Molecules entombed inside pristine magmas suggest that Earth’s water came from soggy dust, not icy comets.
The relative abundance of a heavier variety of hydrogen called deuterium serves as a fingerprint of where in the solar system a reservoir of H2O originated (SN: 5/16/15, p. 18). Previous work hunting for the source of Earth’s water measured deuterium in seawater, but that’s a tainted metric, researchers report in the Nov. 13 Science. Aboveground processes such as hydrogen atoms leaking into space can hike deuterium concentrations in the planet’s surface water (SN: 9/5/15, p. 8).
The researchers found that deuterium levels in water trapped inside molten rock unaltered since the planet’s early days are significantly lower than those in seawater. The lower deuterium fingerprint for Earth’s primordial water hints that the world’s wetness resulted from water-soaked dust grains present during the planet’s assembly, the researchers conclude.
“Water came in during Earth’s formation and has been around ever since,” says study coauthor Lydia Hallis, a planetary scientist at the University of Glasgow. “It wasn’t added later.”
Not all experts agree with this conclusion. But if it holds up, it would cast doubt on previously proposed water sources such as a fortuitous bombardment of icy space rocks around 4.5 billion to 3.8 billion years ago, says study coauthor Karen Meech. “If you need exotic mechanisms to get water to Earth, then just being in a habitable zone doesn’t mean you have all the ingredients for life,” says Meech, a planetary scientist at the University of Hawaii in Honolulu.
Deuterium forms a more stable bond with oxygen than hydrogen does as the temperature drops. So water enriched in deuterium is a sign that the H2O formed in the solar system’s chillier regions, such as the outer areas where comets originate. Water low in deuterium, on the other hand, is more likely to come from the warmer, inner solar system.
Uncovering which region supplied Earth’s water requires collecting H2O molecules left over from the planet’s deep past. Hallis, Meech and colleagues found a source of primitive molecules tucked away in Earth’s deep mantle. Volcanoes occasionally spit out water-containing molten rock from the mantle, providing an accessible record of Earth’s original composition.
The team had a few false starts. Hawaiian lavas were contaminated with water from a nearby aquifer. Lava samples collected from Icelandic and Canadian volcanoes, however, seemed largely unperturbed by processes that alter deuterium levels. The lavas formed with small bits of glass encased inside a mineral called olivine. This glass serves as an unperturbed time capsule preserving the molten rock’s original composition.
Analyzing the hydrogen atoms trapped inside the glass revealed deuterium levels as low as 122 parts per million, roughly 22 percent less than the 156 ppm found in seawater. This scarcity doesn’t match the large deuterium abundances found in ice-laden comets, Hallis says. Earth’s water instead probably originated from a warmer part of the early solar system. Water molecules could have stuck onto dust grains floating in the gas cloud that birthed the sun and planets. As Earth assembled, this waterlogged dust created a wet planet right from the start, Hallis and colleagues propose.
Although cosmic dust may have provided some of Earth’s water, it didn’t provide all of it, contends cosmochemist Conel Alexander of the Carnegie Institution for Science in Washington, D.C. As the planet formed, Earth’s raw materials would have heated up and dried out, he says.