New measurements from the Herschel Space Observatory show that comet Hartley 2, which comes from the distant Kuiper Belt, contains water with the same chemical signature as water in Earth's oceans. (NASA/JPL-Caltech/R. Hurt )

Some of the water that covers much of the Earth may have been carried here by comets from beyond Neptune, new evidence suggests.

The water in a comet called 103P/Hartley 2 is chemically very similar to water on Earth, suggesting that some of Earth's water comes from the same comet family, reports an international study led by Paul Hartogh at the Max-Planck-Institut fur Sonnensystemforschung and published Wednesday in Nature.

103P/Hartley 2 is believed to come from the Kuiper belt beyond the orbit of Neptune.

"Our results...suggest that comets could have played a major role in bringing vast amounts of water to an early Earth," said Dariusz Lis, senior research associate in physics at the California Institute of Technology in Pasadena and co-author of the paper.

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Most of the Earth is very similar in composition to meteorites called enstatite chondrites, which suggests that it similarly started off very dry.

Scientists think asteroids and comets may have subsequently crashed into the Earth, transporting water in the process.

Water is made of hydrogen and oxygen molecules, and hydrogen comes in both a lighter and heavier form called deuterium. The ratio of the two forms on Earth is very distinctive.

Six comets analyzed in the past had water with deuterium-to-hydrogen ratios twice as large as those on Earth. Those comets were thought to originate in the Oort cloud at the very edge of a solar system, about a light year away from the sun.

That led scientists to propose that most of the water on Earth came from asteroids, which have water with a deuterium-to-hydrogen ratio more similar to Earth.

Hartogh and his collaborators measured the deuterium-to-hydrogen ratio in the water of 103P/Hartley 2 using instruments on the Herschel Space Observatory. Frozen water and other materials inside the comet vapourize as the comet approaches the sun, producing a thin, gaseous atmosphere and a streaming tail. The chemical components of that vapour produce signature signals that were analyzed by the Herschel telescope.

The deuterium-to-hydrogen ratio from the comet is only slightly higher than the ratio found in Earth's water.

Researchers had previously expected comets from the Kuiper belt to contain more deuterium than comets from the Oort cloud because of where the two groups of comets are thought to have formed. Therefore, the new results came as a surprise.

"Our study indicates that our understanding of the distribution of the lightest elements and their isotopes, as well as the dynamics of the early solar system, is incomplete," said study co-author Geoffrey Blake, professor of planetary science and chemistry at Caltech, in a statement.

The Herschel Space Observatory is a telescope that orbits 1.5 million kilometres from Earth on the side away from the sun called the second Lagrangian point. One of its primary goals is to observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites in our solar system. It is scheduled to remain in operation until the end of 2012.