Water has made the Earth the planet that it is—a planet known for its blue oceans. Water shapes the land through erosion and is fundamental to Earth's ability to support life. But we have a hard time understanding exactly how Earth ended up with all this water, as the building blocks that created it were likely to be dry, and the collisions that turned these building blocks into a planet should have driven any surface waters off into space.
Various means have been proposed to deliver water to Earth after its formation. But a new study takes information we've gained from examining exoplanets and applies this to Earth. The results suggest that chemical reactions that would have occurred during Earth's formation would have produced enough water to fill the world's oceans. And, as a side benefit, the model explains the somewhat odd density of the Earth's core.
The Earth seems to have primarily been constructed from materials in the inner Solar System. Not only were those materials in the right place, but present material found in asteroids of the region provided good matches in terms of their elemental and isotopic composition. But these materials are also very dry. That's not a surprise; the temperatures in this area would have kept water from condensing out as a solid, as it can further out in the Solar System, beyond a point known as water's "ice line."
Any water present could have been lost to space, as the process of building planets is thought to have occurred via collisions among small bodies, with the larger bodies progressively growing as smaller ones continued to smash into them. Much of the water in these bodies would have been vaporized and potentially lost to space.
But three researchers (Edward Young, Anat Shahar, and Hilke Schlichting) focused on additional factor that would have been present during the formation of the Solar System: hydrogen. Hydrogen is thought to be present in large quantities during the early period of planet formation, but is then driven off by the radiation released once the central star ignites. In our Solar System, some of it was captured by the outer planets before it was lost. But our inner planets seem to have formed with little of the element or lost it early in their history.
But a look at exoplanets suggests that this isn't an inevitable fate. We've found many rocky super-Earths that also seem to lack a hydrogen-rich atmosphere. But there's a gap at around two times the Earth's radius where we see a lot of mini-Neptunes, which seem to have retained thick and likely hydrogen-rich atmospheres. This has led to the proposal that all rocky planets start in a hydrogen-rich environment and form their first atmosphere from that. Below a certain size, however, that hydrogen gets lost later in their history. Any atmospheres found on these planets are likely due to a secondary formation.
Taking that to its logical conclusion, then Earth may have started with a hydrogen-rich atmosphere as well. So, the researchers involved in the new study decided to look into what the consequences of that scenario could have been.
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 (it 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).