Water was essential for the formation and development of life on Earth and remains crucial today. Although some water likely existed in the gas and dust that formed Earth 4.6 billion years ago, much of it would have vaporized due to the Sun’s intense heat.
The origin of Earth’s abundant liquid water is still debated. Research suggests some water came from volcanic vapor that condensed and rained down. At the same time, evidence also points to ice and minerals on asteroids and possibly comets that collided with Earth during intense impacts around 4 billion years ago.
While asteroid water’s link to Earth is strong, the role of comets remains uncertain, despite studies showing similarities between the water in Jupiter-family comets and Earth’s.
Researchers have discovered that the water on Comet 67P/Churyumov–Gerasimenko shares a similar molecular signature with Earth’s ocean water. This finding challenges recent conclusions and reignites the possibility that Jupiter-family comets, like 67P, may have played a role in delivering water to Earth.
This connection is identified through a key molecular marker that scientists use to trace the origins of water in the solar system: the ratio of deuterium (D) to regular hydrogen (H) in an object’s water. Deuterium, a heavier and rarer isotope of hydrogen, provides insights into where the object formed. By comparing this ratio in comets, asteroids, and Earth’s water, scientists can assess potential links between them.
Water with higher deuterium concentrations typically forms in colder environments, so objects farther from the Sun, like comets, tend to have more deuterium than those closer, like asteroids.
In recent decades, measurements of deuterium in the water vapor of several Jupiter-family comets have shown levels similar to those found in Earth’s water.
“It began to seem like these comets were crucial in delivering water to Earth,” said Kathleen Mandt, a planetary scientist at NASA‘s Goddard Space Flight Center in Greenbelt, Maryland. Mandt led the research, which updates the deuterium abundance in comet 67P.
In 2014, ESA’s Rosetta mission to comet 67P challenged the idea that Jupiter-family comets contributed to Earth’s water. Analysis of Rosetta’s water data revealed that 67P had the highest deuterium concentration of any comet, with about three times the amount found in Earth’s oceans, where there is approximately one deuterium atom for every 6,420 hydrogen atoms. This unexpected finding led scientists to reevaluate their previous assumptions.
In response, Mandt’s team used advanced statistical computation methods to automate the process of isolating deuterium-rich water from over 16,000 Rosetta measurements, which were taken from the gas and dust coma surrounding 67P. In collaboration with Rosetta mission scientists, Mandt’s team became the first to analyze the entire set of water measurements collected during the mission.
The researchers sought to understand the physical processes behind the variability in hydrogen isotope ratios observed in comets. Laboratory experiments and observations suggested that cometary dust might affect the hydrogen isotope ratios detected in a comet’s vapor, potentially altering our understanding of the origin of cometary water and its comparison to Earth‘s water.
“So I was just curious if we could find evidence for that happening at 67P,” said Mandt. “And this is one of those rare cases where you propose a hypothesis and find it happening.”
Mandt’s team found a clear connection between deuterium measurements in the coma of 67P and the amount of dust surrounding the Rosetta spacecraft. This indicated that measurements taken near the spacecraft in certain parts of the coma may not accurately reflect the composition of the comet’s body.
As a comet moves closer to the Sun, its surface heats up, causing gases to be released, along with dust that contains water ice. Research indicates that deuterium-rich water adheres more readily to dust grains than regular water. When the ice on these dust grains is released into the coma, it could make the comet appear to have more deuterium than it does.
Mandt and her team reported that dust reaches the outer part of the coma, at least 75 miles from the comet’s body, and has dried out. With the deuterium-rich water gone, spacecraft can accurately measure the deuterium coming directly from the comet’s body.
The paper’s authors suggest that this finding has significant implications, not only for understanding the role of comets in delivering water to Earth but also for interpreting comet observations that provide insight into the formation of the early solar system.
“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 said.
Journal Reference:
- Kathleen Mandt, Jacob Lustig-Yaeger, et al. A nearly terrestrial D/H for comet 67P/Churyumov-Gerasimenko. Science Advances. DOI: 10.1126/sciadv.adp2191