Humanity’s trips to the moon revolutionized our view of this planet. As seen from another celestial body, Earth seemed more fragile and more precious; the iconic Apollo 8 image of Earth rising above the lunar surface helped launch the modern environmental movement. The moon landings made people want to take charge of Earth’s future. They also changed our view of its past.
Earth is constantly remaking itself, and over the eons it has systematically erased its origin story, subsuming and cannibalizing its earliest rocks. Much of what we think we know about the earliest days of Earth therefore comes from the geologically inactive moon, which scientists use like a time capsule.
Ever since Apollo astronauts toted chunks of the moon back home, the story has sounded something like this: After coalescing from grains of dust that swirled around the newly ignited sun, the still-cooling Earth would have been covered in seas of magma, punctured by inky volcanoes spewing sulfur and liquid rock. The young planet was showered in asteroids and larger structures called planetisimals, one of which sheared off a portion of Earth and formed the moon. Just as things were finally settling down, about a half-billion years after the solar system formed, the Earth and moon were again bombarded by asteroids whose onslaught might have liquefied the young planet—and sterilized it.
Geologists named this epoch the Hadean, after the Greek version of the underworld. Only after the so-called Late Heavy Bombardment quieted some 3.9 billion years ago did Earth finally start to morph into the Edenic, cloud-covered, watery world we know.
But as it turns out, the Hadean may not have been so hellish. New analysis of Earth and moon rocks suggest that instead of a roiling ball of lava, baby Earth was a world with continents, oceans of water, and maybe even an atmosphere. It might not have been bombarded by asteroids at all, or at least not in the large quantities scientists originally thought. The Hadean might have been downright hospitable, raising questions about how long ago life could have arisen on this planet.
“Although, if you go back to the original Greek Hell, Hades, you had to cross a river. It’s a cool, wet place. So maybe the joke is on us,” says Mark Harrison, a geologist at the University of California at Los Angeles.
Samples collected by Apollo astronauts helped give rise to the Late Heavy Bombardment idea because of chemical traces embedded within them. Potassium and argon isotopes in the moon rocks suggested they suffered catastrophic heating around 3.9 billion years ago. Along with the moon's many craters, this was evidence of a real battering. But this contradicts evidence in meteorites that have made their way from the moon back to Earth. When geologists analyze those rocks, there is no 3.9-billion-year signal. There is no signal in meteorites from the large asteroid Vesta, either, though presumably it would have also been hammered by the same rocks flung at Earth and the moon. The Apollo rocks might be fooling us.
“If there was a cataclysm, maybe there was an erasing of your slate, in terms of life, at that point,” says Meenakshi Wadhwa, a geologist and meteorite researcher at Arizona State University who is studying Vesta samples. “If there wasn’t a cataclysm, then it is quite possible that whatever life had gotten started before, we might still potentially find evidence of that.”
In a new research paper, Harrison and Patrick Boehnke, also of UCLA, argue the isotope dating methods might have made too many assumptions about a late bombardment. The rocks might not have been hit just once, but many times since their formation. Multiple impacts could make the rocks appear younger than they really are. What’s more, the Apollo samples cover only about 4 percent of the moon, Boehnke says.
“It is very easy to throw ejecta around the moon; if you pick up a loose rock on the moon, you can’t be certain that rock came from that region. How do we know we didn’t just pick stuff up that was affected by the same impact event?” he says.
Geologists started questioning the Late Heavy Bombardment because of evidence in Earth rocks, Boehnke says. If the lunar record was right, the planet would have suffered impacts so often, and so harshly, that it would be hard for the molten, staggering Earth to manage other geological processes of its own, like the formation of continents. It would have had an even harder time holding onto water or an atmosphere. But microscopic crystals from that era say otherwise. The crystals are called zircons, and they can be carefully pried from some of the oldest known rock formations on Earth, in places like western Australia and northern Canada. These zircons from the Hadean era are telling a very different story: They formed at relatively cool temperatures, and with plenty of water around. Some may even contain trace evidence of life.
Jesse Reimink, a geologist at the Carnegie Institution of Washington, studies zircons from the Acasta Gneiss formation near Yellowknife, in Canada’s Northwest Territories. The rocks are some of the oldest pieces of Earth, dating to just over 4 billion years ago. In a recent research paper, Reimink reports zircons from that area interacted with some unusual crust 4.02 billion years ago, opening a new window into what the Hadean Earth might have been like. The zircons joined up with an embryonic crust that was somewhere between oceanic crust, which is thin and dense, and continental crust, which is thick and buoyant. This long-lived crust bolsters the argument that while terrestrial processes at the time were very different than today, they might not have been all that bad. Earth may have had proto-continents. Reimink’s samples also have evidence of water on Earth’s surface.
“So maybe the Hadean was not so Hades-like,” Reimink says.
Wadhwa says lunar samples and zircon samples are so limited that we still can’t paint a full picture of the Earth’s turbulent early days. In one sense, everyone might be a little bit right. Earth might have been nice and calm in the time between major impacts. Or some areas might have been molten, while some areas might have been solid and covered in oceans, she says.
One thing is clear, however: Harrison says there has never been any evidence to support the canonical hellish vision of magma lakes and tar-colored volcanos showered in fiery meteors.
“There is absolutely not a single scrap of observational evidence that requires that scenario ever took place. We as a scientific community created an origin myth that has no more intellectual value than 1 Genesis,” Harrison says. “Although we’re very quick to criticize those that operate on faith, that’s exactly what we did.”
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