From: European Space Agency
Published January 11, 2015 08:00 AM

Why is the water found on comet Comet 67P/Churyumov–Gerasimenko so different from Earth water?

ESA’s Rosetta spacecraft has found the water vapour from its target comet to be significantly different to that found on Earth. The discovery fuels the debate on the origin of our planet’s oceans.

The measurements were made in the month following the spacecraft’s arrival at Comet 67P/Churyumov–Gerasimenko on 6 August. It is one of the most anticipated early results of the mission, because the origin of Earth’s water is still an open question.

One of the leading hypotheses on Earth’s formation is that it was so hot when it formed 4.6 billion years ago that any original water content should have boiled off. But, today, two thirds of the surface is covered in water, so where did it come from?

In this scenario, it should have been delivered after our planet had cooled down, most likely from collisions with comets and asteroids. The relative contribution of each class of object to our planet’s water supply is, however, still debated.

The key to determining where the water originated is in its ‘flavour’, in this case the proportion of deuterium – a form of hydrogen with an additional neutron – to normal hydrogen.

This proportion is an important indicator of the formation and early evolution of the Solar System, with theoretical simulations showing that it should change with distance from the Sun and with time in the first few million years.

One key goal is to compare the value for different kinds of object with that measured for Earth’s oceans, in order to determine how much each type of object may have contributed to Earth’s water.

Comets in particular are unique tools for probing the early Solar System: they harbour material left over from the protoplanetary disc out of which the planets formed, and therefore should reflect the primordial composition of their places of origin.

But thanks to the dynamics of the early Solar System, this is not a straightforward process. Long-period comets that hail from the distant Oort cloud originally formed in Uranus–Neptune region, far enough from the Sun that water ice could survive.

They were later scattered to the Solar System’s far outer reaches as a result of gravitational interactions with the gas giant planets as they settled in their orbits.

Graphic shows measurements of water ratio of comet vs other sources.  Credit ESA.

Read more at European Space Agency.

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