Creation stories tend to be outlandish, in keeping with the near impossibility of explaining why the world is the way it is — from the week-long labours of the Old Testament God, to the eternal cycles of the Hindu creator Brahma, to Raven’s metaphysical trickery in Pacific Northwest First Nations tradition.
Fanciful as they are, however, these myths have nothing on modern science, whose creation story — which already involves a mysterious Big Bang, perhaps one of many creations ex nihilo in infinite succession — this week got even weirder.
At a geophysics conference in California, a series of discoveries was announced that show — not for the first time, but in unprecedented detail — the Earth as we know it today was born of a violent collision between two ancient planets, so forceful it reduced both to molten rock and silicate vapour, and nearly blasted both into cosmic smithereens.
“It would have been a very, very bad day for the Earth,” said Sujoy Mukhopadhyay, associate professor in the Earth and Planetary sciences Department at Harvard University. “One potential outcome could have been complete destruction.”
It came close. The cataclysmic meeting of the proto-Earth and the smaller, Mars-sized planet Theia — known as the Big Splash — set the Earth spinning faster than ever, tilted its axis by 23 degrees, and created the moon from the debris that was blasted into orbit.
Instead of planetary destruction, however, all these calamities were crucial to the eventual formation of the Earth’s protective atmosphere, the cycles of the seasons, the preservation of the polar ice caps, the movement of the moon-driven tides and the emergence of life.
“Subsequent to that bad day, it was a good thing,” Prof. Mukhopadhyay said.
His analysis of material from deep within the Earth, retrieved from places like Iceland and Hawaii where “mantle plumes” force material up through volcanoes, suggests that although the impact released enough energy to totally melt both planets, the opposite side of proto-Earth was shielded, remained partly solid, and likely settled into a layer down near the modern Earth’s molten iron core where it might still be recoverable. If so, he said, “it’s quite possible that by probing material from the deep Earth we may learn something more about the different formation stages of the Earth, how it was put together. … We don’t quite know what fraction melted, what vaporized, what stayed solid.”
He already knows the rough answer, though. The Earth was put together with the monstrous force of a planetary crash, and only gravity held the shrapnel together, settling it over time into the slightly flattened sphere on whose crust we live today. This was not like an asteroid impact, which leaves a crater and throws up a lot of ash, as was likely fatal to the dinosaurs. This was what scientists call, with typical restraint, a “giant impactor.”
Most moons are not created this way. Most already existed fully formed, and were captured by the gravity of planets, or they formed along with the planet from the same spinning disc of stardust. Earth’s Moon, on the other hand, is thought to have formed quickly after the Big Splash, in less than a thousand years. “Geologically that is incredibly fast,” Prof. Mukhopadhyay said.
As the genesis moment for the blue planet and its signature single moon, the Big Splash can be dated quite accurately to about 4.5 billion years ago. One of this week’s discoveries, by a French team, was that the collision happened 60 million years earlier than previously thought, meaning the Earth was formed roughly 40 million years after the solar system began to form.
This solar system itself was the product of perhaps 10 billion years of cosmic evolution, a process that looked roughly like this.
In the beginning was the Big Bang, which was not so much an explosion as an expansion, with space and time hurtling out from a single point, creating a new universe in which matter and antimatter were created in roughly equal measure, each annihilating the other to create the flash of light that telescopes can still detect today, known as the Cosmic Microwave Background.
For some unknown reason, however, there was more matter than antimatter, so some of it remained, with no antimatter to annihilate it. Under the gravitational pull of the mysterious dark matter, these basic elements gathered into fibrous strands, like cosmic cotton candy, and eventually collapsed into stars, vast cauldrons in which the lighter elements like hydrogen and helium were fused to create the heavier atoms. Those stars died and exploded, seeding the universe with stardust that over time formed into spinning discs of diffuse matter, and eventually asteroids, comets and planets, in the orbit of newer stars, themselves evenly distributed in spiral galaxies.
One of these stars was the Sun, and in its early days, it had many more planets in its orbit, perhaps 20, including proto-Earth and Theia. And then, one fateful day 4.5 billion years ago, they approached at a glancing angle at something less than 5 km/second, and collided.
‘It would have been a very, very bad day for the Earth. One potential outcome could have been complete destruction’
This week, one of the discoveries, by a team led by Daniel Herwartz of the University of Cologne in Germany, was that the moon is mostly made of Theia.
“This hypothesis [the Big Splash] can explain a lot of features of the moon, but there is one problem, that if you model this process [by computer], these computer models predict that most of the debris [that forms the moon] is coming from the impacting body [Theia], and because every body in the solar system has its own unique fingerprint, you would expect to have different isotopic compositions of the Earth and the Moon,” said Prof. Herwartz, who studied the composition of lunar meteorites and rocks brought back to Earth by the Apollo Moon missions. “Until now, this difference has not been found, and we’ve now found it.”
He is less optimistic than Prof. Mukhopadhyay about finding traces of proto-Earth in the modern Earth. The intervening 4.5 billion years of “mantle convection certainly erased most, hopefully not all, evidence of distinct materials from proto-Earth or Theia within the Earth,” he said. “We really don’t know if we could find this material anywhere on Earth. … We don’t know if it’s anywhere. It might have been all mixed away.”
Still, a sample of unmelted proto-Earth would be a major prize for earth origins scientists, big enough that Prof. Mukhopadhyay’s idea about a layer down near the core might be worth pursuing, for all the answers it could offer up about the days before the Big Splash.
“If I find this, if you can give me like ten milligrams of proto-Earth, I could tell you how large the impactor was and what its composition was,” Prof. Herwartz said. “This would be very exciting, but currently we have no idea where that might be in the Earth.”