According to a recent study, Earth's water did not come from melted meteorites that had been floating around in space since the solar system's formation four and a half billion years ago. These meteorites had extremely low water content, regardless of their origin in the outer or inner solar system, ruling them out as the primary source of Earth's water.
Scientists dispute that the Earth's water came from melt meteorites.
The Woods Hole Oceanographic Institution is a part of a larger collaboration that seeks to uncover new insights into our lakes, rivers, and oceans' extraterrestrial origins.
Water accounts for 71 percent of Earth's surface, but nobody knows how or when such large quantities of water arrived on Earth.
A new paper published on March 15 in the journal Nature brings scientists one step closer to answering that question. Researchers examined melted meteorites that had been floating around in space since the solar system's formation four and a half billion years ago. They found that these meteorites had a high degree of water content, among the highest ever measured. These findings might have implications for the exploration of other planets' water and life.
Megan Newcombe, an assistant professor of geology at the University of Maryland, was the subject of the investigation, along with other co-authors from the Carnegie Institution of Science.
Because it isn't totally obvious, Newcombe wanted to know how our planet got water. "Getting water and having surface oceans on a planet that is tiny and relatively near the sun is a challenge."
Seven melted, or achondrite, meteorites that crashed into Earth billions of years after splintering from at least five planetesimals that collided to form the planets in our solar system were studied by the same group of researchers. Many of these planetesimals were heated up by the decay of radioactive elements during the early solar system's evolution, causing them to break apart into layers with a crust, mantle, and core.
Liam Peterson, a graduate student at the Carnegie Instution for Science's Earth and Planets Laboratory, measured their water contents with a secondary ion mass spectrometry instrument.
"The challenge in analyzing water in extremely dry materials is that any terrestrial water on the sample's surface or inside the measuring instrument can easily be detected, tainting the results," said co-author Conel Alexander, a scientist at the Carnegie Institution for Science.
Researchers first baked their samples in a low-temperature vacuum oven to remove any surface water, then had them dried out once more.
"I had to leave the samples under a turbo pump -- a really high-quality vacuum -- for more than a month to get rid of the terrestrial water," Newcombe said.
Some of their meteorite samples came from the inner solar system, where Earth is located, and where conditions are generally assumed to have been warm and dry, while others, which are less common, came from the cooler, icier outer reaches of our planet. While water was generally assumed to have flowed to Earth from the outer solar system, it is still undetermined what kinds of objects might have carried that water across the solar system.
"We knew that a good deal of outer solar system objects were distinguished, but it was kind of implicitly assumed that these objects must also contain a lot of water," according to WHOI's Nielsen. "Our paper shows this is absolutely not the case. As soon as meteorites melt, there is virtually no remaining water."
Researchers determined that water constituted less than two-millionths of the total weight of the achondrite meteorites. For comparison, the wettest meteorites, a group called carbonaceous chondrites, contain up to around 20% of water by weight, or 100,000 times more than the meteorite samples studied by Newcombe and co-authors.
Newcombe and his co-authors discovered that, contrary to popular belief, not all outer solar system objects are rich in water. This led them to conclude that water was likely to be delivered to Earth via unmelted or chondritic meteorites.
Newcombe claims that their findings extend beyond geology. Due to its close relationship to life, scientists from many fields and especially exoplanet researchers are interested in the origins of Earth's water.
"We're beginning to explore which of these extraterrestrial systems might be useful hosts for life," Newcombe said.
M. E. Newcombe, S. G. Nielsen, J. Wang, C. M. O'D. Alexander, A. R. Sarafian, K. Shimizu, L. R. Nittler, and A. J. Irving, Nature. DOI: 10.1038/s41586-023-05721-5
NASA (Award Nos. 80NSSC20K0336 and 80NSSC22K0043) funded this research. The author does not necessarily endorse or disagree with this publication.