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

Chuck Bednar for – 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 ESA’s 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 comet’s 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 planet’s 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 Neptune’s 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 Neptune’s 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 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 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 planet’s water. Several other Oort cloud readings have produced similar D/H ration readings.
However, the European Space Agency’s Herschel spacecraft later discovered that the D/H ratio of comet Hartley 2 (believed to be a Kuiper Belt comet) was similar to terrestrial values, according to NASA. The results were unexpected, as most models of the early solar system suggested that Kuiper Belt comets should have an even higher D/H ratio than Oort cloud comets since they formed in a region that was colder.
“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,” the agency said. Altwegg added that the study “disqualifies the idea that Jupiter family comets contain solely Earth ocean-like water” and “supports models that include asteroids as the main delivery mechanism” of terrestrial water.
However, Open University Professor of Planetary and Space Science Monica Grady said that the conclusions of the paper could be “jumping the gun a bit,” telling BBC News, “The measurements that have been made by Rosina are of the gas that has come from the surface of the comet.”
“The amount of hydrogen relative to deuterium changes as the gas escapes from the surface,” she added. “This is why other instruments on the lander were going to make complementary measurements of the ice on the surface. We are going to have to wait to see what comes from the [Philae lander instruments] COSAC and Ptolemy before we can say any more.”
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