New Light Shed on the Origins of Earth’s Water
Scanning electron microscope image of a Baffin Island picrite (type of basaltic rock). The mineral olivine, shown as abundant mid-gray color cracked grains (A), hosts glassy melt inclusions (B) containing tiny amounts of water sourced from Earth’s deep mantle. Image by Lydia J. Hallis.
Water covers more than two-thirds of Earth’s surface, but its exact origins have been something of a mystery. Scientists have long been uncertain whether water was present at the formation of the planet, or if it arrived later, perhaps carried by comets and meteorites. Now researchers from the UH NASA Astrobiology Institute, using advanced ion-microprobe instrumentation, have found that rocks from Baffin Island in Canada contain evidence that Earth’s water was a part of our planet from the beginning.
The research team was led by cosmochemist Lydia Hallis, then a postdoctoral fellow at the UH NASA Astrobiology Institute (UHNAI) and now Marie Curie Research Fellow at the University of Glasgow, Scotland.
The Cameca 1280 ims ion microprobe, housed at the University of Hawai‘i, allowed researchers to focus on minute pockets of glass inside these scientifically important rocks, and to detect the tiny amounts of water within. The ratio of hydrogen to deuterium in the water provided them with valuable new clues as to its origins.
Hydrogen has an atomic mass of one, while deuterium, an isotope of hydrogen also known as “heavy hydrogen,” has an atomic mass of two. Scientists have discovered that water from different types of planetary bodies in our solar system have distinct hydrogen to deuterium ratios.
Hallis explained, “The Baffin Island rocks were collected back in 1985, and scientists have had a lot of time to analyze them in the intervening years. As a result of their efforts, we know that they contain a component from Earth’s deep mantle. On their way to the surface, these rocks were never affected by sedimentary input from crustal rocks, and previous research shows their source region has remained untouched since Earth’s formation. Essentially, they are some of the most primitive rocks we’ve ever found on Earth’s surface, and so the water they contain gives us an invaluable insight into Earth’s early history and where its water came from.”
“We found that the water had very little deuterium, which strongly suggests that it was not carried to Earth after it had formed and cooled. Instead, water molecules were likely carried on the dust that existed in a disk around our Sun before the planets formed. Over time this water-rich dust was slowly drawn together to form our planet.
“Even though a good deal of water would have been lost at the surface through evaporation in the heat of the formation process, enough survived to form the world’s water. It’s an exciting discovery, and one which we simply didn’t have the technology to make just a few years ago. We’re looking forward to further research in this area in the future.”
IfA astronomer and comet expert Karen Meech, a co-author of the study and the head of UHNAI, isn’t so sure that water contributed by comets can be ruled out entirely yet. This is because so far we have samples from only a very small number of comets and their D/H ratios vary by a large amount.
The other UH co-authors are Gary Huss, Kazuhide Nagashima, and G. Jeffrey Taylor of the Hawai‘i Institute of Geophysics and Planetology, and Mike Mottl of the Department of Oceanography.