[meteorite-list] Let's call them "asteromets"

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 24 Jan 2008 23:12:06 -0500
Message-ID: <laoip3lmuvbo9grprisghst2iqdt35qulj_at_4ax.com>

http://www.cosmosmagazine.com/node/1825

News
Stardust blurs line between asteroids and comets
Friday, 25 January 2008Cosmos Online

SYDNEY: A new analysis of comet grains collected by NASA's Stardust probe, has
surprised experts by revealing that they don't contain samples of the most
ancient material in the Solar System.

Rather than primordial dust, the samples contain material that has been more
commonly in asteroids from the inner Solar System.

The find reported today in the U.S. journal Science by cosmochemists led by Hope
Ishii of the Lawrence Livermore National Laboratory in California, blurs the
distinction between asteroids and comets.

Rich source of dust

When the Stardust returned to Earth in 2006 it carried material from the comet
Wild 2 ? the first sample of material from the outer Solar System.

NASA experts thought it would be a rich source of dust, dating from before the
planets formed around 4.5 billion years ago.

"Presolar material acts like a time capsule," said study co-author Anton
Kearsley a mineralogist at the U.K.'s Natural History Museum in London "[It
tells] us about the origins of our Solar System, and what was here before. We
had hoped to find lots of undamaged dust from the birthplace of the Solar System
on comet Wild 2, but have discovered that there are very few unambiguous
presolar grains."

The Stardust samples were definitely collected from an object you'd describe as
a comet, added Kearsley (i.e. icy, with a gaseous head and tail, with an
elliptical orbit and originating from the distant Kuiper Belt). "But it seems
that even if you sample directly from a comet itself, you won't necessarily get
the oldest material."

20,000 kph impact

Much existing knowledge about the composition of primordial dust comes from
meteorites on Earth and a type of interplanetary dust particle (IDP) scooped up
by aircraft flying 15 to 20 km up in the stratosphere.

IDPs contain curious particles of glass with embedded metals and sulphides,
known as GEMS. Planetary scientists believe these were formed in interstellar
space before being swept into the cloud of dust and gas from which our Solar
System formed.

Stardust collected thousands of particles in 2004, each merely micrometres
across. As Wild 2 rushed past the spacecraft at over 20,000 kph, tiny dust
grains were embedded in its collector.

When the samples eventually returned to Earth in 2006, some of the trapped
particles were thought to look like GEMS. However, laboratory analysis has now
shown these GEMS-like structures were more likely created during the heat of
impact on the collector.

Another indicator of primitive interplanetary dust ? a distinctive form of the
mineral enstatite ? is also missing from the Stardust samples, write the
experts.

More diverse than we knew

The mission has revealed that comets are likely more diverse than we knew; some
containing material forged in the swirling disk of gas and dust which then
became the Solar System, while others preserve even more primitive interstellar
material. The distinction between comets and asteroids has been blurred by the
find, said the authors, which shows that Wild 2 may have more in common with
inner some asteroids than comets.

The samples from Wild 2 represented the first new extra-terrestrial material
brought back to Earth since the missions to the Moon in the 1970s. Stardust flew
further than any other sample-return mission ? 4.63 billion km, in looping
orbits, out to between the orbit of Mars and the asteroid belt.
Received on Thu 24 Jan 2008 11:12:06 PM PST