Chemical reactions on the early Earth may have formed its ocean

Planetary-scale chemistry

To explore that idea, the researchers essentially modeled a giant chemical reactor filled with most of the ingredients of the early Earth and scaled up to the size of a large Earth precursor (half the size of present-day Earth). This includes things like iron and sodium oxides, various silicates, carbon dioxide, methane, oxygen, and more. This was all placed under a hydrogen-rich atmosphere and heated up to reflect the magma oceans created by the frequent collisions that took place as planets formed.

This period was likely to have lasted tens of millions of years, in part because hydrogen atmospheres tend to retain heat extremely well (hydrogen can act as a greenhouse gas). This gives the chemical reactions that occur—and the researchers track 18 of them—time to reach an equilibrium and allows enough time for different materials in the planetary interior to partition based on density.

One of the things that happens is that several elements get incorporated into the core's iron, including oxygen, silicon, and hydrogen. Since all of these elements are less dense than iron, this has the effect of making the core less dense than if it were pure iron—something that is true of the actual Earth.

In some of the reactions, the incorporation of hydrogen involves the displacement of oxygen, and a byproduct of these reactions is water. Under the conditions explored here, the reactions produce about the same volume as is present in the oceans of the current Earth. "Even if the rocks in the inner Solar System were entirely dry," the researchers write, "reactions between H₂ atmospheres and magma oceans would generate copious amounts of H₂O. Other sources of H₂O are possible, but not required."

The limits of modeling

On the plus side, the simulations work with a wide range of temperatures—all it requires is enough heat to keep the planet molten while the processes described here reach an equilibrium. It also works for various precursor sizes but fails if the precursor is too small. That's consistent with the extreme dryness of Mars and Mercury. The primary variable ends up being how much water ends up being produced; if more hydrogen ends up in the core, then it's easy to create a water world with three times the volume of today's oceans.

While the model is robust to lots of changes in initial conditions, it's limited by not being a complete picture of the early Earth's chemistry. Notably missing are sulfur and nitrogen, which have played major roles in the Earth's chemistry.

But the big gap in the model is what happens after the water forms. Given the presence of a magma ocean, it would end up in the atmosphere, where it could be split up by solar radiation and lost if the Solar System's hydrogen has already dissipated. The same is true for any later impacts that heat the planet, such as the giant collision that formed the Moon. If there's enough hydrogen around still, this isn't a problem since the water could just reform. And the researchers cite research that shows that a water-rich atmosphere could survive even a massive collision. Finally, you could imagine conditions where there was an initial overproduction of water, but enough was lost through these processes to leave Earth in its present state.

So, while water production doesn't require any fine-tuning of conditions, retaining it might.

But the implications for worlds beyond ours seem a bit larger. These results suggest that a large range of initial conditions should produce water during the formation of rocky planets. So, when we consider planets in exosolar systems, it may be a question of wondering whether they experienced conditions that would cause them to lose water rather than whether they might have had any in the first place.

Nature, 2023. DOI: 10.1038/s41586-023-05823-0  (About DOIs).