|A Comet Comes to the Advanced Light Source|
|Contact: Paul Preuss, [email protected]|
At less than half the cost of a single round trip in the space shuttle, NASA's $200-million, seven-year-long Stardust mission to comet Wild 2 was an economical excursion into space. Yet Stardust scored a remarkable number of firsts:
The spacecraft flew past Wild 2 at a velocity difference of more than six kilometers per second (almost 14,000 miles per hour) while deploying a paddle-shaped collector of wispy aerogel tiles, which when struck by comet particles slowed them abruptly but gently. On other occasions the opposite side of the collector, the one not facing the comet, was used to gather dust particles from interplanetary space.
All this material was enclosed in a sample return capsule that streaked through the skies and parachuted to the Utah desert in January of 2006, which one observer characterized as "a spectacular show, a landing perfect beyond anyone's dream." After the capsule was flown to NASA's Johnson Space Flight Center in Houston and opened, a few of the thousands of captured particles were quickly distributed for inspection by Preliminary Examination Teams (PETs), whose initial reports were published in the 15 December, 2006 issue of Science.
Scores of PET scientists pored over the diminutive samples at experimental facilities around the U.S. and the world. Synchrotron radiation experiments, among many others, were vital to the effort; beamlines at Berkeley Lab's Advanced Light Source among them the infrared (IR) spectroscopy beamline 1.4.3, the polymer scanning transmission x-ray microscope (STXM) at beamline 5.3.2, the micro x-ray absorption spectroscopy (micro-XAS) beamline 10.3.2, and the STXM at the molecular environmental science beamline 11.0.2 played leading roles in several of the PET reports.
Before any of these high-powered tools could be deployed, the samples had to be removed from the aerogel. Upon entering the aerogel, the particles typically excavated a carrot-shaped track, bulbous at the top and thinning to a point. The particles themselves mostly came to rest at this point, but on the way in, depending on its make-up, a particle might shatter and leave multiple tracks. They also shed outer layers and because the collision and deceleration generated lots of heat vaporized some of their more volatile components.
"Keystones" of aerogel, wedges containing complete tracks and the terminal particles at their tips, were removed under the microscope using computer-driven micromanipulators that sliced the aerogel with glass needles, a technique pioneered by physicist Andrew Westphal of UC Berkeley's Space Sciences Laboratory (SSL). In some cases, slices were cut through the track at right angles. Some of the particles were sliced into dozens of invisibly tiny portions, each a few micrometers (millionths of a meter) thick.
"Many different groups analyzed the same particles using many different instruments, but since infrared spectroscopy is virtually nondestructive, we were often first in line," says Saša Bajt, a physicist at Lawrence Livermore National Laboratory who worked with Livermore's John Bradley and other colleagues on infrared beamline 1.4.3. "When we were done we passed the unaltered samples on to others."
Michael Martin, manager of the 1.4.3 beamline, explains that "what makes our beamline distinctive as an IR source is the small spot size, only 2 to 10 micrometers across, and the very high brightness, which gives a signal-to-noise ratio a hundred or even a thousand times better than normal IR sources. Working with small samples, spot size is crucial," he says. "Before synchrotron sources, IR spectroscopy could not have been used on samples like these."
Infrared spectroscopy is uniquely useful for comparing the Wild 2 particles to astronomical observations of comets and interstellar dust, where minerals like crystalline silicates and organic compounds have been revealed using ground- and space-based telescopes for IR spectroscopy. (Here, "organic" has nothing necessarily to do with life but refers only to compounds incorporating carbon.)
The comparison is even more direct between the Wild 2 samples and measurements performed on meteorites called carbonaceous chondrites, found on Earth, and on interplanetary dust particles (IDPs) collected in the upper atmosphere by high-flying aircraft and the MIR spacecraft. IR spectroscopy identified many silicate minerals, including olivine and pyroxene, like those previously found in interplanetary dust particles.
Initially it was planned to do IR spectroscopy only on the terminal particles, says Bajt, the bits of stuff at the bottom of the tracks in the aerogel. While some overlap with the preliminary team investigating organics was inevitable, most of the IR work was expected to concentrate on the silicates in those particles.
The reason for this, Bajt says, is that "IR measurements we made of Stardust aerogels kept on Earth showed organic residue from the aerogel fabrication, and this initially killed any hope for detecting organics. But after I compared those measurements to Stardust aerogels that had actually seen the comet, I noticed different types of carbon-hydrogen molecules present along some of the particle tracks."
Moreover, Bajt says, "We didn't think IR would have a lot to contribute to organic studies, because in studying the aerogels from the MIR spacecraft, which were much denser, I'd found that the spectrum from the aerogel itself saturated all the peaks of interest. But the density of the Stardust aerogels is graded, less dense at the top and growing thicker lower down, which slowed the particles gently. So we were able to capture volatile organics."
This unexpected result occurred because Bajt, concerned about the effect heat during impact might have had on the incoming particles, had decided to look not just at the terminal particle but instead scanned the infrared beam along the whole length of the tracks in the aerogel wedges.
"A lot of heat energy gets dumped along the trail of the incoming particle," Martin says. "Saša found the track coated with different organics, left behind when whatever was carrying them ice, for example melted and vaporized."
"We were able to do IR imaging at each point along the tracks and build up a two-dimensional image of the different organics at different stages of entry," says Bajt. "The distribution of organics was nonuniform along the tracks. Not every track showed organics, but all showed OH bonds" oxygen-hydrogen bonds indicating the former presence of ice.
Says Martin, "IR is good for detecting organic compounds because they all have strong vibrational modes at these wavelengths." He explains that the covalent bonds between atoms in a carbon-hydrogen compound like benzene are like springs tying the atoms together, springs that are excited at infrared frequencies. The resulting vibrations twist the molecules in different ways along different axes and can be mapped as a graph of absorbed frequencies in the spectrum of the substance containing them. Specific patterns indicate specific compounds. "A typical organic molecule may have 20 or 30 absorptions activated by IR."
Another way of identifying organic compounds is the technique known as x‑ray absorption near-edge structure (XANES). When a tightly focused beam of soft x-rays hits an atom in a sample, the beam's electric field is either scattered by the atom's inner electrons or absorbed by them, which kicks them into a higher-energy state. Plotted on a graph, energies where absorption suddenly increases greatly are called absorption "edges," and they form a distinctive signature for each chemical constituent in a sample.
Beamlines 5.3.2 and 11.0.2 are ideal for XANES, combining this technique with scanning transmission x-ray microscopy, or STXM (pronounced sticks-um).
"By scanning across the sample, the STXM can make images showing the physical arrangement of chemical compounds," says the ALS's David Kilcoyne, a scientist at beamline 5.3.2. For years George Cody of the Carnegie Institution of Washington has worked with Kilcoyne and others at beamline 5.3.2, including postdoc Tohru Araki, to codify the characteristics of meteorites and interplanetary dust grains.
"One reason the STXM microscope is important for Stardust is its very high sensitivity in mapping small amounts of carbonaceous material," Kilcoyne says. "Beamline 5.3.2's STXM was designed explicitly to cover the carbon, nitrogen and oxygen edges in a single sweep." Before these compounds could be evaluated in Stardust samples, however, they first had to be distinguished from the background, and especially from any contaminants.
"We were told there was no carbon in the aerogel," says Tolek Tyliszczak of the Chemical Sciences Division (CSD), who worked on the Stardust samples using the STXM at beamline 11.0.2 with Mary Gilles of CSD and with SSL's Andrew Westphal and Anna Butterworth.
Gilles introduced the two SSL scientists to beamlines 11.0.2 and 5.3.2 and has worked with them establishing standards for the aerogel samples. "In fact there was so much carbon in the aerogel we couldn't believe it," says Gilles, who has previously devoted much of her research to characterizing carbon-containing aerosols in Earth's atmosphere. "For the first two months I was not confident we could get results that were truly representative of the comet samples."
Apparently the supposedly pristine silica aerogel tiles had become contaminated during the manufacturing process; in other cases, microtomed slices of the particles themselves had become contaminated with carbon compounds from being embedded in epoxy.
"We saw carbon, all right, but we had to eliminate signals from all these other sources," Tyliszczak says. "We had to sort out what carbon came from the aerogel, what came from epoxy, what came from the comet. The breakthrough came when we were able to examine transverse slices through the track, with cometary fragments inside it and a diffusion of volatile constituents into the neighboring aerogel, and then distinguish these signals from the aerogel far outside the track."
Gilles says, "The wide energy range and excellent energy resolution possible with beamline 11.0.2 was a great help in separating the signals of interest from the noisy background. Since the beamline can look at most elements, including silicon, we were able to correlate silicon with carbon in the aerogel. So when we found carbon in the samples that wasn't correlated with silicon, we knew it wasn't coming from the aerogel."
If the data from the STXM beamlines and the infrared beamline, plus the data from other experimental techniques, yielded an unexpected picture of the Wild 2 organics, at least it was consistent which all the researchers emphasize is essential. As Kilcoyne puts it, "All techniques must agree, or we'd better know the reason why not."
While extraterrestrial organic molecules have nothing to do with life on Earth at present, the impacts of comets and meteorites and the constant rain of interplanetary dust may have played an important role in the origin of life on our planet in the distant past. The inorganic components of Wild 2 are equally interesting, and hold clues to the origin of the solar system itself.
Minerals were the main target of studies at beamline 10.3.2, a beamline originally developed for environmental studies. Here beamline scientists Mathew Marcus and Sirine Fakra worked with Westphal and Butterworth and other researchers to examine what Westphal calls "the most primitive material we've ever had in our hands."
"We used a combination of three techniques for mapping the bulk chemistry and mineralogy of the Wild 2 samples," Marcus says. "In x-ray fluorescence we scan a sample across a small beam spot, as small as five by five micrometers in size, to obtain a color-coded map of the chemistry of its constituents, according to the way they fluoresce. We use XANES and the related technique of extended x-ray absorption fine structure, or EXAFS, to determine the atomic environment of specific elements, even in trace amounts. X-ray diffraction allows us to look at the crystalline structure of minerals using tiny amounts of material."
Fakra notes that "using micro x-ray absorption spectroscopy and diffraction techniques together, on many particles along the tracks, is a good complement to the electron microscopy work. Among the advantages are minimal sample manipulation no embedding or sectioning and the ability to provide some mineralogical statistics for each track."
While the beamline uses high energy (hard) x‑rays, she says, the 10.3.2 bend magnet's beam intensity is low enough that there is little heating of the sample. "We can just put in the whole track, still in the aerogel keystone, and look for all elements simultaneously, everything from sulfur up to selenium that may be present at specific spots."
The proportions of these elements (and how the atoms are arranged) determine which minerals are found in the comet particles, how much of them are present in each sample, and whether and how they have been altered. For years Marcus has been acquiring mineral samples, including meteorite samples from his personal collection, and using the beamline to characterize them and build up a library of data. This information and that from other databases can be compared to the signatures of the minerals found in a sample and often used to identify them .
For example, the mineral commonly known as olivine is well known from meteorites, but olivine is actually a mixed iron-magnesium silicate; the pure magnesium version of the silicate is called forsterite and the pure iron silicate is called fayalite. XANES plots of the olivine in some of the Wild 2 samples could be matched with olivines of various iron-to-magnesium ratios.
One form of mineral that challenged the mineralogy team at beamline 10.3.2 took the form of calcium-aluminum inclusions (CAIs). "Comets have always been cold, but some minerals, like CAIs, can only be formed hot," Marcus says, "What mechanisms in the early solar system formed these minerals before they aggregated in the comet?"
Although CAIs had been seen before in meteorites, just one "definitely real" CAI was found among the particles from the comet. "From a scientific point of view," says Marcus, "one is the worst number of anything to find, because you never know if it's a consistent phenomenon or some kind of fluke or accident."
The problem was that many other calcium-rich particles were found in the comet samples by XRM mapping. "Are these really from the comet, or are they just impurities in the aerogel ?" Marcus wonders. "The aerogel turns out to be far from pure silica it's full of calcium particles." By using XANES to characterize the form of calcium in the aerogel junk and adding it to 10.3.2's data library, the researchers will be able to distinguish contaminants from other inclusions that came from the comet.
Questions and surprises
From silicates to organics, the handful of comet grains subjected to preliminary examination have already produced surprises and opened new avenues for research.
"In the Wild 2 samples we found organics like those in interplanetary dust particles, and some like those in carbonaceous chondrites," says Saša Bajt. "But there were also differences in the organics from the carbonaceous chondrites. There was more oxygen and nitrogen, indicating the Wild 2 particles are more primitive. And there were big differences between the Wild 2 samples and astronomical measurements of interstellar dust" as distinct from interplanetary dust.
In all, Bajt says, the Wild 2 samples proved both diverse and distinctive. "Some contained compounds supposedly formed only near a star, others had compounds requiring high-temperature processing, one had aluminum-titanium-calcium-rich inclusions previously seen mainly in the Allende meteorite. If we didn't know all these came from Wild 2, you might think they were from 20 different parent bodies."
Hundreds of scientists and dozens of experimental techniques in facilities around the world contributed to the preliminary examination of the first Wild 2 samples. Many definitive results were provided by the beamlines and the researchers using them at Berkeley Lab's Advanced Light Source.