Quest to trace origin of Earth’s water is ‘a complete mess’

Space water data too simplistic and seawater may not be best comparison, new analyses suggest


BEYOND THE SEA There’s an awful lot of water in the oceans, but most of Earth’s water could be hiding deep underground and confusing the quest for how the planet got its water. 

Tiago Fioreze/Wikimedia Commons (CC BY-SA 3.0)

HONOLULU — When it comes to wringing out the origins of Earth’s water, planetary scientist Karen Meech has some bad news. Not only do researchers have bad intel on where water-bearing bodies in the solar system formed, our own oceans might be sending them down the wrong path.

“It looks like a complete mess,” says Meech, of the University of Hawaii in Honolulu, who gave a state-of-the-water address August 4 at a meeting of the International Astronomical Union.

There are two big problems, she says. The bulk of Earth’s water, hidden deep underground, has a slightly different composition from that of ocean water. Yet for decades, researchers have used seawater to compare the makeup of Earth’s water against that of icy asteroids and comets in the solar system. To complicate matters, the chemical marker that researchers rely on for tracking water might not even be that useful, Meech says.

When Earth formed, it didn’t have ready access to water, so the planet had to rely on help from farther out, where water ice collected on asteroids and comets (SN: 5/16/15, p. 18). To trace Earth’s water to its source, researchers use a chemical marker known as the D/H ratio. The ratio measures the relative amounts of hydrogen and deuterium (a slightly heavier version of hydrogen) in water. Deuterium shows up more frequently at lower temperatures, so the D/H ratio is useful for figuring out how far from the sun Earth’s water originally came from.

If only it were that simple.

The disk of gas and dust that encircled the young sun was a messy, turbulent place with planet-building material getting sloshed around back and forth. As a result, the D/H ratio bounces all over the place as one moves farther from the sun, recent computer simulations show. And no two calculations agree on what the D/H ratio was at any particular distance from the sun.

“If you get a measurement for a comet, you don’t really know where that comet formed,” says William Irvine, a planetary scientist at the University of Massachusetts in Amherst.

To remove the ambiguity, Meech argues that researchers need more than one fingerprint for water. Ratios of nitrogen and oxygen isotopes also change with distance from the sun. Measuring many ratios for many comets and matching them up with computer simulations could give scientists a much better chance of pinpointing where the water bearers in the solar system formed, Meech says.

Unfortunately, these measurements are impossible for the current generation of telescopes to make except when looking at the nearest and brightest comets. So while waiting for bigger and better instruments to come along, researchers might want to spend a little more time looking down instead of up and asking how well we know Earth’s water, Meech suggests.

“Turns out we may be wrong on that side, too,” she said.

Scientists don’t really know how much water is locked away in Earth. Estimates for the total amount are all over the map. The planet could harbor anywhere from 1.5 times as much water as is found in the oceans all the way up to 11 times or more. “If most of the water is inside Earth,” asks Meech, “does it make sense to compare anything to Earth’s ocean water?”

Irvine agrees. “If the geophysicists are right, and there’s a lot of water in the interior… that certainly makes things difficult.”

Meech and colleagues are hunting for primordial sources of water on Earth. They think they’ve found it near hot spots, such as the Hawaiian Islands, where molten rock from the mantle wells up into volcanoes.

What they find is a bit troubling: The deep water is chemically different from that in the oceans, which means researchers are likely using the wrong D/H ratio for Earth. Scientists need to nail that down first before comparing Earth’s D/H ratio to that of the many families of asteroids and comets buzzing around the sun. The results from this project will appear in an upcoming issue of Science.

Knowing which specific population(s) of asteroids or comets brought water to Earth might seem picky, but getting it right ripples out to science’s understanding of how water arrives on planets throughout the galaxy. Some families of asteroids and comets are easily tossed around by giant planets, for example, while others are stubborn.

“If it requires some sort of exotic mechanism to get water to our planet,” says Meech, “maybe some of these planets in other solar systems might not actually have habitable worlds.”

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