The Earth is believed to be very hot when it was formed more than 4 billion years ago so its waters should have boiled off and escaped. However, the Earth today is mostly made up of water.

One theory proposes that after the planet has cooled down, waters were delivered by extraterrestrial sources when the Earth collided with comets and asteroids. Scientists, however, have not yet determined which object has returned our planet to its water state.

Data from the European Space Agency's Rosetta, which had been studying Comet 67P/C-G in close range since early this year, suggest that asteroid could have been the Earth's water boy.

Measurements made by instruments onboard the spacecraft a month after its Aug. 6 rendezvous with Comet 67P/C-G revealed that the water vapor from Rosetta's target comet is significantly different from what is found on Earth.

To determine the origins of water, scientists look at the proportion of deuterium, also known as heavy hydrogen because of its additional neutron, to normal hydrogen. Water on Earth has a unique signature. Three deuterium atoms can be found for every 10,000 water molecules on Earth.

Measurements made by Rosetta's ROSINA instrument revealed that the ratio of deuterium to hydrogen in Comet 67P/C-G is more than thrice greater than the Earth's oceans.

ROSINA, which stands for Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, is capable of determining the composition of comet Comet 67P/C-G's ionosphere and atmosphere. It can also measure the bulk velocity and temperature of its gas and ions.

"Here we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard ESA's Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10-4, that is, ~3 times the terrestrial value," wrote ROSINA principal investigator Kathrin Altwegg, and colleagues in their study published in the journal Science on Dec. 10.

Altwegg said that the findings, specifically the difference in the D/H ratio means that Comet 67P/C-G, a Jupiter family comet, is not the kind of comet that could have brought water to the Earth. Rosetta's data also suggest that asteroid could have been responsible for bringing water to our planet.

"This surprising finding could indicate a diverse origin for the Jupiter-family comets," Altwegg said. "Our finding also rules out the idea that Jupiter-family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth's oceans."

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