For decades, scientists have debated the origins of Earth’s water. The dominant theories propose that water came from celestial bodies like asteroids or comets, delivered during repeated collisions in the planet’s formative years. While asteroids were considered the more likely source, new findings from the European Space Agency’s Rosetta mission to Comet 67P/Churyumov-Gerasimenko—the so-called “rubber ducky” comet—challenge this assumption.
Mystery of Earth’s Water
Water is one of Earth’s most abundant substances, with its oceans holding about a million trillion tonnes of the liquid. Its molecular composition is simple: two hydrogen atoms bonded with one oxygen atom. However, the question of how water arrived on Earth remains an enigma.
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Some scientists argue that a small fraction of Earth’s water could have originated from geological processes within the planet itself. However, the vast majority, they believe, must have been delivered by external celestial bodies, primarily comets or asteroids, during Earth’s violent early history.
What is the D/H Ratio?
The debate hinges on a specific chemical signature found in water: the deuterium-to-hydrogen ratio (D/H). Deuterium, a heavy isotope of hydrogen with an additional neutron, is less common than its lighter counterpart. The D/H ratio provides insights into where and under what conditions water formed in the solar system.
“The D/H in water tells us at what temperature the ice formed and, from that, how far a comet formed from the Sun,” explained Kathleen Mandt, a planetary scientist at NASA and corresponding author of a new study. Lower D/H values indicate ice formed farther from the Sun, while higher values suggest proximity to the Sun.
Comet 67P’s Initial Measurements and the Puzzle
The Rosetta spacecraft, launched by the ESA in 2004, spent two years orbiting Comet 67P, which is part of the Jupiter-family comets. This group of comets swings past the Sun roughly every 20 years, their trajectories influenced by Jupiter’s gravity.
In 2015, scientists analyzing Rosetta’s data determined that the D/H ratio of Comet 67P was three times higher than Earth’s. This stark discrepancy effectively ruled out comets as major contributors to Earth’s water supply.
However, the findings were puzzling. Comet 67P’s D/H value was significantly higher than that of other Jupiter-family comets, whose D/H ratios were closer to Earth’s. Additionally, researchers noted that Comet 67P had unexpectedly low levels of carbon monoxide and nitrogen, compounds that should have been abundant if it had formed at extremely cold temperatures.
Reanalyzing Rosetta’s Data
In a groundbreaking reanalysis, Mandt and an international team of researchers revisited the Rosetta data using an innovative statistical technique. Led by Jacob Lustig-Yaeger from the Johns Hopkins Applied Physics Laboratory, the team identified signals from deuterium-containing water molecules across 4,000 measurements, a significant improvement from the original 150 measurements.
The new analysis revealed that the D/H ratio varied widely along the comet’s axis. It was highest near the nucleus—where the comet resembles a rubber ducky—and lower along the tail. This variation could be explained by processes occurring as the comet neared the Sun.
Role of Dust Particles
As Comet 67P approaches the Sun, its surface warms, releasing gas and ice-coated dust particles into a glowing halo, or coma, around its nucleus. Laboratory studies have shown that deuterium-containing ice tends to stick more to dust grains than regular ice. When these dust grains enter the coma, they may generate spuriously high D/H values.
To eliminate this bias, researchers analyzed data only from dust particles located 75 miles (120 kilometers) from the nucleus. These particles had lost their deuterium-enriched ice. The results revealed that Comet 67P’s actual D/H ratio was only 1.5 times that of Earth—a much closer match.
Implications for Earth’s Water Origins
This revised D/H ratio suggests that Jupiter-family comets, including 67P, likely played a more significant role in delivering water to Earth than previously thought. “All Jupiter-family comets that we have been able to measure have a D/H closer to Earth’s water D/H,” Mandt confirmed.
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Furthermore, the findings indicate that Comet 67P was born closer to the Sun than scientists had assumed. This has profound implications for understanding the formation and migration of celestial bodies in the early solar system.