Rare yellow desert glass was created as a result of a meteorite impact

The formation of Libyan Desert Glass (LDG) has been the focus of much debate in planetary science. Illumination, strange geological processes, and even volcanoes on the Moon have been suggested as possible sources. For the past 25 years, researchers have focused on two scenarios: it was either created by a meteorite hitting the desert or by a meteorite exploding in the air. New evidence suggests that the first hypothesis is likely the correct one.

Glass is made by melting sand, so most scenarios involve delivering large amounts of energy to the desert sands between Egypt and Libya. When it comes to temperatures that can turn sand into glass, both meteorite impacts and airbursts have the potential to get there, where they can produce high-temperature fusion.

“The main goal of our study was to distinguish between an airburst, such as over Chelyabinsk or Tunguska, and a surface impact,” said the lead author of the study. Dr. Elizaveta Kovalevafrom the University of the Western Cape, for IFLScience.

The team set out to find evidence that could separate one scenario from the other. In fact, there is an important difference between collision and airburst. Despite the high temperatures and shock waves in the air, air explosions cannot provide enough pressure in the ground to produce impact minerals, so the researchers looked into the detailed composition of the glass.

The international team of researchers used transmission electron microscopy to study how minerals are organized in matter. In particular, they found small crystals of zircon oxides. Crystals can have the atoms within them arranged in different ways, and some arrangements can only form under certain specific conditions.

One of the formations found is known as cubic zirconia. This is often seen in jewellery, where it becomes stable thanks to intentional inclusions – here, it is the surrounding glass that keeps it stable. It is formed at a high temperature between 2250 and 2700 degrees Celsius (4082 and 4892 degrees Fahrenheit). But the actual temperature may have been even higher, based on the melting of other minerals found during the observations.

Temperature indicators are not sufficient to differentiate between impact and airburst, but another zirconia formation seen in glass is very rare and also indicates significant impact pressures. For this mineral to form, it requires not only heat, but also very high pressure – about 130 thousand atmospheres. If the minerals inside the glass formed under those conditions, there is an obvious natural scenario: a meteorite impact.

“Those are very small particles of mineral phases that only form at very high pressures. Those high pressures can only be achieved in the Earth’s crust after meteorite impacts. They are preserved in LDG because they are very small,” explained Dr. Kovaleva.

The evidence for the impact scenario is becoming more solid. But many questions remain, including a big one: If there was an impact, where is the crater located? This is currently unknown. A team of researchers is currently looking into potential locations (and if you speak French, You can help too).

The study is published in American mineralogist.