Mars, Earth and Moon from 'unique planetary nursery' (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 27 Mar 2008 01:54:30 -0400
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25 Mar 08
Mars, Earth and Moon from 'unique planetary nursery'
A study of meteorites suggests that Mars, the Earth and the Moon share a
common composition from 'growing up' in a unique planetary nursery in the
inner solar system.
The finding could lead to a rethink of how the inner solar system formed.
The international team of scientists, which includes Professor Alex Halliday
from Oxford University's Department of Earth Sciences, report in Nature how
they analysed 16 meteorites that fell to Earth from Mars. They found that
the amounts of neodymium-142 these contain are subtly different from those
of objects found in the asteroid belt. This isotopic fingerprint is proof
that the chemistry of the inner solar system was different even for elements
that are hard to vapourise.
Professor Halliday said: "The Earth, Moon and Mars appear to have formed in
a part of the inner solar system with a ratio of samarium to neodymium that
is around 5 per cent more than could be found in the asteroid belt. It is
this 'family resemblance' that we see today when we compare oceanic basalts
from Earth with Moon rocks and Martian meteorites. Such differences may be
the result of the erosion of planetary crusts during formation events,
alternatively, this composition arose from the sorting of clouds of
partially melted droplets or grains -- known as 'chondrules'."
Earth has a long geological history of recycling the materials that make up
its crust and mantle, which could help explain why its composition is
different from that of other planetary bodies -- it could, for example, have
deeply buried reservoirs of certain elements. However Mars and the Moon are
believed to have been nothing like as active during their lifespan: making
it much more difficult for any theory involving material recycling to
explain why their composition should differ from other planetary bodies and
yet have such similarities with the composition of the Earth.
Professor Halliday said: "What our results suggest is that the sorting of
the elements that make up these planets may have happened at a much earlier
stage than had been believed. It may even be that this sorting happened in
the accretion disk out of which Mars and the early Earth first formed. What
we can say is that the composition of these worlds is inconsistent with them
simply forming out of large 'lumps' of stony meteorites, like those we see
today in the asteroid belt."
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