Comets Not Source of Water on Earth, Mission Suggests
Findings from Europe’s Rosetta spacecraft undercut the theory that most of the Earth’s water was brought here by comets, lending new credence to the alternate view that the oceans were seeded by asteroids.
Ever since Rosetta entered into close orbit around a comet called 67P/Churyumov-Gerasimenko in August, it has been analyzing the chemical signature, or makeup, of the comet’s gases. One goal was to check whether the comet’s water was the same as that found on our planet. If it was, it would support the view that icy comets crashed into the early Earth and brought water for the first time. But the latest study shows that the chemical signature of water on comet 67P is unlike that on Earth. As 67P is believed to be typical of many comets that originate in the far reaches of the solar system, the findings undermine the view that comets were the main source of water for Earth’s oceans.
Some water is made from regular hydrogen atoms, which, in turn, are made from one proton and one electron. But in other cases, the hydrogen is replaced by its heavier isotope called deuterium, which also includes a neutron. Rosetta’s instruments found that 67P’s water had a deuterium-hydrogen ratio that is three times that of Earth’s water.
“It’s not the same as terrestrial water—it’s much heavier,” said Kathrin Altwegg, physicist at the University of Bern, Switzerland, and lead author of the study, published Wednesday in the journal Science.
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