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Sandford et al. (Reports, 15 December 2006, p. 1720) reportedon organic compounds captured from Comet 81P/Wild 2 by the Stardustspacecraft. We emphasize the difficulty in assigning the originof compounds detected diffusely along particle impact tracksand show that rapid heating of aerogel that has never been exposedto cometary particle capture can generate complex aromatic moleculesfrom low-mass carbon impurities present in the aerogel.
Department of Chemistry, Stanford University, Stanford, CA 94305–5080, USA.
* To whom correspondence should be addressed. E-mail: zare{at}stanford.edu
The NASA Stardust Mission, which robotically returned samplesof cometary dust from comet 81P/Wild2 from deep space to Earth,was an extraordinary triumph (1–5). One limitation ofthe mission design is the known carbon impurity content of thedust collection material, aerogel. This residual carbon cangive rise to an artifactual signature that makes the interpretationof some classes of organic compounds problematic (1, 6). Thepotential for misinterpretation of organic compounds lying diffuselyalong particle impact tracks in aerogel was mentioned brieflyin (1), which stated that "not all the collected organics inthe samples will be fully representative of the original cometarymaterial because some may have been modified during impact withthe aerogel collectors." We emphasize here that the generationof low-mass polycyclic aromatic hydrocarbons (PAHs) during hypervelocityimpact of particles into Stardust silica aerogel can complicatethe interpretation of related observations.
Thermal energy dissipated during hypervelocity particle captureis calculated to induce temperatures greater than the silicamelting temperature (>1200 K) along portions of the impacttrack (7). Such temperatures may induce synthesis of complexorganic molecules from innate aerogel carbon, present at thelevel of 2% (8). To assess this problem, we used high-powerinfrared (IR) laser pulses on a sample of Stardust witness couponaerogel, as part of our Stardust Organics Preliminary ExaminationTeam (PET) studies (9). The witness coupon is an aerogel tileemployed for contamination assessment during the mission flightperiod. It was located near the cometary collection aerogeltiles but shielded from particle impacts. Witness coupon aerogelacts as an ideal organic contamination control for the entireStardust Mission period.
It has been shown that laser pulses can approximate the typeof rapid heating introduced during hypervelocity particle impact(10–12). We performed a laser heating experiment at variousdepths in witness coupon aerogel to allow for the distinctionbetween external and internal contamination sources. Externalcontamination sources would be most concentrated on exposedareas of the witness coupon, decreasing in concentration withincreasing depth. In contrast, internal contamination sources(i.e., introduced during Stardust aerogel preparation) wouldbe distributed uniformly throughout the aerogel tile.
Analysis of PAHs on this aerogel was performed using microprobelaser desorption laser ionization mass spectrometry (µL2MS)(13, 14) (Fig. 1). The µL2MS technique is particularlywell suited for the spatially resolved surface analysis of PAHs,having sensitivity in the attomole regime for some PAHs (15).Initial µL2MS analysis of the witness coupon (WCARMI1CPN0,6 and 7) revealed the presence of no PAHs at normal operatingparameters: 22 µJperCO2 laser pulse and 106 W/cm2 powerdensity (Fig. 1E). As the laser desorption power was increased,however, a low-mass envelope of aromatic compounds was detectedthat was uncorrelated with depth in the witness coupon (Fig. 1, B to E).Similar masses were found in previous aerogel studies that useda laser microprobe mass spectrometer, and these were attributedto volatiles trapped in the aerogel macrostructure (16, 17).Laser desorption parameters used in (17) [e.g., 1.06 µm;4 x 109 W peak power (18)] differ from those used here, butin both cases the laser energy is high enough to release trappedorganic volatiles and/or to disrupt carbon-carbon bonds in theaerogel macrostructure.
Fig. 1. µL2MS mass spectra comparing PAHs detected during laser pulse heating of Stardust witness coupon aerogel to those detected on a cometary impact track. (A) Mass spectrum taken at the entry portion of Wild2 cometary particle impact track (C2115,26,22 track 6); scaled down by a factor of 4. Main m/z peaks are indicated. This distribution consists of aromatic compounds with masses between 78 (benzene) and 178 (phenanthrene) atomic mass units (amu). A series of alkylated components was found form asses 78 amu (benzene), 128 amu (naphthalene), and 178 amu (phenanthrene), as evidenced by mass peaks separated by 14 amu (CH2) corresponding to the loss of H and the addition of CH3 (Fig. 2A). No PAHs in the area of interest were detected on aerogel away from the track. The asterisk indicates mass calibration peaks for D8-toluene. The inset shows an optical microscope image of dissected impact track C2115,26,22. The arrow points to the location of the mass spectrum shown in (A). (B to E) Mass spectra, which are each composed of 50 averaged mass spectra taken over the surface of a witness coupon sample (WCARMI1CPN,0,6) at various laser desorption powers. (B) High power [two attenuation grids (15)]. Detected compounds range in mass from 78 (benzene) to 206 amu (phenanthrene + 2CH2) and include mainly volatile aromatic compounds. (C) Intermediate power (three attenuation grids). (D) Normal operating power (four attenuation grids). (E) Low power (five attenuation grids).
[View Larger Version of this Image (31K GIF file)]
To examine the source of detected PAHs, high-power laser pulsepositions were reanalyzed 14 days after the initial analysis(Fig. 2). Using normal, low-power operating parameters, PAHswere detected at earlier high-power laser pulse sites (Fig. 2B).These compounds are identical in mass-to-charge ratio (m/z)and close in relative abundance to those detected previouslyduring high-power IR laser pulses (Fig. 2A). Synthesis of neworganic compounds from carbon pre-cursors in Stardust aerogelby rapid pulsed heating would likely leave localized residualproducts on the surface of the aerogel, as we found. The dataimply that PAHs detected after high-power laser shots were synthesized,not just released, during rapid heating of the aerogel. Comparisonof PAHs found during high-power laser pulse studies of witnesscoupon aerogel to those found diffusely along a Wild 2 cometaryparticle impact track [C2115,26,22 track 6; (9)] reveals thatthey are very close in identity and relative abundance (compareFig. 1, A and B). Although it is difficult to assess quantitativelythe relevance of using IR laser pulses as a hypervelocity particleimpact analog for aerogel, the masses found in these two samplesare strikingly similar, which suggests a close correlation inthermal processing for these two processes. This study demonstratesthe difficulty in distinguishing between cometary and noncometaryorganic compounds lying along particle impact tracks in aerogel.
Fig. 2. µL2MS mass spectra comparing PAHs detected during laser pulse heating of Stardust witness coupon aerogel to those detected at the same position 14 days later using normal operating parameters, both on and off of the original high-power laser shot position. Main m/z peaks are indicated. The asterisk indicates mass calibration peaks for D8-toluene. (A) Mass spectrum composed of 50 averaged mass spectra taken over the surface of a witness coupon sample (WCARMI1CPN,0,6) at high laser desorption power (two attenuation grids). Prominent peaks include 78 amu (benzene), 92 amu (toluene), and 104 amu (styrene). Masses at 104 amu (styrene) and 128 amu (naphthalene) are the most intense peaks observed in the mass spectrum. Several higher order PAHs are detected with relatively low abundance, including 178 amu (phenanthrene), 202 amu (pyrene), and 228 amu (chrysene). (B) Single mass spectrum, taken 14 days after (A), on the previous high-power laser pulse position. Normal operating parameters used; mass spectrum scaled up by a factor of 3. (C) Single mass spectrum, taken 14 days after (A), on aerogel away from the previous high-power laser pulse position. Normal operating parameters used; mass spectrum scaled up by a factor of 3.
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Our study highlights the need for extreme caution in interpretinganalyses of this type. As pointed out in (1), our work showsthat simple correlation of low-mass PAHs with an impact trackin aerogel is not conclusive evidence that they belong to theoriginal impactor, although it does not rule out this conclusion.It has been shown that higher-mass PAHs can be detected alongWild 2 particle impact tracks in aerogel (1). These PAHs donot correlate with those detected in our experiment, which suggestsa cometary origin, albeit potentially thermally altered fromtheir pristine condition. For the analysis of organic compoundsalong impact tracks in aerogel, our study emphasizes the essentialneed for comparison with control experiments.
We stress that these results do not call into doubt that cometaryorganic compounds have been detected in the Stardust returnas evidenced by nonterrestrial D/H and 15N/14N isotopic ratios,which are found in organic particles intimately associated withterminal grains (1, 4). Large particles, which survive aerogelcapture and are found at impact track termini, likely experiencemuch lower temperatures in comparison with ablated materiallying along the track in aerogel. Several experiments have producedresults that show organic compounds located within large terminalparticles retain their original composition, owing to theirdecreased exposure to high temperatures (7, 19). It is continuedwork on these materials that will likely best further our growingunderstanding of cometary organic compounds.
10. G. G. Managadze, J. Exp. Theor. Phys.97, 49 (2003). [CrossRef]
11. S. Sugita, T. Kadono, S. Ohno, K. Hamano, T. Matsui, paper presented at the Lunar and Planetary Science Conference XXXIV, Houston, TX, 2003.
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13. S. J. Clemett, R. N. Zare, in Molecules in Astrophysics: Probes and Processes, E. F. v. Dishoeck, Ed. (Kluwer Academic Publishers, Leiden, Netherlands, 1997), pp. 305–320.
14. L. J. Kovalenko et al., Anal. Chem.64, 682 (1992).
15. Materials and methods are available as supporting material on Science Online.
16. R. A. Barrett, M. E. Zolensky, F. Horz, D. J. Lindstrom, E. K. Gibson, Proc. Lunar Planet. Sci.22, 203 (1992).
17. C. P. Hartmetz, E. K. Gibson Jr., G. E. Blanford, Proc. Lunar Planet. Sci. Conf.20, 343 (1990).
18. E. K. Gibson, C. P. Hartmetz, G. E. Blanford, in New Frontiers in Stable Isotope Research: Laser Probes, Ion Probes, and Small Sample Analysis (U.S. Geological Survey Bulletin No. 1890, Reston, VA, 1989), pp. 35–49.
19. M. J. Burchell, J. A. Creighton, A. T. Kearsley, J. Raman Spectrosc.35, 249 (2004). [CrossRef]
20. The authors thank M. Bernstein, S. Sandford, and the entire Stardust Preliminary Examination Team for many helpful discussions. Samples were prepared by A. Westphal and C. Snead (University of California, Berkeley, Space Sciences Laboratory) and K. Messenger (NASA Johnson Space Center). Funding was provided by NASA grants NNG05GI78G (Stardust Participating Scientists Program), NNG05GN81G (Cosmochemistry Program), and NNA04CK51H (M.K.S.).
Received for publication 12 March 2007. Accepted for publication 1 August 2007.
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