March 16, 2004
The Origin of Organic Matter in the Solar System: Evidence from the Interplanetary Dust Particles
G.J. Flynn1, L.P. Keller2, M. Feser3, S. Wirick3, and C. Jacobsen3
1Deptartment of Physics, SUNY-Plattsburgh, Plattsburgh, NY;
2NASA Johnson Space Center, Houston, TX;
3Deptartment of Physics, SUNY-Stony Brook, Stony Brook, NY
Researchers from SUNY-Plattsburgh, the NASA Johnson Space Center, and SUNY-Stony Brook measured the types and abundances of organic matter in interplanetary dust particles, originating from asteroids and comets, that NASA research aircraft collected from the Earth’s atmosphere. They found the same amount and types of organic matter in both anhydrous and hydrated interplanetary dust particles. This suggests that aqueous processing, long believed to have played an important role in the production of organic matter, was much less important, and that most of the pre-biotic organic matter in the Solar System formed at about the same time as the first dust that condensed from the Solar Nebula. It is also possible that this matter formed even earlier, in circumstellar or interstellar space.
Extraterrestrial materials, including asteroids, comets, and grains in interstellar
space, contain organic compounds that may have been important
starting materials for the origin of life. How this pre-biotic
organic matter formed is not known, but a wide variety of processes,
ranging from catalyzed reactions on the surfaces of grains to
reactions involving liquid water, have been proposed. If this
organic matter was produced by aqueous alteration of elemental
carbon, which is believed to happen on wet asteroids, then we would
expect to see organic matter occurring preferentially in
interplanetary materials that exhibit evidence of aqueous activity,
such as the presence of hydrated silicates. If the organic matter
were produced either during the nebula phase of Solar System
evolution or in the interstellar medium, we would expect this
organic matter to be incorporated into the dust as it formed, so
that it would be present in the anhydrous interplanetary materials
as well. Earlier studies of meteorites showed abundant organic
matter in the hydrated meteorites, but mostly amorphous carbon, with
little or no organic matter, in the anhydrous meteorites. This
suggested aqueous activity was important in the production of the
pre-biotic organic matter. However, all the anhydrous, carbon-rich
meteorites show significant depletions of the moderately volatile
elements in a pattern that indicates these meteorites were once
exposed to temperatures as high as 1200 degrees Celsius, hot enough
to destroy any organic matter initially present.
To make a proper comparison, we were forced to examine interplanetary dust particles (IDPs) like the one shown in Figure 1, which are fragments from asteroids and comets, approximately 10 micrometers in size, that NASA collects from the Earth’s stratosphere. These particles are so small that we required the high sensitivity of the synchrotron-based instruments at the National Synchrotron Light Source to perform the analyses. We mapped the carbon distribution in approximately 100 nanometer (nm) thick slices of seven anhydrous and four hydrated IDPs, determining the carbon abundance, and we performed carbon x-ray absorption near-edge structure (XANES) and infrared spectroscopy, both of which identify specific carbon functional groups. The carbon-XANES spectra of the hydrated and anhydrous IDPs (Figure 2) are very similar, with strong absorptions at about 285 electron volts (eV), identifying C=C, and at about 288.5 eV, identifying C=O.
The infrared spectra of the hydrated and anhydrous
IDPs are also very similar (Figure 3), with the pair of features
near the 2926 and 2854 cm-1 wavelengths identifying aliphatic C-H2,
and the feature near 2960 cm-1 identifying aliphatic C-H3 (where
‘aliphatic’ refers to organic compounds containing short-chain
arrangements of carbon atoms). In some anhydrous IDPs the individual
mineral grains are coated with this carbonaceous material,
apparently the “glue” holding the aggregate particle together. We
found that organic matter is present in similar types and abundances
in both the anhydrous and the hydrated IDPs, indicating that the
bulk of the pre-biotic organic matter in the Solar System did not
form by aqueous processing. Instead, this organic matter had already
formed at the time that primitive, anhydrous dust was being
assembled. The hydrated meteorite, Murchison, has a higher ratio of
aliphatic C-H3 to C-H2 (indicating a shorter mean aliphatic chain
length) and Murchison also contains more aromatic C-H (the broad
absorption at approximately 3050 cm-1 in Figure 3) than either type
of IDP, consistent with Murchison being more thermally processed
than the hydrated IDPs.
BEAMLINE
X1A, U10B
FUNDING
NASA Exobiology Research Program
PUBLICATION
Flynn, G. J., Keller, L. P., Feser, M., Wirick, S., and Jacobsen, C.
The origin of organic matter in the solar system: Evidence from the
interplanetary dust particles, Geochimica et Cosmochimica Acta,
67(24), 4791-4806 (2003).
FOR MORE INFORMATION
Dr. George J. Flynn
Dept. of Physics
SUNY-Plattsburgh
Plattsburgh, NY
Email: george.flynn@plattsburgh.edu