[meteorite-list] Mysterious Meteorite Dust Mismatch Solved

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:25:33 2004
Message-ID: <200305011641.JAA26034_at_zagami.jpl.nasa.gov>

http://abc.net.au/science/news/stories/s843594.htm

Mysterious meteorite dust mismatch solved
Danny Kingsley
Australian Broadcasting Corporation
May 1, 2003

Japanese scientists think they might have worked out
the reason for a curious mismatch in composition
between the dust that falls to Earth from colliding
asteroids, and the asteroids that are actually out there.

A team of researchers led by Professor Kazushige
Tomeoka from Kobe University in Japan report on their
experiments with meteorites in today's issue of the
journal Nature.

Most asteroids orbit between Mars and Jupiter, but
collisions between them can knock the asteroids off
course and put them out of orbit, sometimes causing
them to crash to the Earth as meteors and meteorites.

Each year tens of thousands of tonnes of meteorite dust
from colliding asteroids rains down on the Earth. The
researchers set out to explain the reason why, although
the vast majority of asteroids are hydrous (containing water),
nearly 98% of the dust that falls to Earth is anhydrous
(water-free) - albeit with a similar chemistry and
minerology to the watery dust.

In a series of laboratory experiments Tomeoka and team shot
at hydrous and anhydrous samples of meteorites to simulate
the 'shock pressure' that occurs when asteroids collide in
space. They subjected the meteorites to different levels of
pressure, from 4 to 49 Giga-Pascals (GPa). To give an
indication of the level of pressure involved, 26Gpa is the
pressure 660km inside the Earth at which the crust begins to
turn into molten lava.

The researchers found that hydrated meteorites
broke up more easily under the pressure of
impact and when they did their water content
vapourised. This means that what might end up
as water-free dust on Earth may have started off
being part of a hydrated asteroid, the researchers
argue.

"It has been speculated that what hits the Earth
is not representative of what is out there,"
commented Dr Hugh O'Neill from the School of
Earth Sciences at the Australian National
University, who does research into the chemistry
of meteorites.

"What they have shown, and its neat stuff, but
not all that surprising, is the kind of shocks
that occur when two asteroids collide causes complete
disintegration of the hydrated ones," he told
ABC Science Online.

Astronomers know that asteroids have been exposed
to shock pressure because some asteroids contain the
same sort of high pressure minerals found deep inside
the Earth. Asteroids are much smaller than the Earth,
which means that impacts with other asteroids are the
only explanation for these minerals.

Our knowledge of the composition of asteroids comes
from spectral imaging which analyses the surface of
asteroids in space. This form of imaging has limitations
in that it can only observe what is on the surface of
the asteroid and this may be contaminated by dust
resulting from weathering in space.
Received on Thu 01 May 2003 12:41:37 PM PDT