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.
Waterproof
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 farther 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 an 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.
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John Timmer