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September 29, 2004

 

 

Organic Matter in Interplanetary Dust

Since long before life appeared on Earth, a constant flux of carbonaceous (carbon-containing) dust has rained down from the sky, at times accreting as much as a centimeter deep over the Earth's surface every million years. Such dust particles are complex assemblages of primitive interplanetary material left over from the formation of the solar system. One such interplanetary dust particle (IDP), collected by NASA from Earth's stratosphere and nicknamed "Benavente," was analyzed by researchers from Washington University and Lawrence Livermore National Laboratory using a variety of techniques, including synchrotron infrared spectroscopy at the ALS. The measurements revealed, for the first time in an IDP, an isotopically anomalous region both depleted in carbon-13 and enriched in nitrogen-15, located in a host phase that is organic in nature. These isotopic anomalies provide valuable clues to the history of the particle and, by extrapolation, the solar system.

Quietly Collecting Dust

They are not among NASA's most glamorous missions. They don't normally make headlines unless something goes spectacularly awry, as with the recent crash of the Genesis capsule in the Nevada desert. Genesis' mission was to collect particles thrown off by the sun in what's called the solar wind. A related mission, dubbed Stardust, was launched in 1999 to collect dust particles from the comet Wild 2 (pronounced "Vildt 2"). It is scheduled to return to Earth in 2006 (also by parachute in Nevada). Benavente, the dust particle studied here by Floss et al., was collected closer to home, in the Earth's stratosphere, by high-flying aircraft. NASA has been collecting dust particles from the stratosphere since the 1970s, but their small size makes chemical and petrographic analysis challenging. With new and improved analytical techniques, scientists can extract from the grains of dust a "geological" record, not of the Earth, but of the solar system. By studying the components of the dust particles, specifically their isotopic content, we can gain knowledge, not only about the age of a sample, but about the physical conditions under which it formed in the dusty, distant past.

EM image of IDP

Electron microscope image of an interplanetary dust particle collected in the stratosphere.

In the Milky Way galaxy, interstellar molecular clouds are the principal formation sites of organic matter—that is, molecules based on carbon and hydrogen, where the carbon is arranged in chains (aliphatic hydrocarbons) or rings (aromatic hydrocarbons). Isotopic anomalies provide a fingerprint for tracing how abiotic interstellar organic matter was incorporated into the solar system. Organic molecules formed in cold, dense interstellar clouds will have isotopic ratios that differ from those formed in the relative warmth of the solar system, because the pathways for mass fractionation (which determines isotopic composition) depend on physical or chemical conditions. A "heavy" isotope's greater mass results in slightly different chemical bonding characteristics. These differences become significant at low temperatures, when the difference in the binding energy between isotopes exceeds thermal energy. Past measurements of IDPs have found high levels of deuterium (heavy hydrogen) and nitrogen-15, consistent with origins in low-temperature interstellar clouds. Because these anomalies are associated with carbonaceous material, the lack of similar anomalies in carbon has been a major conundrum.

Using a new technique called NanoSIMS (secondary ion mass spectrometry) that allows isotopic imaging down to 100 nm, the researchers looked for carbon and nitrogen isotopes simultaneously. They noted that one region of the IDP Benavente, about 0.5 by 2 µm2 in size, was strongly enriched in nitrogen-15 and depleted in carbon-13 relative to terrestrial isotopic compositions, the first observation of correlated nitrogen and carbon isotopic anomalies in an IDP. Transmission electron microscopy of this material at Lawrence Livermore National Laboratory showed that the region is rich in GEMS (glass with embedded metal and sulfides) and amorphous carbonaceous material. Infrared spectra of the carbonaceous material taken at ALS Beamline 1.4.3 showed a prominent absorption feature at a wavelength of 3.4 µm, corresponding to the stretching mode of the carbon–hydrogen (C-H) bond. The positions of the bands within the feature are consistent with those of aliphatic hydrocarbons, confirming the organic nature of the carbonaceous material. Aromatic hydrocarbons are likely to be present too, but are probably dominated by the strong resonance of the C-H feature.

NanoSIMS images

Secondary ion mass spectrometer (NanoSIMS) images of the dust particle, showing a region of nitrogen-15 enrichment (left) associated with carbon-13 depletion (right).

IR spectrum

Infrared spectrum of a section of the dust particle, showing the C-H stretch feature at 3.4 µm indicative of the presence of aliphatic hydrocarbons.

In the absence of carbon isotopic anomalies, it was not possible to determine whether the carbonaceous material was itself presolar or simply a relatively recent solar-system host substrate for the anomalous deuterium and nitrogen-15 species commonly found in IDPs. The observation of correlated carbon and nitrogen anomalies establishes that IDPs contain heteroatomic organic compounds of presolar interstellar origin: the material took organic form before solar system existed and was then incorporated into the IDP. Theoretical models indicate that low-temperature formation of organic molecules in cold molecular clouds can produce C and N fractionations through gas-phase ion–molecule reactions. However, additional studies are required to determine whether processes exist that can produce both effects in the same material.

Research conducted by C. Floss and F.J. Stadermann (Washington University) and J. Bradley, Z.R. Dai, S. Bajt, and G. Graham (Lawrence Livermore National Laboratory).

Research funding: National Aeronautics and Space Administration, U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: C. Floss, F.J. Stadermann, J. Bradley, Z.R. Dai, S. Bajt, and G. Graham, "Carbon and nitrogen isotopic anomalies in an anhydrous interplanetary dust particle," Science 303, 1355 (2004).

ALSNews Vol. 245, September 29, 2004

 

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