A new look at the extraterrestrial origin of the Earth’s lakes, rivers and oceans

Water on Earth was not derived from molten meteorites, according to a new study that analyzed molten meteorites that have been floating in space since the formation of the solar system four and a half billion years ago. These meteorites had an extremely low water content, regardless of their origin in the outer or inner solar system, which excludes them as the main source of water on Earth. The dotted white line in the accompanying drawing is the boundary with the outer solar system, showing the transfer of material from the outer solar system to the inner solar system. Image Credit & Copyright: Illustration by Jack Cook, © Woods Hole Oceanographic Institution.

Where did water come from on Earth? Not molten meteorites, according to scientists.

Woods Hole Oceanographic Institute is part of a collaborative study offering new insights into the extraterrestrial origins of our lakes, rivers and oceans.

Water makes up 71% of the Earth’s surface, but no one knows how and when such a huge amount of water came to Earth.

The new study, published March 15 in the journal Nature, brings scientists one step closer to answering this question. Sune Nielsen, a researcher in the department of geology and geophysics at the Woods Hole Oceanographic Institution (WHOI), co-authored a study that analyzes molten meteorites that have been floating in space since the formation of the solar system four and a half billion years ago. The researchers found that these meteorites had an extremely low water content – in fact, they were among the driest extraterrestrial materials ever measured. These results, which allowed researchers to rule them out as the main source of water on Earth, could have important implications for the search for water and life on other planets. It also helps researchers understand the unlikely conditions that made Earth a habitable planet.

The study was led by University of Maryland (UMD) associate professor of geology Megan Newcomb with additional co-authors from the Carnegie Institute of Science.

“We wanted to understand how our planet managed to get water because it’s not entirely obvious,” Newcomb said. “Producing water and having surface oceans on a small planet relatively close to the Sun is not an easy task.”

A team of researchers analyzed seven molten, or achondrite, meteorites that crashed into Earth billions of years after they broke away from at least five planetesimals — objects that collided and formed planets in our solar system. In a process known as melting, many of these planetesimals were heated by the decay of radioactive elements in the early history of the solar system, causing them to separate into layers with a crust, mantle, and core.

Because these meteorites only recently hit Earth, this experiment was the first time anyone had ever measured their water content. UMD geology graduate student Liam Peterson used an electron microprobe to measure levels of magnesium, iron, calcium, and silicon, and then joined Newcomb at the Carnegie Institute of Science’s Earth and Planetary Laboratory to measure their water content with a secondary ion mass spectrometry instrument.

“The problem with analyzing water in extremely dry materials is that any terrestrial water on the surface of a sample or inside a measuring instrument can easily be detected, which skews the results,” said co-author Conel Alexander, a scientist at the Carnegie Institution of Science.

To reduce contamination, the researchers first baked their samples in a low-temperature vacuum oven to remove any surface water. Before the samples could be analyzed in the secondary ion mass spectrometer, the samples had to be dried again.

“I had to leave the samples under a turbopump — a really high quality vacuum — for more than a month to get enough terrestrial water,” Newcomb said.

Some of their meteorite samples came from the inner solar system, where the Earth is located and where conditions are generally considered to be warm and dry. Other, rarer specimens have come from the colder, icier fringes of our planetary system. Although it was generally believed that water came to Earth from the outer solar system, it remains to be determined what types of objects could have carried this water through the solar system.

“We knew that a lot of the outer solar system objects were differentiated, but it was sort of implicitly assumed that since they originate from the outer solar system, they must also contain a lot of water,” WHOI’s Nielsen said. “Our newspaper shows that this is definitely not the case. As soon as the meteorites melt, there is practically no water left.”

After analyzing samples of meteorite achondrites, the researchers found that water makes up less than two millionths of their mass. By comparison, the wettest meteorites — a group called carbonaceous chondrites — contain up to 20% water by weight, or 100,000 times more than the meteorite samples studied by Newcomb et al.

This means that the heating and melting of planetesimals leads to an almost complete loss of water, regardless of where in the solar system these planetesimals originated and how much water they were originally associated with. Newcomb et al found that, contrary to popular belief, not all objects outside the solar system are rich in water. This led them to conclude that water probably entered Earth through unmelted or chondrite meteorites.

Newcomb said their findings have applications beyond geology. Scientists from many disciplines, especially exoplanet researchers, are interested in the origin of Earth’s water because of its deep connection with life.

“Water is considered an ingredient for life to thrive, so when we look into the universe and find all these exoplanets, we start to figure out which of these planetary systems could be potential hosts. life,” Newcomb said. “To be able to understand these other solar systems, we want to understand our own.”

Reference: “Degassing of early-forming planetesimals has limited the flow of water to the Earth” M. E. Newcomb, S. G. Nielsen, L. D. Peterson, J. Wang, C. M. O’D. Alexander, A. R. Sarafian, K. Shimizu, L. R. Nittler, and A. J. Irving, March 15, 2023, Nature.
DOI: 10.1038/s41586-023-05721-5

This research was supported by NASA (award number 80NSSC20K0336 and 80NSSC22K0043) and a Carnegie Institution of Science fellowship. This story does not necessarily reflect the views of these organizations.