Jupiter-family comets, including Comet 67P, may have contributed more to Earth's water supply than previously thought.
A groundbreaking reanalysis of data collected nearly a decade ago from Comet 67P/Churyumov-Gerasimenko suggests comets may have contributed significantly to Earth’s water supply. This challenges the prevailing belief that asteroids, rather than comets, were the primary carriers of water to our planet.
Water, a molecule essential for life, is surprisingly simple in its structure: two hydrogen atoms bonded to one oxygen atom. Despite this simplicity, the origins of Earth’s vast water reserves—an estimated million trillion tons—have puzzled scientists for decades.
Theories about how Earth acquired its water fall into two camps. One suggests Earth’s geological processes may have created small amounts, but the majority likely came from extraterrestrial sources, delivered during violent impacts with comets or asteroids. Deciphering the main contributors requires examining a unique chemical fingerprint: the ratio of deuterium (a heavy isotope of hydrogen) to regular hydrogen, known as the D/H ratio.
The D/H Ratio: A Cosmic Clue
The D/H ratio acts as a thermometer for where water ice formed in the early solar system. Water ice with a lower D/H ratio typically forms farther from the Sun. For decades, scientists have observed that Earth’s D/H ratio closely matches that of many asteroids and some comets, specifically a group known as Jupiter-family comets, which are influenced by Jupiter’s gravity and pass the Sun every 20 years or so.
However, a 2015 study on Comet 67P challenged this narrative. Data from the European Space Agency’s (ESA) Rosetta spacecraft revealed a D/H ratio roughly three times higher than Earth’s. This discrepancy seemed to exclude comets like 67P from playing a significant role in delivering Earth’s water.
A Closer Look at Comet 67P’s “Rubber Ducky” Shape
The unusual shape of Comet 67P, resembling a rubber duck, provided clues to its behavior. As the comet approaches the Sun, its surface warms, releasing gas and ice-coated dust particles into its coma, the glowing halo surrounding its rocky core. Earlier lab studies indicated that deuterium-rich ice tends to adhere more strongly to dust grains than regular ice, potentially skewing D/H measurements taken from the coma.
In a recent study published in Science Advances, researchers, including Kathleen Mandt from NASA, analyzed 4,000 individual D/H measurements from the Rosetta dataset. They found that the D/H ratio varied significantly along the comet’s axis, with higher ratios near its core and lower ratios farther along its tail. Importantly, particles measured farther from the nucleus, about 120 kilometers away, were largely devoid of deuterium-rich ice. By focusing on these measurements, the team recalculated Comet 67P’s D/H ratio to be just 1.5 times that of Earth’s—a figure much closer to our planet’s water signature.
These findings suggest that Jupiter-family comets, including Comet 67P, may have contributed more to Earth’s water supply than previously thought. “All Jupiter-family comets that we’ve been able to measure now have D/H ratios closer to Earth’s water,” Mandt explained.
Interestingly, the revised D/H ratio also suggests that Comet 67P may have formed closer to the Sun than initially believed, reshaping our understanding of its origins.
While mysteries about Earth’s water origins remain, this study revives the idea that comets could have played a central role in shaping our planet’s habitability.
