ESA's Rosetta mission has come up trumps again. One of its prime missions was to explore whether "Jupiter family" comets could have seeded the Earth with water. To do this, it measured the ration of "heavy water", where the hydrogen atom is replaced by deuterium, to ordinary H2O.
Results published today in Science from the Rosina mass spectrometers aboard Rosetta are definitive:
The provenance of water and organic compounds on the Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the D/H ratios in the reservoirs for comets and the Earth’s oceans. Here we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard ESA’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10−4, that is, ~3 times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean-like water.
It does not preclude that other objects could have rained down from space.
Prof Altwegg [of the University of Bern in Switzerland who leads on the Rosina data] believes that Kuiper Belt comets did not bring water to Earth.
She said: "The conclusion here is that in the reservoir of the Kuiper Belt, we have very diverse comets that probably came from different regions of the early Solar System.
"We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water."
Instead, she thinks that asteroids - dense, rocky objects that formed closer to the Sun than comets - seeded our oceans.
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