A new study challenges the prevailing theory that icy comets or asteroids delivered water to Earth during its early years. Instead, researchers suggest that the planet may have produced its earliest water supply from chemical interactions between a hydrogen-rich atmosphere enveloping the young Earth and massive oceans of magma on its surface. In these conditions, water would have formed as a natural byproduct of the chemistry that took place.
According to Anat Shahar, a scientist at the Carnegie Institution for Science in Washington D.C. and co-author of the study, “simply by changing the early conditions with which Earth formed, we are able to produce lots of water that goes both into the planet and into its atmosphere.” Earth’s abundance of water is one of its defining features, making up over 70% of its surface, more than any other planet in our solar system. However, the origins of this vast amount of water have remained a mystery for scientists.
One popular theory suggests that asteroid impacts delivered most of the planet’s water, but research has shown that the water locked inside asteroids is chemically different from that on Earth. This new study proposes that a hydrogen-rich atmosphere early in Earth’s history would have enabled the planet to retain more hydrogen, producing more water.
The team behind the study developed a model to study 25 compounds and 18 different chemical reactions, finding that massive amounts of hydrogen from the atmosphere would have combined with the molten magma oceans on the surface below, which later solidified to form Earth’s largest and thickest layer, the mantle. The planet’s abundant water stock then formed as a natural byproduct of these chemical reactions.
The study also suggests that a hydrogen-rich atmosphere would have allowed the young Earth to hold onto its atmosphere for a long time, meaning that it would have been far richer in hydrogen than it is today. Earth’s current atmosphere is 78% nitrogen, with hydrogen comprising just less than one part per million of the planet’s protective blanket.
The study proposes that Earth’s early conditions were responsible for the planet’s water content and core density, answering two long-standing questions: how large amounts of liquid water came to exist on Earth’s surface, and why the planet’s core, which is mostly iron, is less dense than scientists think it should be.
The team behind the study used findings from exoplanet studies to develop a model and found that hydrogen-rich atmospheres are commonly found around many newly formed rocky planets beyond our solar system. Such atmospheres are regularly spotted around super-Earths and mini-Neptunes, which are the most common types of exoplanets. Young exoplanets commonly host hydrogen-rich atmospheres “during their first several million years of growth,” Shahar said.
“Eventually these hydrogen envelopes dissipate, but they leave their fingerprints on the young planet’s composition.” The team treated Earth as an exoplanet to understand its early years in a new way, reversing the norm and using exoplanet studies to shed light on Earth’s unique characteristics.
The research was published in the journal Nature on April 12, 2023.
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