Scientists have found evidence of an asteroid strike that dwarfs the giant rock that wiped out the dinosaurs.
The startling find came after studies of core samples from some of the oldest sediments on Earth revealed the presence of spherules. These tiny glass beads, created from vaporized rock condensing into a kind of molten rain, act as a marker for impact events long after identifying craters have been wiped away from the Earth's surface. The larger the spherules, the bigger the asteroid.
Researchers analyzing the core samples, which were retrieved from northwestern Australia, measured spherules in the range of 1 to 2 millimeters. They were also found to contain levels of platinum, nickel and chromium, elements consistent with those in asteroids.
Polarizing microcopy images of spherules discovered during analysis of core samples. Based on diameter, they indicate an asteroid strike of between 20 to 30 kilometers wide. (Photo: Andrew Glikson, Arthur Hickman)
According to Andrew Glikson from Australian National University, the diameters of the spherules indicate an asteroid between 12 to 18 miles wide. For comparison, the asteroid that struck Earth 66 million years ago and wiped out the dinosaurs was estimated to have been "only" 6 miles in diameter.
"The impact would have triggered earthquakes orders of magnitude greater than terrestrial earthquakes, it would have caused huge tsunamis and would have made cliffs crumble," Glikson said in a statement on the discovery. The team's research was published this month in the journal Precambrian Research.
So why is there no evidence on Earth's surface of this massive collision? The biggest reason has to do with the age of impact, estimated from the core samples to have happened 3.46 billion years ago.
"Exactly where this asteroid struck the earth remains a mystery," Glikson added. "Any craters from this time on Earth's surface have been obliterated by volcanic activity and tectonic movements."
Spherules remain our best guides to understanding the frequency of impacts that occurred billions of years ago, a period known as the Late Heavy Bombardment. The video below explains more about that time:
While evidence of cratering from the Late Heavy Bombardment has disappeared from Earth, these little glass beads offer tremendous insight into the kinds of threats Earth faced on a more regular basis.
"We can look at these spherules, see how thick the layer is, how big the spherules are and we can infer the size and velocity of the asteroid," Jay Melosh, a professor of Earth and atmospheric sciences, physics and aerospace engineering at Purdue University, explained in 2012. "We can go back to the earliest era in the history of the Earth and infer the population of asteroids impacting the planet."
According to Glikson, our planet's violent early history has many chapters left to be discovered.
"This is just the tip of the iceberg," he said. "We've only found evidence for 17 impacts older than 2.5 billion years, but there could have been hundreds."