A study published this week in Nature Communications posits that Earth’s core holds a lot of hydrogen. The range is from nine to 45 oceans’ worth of hydrogen, mostly locked up in the form of iron compounds.
If true, it could negate the need for some theories about the origins of water on Earth and, by extension, explain how Earth had the conditions needed for the origin of life.
According to this study, Earth’s oceans may not have arrived via a comet; instead, they may be of terrestrial origin. In this way of thinking, the hydrogen that sequesters oxygen in the form of water would have been part of the Earth’s composition from the beginning.
But let’s talk chemistry. In the formation of water from hydrogen and oxygen gas, the formula is 2H₂ + O₂ -> 2 H₂O. These gases want to form water, since by fixing the same six atoms into two molecules, rather than the starting three molecules, we’ve reduced the overall energy they hold.
However, the above applies to two free-mixing gases, whereas oxygen on the early Earth would not have been in gaseous form. Instead, it would have been mostly locked away in compounds, especially silicates and oxides. There would have been a bit of free hydrogen gas interacting with oxygen-rich compounds in rock, but that hydrogen gas also wouldn’t have been nearly abundant enough to explain the planet’s vast oceans.
More likely, those oceans originally came from reactions of oxygen and hydrogen within single compounds, with the energy needed to release them coming from the much higher level of volcanism on primordial Earth.
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Some compounds rich in both oxygen and hydrogen will “dehydrate” when superheated, with an oxygen atom and two hydrogen atoms leaving as water. This is why, when lava cools, it forms a different type of rock than originally melted; lava’s outgassing irreversibly alters its composition.
Perhaps the largest source of water on early Earth would have been volcanic outgassing of water vapor.
Models for such terrestrial water formation have long struggled to demonstrate that sufficient hydrogen was present in Earth’s composition, which is why scientists sought alternate explanations, such as hydrogen-rich comet impacts. This paper proposes that the Earth’s core is hydrogen-rich enough to suggest that the early Earth would have had plenty of hydrogen available.
The volume of the ocean, its average purity, and the chemical makeup of H₂O allow a simple estimate of the amount of hydrogen contained in all the water on Earth. These researchers show that hydrogen trapped in compounds in the Earth’s core could outweigh that amount by tens of times.
Historically, quantifying hydrogen has been difficult, since it’s so small and light that it can hide in compounds with little signature. This team subjected different iron compounds to conditions similar to those in the planet’s core, using their properties under such pressure to infer which one is closest to those in the core.
The origin of the very first life on Earth, or “abiogenesis,” is one of the least understood aspects of life. From the first replicator onward, we have a rough idea of how things played out, but science is still searching for evidence of the conditions that allowed the very first proto-cell to form. Liquid water is one of the most important prerequisites for abiogenesis, and this study helps narrow down exactly how it came about in the first place.