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Earth's water probably didn't come from comets, Rosetta scientists say

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The new data is important for answering questions about the origins of water on Earth, but not everyone is convinced

European Space Agency

The researchers behind Rosetta, the ESA spacecraft currently orbiting the Churyumov-Gerasimenko comet, think they can help answer one of the fundamental questions about how water came to Earth: most likely not from comets. They published their findings today in Science.

Around 4 billion years ago, meteorites repeatedly slammed into the young Earth in a violent event called the Late Heavy Bombardment. This happened right after our planet’s crust had started to cool, so instead of burning up and disintegrating into the molten surface of an older Earth, these chunks of comets and asteroids made landfall. In the process they likely brought the first organic molecules to Earth and maybe even enough water to form oceans, setting the scene for life to take hold.

Since the average asteroid and comet are much older than most of Earth’s crust today by several billion years, scientists have been chasing them down to better understand the conditions under which life on Earth may have started. In the eight analyses of comets that scientists have done so far, water has been a central question; instruments both near and far to the comet can detect the concentrations of certain elements in comet ice to see if they match what we see on Earth. But with data from only a few comets, not all planetary scientists are convinced that the answer is quite so simple.

Although the Rosetta orbiter is not the first to do close analysis of ice on a comet, Kathrin Altwegg, the principal investigator of the mass spectrometer on the Rosetta craft, an astronomer at the University of Bern in Switzerland and one of the study authors, says this data is a surprising addition to the information we already know. "When we launched this mission 10 years ago, I wouldn’t have been surprised at all about the findings," she says. But then, three years ago, a mission to the comet Hartley–2 three years ago yielded very different results — suggesting that Earth’s water may have been deposited by comets. Rosetta’s findings don’t support that conclusion. "Not everything is as simple as it seems — we have real science here," she says.

The scientists are addressing the dispute about water’s origin by looking at hydrogen isotopes, molecules that all have the same number of protons and electrons but different numbers of neutrons in their nuclei. They’re still hydrogen — but one isotope dubbed deuterium has one more neutron, which means it’s heavier than regular hydrogen. Both hydrogen and deuterium are found in water and ice. The unique ratio of hydrogen to deuterium can help identify where in the solar system the water came from. The Earth has a pretty small ratio, only three heavy molecules out of 10,000, Altwegg says, but it’s a ratio that is characteristic of our planet. Because Rosetta was so close to the comet, scientists were able to use a technique called mass spectrometry to determine the quantity and type of chemicals present in a sample. It's also possible to detect the ratio in comets that are farther away; to do so, scientists have used remote observations based on the light waves coming from the sample.

In the 30 years that astronomers have been looking at this ratio of deuterium to hydrogen in comets, they’ve seen a range of ratios, which means individual comets formed under very different conditions. The first three comets that researchers analyzed had ratios double those found on Earth, says Humberto Campins, a physics and astronomy professor at the University of Central Florida in Orlando. Another two have the same ratios as on Earth. But on the Churyumov-Gerasimenko comet that Rosetta is orbiting, the ratio is even higher, with deuterium molecules present at three times the concentration that they appear on Earth.

The ratio on Churyumov-Gerasimenko is so different from other observations that comets like it couldn’t have been responsible for bringing water to Earth, Altwegg says. "Terrestrial water was more likely brought by asteroids than comets," Altwegg says. Although asteroids are now much rockier than comets, they probably used to have more ice with ratios that were closer to those that we see on Earth, she suggests.

But Florida’s Campins isn’t ready to draw any conclusions. "Rosetta’s findings are interesting and important, but I wouldn’t necessarily say that this information makes asteroids a more likely source," he said. The small amount of data that scientists have from comets in this part of the solar system aren’t enough to draw any definite conclusions, especially given the variation in readings they’ve already found, Campins said. Also, no one is really sure what can affect the ratio of deuterium to hydrogen. What’s more, we’re not even sure if our ratio for Earth is right — getting readings from water in Earth’s lower layers, which scientists don’t yet have, might change the ratio. "When you look at this data carefully, the conclusion isn’t so black and white, but it’s still an interesting result," Campins said.

Both Campins and Altwegg agreed that they need more research on comets and asteroids to understand how water came to be on Earth. Altwegg mentioned that Japan’s prospective Hayabusa-2 mission, intended to collect a sample from a nearby asteroid and bring it back to Earth, will likely provide more data. "If Hayabusa-2 brings a sample home, that’s always the best thing," Altwegg said. That’s because the tests here on Earth are more sensitive than what can be done in space, Altwegg said.

But Campins has more questions. He is working on NASA’s OSIRIS-REx mission, set to launch in 2016, which has a similar goal of bringing back a sample from a nearby asteroid to understand not only the water it may have contained, but also the organic molecules that might have helped start life on Earth. "By understanding how life came to Earth, we might understand how we may find life elsewhere," Campins said.