[meteorite-list] Dust in the wind

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Mon, 02 Nov 2009 18:13:49 -0500
Message-ID: <brpue5tgsa4762kpd4q074ra7mbi9quu25_at_4ax.com>

http://www.eurekalert.org/pub_releases/2009-11/ci-pf110209.php

'Ultra-primitive' particles found in comet dust

Washington, D.C.?Dust samples collected by high-flying aircraft in the upper
atmosphere have yielded an unexpectedly rich trove of relicts from the ancient
cosmos, report scientists from the Carnegie Institution. The stratospheric dust
includes minute grains that likely formed inside stars that lived and died long
before the birth of our sun, as well as material from molecular clouds in
interstellar space. This "ultra-primitive" material likely wafted into the
atmosphere after the Earth passed through the trail of an Earth-crossing comet
in 2003, giving scientists a rare opportunity to study cometary dust in the
laboratory.

At high altitudes, most dust in the atmosphere comes from space, rather than the
Earth's surface. Thousands of tons of interplanetary dust particles (IDPs) enter
the atmosphere each year. "We've known that many IDPs come from comets, but
we've never been able to definitively tie a single IDP to a particular comet,"
says study coauthor Larry Nittler, of Carnegie's Department of Terrestrial
Magnetism. "The only known cometary samples we've studied in the laboratory are
those that were returned from comet 81P/Wild 2 by the Stardust mission." The
Stardust mission used a NASA-launched spacecraft to collect samples of comet
dust, returning to Earth in 2006.

Comets are thought to be repositories of primitive, unaltered matter left over
from the formation of the solar system. Material held for eons in cometary ice
has largely escaped the heating and chemical processing that has affected other
bodies, such as the planets. However, the Wild 2 dust returned by the Stardust
mission included more altered material than expected, indicating that not all
cometary material is highly primitive.

The IDPs used in the current study were collected by NASA aircraft in April
2003, after the Earth passed through the dust trail of comet Gregg-Skjellerup.
The research team, which included Carnegie scientists Nittler, Henner Busemann
(now at the University of Manchester, U.K.), Ann Nguyen, George Cody, and seven
other colleagues, analyzed a sub-sample of the dust to determine the chemical,
isotopic and microstructural composition of its grains. The results are reported
on-line in Earth and Planetary Science Letters.*

"What we found is that they are very different from typical IDPs" says Nittler.
"They are more primitive, with higher abundances of material whose origin
predates the formation of the solar system." The distinctiveness of the
particles, plus the timing of their collection after the Earth's passing through
the comet trail, point to their source being the Gregg-Skjellerup comet.

"This is exciting because it allows us to compare on a microscopic scale in the
laboratory dust particles from different comets," says Nittler. "We can use them
as tracers for different processes that occurred in the solar system
four-and-a-half billion years ago."

The biggest surprise for the researchers was the abundance of so-called presolar
grains in the dust sample. Presolar grains are tiny dust particles that formed
in previous generations of stars and in supernova explosions before the
formation of the solar system. Afterwards, they were trapped in our solar system
as it was forming and are found today in meteorites and in IDPs. Presolar grains
are identified by having extremely unusual isotopic compositions compared to
anything else in the solar system. But presolar grains are generally extremely
rare, with abundances of just a few parts per million in even the most primitive
meteorites, and a few hundred parts per million in IDPs. "In the IDPs associated
with comet Gregg-Skjellerup they are up to the percent level," says Nittler.
"This is tens of times higher abundances than we see in other primitive
materials."

Also surprising is the comparison with the samples from Wild 2 collected by the
Stardust mission. "Our samples seem to be much more primitive, much less
processed, than the samples from Wild 2," says Nittler, "which might indicate
that there is a huge diversity in the degree of processing of materials in
different comets."
Received on Mon 02 Nov 2009 06:13:49 PM PST