The spacecraft Rosetta has been circling a "dirty snowball" of a comet since late summer, and now the first scientific paper about this comet has been published. In Science today, scientists analyze the comet's water, giving some new clues on how water may have come to Earth.

Back in November, Rosetta's data showed a high ratio of deuterium to hydrogen on the comet, which this paper confirms. That suggests asteroids, rather than comets, may have been the source of Earth's water. You can read about why below. [Science]

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Top image: ESA