University Times

When Argentine geologist Juan Nagera first visited Campo del Cielo, "Field of the Sky," in northern Argentina in 1926, he concluded that the four shallow pits he explored in the region had been dug by ancient Indians. He also believed that the fragments of meteorites he uncovered in the pits had been placed there by the Indians.

It was not until seven years later, in 1933, that L.J. Spencer, keeper of minerals at the British Museum in London, read Nagera's paper on Campo del Cielo and ventured the opinion that the pits the Argentine geologist had located actually were meteorite craters.

Spencer's interpretation of Nagera's work in turn led William Cassidy, a professor in Pitt's Department of Geology and Planetary Science, to conclude that there was a very high likelihood the pits in Campo del Cielo had been made by a meteorite. And that is exactly what Cassidy, then a young researcher at Columbia University's Lamont Geological Observatory, finally confirmed when he traveled to the region in 1963 as part of the first scientific expedition to investigate Nagera's craters.

Over the next 10 years, Cassidy, who joined the Pitt faculty in 1968, located 16 additional craters, for a total of at least 20, in the area. Based on the carbon dating of materials found at the site, he and his colleagues determined, too, that the crater field was formed when a meteorite of several hundred tons from the asteroid belt between Mars and Jupiter broke up and struck the Earth about 4,000 years ago.

Although Cassidy's work in Argentina ended in 1973, its significance to the field of meteorite studies recently became news again when the Meteoritical Society honored the Pitt professor with the Daniel Moreau Barringer Award. Given in recognition of outstanding research on meteorite craters, the Barringer award is named for the mining engineer who first convincingly identified meteorite craters in Arizona in the early 1900s. Cassidy says it took so long for his work to be recognized by the Meteoritical Society because the Barringer award was created only five years ago and there is a big backlog of scientists whose work in the field deserves to be honored.

"It was kind of a surprise," he adds. "I didn't really think that there had been much notice taken of the Argentine work. But apparently somebody knew about it." That is more than can be said for the craters themselves. Despite the fact that the local people were well aware of the existence of the pits when Cassidy first arrived in Campo del Cielo in 1963, like Nagera, they never imagined them as being made by meteorites. "We were talking to the local people about meteorite craters and getting nowhere," Cassidy recalls. "Finally, we figured out the magic word. And it wasn't money. We asked if there were any pits in the area dug by the Indians. They knew about lots of them. They took us around to all of the ones we ended up finding." Once the pits were located, Cassidy says, it was easy to identify them as meteorite craters because some had meteorite fragments scattered around them, while others had "tremendous magnetic anomalies that indicated big metallic masses buried at some depth." Cassidy's research team eventually excavated two of the craters. One of them contained a pair of meteorite fragments that together weighed 5.3 tons. The other had "a real buster of a magnetic anomaly" that turned out to be a 34-ton meteorite fragment, the third largest ever found. The largest meteorite fragment discovered to date is one of 60 tons uncovered in Namibia in southwestern Africa. According to Cassidy, it is highly unusual to find specimens of such size because most large meteorites are disrupted or destroyed when they enter the Earth's atmosphere and strike the ground. The ones that survive appear to have entered the atmosphere and struck Earth at a shallow or low angle like a rock skipping across a pond.

Even though the Barringer award honors Cassidy's work in Argentina, Cassidy himself believes "my possibly more important work has been with Antarctic meteorites." Since 1976, when he was given his first research grant by the National Science Foundation, Cassidy's meteorite studies have focused on Antarctica. Working with Japanese researchers on that southern-most continent, he has helped to find more than 15,000 meteorite fragments produced by about 5,000 meteorites. He and his colleagues have more than doubled the number of meteorites known in the world. "Antarctica is the best place in the world to find meteorites," Cassidy says. "Concentrations can build up because meteorites are in a kind of a deep freeze and they don't weather away over time. If you have a million years, you can accumulate a lot of meteorites just from random falls." Cassidy and his colleagues locate meteorites in Antarctica by searching satellite photographs for patches of bare ice, "places where the wind is strong enough to keep the ice free of snow." Then they travel to those areas by snowmobile and search visually for meteorites.

Among the thousands of meteorite fragments that Cassidy and his Antarctic colleagues have unearthed are some of the first meteorites ever to have been identified as coming from the surface of the planet Mars. According to Cassidy, most meteorites come from the asteroid belt between Mars and Jupiter, and are composed mainly of nickel and iron. Some meteorites, though, are igneous in origin, meaning they were formed from magma or molten rock. Several groups of igneous meteorites are believed to have come from Mars.

When Cassidy and his colleagues first advanced the idea that the igneous meteorites they were finding on Antarctica had come from Mars, some researchers claimed that was impossible. They said the amount of energy that would be needed to knock a piece of material off the surface of Mars into space and send it to Earth would be so great that the material would melt.

Cassidy and his colleagues believed that the igneous meteorites they were finding in Antarctica had come from Mars because their age of solidification was about 1.3 billion years ago. Other igneous meteorites solidified much earlier in the solar system's history, he says, about 4.6 billion years ago.

"So what we found came from some place that had igneous activity as recently as 1.3 billion years ago," Cassidy says. "Since there are only a limited number of planets — the Earth is one, Mars is another, possibly Venus and Io, a satellite of Jupiter — that have had recent igneous activity, that limited the number of places igneous meteorites could have come from." Mars was finally pinpointed as a source of Antarctica's igneous meteorites because when the meteorites Cassidy found entered the Earth's atmosphere, they partially melted and pockets of glass formed on some of them. When researchers melted and degassed the glass, they found that the ratio of elements and isotopes in them to be very similar to the Martian atmosphere.

Cassidy himself originally doubted that the meteorites he and his colleagues found in Antarctic came from Mars because "Earth seems like a very tiny target from Mars and you don't have mid-course corrections like we have with rockets." He also wondered why he and his colleagues were finding meteorites that had been chipped off the surface of Mars, but not the much closer moon. No sooner had Cassidy asked that question, though, than the Antarctic researchers began to find meteorites that came from the moon.

"These are undoubtedly lunar samples," Cassidy says. "They have been examined by the same group of scientists worldwide who do research on lunar samples and they find no difference between these meteorites and lunar samples brought back by the astronauts." Igneous meteorites from Mars and the moon are probably the results of large impacts on those bodies. Cassidy believes they were the result of very low angle impacts that sent a jet of material toward the Earth.

"When a meteorite falls, it is not really sucked in by the Earth's gravity," he points out. "It's more of a chance collision between two orbital bodies in a solar system." Such jets of asteroids or igneous material would seem to be the logical source for meteor showers, but Cassidy says that scientists who study meteorites tend to believe that meteor showers are related more to comets than to asteroids. The tails of comets contain large amounts of space debris such as gases, dust and ice. Meteor showers appear to be caused by the Earth crossing the orbit of a comet. Most of the Antarctic meteorites uncovered by Cassidy and his colleagues are now in either NASA's Lyndon B. Johnson Space Center in Houston, Tex., or the National Institute of Polar Research in Japan. Cassidy is not sure if any have been released to museums, but says the ultimate destination of the American collection is the Smithsonian Institute in Washington, D.C.

"Eventually, they will be in a place where the public will be able to see them," he says.

–Mike Sajna