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Most of the water that sustains life on Earth probably came from asteroids rather than comets. That is the conclusion of scientists working on the Rosetta space mission, who have measured the levels of hydrogen isotopes in the comet 67P/Churyumov–Gerasimenko. The ratio of deuterium to hydrogen in the comet is much greater than the ratio found on Earth, which suggests that comets supplied Earth with only a small fraction of its water.

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