[meteorite-list] Interstellar Organic Matter in Meteorites

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue May 30 00:09:59 2006
Message-ID: <200605300011.RAA22518_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/May06/meteoriteOrganics.html
  
Interstellar Organic Matter in Meteorites
Planetary Science Research Discoveries
May 26, 2006

--- Carbonaceous chondrites contain organic compounds with high
deuterium/hydrogen ratios, suggesting they formed in interstellar space.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology


Organic compounds in carbonaceous chondrites contain microscopic regions
with surprising enrichments in the ratios of deuterium (D) to hydrogen (H)
and nitrogen-15 (15N) to nitrogen-14 (14N). Henner Busemann and his
colleagues Andrea Young, Conel Alexander, Sujoy Mukhopadhyay, and Larry
Nittler at the Carnegie Institution of Washington, and Peter Hoppe
(Max-Planck-Institut f??r Chemie, Mainz, Germany) demonstrate that
organic matter resistant to dissolution by strong acids carry
significant isotopic anomalies. They suggest that these anomalies most
likely formed in interstellar space before the solar system formed and
survived the long journey from molecular cloud to protostellar disk to
asteroids.

Reference:

    * Busemann, H., A. F. Young, C. M. O'D. Alexander, P. Hoppe, S.
      Mukhopadhyay, and L. R. Nittler (2006) Interstellar chemistry
      recorded in organic matter from primitive meteorites. Science, v.
      312, p. 727-730.

------------------------------------------------------------------------

Gunky Meteorites

Some carbonaceous chondrites smell. They contain volatile compounds that
slowly give off chemicals with a distinctive organic aroma. Most types of
carbonaceous chondrites (and there are lots of types) contain only about
2% organic compounds, but these are very important for understanding how
organic compounds might have formed in the solar system. They even contain
complex compounds such as amino acids, the building blocks of proteins.

The presence of amino acids certainly sounds like they could contain
life. Maybe carbonaceous chondrites are crawling with micro-organisms. I
was amazed when I first learned decades ago while still in graduate
school that some meteorites contain amino acids. I admit I was a bit
disappointed when I read that the amino acids have equal amounts of
left- and right-handed molecules (a description of their symmetry, a
property called chirality). Most biological amino acids on Earth have
the same handedness, so the amino acids in carbonaceous chondrites
formed inorganically. No wee creatures were involved. No big ones, either.

The organic compounds in carbonaceous are very important, however. They
may represent the type of materials that seeded the Earth with organic
molecules, producing a complex, smelly soup in which life arose. Most of
the studies of organic compounds in carbonaceous chondrites have focused
on the origin of organics to the early Earth and on the processes in the
cloud of gas and dust from which the solar system formed (the solar
nebula or protoplanetary disk). Now, advanced instrumentation allows
cosmochemists to investigate the origin of the carbonaceous gunk, and it
appears that at least some of it formed in interstellar space before the
Sun formed. It is presolar organic matter.

The Murchison carbonaceous chondrite (a CM chondrite), shown on the
left, was the first meteorite in which unambiguous evidence for organic
compounds, including amino acids, was found. It fell in Australia in 1969.


------------------------------------------------------------------------

Searching for Presolar Organic Compounds

Henner Busemann and his colleagues concentrated their work on CR
chondrites. These are carbonaceous chondrites that contain metallic iron
and appear to be primitive (relatively unaltered since their parent
asteroid formed). Previous research on CR chondrites hinted that they
have unusual deuterium/hydrogen ratios. (Deuterium is an isotope of
hydrogen. Its nucleus contains a proton and a neutron; hydrogen's
nucleus contains only a proton.) The team analyzed samples using two
different ion microprobes (see PSRD article: Ion Microprobe capable of
making images of the distribution of hydrogen and nitrogen isotopes.
They studied two types of samples. One type was extracted by using a
highly acidic solution of cesium fluoride and hydrochloric acid. This
procedure dissolves everything but insoluble organic matter, which is
nicknamed IOM. The other material was small chips of the dark,
fine-grained matrix of the chondrites.

lightbulb The results are quite startling. Instead of the modest
enrichments in deuterium/hydrogen (D/H) observed in bulk analyses of
meteorites, Busemann and his coworkers found very high values of D/H,
higher than even observed in interplanetary dust particles. (The D/H
ratio is expressed as delta D (??D), a measure of the deviation in the
D/H ratio from a terrestrial standard.) The highest values were found in
pure separates of insoluble organic matter. Tiny spots in two of the
meteorite organic separates record the highest values ever measured in
meteoritic material. See maps below.

map of hotspots

Maps of ??D (a measure of the deviation of the D/H ratio from Earth
values) and ??15N (the deviation in the nitrogen-15 to nitrogen-14 ratio
compared to Earth values). Both scales are in parts per thousand (10
parts per thousand is 1 percent) also known as parts per mil and
indicated with this symbol ???. Hot spots (outlined in white) for ??D do
not correspond to those for nitrogen. These maps are in a sample of
insoluble organic matter extracted from the CR chondrite EET 92042.

Nitrogen also shows isotopic anomalies in the insoluble organic matter,
but the locations of these are almost always different from those
showing deuterium anomalies. These isotopic differences suggest that
they formed in different environments in interstellar space (or possibly
in the outer fringes of the solar system).

------------------------------------------------------------------------

A Long Journey Through Space and Time

No matter how this isotopically anomalous organic matter formed,
Busemann and his colleagues have shown that such material survived their
long journey from interstellar space into the solar nebula (the Sun's
protoplanetary disk), then into a carbonaceous asteroid. It was not even
affected by processes that operated inside the asteroid, such as those
driven by migrating water. Busemann and coworkers point out that the
preservation of the anomalous areas, small though they are, shows that
organic matter was not completely homogenized when the solar system
formed. This is consistent with preservation of other types of
interstellar grains in chondrites and interplanetary dust particles (see
PSRD articles: Silicate Stardust in Meteorites
<../June04/silicatesMeteorites.html>, A New Type of Stardust
<../Aug03/stardust.html>, and Moving Stars and Shifting Sands of
Presolar History <../July97/Stardust.html>.)

How did these interesting isotopic anomalies form? One idea is that they
originated in cold interstellar clouds. Another is that they formed in
the fringes of the protoplanetary disk. In either case, the environment
was very cold (about 10 Kelvin) and reactions between ions and molecules
or reactions on the frigid surfaces of grains can cause isotopic
fraction. Busemann and his colleagues favor an interstellar origin
because infrared and ultraviolet spectra of insoluble organic matter is
similar to spectra taken of the interstellar medium. They also point out
that other types of presolar grains survived formation of the solar
system, so why shouldn't at least some organic matter? On the other
hand, they are less sure of how the nitrogen anomalies formed. Current
theory cannot explain the high ??15N in any astrophysical environment.
Its formation probably does not involve the same chemical processes that
led to enrichments in deuterium.

Interstellar clouds such as IC 1396 may be the birthplace of
isotopically anomalous organic matter in primitive chondrites. The solar
system most likely formed in an environment like this. For more
information about interstellar clouds, see an article
<http://www2.ifa.hawaii.edu/newsletters/article.cfm?a=138&n=14> by
University of Hawaii scientists Karen Meech and Eric Gaidos. For more
information about organics in the interstellar medium, see article
<http://interstellar.jpl.nasa.gov/interstellar/probe/solar_system/organic.html>
from the Jet Propulsion Lab. For more images see Hawaiian Starlight
<http://www.cfht.hawaii.edu/HawaiianStarlight>. [These links will open
in a new window.]

These detailed studies of organic matter in carbonaceous chondrites have
important implications for the origin of asteroids, too. Busemann points
out that organic molecules are much more fragile than silicates, oxides,
and other presolar grains found in meteorites. If heated too much
organic compounds break down. This indicates that the asteroids that
gave rise to carbonaceous chondrites (at least the types rich in organic
compounds) either formed in regions of the protoplanetary disk that were
always cold or in places where the temperature had dropped enough to
assure survival of the organics. Busemann and his colleagues speculate
that the organic matter could have been transported into the asteroid
belt from much farther out in the solar system, where temperatures were
colder. This kind of transport is consistent with the presence of
crystalline silicates in comets (see PSRD article: Cosmochemistry from
Nanometers to Light Years <../Jan06/protoplanetary.html>.)

This study, like all studies of presolar grains in meteorites and
interplanetary dust particles, bridges the gap between astronomy and
cosmochemistry. The meteorites contain a wide variety of presolar
materials that exist in molecular clouds or in star-forming
regions--little pieces of the clouds examined by powerful telescopes.

------------------------------------------------------------------------
ADDITIONAL RESOURCES

    * Busemann, H., A. F. Young, C. M. O'D. Alexander, P. Hoppe, S.
      Mukhopadhyay, and L. R. Nittler (2006) Interstellar chemistry
      recorded in organic matter from primitive meteorites. Science, v.
      312, p. 727-730.
    * Clayton, D. D. (1997) Moving Stars and Shifting Sands of Presolar
      History. Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/July97/Stardust.html
    * Martel, L. and G. J. Taylor (2006) Ion Microprobe. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/Feb06/PSRD-ion_microprobe.html
    * Taylor, G. J. (2003) A New Type of Stardust . Planetary Science
      Research Discoveries.
      http://www.psrd.hawaii.edu/Aug03/stardust.html
    * Taylor, G. J. (2004) Silicate Stardust in Meteorites. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/June04/silicatesMeteorites.html
    * Taylor, G. J. (2006) Cosmochemistry from Nanometers to Light
      Years. Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Jan06/protoplanetary.html
Received on Mon 29 May 2006 08:11:13 PM PDT