ScienceShot: The First-Known Comet to Strike Earth

Terry Bakker

A black, diamond-spackled pebble just a few centimeters across is the remainder of a comet that struck Earth almost 29 million years ago—making it the first direct evidence of a comet exploding in our atmosphere, scientists say. The stone, which the scientists named “Hypatia” after an Alexandrine mathematician and philosopher, was found in 1996 among tumbled bits of yellow sand glass (also known as the Libyan Desert Glass) scattered across tens of kilometers in southwestern Egypt, near the border with Libya. The glass itself, one large polished piece of which has a prominent place in a necklace that belonged to Egyptian pharaoh Tutankhamen, has been dated to 28.5 million years and has long been thought to be the result of a meteorite impact or an airburst caused by a comet breaking up in Earth’s atmosphere. To determine its origin, scientists performed a range of tests on the tiny pebble, examining its mineralogy, bulk chemistry, carbon isotope, and noble gas content. The stone’s noble gas content supports an extraterrestrial origin, while the presence of tiny diamonds—larger than nanodiamonds found in a common kind of meteorite called chondrites, but similar in size to diamond aggregates known to be formed by impacts—supports a cometary origin. The stone is also markedly carbon-rich, more so than other known extraterrestrial material aside from comets, the researchers will report in an upcoming issue of Earth and Planetary Science Letters. All of this, they say, points to a cometlike object entering Earth’s atmosphere, where it exploded, cooking the desert sand below to 2000°C and forming the Libyan Desert Glass.

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