"The most primitive objects in the solar system record chemistry that started in the dark regions of cold molecular clouds," says geochemist George Cody of the Carnegie Institution of Washington (CIW). "We can identify such materials by their unusually rich abundances of certain isotopes for example, deuterium."
Astronomers theorize that these dust clouds, once nudged by an outside shockwave, gradually collapsed under their own gravity to form a glowing star surrounded by an accretion disk, where protoplanets took shape. In them the constituents of the interstellar dust were subjected to heat and pressure and reactions with water. Accretionary processes ultimately led to the formation of planets.
Within a band between Jupiter and Mars, however, perturbation by Jupiter's enormous mass interfered with the formation of a planet, leaving the vast rubble field we call the asteroid belt.
"Continuous collisions of asteroids over the past 4.5 billion years yield small fragments that in rare cases crash into the Earth," Cody says. "Most meteorites are pieces of failed planets."
In particular, he says, "organic-rich meteorites tell a spectacular story, but it's one we don't completely understand." To unravel the tale Cody is using a scanning transmission x-ray microscope (STXM) on beamline 5.3.2 at the Advanced Light Source, an instrument initially built by a team led by Harald W. Ade from North Carolina State University and now under the direction of David Kilcoyne of the ALS Scientific Support Group.
A matrix of clues
Cody uses STXM to analyze samples from a class of meteorites called carbonaceous chondrites. What he calls their "rich inventory of organic matter" holds important clues to the evolutionary phases through which they have passed: how much water was present during the processing of their parent bodies, for example, and whether chemical reactions occurred at high or low temperatures. The meteorites take their name from chondrules, or glassy melt droplets, "that must have formed at 1,200 to 1,500 degrees Celsius, embedded in a carbon-bearing matrix that in many cases probably experienced temperatures of not more than 20 degrees C. How is this possible? It's a raging debate."
The carbon in a carbonaceous chondrite is distinctive in another way: "70 to 90 percent of it is insoluble in any solvent," so classic analytic techniques like gas chromatography and mass spectrometry are impractical. Because meteorites typically come in chunks although Cody jokes that one of the hardest things about studying them is "getting the meteorite from the curator's hoard" he and his colleagues have developed analytical methods for applying solid-state nuclear magnetic resonance spectroscopy, or solid-state NMR.
By using NMR to record the chemical shifts of carbon compounds in meteorites, Cody has charted differences like varying ratios of hydrogen to carbon and oxygen to carbon, which reflect chemical processes characteristic of their parent bodies' stage of evolution, and point backward to the unprocessed state of the primal interstellar dust.
Cody has supplemented these solid-state NMR studies with x-ray microscopy and spectroscopy. Because each chemical constituent of a sample absorbs x-rays differently, STXM can make images showing the physical arrangement of chemical compounds in a sample. And STXM has other advantages. Unlike the NMR experiments, it can use very small samples, and it produces results in a hurry.
Says Cody, "All these factors led to STARDUST," a NASA mission led by Donald Brownlee of the University of Washington that will soon bring pieces of a comet and samples of interstellar dust to Earth unaltered material expected to date from before the origin of the solar system.
To catch a flying comet
Finding out what comets are made of requires more than whacking them with heavy objects, as the Deep Impact mission did to Comet Tempel 1 on July 4, 2005. The plume from that impact was imaged by the Deep Impact fly-by spacecraft, which closed to within 500 kilometers of Tempel 1's nucleus, and by other satellites and ground-based telescopes. Once spectra and images and other data are beamed to Earth, Deep Impact will not be heard from again.
A year and a half earlier, on January 2, 2004, the armored STARDUST spacecraft flew through a hail of ejecta to within 263 kilometers of Comet Wild 2's nucleus. It stuck a tennis-racket-shaped panel of aerogel tiles into the debris storm, in which some of the shrapnel embedded itself. The spacecraft is now on its way home and will drop its singular cargo in the Utah desert on January 15, 2006.
Cody showed Scott Sanford, the STARDUST science team leader, his x-ray microscopy and spectroscopy of meteorite samples to make the case for, among its other strengths, beamline 5.3.2's unique ability to make full-edge spectra (features that stand out on an x-ray absorption graph when an atom's inner electrons are kicked into a higher-energy state) of elements like carbon, nitrogen, and oxygen in each sample.
"There will be a lot of microscopes in play," Cody argued, "but this one will get the most information out fast. You have the full bandwidth to look at." By calibrating the x-ray observations against earlier NMR studies, beamline 5.3.2 has created a database of extraterrestrial matter with which the STARDUST samples can be accurately compared.
Partly as a result of his demonstration of what STXM could do, Cody was invited to become a member of the STARDUST experimental team, joining colleagues Andrew Westphal and Anna Butterworth of the University of California at Berkeley's Space Sciences Laboratory (SSL) in the Berkeley Hills.
Westphal and Butterworth are interested not only in comet samples but in another kind of primitive matter STARDUST is collecting the stuff that gives the mission its name. Only one side of STARDUST's collector was aimed toward the comet; during the mission the other side of the collector was oriented away from the comet to collect interstellar dust.
"Westphal and Butterworth are targeted to receive the first grains of this precious cargo for projected measurements at the STXM at beamline 5.3.2," says Kilcoyne of the ALS.
Building an extraterrestrial database
Kilcoyne reports that over the past year or so the beamline has produced definitive measurements of the organics in more than two dozen meteorites, against which all future measurements on meteorites, interstellar dust grains, and pieces of comet will likely be referenced. "It's an incredible database," says SSL's Butterworth, an analytical chemist, of the database of meteorite organics that Cody is compiling with the 5.3.2 STXM.
Looking to STARDUST, she says, "No one has knowingly sampled a comet before, although it's possible some of the tens of thousands of meteorites we have came from comets. STARDUST will tell us what comets are made of, and the meteorite database will tell us if we've seen it before. As for interstellar dust grains, we've found microscopic particles embedded in meteorites, but STARDUST will give us the first-ever contemporary interstellar dust grains."
Many factors work together to determine the characteristic chemical signature of a specimen from space. One is temperature. Comets, for example, "formed cold and stayed cold; they were never part of a planet," says Cody. He hopes to find that Comet Wild 2 "sits way out" on his chart of how the solar system evolved chemically.
But what happens to a particle of dust or a piece of a comet when it slams into an aerogel tile at up to six kilometers a second? That's the velocity difference between the comet and the passing spacecraft, six times faster than a rifle bullet. Even though aerogel is 99 percent nothing, the remaining one percent of silicon froth brakes the particle fast; friction heats it so much the outer part melts. To understand these effects, the researchers are heating test particles up to 1,400 °C and measuring the chemical changes.
Just finding microscopic dust particles buried in each six-centimeter square of aerogel will be "like looking for 45 ants on a football field," says astrophysicist Westphal of SSL. No fully automated system of image analysis can do the job unaided, so he and Butterworth are recruiting amateur astronomers to visually help with the search for interplanetary dust particles using their home computers.
Once found, "each of these grains of dust is worth about $150 thousand," Westphal says. He's invented a way to extract them with a glass needle inserted along the particle track, a gentler method than existing laser-slicing techniques. SSL's extraction system will be duplicated at the Johnson Space Center in Houston, where the particles will eventually be stored for distribution to researchers around the world.
Anticipation runs high as CIW's Cody and his SSL colleagues prepare for the first samples of dust and cometary material from STARDUST. Cody says, "For a cosmochemist, being able to work with the material from STARDUST is the most privileged thing in the world."
Says Butterworth, "My preparations on 5.3.2 towards STARDUST are focused on challenging sample-prep issues finding, extracting, and preparing comet material containing just a few percent by weight of carbon for STXM. One reason the STXM microscope at beamline 5.3.2 will be important for STARDUST is its very high sensitivity in mapping small amounts of carbonaceous material."
"The beamline and STXM were designed explicitly to cover the carbon, nitrogen and oxygen edges in a single sweep, without any manual adjustments, and to operate in a stable, controlled, low-pressure helium environment that preserves the chemistry of these precious samples," says Kilcoyne. "This means the 5.3.2 STXM is ideally suited to cover the energy range of interest, and it uses a broadband source that does not need time-consuming tuning. Additionally, the dedicated instrumentation has led to a growing microscopy community, who enjoy all the advantages of being at the ALS."
"It's a fantastic time to be a planetary scientist," says Butterworth, who notes one of the reasons the STARDUST material is so precious: "These will be the first samples of an extraterrestrial body to be brought back to Earth since Apollo went to the moon."
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