Right after the Mifflin Meteorite fell in SW Wisconsin in April 2010 the Robert A. Pritzker Assistant Curator of Meteoritics and Polar Studies Dr. Philipp R. Heck coordinated an international study to determine the time it spent in space and to calculate its size in space before it got ablated and broke apart in our atmosphere. Now, first results obtained from this study are published as extended abstracts, and will be presented in more detail in March at the Lunar and Planetary Science Conference in Texas: The new results show that Mifflin was travelling through space as a small 3 feet object for about 20 Million years before it landed in Wisconsin.
At the time of the meteorite fall Heck was visiting a lab in Europe and stayed in touch with the Robert A. Pritzker Center in Chicago by phone and e-mail. Collections manager for meteorites James L. Holstein sent out the meteorite as quickly as possible. “It is critical to do measurements as quickly as possible after the meteorite fall, because many of the radioactive elements that were produced in space, who are essential to measure, decay rapidly”, Heck says, “only thanks to the rapid donation of a large piece of Mifflin to us by private collector Mr. Terry Boudreaux, this study was possible.”
The natural radioactivity from the meteorite is not harmful for humans and is in fact so low, that the measurement was done in a special laboratory shielded from other natural sources of radioactivity. The Gran Sasso National Laboratory is located deep beneath the Apennines Mountains in Italy. Collaborator Dr. Matthias Laubenstein was ready to do this high-sensitivity measurement right after the meteorite fall. Longer-lived radioactive elements were analyzed by collaborators Dr. Kees Welten at the University of California at Berkeley and Dr. Marc Caffee at Purdue University in West Lafayette, Indiana. Radioactive elements are produced when cosmic rays hit the meteorite in space. Their concentrations depend on the size of the meteorite in space and the results from the three different labs show that Mifflin was about 3 feet in diameter before it entered Earth’s atmosphere. This confirms the first size estimates from video footage of the fireball.
The same cosmic rays also produce stable elements in the meteorite such as the noble gases, neon and argon. The concentration of these noble gases and their production rates tell us how much time the meteorite spent in space – from its ejection from its parent asteroid to its fall on Earth – its interplanetary flight time or cosmic-ray exposure age. Matthias Meier, a graduate student at ETH Zurich in Switzerland analyzed a piece of a Field Museum’s Mifflin specimen with a noble gas mass spectrometer dedicated only for meteorites and determined its interplanetary flight time to be 20 million years. This is not unusual for this type of meteorite. He also found that Mifflin has not been shocked much by impacts since almost two billion years, and was only slightly affected by the large parent asteroid breakup event 470 million years ago. Dr. Heck, who is also a co-advisor of Meier, comments, "This is a good example that meteorites do not just sit around and ‘gather dust’ once they arrive at a natural history museum."
The results from this study are interesting on their own but also serve as puzzle pieces to improve our understanding of the evolution of the Solar System.
The public Mifflin Meteorite exhibit at the Field Museum (upper level, south) now features a beautiful cut slice of the meteorite which reveals the brecciated nature of the rock, signs of impacts on the parent asteroid in space. Numerous clasts and shiny specks of metal can be easily seen with the naked eye. Also new on exhibit is the piece of Mifflin found by an elementary school student on the Iowa-Grant school ground. Private meteorite collectors Terry Boudreaux and Michael Farmer each donated two pieces.