For a brief period in Earth’s tumultuous early history, a mineral that no longer exists on our planet may have safeguarded the ingredients of water long enough to enable the emergence of oceans that eventually nurtured life.
That’s the conclusion of a recent study led by Xiao Dong, a materials scientist at Nankai University, that presents a new possible origin story for Earth’s water—the most essential ingredient for life as we know it. In addition to yielding new insights into Earth’s ancient oceans, the new study also has implications in the search for water, and therefore alien life, on other planets.
As our young planet was bombarded by asteroids and comets more than four billion years ago, a “now-extinct” permutation of magnesium silicate might have kept hydrogen and oxygen atoms securely locked away deep underground so that they could eventually survive and upwell as liquid water at the surface, according to the study, which appears in Physical Review Letters.
“The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements,” said Dong and his colleagues in the study.
The team undertook just such a search with the help of an algorithm called Universal Structure Predictor: Evolutionary Xtallography (USPEX) developed by study co-author Artem Oganov, who is a professor at the Skolkovo Institute of Science and Technology. USPEX is able to predict exotic crystal structures to fit a variety of parameters, including compounds that would have existed within the extreme conditions in the interior of our infant planet.
The researchers used USPEX to search for compounds that contain hydrogen and oxygen, the two constituents of water, that would be stable at the high temperatures and pressures that existed hundreds of miles under our planet’s ancient surface. The results revealed a magnesium silicate that is two parts magnesium, one part silicon, five parts oxygen, and two parts hydrogen. This compound “must have existed in the Earth, hosting much of Earth’s water” during “the first 30 million years of Earth’s history, before the Earth’s core was formed, according to the study.
In an email, Oganov noted that his team’s hypothesis presents an alternate origin of Earth’s oceans that might explain some of the mysteries of another popular explanation that suggests our planet’s early water was delivered by comet impacts.
“Only a minor fraction of Earth's water can be from comets,” Oganov said. “This follows from very different isotope compositions of Earth's water and water in comets.”
According to the team, the magnesium silicates would have disintegrated as Earth’s core formed, a process that released hydrous constituents as water. Over the course of 100 million years, this water made its way to Earth’s surface, where it became the life-sustaining force that still exists today. In this way, these silicates “likely contributed in a major way to the evolution of our planet,” the team said.
“Now we are asking ourselves whether other components could be brought to the Earth during this or similar process,” Oganov said.
These now-extinct compounds may also contribute to the evolution of other planets, which makes them relevant to the search for extraterrestrial life.
Planets that are smaller than Earth, such as Mars, cannot achieve the interior pressures necessary to create these magnesium silicates, which means any water on these worlds had to have a different origin. Meanwhile, planets larger than Earth, such as the tantalizing “Super-Earths” observed in other star systems, would likely support pressures that could preserve huge volumes of these hydrous compounds.
“For large Earth-like planets (Super-Earths) the ‘window’ of habitability is wider than previously thought,” Oganov said. “Previously, it was shown that if a planet contains more than 0.2 weight percent of water, its surface will be completely flooded, which will result in an unstable climate and will be adverse for emergence of life.”
“With the new hydrosilicates, a planet can contain a lot more water ‘hidden’ in the interior of the planet and not flooding its surface,” he added. “Such water-rich planets are therefore still suitable for life.”
While some scientists have speculated that Earth’s water may have been delivered from outer space by comets, the new study shows that our precious oceans may have emerged from the opposite direction—as byproducts of long-lost compounds hidden deep underground.
Update: This article has been updated to include comments from study co-author Artem Oganov.