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Posted by George on June 28, 2007, 1:38 pm
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> **Contact data appear below**
>
> Scientists find that Earth and Mars are different to the core
> Science and Technology Facilities Council
> June 28, 2007
>
> Research comparing silicon samples from Earth, meteorites and
> planetary
> materials, published in Nature (28th June 2007), provides new evidence
> that the Earth's core formed under very different conditions from
> those
> that existed on Mars. It also shows that the Earth and the Moon have
> the
> same silicon isotopic composition supporting the theory that atoms
> from
> the two mixed in the early stages of their development.
>
> This latest research which was carried out by scientists from Oxford
> University along with colleagues from University of California, Los
> Angeles (UCLA) and the Swiss Federal Institute of Technology in Zurich
> (ETH) compared silicon isotopes from rocks on Earth with samples from
> meteorites and other solar system materials. This is the first time
> that
> isotopes have been used in this way and it has opened up a new line of
> scientific investigation into how the Earth's core formed.
>
> On Earth rocks that make up volcanoes and mountain ranges and underlie
> the ocean floor are made of silicate - compounds made of silicon and
> oxygen linked with other kinds of atoms. Silicate dominates down to a
> depth of 2,900 km - roughly half way to the centre of the Earth. At
> this
> point there is an abrupt boundary with the dense metallic iron core.
> Studies by Birch in the 1950's demonstrated that the outer core had a
> density too low to be made of pure iron and that it must also be made
> up
> of some lighter elements (see notes to editors for further details).
>
> Research team member, Bastian Georg, a post doctoral researcher from
> Oxford University's Earth Sciences Department said, "We dissolved
> meteorites, provided by the Natural History Museum in London, in order
> to compare their isotopic composition with those of rocks from the
> Earth. The silicon was separated from other elements and the atomic
> proportions of isotopes measured using a particularly sophisticated
> mass
> spectrometer at the ETH in Zurich".
>
> Professor Alex Halliday, also from Oxford University explains, "We
> were
> quite startled at our results which showed that the heavier isotopes
> from silicate Earth samples contained increased proportions of the
> heavier isotopes of silicon. This is quite different from meteorites
> from the silicate portions of Mars and the large Asteroid Vesta -
> which
> do not display such an effect even though these bodies also have an
> iron
> core."
>
> Silicate samples from Mars and Vesta are identical to a primitive
> class
> of meteorites called chondrites that represent average solar system
> material from small "planetesimals" that never underwent core
> formation.
>
> Professor Halliday continues, "The most likely explanation is that,
> unlike Mars and Vesta, the Earth's silicon has been divided into two
> sorts - a portion that became a light element in the Earth's core
> dissolved in metal and the greater proportion which formed the
> silicon-oxygen bonded silicate of the Earth's mantle and crust."
> At depths the silicates change structure to denser forms so the
> isotopic
> make-up would depend on the pressure at which metal and silicate
> separate. Quantifying this effect is the subject of ongoing studies.
> Co-author on the paper Edwin Schauble from UCLA, has produced
> preliminary calculations that show that the isotopic effects found are
> of the right direction and magnitude.
>
> This research provides new evidence that the Earth's core formed under
> different conditions from those that existed on Mars. This could be
> explained in part by the difference in mass between the two planets.
> With Earth being eight times larger than Mars the pressure of core
> formation could be higher and different silicate phases may have been
> involved. The mass of a planet also affects the energy that is
> released
> as it accretes (or grows).
>
> The Earth accreted most of its mass by violent collisions with other
> planets and planetary embryos. The bigger the planet, the greater the
> gravitational attraction and the higher the temperatures that are
> generated as the kinetic energy of impacting objects is converted to
> heat. Some have proposed that the outer Earth would have periodically
> become a "magma ocean" of molten rock as a result of such extreme high
> temperature events.
>
> There is evidence that Mars stopped growing in the first few million
> years of the solar system and did not experience the protracted
> history
> of violent collisions that affected the Earth. There already exists
> compelling evidence for relatively strong magnetic fields early in
> martian history but a thorough understanding of the martian core must
> await geophysical measurements by future landers. It is however
> thought
> that the core of Mars is proportionally smaller than that of the Earth
> and it probably formed under lower pressures and temperatures.
>
> The research also shows that the Moon has the same silicon isotopic
> composition as the Earth. This cannot be caused by high pressure core
> formation on the Moon which is only about one percent of the mass of
> the
> Earth. However, it is consistent with the recent proposal that the
> material that made the Moon during the giant impact between the
> proto-Earth and another planet, usually called "Theia", was
> sufficiently
> energetic that the atoms of the disk from which the Moon formed mixed
> with those from the silicate Earth. This means the silicon in the
> silicate Earth must have already had a heavy isotopic composition
> before
> the Moon formed about 40 million years after the start of the solar
> system.
>
> The research was supported from grants provided by the UK's Science
> and
> Technology Facilities Council, and the USA's and Switzerland's
> National
> Science Foundation.
>
> Contacts
> Gill Ormrod - Science and Technology Facilities Council Press Office
> Tel: 01793 442012. Email: gill.ormrod@stfc.ac.uk
>
> Pete Wilton - Oxford University Press Office
> Tel: 01865 283877
> Email : pete.wilton@admin.ox.ac.uk
>
> UK Science contact
> Professor Alex Halliday - Department of Earth Science, Oxford
> University
> Tel: 07769728153
> Email: alexh@earth.ox.ac.uk
>
> Notes to Editors
> The information in this release in based on the following Nature
> paper,
> which appears in 28th June issue.
>
> Silicon in the Earth's core
> R Bastian Georg 1,2, Alex N. Halliday 1, Edwin A Schauble 3 and Ben C
> Reynolds 2.
> 1. Department of Earth Sciences, University of Oxford 2. Department of
> Earth Sciences, ETH Zentrum, Zurich 3. Department of Earth and Space
> Science, University of California, Los Angeles.
>
> In 1952 the distinguished Harvard geophysicist Francis Birch showed
> that
> the Earth's liquid outer core, where the magnetic field is generated,
> is
> chemically different from the solid metallic iron inner core that
> occupies the very centre of the Earth. Birch argued that the outer
> core
> had a density that was too low to be made of pure iron; it had to
> contain several percent of a lighter element or elements of lower
> atomic
> weight. Since then scientists have sought to determine what these
> light
> elements are and what their concentrations tell us about the
> conditions
> of core formation.
>
> The Science and Technology Facilities Council ensures the UK retains
> its
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>
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> Physics, Particle Astrophysics, Nuclear Physics, Space Science,
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Thanks for posting this, Ron. It answers a lot of my questions.
George
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