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Posted: Dec 11, 2014
Rosetta findings fuel debate on origin of Earth's water
(Nanowerk News) First measurements by the Rosetta mission's ROSINA instrument show that the deuterium-to-hydrogen (D/H) ratio of the comet 67P/Churyumov-Gerasimenko is highly enriched in deuterium. These results contradict the theory that the water present in the Earth's atmosphere and oceans has a cometary origin. They also indicate that 67P comets do not all come from one region, the Kuiper Belt: some may have originated in the Oort Cloud.
Values of D/H ratios in different objects in the Solar System, grouped together by color: planets and satellites (blue), chondrites in the asteroid belt (grey), comets from the Oort Cloud (violet), and Jupiter family comets (pink). The comet 67P/Churyumov-Gerasimenko (yellow) has a different D/H ratio from the other comets in the same family.
Born 4.55 billion years ago, the various bodies that make up the Solar System-the Earth and planets, asteroids and comets-originally formed from the same cloud of gas and dust, the protosolar nebula. From this common origin they have developed in different ways, depending on their orbit and therefore on their exposure to solar radiation. Comets, which have been at great distances from the Sun for most of their lives, have hardly changed at all, and are thus privileged witnesses of the conditions that prevailed at the birth of the Solar System. The isotopic compositions of their principal constituents are therefore likely to provide unique information about the conditions and processes underlying the formation of the Solar System, and especially about the origin of the Earth's water.
The D/H ratio is a key marker for determining the origin of the Earth's water and understanding the role that the comets and/or asteroids may have played. The detailed study of the first spectra obtained by the ROSINA instrument since it arrived in the neighborhood of the comet gives a value for the D/H ratio of 5.3 ± 0.7 10-4, whereas its value on Earth is 1.55 10-4. This ratio, highly enriched in deuterium in comparison with the Earth, therefore conflicts with the assumption that the water present in the Earth's atmosphere and oceans has a cometary origin, contrary to what other results from Jupiter family comets suggested1. Since the value of the terrestrial D/H ratio falls within the range of the D/H ratio of asteroids located between Mars and Jupiter, the water in the Earth's oceans may have come mainly from asteroids and/or certain comets. In addition, cometary reservoirs are located at considerable distances from the Sun: the Oort Cloud, for instance, stretches out to over 105 AU2, and is the source of long-period comets such as Halley. The Kuiper Belt, located at a distance of over 50 AU, is known to be the origin of the comets of the 67P/Churyumov-Gerasimenko family, known as Jupiter family comets since the distant part of their orbits are in the neighborhood of Jupiter's orbit. According to ROSINA's new results, the comets in this family may not all come from a single source region, the Kuiper Belt: some may have originated in the Oort Cloud.
The researchers now aim to continue elucidating the chemical and isotopic composition of the atmosphere of 67P/Churyumov-Gerasimenko's, especially for gaseous species other than hydrogen, such as noble gases and nitrogen. These measurements should make it possible to undertake a detailed exploration of the origin of the volatile elements on Earth that allowed the emergence of life.
 For instance, comet 103P/Hartley 2, whose D/H ratio is similar to that of the water in the Earth's oceans.
 1 AU (astronomical unit) = the average Sun-Earth distance, approximately 150 million kilometers.