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Earth contains a vast amount of water, but scientists are unsure of its origins

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By Ben Harder
Monday, November 8, 2010; 4:10 PM

Imagine you own a gigantic swimming pool that's a thousand miles long and a thousand miles wide. Now imagine you decided to pour all of the world's oceans, lakes and rivers into your pool. It had better be about 333 miles deep (all the way across - no shallow end) to accommodate all that water.

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In other words, there's a reason the Earth is called the blue planet: It's covered in a lot of H2O. And its surface waters are only the tip of the proverbial iceberg. Beneath the planet's crust lies more water, probably a great deal. Experts estimate that the planet's interior contains perhaps 10 times more water than exists on the surface. And that might be a serious underestimate.

Where did all the water in our world come from? If you've never paused to contemplate that question, you're clearly not a planetary scientist. Those experts have been debating the subject for years and still can't agree on an answer. They've come up with some pretty interesting theories, though, including one called the "Late Heavy Bombardment."

But before we debate water bombs, let's back up a bit and consider the early history of the sun and planets.

When our solar system began to take shape, roughly 4.5 billion years ago, it was a disk-shaped cloud of gas and dust spinning around a dense core, which became the sun. Close to this core, the cloud was very hot - too hot for compounds such as H2O to condense, so they got blown outward by a powerful solar wind. When they got far enough from the nascent sun, they condensed into water and ice. This happened beyond the orbits of the inner planets, including Earth, which coalesced out of heavier dust particles.

So planetary scientists have long assumed that the rocky inner planets - Mercury, Venus, Earth and Mars - lacked water when they formed. Only celestial bodies born farther from the sun, beyond what scientists call the snow line or frost line, could have contained significant amounts of water, the thinking went.

How then, scientists have wondered, did water and ice from out there get in here?

Comets, they used to think. Comets are among the wettest objects in the solar system. Those that have been examined up close by space probes are about 50 percent frozen water and 50 percent rock and dust; some experts call comets dirty snowballs. By comparison, water probably makes up less than 1 percent of our "wet" planet's mass.

A comet can be many miles across, so it wouldn't take an extraordinary number of them to fill that gigantic imaginary swimming pool. And toward the end of the Earth's formation, it's likely that many comets were crisscrossing space in irregular orbits that put them on collision courses with the young planet. If the Earth did indeed get pelted by a bunch of those dirty snowballs, that so-called Late Heavy Bombardment could easily have delivered enough water to explain how wet the planet is today. (Other inner planets would have been pelted, too, but scientists, for a variety of complex reasons, don't think much water would have stayed on the surfaces of our planetary neighbors.)

Unfortunately, such a bombardment would leave other mysteries unexplained. For one thing, the water in comets that scientists have managed to probe is heavier than the water on Earth. That's because comet water contains about twice as many deuterium atoms. Deuterium is an isotope, or variant, of hydrogen; both types of atoms contain a single proton and an electron. But deuterium also contains a neutron. Standard hydrogen, which is much more common on Earth has none. The extra neutron makes water molecules containing deuterium heavier than regular H2O. If ancient comets had delivered most of the water that's now on Earth, then our planet's water would be ever-so-slightly heavier than it is.

While the high deuterium content in comets makes it unlikely that they were the primarily source of Earth's water, it doesn't rule out the Late Heavy Bombardment theory. Some meteorites contain water, and don't have an excess of deuterium, and during Earth's formation, some experts hypothesize, numerous such objects - some the size of small planets - were careening around the solar system. If the bombardment involved not only comets but also a bunch of these wet meteorites, called carbonaceous chondrites, then the overall deuterium-to-hydrogen ratio of the water raining down from the heavens could have been about right for forming today's oceans and lakes.

However, carbonaceous chondrites would have delivered not only water but also other compounds and elements that would not have condensed in the heat of the early solar disk. Those compounds and elements include gases such as argon, krypton and xenon, and the proportions of those elements in carbonaceous chondrites don't match what's found on Earth. That discrepancy undermines the idea that meteorites brought much water to Earth.

It's possible, experts say, that the water-bearing bombs were some type of comet or meteorite with just the right deuterium and gas ratios. But why haven't scientists found such objects? Maybe all those bodies long ago crashed into Earth and other planets, and none exist anymore. Or perhaps Earth got doused not by a swarm of small water bombs but by a single giant one, a wet mini-planet.

Michael Drake of the Lunar and Planetary Laboratory at the University of Arizona in Tucson, thinks there's a better explanation. The majority of Earth's water, Drake believes, was there from the beginning, despite Earth's having formed inside the solar system's snow line. He and his colleagues have speculated that individual molecules of water vapor could have glommed onto dust particles inside the snow line, much as dew forms on grass. Then, when the dust particles drifted together into larger and larger objects, eventually growing to become the inner planets, the moisture stayed with them. Eventually, there was enough to form Earth's oceans.

Or a giant swimming pool.

Harder is general manager of health and science at U.S. News and World Report.


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