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.