Rosetta reignites debate on the origin of Earth’s water
The European Space Agency’s (ESA) comet-chasing spacecraft, Rosetta, is on a mission of discovery, aiming to unlock the secrets of the early solar system. After deploying its Philae lander on Nov. 12, Rosetta has continued to orbit and analyze Comet 67P/Churyumov-Gerasimenko. Scientists have long hypothesized that comets were responsible for supplying the infant Earth with water, and ultimately life. This week ESA announced that water vapor collected from the comet varies significantly from the water we find on Earth, thus reigniting the debate on where Earth’s water came from.
The Earth formed over 4.6 billion years ago, and in its infancy was a much hotter world than we know today. As such, any surface water present would have boiled off by the searing heat of an early Earth. Today, our home planet is very different, with two-thirds of its surface covered by water. So how did that water get here?
One hypothesis is that the water was delivered to Earth after the planet significantly cooled, further on in its development. The delivery method was most likely via comet or asteroid collision; however, this hypothesis is strongly debated. How can we tell where water came from? By analyzing the water and determining its “flavor” or composition, researchers can tell the proportion and variety of hydrogen isotope present, thus determining its origin. In the case of comet 67P’s water, Rosetta’s analysis discovered the water contained a different ratio of hydrogen to deuterium — a form of hydrogen with an additional neutron.
“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 ratio plays an integral role in the formation and evolution of the early solar system, and could change depending on how far the water was from the Sun within the first few million years of development. After analyzing different water sources from various cosmic bodies, like comets and asteroids, and comparing the flavors to that of water samples from Earth’s oceans in order to determine how any of the objects have contributed to Earth’s water.
Comets are cosmic time capsules, harboring protoplanetary material left over from the early days of planet formation. These icy bodies form in various regions of the solar system and contain traces of material from where they were forged. However, solar system dynamics does make the origin easy to discern.
Long-period comets can originate out in far reaches of the solar system in a region known as the Oort Cloud, and typically form in the region around Uranus and Neptune. This area is far enough away from the Sun, that water ice would be present. So how do they get to the Oort Cloud? As the outer planets settled into their orbits, gravitational interactions between the planets scattered the comets to the outer solar system.
On the other hand, Rosetta’s comet, belongs to the Jupiter-family of comets and are thought to have formed way out, beyond Neptune, in a region known as the Kuiper Belt. From time to time, the orbits of these comets are disturbed and as they travel through the solar system, they are captured by Jupiter’s massive gravitational pull.
Scientists have measured the ratio of deuterium to hydrogen (D/H) in 11 different comets, thought to originate in different regions of the solar system, only the Jupiter-family of comets, like Comet 103P/Hartley 2, was observed to contain the same D/H ratio as the Earth’s oceans. Meteorites within the main asteroid belt between Mars and Jupiter were also studied and determined to be a match to Earth’s oceans. Asteroids and meteorites tend to have a significantly lower water content than comets, but could still contribute to the presence of water on Earth.
“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.
Rosetta’s instrument Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, or ROSINA, determined the D/H ratio of comet 67P is three times higher than that of Earth’s oceans, higher than the Jupiter-family comets, and higher than any Oort cloud comet.
“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,” Atlwegg said.
Over the coming months, Rosetta will continue to follow along with comet 67P, taking measurements and beaming data back to us on Earth. The operations team will continue to monitor how the comet evolves and behaves as it approaches the Sun.
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