.nature.com/nature/journal/v517/n7535/full/nature14114.html
> Long-lived magnetism from solidification-driven convection on the
> pallasite parent body
> James F. J. Bryson et al.
> Nature 517, 472?475 (22 January 2015)
> doi:10.1038/nature14114
> Palaeomagnetic measurements of meteorites suggest that,
> shortly after the birth of the Solar System, the molten
> metallic cores of many small planetary bodies convected
> vigorously and were capable of generating magnetic fields.
> Convection on these bodies is currently thought to have
> been thermally driven, implying that magnetic activity
> would have been short-lived. Here we report a
> time-series palaeomagnetic record derived from nanomagnetic
> imaging of the Imilac and Esquel pallasite meteorites, a
> group of meteorites consisting of centimetre-sized metallic
> and silicate phases. We find a history of long-lived magnetic
> activity on the pallasite parent body, capturing the decay
> and eventual shutdown of the magnetic field as core
> solidification completed. We demonstrate that magnetic
> activity driven by progressive solidification of an inner
> core, is consistent with our measured magnetic field
> characteristics and cooling rates. Solidification-driven
> convection was probably common among small body cores, and,
> in contrast to thermally driven convection, will have led
> to a relatively late (hundreds of millions of years after
> accretion), long-lasting, intense and widespread epoch of
> magnetic activity among these bodies in the early Solar
> System.
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Received on Thu 22 Jan 2015 12:02:58 PM PST

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