Scientists have discovered a beach ball-sized meteorite a half-mile below a giant crater in South Africa.
The 145-million-year-old meteorite, found in the Morokweng crater, has a chemical composition unlike any known meteorite.
It is also an unusual find because it was largely unaltered by the extreme heat from the impact.
The study is detailed in the May 11 issue of the journal Nature.
Scientists have collected thousands of various meteorites over the years and tell them all apart by their various structural, chemical, and mineralogical compositions. The specific concentrations of platinum group elements in the newfound 10-inch (25 centimeter) meteorite place it in the "LL-ordinary chondrite" group of meteorites.
But other characteristics set it apart from the group, such as having silicate and sulfide minerals rich in iron, but no metallic iron-nickel phase.
"So it is 'another kind' of LL-ordinary chondrite that we do not have in our collections," said study co-author Alexander Shukolyukov of the Scripps Institution of Oceanography and the University of California, San Diego.
A potential implication of this odd meteorite, he said, is that the bombardment of meteorites 145 million years ago was different than those crashing into Earth more recently.
The researchers can't say for sure why this fragment is preserved. Current models indicate that no unaltered fragments can survive large impacts, which, Shukolyukov suggests, implies the models are incomplete.
Just a remnant
It is also clear that it was much, much larger than 10 inches in diameter when it smacked the surface.
An 820-foot-diameter (250 meters) meteorite would slam into the planet and release energy on the order of about 1,000 megatons--about 66,000 times the strength of the 15-kiloton bomb dropped on Hiroshima. The impact would create a crater 3 miles (5 kilometers) across, Shukolyukov told SPACE.com.
In contrast, the Morokweng crater is a whopping 43 miles across (70 kilometers), so the meteorite that created it must have been substantially bigger than 820 feet.
The raw energy produced by the impact also generates a lot of heat--3,100 to 24,700 degrees Fahrenheit (1700 to 13,700 Celsius)--so it's surprising that anything remains of the rock at all, and even more so that it is unaltered. Generally the heat completely melts or vaporizes meteorites, and if anything is left, it doesn't resemble its original state.
"It may be that this fragment was just a separate small meteorite that accompanied a 'big guy," Shukolyukov said.
However, according to the models, a trailing object would have completely melted.