New Proof That Comets Watered the Earth

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By rights, the earth should not be the cosmic garden it is. In a solar system of planets and moons that are solid rock or mostly gas, shrouded in clouds or atmosphere-free, scorchingly hot or bitterly cold, there's only one that's dripping wet. Earthlings like to refer to our home planet as the solar system's water world, and it's a jolly good thing it's as wet as it is, because without plenty of water, life (at least as we know it) would be impossible.

All the same, it's likely our planet was once a far drier, dustier place. You need only look at two of our nearby rocky neighbors — Mercury and Venus — for a reminder of what living so close to the blast furnace of the sun can do to you. Our atmosphere helps us retain the abundant water we do have, but how did it get to us in the first place?

One popular theory has long been comets. The solar system swarms with these little rogue bodies — perhaps a trillion of them, according to astronomers' back-of-the-envelope estimates — and shortly after the sun and planets formed, they were everywhere, flying randomly and free to collide with anything in their way. Since comets are essentially dirty snowballs made of rock, gas and water ice, a few crash landings on earth could have provided all the water we needed quite nicely.

But there was a problem with that theory. All of the comets astronomers observed were indeed packed with water ice, but a lot of it was what's known as heavy water, in which the hydrogen in the H2O mix is an isotope known as deuterium, with one proton and one neutron in its nucleus. The hydrogen found in ordinary water has no neutron. Since the overwhelming share of the water in earth's oceans is made with the light hydrogen atom, astronomers calculated that comets could have accounted for only about 10% of what's there. Now, according to a new paper published in the journal Nature, it appears that those scientists may have been wrong — and the reason for their error is that they were simply looking at the wrong comets.

The paper, co-authored by researchers at the California Institute of Technology, is based on observations conducted by the Herschel Space Observatory, a spacecraft launched by the European Space Agency in 2009. Herschel looked specifically at comet Hartley 2, a small comet discovered in 1986 with an estimated diameter of .75 to .99 mi. (1.2 to 1.6 km). Analyzing the chemical composition of Hartley 2's corona — or the gassy veil surrounding the main comet body — Herschel discovered that its concentration of heavy water was only about half that of any comets observed before. While that wouldn't entirely explain earth's particular heavy- and light-water mix, it does bring the chemistry a lot more into line — and gives the cometary explanation for earthly water a big boost.

"Our results with Herschel suggest that comets could have played a major role in bringing vast amounts of water to an early earth," says physicist Dariusz Lis of Caltech, a co-author of the paper.

What distinguishes Hartley 2 from the other comets previously studied — apart from its chemistry — is the place it was born. The trajectory the comet follows in its vast looping swings toward and away from the sun suggests it originated in the Kuiper Belt, a ring of icy bodies circling the solar system some 50 times farther from the sun than the earth is. The trajectory of the other comets makes it likely they are natives of the Oort cloud, a vast swarm of comets completely surrounding the solar system up to 10,000 times more distant than the Kuiper Belt.

It's not clear why Oort comets and Kuiper comets would have different water chemistry, but the time each spent in close proximity to the sun before being gravitationally ejected into deep space may play a role. That, however, is as much of a guess as astronomers want to make. "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," conceded Caltech planetary scientist Geoffrey Blake.

But if the science is incomplete, it's still more complete than it ever was before — making scientists more certain, too, about how our planet's water was delivered. Getting banged about by comets today could spell the end of all life on earth. But some 4 billion years ago, it may well have spelled the start.