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New Scientist: Although comets are partly composed of water ice and periodically visit the inner solar system, it is unlikely that Earth got its water from such a source, according to a new study. Data from the European Space Agency’s Rosetta spacecraft, which landed a probe on the surface of comet 67P/Churyumov–Gerasimenko last month, reveal that 67P’s water has about three times as much deuterium as water on Earth. However, the data may be incomplete, say the researchers: It is possible that pockets of water on the comet have different deuterium-to-hydrogen ratios or that the two halves of the odd-shaped comet have different D/H ratios, which would indicate that the comet is actually composed of two distinct bodies that collided long ago. "In the end, Earth's oceans are probably a mix of many things," says Kathrin Altwegg of the University of Bern and lead author of the paper.
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Comet 67P’s water differs from that of Earth's Free
11 December 2014