Earth’s first 600 million years are called the Hadean eon, a name that offers a big clue as to why the planet’s baby years were not cute. Early Earth was violent, hence the nod to Hades, the Greek god of the underworld. The planet’s surface was still roiling with volcanism and asteroids repeatedly slammed its face. It wasn’t a great place for life, especially when several giant impacts apparently boiled the Earth’s oceans for more than a year, according to Maddie Stone at Gizmodo.
Researchers led by Don Lowe of Stanford University describe the effects of two asteroids measuring 30 to 60 miles across that hit about 3.29 and 3.23 billion years ago. (For context, the asteroid that killed the dinosaurs was probably a measly six miles across.) The dual impacts sent temperatures in the atmosphere up to 932 degrees Fahrenheit for weeks and boiled the oceans for a year, long enough that seawater evaporated and they dropped perhaps 328 feet. The researchers reported their findings in the journal Geology.
“These impacts would have a profound influence on any life trying to evolve into more complex, low-temperature organisms,” Lowe told Thomas Sumner, writing for Science News. “They’d keep getting whacked by these giant impactors and driven to extinction or near extinction.”
Heat loving bacteria called thermophiles, which can tolerate intense heat near oceanic volcano vents, would have been among those that could tolerate these ocean-searing events, Sumner confirms in the article’s comment section. Deep sea dwellers would also have survived. Sumner writes:
These mass die-offs could have driven early evolution, says planetary scientist Kevin Zahnle of the NASA Ames Research Center in Moffett Field, Calif. A common ancestor to all modern life is thought to have thrived at hot temperatures. That could be because everything that preferred cool conditions got killed in the hot aftermaths of asteroid impacts, Zahnle says. “You can picture these asteroids as a crazed tree trimmer showing up and chopping branches off the tree of life.”
However, the studies findings are hard to conclusively verify, points out geologist James Day of the Scripps Institution of Oceanography in La Jolla, California in the Science News article. The researchers have to grapple with the same problem any scientist does when studying the Hadean. Plate tectonics and erosion have long ago wiped craters from impacts in that ancient time from the face of the Earth. Sumner writes: "Since the craters from the impacts aren’t around anymore, it’s hard to precisely nail down how large, and therefore how potentially destructive, the asteroids were, [Day] says." Previously, other researchers have estimated the general violence of the Hadean by look at the moon, which still has pockmarks from every collision.
So how did Lowe and his colleague Gary Byerly find out about those two giant asteroids? They looked at an ancient geological formation in South Africa called the Barberton greenstone belt, a very rare bit of ancient Earth’s crust that survived. In that belt are two rock layers that have round spheres of silica. These are bits of rock made molten and tossed skyward by a distant impact before raining down again. The rock layers they now sprinkle indicate it was once a shallow seabed. The layers both above and below, however, are from a deeper ocean floor. This led Lowe and Byerly to conclude that the ocean must have evaporated enough, temporarily, for the silica rain to reach it after an asteroid strike. A drop in the ocean level that extreme means the asteroids responsible were enormous.
Even if the findings are somewhat speculative, the fact that there were such massive impacts seems likely. One of Lowe’s previous studies found evidence that an asteroid the width of Rhode Island shook the Earth for a half hour. It may have been one of the rocks the new study describes.