Where did our planet get its oceans? Among planetary scientists, this is one of the most important and perplexing questions about the origins of Earth. One popular theory holds that water was brought to Earth by the ancient impacts of comets and asteroids. However, new data from the European Space Agency's Rosetta spacecraft indicate that terrestrial water did not come from comets like 67P/Churyumov-Gerasimenko. The findings were published Dec. 10th in the journal Science.

Researchers agree that water 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 Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument has found that the composition of comet 67P/Churyumov-Gerasimenko's water vapor is significantly different from that found on Earth.

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.

The D/H ratio is the ratio of a heavier hydrogen isotope, called deuterium, to the most common hydrogen isotope. It can provide a signature for comparison across different stages of a planet's history.

"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."

Almost 30 years ago (1986) the mass spectrometers on board the European Giotto mission to comet Halley could, for the first time, determine D/H ratio in a comet. 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."

Comets are time capsules containing primitive material left over from the epoch when the sun and its planets formed. Rosetta's lander obtained the first images taken from a comet's surface and will provide analysis of the comet's possible primordial composition. Rosetta will be the first spacecraft to witness at close proximity how a comet changes as it is subjected to the increasing intensity of the sun's radiation.

Observations will help scientists learn more about the origin and evolution of our solar system and the role comets may have played in seeding Earth with water, and perhaps even life.