New measurements from the Herschel Space Observatory show that comet Hartley 2, contains water with the same chemical signature as water in Earth's oceans.
Credit: NASA/JPL-Caltech/R. Hurt
NASA EPOXI image of the nucleus of comet Hartley 2 with overlaid spectra of the 'normal' and 'heavy' water, as observed with the Heterodyne Instrument for the Far Infrared (HIFI) aboard the Herschel Space Observatory.
Credit: NASA/JPL-Caltech/R. Hurt
PORTLAND: We're one step closer to unlocking the mystery of where Earth's water came from, with new research suggesting it came from comets, not asteroids.
Debate over the source of the Earth's water has been raging for decades. In the past, scientists have suggested that comets were the main source, but were unable to prove it because they could find none with the same chemical signature as the water in Earth's oceans.
New measurements from the Herschel Space Observatory have now shown that comet Hartley 2, which comes from the distant Kuiper Belt, contains water with a matching chemical signature. The findings may help explain how Earth’s surface ended up covered in water.
"We determined for the first time the deuterium to hydrogen ratio in a comet from another area, the Jupiter family ... It turned out that its water composition was almost exactly the same [as] the one on Earth," said lead author Paul Hartogh from the Max Planck Institute for Solar System Research in Germany of the study published in Nature today.
Heavy water problem
Since comets consist mainly of water ice, scientists more than 50 years ago started to believe that comets delivered our water.
But in the 1980s, the European Space Agency sent the Giotto spacecraft to Halley's comet and analysed the isotopic composition of the water. What they found was that the ratio of deuterium - a heavy hydrogen - to hydrogen (D/H) wasn't compatible with the Earth's water, but contained twice as many deuterium atom. In semi-heavy water, deuterium replaces one of the two hydrogen atoms. "The ratio of HDO relative to H2O is some kind of fingerprint telling us where the water may come from," said Hartogh.
This finding seemed to discount the idea that comets were the main source of water on Earth and scientists came to the conclusion that, at most, 10% of the ocean's water came from comets.
Asteroids vs comets
Now, with the analysis of comet Hartley 2's water signature, scientists have to once again reconsider the role of comets in contributing to the early Earth's oceans.
Scientists believe that water was most likely delivered when the Earth was colliding with water-rich material not yet incorporated into planets.
According to Francis Nimmo, planetary scientist at the University of California in Santa Cruz who was not involved in the study, this material could have resembled either asteroids or comets. He said the difference between the material in asteroids versus that in comets is in how much heavy water they contain. Heavier water contains hydrogen and deuterium - a heavier version of hydrogen - whereas normal water only has hydrogen.
"Asteroids have about as much heavy water as the Earth's oceans do, whereas comets have about twice as much heavy water. So it seemed as if most of Earth's oceans must have come from asteroids," Nimmo said.
Making the oceans
The discovery that Hartley 2 has only as much heavy water as the Earth's oceans suggests that this kind of comet could have made up our planet's oceans without requiring any asteroid contribution. "That is important because it opens up the possibilities of how the Earth's oceans could have formed," said Nimmo.
The research also highlights the fact that scientists don't really understand why various objects have different amounts of heavy water, he added. "People generally think that objects which formed further from the Sun should have more heavy water, but the new measurements don't really fit."
The new comet formed further from the Sun than Enceladus (a moon of Saturn), but it has less heavy water. "One possibility is that the original material moved around a lot after it formed," said Nimmo. "So this particular comet could have formed close to the Sun, and then moved outwards. That actually agrees with ideas about how the architecture of the early solar system may have been reorganised, causing a lot of material to move around as it did so."
The new findings suggest that either the material moved around during the early Solar System, or that our current understanding of how heavy water is incorporated into solid bodies is incorrect, Nimmo said.
"It is possible that a higher percentage [of the Earth's water] came from comets, perhaps all," added Hartogh.
