| | October 2002: About 3.9 billion years ago, the birth of an asteroid in outer space set off what is probably the longest-running crystal growth experiment ever. Now NASA researchers are working to peer inside a piece of that asteroid to glean the results. The understanding they gain of the asteroid's composition and crystalline structure, which is unlike any scientists have yet had the chance to study, should yield clues to how planetary bodies form and how to improve processing on Earth of important materials. The focus of the work is the metallic core from an asteroid that eventually entered Earth's atmosphere thousands, if not millions, of years ago, judging from the weathering on the surface. The meteorite pieces that survived the trip landed in Western Australia. Small chunks were found in 1911, but in 1966 two multiton pieces were discovered and collectively dubbed the Mundrabilla meteorite after the area in which they were found. The largest piece, weighing some 11 tons (9,980 kilograms), is still in Australia, but a smaller piece, roughly 6 tons (5,440 kilograms), was cut up and sent to various institutions for study. The Smithsonian Institution's National Museum of Natural History has several pieces, including one on exhibit and a 100-pound (45-kilogram) sample that is now on loan to Donald Gillies, discipline scientist for materials science at Marshall Space Flight Center's Micro-gravity Science and Applications Department. Above: Image of a single slice of the Mundrabilla meteorite sample taken with computed tomography. Working with Peter Engel, an engineering specialist at Kennedy Space Center, Gillies is using a technique called computed tomography (CT) [see sidebar below "Computed Tomography"] to study the internal structure of the meteorite piece. CT, known to most people for its use in medical applications, is most effective on objects composed of more than one type of material because it detects changes in density. Gillies says the materials in most meteorites are well-mixed, so CT would be ineffective on them, but the structure of the Mundrabilla meteorite includes distinct regions, or phases, of different materials. It is about 75 percent iron-nickel alloy and 25 percent iron sulfide, with the iron-nickel alloy forming a metal maze that runs through the iron sulfide. The metallic crystals that form the maze inside the meteorite are surprisingly large, and scientists have so far been baffled by how they could have formed the structure now seen. Sulfur in asteroids is normally found between the metallic inner cores of the formations and their rocky outer parts. Usually the inner metal solidifies slowly, forcing out lighter, sulfur-rich liquid that can float away even under microgravity conditions if given enough time. For some reason, in the Mundrabilla meteorite, the metal's initial solidification was too rapid for the sulfur to float away. Estimates are that the meteorite must have initially cooled by 500ºC per year, a sprint in geological terms. However, researchers believe that the present structure of the iron-nickel regions may have formed by very slow cooling in astronomical terms, at a rate of about one degree per million years, perhaps much later than the rapid initial solidification. This is conceivable only if the asteroid were large enough to provide good insulation against cold space temperatures and the internal temperature was maintained by internal radioactivity. Yet the fast initial cooling would not have been possible with an insulating rocky surface. This is a paradox scientists find most puzzling. Over the decades, very little of the meteorite's internal structure has been observed, because most techniques would require tearing apart irreplaceable samples. But using nondestructive CT, Gillies and Engel do not face that problem. Gillies says their work, the first real "look" inside the meteorite, could help solve the mystery of the meteorite's formation. As many materials solidify, they form crystals, or patterned arrangements of atoms or molecules. The key question for Gillies is whether the crystalline structure of the metal maze in the meteorite has a dendritic structure, which means it grew in a treelike fashion, or has some other less common structure. "Are we looking at branches of it? Are we looking at leaves on branches of the dendrite?" asks Gillies. The researchers may even be able to spot a pattern in the metal branches that would not only point to the physical starting point of the original asteroid's crystal structure but would also give clues about its formation. So far Gillies and Engel have not imaged enough of the meteorite chunk to say, but they are working toward a complete scan of the Smithsonian sample, which is over 2 feet long. Gillies says once the three-dimensional structure of the sample is revealed, it may allow him to confirm that the structure is indeed dendritic and in what direction it grew, or it could even point to some new, previously unrecognized mechanism for growth. In any case, just how the sulfide was trapped might become clear. Above: Peter Engel places an explosive mounting flange on the object positioning unit of the Kennedy Space Center computed tomography scanner. The flange, which is used to attach solid rocket boosters to the shuttle, was checked for cracks before use on a launch. Most of Gillies's NASA research has focused on growing crystals in microgravity. In his experiments, he is usually setting up initial conditions to study how they influence the structure and properties of a material once it has solidified. For the meteorite analysis, Gillies must reverse his usual process, as he is looking at a completed crystal structure and trying to figure out what the initial conditions were that produced it. One of the reasons Gillies was attracted to studying the meteorite is that, for the most part, its crystal structure likely formed in microgravity while the entire asteroid was orbiting in the asteroid belt. If that's the case, then the meteorite is really the longest-running microgravity experiment ever and could provide vital information about crystal formation that could not be obtained any other way. Gillies says learning about the meteorite and how it formed could lead to a better understanding of how other materials crystallize. For instance, the meteorite's large metallic crystals may have grown as a result of a process called coarsening, in which larger particles grow at the expense of smaller particles. Coarsening weakens materials over time, so understanding the process could prove helpful to industry. Likewise, the formation of dendrites is fundamental to solidification theory, as most materials, particularly metals, begin their solid lives as dendrites. Dendrite growth and pattern formation in general are critical elements in the production of everything from industrial metal structures like turbines to some biological systems. Gillies says the asteroid's long life should have given its structure time to grow nearly as large as it ever could, a state of equilibrium that cannot be duplicated in normal experiments. And information about the ultimate fate of dendrite formation could advance research on shorter timescales, he says. Both coarsening and dendrite growth have been much studied in NASA's materials science program. Another reason researchers have long been interested in learning about the meteorite's metallic structure and the possibility that it is dendritic is that it could provide critical general information about how planetary bodies form. These bodies would include the Earth, whose core some researchers believe contains similar materials. There is ongoing debate, says Gillies, about whether the center of the Earth is in fact dendritic. "The origins of the universe are still very mysterious to us. The status of our own planet is very mysterious to us," says Gillies. He and other researchers he is consulting with are hoping that solving some of the Mundrabilla meteorite mysteries will soon shed light on these elemental questions.
Computed Tomography Computed tomography (CT) was first used decades ago as a medical tool, but it has long since been put to use imaging everything from engine parts to meteorites. The technique allows researchers to create an image of any object that has density changes within it. That includes objects made of more than one material, objects made of a single material of nonuniform density, or objects that have air pockets inside them. Computed tomography does not, in fact, take the best possible image of an object's insides, but better techniques require destruction of the object. When dealing with human brains, space shuttle parts, or a host of other things that perform better intact, CT is a welcome alternative. The idea behind computed tomography is to create a picture of a very thin cross section of an object by passing a very thin fan of X-rays or gamma rays through it and then repeating the process until every slice of an object is imaged in order to create a three-dimensional representation of the object. Researchers Donald Gillies and Peter Engel are conducting the meteorite CT work at a Kennedy Space Center facility using gamma rays given off by a piece of radioactive cobalt, about the length and diameter of a pencil lead, as it decays. The cobalt is shielded by 700 pounds (318 kilograms) of depleted, nonradioactive uranium. During imaging, a narrow slit in the container is opened automatically, and the cobalt is positioned in back of it so the gamma rays can be emitted. About 6 feet away is a line of 125 detectors, parallel to the floor and spaced just under half an inch (10 mm) apart, that detect the incoming gamma rays. Samples are placed between these detectors and the gamma ray or X-ray source. CT imaging is possible because denser materials reduce the number of rays more than less dense materials. For CT using the cobalt source, this is due to a phenomenon known as Compton scattering. Compton scattering generally occurs when the gamma ray photons, which like visible light are forms of electromagnetic radiation but vibrate much faster, are scattered by the electrons around atoms. This prevents some from reaching the detectors and, hence, reduces the signal ultimately detected. The atoms of denser materials have more electrons, so they cause relatively more scattering and weaker signals. The full imaging process begins with the measurement of the gamma rays through air, which causes almost no scattering. Then the object to be imaged is moved in front of the gamma ray source and the detectors take measurements that are compared to air readings. This produces one detector data set relative to the density of the material in the path of the rays, but not enough to get a complete image of the cross section. Getting a full and accurate image depends on determining the density of each specific point on the imaged slice. One way to do that is to move the object back and forth in front of the gamma rays and rotate it to various angles. Ultimately the CT scanner takes about a million or so readings. The density of a specific point is determined by mathematically comparing the many detector readings of gamma rays that went through that point from different angles with the rays sent through the air. Then each point on the slice can be put together, or mapped, to create a picture of the slice. "A good analogy might be if you sawed a tree trunk in half, gathered up all the sawdust, and were able to put all the sawdust grains back together the way they were before," says Engel, who serves as head of the Kennedy Space Center CT facility. "Those sawdust grains would make you a flat, two-dimensional picture of that particular cross section of the tree trunk," he explains. The CT images created can show even minute differences in densities of materials, so the CT slice pictures reveal everything from the internal structure of a piece of equipment, to defects on its surfaces, to cracks, which show up because of air or foreign substances in the crack. Multiple slice images can be stacked digitally on top of each other to create a computerized three-dimensional image of the object. Besides peering into the Mundrabilla meteorite, Engel uses CT to examine various shuttle components for defects, as well as for numerous other applications. |
|
