Bruce Lilienthal has picked a lot of rocks from his cornfield in southern Minnesota but none like the 33-pound (15 kg) rusty slab he recently saw poking out of the soil. The odd, flat rock not only caught his eye but was unusually heavy for its size.
Bruce suspected it might be a meteorite so he called an expert with a description and then got a hold of Dr. Calvin Alexander, Earth Sciences professor and curator of meteorites at the University of Minnesota. Alexander drove out to the farm to visit Bruce and his wife Nelva.
“It has a very unusual shape,” said Alexander. The Lilienthals allowed him to remove a 0.6-gram sample in four small pieces. Back at the university, Alexander placed a fragment in an electron microprobe, a specialized instrument that determines the chemical makeup of a substance by bombarding it with beams of electrons. When excited by the little buggers, each element emits X-rays of a particular energy with a unique fingerprint.
The professor’s eyes must have lit up when he saw the results – the crumbs contained 8 percent nickel, an element rare in Earth rocks but common in meteorites and frequently used to tell the two apart.
Not only that, but the specimen flashed the telltale criss-cross pattern of interwoven iron-nickel crystals called the Widmanstatten pattern, unique to iron meteorites.
But what about that shape – why so flat? In the early solar system, nonstop meteorite impacts and heat from the decay of radioactive elements melted the larger asteroids, causing heavier materials like iron to sink to the core and lighter rocks to float to the surface and eventually harden into crust. Most iron meteorites originate in the cores of asteroids torn asunder by impacts from other asteroids. Not this one.
“It didn’t form in the interior,” said Alexander. “The object’s unusual shape indicates it probably formed as a thin layer or pool of melted surface rock created in an asteroid collision.”
In the video below, Nelva describes two other meteorites found three miles either side of their cornfield. Indeed, in 1894 a farmer plowed up a similar flat stone weighing 19.7 pounds (8.9 kg) named Arlington and classified as a rare II-E anomalous iron meteorite. Translation: it probably formed as melted surface rock instead of inside the asteroid’s core. The bulk of this rock forms part of the University of Minnesota collection currently curated by the Smithsonian.
“I’m 99 percent sure it’s the same as Arlington,” said Alexander, who hopes to acquire the piece for the university’s collection and put it on display. If it does turn out to be one and the same, it will likely be named ‘Arlington II’.
KSTP-TV report on finding the Lilienthal space rock
For now, Bruce and Nelva haven’t decided exactly what to do with their new-found rock from space. Wherever it ends up, they can say they’ve cradled a rock from the asteroid belt as old as the solar system itself thanks to their cornfield’s cosmic connections.