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The Planetary Society Blog
By Emily Lakdawalla
LPSC, Monday: Notes from the Stardust session
Mar. 13, 2006 | 14:20 PST | 22:20 UTC
My choice among the morning's four sessions was to go learn about the first results from Stardust. To do that, I had to skip hearing the talks about new results from Mars Express MARSIS, which was kind of a bummer, but this conference will be full of such difficult choices, and I was hoping that there would be exciting things reported from the Stardust team's first look at their samples.
I wasn't disappointed. Before I dive into the more technical details, let me report something that probably a large number of you are interested in, and that's what the team had to say about the status of the Stardust@home project. In brief, a couple of the members of the team have said that they have just not had time to even look at the tray containing the interstellar samples -- it's still sitting in nitrogen storage. But they repeatedly said that next week or the week after they plan to start doing the microscopic scanning that will produce the "movies" slicing through the aerogel samples, which will produce the data set that Stardust@home participants will analyze. Mike Zolensky remarked during his talk that he had originally planned to schedule the tray scanning for April 1, but thought better of it, and is now saying April 2. (For those of you who don't understand the significance of April 1, that date is known here in the U.S. as April Fool's day and it's traditional to play practical jokes on people on that date.)
So, moving along into the technical talks, Principal Investigator Don Brownlee gave the first one, an overview of the samples (here's the abstract. This and all other links to abstracts will get you to PDF files containing 1- or 2-page summaries written by the scientists about their work.) He asserted that he believes that the particles that Stardust picked up came from subterranean ice-rich regions of the comet and were only two hours old when they were captured, so they should be quite pristine. (Of course the capture process vaporizes any volatile minerals in the samples, so they're not the same as they were when they came out of the comet -- more on that later.) Brownlee got a laugh from the audience when he showed a slide that he titled "Progess in our view of comets." One side of the slide was a 19th century engraving of a comet stretching across the sky of Paris. The other side was an atomic level view of an olivine crystal from the comet. He said that it was a view 17 orders of magnitude more detailed!
Brownlee went on to describe the particles they are seeing, and they are big. Some he called "rocks" because they are larger than 10 micrometers in size. (For context, a human hair is roughly 100 micrometers in diameter.) "We could have done everything right and the comet could have given us stuff that we couldn't collect and we couldn't work with" because it was too small, he said. "Instead, we have what we consider huge rocks -- we don't really know how to deal with 10- to 15-micron grains, the slicing techniques we use tend to chatter them" or produce striations on their surfaces when they are cut.
But the most astonishing thing was not the size of the grains, but what they were made of. Many of these largest grains were what mineralogists call "refractory," which means that they formed at high temperatures, temperatures up to 1400 Kelvin or so. Minerals like olivine and pyroxene that we on Earth are familiar with as being the constituents of basalt, the highest-temperature lava that erupts on the surface of the Earth. But remember, these grains were in a comet, which must have formed in the outermost, coldest part of the solar system in order for it to retain its primordial ice. "We have hot minerals in the coldest place in the solar system. Where did they come from? They didn't come from there. They either came from the inner regions of the solar system" meaning the location of the terrestrial planets like Earth and Venus "or from other stars. If this was astronomy, we would stop there, with that question. But we have samples in hand. We will solve the mystery. Stay tuned; and I encourage you to join in the analysis," he concluded.
Impact tracks in aerogelThese tracks were made by two particles from comet Wild 2 after they crashed into an aerogel sample cell on the Stardust spacecraft. The largest comet particles fragmented into multiple pieces on impact; you can see several particles at the ends (on the left) of these tracks. Credit: NASA / JPL-Caltech / University of Washington |
Next up was Peter Tsou, who is the Deputy Principal Investigator and who was so excited about the quality of the samples that he didn't seem to stop to draw breath (here's the abstract). He said that the mission exceeded all expectations. The aerogel exceeded its design goals, the aluminum foil exceeded its design goals, there were 45 impacts large enough to be visible to the naked eye, and that none of the tracks penetrated the entire 3-centimeter thickness of the sample paddle. To date, he said, they have removed just six cells from the sample paddle: two in January and four in February. He was particularly proud of having thought of using the thin aluminum foil that was laid between each of the aerogel sampling cells as a second sampling medium (its main purpose was essentially to act as a handle to use to hold the aerogel and slide it out of the grid). The aluminum foil strips that separated each aerogel cell are being cut off separately and analyzed to find incredibly tiny impact craters from particles so small that they weren't captured in the aerogel.
Next up was Peter Flynn, who talked about the preliminary chemical analysis of some of the first grains that were removed from the aerogel cells (here's the abstract). Rather than diving right in to the chemistry, he opened by explaining the context of their work. Basically, their first interest is to figure out just how many individual grains they were going to have to analyze to get a good representative sampling of what's in the comet. One prediction is that the Stardust particles would look like interstellar dust particles (often abbreviated IDPs), which are pretty well mixed up at a scale of 10 micrometers or so. If that were the case, Flynn said, they'd need to look at 30 grains to get a good bulk composition for Wild-2. But the more they look at the grains, the more variety they see. Furthermore, not only is there variation from one grain to another, but when they analyze all the little bits of grains that got deposited along those carrot- or turnip-shaped tracks made by the big particles, they find varying compositions along the tracks. For example, Flynn showed one track where they saw iron and nickel deposited pretty much homogeneously along the whole track, whereas zinc was there only along one side of the track, and chromium was there only at the end of the track. Some individual grains have strangely elevated abundances of one element or another. All in all, the composition of the grains is going to be a big, big, long-term analysis project. Some analysis will have to wait for the development of new technology. Flynn talked about one technique they were using that is able to analyze samples only 200 nanometers across; that's smaller than individual light waves! But even that tool is too coarse to analyze some of the particles.
That's all I've got time for now -- I have to run to "NASA night," where NASA headquarters gives their spiel about their future plans. In light of the implications of their FY 2007 budget, it is probably going to be a bit of a bloodbath. I have to go early to try to get a seat from which I'll be able to get to a microphone. More later…
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