Water arrived on Earth gradually, not in one big event

Earth, the only known planet to support life, owes much of its habitability to water. Understanding when and how water arrived on Earth is crucial for piecing together the story of life’s origins.

Scientists have long debated whether water was present from the planet’s formation or if it arrived later through external sources. A recent study led by a Rutgers-New Brunswick scientist challenges existing theories, suggesting that water arrived much later than previously believed.

The findings indicate that Earth received water toward the final stages of its formation. This discovery could reshape scientific understanding of the conditions necessary for life.

By examining isotopes in Earth’s rocks and comparing them with meteorites, researchers have provided new insights into the timing and process of water’s arrival on our planet.

Timing of life’s origins

Scientists seek to pinpoint when key ingredients for life first appeared on Earth. Water, energy, and a mix of organic chemicals known as CHNOPS – carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur – are essential for life to emerge.

Determining when these components arrived can offer clues about the timing of life’s formation.

Katherine Bermingham is an associate professor in the Department of Earth and Planetary Sciences at Rutgers and lead author of the study.

“When water was delivered to the planet is a major unanswered question in planetary science,” said Bermingham. “If we know the answer, we can better constrain when and how life developed.”

Her research suggests that water was not present during the early phases of Earth’s formation, as some theories propose. Instead, the planet acquired it much later through a process known as late accretion, when additional material from space continued to shape Earth.

Examining the building blocks of planets

Bermingham specializes in cosmogeochemistry, a field that examines the chemical composition of planetary materials to understand their origins. By analyzing Earth rocks and extraterrestrial materials such as meteorites, she investigates how rocky planets, including Earth, formed and evolved.

In this study, Bermingham’s team used thermal ionization mass spectrometry, a precise method for measuring isotopic compositions. They developed a new analytical approach to examine isotopes of molybdenum, an element found in both Earth’s crust and meteorites.

Isotopes are variations of an element with the same number of protons but different numbers of neutrons, which gives them slightly different atomic masses while maintaining identical chemical properties.

“The molybdenum isotopic composition of Earth rocks provides us with a special window into events occurring around the time of Earth’s final core formation, when the last 10% to 20% of material was being assembled by the planet. This period is thought to coincide with the Moon’s formation,” explained Bermingham.

Comparing meteorites and Earth rocks

To track Earth’s early chemical history, the team examined meteorite samples from the National Museum of Natural History at the Smithsonian Institution. Scientists classify meteorites into two main groups based on their place of origin in the solar system.

The first group, known as “CC” meteorites, likely formed in the outer solar system, where conditions were wetter. The second group, “NC” meteorites, originated in the inner solar system, where conditions were much drier.

This study focused on meteorites from the NC group to compare their chemical composition with Earth rocks.

The researchers analyzed molybdenum isotopes in rocks from Greenland, South Africa, Canada, the United States, and Japan. These rocks date back to the time of the Moon’s formation and are believed to contain elements added to Earth during this period.

By comparing the isotopic signatures of meteorites and Earth rocks, the team sought to determine whether the Moon-forming event delivered a significant amount of water to Earth.

Earth’s water may have arrived late

“Once we gathered the different samples and measured their isotopic compositions, we compared the meteorites’ signatures with the rock signatures to see if there was a similarity or a difference,” Bermingham said. “And from there, we drew inferences.”

The analysis revealed that the molybdenum composition of Earth rocks closely matched that of NC meteorites, which originated in the drier inner solar system.

This finding suggests that Earth’s building blocks primarily came from a region of the solar system where water was scarce.

This contradicts a popular theory that a significant amount of Earth’s water arrived during the Moon-forming event. Instead, the study indicates that water was delivered in smaller amounts long after the Moon formed, during the later stages of planetary development.

Rethinking the source of Earth’s water

“We have to figure out from where in our solar system Earth’s building blocks – the dust and the gas – came and around when that happened,” Bermingham said. “That’s the information needed to understand when the stage was set for life to begin.”

If most of Earth’s original material came from the drier inner solar system, then the Moon-forming event was likely not a major contributor of water. Instead, water must have arrived in smaller portions later.

This conclusion challenges the idea that a single event provided Earth with the majority of its water.

The study supports the view that water accumulated gradually over time, arriving through a combination of asteroid impacts and other processes occurring well after the planet had taken shape.

Questions about life’s early conditions

“Our results suggest that the Moon-forming event was not a major supplier of water, unlike what has been thought previously,” Bermingham said. “These findings, however, permit a small amount of water to be added after final core formation, during what is called late accretion.”

This discovery raises new questions about how long it took for Earth to become habitable. If water arrived later than previously thought, then life may have taken longer to emerge. This could also have implications for understanding other planets, particularly those that scientists consider candidates for life.

By refining the timeline of Earth’s water history, this study contributes to a broader effort to understand planetary evolution. If water arrived late on Earth, then other planets might have followed a similar pattern.

This could mean that life-supporting conditions take longer to develop than scientists once assumed.

What’s next in the search for Earth’s water?

The study represents a step forward in unraveling Earth’s past, but it also highlights the many unanswered questions about the origins of water and life.

Future research will likely focus on refining models of planetary accretion, analyzing additional meteorite samples, and exploring other isotopic markers that could provide further insights.

Other Rutgers contributors to the study include Linda Godfrey, an assistant research professor, and Hope Tornebene, a laboratory researcher in the Department of Earth and Planetary Sciences.

The research continues to push the boundaries of planetary science, bringing us closer to understanding how Earth became a world capable of supporting life.

The study is published in the journal Geochimica et Cosmochimica Acta.

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