[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
Here is a write-up of some interesting research.
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:
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)
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
Received on Wed 21 Jan 2015 08:26:42 PM PST