Where did Earth's water come from? Comet Hartley 2 offers new clues.

For years, astronomers have been drafting a Kipling-like "Just So" story one might call "How the Earth Got Its Oceans." But they have had a tough time figuring out how to divvy up the credit between two potential sources – comets and asteroids.

Now, it seems, comets may have played a more significant role in drenching the third rock from the sun that previously thought.

Comet 103P/Hartley 2, which made its closest approach to the sun last October, contains water with virtually the same chemical signatures as water in the oceans, according to a study published Thursday in the journal Nature.

That signature shows up in the relative abundance of two forms of water: a typical water molecule, H2O; and a much rarer type known as heavy water, in which one of the two hydrogen atoms has a neutron in its nucleus and the other doesn't.

But the findings raise new questions. The proportion of heavy water in the vapor spewed by Hartley 2 is much lower than theory says it should be, given where astronomers believe the comet formed. It's also lower than the proportion astronomers have measured in other comets so far.

"To me, this changes the problem," says Edwin Bergin, a University of Michigan astronomer and member of the team reporting the results.

Questions of the source for Earth's oceans are giving way to trying to figure out why comets have these differences in their water's chemistry and what that might imply for the formation and evolution of the solar system.

"That wasn't yesterday's problem," he acknowledges with a chuckle.

The team, led by Paul Hartogh with the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, used the European Space Agency's Herschel Space Observatory to analyze Hartley 2's halo, or coma, when the comet passed within 11 million miles of Earth shortly before its closest approach to the sun last year.

Much of the significance of Hartley 2 is where it comes from – a broad swath of frigid objects orbiting the sun beyond Neptune, called the Kuiper Belt.

Until now, scientists have only been able to measure the chemical signatures of comets from the Oort Cloud – a halo of comets much farther away from the sun at a distance of more than 5,000 astronomical units. (The Earth is 1 AU from the sun.)

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