Rosetta casts doubt on comets as Earth’s water providers

Data from the Rosetta mission are raising doubts about the idea that Earth’s oceans are filled with water from comets.

The water in comet 67P/Churyumov–Gerasimenko’s thin, hazy atmosphere doesn’t chemically match Earth’s oceans, suggesting that asteroids, not comets, brought water to Earth billions of years ago, said planetary scientist Kathrin Altwegg of the University of Bern in Switzerland in a news conference December 9.

Altwegg and colleagues used an instrument aboard the Rosetta spacecraft to measure deuterium, a heavy form of hydrogen, in comet 67P’s meager atmosphere, known as its coma. The results revealed that the comet’s water has a deuterium-to-hydrogen ratio roughly three times as high as water on Earth does, the team reports December 11 in Science.

Understanding how Earth got its oceans would give scientists clues to events in the solar system’s formative years and could hint at how common water may be in Earthlike planets beyond the solar system. Scientists use deuterium and hydrogen as tracers of the origins of water in the solar system. If two planetary bodies have similar ratios of deuterium to hydrogen, then their water probably came from the same place.

Some scientists suggest that Earth may have had water ever since it formed. Others argue that the planet would have been bone-dry early on and needed a special delivery from comets or asteroids to become so wet. To test these ideas, scientists have measured deuterium-to-hydrogen ratios in comets and meteorites (SN Online: 11/1/14).

In the 1980s, scientists thought comets could have ferried water to Earth. Then, in 1986, the European Space Agency’s Giotto spacecraft flew through the coma of comet Halley and found a deuterium-to-hydrogen ratio twice as high as Earth’s. Other studies found that comets originating in the far-off Oort cloud had ratios similar to Halley’s ratio.

But more recently, the pendulum has swung back: Researchers have begun to find comets with ratios similar to Earth’s. In 2011, scientists announced that comet 103P/Hartley 2, which originated in the Kuiper belt, had a deuterium-to-hydrogen ratio nearly identical to Earth’s. Another comet that formed in the Kuiper belt, 45P/Honda–Mrkos–Pajdušáková, also had a deuterium-to-hydrogen ratio close to Earth’s. Scientists started to consider the possibility that Earth’s water came only from comets originating in the Kuiper belt, an icy debris disk in the outer solar system that includes Pluto (SN: 10/19/13, p. 19).

Measuring the deuterium-to-hydrogen ratio of comet 67P, which originated in the Kuiper belt, allowed Altwegg and her team to test the idea. Its ratio turned out to be about 5.3×10⁻⁴; Earth’s is 1.5× 10⁻⁴.

“These new data require us to think harder,” says Edward Young, a geochemist at UCLA who was not involved with the Rosetta mission. The disparity in deuterium-to-hydrogen ratios between Hartley 2 and 67P rules out simple ideas suggesting that comets formed in restricted regions of the early solar system.

Young thinks it is naive to assert that one type of primitive body was the sole source of Earth’s water. Perhaps water came from a mix of comets with high deuterium-to-hydrogen ratios and asteroids with low ones, which would average out to what’s seen on Earth, he suggests. Altwegg agrees that this scenario is possible.

Young also notes the school of thought holding that comets and asteroids are on a continuum (SN: 11/1/14, p. 22), and that the distinction between the two types of bodies may not be as great as scientists once thought.