The Rosetta spacecraft has determined that the water vapor on Comet 67P Churyumov-Gerasimenko doesn’t match that found on Earth, other Jupiter-family comets, or even any known Oort Cloud comet.
The European Space Agency’s (ESA) Rosetta spacecraft has found the water vapor from its target comet to be significantly different from 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 August 6. 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?
First measurements of comet's water ratio. (Spacecraft: ESA/ATG medialab)
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 “flavor,” 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 objects 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 harbor material left over from the protoplanetary disk 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 the 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 toward the inner solar system where their orbits become controlled by the gravitational influence of Jupiter.
Indeed, Rosetta’s comet now travels around the Sun between the orbits of Earth and Mars at its closest and just beyond Jupiter at its furthest, with a period of about 6.5 years.
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.
“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,” said Matt Taylor from ESA. “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.”
Read more: astronomy.com