Mars, Earth And Moon
From 'Unique Planetary Nursery'
(5 April 2008) 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 vaporise.
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."
(source: University of Oxford)
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