A Record of Ancient Impacts
In the rolling hills of Mpumalanga, South Africa, lies a geological formation known as the Barberton Greenstone Belt. These rocks record, some of the oldest and best-preserved on the planet, dating back 3.2 to 3.5 billion years. Within this ancient crust, scientists have discovered evidence of a violent period in Earth's history. The proof comes in the form of thin layers of millimeter-sized beads called impact spherules. These are not ordinary geological deposits; they are the condensed droplets of rock that was vaporized by colossal asteroid impacts.
Geologists have identified at least eight distinct spherule layers, labeled S1 through S8, in formations like the Fig Tree and Onverwacht Groups. These layers record, each one marking a separate, catastrophic impact event. To confirm their extraterrestrial origin, scientists analyzed their chemical makeup. The spherules contain high concentrations of elements like iridium, which is rare in Earth's crust but common in asteroids. More definitive evidence comes from chromium isotope ratios; the specific isotopic signature found in the spherules matches that of carbonaceous chondrite meteorites, providing a clear chemical signature of their cosmic origins.
A Harsher Early Earth
The eight spherule beds were deposited over a 240-million-year period between 3.47 and 3.23 billion years ago. This timeline shows: the early Earth was being pummeled by giant asteroids far more frequently than previously understood. The rate of these massive impacts was orders of magnitude greater than it is today. This was not the tail end of the Late Heavy Bombardment, which is thought to have ended around 3.8 billion years ago, but a prolonged period of intense bombardment that continued for hundreds of millions of years.
The asteroids responsible for these layers were immense. Estimates place their diameters between 20 and 50 kilometers (12 to 31 miles), and some may have been even larger. An impactor of this size, striking the planet at roughly 20 kilometers per second, would create a crater nearly 500 kilometers (300 miles) across. The energy released would surpass a magnitude 10.8 earthquake, triggering seismic waves that shook the entire planet for half an hour and creating tsunamis thousands of meters deep. Such events would have repeatedly boiled the oceans, sterilized the surface and thrown Earth's tectonic plates into disarray, possibly delaying the establishment of stable environments where life could take hold.