Researchers working in South Africa have found evidence that Earth may have been blitzed by enormous chunks of rock for a billion years longer than we previously thought: The impacts stopped around only 3 billion years ago, according to work published in Geology last month. 

If that’s the case, that means early life endured bombardment that would periodically melt the surface of Earth, New Scientist reports. “Its termination was not an abrupt drop-off but a gradual waning until 3 billion years ago," Donald Lowe of Stanford says.

A formation called the Barberton Belt in South Africa includes eight layers that contain spherical particles (or spherules) that have condensed from rock vapor clouds formed by the impact of large meteorites and asteroids around 3.47 billion to 3.23 billion years ago. The spherules represent space rocks that are at least 20 to 70 kilometers across. 

While spherule beds S1 to S4 have already been reported in detail, Lowe and colleagues describe the more recently discovered beds S5 to S8 for the first time. For comparison, the dinosaur-killing asteroid left a layer of spherules a few millimeters thick. "Our layers are 30 to 40 centimeters," Lowe says. "They were big boys.” 

The formation of these eight major impact layers representing 70-kilometer bolides over the course of 240 million years suggests that impact rates greatly exceeding those of later geologic time. Terrestrial bombardment by these colossal bolides didn’t end abruptly at 3.8 billion years ago -- as suggested by previous work on this violent “late heavy bombardment” era -- but waned gradually until 3.0 billion years ago, or even later.

Each impact by one of these city-sized space rocks would have blasted a crater up to 800 kilometers across. In bed S6, for example, they found a thick sequence of volcanic rocks overlain by a tsunami layer. "A very large impact has the potential to evaporate the top 100 meters of the ocean," Lowe explains. "The atmosphere would have heated up to hundreds of degrees Celsius. That would be apocalyptic.” 

Although, he adds, “we don't know if this was apocalyptic for the microbes.” If something like that happened today, most animals and plants would be destroyed, but life was singled-celled way back then. Any life existing below the photic (sunlight) zone that required photosynthesis would have been obliterated. Those on the far side of the planet, Lowe says, "would have to ride out some large waves" and a rain of hot rocks, but some would surely have survived.

[Via New Scientist]

Image: Simone Marchi via NASA