To date, the origin of water on the Earth remains a huge mystery. There are many different hypotheses, although they are beginning to point in one direction: the significant temperatures which must have governed the primordial Earth support hypotheses suggesting that the Earth’s oceans have an extraterrestrial origin and must have come into existence after the period when the Earth was formed. The impressive number of craters on the Moon’s surface indicates that many collisions did indeed occur in the primordial Solar System. Because comets are essentially blocks of ice, astronomers naturally turned to them in the first instance in order to understand the origins of the Earth's water. All the water in the oceans, or a part of it, may have been brought to Earth during a relatively short period of intense comet bombardment.

Comparison of isotopic ratio measurements of water from the Earth and from comets provides a means of testing this hypothesis. The value of an isotopic ratio is given by the relative abundance of two isotopes of the same chemical element. For example, the isotopic ratio of carbon, noted as 14C/12C, compares the chemical abundance of the rare carbon isotope (14C) to that of the common carbon isotope (12C) in this environment.

 “If a comet hit the surface of the earth, it would leave all its components behind including water which would be diluted, possibly resulting in the Earth's primitive water”, explains astronomer Damien Hutsemékers, FRS – FNRS senior researcher at the department of Astrophysics, Geophysics and Oceanology (AGO) at ULg.

Any primitive isotopic ratios of the Earth’s water would thus be changed and replaced with a mean ratio, the value of which would rest between the Earth’s value before the bombardment and the comet’s value. From this perspective, if comets were really the main source of the Earth's primitive oceans, we would expect to find similar isotopic ratios in the Earth's oceans today and in comets ... based on theories of their non-variability over time.”

Measuring isotopic ratios in comets requires very high resolution spectroscopy. It involves detecting rare isotope beams which are several hundred times less intense than common isotope beams. Until the 2000s, this operation remained fairly unpredictable: astronomers were reduced to waiting patiently for an extremely bright comet to move sufficiently close to the sun to make measurement possible, as was the case with Halley in 1986 or Hale-Bopp in 1996 and 1997. Another possibility was to send a space probe towards or onto a particular comet, as was the case with the impactor mounted on the Deep Impact probe which collided with the Tempel 1 comet in 2005. These occasions, however, can be singled out for their rarity...