Although existing theories of where Earth’s water comes from assume that ice-rich comets or asteroids hit the planet after it formed, new research leaves those ideas in the dust. A team of researchers has found evidence that “Earth has had water since the beginning of its formation,” according to planetary scientist Lydia Hallis, who led the team.
A new paper about this evidence, published earlier this month in the journal Science, points to Earth’s water coming from motes of space dust with water molecules attached getting incorporated into the planet during its formation.
Hallis and her colleagues tracked the water back to its dusty source by looking at 60-million-year-old lava flows from Baffin Island, Canada, that originated deep within Earth’s mantle. Within these lava flow samples, which cooled into rock after erupting, the team found a low ratio of two isotopes of hydrogen—specifically, deuterium to ordinary hydrogen (D/H). The low value strongly indicates that Earth’s water came from the solar nebula—a swirling disk of dust around the early Sun from which the planets and other bodies in our solar system formed.
Heavy Water Holds the Answer
Deuterium is a neutron-containing hydrogen isotope; water containing deuterium is sometimes referred to as heavy water. Planetary scientists can use the ratio of deuterium to hydrogen within a planet’s water reservoirs to determine the water’s origin. “It’s a signature for where that water came from in the solar system,” said Hallis. Earth’s water resides not just in such familiar locations as oceans, aquifers, and clouds but also in rock or magma permeated with water molecules and located deep within the planet.
Easily accessible reservoirs of water near Earth’s surface, such as the oceans, contain deuterium, but not at the same concentrations as when the planet first formed. The concentrations have changed due to contact with the atmosphere, crust, and upper mantle and exchanges of molecules and atoms between those different layers. Although portions of Earth’s mantle do mix, Hallis thinks that the area of the deep mantle where the Baffin Island lava flows came from was an isolated region of the heterogeneous mantle. Therefore, those flows didn’t mix with magma from the upper mantle or the crust before erupting, which allowed them to preserve their original D/H ratio, she and her team reported in their 13 November paper.
“I think the most important thing is that they’ve made a good attempt to measure Earth’s original D/H ratio,” said planetary scientist Benjamin Weiss of the Massachusetts Institute of Technology (MIT) in Cambridge, who was not involved in this research. He said that measurement marks the first step toward really understanding where our water came from.
Comets and Meteorites Don’t Match Up
Using the Sun as a proxy for the solar system–forming nebula, Hallis and her colleagues found that it is the only body in the solar system with as low a D/H ratio as the lava flows. However, when they compared their lava samples to meteorites, which are shards of comets or asteroids that landed and were found on Earth, all the meteorites had higher D/H ratios. The mismatch indicates that, most likely, neither the parent comets or asteroids nor the shards from such bodies are the source of Earth’s water, Hallis told Eos.
A study published a little more than a year ago in Science also argued that Earth received its water as it was forming, but from really old meteors called carbonaceous chondrites. However, the D/H ratio that Hallis and her colleagues determined for the deep mantle is lower than that of the chondrites, Hallis said, suggesting that something that preceded those meteors—i.e., the nebula’s dust—delivered the water.
Still, MIT’s Weiss said we know too little about the D/H ratios of all asteroids and comets to rule them out as a source of Earth’s water. “The trouble is, we don’t know the full range of meteorites,” he explained. “Our meteorites are just a tiny sample of the enormous diversity of asteroids that are out there.”
In the future, meteorite specialists might discover some of the space rocks with D/H ratios closer to what Hallis and her colleagues found in the lava flows. In the meantime, their findings “definitely make it less probable that [asteroids] are the source,” Weiss said.
Other Planets Formed Similarly
Some scientists challenge the idea that water always existed on Earth because the extreme heat of the early solar system would evaporate any water. Hallis said that although the Earth probably did and still does lose hydrogen and therefore water, she believes the Earth kept most of its water. Earth’s thick atmosphere and its magnetic field, which slows down atmospheric erosion by fending off the wind of charged particles from the Sun, help to reduce water losses. Other rocky planets probably formed with water in the same way as Earth but then lost their water—Mars lost its protective magnetic field and atmosphere, whereas conditions on Venus caused water molecules to break up and release hydrogen into space.
“There’s no reason why, if Earth accreted in this way, that Venus, Mars, and Mercury, and maybe the other rocky bodies in the solar system, wouldn’t have accreted in this way as well,” said Hallis.
—Cody Sullivan, Writer Intern
Citation: Sullivan, C. (2015), Earth’s water came from space dust during planetary formation, Eos, 96, doi:10.1029/2015EO040147. Published on 23 November 2015.