[meteorite-list] Scientists Discover Olivine Mineral Comes From Ancient Supernova

[Ron Baalke]

SCIENTISTS DISCOVER MINERAL COMES FROM ANCIENT SUPERNOVA
>From Lori Stiles, UA News Services, 520-621-1877
June 30, 2005

 NASA and University of Arizona researchers have discovered pristine mineral
grains that formed in an ancient supernova explosion.

 The grains were among other extraterrestrial dust plucked by high-flying
NASA research aircraft from Earth's upper atmosphere after they were
delivered to Earth by a comet or primitive asteroid.

 It is the first time anyone has ever discovered silicate grains, in this
case olivine, from a supernova. They reveal important new information on how
much material supernovae contributed to making our sun and planets,
including radioactive material used in isotope age-dating techniques. The
discovery also gives astrophysicists important new physical evidence they
need to verify complex numerical models of supernovae explosions.

 Scott Messenger and Lindsay P. Keller of the NASA Johnson Space Center in
Houston and Dante S. Lauretta of UA's Lunar and Planetary Laboratory are
publishing their findings in the current (June 30) issue of Science.

-------------------------------------------------
Contact Information
 Dante S. Lauretta
 520-626-1138
lauretta at lpl.arizona.edu
http://www.lpl.arizona.edu/people/faculty/lauretta.html/
------------------------------------------------

 A supernova is a massive star that burns through its nuclear fuel -- layers
of hydrogen, of helium, of carbon, oxygen, silicon, sulphur, etc. -- all the
way down to the element iron. Iron is the end point for stellar
nucleosynthesis -- a star can't get energy by fusing iron. When the star
runs out of fuel it collapses, forms an incredibly dense neutron ball and
rebounds in a cosmically violent supernova explosion. The star's successive
shell-like layers of burning hydrogen, helium, carbon, oxygen, etc. undergo
catastrophic mixing and eject tremendous amounts of dust and gas into the
Galaxy. 

 Researchers don't know whether a supernova led to the formation of our own
solar system, but some have found evidence that a supernova produced
radioactive atoms a few million years before our solar system formed.

 Messenger used a new kind of ion microprobe call the NanoSIMS to measure
oxygen isotopes in the unusual grains.

 Results showed that the olivine doesn't come from anywhere in our solar
system, plentiful as olivine is in our solar system. "Olivine, which
includes gem-quality peridot, is a very common mineral in meteorites and
makes up the bulk of the mantle of the Earth," Lauretta said. "That's why
it's been so hard to identify olivine that came in from another star
system."

 "The supernova grains have oxygen isotopic ratios that have never been seen
before in meteorites or comet dust, but are predicted in astrophysical
models of supernova explosions," Messenger said.

 Keller identified the mineral composition using a transmission electron
microscope. The NASA scientists then asked Lauretta if the grain could
possibly be a supernova grain.

 Lauretta used well-known supernova structure models to calculate whether
the grain could have condensed directly from cooling supernova gas, as its
mineral make-up suggested.

 Lauretta said his computational chemical analysis matched the grain's
actual isotopic and mineral composition "dead on." Messenger's isotopic
ratios enabled the team to pinpoint where in the supernova explosion the
grains formed.

 Although there is no way to directly measure the age of the supernova
olivine grain, the scientists said that mineral has remained so strikingly
unaltered since it formed that it probably hasn't spent much time in the
interstellar medium.

 "We know from astronomical observations that crystalline silicates formed
in stars are quickly destroyed by the harsh environment in the interstellar
medium," Keller said, "so the survival of these grains in pristine condition
is remarkable." 

 The scientists have pieced together the olivine's intriguing history:

    o The mineral comes from elements that mixed during the violent
explosion of a collapsed, dying star about 15 times as massive as the sun.

    o The olivine crystallized when the supernova gas cooled to form
dust. Numerous tiny olivine grains in one parcel of gas condensed and stuck
together to form a submicron-sized olivine rock.

    o The olivine bided its time in the interstellar medium for millions
of years, until it was swept up into a cold dust cloud and coated with a
thin veneer of organic matter.

    o  At some stage, the cloud collapsed to form our solar system, and
the grain became trapped within a comet or asteroid for 4.5 billion years,
the age of our solar system. If it was an asteroid, the asteroid would have
to be a primitive asteroid, the kind that hasn't been heated enough to
destroy such presolar grains.

    o The pristine olivine was recently delivered to Earth's upper
atmosphere, where it was snatched up with other interplanetary dust by an
oily collector on a high-altitude NASA research aircraft

 "These are the closest analogs we have now to what we think Stardust
samples will look like," Lauretta said.

 The NASA Stardust mission rendezvoused with comet Wild 2 in January 2004.

 "We basically stuck out an ice cube tray full of aerogel as we flew through
the coma, picked up a bunch of dust, and retracted the sample tray for the
trip back home," Lauretta said.

 Stardust returns to Earth in January 2006.