Where did the water of the earth come from? According to scientists, no molten meteorites

The dashed white line in this figure shows the boundary between the inner solar system and the outer solar system, with the asteroid belt positioned roughly between Mars and Jupiter. A bubble at the top of the image shows water molecules attached to a rock fragment, demonstrating the type of object that could have transported water to Earth. Credit: Jack Cook/Woods Hole Oceanographic Institution

Water makes up 71% of the earth’s surface, but no one knows how or when such huge amounts of water reached the earth.

A new study published in the journal Nature brings scientists one step closer to answering this question. Researchers led by Megan Newcombe, assistant professor of geology at the University of Maryland, analyzed molten meteorites that have been floating around in space since the solar system formed 4 1/2 billion years ago. They found that these meteorites had extremely low water content – in fact, they were among the driest extraterrestrial materials ever measured.

These findings, which allow researchers to rule them out as the main source of Earth’s water, could have important implications for the quest for water – and life – on other planets. It also helps researchers understand the unlikely conditions that led to Earth becoming a habitable planet.

“We wanted to understand how our planet managed to get water because it’s not very obvious,” Newcombe said. “Getting water and having surface oceans on a planet that’s small and relatively close to the sun is challenging.”

The research team analyzed seven molten, or achondrite, meteorites that fell to Earth billions of years after the fragmentation of at least five planetesimals – objects that collided to form the 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 containing the crust, mantle, and core.

Since these meteorites only recently fell to earth, this experiment was the first time anyone had measured their volatiles. Liam Peterson, a graduate student in UMD geology, used an electron microprobe to measure their magnesium, iron, calcium and silicon levels, then joined Newcombe at the Carnegie Institution for Science’s Earth and Planets Laboratory to compare their water content with a Measure secondary ion mass spectrometry instrument.

“The challenge in analyzing water in extremely dry materials is that terrestrial water can easily be detected on the surface of the sample or inside the meter, skewing the results,” said study co-author Conel Alexander, scientist at the Carnegie Institution for Science.

To reduce contamination, the researchers first heated 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 pull the terrestrial water down sufficiently,” Newcombe said.

Some of their meteorite samples came from the inner solar system, where Earth is located and where conditions are generally believed to have been warm and dry. Other rarer samples came from the colder, icier outer reaches of our planetary system. While it has been widely believed that water came to Earth from the outer solar system, what types of objects might have carried that water through the solar system has yet to be determined.

“We knew that many outer Solar System objects were distinguished, but it was kind of implicitly assumed that because they came from the outer Solar System, they must also contain a lot of water,” said Sune Nielsen, co-author of the study and geologist at the Woods Hole Oceanographic Institution. “Our paper shows that this is definitely not the case. Once meteorites melt, there is no water left.”

After analyzing the Achondrite meteorite samples, the researchers discovered that water made up less than two-millionths of their mass. For comparison, the wettest meteorites — a group called carbonaceous chondrites — contain up to about 20 percent water by weight, or 100,000 times more than the meteorite samples studied by Newcombe and her co-authors.

This means that heating and melting of planetesimals results in almost total loss of water, regardless of where in the solar system those planetesimals formed and how much water they started with. Newcombe and her co-authors discovered that, contrary to popular belief, not all objects in the outer Solar System are water-rich. This led them to conclude that water likely reached Earth via unmelted or chondritic meteorites.

Newcombe said their findings have applications beyond geology. Scientists of many disciplines – and exoplanet researchers in particular – are interested in the origin of water on Earth, as it is deeply connected to life.

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

The research paper “Degassing of early-formed planetesimals limited water delivery to Earth” was published in Nature on March 15, 2023.

More information:
Megan Newcombe, Outgassing of early formed planetesimals restricted water supply to Earth, Nature (2023). DOI: 10.1038/s41586-023-05721-5. www.nature.com/articles/s41586-023-05721-5

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