[meteorite-list] Isotopes in Meteorites Suggest Solar System Formed in a Rough Neighborhood

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 24 May 2007 16:01:49 -0700 (PDT)
Message-ID: <200705242301.QAA06561_at_zagami.jpl.nasa.gov>

http://www.sciencemag.org/cgi/content/full/316/5828/1111a

Science
25 May 2007:
Vol. 316. no. 5828, p. 1111
DOI: 10.1126/science.316.5828.1111a

News of the Week
GEOCHEMISTRY:
Isotopes Suggest Solar System Formed in a Rough Neighborhood

Richard A. Kerr

Astrophysicists have long assumed that a supernova played midwife to the
solar system. An exploding star could have collapsed wispy interstellar
gas and dust into a dense swirling disk to get things started and loaded
it with the intensely radioactive aluminum that cooked up chunks of the
nascent solar system. But on page 1178
<http://www.sciencemag.org/cgi/content/full/316/5828/1178>, a group of
cosmo-chemists presents evidence that the sun was born into an even more
brutal environment.

What's rougher than a supernova next door? A supernova that, before
detonating, blasts its neighborhood with eons' worth of energy in an
astrophysical instant. Astrophysicists think such behavior is typical of
stars dozens of times as massive as the sun. And if one of those massive
stars was so close, our home system must have formed in a dense,
swirling cluster of stars. The newborn solar system's neighborhood would
have been "a much more violent and turbulent" place than had been
assumed, says theoretical astrophysicist Alan Boss of the Carnegie
Institution of Washington's Department of Terrestrial Magnetism.

The evidence for our violent beginnings comes from some of the most
precise isotopic measurements yet of nickel in samples of Earth, Mars,
and meteorites. Martin Bizzarro of the University of Copenhagen in
Denmark and colleagues had gone looking for signs of radioactive iron-60
in the oldest meteorite from an asteroid that had melted in the earliest
solar system. The iron-60 itself wouldn't be there. It was forged in the
heart of a star and spewed into the material that would become the solar
system after the star went supernova. Then the iron-60 promptly decayed
away into nickel-60. So the researchers looked for the nickel "ash"
using a type of mass spectrometer that can ionize all the nickel in a
sample. That allows sensitive detection of the isotopes following
magnetic separation. They also analyzed each sample many times to drive
down the analytical error.

To their surprise, Bizzarro and colleagues did not find the expected
extra dose of the iron-60 marker. Instead, the samples contained less
nickel-60 than found in younger meteorites. Apparently, the solar
system's shot of iron-60 had not arrived when this old meteorite
solidified about a million years after the solar system's start. Yet
radioactive aluminum-26--also made in stars--had been there all along.

"Iron-60 and aluminum-26 don't seem to be coming into the solar system
at the same time," says Bizzarro. "There's only one stellar environment
that can do that: very, very massive stars." The bigger the star, the
faster it burns its hydrogen fuel. If it has more than 30 times the mass
of the sun, a star will blow away much of its outer layers--including
its aluminum-26--in the last million years of its brief life of 4
million years or so. That stellar wind could have driven the collapse of
interstellar gas and dust to form our sun and the protoplanetary disk
that once surrounded it. Later, the massive star exploded, spewing
iron-60 from its deep interior.

The Bizzarro paper "has a great story to tell - based on some truly
spectacular nickel-isotope data," says cosmochemist Meenakshi Wadhwa of
Arizona State University in Tempe. There is a caveat, however. Three
other labs, including her own, have analyzed similar samples with
similar levels of precision--albeit using a different data-analysis
approach--without finding a deficit of nickel-60 in the oldest samples.
Wadhwa still believes the authors make "a pretty good case for the
accuracy and precision of their data." But you can bet that "pretty
good" won't stop competing labs from gearing up for more analytical runs.
Received on Thu 24 May 2007 07:01:49 PM PDT