Earth’s Water Probably Came from Asteroids, Not Comets

When comet-chasing spacecraft Rosetta was poised to rendezvous with its rocky prey, scientists eagerly anticipated one observation in particular: water.

One theory held that comets like 67P/Churyumov-Gerasimenko could have been the thing that brought water to the Earth millions of years ago. Now, a paper published in the jo​urnal Science gives the data on the composition of its water, which could be compared to Earth's to see if they're a match.

The result: they're not. This raises the odds that another type of space rock could have been our water benefactor: asteroids.

The paper details the deuterium-to-hydrogen ratio in the water on 67P. It's based on results gathered using the ROSINA mass s​pectrometer instrument on the Rosetta spacecraft.

"Water consists mainly of hydrogen and oxygen, but some of the water molecules can contain deuterium, which is a heavy isotope of water—it has double the mass of hydrogen," first author Kathrin Altwegg, who hails from the University of Bern in Switzerland, explained in a phone interview.

She said that the idea of Earth's water coming from comets had met a setback when Oort cloud comets like 1P/Halley were found to have water with more deuterium than that on Earth. But interest in the idea was revived in 2011 when water on 103P/Hartley 2, a Jupiter-family comet, was​ found to have a closer composition to Earth's.

In the new work, the researchers found the Comet 67P/Churyumov-Gerasimenko, which is also a Jupiter-family comet and therefore expected to have originated from the Kuiper Belt, didn't follow the same pattern as Hartley 2. In fact, they wrote that the deuterium-to-hydrogen ratio is the highest measured, "even higher than values characteristic of OOCs [Outer Oort Cloud comets]," with a measurement around 30-120 percent more than that of Comet Halley and around three times terrestrial levels.

"From this we have to conclude that, first of all, the Kuiper Belt family is probably very diverse—it contains very different comets," said Altwegg. "And secondly, that these comets could not have brought terrestrial water, at least not a big part of it."

With Oort Cloud comets and Kuiper Belt comets ruled out, that leaves asteroids as a far likelier source of Earth's original water.

The paper concluded that "the new measurement supports models advocating an asteroidal (i.e. carbonaceous chondrite-like), rather than cometary origin for the oceans, and by extension for the terrestrial atmosphere."

I asked Altwegg how certain that was, and she admitted that in an ideal world we'd have 50 spacecraft up there sampling comets, especially given the contrast in the Hartley 2 and Churyumov-Gerasimenko results. But, she said, the Churyumov-Gerasimenko finding was in fact more expected; Hartley 2 was the outlier.

"Actually, models predicted a high D/H [deuterium-hydrogen ratio] for the Kuiper Belt," she explained. "The exception is Hartley 2, not Churyumov Gerasimenko."

This is because Kuiper Belt comets are formed out past Neptune, and low temperatures are expected to lead to more deuterium. But there can be a wide range in distance at which Jupiter-family comets form, which may account for the difference.

Using ROSINA, Altwegg and her team will continue to measure the molecules around the comet, including noble gases and organics, for which they need to wait for the comet to become more active. They'll also look at the homogeneity (or lack thereof) within the comet's nucleus, which will help shed light on how comets form. "That's one of the least understood topics in early Solar System formation," Altwegg said.

Churyumov-Gerasimenko is currently continuing on course with the Rosetta probe in tow at a safe distance after the Philae lander fell asleep on its drained b​atteries. The European Space Agency is optimistic that it will wake up again once the comet brings it closer to the Sun so it can charge its solar panels.

In the meantime, we can expect a ton more scientific findings to come out from instruments on both Rosetta and Philae.