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In the early hours of February 8, 1969, a large fireball rumbled out of the southwest, increasing in luminosity while its noise level grew to that of sonic booms. It illuminated the night sky over vast areas of northern Mexico, even extending into parts of New Mexico. Approaching the southern Chihuahuan village of Pueblito de Allende—some 340 miles (547 km) south of El Paso, Texas—it exploded, sending thousands of stones over a nearly 200-square-mile (300 km) area—the most important stony meteorite shower on record. The Allende meteorite, as it came to be called, is a rare type of meteorite, a Type III carbonaceous chondrite. It turned out to contain some of the oldest matter known in our planetary system, and studying it has opened new windows on the formation of the early solar system.
Within five days of the fall, Smithsonian scientists Brian Mason and Roy Clarke, Jr. were on the ground in Mexico collecting materials. The strewnfield—the term for a meteorite’s dispersal area—was vast, one of the largest ever known, with the head of the field lying near Rancho el Cairo, to the east of the Sierra de Almoloya hills and the tail some 30 miles (50 km) to the south near Rancho Polanco. The scientists enlisted local help in the collecting, and a number of school children joined in the search. More than a ton of material was collected, with some fragments weighing as little as one gram, and one enormous stone weighing 242 pounds (110 kg).
In 1969 NASA had readied laboratories all across America, outfitting them with the latest equipment in order to analyze the first ever lunar materials, which were to be brought back by the historic Apollo 11 mission to the moon later that year. Allende appeared just as everyone was waiting, ready to put their state-of-the-art equipment into action. With the tremendous amount of Allende material collected, the Smithsonian was able to distribute specimens to many different institutions. Even the Smithsonian effort was a joint one, with individuals from both the National Museum of Natural History and the Smithsonian Astrophysical Observatory getting involved.
The Allende material turned out, in fact, to be much more important than the rocks that were brought back from the moon. When scientists sliced samples of the Allende meteorite open, they discovered curious small white objects, which didn’t look like anything anyone had studied before. What were these objects? They are called calcium-aluminum inclusions, or CAIs. Typically no more than a few millimeters wide, they are mixtures of high-temperature oxides and silicates of calcium, aluminum, and titanium. They represent some of the first solid matter to form in the solar system, the swirling dust around the young sun, and are more than 4.5 billion years old. In fact, in composition, the CAIs have much the same composition as our sun (minus the hydrogen and helium). Allende is in essence pre-planetary material. In studying it, scientists can read the history of the beginning of the solar system. And with it, for the first time, scientists had found a key to unlocking the first half billion years of Earth’s history.
With the Allende fall, the study of meteorites became an integral part of earth science. It was in fact a catalyst for the birth of a new science: cosmo-chemistry, at the intersection of geology, planetary science, astronomy, and astrophysics. Prior to the Allende fall, this particular type of meteorite, a CV3 carbonaceous chondrite, was exceedingly rare. Only sixteen meteorite falls of this type are known. Allende is the largest ever found. The Smithsonian’s collection of Allende specimens—which numbers some 2,100 specimens—is especially valuable, as it is the only collection to contain detailed information on the locality of each collection and comprehensive coverage of samples from the entire strewnfield. It continues to be studied intensively. Since Allende landed in the desert in 1969, not a year has passed without additional publications appearing on this remarkable meteorite—often as frequently as one a month!
Read more about Allende in Smithsonian Contributions to Knowledge (1971)
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