The origin of Earth's water is an open question. The Earth should have formed from pretty dry materials, so at least some of the water comes from extraterrestrial sources. Comets were the first possible culprit considered, but observations suggested that it was not the case. Now, new work brings the comet back to the table. The answer to the question "did comets help make Earth habitable?" is a solid maybe. The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

Water on Earth is mostly your classic H 2 O: two parts regular hydrogen and one part oxygen. It also has a sprinkling of heavy water (HDO), where one of the hydrogen atoms has an extra neutron, making it the isotope deuterium. The Earth’s water has a very specific isotopic signature.

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