Stardust workshop at Timber Cove, California (Feb. 4 to 9, 2011)

Stardust Space Probe at Comet Wild 2

Artist rendering of the Stardust space probe's encounter with comet Wild 2. (Courtesy NASA/JPL-Caltech)

The focus of this meeting was to discuss recent results from the Stardust mission which was the first space mission ever to collect and return cometary material (from comet 81P/Wild 2) and contemporary interstellar dust in 2006. Material collection was mainly achieved by using a special capture medium called aerogel, an extremely low density glass ‘foam’. Additionally, particles impacting into aluminum foil often left some residues behind. Comets are thought of being some sort of ‘space fridge’, having locked away and preserved some of the matter present at the time our Solar System formed 4568 million years ago. On the other hand, the contemporary interstellar dust represents the sort of condensed matter that would contribute to the formation of new solar systems right now.

The workshop started with a session on early processes during our Solar System formation. Q.-Z. Yin (UC Davis, USA) pointed out that the two oldest types of processed and recrystallized matter we know, i.e., calcium and aluminum rich inclusions (CAIs) and the so called chondrules, overlap in their formation times much more than previously thought, with the oldest chondrules being as old as the oldest CAIs. Both objects are found incorporated into meteorites. Chondrules are recrystallized melt droplets that had been produced by an unidentified flash-heating mechanism in the early Solar System and CAIs are condensates of the early Solar Nebular: Their oldest age is used to define the age of the Solar System. While neither chondrule nor CAI formation is really understood, CAIs provide an additional riddle by being isotopically enriched in 16O. Several new models were presented and discussed, including the change of isotopic ratios by photochemical processing of oxygen on dust grain surfaces (G. Dominguez, UC San Diego, USA) or the formation of chondrules through flash heating in spiral density waves caused by orbit changes of the young planet Jupiter (M. Gong, U Tsinghua, China). Also, an older idea was reviewed by M. Gritschneder (U Peking, China): A nearby supernova could have triggered the formation of our Solar System and could have introduced some isotopic anomalies as well.

Then the status quo of the research on the collected contemporary interstellar dust was presented. A. Westphal (UC Berkeley, USA) pointed out that some candidate grains captured in aerogel have been identified, but great care and effort is still necessary to exclude contamination, i.e., that these candidates are not secondary particles knocked off the space craft. R. Stroud (Naval Res. Lab., USA) and R. Ogliore (U Hawaii, USA) presented advances in finding the often less than a thousandth of a millimeter small impact craters of interstellar dust in the aluminum foils.

Another interesting session dealt with new ways to retrieve more cometary material. Compared to an expensive space mission, the collection of dust in Earth’s stratosphere is rather inexpensive. S. Messenger (NASA, USA) and G. Flynn (SUNY Plattsburgh, USA) pointed out the opportunities of timing the collection well after an Earth-crossing of a cometary dust band, as has been best demonstrated in case of comet 26P/Grigg-Skjellerup in 2003: H. Busemann (U Manchester, UK) has found the highest concentration of matter predating our Solar System (presolar grains) in one particle of this collection. One important caveat is, however, that an individual particle from a timed collection cannot be linked for certain to the respective comet, it could still be an interplanetary dust particle from the asteroid belt or even terrestrial, though usually the latter case is easily recognized.

One session was dedicated to isotopic anomalies found in comet Wild 2 material, more specifically, in presolar grains that had survived within the cometary matrix. Scientists are puzzled over the observation that the Stardust mission samples seem to contain fewer presolar grains than one would expected for an object as old and primitive as a comet. This, however, holds true only for material captured in the aerogel and might still be explained by the statistics of small numbers. As J. Leitner (MPI Mainz, Germany) showed for residual matter found in the aluminum impact craters, the concentration of presolar grains in comet Wild 2 seems to be of the same order as for primitive meteorites after all. This session was dedicated to Frank Stadermann (1962–2010, WU St. Louis, USA) who passed away unexpectedly last year. He has contributed a tremendous amount of pioneering work to this field introducing cutting-edge instrumentation and will be greatly missed.

Obviously, collecting cometary grains through high velocity impacts is a violent process, even when using fluffy aerogel: The relative capture velocity was 6.1 km/s or 14,000 mph! Therefore scientists still conduct test experiments to better understand the behavior of cometary matter under these extreme conditions. This helps to assess how biased our results are, i.e., we need to know which minerals survive alright, which phases are probably severely altered (melted/recrystallized) and which phases are likely to be destroyed completely during the capture process. New laboratory data was presented by M. Burchell (U Kent, UK) who showed results from aerogel impact experiments with organic projectiles (glycine), while A. Kearsley (NHM London, UK) demonstrated links between the shape of aluminum impact craters and the composition and structure of the original grains, and P. Wozniakiewicz (Lawrence Livermore, USA) studied the chances of survival for various minerals when shot into aluminum.

Organic compounds are very abundant in primitive Solar System material including comets. Some scientists even think that comets could have brought the first organic matter to our young planet Earth, triggering the formation of life. While organic compounds are not expected to survive the harsh impact into aluminum, they have been found associated with some aerogel impacts. A study by Cody et al. (2008, Meteoritics & Planetary Science) found a surprisingly complex range of functional groups and specific organic bonds in aerogel-captured particles, more than have been known so far from primitive meteorites. Now, B. DeGregorio (NASA, USA) has thoroughly assessed the findings and could link many types of organics to contamination either indigenous to the aerogel or stemming from other parts of the space craft. What remains is about the same organic inventory as was previously known for extraterrestrial materials. S. Clemett (NASA, USA) pointed out that wrong sample preparation (e.g., embedding a sample in epoxy) or certain measurement conditions (including all electron beam methods, e.g. scanning or transmission electron microscopes) jeopardize the accurate measurement of organic compounds due to the fabrication of spurious new compounds. Finally, I presented the results from our group using a new type of mass spectrometer that can analyze untreated (not compressed, coated or embedded) aerogel for organic compounds. We found none associated with the one impact track we could analyze; however, we found an enrichment of organic compounds associated with the aerogel surface that had faced the cometary particle and ion flux.


About D Rost

Research Associate at the University of Manchester, UK
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3 Responses to Stardust workshop at Timber Cove, California (Feb. 4 to 9, 2011)

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