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Projects: Stardust@homeTwo Years after the Launch, Scientific Results are Trickling inStardust@home Update, August 11, 2008
It's been two years since Strardust@home opened for business on August 1, 2006, inviting members of the public to join in the search for interstellar dust particles brought to Earth by the spacecraft Stardust. Two years in which tens of thousands of volunteers have been pouring over hundreds of thousands of movies on their home computers looking for the telltale signs of particle tracks in the aerogel, and reporting their finds to the team in Berkeley. It's been even longer since the Stardust@home crew began perfecting the technique of scanning the collector, producing aerogel movies, sending them to users and collecting the results. And it's been even longer than that since they began, through a slow and difficult process of trial and error, to perfect a unique method of cutting picokeystones out of the aerogel collector, each containing candidate tracks of interstellar dust particles. It's been a long road. And so it is particularly satisfying to all those who have been along for the journey that two years after its launch, Stardust@home is finally bearing scientific fruit. In July Westphal and his team sent out the first batch of candidate tracks detected by Stardust@home volunteers for preliminary scientific analysis. Five candidates were sent to European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and one was kept closer to home, analyzed at the Advanced Light Source synchrotron at the Lawrence Berkeley National Laboratory. The instruments in Grenoble and Berkeley used different methods for their analysis, but both were able to determine the precise composition of the particles. The results are now in, and it appears that none of the particles in this first batch are of extraterrestrial origin. While this result may seem disappointing to those hoping for a quick breakthrough, it is not, in fact, surprising to the Stardust@home team. The six candidates are the first in a list of 100 potential particles found by Stardust@home volunteers in the 40% of the collector that has been scanned so far. Overall, Westphal and his crew expect to find around 20 true interstellar dust particles in this area. This means that only about one in five of the tracks found by volunteers is expected to be the real thing. With these odds it is hardly a surprise that the first six, selected at random, turned out not to be interstellar dust particles. But even if the analysis has yet to yield the hoped for results, the Stardust@home team can see a many positives coming out of this first batch of tests. First there is the persistent problem of how to mount the samples in a way that would both protect them and make them available for analysis. Westphal and his crew has been experimenting with different types of mountings ever since the first experimental keystones were extracted last fall. Now they have finally settled on what appears to be an optimal solution. In this configuration, the keystone is sandwiched between two "windows" composed of silicon nitride. Each window is a mere 70 nanometers in thickness and extremely clean and clear, which makes it relatively easy to see the keystone and place it in any desired location in the window. Such flexibility can be very helpful in positioning the sample for analysis by a range of different instruments, which may require different configurations. Furthermore, the window frames are comparatively large, at 1.5 millimeters to a side, which makes it possible to analyze these samples on a variety of beamlines. While this type of mounting has proven ideal for a range of analysis techniques, it is not suitable for one particular type of examination known as fluorescence tomography. For this a different mounting will be used, known as an "ultralene bubble." In this configuration the keystone is sandwiched between two 4-micron thick ultralene plastic sheets, which are then sealed with a soldering iron. While this mounting is not as user friendly as the silicon nitride windows, and is not as easy to handle and manipulate, it does allow for fluorescence tomography. Beyond the improved techniques and the experience gained in preparing and analyzing the candidate tracks, there is also this: Westphal and his crew now know that the particles in the aerogel collector can be successfully analyzed and characterized. And while this may sound simple, it is in fact no mean accomplishment. During the analysis the samples are subject to an intense bombardment by photons, and it is extremely difficult to predict beforehand what effect this would have on the possible particle tracks in the aerogel. It is now evident that the samples can safely be exposed to this type of radiation and survive to be studies again another day. Then there is the fact that the analysis clearly resolved the composition of the candidate particles, and determined with a high degree of certainty whether they are, in fact of extraterrestrial origin. "This has been a serious problem with the particles that Stardust collected at comet Wild 2" explained Westphal. "These grains are not trivial to characterize, and it happened that we sent particles to researchers for study, only to discover later the particles were simply condensed aerogel." It is therefore very reassuring for researchers to know that they can safely study the Stardust@home candidates, and characterize them accurately. The actual analysis of the candidate particles occurred at the synchrotron facilities in Berkeley and Grenoble. "A synchrotron is an instrument the size of a shopping mall" Westphal likes to say, to give an idea of the vast size of the operation. Inside, electrons moving in a circular tube are accelerated to phenomenal speeds, kept in place by a series of powerful magnets placed at strategic locations along the circumference. At each of those magnets the speeding electrons emit an extremely clear and powerful beam of white light, covering the entire spectrum from x-rays to the infrared. Scientists can then filter out unwanted wavelengths from these beamlines and still be left with a powerful beam at the precise frequency they need.
At the ESRF synchrotron in Grenoble researchers studied the samples at a beamline known as the "x-ray nanoprobe," using a technique known as Synchrotron X-Ray Fluorescence (SXRF). In this method the clear x-ray beam from the synchrotron excites the atoms in the sample, causing them also to radiate in the x-ray range. By measuring the precise energy level of the radiation from these atoms, scientists can deduce the precise composition of the sample. A different technique known as "Scanning transmission x-ray microspectroscopy" (STXM) was used at the ALS in Berkeley. In this method method an x-ray beam from the synchrotron is focused on the sample, and the precise frequency of the radiation is varied. A detector on the other side of the sample then registers the degree to which different wavelengths from the beam are absorbed by the sample. Since each element and compound has its own unique absorption spectrum, researchers can deduce which elements are present in the sample and at what ratios. Another synchrotron-based technique is "Fourier transform infrared spectroscopy" (FTIR). Here the sample is exposed to a single pulse of relatively broad-band infrared radiation, and its total absorption profile recorded. Using the mathematical technique known as Fourier transforms, researchers can then unpack the data and determine which frequencies were absorbed and to what degree. As with STXM, this effectively indicates the composition of the entire sample. The FTIR method, Westphal noted, is particularly effective at detecting organic materials. The six Stardust@home candidates were all analyzed by FTIR at the ALS in Berkeley. Five were then sent to Grenoble for further analysis by SXRF, and one remained in Berkeley to be studied by STXM. The results in all cases were consistent with particles of terrestrial origin. Three of the particles sent to Grenoble showed a high ratio of zinc to iron, which makes it highly unlikely that they are of interstellar, or even interplanetary origins. This is because zinc is relatively rare in the universe, whereas iron is common, and an undifferentiated space-dust particle would almost certainly present the elements in these proportions. That this is not the case strongly suggests that the particles came from a highly differentiated environment – our Earth. A fourth particle studied in Grenoble showed the presence of silicone, sulfur, and nickel, but no iron, which is again highly improbable for a particle that came from space, while a fifth did not register at all in x-ray fluorescence. As for the particle studied at the ALS in Berkeley, it turned out to be a grain of alumina (or aluminum oxide) trapped near the surface of the aerogel. This too is almost certainly not of extraterrestrial origins. Nevertheless, despite the early indications that they came from our own home planet rather than the interstellar medium, the particles will be preserved for further analysis. They will be available for researchers in the near future, and one can never rule out surprises. Meanwhile, two years into the project, the work of Stardust@home goes on. In Houston, at the Johnson Space Center, Dave Frank is monitoring the extraction of additional tracks from the aerogel collector. In Chicago, at the Argonne National Laboratory's Advanced Photon Source (APS), three more candidate particles are being analyzed. Will any of them prove to be the first confirmed interstellar dust particle retrieved from the Stardust collector? Stay tuned. Did you like this story? Send
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