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The giant impact that created the Moon also seeded Earth with life-giving carbon

Why did our planet turn out like it did — a temperate, life-supporting place? Why weren’t the volatile elements (sulfur, carbon, and the like) in our planet boiled away into space or locked in the core, like on Mars or countless other cold, rocky exoplanets? A team of scientists from Rice University have published a new explanation for the chemistry of our planet’s surface, and it has to do with the giant impact hypothesis.

The story goes like this: About four and a half billion years ago, the Earth first settled together into a planet. Only about a hundred million years afterward, it probably had an oblique collision with a slow-moving, massive body somewhere between the sizes of Mercury and Mars. According to the giant impact hypothesis, that massive body was Theia, and the impact sent a bolus of mashed-together molten planet sloshing into space, where it coalesced into the Moon. Visualize water droplets combining in zero gravity, except with two huge globs of partially melted rock glowing hot through the cracks.

This is all extremely difficult to verify, because beyond about 4 billion years ago, the Earth’s face has been so changed by its own tectonic activity that all we have left to bear witness to the aptly named Hadean Eon are scattered zircon crystals embedded in other, younger rocks. But zircons don’t give us a conclusive history of our planet through deep time, other than having the chemistry they do and being as old as they are. They don’t tell us about what happened before the Late Heavy Bombardment. But these scientists think there are still clues written in the bulk chemical composition of the planet as a whole.

Before the oxygen catastrophe about 2.3 billion years ago, our planet’s atmosphere was a reducing environment — which is to say, the planet’s chemistry was dominated by sulfur compounds instead of oxygen. But the cyanobacteria changed everything. Their shiny new talent for photosynthesis came with a catch: they excreted diatomic oxygen, good old O2 gas, as a waste product. Oxygen built up in the environment to such concentrations that it permanently changed the atmospheric composition from a reducing, sulfurous atmosphere to the breathable, oxidizing one we have now.

Atmospheric chemistry is really important here, because it totally changes the path of chemical reactions on the planet. In anoxic environments, for example, iron acts differently than its familiar oxidation (rusting) behavior. Under anoxic conditions, like the sulfurous environment on the still-molten early earth, metals like iron tend to form sulfides and carbides, which are heavy enough to sink. But, the scientists contend, sufficient concentrations of silica can force metallic compounds to play hot-potato with their functional groups, accepting silica and rejecting carbon to the mantle. Dumping a ton of silica into the planet’s metal-sulfide and carbide core could switch the equilibrium and induce the formation of less-dense metal silicates and carbonates, like those found in terrestrial igneous and sedimentary rocks. This is where the great impact comes in.

An embryonic planet — one that had been around long enough to make carbon-rich surface rocks, long enough for the silicon to sink to its core — could explain the mixing we see. If a young, slow-moving, Mercury- or Mars-sized planet collided with our own, the two might not completely mix; the lighter, cooler mantles would be able to intermingle, well excluded from the relatively liquid core and its different chemistry balance.

“Because it’s a massive body, the dynamics could work in a way that the core of that planet would go directly to the core of our planet, and the carbon-rich mantle would mix with Earth’s mantle,” explains Rajdeep Dasgupta, coauthor of the study.

Given Mercury has a silicon-dominated core chemistry, the idea of finding a nearby Mercury-sized planet with a silicon-rich core isn’t too far-fetched. The sulfur chemistry of the early Earth provides all the conditions required for this hypothesis. And the Late Heavy Bombardment would have done the rest of the surface mixing to achieve the surface chemistry we have, with the planetary carbon and sulfur budget we have. As with so many things in science, only further observation will help to form a conclusion. But it’s really something to think about, the idea Earth only became this lush, lovely planet after total surface-melting cataclysm.

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