[meteorite-list] Comet Composition Show Striking Differences (Wild 2 & Tempel 1)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Oct 11 20:46:38 2006
Message-ID: <200610120046.RAA11499_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn10280-comet-compositions-show-striking-differences.html

Comet compositions show striking differences
David Shiga
New Scientist
11 October 2006

The comets visited by NASA's Stardust and Deep Impact missions are
remarkably different in composition, a new analysis reveals. The
research suggests the two comets have been chemically altered by heat
and water at different levels during their lifetimes.

On 15 January 2006, NASA's Stardust mission returned to Earth
with material collected from comet Wild 2. After analysing about 50
grains returned to Earth from Wild 2, the Stardust team has discovered
that this material is very different from the stuff dredged up by the
Deep Impact collision with comet Tempel 1 on 4 July 2005.

Two of the most common materials found in Tempel 1 are an iron-silicon
mineral called ferrosilite and a glassy form of a magnesium-iron mineral
called olivine, which make up 33% and 17% of the comet, respectively,
according to observations by the Spitzer Space Telescope. However, these
minerals are entirely absent from the Wild 2 samples analysed so far.

Ancient collisions

It is not clear how to explain this difference, says Stardust mission
leader Donald Brownlee of the University of Washington in Seattle, US.
But he says one possibility is that the material on Tempel 1 was
chemically modified by ancient collisions - the Deep Impact spacecraft
struck Tempel 1 near two impact craters.

Another possibility is that the two comets were born with different
compositions, he says. Both are thought to have formed in the Kuiper
Belt, a ring of icy objects beyond Neptune, and both are a few
kilometres wide.

Such small objects would normally be expected to be very similar in
composition. However, if one had broken off of a much larger parent
body, they could have different makeups. That is because very large
objects in the outer solar system, like Pluto, are massive enough to
have had hot interiors with liquid water in the past, he says.

Some of these objects would have collided, producing fragments. Comets
such as Tempel 1 might contain minerals that only form in warm, wet
environments as a result, Brownlee says.
          
Other stars

By analysing atomic isotopes from Wild 2, the Stardust team has also
verified that the comet is made up of material from our own solar system
as well as that expelled from other stars.

But the team has proven that some minerals once suspected of coming from
other stars were actually locally grown. Pyroxene, which forms at
temperatures greater than 1000 K, is one such mineral: "It almost
certainly came from the innermost region of the solar system," Brownlee
told New Scientist.

That bolsters a theory that in the first few million years after the Sun
formed, magnetic currents near the infant Sun lifted material out of the
disc from which the planets formed and channelled it to the outer solar
system. There, it would have fallen back into the disc to form comets
such as Wild 2.

Brownlee presented the results on Tuesday at a meeting of the American
Astronomical Society's Division for Planetary Sciences in Pasadena,
California, US.
Received on Wed 11 Oct 2006 08:46:35 PM PDT