Purdue News

July 31, 1992 Purdue Scientists Find Evidence To Back Meteorite Theory COPENHAGN, DENMARK — Purdue University researchers now have rock-solid evidence that meteorites sometimes travel in streams, debunking conventional theories of how the stony fragments travel through space. Theoretically, a meteorite stream is a group of meteorites traveling through space in the same orbit. These streams differ from meteor streams, which originate from comets and create visible, sometimes dazzling, showers when entering the Earth's atmosphere. Using sophisticated chemical analyses, the Purdue team has established that 13 meteorites that fell to Earth between 1855-1895 are part of a single meteorite stream. The findings mark the first time meteorites have been identified as part of a stream. Details of the study will be presented Friday (7/31) in Copenhagen, Denmark, during the annual meeting of the Meteoritical Society. "Conventional theory was that there was no such thing as a meteorite stream, as meteorites would be pulled away from each other by the gravitational effects of the planets," says Michael Lipschutz, professor of cosmochemistry at Purdue. "Now, for the first time, we have unambiguous evidence for a meteorite stream of asteroidal origin." The existence of meteorite streams was first proposed by Lipschutz in 1986 to explain chemical differences found in meteorites between Antarctic and non-Antarctic meteorites. Only meteorites from a constant source, or "stream," could account for these differences, he argued. Later that year, scientists at the University of Texas at Austin found evidence to back the theory when they examined data collected from seismometers placed on the moon in the mid-1970s. The data showed a series of repeated impacts for a period of about a month. "Conventional theory, which viewed meteorite falls as rare and random events, simply could not explain this continuous shower of meteorites on the moon," Lipschutz says. Meteorites traveling together in the same orbit or "stream," however, could cause such an event, the scientists said, noting that a planet or satellite intersecting a stream's orbit would likely pick up a number of meteorite samples from the stream. Additional studies in 1989 and 1990 also indicated that such streams exist. Hoping to explore some of these hypotheses, the Purdue group looked for a similar incidence of repeated impacts on Earth. In collaboration with Robert Dodd, a researcher at the State University of New York at Stonybrook, the researchers found evidence that late last century, the Earth was bombarded with a number of meteorites. Using data to plot precise information on when and where each meteorite fall occurred during that time, the team discovered a unique fall pattern in one group of meteorites. This group, made up of 13 meteorites that landed on the Earth's surface from 1855 to 1895, created a "line" that extended from the lower latitudes to regions in the higher latitudes. "These meteorites seemed to just line-up," Lipschutz says. "We knew from the data that this was probably not a random event." With the help of the Nininger Collection at Arizona State University and several museums, including the British Natural History Museum, the American Museum of Natural History in New York and the Field Museum of Natural History in Chicago, Lipschutz gathered samples of the meteorites that fell along this line to study their chemical composition. The researchers found that the meteorites were nearly identical to each other and were distinct from all other meteorites, indicating that they likely came from a single source, Lipschutz says. "The fact that these meteorites are so similar and are chemically distinct from others, along with the linear pattern created by the falls, leads us to conclude that this group of meteorites came to Earth from a meteorite stream," Lipschutz says, noting that meteorites traveling in streams may represent fragments of the same asteroid. Stephen F. Wolf, a doctoral student working with Lipschutz on the project, says they were fortunate in that they were able to obtain comprehensive data on each of these meteorite falls. "In each case, someone witnessed the meteorite's landing and was able to record the exact time and location of the fall," he says Lipschutz says the findings also open new questions on how meteorites travel to Earth. "Several of the stream members had different cosmic ray exposure, indicating that they had been in space for different periods of time," he says. "This may mean that some of the meteorites were involved in multiple collisions before landing on Earth." It could also mean that the fragments were somehow eroded, or swept clean, by small particles of dust before reaching Earth, Lipschutz says. "At the very least, these findings tell us that we don't know as much as we thought we knew about the dynamics of how things get to Earth," Lipschutz says. The studies at Purdue were funded by the National Aeronautics and Space Administration and by the North Atlantic Treaty Organization. Purdue News Service: (765) 494-2096; purduenews@purdue.edu