The deuterium-to-hydrogen ratio, noted as D/H, is difficult to determine, but provides direct information about the water in which the measurement is made. The Giotto probe has estimated this ratio in three Oort cloud comets as well as in Halley’s Comet. These four comets present the same D/H ratio value, which is twice as high as that observed on Earth. Coupled with other independent measures (such as the number of lunar craters), this result indicates a maximum cometary contribution of 10% to the Earth’s water. However, because the D/H ratio measurement was only conducted on a particular family of comets, it remains possible that a second family, from the Kuiper belt, may have contributed more to our oceans. In the future, the Herschel satellite, which was launched in May 2009, should be able to conduct the first D/H ratio measurements in comets in the Kuiper belt, which are more difficult to observe than those in the Oort cloud.

“The results of this programme have already provided a collection of nitrogen isotopic ratios (15N/14N) measured using the CN molecule in twenty or so comets from the Kuiper belt as well as the Oort cloud," says the Liegeois astronomer. “These ratios show the same value in all comets, regardless of their distance from the Sun, but this value is twice as high as that observed on Earth. This result supports other calculations of nitrogen isotopic ratios made using millimetric measures in the HCN molecule.”

But what is the nitrogen-to-water ratio? The isotopic ratio of nitrogen directly limits the contribution that comets can have made not only to the water of the oceans but also to the Earth’s atmosphere: the VLT measurements confirm that the contribution which cometary nitrogen makes to the Earth’s atmosphere cannot be more than 40%. This result indirectly limits the contribution which comets can have made to the water in the oceans. Indeed, comets are made up of water and nitrogen, in estimated proportions. Thus, if the observations show that they cannot have contributed significantly to terrestrial nitrogen, the same goes for the primitive oceans: a cometary contribution of 40% to atmospheric nitrogen on the Earth corresponds to a cometary contribution of, at the most, a few percent of the oceans’ water.