Karen Meech doesn't spend much time digging in the rocks of the earth. An astronomer by trade, she is often behind the telescope, examining comets and looking for clues as to how Earth got its water. But a field trip to Iceland in 2004 almost a decade later took them to the craters of Hawaii to look for evidence of the liquid that helped birth life on that planet.
On that fateful trip to Iceland, Meech saw geothermal areas where gas was pouring out of the ground. The guide told the group not to worry it was just water. "Then she said, 'That's probably primeval water,' and she lit a lamp," says Meech.
Cliffs near Baffin Island, Canada, are giving researchers access to material from Earth's deep mantle that may contain the oldest water fingerprints on our planet. (Source: Timkal/Wikimedia Commons)
The flavors of the water
The source of water on earth has long been a mystery; Meech himself has been trying to figure it out for at least 20 years. Most of this research has focused on classifying the different isotopes of hydrogen that make up water, or "the taste of water" as Lydia Hallis of the University of Glasgow calls it. One such "flavor" is heavy water, a form of water that contains deuterium, an isotope of hydrogen whose nucleus contains a proton and a neutron. Normal hydrogen has no neutron, so deuterium water weighs more than normal water.
By simulating conditions in the early solar system, researchers can calculate the ratio of heavy water to coarse water at planet formation. On Earth, the observed ratio is higher than in the early solar system, leading many astronomers to believe that the water was imported since the ratio was thought to remain constant over time. Today, most scientists believe that asteroids brought water to the young, dry earth.
Meech was wary of this idea because measurements of Earth's deuterium-to-hydrogen (D/H) ratio, which is related to the heavy-to-normal water ratio, are generally based on the composition of present-day oceans. Reservoirs with a lot of heavy water have a high D/H ratio, while reservoirs low in deuterium have a lower ratio.
But the proportion of earth is said to have changed over time. Like most planets, Earth has likely lost some of its atmosphere to space, and lighter hydrogen would be easier to remove from the planet than its heavier counterpart. Geological processes, such as the evaporation of water from reservoirs such as lakes and oceans, can also change the ratio, as can biological reactions, because lighter isotopes are used differently in metabolic processes than heavier ones. All of these processes would give modern Earth a higher D/H ratio compared to when the planet was recently formed.
When Meech heard that primeval water might be gushing from the surface of Iceland, she was excited for the opportunity to study the oldest taste of water. But after speaking to a geologist, he discovered that the plumes were actually from recent activity; after all, they weren't primeval. However, the geologist revealed that some rock materials brought out of the mantle contain small traces of water. This material may never have mixed with surface material and may represent water from early Earth. No one had examined the D/H ratio in these samples because the technology to do so was new. But the University of Hawaii, where Meech is based, had just bought a new ion microprobe that could do the job.
"I thought, wow, here's a way to measure the original fingerprints," says Meech. "I got very emotional at that moment."
What is heavy water?
Heavy water or D2O contains deuterium instead of hydrogen. Deuterium is an isotope of hydrogen whose nucleus contains a proton and a neutron, while normal hydrogen only contains a proton. The ratio of heavy water to normal water in a sample tells scientists how it formed, information researchers are now using to try to unravel the origin of water on Earth.
looking for the culprit
Earth and the rest of the planets formed in a nest of gas left over from the birth of the sun. Known as the solar nebula, this material contained all the elements that made up the planets, and the composition varied with distance from the Sun. The region near the star was too hot for any material, such as ice, forming in the outer solar system to stick together. On Earth, hydrogen and other elements could still be just a gas. Because the nebula was short-lived, most scientists suspect Earth didn't have enough time to collect these gases before they escaped into space. This idea, along with the planet's high D/H ratio, has led many to believe that Earth's water must have arrived after Earth cooled.
When Europe's Giotto spacecraft visited Comet Halley in 1986, researchers found that its heavy water content was greater than that of near-Earth gas that was part of the early Solar System. A new theory has emerged: Comets may have brought water to early Earth. After the planets formed, the giant bodies continued to shake things up, with giant planets like Jupiter spewing some material into the inner solar system. Icy objects that formed in the outer solar system may have been hurled toward Earth to rain down in giant water-laden impacts.
But as other missions studied more comets, it became clear that the amount of heavy water was not constant across all comets. In fact, the heavy water ratios of most comets were too high to account for the fall of water to Earth. Another culprit must be responsible.
Comets weren't the only thing ejected by the gas giants. When Jupiter passed the asteroid belt early in our solar system's history, it threw rocky debris in all directions. Like comets, some material fell to earth. Unlike comets, asteroids do not store water as ice. Instead, they lock their components (hydrogen and oxygen) in the minerals. Also, the heavy water content in asteroids falls much closer to Earth's current ratio. Therefore, asteroids are the main suspects for the formation of water on our planet.
“Really, we're not talking about water; we're talking about hydrogen," says Anne Peslier, a geochemist at NASA's Johnson Space Center. Peslier studies the geochemistry of Earth's mantle and other terrestrial planets, including hydrogen trapped in minerals.
As the Earth formed, hydrogen became trapped in its rocks and minerals around the growing planet. When hydrogen- and oxygen-rich minerals melt from the mantle due to the heat, the resulting water can be expelled from the earth's crust.
Most of the mantle is rocky, and it can trap enormous amounts of hydrogen and oxygen. Researchers estimate there may be as many as 10 oceans of water in the mantle.
Erupting volcanoes usually bring magma from the upper part of Earth's mantle, the region closest to the surface. This material is most likely contaminated by crustal hydrogen, which contains the same higher D/H ratios measured in the oceans today. More primitive samples are found much deeper in the mantle. Though it's hot there, less than 20% of the mantle rock has melted, says Peslier. When molten material erupts, it can have violent impacts on solid rock.
"If [lavas] are fast enough and violent enough, they sometimes break off chunks of whatever they're passing through," says Peslier. She describes the result, named mantle xenolite after the Greek word for 'foreign rock', as crystals of light green olivine and black pyroxene embedded in black lava.
Green olivine crystals in lava may contain and shield hydrogen collected during Earth formation, allowing researchers to determine the ratio of deuterium to hydrogen. This xenolite sample from the Peridot Mesa Mantle in Arizona features high-grade green (olivine) peridotite hosted in gray (volcanic) phonophrite host rocks. (Image credit: James St. John/Flickr)
If the hydrogen-rich olivine crystals were captured early enough in Earth formation and remained intact throughout the planet's 4.5-billion-year lifespan, they could show how much, if at all, the ancient ratios of heavy and normal water have changed. The tiny time capsules could provide answers to long-held questions about the origins of water on Earth.
But first they had to be found.
primeval water hunt
While Meech knows a lot about water in the solar system, she wasn't as familiar with rocks on Earth. He hired Hallis, then a post-doctoral student, to lead geological excavations looking for the first fingerprints of normal and heavy water. Hallis was intrigued by the ability to climb craters in Hawaii and along the coast of Baffin Island, Canada, looking for clues. Baffin is one of the few places where the deep mantle is accessible. The chain of eruptions that formed the island also created Greenland and Iceland. "The Baffin Island samples are the most primitive examples of the deep mantle that we have," says Hallis.
Hallis also received samples collected by Don Francis, now Professor Emeritus at McGill University in Montreal, from a small uninhabited island called Padloping, off the east coast of Canada and northwest of Baffin Island. According to Hallis, Francis collected the first of his samples in 1985. Padloping Island's isolation forced researchers to travel there by boat and set up camp. The sheer cliffs dropped rocks in abundance, and Francis collected the finest minerals on the beach. A return trip in 2004 yielded even more samples.
"One thing I'd really like to do is go back [to Padloping Island]," says Hallis. The towering cliffs make sampling difficult, but if you could get to some of the sheer ledges you could pinpoint where and when the material rose to the surface.
If the ingredients of the water were absorbed by the earth at the beginning of their formation, they can remain unchanged in the interior of the planet. Researchers are searching in the deepest mantle for samples that volcanic activity could bring to the surface in order to pinpoint the exact taste of our planet's primordial water and determine whether it came from the solar nebula or from an external source. (Image credit: Roen Kelly/Discover)
With the well-preserved samples in hand, Hallis and his colleagues began to systematically destroy them. The rocks have been reduced to sand-like dust. The scientists used the microprobe to classify the closed crystals by color.
Meech helped classify the crystals. "It was difficult for me to handle the pieces of sand without dropping them on the ground," he admits sadly.
Part of the process was making sure the samples were taken from the mantle and not the crust when the volcanic plume exploded. Previous studies of the Baffin Island minerals suggested they originated deep in the mantle, and mineralogical evidence indicated that the samples Hallis had in the lab were likely pristine. The tiny glass beads have been partially protected by olivine crystals, which act as a barrier against wear when the stones are on the surface. Still, they weren't quite perfect.
"Even with the most pristine samples we have, it's not 100% accurate deep coat," says Hallis. "There will always be some incorporation of the [upper] mantle just because it has to traverse much of the mantle to get to the surface."
While the Baffin Island samples were free of crustal contamination, the team wasn't so lucky with rocks collected near their university. Hawaiian minerals suffered from weathering and were heavily affected by surface water, probably rain. The contamination prevented these samples from revealing the taste of pure water.
After the first fingerprints of Earth's waters were finally collected, Meech and Hallis began comparing them to other samples. Hallis expected to see a large water content closer to the meteorites thought to have provided water to the young planet. Instead, the samples weighed about 25% less heavy water compared to regular water, much less than expected.
"It was a surprise," says Hallis. "This suggests that carbonaceous chondrites [a class of meteorites] are not well suited to Earth's water source." While meteorites may have provided some of Earth's water, he doesn't think they provided all of it.
Young star HL Tauri, pictured here at radio wavelengths, has a protoplanetary disk with gaps where planets could form and collect hydrogen, oxygen and water from their surroundings. (Source: ALMA (ESO/NAOJ/NRAO))
The source of the water of the earth
What do the samples suggest is the source of Earth's water? Hallis suspects it came from the solar nebula. While many scientists argue that the nebula would have dissipated in 6 million years, long before our planet would have grown large enough to capture it, he notes that several young stars have been found with gas around them for up to 10 million years . That would give the tiny rocks that make up the Earth enough time to incorporate elements like hydrogen and nitrogen into their structure. Hallis says that nitrogen and hydrogen tend to follow each other in the solar system: "If you taste a certain amount of hydrogen, you taste a certain amount of nitrogen," she says.
"Perhaps there are still places on Earth that preserved this early source of hydrogen," says Zachary Sharp, a researcher at the University of New Mexico, who also suspects that Earth's D/H ratio changes over time Has.
Hallis' findings aren't the only ones that suggest Earth may have captured most of its water early on. Although the moon was thought to be completely dry, recent reexaminations of Apollo lunar rocks have revealed traces of water. The leading theory for the moon's formation is that it was formed when an object the size of Mars collided with the young Earth. Liquid water on the surface would have evaporated, leading many to conclude that the Earth needed to get more water from elsewhere. But the low D/H ratios of the lunar samples suggest the moon may have collected water in the minerals trapped within, a region that neither comets nor asteroids could have contaminated. Subsequent volcanic eruptions spewed this material to the surface to be returned to Earth by astronauts.
Because it's important? The high temperatures after the collision would have been similar to those in the solar nebula, Hallis says. This contributes to the accumulation of volatiles and water even in early hot solar systems.
But hydrogen comes in heavy and light flavors, doesn't that mean the ratio can change either way? Not really, according to Sharp, who has revisited the idea that most of Earth's water was collected from the nebula rather than from subsequent collisions. "It's easy to increase the isotope ratio of the samples, but it's difficult to decrease it," he says. This is because the lighter hydrogen is easier to remove. For example, hydrogen rises more easily to the top of the atmosphere where it can be removed by the solar wind. The heavier deuterium tends to settle closer to the bottom.
Asteroids also provide evidence that Earth's water may have come from the gas that formed the planets. Meteor studies of the large asteroid Vesta have revealed severe water conditions similar to Baffin Island estimates.
"Now that we're finding low levels on Earth, the Moon and Vesta, and also in the asteroid water reservoir, maybe the [nebula] story is now possible," says Alice Stephant of Arizona State University, which studies Vesta. . "It seems they all have a common stockpile that is less [of deuterium] than we thought."
Asteroid Vesta photographed by NASA's Dawn spacecraft. (Source: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA)
The Smoking Gun
The lower D/H ratios reported by Hallis, Meech and their colleagues are not yet widely accepted. Conel Alexander, a cosmochemist at the Carnegie Institution in Washington, says there are two reasons other researchers didn't immediately change their minds about where Earth's water came from.
An argument against the results arises from the way Hallis extrapolated isotopes and elemental abundances in his measurements; Alexander says some scientists disagree with the way the final numbers work using his method. The other problem is how Hallis explained his findings. "Lydia's performance was unique," says Alexander. "There might be other ways to introduce hydrogen into the molten inclusions I measured."
Alexander's greatest concern stems from the fact that only a single source of rocks, the Baffin Island samples, were used to estimate the ancient proportions of the entire planet. "Most of the Earth could have a completely different composition, and there could be something odd about the basalts on oceanic islands," he says. He hopes other scientists will follow Hallis' example and measure the D/H ratio of a variety of deep mantle plumes.
Hallis is ready to make her own trip to Padloping Island to collect more samples. Among other things, you want to investigate not only the hydrogen involved, but also the nitrogen. However, analyzing the nitrogen in the samples is more difficult than looking for hydrogen, in part because there is even less nitrogen than hydrogen in these samples. Nitrogen measurement also requires instruments with very high precision. Hallis says he's pushing the boundaries of what current technology can do.
Alexander says Hallis' goal to look for nitrogen in future samples will also help answer any questions about the pristine nature of the Baffin Island samples. "If you can show that both light hydrogen and light nitrogen are present in these inclusions, I think that would be irrefutable evidence," he says.
"If nitrogen follows hydrogen, then we've proved our theory that [the samples] are primitive," says Hallis.
Nola Taylor Reddis a freelance science writer specializing in space and astronomy.