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

From: Robin Whittle <rw_at_meteoritecentral.com>
Date: Thu, 22 Jan 2015 12:26:42 +1100
Message-ID: <54C051D2.8000503_at_firstpr.com.au>

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.
Received on Wed 21 Jan 2015 08:26:42 PM PST