NASA science team links comet water to Earth's oceans

Where does water in Earth's oceans come from? It is an open question that has scientists constantly researching, and comets may have part of the answer.

This image, taken by ESA's Rosetta navigation camera, was taken about 85 kilometers from the center of comet 67P/Churyumov-Gerasimenko on March 14, 2015. The image resolution is 7.3 meters per pixel and It is cropped and processed to highlight the details of the comet's activity. Credit: ESA/Rosetta/NAVCAM

Researchers have discovered that the water from comet 67P/Churyumov–Gerasimenko has a molecular signature similar to that of water in Earth's oceans, contradicting some recent results. This finding reopens the possibility that Jupiter family comets like 67P could have helped bring water to Earth.

Water was essential to the formation and flourishing of life on Earth and remains essential to life on Earth today. While some water likely existed in the gas and dust from which our planet materialized about 4.6 billion years ago, much of the water would have vaporized because the Earth formed near the intense heat of the sun. How the Earth eventually became rich in liquid water remains a source of debate for scientists.

Research has shown that some of the Earth's water originated from steam emitted by volcanoes, that vapour condensed and fell as rain on the oceans. But scientists have found evidence that a substantial part of our oceans came from ice and minerals from asteroids, and possibly comets that collided with Earth. A wave of comet and asteroid collisions with the solar system's inner planets 4 billion years ago would have made this possible.

Comets and Earth's water

Although the relationship between asteroid water and Earth's water is strong, the role of comets has baffled scientists. Several measurements of Jupiter-family comets (which contain primitive material from the early solar system and are thought to have formed beyond Saturn's orbit) showed a strong link between their water and that of Earth. This link was based on a key molecular signature that scientists use to trace the origin of water throughout the solar system.

This signal is the ratio of deuterium (D) and normal hydrogen (H) in the water of any object, and it gives scientists clues about where that object formed. Deuterium is a rare and heavier type (or isotope) of hydrogen. When compared to water on Earth, this ratio of hydrogen in comets and asteroids can reveal whether there is a connection.

Because deuterium-containing water is more likely to form in cold environments, there is a higher concentration of the isotope in objects that formed far from the Sun, such as comets, than in objects that formed closer to the Sun, such as asteroids.

Measurements over the past two decades of deuterium in the water vapor of several other Jupiter-family comets showed levels similar to those in water on Earth.

"It was starting to look like these comets played an important role in providing water to Earth," said Kathleen Mandt, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Mandt led the research, published in Science Advances on November 13, 2024, which revisits the abundance of deuterium in 67P.

But in 2014, ESA's Rosetta mission to 67P challenged the idea that Jupiter-family comets help fill Earth's water reserves. Scientists analysing Rosetta's water measurements found the highest concentration of deuterium of any comet, and about three times as much deuterium as there is in Earth's oceans, which have about one deuterium atom for every 6,420 hydrogen atoms.

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What are comets made of? It is one of the questions that ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko wanted to answer. Credit: NASA

"It was a big surprise and made us rethink everything," Mandt said..

Mandt's team decided to use an advanced statistical calculation technique to automate the laborious process of isolating deuterium-rich water from more than 16,000 Rosetta measurements. Rosetta made these measurements in the "coma" of gas and dust surrounding 67P. Mandt's team, which included Rosetta scientists, was the first to analyse all of the European mission's water measurements spanning the entire mission.

The researchers wanted to understand what physical processes caused the variability in the hydrogen isotope ratios measured in comets. Laboratory studies and comet observations showed that comet dust could affect readings of the proportion of hydrogen that scientists detect in comet vapor, which could change our understanding of where comet water comes from and how it compares with the water of the Earth.

"I was curious if we could find evidence of that happening on 67P," Mandt said. "And this is one of those very rare cases where a hypothesis is proposed and it is found to actually happen."

In fact, Mandt's team found a clear connection between measurements of deuterium in the 67P coma and the amount of dust around the Rosetta spacecraft, showing that measurements taken near the spacecraft in some parts of the coma may not be representative of the composition of the body of a comet.

As a comet approaches the Sun in its orbit, its surface heats up, causing surface gases to be released, including dust with chunks of water ice. Deuterium water sticks to dust grains more easily than regular water, research suggests. When ice from these dust grains is released into the coma, this effect could make the comet appear to have more deuterium than it does.

Mandt and his team reported that by the time the dust reaches the outer part of the coma, at least 120 kilometres from the body of the comet, it is already dry. Once the deuterium-rich water is gone, a space probe can precisely measure the amount of deuterium coming from the comet's body.

This finding, say the authors of the article, has great implications not only for understanding the role of comets in transporting water to Earth, but also for understanding observations of comets that provide information about the formation of the early solar system.

Mandt concludes "This means there is a great opportunity to review our past observations and prepare for future ones so we can better take into account the effects of dust."

Source: NASA

Reference

A nearly terrestrial D/H for comet 67P/Churyumov-Gerasimenko, 13th November 2024, Kathleen E. Mandt et al