Rosetta Sheds New Light on Origin of Earth’s Oceans

One of the most important science goals of the Rosetta mission is to determine the conditions in the early solar system, as well as the element abundance and climate that led to the development of life on Earth.

By studying a comet that has been isolated in Space for 4.5 Billion years, we can learn about the formation of the Solar System and the origin of the raw materials for life on Earth.  The Earth was thought to be so warm when it formed that any water would have evaporated away into space.  If so, where did our vast oceans come from? The leading theory is that as the planet cooled, bombardment by other solar system bodies gradually saturated the Earth with water.  But which ones? Where did they come from and how did they form?

Now we have our first answers.  The ROSINA instrument took measurements of the composition of the water vapour present on the comet 67P/Churyumov-Gerasimenko during its first month of orbit in August/September 2014.  It measured the relative amount of heavy water to normal water, the Deuterium/Hydrogen ratio. Heavy water contains hydrogen atoms with an extra Neutron, and its abundance is a key indicator of where water has migrated through the Solar System.  This D/H ratio is thought to vary with distance from the Sun, and over the first few Million years of planet development.  If we can find objects with similar D/H ratios to that of Earth’s oceans, we can pinpoint where Earth’s water came from.

Credit: Copyright Spacecraft: ESA/ATG medialab; Comet: ESA/Rosetta/NavCam; Data: Altwegg et al. 2014

The findings are that Rosetta’s comet has more than three times the relative amount of Deuterium that Earth’s oceans have.  This leads us to the conclusion that Jupiter-family comets like 67P were likely formed in a different environment than Earth, somewhere else in the solar system.  This also adds more emphasis to the idea that Asteroids, not Comets, are responsible for Earth’s water.

The result also gives insight into the formation of Comets near Jupiter. “This surprising finding could indicate a diverse origin for the Jupiter-family comets — perhaps they formed over a wider range of distances in the young Solar System than we previously thought,” says Kathrin Altwegg, principal investigator for ROSINA and lead author of the paper reporting the results in the journal Science this week.

“We knew that Rosetta’s in situ analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science, and this outstanding observation certainly adds fuel to the debate about the origin of Earth’s water,” says Matt Taylor, ESA’s Rosetta project scientist.

This result is some of the first science done on the data from the Rosetta mission.  As more data trickles in we will learn about the comet and its formation in the early solar system, and how objects like it contributed, if at all, to the development of life on Earth.


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