Every few million years, give or take, a sizeable asteroid slams into our planet. Researchers have now found evidence of such a cataclysm roughly 11 million years ago, thanks to shards from a more recent earthly encounter with a giant space rock.
The scientists described their discovery last month in a paper published in the journal Earth and Planetary Science Letters.
Thousands of pieces of black glass are stored in a collection at the South Australian Museum in Adelaide. They are tektites, which sometimes form when terrestrial rocks melt in the intense heat and pressure of a powerful asteroid impact.
Researchers long believed that the tektites at the museum came from an impact that occurred roughly 8,00,000 years ago, likely somewhere in Southeast Asia. That event scattered tektites across 10 per cent of Earth’s surface. These objects are named Australasites, and they are the most common of all known tektites.
In 1969, two scientists reported the discovery of eight Australasites with an odd chemical composition. Follow-up work suggested that those chemical outliers were at least 2 million years old, far older than other Australasites.
In 2023, researchers from Aix-Marseille University in France led by Anna Musolino, a geoscientist, travelled to the Adelaide museum in search of unusual tektites.
The team started by dunking more than 5,000 Australasites in liquid to estimate their density. The researchers kept tektites that sank and were therefore particularly dense. They also retained tektites that had high levels of iron.
They were left with 417 tektites, and they arranged to bring the roughly penny-size objects to France for further analysis. “I travelled with the samples in my suitcase,” said Pierre Rochette, a geophysicist who was part of the Aix-Marseille research team.
Musolino and her collaborators analysed each tektite. In the end, six tektites stood out as being chemically perplexing. And when the team age-dated two of those oddballs, they recovered an age of 10.8 million years.
These chemical outliers, along with the original sample of eight identified back in the 1960s, constitute a group of tektites distinct from Australasites, Musolino and her team concluded. It was clearly time for a name.
All 14 tektites were found in South Australia, and some had fallen on lands home to Aboriginal people known as the Anangu. The name the team selected — Ananguites — is a nod to that history. “It was a way to honour the local people,” Rochette said.
But where did the space rock that produced those Ananguites land? Musolino and her collaborators have laid out a map of sorts for would-be crater seekers.
They identified areas with rocks older than Ananguites
with the same elemental composition. Ananguite chemistry indicates that their parent rock came from deep within the earth’s mantle, so Musolino and her colleagues homed in on volcanically active regions. Nothing in Australia was a match, but chains of volcanoes in the Philippines, Indonesia and Papua New Guinea were all contenders.
Looking for a crater in a field of volcanoes will be challenging. “You have circular structures everywhere,” Rochette said. And the impact crater might not even be visible today, he added, as it could have been buried by lava or eroded away.
It makes sense to narrow down crater locations this way, said Steven Jaret, a planetary geologist at the City University of New York, Kingsborough Community College, US, who was not involved in the study. “They did something very reasonable.”
Musolino hopes that future research will identify more Ananguites hiding in plain sight in museums and private collections.
“I’m very confident that there are more,” she said.
NYTNS