The missing 500 million: Cosmic bombardment melted Earth's first crust - Ars Technica

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Earth is the only planet we know of with buoyant, silica-rich continents. But, despite decades of research, geologists still don’t agree on how they formed. “The continents started appearing around about four billion years ago—that’s the oldest continental rock we know about,” said Tim Johnson, a geologist at Curtin University in Perth, Australia. “The Earth is four and a half billion years old, so why they started appearing then is unknown, as is the mechanism to make that continental crust.”

Johnson and his colleagues are now arguing that the formation of continents on Earth was caused largely by an intense, sustained barrage of asteroid impacts that kept the early crust hot and thin enough to make buoyant continents possible. In short, the lands we live on are here because of ancient bombardment from space.

Plates and plumes

The problem with studying the formation of continents is that the geological evidence of this process is almost gone. The oldest known continental-type rocks crystallized around 4.03 billion years ago, right at the end of the Hadean eon (the earliest era in Earth’s history, spanning the first 500 million years of its existence). Rare basaltic rocks date back about 4.2 billion years, and a handful of the oldest zircon crystals push the record back to 4.4 billion years. Beyond that, there’s hardly anything else. So, scientists looking into the origin of continents had to rely largely on educated guesses. “There are huge debates about what was going on in the early Earth, because the data is so scarce,” Johnson said.

One dominant idea holds that plate tectonics, much like today’s, was already running in the Hadean, with continental crust forming above subduction zones—areas where tectonic plates collide. The other claims that early Earth was too hot for rigid plates, and that crust instead formed above mantle plumes rising from deep within the planet, a phenomenon comparable, Johnson said, to the wax blobs rising inside a lava lamp.

The issue with both these ideas, though, was that Earth, based on most models, appeared too cold for all this to happen. “People have tried to understand Earth’s heat budget through time, and nobody could make it fit,” Johnson said. “Nobody could make it fit because we did not consider the energy coming from outside of Earth.” This energy, he argues, came from asteroid and meteorite impacts that were far more frequent back when the solar system was young. Adding these impacts to the early Earth’s heat budget, though, proved rather challenging because Earth has a peculiar way of healing its scars.

The moon shot

The reason we don’t really know what was happening on Earth four billion years ago is that plate tectonics effectively recycles the surface of the planet back into the mantle. “One place where we do know what was going on back then is the Moon,” Johnson said. “We have sent people there. We have collected sample from there. We have immense amounts of high-quality data from the Moon.” Because the Moon does not have plate tectonics, its crust is a single, solid, continuous shell. And this shell, Johnson’s team noted, is peppered with impact craters.

Calibrated against dated lunar samples, crater counts on the Moon let Johnson’s team estimate how frequently large bodies were hitting our closest celestial neighbor shortly after the Earth had formed. “Scaling that flux up to Earth’s larger size and stronger gravity makes it clear the planet must have been hit by thousands of impactors that were greater than 10 kilometers in diameter,” Johnson said. When his team determined the most probable frequency of impacts and the size of impactors, they could calculate how much energy this immense bombardment delivered to Earth and, consequently, how much heat it produced.

It turned out it was a lot of heat.

Most prior modeling of early Earth’s heat budget focused on internal sources like heat left over from accretion and core formation plus the ongoing decay of radioactive isotopes—we thought these were absolutely dominant. Johnson’s space bombardment model showed they were not.

Bringing the heat

The team focused on modeling how the kinetic energy of each impact would ultimately end up as heat. The physics, Johnson said, is straightforward even if the details are complex. “It really is as simple as converting the size and the velocity of the impactor into energy,” he explains. When a large body hits, some of the impact energy goes...