Scientists Find That Earth and Mars are Different to the Core
- From: baalke@xxxxxxxxxxxxx
- Date: Thu, 28 Jun 2007 09:28:19 -0700
**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@xxxxxxxxxx
Pete Wilton - Oxford University Press Office
Tel: 01865 283877
Email : pete.wilton@xxxxxxxxxxxxxx
UK Science contact
Professor Alex Halliday - Department of Earth Science, Oxford
University
Tel: 07769728153
Email: alexh@xxxxxxxxxxxxxx
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.
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