This result confirms earlier estimates based on measurements of deuterium in four comets in the Oort cloud. Nitrogen isotopic ratio measurements nevertheless allow us to clarify and apply this initial conclusion to two types of comets, those from the Oort cloud, but also those from the Kuiper belt, and thus to reasonably exclude all comets as principle sources of water on the primitive Earth. But if the water in the oceans does not come from comets, how did it get to our planet? Meteorites are an interesting alternative to comets. However, they have the inconvenience of presenting identical isotopic ratios to that observed on the Earth, thus providing no parameters as to the proportion of water they may have brought to the Earth: all the oceans could come from meteorites, or only a tiny fraction could.

Before drawing any final conclusions, Damien Hutsemékers remains prudent, because any conclusions depend upon the initial conditions imposed upon the basic model: “Given the number of hypotheses underlying any attempt at explaining the origin of the oceans, it is important to cross-reference all attempts at an answer: no argument, taken in isolation, has the force of law in itself. This is why in the future we want to routinely measure the D/H isotopic ratio in a larger number of comets, using the UVES spectrograph installed at the VLT.”

In order to cross-reference independent results, ideally the Earth and that of Mars should be treated simultaneously, because if the primitive Earth was bombarded, Mars must also have been. The problem is that we do not have baseline isotopic ratio measurements for Mars in the same way as we have for the Earth. “Prudence is also required here,” continues the astronomer. “Even if we had these Martian isotopic measures, things would be far from simple. We cannot simply compare the Earth and Mars: Mars is much smaller than the Earth, and could have more easily escaped light isotopes than heavy ones, thus altering the isotopic ratios over time. On the Earth, this phenomenon plays a lesser role, as the planet’s mass is much bigger. Also, it is difficult to transpose valid arguments about the Earth to Mars. Once again, from whatever angle the problem is approached , independent results have to be put together in order to draw a conclusion, because of the large number of unknowns."
The study of isotopic ratios in comets and planets goes beyond the question of the origin of oceans and even of the Earth’s atmosphere: these can be used as tracers of the history of the Solar System, because comets are the most primitive objects in the Solar System. For example, we expect the D/H ratio in the developing Solar System to be related to the region we are looking at it from. Thus the D/H ratio measurement of a comet may be an indication of where it developed in the Solar System. Accumulating such measurements is a mine of information on the origin of the Solar System. Another example: the fact that the isotopic ratio of nitrogen measured in the comets differs from that of the Solar System means that isotopic fragmentation mechanisms must have occurred before the development of the Solar System, and hence before these isotopes were incorporated into comets, giving some insight into the chemistry of the interstellar environment before or at the time of the development of the Solar System. Finally, the comparison of isotopic ratios of comets and a planet gives some insight into part of the evolutionary history of the planet. The growing interest in isotopic ratio measurements in the solar system can thus be understood.

(1)  Hutsemékers D., Manfroid J., Jehin E., Arpigny C. New constraints on the delivery of cometary water and nitrogen to Earth from the 15N/14N isotopic ratio, Icarus 204 (2009), 346