The search heats up for the source of Earth’s water

In December 2020 a 40-centimetre steel capsule parachuted to Earth, landing next to a bush in the remote Australian outback. Inside the capsule were rock samples from an ancient asteroid that formed during the earliest days of our solar system.

To gather those rocks, the Japanese spacecraft Hayabusa2, spent six years traveling 3.2 billion kilometres to the asteroid, Ryugu, which orbits between Earth and Mars, blew a hole in it with a copper cannonball, sampled rock from the blast site, and made the long journey back to Earth. Those rocks could help reveal the source of the water that ultimately filled our oceans.

“We still don’t have the early history of water on Earth locked down – but the ideas are evolving fast,” says Jessica Barnes, a cosmochemist at the Lunar and Planetary Laboratory at the University of Arizona, who studies water in the early solar system by analysing extraterrestrial materials.

Scientists believe that Earth accreted from rocky building blocks as the Solar System was taking shape 4.6 billion years ago. At the time, the inner Solar System (which today includes Mercury, Venus, Earth and Mars) was too hot for water ice, and therefore water, to be retained.

To explain where Earth’s water came from, researchers had hypothesized that icy asteroids, scattered into the Earth’s path by the formation and growth of Jupiter, had bombarded Earth and delivered tremendous amounts of water in the first few million years of its existence. But new analyses of the chemical and isotopic composition of asteroids and meteorites, using advanced analytical techniques, including secondary ion mass spectrometry (SIMS) and atom probe tomography (APT), suggest such elaborate theories to explain how Earth’s oceans were filled may not be needed.¹

Hunting for hydrogen

Meteorites are chunks of asteroids that fall to Earth, and they come in many forms, from hunks of iron-nickel alloy to highly heterogeneous accretions, known as chondritic meteorites, that can contain metals, silicates, water and organic matter. One rare type of meteorite, enstatite chondrites, contains oxygen, titanium, calcium and molybdenum in isotopic ratios that very closely match that of Earth. “We think that Earth mostly accreted from enstatite chondrite-like materials,” Barnes says.

Until recently enstatite meteorites were considered too dry to have supplied Earth’s water. Specifically, scientists thought enstatite chondrites did not have enough of one of water’s atomic building blocks, hydrogen. “Oxygen is everywhere in rock, in silicates, in oxides,” says Laurette Piani, a cosmochemist at the University of Lorraine. “But hydrogen is much more difficult to gather into solids than oxygen because it is a much more volatile element.” Nevertheless, Piani suspected there might be enough of it in enstatite chondrites to supply early Earth with water. So, the hunt for water boiled down to a hunt for hydrogen.

Specifically, Piani set out to measure the isotopic signature of hydrogen in enstatite chondrites to see if it matched that of Earth’s water. And to determine that signature — that is, the ratio of hydrogen to its heavier isotopic form, deuterium — Piani’s team used a specialized version of secondary ion mass spectrometry (SIMS).

SIMS focuses a high-energy stream of ions on a small part of the sample surface, ejecting material for analysis by high-resolution mass spectrometry. This enables researchers to measure the abundance of elements and their isotope ratios in that part of the sample. Piani used a specialized version of the technique called large-geometry SIMS (LG-SIMS), using an instrument from CAMECA, an instrumentation company in Gennevilliers, France, that provides enhanced precision for cosmochemistry applications. Another specialized version of the technique, NanoSIMS, enables the analysis of features as small as 50 nm.

“If you did a bulk analysis of the meteorite you would just average everything, and then you cannot tell anything useful,” explains Céline Defouilloy, an application scientist at CAMECA, which supplied the LG-SIMS instruments that Piani’s team used. “With SIMS, you can analyse mineral by mineral, and even within individual minerals.”

Tell-tale isotopes

Previously, cosmochemists had used mass spectrometry on crushed samples of bulk enstatite meteorite, but traces of terrestrial water would often contaminate samples, leading researchers to conclude that the meteorites contained little hydrogen of their own. SIMS allowed the researchers to make precise measurements of isotope ratios, even for light elements such as hydrogen, while consuming little sample.²

“For us it was very important to be able to use the SIMS instrument to do in situ analysis at the scale of the mineral, to show that in some of the minerals there are high amounts of hydrogen,” Piani says.

What’s more, the hydrogen in those minerals contained an isotopic signature — a ratio of hydrogen to deuterium — very similar to that of Earth’s water. “Our discovery shows that the Earth's building blocks might have significantly contributed to the Earth’s water,” Piani says. In fact, the team’s analysis shows, they had enough hydrogen to produce enough water to fill our oceans three times over.

Still, Earth’s early building blocks were probably not the sole source of water on the Earth, Piani says. Barnes add that, “I think general consensus is we do still need some contribution of carbonaceous chondrite like material – the type of material we hope to get from the asteroids Bennu and Ryugu.”

“Those of us who study meteorites are really at the mercy of what falls from the sky,” says Tom Zega, a cosmochemist at the Lunar and Planetary Laboratory at the University of Arizona. “A geologist can go out to the field and collect rock, and have context for each sample, but a cosmochemist doesn’t have that luxury.”

Sampling asteroids

In addition to artefacts arising from a meteorite’s journey to Earth’s surface, scientists cannot be quite sure where in the solar system it originated. Asteroid sample-return missions could fill these knowledge gaps, Zega says. “The sample-return missions are very important because we would be able to link one asteroid and its history to one sample for sure.”

In addition to the successful return of Hayabusa2 from the asteroid Ryugu, NASA’s OSIRIS-REx mission has spent more than a year orbiting and studying Bennu, a near-Earth asteroid. After collecting rock samples, it set out back to Earth, and it’s scheduled to arrive in 2023.

Scientists are already planning how to analyse returned samples from both missions, says Barnes, who is part of the OSIRIS-REx mission. “Cosmochemistry is increasingly driven by coordinated microanalysis, where you go from the least destructive to the most destructive technique, she says. “We can’t go back to Bennu to collect more.”

For Bennu samples, this analysis is likely to start without even touching the sample. “Micro x-ray computer tomography, a non-destructive 3D x-ray scan of the object, lets you see into the sample without even making a cut,” Barnes says. Isotopic analysis with SIMS and NanoSIMS will follow, then atom probe tomography that takes apart a small section of the sample atom by atom to capture its 3D atomic structure, and even do an isotopic ratio analysis, she adds.^3,4

Analysis of returned asteroid samples promise something meteorite analysis cannot. From decades of lab research into meteorites, we think we have a handle on Earth’s earliest history, but there is always a chance we have misunderstood a meteorite family’s origins, or misinterpreted the isotopic messages meteorites contain, Zega says. Putting pristine pieces of Bennu and Ryugu under a SIMS ion beam will offer a reality check. If the results align with predictions, it will help corroborate our entire understanding of Earth’s origins. Which could be the launchpad for understanding the origins of life itself.