December 11, 2014

Image Caption: This four image mosaic comprises images taken with Rosettas NAVCAM on 2 December from a distance of 30.1 km from the centre of Comet 67P/Churyumov-Gerasimenko. The image resolution of the mosaic is about 3.1 m/pixel, and it has been cropped to measure 4.5 x 3.6 km. Credit: ESA/Rosetta/NAVCAM CC BY-SA IGO 3.0

Chuck Bednar for redOrbit.com Your Universe Online

Terrestrial water most likely did not come from comets like 67P/Churyumov-Gerasimenko, meaning that the H2O found on Earth was most likely brought here by asteroids, scientists involved with the ESAs Rosetta mission reported Wednesday in the journal Science.

The study, which was led by Kathrin Altwegg of the University of Bern in Switzerland using information provided by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument, measured the amount of deuterium (a heavier isotope of hydrogen) found in water vapor on the comets surface, said Dan Vergano of National Geographic.

While normal water contains regular hydrogen atoms, water that contains deuterium is known as heavy water, and Altwegg found that ice on the surface of 67P/C-G had a ratio of heavy water to normal water that is roughly three times that of the planets oceans. As a result, the researchers said that it is unlikely that terrestrial water came from Kuiper belt comets, as there would have been more deuterium-rich heavy water here on Earth.

Kuiper belt comets, which are formed outside of Neptunes orbit, have long been one of three entities believed to have been responsible for bringing water to the Earth during the later stages of its evolution, according to NASA. The other two likely sources are asteroid-like objects from the region of Jupiter or Oort cloud comets formed inside of Neptunes orbit, the US space agency added. The ROSINA data effectively eliminates Kuiper belt comets as a possible source.

This composite is a mosaic comprising four individual NAVCAM images taken from 19 miles (31 kilometers) from the center of comet 67P/Churyumov-Gerasimenko on Nov. 20, 2014. The image resolution is 10 feet (3 meters) per pixel. Credit: ESA/Rosetta/NAVCAM

We knew that Rosettas in situ analysis of this comet was always going to throw us surprises, said Matt Taylor, Rosettas project scientist from the European Space Research and Technology Center in the Netherlands. The bigger picture of solar-system science, and this outstanding observation, certainly fuel the debate as to where Earth got its water.

Nearly three decades ago, mass spectrometers on board the European Giotto mission to comet Halley were able to measure the ratio of deuterium to hydrogen (D/H ratio) in a comet. Those readings revealed a deuterium level twice that of Earth, concluding that Oort cloud comets such as Halley could not have been the original source of the planets water. Several other Oort cloud readings have produced similar D/H ration readings.

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Rosetta Data Suggests Comets Were Not The Source Of Earth's Water