28.5 million years ago an extreme surface-heating event produced the Libyan Desert Glass. One famous fragment of this glass was carved into a scarab and incorporated into the pectoral of Tutankhamun (see image below). The glass was thought to be what remains from a glassy surface layer that resulted from a high temperature event. This event would have melted the desert sand, and was believed to be the product of either a meteorite impact or by a comet exploding in the atmosphere (known as an airburst).

Image: An artist’s impression of the comet exploding above Egypt 28.5 million years ago. Image credit: Terry Bakker.

A paper recently published online in Earth and Planetary Science Letters has revealed new insights into the formation of the Libyan Desert Glass (LDG). The authors, led by Prof Jan Kramers of the University of Johannesburg, had a new line of evidence to investigate: a small black stone. This stone, which has been named Hypatia in honour of the 4th century female philosopher, mathematician and astronomer of Alexandria, was found in December 1996 by Egyptian geologist Aly Barakat in a pebbly corridor in the southwest section of the LDG field, Egypt.

Hypatia is a carbonaceous stone that was found to contain sub-micrometre diamonds. It is angular, shiny, and extremely hard and fractured. A lot of work went into determining the origin of the stone. Studies included including X-ray diffraction, scanning electron microscopy, carbon (C) isotope and noble gas analyses. The C-isotope and noble gas analyses yielded particularly interesting results that pointed towards an extraterrestrial origin. Kramers described the elimination of all other options and the realization of what Hypatia must be as a “scientific euphoria”.

The C isotope (d13C) values exclude the possibility of this being terrestrial carbonaceous matter. The noble gas analyses suggested the same, with the argon isotopic data showing the clearest evidence for an extraterrestrial origin of at least part of Hypatia. The evidence led the team to hypothesize that an object similar in composition to cometary nuclei entered the Earths atmosphere and impacted with enough velocity to create shock diamonds. They believe Hypatia to be a fragment of a larger bolide that would have generated the airburst that formed the LDG.

Although more work is being carried out, Hypatia thus far provides evidence that a comet struck Earth 28.5 million years ago. Although frequently seen streaking across our skies, this is the first macroscopic cometary matter to have been found on Earth. The implications of this are huge. “NASA and ESA (European Space Agency) spend billions of dollars collecting a few micrograms of comet material and bringing it back to Earth, and now we’ve got a radical new approach of studying this material, without spending billions of dollars collecting it,” said Kramers. This stone now provides scientists with tens of grams of cometary material to work with, which will surely open new windows into the stories behind comets.

Author: Clare Stead

Ph.D. student, Department of Geology, Trinity College Dublin

P.S. For a run down on the often confusing terminology surrounding meteors, meteoroids and meteorites, read Michael Babechuk’s illuminating post here.

Image sources:

Artist’s impression of comet exploding over Egypt and Tutankhamun’s pectoral: http://www.wits.ac.za/newsroom/newsitems/201310/21649/news_item_21649.html

Hypatia image:

http://www.sciencedirect.com/science/article/pii/S0012821X13004998

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