Asteroids, not comets, likely delivered Earth's ancient oceans from space, concludes a Wednesday study from the Rosetta spacecraft, now in orbit around a comet that is a frozen relic from the dawn of the planets.
Where did the Earth's oceans come from? the new study asks, investigating a long-debated question of whether the water on our planet's surface was delivered during a bombardment of comets some 3.8 billion years ago. Not likely, mission scientists conclude, pointing instead to ancient asteroids, which were covered with frost in the early solar system.
"Terrestrial water was probably brought by asteroids," says Rosetta study leader Kathrin Altwegg of the University of Bern in Switzerland. She finds that source "more likely than comets."
These are the first scientific results from the European Space Agency craft, which is orbiting the lumpy 2.5-mile-wide (4.1 kilometers) comet 67P/Churyumov-Gerasimenko, the mission team reports in the journal Science. (See: "Touchdown! Comet Landing to Offer Clues to Solar System's Birth.")
Rosetta arrived at the lumpy ice ball last month, delivering a probe that lost power and went into hibernation during its first days on the comet. Comet 67P is now more than 260 million miles (418 million kilometers) from the sun, awaiting a solar warm-up that will spark its cometary tail.
Comets are chunks of ice and dust zipping through the far reaches of space and occasionally zooming past the sun. A shooting gallery of them accompanied the birth of the solar system some 4.6 billion years ago, with comets and asteroids slamming into each other for another 800 million years. The epoch was capped by a pummeling of the Earth, the moon, and other planets known as the Late Heavy Bombardment.
Some of the bullets stopped by the early Earth were undoubtedly comet impacts, and planetary scientists have long suggested that these icebergs in space may have provided the waters of the early oceans.
When Earth first formed into a sphere, it was likely a ball of magma that would have boiled off any surface water, Altwegg notes, which is why scientists are looking to the skies to explain the origins of oceans in the first place.
Scientific suspicions that comets brought Earth's water were reinforced three years ago, when Europe's Herschel space telescope spotted ice with a chemistry signal similar to Earth's in the comet Hartley 2. That comet, like comet 67P, is thought to have originated in the Kuiper belt, just outside the orbit of Neptune.
It looks like those suspicions have been doused. "We found something different," says Altwegg, and "more exciting." She heads the team operating a spectrometer aboard Rosetta, which is circling in orbit some 14 miles (23 kilometers) above comet 67P.
With the spectrometer, the researchers measured the amount of deuterium, a heavier isotope of hydrogen, found in the ice on the surface of the comet. Normal water is built with hydrogen atoms, but "heavy" water contains deuterium instead.
On comet 67P's icy surface, the study concludes, the ratio of heavy water to normal water is roughly three times more than the ratio in Earth's oceans.
That rules out comets from the Kuiper belt delivering water to the early Earth, Altwegg says, because even a few deuterium-loaded comets like comet 67P would have spiked the punch bowl of the early Earth with heavy water.
"It is a very exciting study that raises more questions than it answers," says geophysicist Nicolas Dauphas of the University of Chicago, who was not part of the discovery team. "The mission is a tremendous success for space exploration and for science in Europe."
Since Kuiper belt comets apparently don't share Earth's heavy water ratio, Altwegg argues that impacts with asteroids, small rocky bodies largely found between Mars and Jupiter, must have delivered the water in our oceans. (See: "Asteroids and Comets.")
In its earliest era, the sun wasn't as warm as it is today, she says, which would have allowed frozen water to remain on asteroids instead of boiling off. Deuterium is thought to have more often agglomerated in the coldest and most distant reaches of the early solar system, where comets dwell, so asteroids' ice likely packed much less of the heavy stuff.
But some outside experts are less convinced, citing the low numbers of comets so far investigated. Altwegg also speculated that some water baked out of the Earth's crust during the Late Heavy Bombardment era, adding to extraterrestrial contributions. (See: "Mystery of Earth's Water Origin Solved.")
"Each comet is different, and the question of their contribution to Earth's atmosphere and oceans is still open," Dauphas says.
The questions remain as to what sort of water emerges from comet 67P once it starts melting in the sun's glare next year, producing a cometary tail. Perhaps the surface ice of the comet is enriched with heavy water, while the interior isn't. (Related: "Comet Catcher: The Rosetta Landing.")
Once the comet reaches a distance of about 250 million miles (400 million kilometers) from the sun this spring, that activity should pick up, Altwegg says, and a tail should sprout from comet 67P. Until then, the spectrometer aboard the spacecraft will continue to refine its measurements.
"The Herschel observation cannot prove that comets delivered water to Earth," says planetary scientist Paul Hartogh, of the Max Planck Institute for Solar System Research in Göttingen, Germany, "while the Rosetta observation cannot prove that Earth water stems from [asteroids]."
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