The mystery of how Earth got all its water has long baffled scientists, with many believing that icy comets and asteroids crashed into the planet, leaving a liquid legacy that formed our oceans.
US researchers have now found a much simpler explanation that doesn’t require space rocks at all.
While studying young exoplanets orbiting stars outside the Solar System, scientists discovered that they are enveloped in a thick hydrogen atmosphere, which may have reacted with early magma oceans to form huge amounts of water.
While some may have come from comets, experts say there’s no need to explain Earth’s current watery state.
“Increasingly powerful telescopes are enabling astronomers to understand the compositions of exoplanet atmospheres in detail never seen before,” said Anat Shahar of the Carnegie Institution for Science.
“Discoveries of exoplanets have allowed us to appreciate much more how common it is for newly formed planets to be surrounded by molecular hydrogen-rich atmospheres during their first million years of growth.
“Eventually, these hydrogen envelopes dissipate, but leave their imprints on the composition of the young planet.”
To test the theory, the researchers developed a new computer model that replicated the chemical makeup of the early Earth.
The planet was formed about 4.6 billion years ago, from matter circling the Sun, which coalesced into a vast ocean of magma.
Over time, as the Earth cooled, denser material sank inward, separating the planet into three distinct layers: the metallic core, the rocky mantle, and the crust.
While still in its hot, molten phase, modeling showed it could gain enough mass to maintain a large hydrogen atmosphere. This mixed with the molten surface before it solidified, generating an enormous amount of water.
The model showed that even if all the rocky material that collided to form the growing Earth were completely dry, these interactions between the molecular hydrogen atmosphere and the magma ocean would generate enough water to explain the oceans, seas , rivers and lakes.
About 70% of the planet is covered in water, totaling about 332.5 million cubic miles.
Past space rock studies
Previous studies have shown that asteroids and comets contain enough ice to bring all of the water to Earth.
In 2021, a meteorite landed on a driveway in Gloucestershire and was found to contain extraterrestrial water much like that on Earth, as well as amino acids, important building blocks of life.
Nicknamed the Winchcombe meteorite, it came from the asteroid belt between Jupiter and Mars.
Most of the space rocks studied contain water of a different composition than that found on Earth, suggesting that it was not the source.
When the European Space Agency’s Rosetta mission rendezvoused with comet 67P/Churyumov-Gerasimenko in 2014, only 10% of the water found resembled that on Earth.
The new theory helps explain why Earth’s inner core is less dense than pure iron — less dense than it should be — by suggesting that a light element did get mixed in at some point.
The authors suggest that the element could be hydrogen, which entered the core before it solidified.
The research was published in the journal Nature.