April 13, 2024

Australian ‘bush glass’ bears fingerprints of cosmic collision with iron meteorite

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How Earth and the other planets in the solar system formed and evolved over the eons is an important question for planetary scientists like me. One of the best ways to find out is to look at rocks from space.

Picking up the rocks is the hard part. Sending spacecraft to asteroids or other planets to collect samples and bring them home is possible, but extremely difficult and expensive.

Another option is to study space rocks that fall to Earth: meteorites. However, they are relatively rare, and the journey through our planet’s atmosphere followed by a high-speed collision with the ground often means they are not in very good condition when we observe them.

That said, meteorites leave fascinating traces. In a new study, my colleagues and I analyzed pieces of glass found around a 5,000-year-old meteorite impact site in the Northern Territory and found that it contains a surprisingly large amount of metal from the meteorite itself – proving that the craters at the site were formed by a cosmic intruder and giving clues about the intruder’s composition.

Natural glasses

We are all familiar with the type of man-made glass found in window panes and kitchen utensils. But glass also occurs in nature. Most of it is obsidian, the glass produced in volcanoes that has been known since antiquity.

A much smaller amount of natural glass is produced by lightning strikes and asteroid impacts. When we find glass in nature, it may require careful forensic work to identify what created it. However, forensic analysis can reveal a surprising amount of information about the origin of the glass.

The Henbury IIIAB iron meteorite. Credit: Museums Victoria

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The Henbury IIIAB iron meteorite. Credit: Museums Victoria

In our study, published in Geochemistry and Cosmochemistry Actawe analyzed glass from a site in the NT called the Henbury crater field.

Meteorite fragments were recovered from the site, where there are at least 13 impact craters formed in an event around 5,000 years ago. The crater field is also called Tatyeye Kepmwere, and accounts of it are found in Aboriginal oral traditions.

The meteorites recovered from the Henbury field are of a type called IIIAB irons. They are the remains of the metallic core of an ancient destroyed world and were eventually delivered to Earth. They are essentially pieces of metal, composed mainly of iron, nickel and cobalt.

Classic heavy metal-rock fusion

When the space rock hit Henbury, the heat from the impact melted the meteorite along with the rock in the ground. Some of this molten material formed molten droplets that were thrown from the craters and cooled to form thumb-sized pieces that look a lot like volcanic glass.

To learn more about this “glass,” we took samples to the lab and drilled holes in them with a laser, heating the glass until it formed a hot plasma that we could study with a mass spectrometer, which can determine which elements are present.

This revealed that the glass contained elements from the local sandstone, as well as high levels of iron, nickel and cobalt – much more than we found in the rocks exposed in the craters. These results suggest that the glass is made of about 10% molten meteorite.

A 10% contribution from meteorites may not seem like much, but it is a relatively huge amount. By comparison, molten rocks from Chicxulub, the giant asteroid strike in Mexico believed to have killed the dinosaurs, are typically less than 0.1% meteorites.

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. Its great abundance in Henbury glass is another hallmark of cosmic origin.

Meteorite glass around the world

Such high levels of meteorite residue in glass have not been reported in other Australian craters.

Similar glass has been described at two other sites, both younger and smaller than the larger Henbury crater (145 m in diameter). One is the 45-meter Kamil crater in Egypt, and the other is the 110-meter Wabar crater in Saudi Arabia.

About 200 meteorite impact structures have been documented on Earth, with 32 located in Australia.

We believe that meteorite-rich glass, like what we found at Henbury, forms in all craters, regardless of size. However, it probably represents a very small volume of the melt formed in large craters and is best preserved in young craters that have not been eroded.

OSIRIS-REx mission collecting samples from the asteroid Bennu.

Our main motivation for looking for meteorite residues in natural glass is that they provide real evidence of an impact with a celestial object. Many circular crater-like formations occur on Earth’s surface, but few have a truly cosmic origin.

The discovery of meteorite residue in glass is an unambiguous method of confirming that a suspected location was hit by an asteroid.

Most enigmatic glasses

There are many reports of enigmatic natural glass, in places like Argentina, Australia and elsewhere, whose origins are ambiguous. In many cases, no craters are known nearby, such as the Libyan Desert Glass. Determining whether they have an impact source requires careful detective work to look for telltale signs.

NASA is currently considering spending about $11 billion to bring back a few hundred grams of rock samples from Mars collected by the Perseverance rover. Missions to Itokawa, Ryugu and Bennu have returned asteroid samples, and a flurry of new missions to the Moon is expected to return new samples from our planetary neighbor.

In the meantime, there are plenty of interesting glasses that deserve a second look for clues to a cosmic heritage.

Diary information:
Geochemistry and Cosmochemistry Acta

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