Rosetta Mission Data Rules Out Comets as a Source for Earth’s Water
By KENNETH CHANGDEC. 10, 2014
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The water that fills Earth’s oceans almost certainly did not come from melted comets, scientists working on the European Space Agency’s Rosetta mission reported on Wednesday.
Water vapor streaming off Comet 67P/Churyumov-Gerasimenko, the subject of a yearlong examination by the Rosetta spacecraft, contains a much higher fraction of “heavy hydrogen” than water on Earth does.
“That now probably rules out” comets as the primary source of terrestrial water, said Kathrin Altwegg, a scientist at the University of Bern in Switzerland and the principal investigator for the Rosetta instrument that made the measurements.
With comets unlikely, most astronomers now think the Earth’s water came from asteroids.
The new findings, published in the journal Science, are among the first from Rosetta, which arrived at Comet 67P in August. Last month, Rosetta successfully sent a small lander named Philae to the surface of the comet.
The lander failed to hold on, bounced high off the surface and settled in a shaded area — depriving it of sunlight to replenish its batteries and cutting short its scientific mission.
The Rosetta spacecraft’s Philae lander is attempting to land on Comet 67P/Churyumov-Gerasimenko.
But the Rosetta orbiter continues its observations, and mission managers are hopeful that Philae will rouse itself from hibernation in May, when the comet is closer to the sun and the lander’s solar panels are able to generate more electricity.
“It’s a nice start to this phase of the mission,” Matt Taylor, the project scientist, said of the water findings.
Earth’s water has long been a puzzle. Scientists had long presumed that the planet was dry when it formed 4.5 billion years ago, and that the water came later, perhaps during the “late heavy bombardment” period more than 3.8 billion years ago. Comets, often called dirty snowballs, seemed a likely candidate.
But comets’ water turned out to be different from Earth’s. Some water molecules have a heavier version of hydrogen called deuterium that replaces one of the two hydrogen atoms, forming what is known as heavy water.
Comets that originate in the Oort cloud, the outermost reaches of the solar system, have twice the concentration of heavy water found in Earth water. Thus, when the Rosetta spacecraft was launched in 2004 on its long journey, most planetary scientists had already crossed comets off the list of possibilities.
But in 2011, a team using the Herschel Space Observatory, an infrared telescope operated by the European Space Agency, took a look at water vapor from the comet Hartley 2 and found that its deuterium signal perfectly matched Earth’s water. That opened the possibility that Earth’s water could have come from closer comets, like Hartley 2, whose orbit does not go much farther out than Jupiter’s.
The new measurements of 67P, another Jupiter-family comet, appear to rule out comets again. Its fraction of heavy water is three times that of Earth, higher than those of the Oort cloud comets.
“Ten years ago I would not have been surprised at all by this result, because that’s what I expected,” Dr. Altwegg said. “But then three years ago we got this Hartley 2 measurement, and that was a real big surprise. Now we’re back to what I actually expected.”
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Much of the solar system’s water, including up to half of Earth’s, appears to predate the solar system, formed within expanses of interstellar space billions of years ago. The sun, like the universe over all, has very low levels of deuterium
However, under certain conditions — cold temperatures along with radiation that knocks electrons off hydrogen — chemical reactions create water molecules with a much higher fraction of deuterium.
In a paper published in September, scientists led by L. Ilsedore Cleeves, a graduate student at the University of Michigan, found that these conditions did not exist in the early solar system, so the high-deuterium water must have been present in the cloud of matter that collapsed to form the solar system. While the outer solar system was cold enough, there was no radiation to eliminate f the electrons.
“Our paper showed that the outer part of the disk cannot be an engine for creating the deuterium fingerprint,” said Edwin A. Bergin, a professor of astronomy at Michigan who was another of the paper’s authors.
http://www.nytimes.com/2014/12/11/science/rosetta-mission-data-rules-out-comets-as-a-source-for-earths-water.html