By Sid Perkins
Ancient rocky sediments in what are now western Australia and eastern South Africa contain remnants of what may have been an extraterrestrial-object impact large enough to disperse debris over the entire planet.
The sediments in question formed from particles deposited about 3.47 billion years ago in the normally quiet and probably shallow waters off the coasts of ancient continents, says Gary R. Byerly, a geologist at Louisiana State University in Baton Rouge. Some layers include BB-size and smaller spheres that typically form from airborne droplets of molten rock. Although the spherules could have come from volcanoes, the droplets’ widespread distribution and mineral composition suggest they condensed from a cloud of rock vapor blasted into and above the atmosphere after a massive impact.
Chemical analyses of the South African sediments show that, on average, the layers that contain the spherules have more than 10 times the concentration of iridium that’s normally found in Earth’s crust, as well as elevated concentrations of certain chromium isotopes. These chemical traits indicate an extraterrestrial origin for those materials, says Byerly.
“There’s no doubt about the impact origin of these spherules,” comments Bruce M. Simonson, a geologist at Oberlin College in Ohio. The spheres form a much larger proportion of the sediment layers than would be expected from a volcanic eruption, he adds.
Byerly and his colleagues, who report their findings in the Aug. 23 Science, estimate that the object that struck Earth 3.47 billion years ago was between 20 and 50 kilometers across. Therefore, it could have been as much as 100 times as massive as the object thought to have wiped out the dinosaurs and caused worldwide climate change when it struck near Mexico’s Yucatn peninsula about 65 million years ago.
The researchers haven’t discovered any so-called shocked minerals in the spherule layers. Such battered crystals, which show multidirectional patterns of parallel fractures, are a hallmark of impacts from space (SN: 6/15/02, p. 378: Presto, Change-o!). Evidence of shocked minerals in the rocks may have been erased by billions of years of heat and pressure, but Byerly says it’s more likely that it’s missing because the object struck Earth in deep ocean far from where the spherules landed. Rocks from a deep-ocean site would have been destroyed long ago by seafloor spreading.
Even if the impact occurred in deep water, however, the large amount of debris blasted from the impact site should contain some shocked minerals, says Simonson. He also notes that the South African and Australian sites could have been geographically much closer to each other at the time of the purported impact, and the effects of any blast actually may have been regional, not worldwide.
