How Earth and other planets in the solar system formed and evolved over the eons is a pressing question for planetary scientists like me. One of the best ways to find out is to look at rocks from space. Getting the stones is the hard part. It is possible to send a spacecraft to asteroids or other planets to collect samples and bring them home, but it is extremely difficult and expensive.

Another option is to examine space rocks, i.e. meteorites, that have fallen to Earth. However, they are relatively rare, and the journey through our planet’s atmosphere and subsequent high-speed collision with the ground means that they’re usually not in great shape when we see them.

But meteors leave fascinating traces. In a new study, my colleagues and I analyzed glass fragments found around the impact site of a 5,000-year-old meteorite in the Northern Territory and found that they contained a surprisingly large amount of metal from the meteorite itself. The site was created by a space intruder and provides clues about the attacker’s nature.

natural glasses

We are all familiar with man-made glass in window panes and pots. However, glass is also found in nature. It is mostly obsidian, a glass known since ancient times and produced in volcanoes.

Much smaller amounts of natural glass are formed from lightning strikes and asteroids. When we find glass in nature, careful forensic work may be required to determine exactly what made it. But forensic analysis can reveal a surprising amount of information about the origin of the glass.

In a published study Geochimica and Cosmochimica ActaWe analyzed glass from an area in the northwest called the Hanbury Crater Field. The meteorite debris was found in an area containing at least 13 impact craters created by an event about 5,000 years ago. The crater area is also called Tatyeye Kepmwere and is reported in Aboriginal oral tradition.

Meteorites unearthed from the Hanbury deposit belong to type IIIAB iron. They are the remnants of the metal core of an ancient, destroyed world, eventually brought to Earth. Basically these are pieces of metal mostly composed of iron, nickel and cobalt.

Heavy metal-classic rock fusion

When the space rock hit Hanbury, the heat from the impact melted the meteorite and the rock in the ground. Some of this molten material formed molten droplets that emerged from craters and cooled to form thumb-sized pieces that looked very similar to volcanic glass.

To learn more about this “bush glass,” we took samples to the lab and drilled holes in them with a laser, heating the glass to a hot plasma so we could examine it with a mass spectrometer that could determine what elements were present.

This showed that the glass contained high levels of iron, nickel and cobalt, as well as elements from the local sandstone; This was much more than what we found in rocks exposed in craters. These results suggest that approximately 10 percent of the glass contains molten meteorite.

A 10 percent meteorite contribution may seem small, but it is a relatively large amount. By comparison, molten rock from Chicxulub, the giant asteroid impact in Mexico believed to have killed off the dinosaurs, generally accounts for less than 0.1 percent of the asteroid.

Henbury glass also contained high levels of chromium, iridium and other platinum group elements. All of these are extremely rare in most rocks on the Earth’s surface. Their abundance in Hanbury glass is another sign of cosmic origin.

Meteorite glass around the world

Such high levels of meteorite debris in glass have not been found in other craters in Australia. Similar glasses have been identified at two other sites that are younger and smaller than the largest Hanbury Crater (145 meters in diameter). One is the 45-meter Kamil Crater in Egypt and the other is the 110-meter Wabar Crater in Saudi Arabia. Nearly 200 meteorite impact structures have been documented on Earth, 32 of which are in Australia.

We believe that the meteorite-rich glass we found at Hanbury occurs in all craters, regardless of size. However, it is probably a very small volume of melt produced in large craters and is best preserved in young uneroded craters.

The main motivation for finding meteorite debris in natural glass is that it provides actual evidence of a collision with a celestial body. There are many circular, crater-like craters on the Earth’s surface, but very few are truly cosmic in origin. Detecting meteorite debris in glass is a definitive method of confirming that a suspected area was caused by an asteroid impact.

More mysterious glasses

In Argentina, Australia and elsewhere, there have been many reports of mysterious natural glasses of uncertain origin. In most cases, there are no known craters nearby, such as the glass of the Libyan desert. Determining whether these were caused by a collision requires careful detective work, looking for clues.

NASA now plans to spend about $11 billion to deliver several hundred grams of rock samples collected from Mars by the Perseverance rover. Missions to Itokawa, Ryuga, and Bennu returned asteroid samples, and we hope that a stream of new missions to the Moon will also return fresh samples from our planetary neighbor. In the meantime, there are many interesting spectacles that deserve to be re-investigated for clues to cosmic legacy.