Asteroids boiled young Earth’s oceans

It didn’t take long for early life on Earth to get into hot water. Very hot water.

Evidence from ancient rocks suggests that asteroids slammed into our planet around 3.3 billion years ago. The impacts would have released huge amounts of energy. So much energy that this heat baked the skies and boiled the oceans.

These dramatic events would have shaped the evolution of early life, says Donald Lowe. A geologist, he works at Stanford University in Palo Alto, Calif. His team detailed the evidence for this hellish scenario in Geology. It appeared early online May 7.

Life had been around for only about 500 million years when these asteroids hit. At once, things got tough. Following the impacts, air temperatures for weeks would have exceeded 500° Celsius (932° Fahrenheit). The ocean surface would have boiled for more than a year. Global sea levels might have plummeted by as much as 100 meters (333 feet).

Such impacts “would have a profound influence on any life trying to evolve into more complex, low-temperature organisms,” says Lowe. “They’d keep getting whacked by these giant [asteroids] and driven to extinction or near extinction.”

Early Earth was hostile

Asteroids and other space debris left over from the solar system’s creation regularly slammed into Earth early in its history. These massive impacts initially were quite common. Over time, they became smaller and less frequent.

Around 3.3 billion years ago, several large asteroids hit Earth. They had been traveling at about 36,000 kilometers per hour (22,000 miles per hour). Lowe estimates that the massive rocks measured 50 to 100 kilometers (up to 62 miles) across. Each would have dwarfed the roughly 10-kilometer rock blamed for killing off the dinosaurs.

The force of these collisions heated the atmosphere. It vaporized rock. And it sent debris flying. Geologists had speculated that these impacts had global effects. However, until now geologic evidence of any environmental changes had been lacking.

The newfound evidence

Lowe teamed up with geologist Gary Byerly of Louisiana State University in Baton Rouge. In South Africa, they went looking for evidence of those massive collisions, focusing on rock layers that had formed about 3.3 billion years. In them, they found spherical bits of rock about the size of BB pellets. Solidifying rock vapor and debris had sent this big grit raining down following the impacts.

Lowe and Byerly found sediments below and above each layer. This suggests that the location had once been deep underwater. The impact layers, however, show evidence of having eroded in shallower water. That suggests sea levels dropped dramatically after the asteroid impacts. Eventually, the sea levels returned to normal.

The researchers also found the mineral silica coating the impact layer. That indicates that the sea surface would have boiled away. As the liquid turned to gas, any dissolved silica was left behind. The ocean surface had turned into a global hot spring beneath a scorching sky, Lowe now concludes.

The findings are exciting but speculative, says geologist James Day. He works at the Scripps Institution of Oceanography in La Jolla, Calif. No trace of the craters created by the impacts remains. So it’s hard to pinpoint how large the asteroids were, he says.

Understanding the destructive effects of such giant impacts is important. It can help experts understand the environment in which early life lived and adapted, Lowe says. Primitive microbes that conducted photosynthesis probably lived near the ocean surface. These organisms could not have survived temperatures above about 73 °C (163 °F). The only life that could have survived in this boiling world must have been underground, been living deep in the ocean or been capable of coping with high temperatures, Lowe says.

These mass die-offs could have shaped early evolution, says planetary scientist Kevin Zahnle. He works at NASA’s Ames Research Center in Moffett Field, Calif. A common ancestor to all current life is thought to have thrived at hot temperatures. The reason could be that all life forms that preferred cooler conditions died off, he now says. They couldn’t survive the hot aftermath of asteroid impacts. He says: “You can picture these asteroids as a crazed tree trimmer showing up and chopping branches off the tree of life.”


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