NEW DELHI (TIP): First results of studies done by the comet-chaser spacecraft Rosetta of its target comet have dealt a blow to the popular theory that water was brought to Earth by comets. Data from the ROSINA instrument aboard the European Space Agency’s spacecraft that had a spectacular rendezvous with comet 67P/Churyumov-Gerasimenko last month shows that the composition of its water vapor is significantly different from that found on Earth. When the Earth formed some 4.6 billion years ago, it was so hot that all the water on it would have vaporized. Yet today, two-thirds of the surface of Earth is covered by water.

Researchers agree that it must have been delivered to Earth by small bodies at a later stage of the planet’s evolution. It is, however, not clear which family of small bodies is responsible. There are three possibilities: asteroid-like small bodies from the region of Jupiter; Oort cloud comets formed inside of Neptune’s orbit; and Kuiper Belt comets formed outside of Neptune’s orbit. The key to determining where the water originated is in its isotopic “flavor.” That is, by measuring the level of deuterium – a heavier form of hydrogen.

By comparing the ratio of deuterium to hydrogen in different objects, scientists can identify where in the solar system that object originated. And by comparing the D/H ratio, in Earth’s oceans with that in other bodies, scientists can aim to identify the origin of our water. The ROSINA instrument on Rosetta has found that the value for the D/H ratio on the comet is more than three times the terrestrial value. This is among the highest-ever-measured values in the solar system.

That means it is very unlikely that comets like 67P/Churyumov-Gerasimenko are responsible for the terrestrial water. “We knew that Rosetta’s in situ analysis of this comet was always going to throw us surprises,” said Matt Taylor, Rosetta’s project scientist from the European Space Research and Technology Center, Noordwijk, the Netherlands. “The bigger picture of solar-system science, and this outstanding observation, certainly fuel the debate as to where Earth got its water.” In 1986, the European Giotto mission to comet Halley had found the D/H ratio in a comet for the first time.

It turned out to be twice the terrestrial ratio. The conclusion at that time was that Oort cloud comets, of which Halley is a member, cannot be the responsible reservoir for our water. Several other Oort cloud comets were measured in the next 20 years, all displaying very similar D/H values compared to Halley. Subsequently, models that had comets as the origin of the terrestrial water became less popular. This changed when, thanks to the European Space Agency’s Herschel spacecraft, the D/H ratio was determined in comet Hartley 2, which is believed to be a Kuiper Belt comet. The D/H ratio found was very close to our terrestrial value — which was not really expected.

Most models on the early solar system claim that Kuiper Belt comets should have an even higher D/H ratio than Oort cloud comets because Kuiper Belt objects formed in a colder region than Oort cloud comets. The new findings of the Rosetta mission make it more likely that Earth got its water from asteroid-like bodies closer to our orbit and/or that Earth could actually preserve at least some of its original water in minerals and at the poles. “Our finding also disqualifies the idea that Jupiter family comets contain solely Earth ocean-like water,” said Kathrin Altwegg, principal investigator for the ROSINA instrument from the University of Bern, Switzerland, and lead author of the Science paper. “It supports models that include asteroids as the main delivery mechanism for Earth’s oceans.”