A scientific study published in October 2026 about the interstellar comet 3I/ATLAS brought back to the forefront a question that has puzzled scientists for decades: Where did the water that covers most of the Earth's surface come from? Did icy comets carry it to our planet at the beginning of its formation, or was its main source other bodies?
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The James Webb Space Telescope revealed that the methane gas emitted from the comet contains an exceptional proportion of deuterium, a form of hydrogen, about 13 times greater than its proportion in the methane of another comet studied by the European Rosetta spacecraft.
The story began in March 1986, when the European spacecraft Giotto approached the nucleus of Halley's Comet and studied the gases and dust emitted from it. Subsequent scientific analyses revealed that the comet's water contains about twice the amount of deuterium found in Earth's oceans.
Deuterium is a heavier form of hydrogen, and scientists use its proportion in water as a chemical fingerprint to help determine the conditions under which it formed. The discrepancy was puzzling; comets are icy bodies, and scientists long believed they might have delivered large quantities of water to Earth during its early stages of formation.
In 1986, Giotto's encounter with Comet Halley provided the first ever opportunity to take images of a comet nucleus. The images were obtained with the Halley Multicolour Camera on Giotto.
The nucleus of Halley's Comet as observed by the Giotto spacecraft in 1986, in a mission that established comparisons between cometary water and Earth's water (European Space Agency).
Doubts were reinforced in the 1990s when studies of comets Hyakutake and Hale-Bopp showed similarly high levels. In 1998, researchers concluded that the comets studied could not be the sole source of ocean water.
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