[meteorite-list] Asteroid Fragments On A Fast Collision Course

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Jul 14 17:28:08 2004
Message-ID: <200407142127.OAA14632_at_zagami.jpl.nasa.gov>

http://fm-eth.ethz.ch/eth/media/FMPro?-db=pressemitteilungen.fp5&-format=pr_detail_de.html&-lay=html&-op=eq&pr_id=2004-50&-find

Swiss Federal Institute of Technology Zurich (ETH Zurich)
Zurich, Switzerland

Contact:

Philipp Reza Heck
Institute for Isotope Geology
Department of Earth Sciences
Tel. +41 1 632 79 72

Prof. Rainer Wieler
Institute for Isotope Geology
Department of Earth Sciences
Tel. +41 1 632 37 32

July 14, 2004

Asteroid fragments on a fast collision course

ETH researchers determined travel times of asteroid
fragments

Over a million large asteroids, each several
kilometres wide, are orbiting the Sun in a belt
between Mars and Jupiter. There are sometimes
violent collisions. Until now, it has been thought
that the resulting asteroid fragments would need
several million years to reach the Earth. New
measurements from the Noble Gas Laboratory of ETH
Zurich show however, that the Earth could be
reached much sooner. This knowledge is significant
for the prediction of future meteorite impacts on
Earth.

Collisions in the Asteroid Belt result in the
asteroids being completely destroyed and shattered
into countless pieces. Computer simulations predict
that most of these fragments will eventually fall
into the Sun. Some of them, however, will hit the
Earth after millions of years as meteorites. It is
possible that this could also occur much earlier.
In certain positions in the Asteroid Belt, the
orbiting time of an object around the sun is a
multiple of the orbit of the giant planet Jupiter.
The so-called orbital resonance can lead to a
disruption in the object's orbit. It can change the
orbit so much that the object would cross the
Earth's orbit and collide with the Earth. Up until
today, when this might occur has only been
theoretically calculated. But, a new measurement
method developed by a research team at the
Institute for Isotope Geology at ETH Zurich can now
bring more certainty to the subject. The team has
established that it could take just a few hundred
thousand years for such an object to collide with
our planet.

Concentrations of noble gases tell the travel time
of an asteroid

Collisional fragments from asteroids in space are
constantly being hit by cosmic radiation. This
creates noble gases from nuclear reactions. These
gases do not enter into any further chemical
reactions. Therefore, during the entire duration of
the radiation, i.e., the travel time of the
fragment in space, they accumulate in the fragment.
After measuring the concentration of these so-
called cosmogenic inert gases, the travel time from
original body to Earth can be calculated. The
higher the concentration, the longer the meteorite
was underway.

Fossil meteorites: participants in a catastrophe

The researchers used meteorites for their tests
that are assumed to be the results of a huge
asteroid collision in the recent history of the
solar system. These meteorites were found in a
stone quarry in southern Sweden in a 480 million
year old seabed deposit. What is astonishing is
that the fragments still show the traces of the
inert gases absorbed more than 500 million years
ago.

"Tom Dooley" allows measurement of very small
amounts of gas

The Noble Gas Laboratory at ETH Zurich has a highly
sensitive mass spectrometer, nicknamed "Tom
Dooley", that is specialized for the measurement of
extremely small amounts of gas. This instrument,
developed at ETH, condenses the test gas into a
tiny volume in order to raise the concentration to
the point that even rare gases, such as the helium
or neon in a single dust particle, can be measured.
The sensitivity of this instrument is more than a
hundred times higher than conventional mass
spectrometers. The device is unique, worldwide.
Using this instrument, the young researcher Philipp
Reza Heck found a method for measuring a very small
amount of cosmic inert gas. To do this, just a few
micrograms of lightweight meteorite sample is
melted with an infrared laser and the gas is then
set free and cleaned. Heck then measures the
isotopes of the elements helium and neon with "Tom
Dooley" spectrometer.

Confirmation of the shorter travel time

With the new method, it could be proven for the
first time that the noble gases in the meteorites
in southern Sweden were already in the meteorites
480 million years ago. The calculated travel time
was reduced to a few hundred thousand years, which
corresponds to the lower limits predicted by the
computer simulations. These were the first
fragments to arrive on Earth after a great
collision. The short radiation age is a clue that
the collision took place in the proximity of an
orbital resonance in the Asteroid Belt. In
addition, it could be proven that the fossil
meteorites from southern Sweden all stem from the
same event. The newly developed method from the
Institute for Isotope Geology makes it possible to
confirm the theories about the behaviour of
asteroid fragments in space. This will make it
significantly easier for researchers to predict
future collisions with our planet.

Publication in Nature on 15 July 2004

Title: 'Fast delivery of meteorites to Earth after
a major asteroid collision'

Authors: Philipp R. Heck [1], Birger Schmitz [2],
Heinrich Baur [1], Alex N. Halliday [1] & Rainer
Wieler [1]

[1] ETH Zurich, Isotope Geology, NO C61, CH-8092
Zurich, Switzerland

[2] University of Lund, Department of Geology,
Solvegatan 12, SE-22362 Lund, Sweden.
Received on Wed 14 Jul 2004 05:27:15 PM PDT