Special delivery: New study suggests water came to early Earth via comets

Scientists uncovered new insights regarding the potential role of comets, particularly Comet 67P/Churyumov-Gerasimenko, in delivering water to early Earth. A new study led by Kathleen Mandt, a planetary scientist at NASA's Goddard Space Flight Center, has renewed focus on the role of Jupiter-family comets in Earth's water history, according to Phys.org.

By analyzing thousands of water measurements from Comet 67P, Mandt's team revisited previous findings from the European Space Agency's Rosetta mission. The Rosetta spacecraft, which rendezvoused with Comet 67P in August 2014, had earlier cast doubt on the idea that Jupiter-family comets contributed water to Earth. During its mission, Rosetta made measurements in the coma of gas and dust surrounding the comet and found a deuterium level three times higher than Earth's water—the highest level of any comet—suggesting a disconnect between cometary and terrestrial water.

However, Mandt's team applied advanced statistical techniques to analyze over 16,000 water measurements from the Rosetta mission to investigate deuterium levels in Comet 67P. They discovered that cometary dust may have skewed earlier readings of deuterium levels, which could lead to misleadingly high deuterium readings in the coma. According to Interesting Engineering, laboratory studies and previous observations indicate that dust around comets can affect deuterium readings, potentially skewing the results.

Their analysis revealed a direct correlation between deuterium levels and dust presence in Comet 67P's coma, suggesting that previous assessments may not accurately represent the comet's true water composition. Mandt emphasized the importance of reevaluating past observations and preparing for future studies to account for dust effects in cometary measurements. "This means there is a great opportunity to revisit our past observations and prepare for future ones so we can better account for the dust effects," Mandt stated, according to India Today.

By the time dust reaches the outer part of the coma, at least 75 miles from the comet body, it is dried out and has lost its deuterium-rich water, allowing for accurate measurement of the comet's true deuterium content. With the deuterium-rich water gone, a spacecraft can accurately measure the amount of deuterium coming from the comet body, according to Phys.org.

"I was just curious if we could find evidence for that happening at 67P. And this is just one of those very rare cases where you propose a hypothesis and actually find it happening," Mandt said, as reported by Newsweek. Her team found that the true deuterium levels of Comet 67P's water align more closely with Earth's, which revives the possibility that Jupiter-family comets have played a role in supplying water to Earth, contradicting previous studies that questioned this connection.

The discovery suggests a potential connection in water delivery to Earth, bolstering the hypothesis that comets contributed to Earth's oceans alongside asteroids. 

Over the past two decades, measurements of water vapor in several Jupiter-family comets have shown deuterium levels similar to those found in Earth's water, indicating a potential connection in the transfer of water to Earth. "It was really starting to look like these comets played a major role in delivering water to Earth," Mandt noted, according to Phys.org.

Deuterium is a rare, heavier type of hydrogen, and its concentration is higher in objects that formed far from the Sun, such as comets, compared to those that formed closer, like asteroids. The deuterium-to-hydrogen ratio offers insights into where these bodies formed in the solar system. Asteroids, made mostly of rock, have long been considered a primary source of Earth's water, and their deuterium-to-hydrogen ratio matches well with Earth's, supporting this idea.

However, the role of comets in supplying water to Earth remains uncertain. Comets that form in the cold outer solar system have higher deuterium levels compared to asteroids that are closer to the Sun. The connection between water present in asteroids and Earth's oceans is generally well-supported, but the role of comets in bringing this life-giving liquid to Earth has been questioned.

In 2014, Rosetta's analysis of Comet 67P surprised scientists by showing a deuterium level three times higher than Earth's water. "It was a big surprise and it made us rethink everything," Mandt said, according to Newsweek. This led scientists, including Mandt, to reconsider the idea that Jupiter-family comets have played a role in supplying water to Earth.

By analyzing Rosetta's full dataset, Mandt's team found that the measurements taken near the spacecraft in some parts of the coma may not be representative of the comet's true water composition. Research suggests that deuterium-rich water sticks to dust grains more readily than regular water does. Laboratory studies and previous observations indicate that dust around comets can affect deuterium readings, potentially skewing the results.

Mandt's team, which included Rosetta scientists, was the first to analyze all of the European mission's water measurements spanning the entire mission.

This article was written in collaboration with generative AI company Alchemiq