*Originally seen on Phys.org.*
Recently, the Rosetta space probe analyzed water from a Kuiper Belt comet theorized to have originally brought water to Earth billions of years ago. It was found to contain more of a hydrogen isotope called deuterium than water found on Earth, fueling the debate on the origin of our planet’s oceans.
About 97% of Earth’s surface water is found in our oceans, meaning that all of the water from lakes, rivers, mountains, glaciers, ice caps and groundwater only make up 3% of Earth’s surface water. But the Earth was once thought to be a dry planet some 4.5 billion years ago, as the oceans are thought to be only 3.8 billion years old.
The two current prevailing theories about the ocean’s formation state that 1) the Earth was formed with water and that this water came from the inside out, as well as 2) the Earth’s water was also brought here by other sources.
The current theory proposes that during the differentiation of Earth’s layers, (i.e., core, mantle, etc.), there were massive amounts of gases being released from volcanic activity, and while this was taking place the Earth was also thought to be bombarded by objects such as comets. These two processes are thought to have provided the Earth with its water.
But the comet aspect of this theory was recently displaced, according to water samples analyzed from the Rosetta comet 67P/C-G.
Different Type Of Water Found On Comets Compared To Earth Water
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, Phys reports.
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?
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.
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.
Conversely, Jupiter-family comets like Rosetta’s comet were thought to have formed further out, in the Kuiper Belt beyond Neptune. Occasionally these bodies are disrupted from this location and sent towards the inner Solar System, where their orbits become controlled by the gravitational influence of Jupiter.
Previous measurements of the deuterium/hydrogen (D/H) ratio in other comets have shown a wide range of values. Of the 11 comets for which measurements have been made, it is only the Jupiter-family Comet 103P/Hartley 2 that was found to match the composition of Earth’s water, in observations made by ESA’s Herschel mission in 2011.
By contrast, meteorites originally hailing from asteroids in the Asteroid Belt also match the composition of Earth’s water. Thus, despite the fact that asteroids have a much lower overall water content, impacts by a large number of them could still have resulted in Earth’s oceans.
It is against this backdrop that Rosetta’s investigations are important. Interestingly, the D/H ratio measured by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, or ROSINA, is more than three times greater than for Earth’s oceans and for its Jupiter-family companion, Comet Hartley 2. Indeed, it is even higher than measured for any Oort cloud comet as well.
“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.
“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.”
“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.
“As Rosetta continues to follow the comet on its orbit around the Sun throughout next year, we’ll be keeping a close watch on how it evolves and behaves, which will give us unique insight into the mysterious world of comets and their contribution to our understanding of the evolution of the Solar System.”
What are your thoughts on this discovery? Do you think extraplanetary sources helped to provide our planet’s oceans? Share with us below!
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