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.

Planetary Pinball

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.

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Rosetta Spacecraft Suggests Asteroids, Not Comets, Birthed Earth's Oceans