We have several developments in the search for interstellar dust impacts in the Stardust Interstellar Collector. Expect the unexpected!
Last year, we reported on three interstellar dust candidates, Orion/Sirius (track 30), Hylabrook (track 34) and Merlin (track 37). All of these tracks look promising because their orientations are consistent with an origin in the interstellar dust stream, and because their compositions do not look consistent with secondary ejecta from the spacecraft. Further, the tracks were narrow, which looked consistent with some experiments that we did with our colleagues at Heidelberg, in which submicron dust particles accelerated in an electrostatic Van de Graaff accelerator were captured in flight spare aerogel tiles. These first, extremely challenging experiments showed tracks very similar to Orion/Sirius, Hylabrook and Merlin. Now we have done new experiments with a dramatically improved electronic filter, which resulted in very tight spreads in the masses and speeds of the accelerated dust particles. These experiments were done with orthopyroxene, polystyrene and iron submicron particles at speeds between 10 and 20 km/sec. Our conclusion from the new experiments is that Orion/Sirius, Hylabrook and Merlin are probably much slower particles than we originally thought.This does not mean that they are not interstellar, just that the situation is more complicated than we understood just a few months ago.
We have continued our analyses of these particles by Scanning Transmission X-ray Microscopy (STXM) at the Advanced Light Source at Berkeley and Synchrotron X-ray Fluorescence (SXRF) and Synchrotron X-ray Diffraction (SXRD) at the European Synchrotron Radiation Facility in Grenoble. We have learned that Orion, discovered by duster Bruce Hudson, is aluminum rich and appears to contain small crystals of a mineral with a spinel structure. Most of the aluminum is in the amorphous (glassy) phase. Sirius is Mg-rich and the diffraction data show no evidence for crystallinity. Hylabrook, discovered by duster Naomi Wordsworth, is Mg-rich, like Sirius, but the similarity ends there: the diffraction data show that Hylabrook is crystalline, and the shape of the Mg absorption edge is quite unlike that of Sirius, indicating that they are mineralogically distinct. Neither of the phases matches any standards in our library of Mg absorption edge spectra. Finally, Merlin (track 37), which was named by its discoverer, Patrick Fougeray, is carbon-rich and also contains iron. While carbonaceous material might be expected from the heat shield material of the spacecraft, iron in this material would be unexpected.
Meanwhile, totally by accident, we recently discovered a new track, track 40. This track was discovered as we were setting up tile I1003 for automated scanning in Berkeley. Track 40 appears to be very high speed from its morphology — in fact, it looks very much like the original calibration tracks from Phase I of Stardust@home, and is very similar in shape to the 15-20 km/sec tracks in our most recent and highest fidelity calibrations at Heidelberg. It was near the edge of a tile and so is not in the Stardust@home imagery. We have already extracted it in a picokeystone and analyzed it at the Advanced Light Source. (STXM image here.) It appears to have carbon and some iron in it. We deliberately have done no further work on it to avoid altering it.
The two synchrotron analyses that were done last summer at the European Synchrotron Radiation Facility in Grenoble appear, very surprisingly, to have had an effect on two of our particles (Sirius and Hylabrook). We are still in the process of assessing the effect, and we are establishing new protocols on these bright, hard x-ray microprobes to prevent any further effects. Tentatively, it appears that the effect was mostly in breaking the particles up and dispersing them slightly, rather than damaging them chemically or mineralogically. If this is indeed the case, the science is unlikely to be significantly compromised. The effect appears to be related to charging of particles in the aerogel, which is an excellent electrical insulator. If this is the case, we can avoid any problems in the future straightforwardly by limiting the brightness of the x-ray probe beam. We are testing this hypothesis on analog materials now.
The new and very important Heidelberg calibrations have given us for the first time sufficiently high-quality data that understand much better the size of tracks as a function of particle size and particle capture speed. During phase 1 and 2 of Stardust@home, while we found many low speed tracks (Orion/Sirius, Hylabrook, Merlin, plus 51 other tracks that we think come from ejecta from impacts on the spacecraft), very unexpectedly we did not find any high-speed (>10 km/sec) tracks, despite diligent searching by thousands of Stardust@home dusters in about 210 cm^2 of aerogel area. These tracks should appear to be essentially identical to our phase 1 calibration tracks. This is quite unexpected, and it is not clear at all what this is telling us. It appears that high-speed interstellar dust grains are much rarer than we originally thought. But perhaps low-speed interstellar dust grains are more common than we expected. In fact, this would be good news, because the slower the particles, the less altered they are by the capture process.
We have submitted abstracts on this work to the Lunar and Planetary Science Conference in Houston. You can download and read the abstracts here:
http://www.lpi.usra.edu/meetings/lpsc2011/
You can also read about the efforts of the Stardust Interstellar Preliminary Examination foils team here as well.
Finally, we are working hard on launching Stardust@home Phase 4. More details soon…
Stay tuned, and thank you for all your hard work!
Best,
Andrew