Earth has had many run-ins with space rocks. They’ve triggered the demise of the dinosaurs 66 million years ago, lit up daytime skies over Russia in thousands of dash cam videos and even struck a human.
But one major meteorite impact that occurred roughly 800,000 years ago has long baffled researchers. They know it happened because millions of blobs of glass known as tektites were launched over 10 percent of the planet’s surface, from Southeast Asia to Antarctica and across wide swathes of the Indian and Pacific Oceans.
The impact that flung these Australasian tektites would have excavated a crater at least several miles in diameter and hundreds of feet deep. But nearly a century of sleuthing failed to turn up any direct trace of the strike.
“That’s a very difficult size hole to make go away,” said Aaron Cavosie, a planetary scientist at the Space Science and Technology Centre at Curtin University in Perth, Australia.
Now a team of researchers think they’ve found it, buried beneath a lava bed in Laos.
The team, led by Kerry Sieh, a geologist at the Earth Observatory of Singapore, published its evidence last week in Proceedings of the National Academy of Sciences. While some researchers suggest that additional fieldwork is needed to confirm the findings, these results provide important clues toward piecing together a catastrophic encounter between Earth and a fiery visitor from space.
Dr. Sieh has been hunting for the crater for years, but many leads turned out to be dead ends. While most scientists agree the meteorite impact likely occurred in Southeast Asia, that presents a puzzle.
Many ancient impact craters have disappeared from Earth’s surface as a result of volcanic, tectonic and other erosional forces. But most places in Southeast Asia experience low rates of erosion and sedimentation, insufficient to have erased such a large crater in a relatively short amount of time.
However, there’s at least one exception: a Delaware-sized plateau near the Mekong River in southern Laos. Here, volcanic eruptions have created lava beds up to 1,000 feet deep. That’s thick enough to hide a large crater, the scientists realized. The hunt was on.
Dr. Sieh and his colleagues began by studying data showing that Australasian tektites contain elements characteristic of the Laotian volcanic field. The scientists also age dated the lava flows. Some predated the impact while others postdated it. That makes sense, said Dr. Sieh, since the lavas that buried the crater must be younger than the impact and the lavas that were incorporated into the tektites must be older.
Dr. Sieh and his team also measured the local gravitational field around the lava beds. Craters often exhibit a slightly weaker gravitational tug than surrounding areas because they can be filled with broken-up, less densely packed material. The scientists found a gravity signal consistent with an elongated crater roughly 11 miles long by 8 miles wide filled with about 300 feet of jumbled rock.
The team then set off looking for debris — any large impact would have scattered boulders and soil. A few miles from the summit of the volcanic field, Dr. Sieh and his colleagues stumbled upon two places where the hill slope had been excavated to make room for a road. Those cuts revealed sandstone boulders that fit together, “like a jigsaw puzzle,” said Dr. Sieh.
“We calculated that they were ejected from the crater and landed at about 450 meters per second, fast enough to shatter them upon impact,” said Vanpheng Sihavong, a geoscientist at the Ministry of Energy and Mines in the Lao People’s Democratic Republic and a member of the research team.
The scientists also examined quartz grains in some of the boulders and observed that they were fractured, often considered a smoking gun of an impact.
“We think we’ve found that,” Dr. Sieh said.
While these results are consistent with a buried crater, they are not unambiguous proof, said Dr. Cavosie.
“It’s a great lead on a new site worthy of investigation,” he said, but added that the next step would be to drill through the lava.
“Finding shock-deformed rocks would seal the deal.”