- One of the best ways to find out is by looking at rocks from space.
- Sending spacecraft to asteroids or other planets to gather samples and bring them home is possible, but extremely difficult and expensive.
- Another option is to study space rocks that fall to Earth: meteorites.
Natural glasses
We’re all familiar with the human-made kind of glass found in windowpanes and kitchenware. But glass occurs in nature, too. Most of it is obsidian, the glass produced in volcanoes which has been known since ancient times.
- A much smaller amount of natural glass is produced by lightning strikes and asteroid impacts.
- When we find glass in nature, it can take careful forensic work to pinpoint what created it.
The meteorites recovered from the Henbury field are a type called IIIAB irons. They are remnants of the metallic core of an ancient shattered world and were eventually delivered to Earth. They’re essentially lumps of metal, comprised mostly of iron, nickel and cobalt.
Heavy metal–classic rock fusion
When the space rock struck at Henbury, the heat of the impact melted the meteorite along with rock from the ground. Some of this fused material formed molten droplets which was thrown from the craters and cooled to form thumb-sized lumps that look a lot like volcanic glass.
- This revealed the glass contained elements from the local sandstone as well as high levels of iron, nickel and cobalt – much more than we found in exposed rocks in the craters.
- These results suggest the glass is made of about 10% melted meteorite.
- The Henbury glass also contained elevated levels of chromium, iridium and other elements from the platinum group.
Meteorite glass around the world
- Such high levels of meteorite residue in glass have not been reported from other Australian craters.
- Similar glass has been described at two other sites, both younger and smaller than the largest Henbury crater (145m across).
- Our main motivation to search for meteorite residue in natural glass is that it provides ground-truth evidence for an impact with a celestial object.
More enigmatic glasses
- There are many reports of enigmatic natural glasses, in places such as Argentina, Australia and elsewhere, whose origins are ambiguous.
- In many cases no crater is known in the vicinity, such as Libyan desert glass.
- In the meantime, there are lots of interesting bush glasses that deserve a second look for clues of a cosmic heritage.
Aaron J. Cavosie has received funding from Australian Research Council and the Space Science and Technology Centre at Curtin University.
