They are the “Mona Lisas” of meteorites – out of over 50,000 known meteorites, only 101 of them are fossilized, and four of them will be on display at The Field Museum.
These meteorites have a fascinating story! Back in 1952, the manager of a limestone quarry in Sweden that was once an ancient sea floor noticed an unusually dark object in a slab that one of his workers had cut and set aside. He approached a paleontologist about it, who set it on a shelf in his office and forgot about it. 27 years later, a mineralogist who was interested in meteorites walked into that very same office, and exclaimed, “That looks like a meteorite!”
The discovery of this meteorite nearly three decades after it was removed from the quarry led to a systematic search for more—in fact, workers were trained to recognize them. Over the next 20 years, 101 fossilized meteorites were uncovered in the quarry, which is 100 times more than we would expect. Philipp Heck, Robert A. Pritzker Associate Curator of Meteoritics and Polar Studies at The Field Museum, tells us why.
“These meteorites all came from the same source,” said Heck. “The breakup occurred somewhere between Jupiter and Mars – nowhere near Earth. We know that because the collision generated fragments ranging from millimeters to feet in diameter that have been found in Russia, China and Sweden, indicating that this was a global event. In fact, I would venture to guess that fragments of this meteorite can even be found on Mars.”
Like all fossils, the 101 meteorites that have been recovered in the quarry became fossilized when the original material was replaced by other minerals during the transformation of the soft seabed to sedimentary rock. However, these meteorites contain a mineral called chromite that was preserved in its pristine form that matches the composition of L chondrites – the second most abundant type of meteorites that fall to Earth today.
“There is one exception,” said Heck. “Out of the 101 fossilized meteorites, one of them did not match the composition of the L chondrites. We do know that it took the same amount of time to reach Earth as the others, so we think it may be part of the projectile that caused the parent asteroid to break up.”
Meteorites are dated using products of cosmic ray irradiation. Cosmic rays hit everything that flies through space and is not protected by a magnetic field or a thick layer of rock. For example, when a cosmic ray hits a silicon atom that has been exposed on the surface of a meteorite, it causes the atom to break apart and form noble gases, which are inert – meaning that once they form, they stay put. Scientists can measure the concentration of these gases and use them to determine the time it took for a meteorite to fall to Earth, since the meteorites were protected inside the parent body prior to the breakup.
“The first of these fossil meteorites arrived on Earth about 100,000 years after the breakup, and the most recent ones that were found took about one million years to arrive.” said Heck.
It sounds like a long time, but it’s actually extremely fast! The orbits of meteorites change over time, until they cross the Earth’s path, and the probability of that ever occurring is very low, given where the collision occurred.
“That being said, these meteorites are extremely rare, and we will definitely be keeping our eyes out for more,” said Heck.