We have finished the analysis now of six of our interstellar candidates — one by Scanning Transmission X-ray Microscopy (STXM) at the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Laboratory, and five by high-resolution Synchrotron X-ray Fluorescence (SXRF) Microscopy at the European Synchrotron Radiation Facility (ESRF). All candidates have also been analyzed using Fourier-Transform Infrared (FTIR) spectroscopy at the ALS. The STXM work was done in collaboration with Tolek Tyliszczak and Anna Butterworth; the SXRF work was done with Alexandre Simionovici, Laurence Lemelle, Pierre Bleuet, Peter Cloetens, and Romain Basset. The FTIR work was done in collaboration with Hans Bechtel and Sasa Bajt.
First, the mildly disappointing but unsurprising news — it does not appear that any of our candidates are likely to be interstellar. The one that we looked at by STXM (4589365V1) was a water-clear alumina inclusion near the surface of the aerogel. Of the five that we analyzed at the ESRF, three (9471219V1, 9267050V1, 404198V1) show high Zn/Fe, one (5637295V1) shows Si, S and Ni but no Fe , and one (3602277V1) shows nothing at all in x-ray fluorescence.
The reason that we conclude that these are unlikely to be interstellar candidates is because Zn is a relatively rare element in the universe, while Fe is one of the most common. A high Zn/Fe ratio is most likely an indication of terrestrial material. Similarly, it is unexpected to find Ni without Fe. Lastly, it is possible that an impact could show no detectable material using these techniques if it is purely made of H, C, N and O (“organic” material), but feature 3602277V1 appears to be crater-like rather than track-like, which is unexpected.
While our assessment is that none of these features are likely to be interstellar, these samples will be available to investigators for more detailed analyses in the near future, and we could be surprised.
But there is also good news. We have successfully demonstrated the critical techniques that will be important for this project.
First, identification of candidates through the Stardust@home collaboration:
We know from two lines of evidence that the detection efficiency for tracks with the expected size and morphology of interstellar dust tracks is high — first, because we have measured duster efficiency using calibration images and found the efficiency to be very high, and second, because many very thin (2 micron diameter) bona-fide off-normal tracks have been identified by Stardust@home dusters — typically >300 dusters have identified each of these. Because of their weaker contrast, these should be more difficult to identify than high-velocity interstellar tracks.
Second, we can now reliably extract tracks directly from the tray with low (but not zero) risk, although the process is slow and tedious (and fortunately mostly automated), and must be tailored to each extraction since the local topography is often complex. Dave Frank has now completed the move from Berkeley to JSC and is working hard at extractions on the interstellar tray. We now permanently preserve trajectory information right in the picokeystone using a thin artificial track.
Third, we can analyze these candidates on at least four different synchrotron beamlines (ALS/1.4.3 FTIR, ALS/11.0.2 STXM, ESRF/ID13 SXRF, ESRF/ID22 SXRF) and next week we expect to have an analysis at the 2-ID SXRF beamline at the Advanced Photon Source (APS) at Argonne National Laboratory with Steve Sutton and George Flynn on at least two more IS candidates. We can do synchrotron fluorescence mapping with ~100nm resolution, map abundances of many major elements (including Mg and Si) and do X-ray Absorption Near-Edge Spectroscopy (XANES) using x-ray absorption mapping with ~25nm resolution, and search for organic materials with ~0.1% sensitivity in micron-scale particles using FTIR, all essentially non-destructively and in situ in the aerogel picokeystones. We expect to have a fluorescence tomography analysis done on one of the IS candidates by Alexandre Simionovici and colleagues at ESRF in the next month or so.
Finally, an important point is that although none of our first candidates appear to be interstellar, it is quite amazing that we are able to say that at all. The fact that we are able to do detailed characterizations and make sensible assessments of these tiny candidates is very important. A lesson from the cometary side of Stardust is how critical but difficult it is to adequately characterize samples before they are sent out to investigators.
Although this is mildly disappointing, it is not unexpected. We are just starting to look at our candidates, and we knew from the beginning that our signal to noise was low. These are still early days — these candidates comprise about 6% of our current list of candidates. Statistically we could have expected to find perhaps one real interstellar track among these first candidates, so finding none is not at all surprising. We are not at all discouraged by these early results, but even more determined to press on to identify and analyze the first contemporary interstellar dust particles ever returned to earth for analysis.