To host life, our home planet needed some key ingredients: carbon, nitrogen, and water. Information on where these crucial chemical entities came from, and when, isn’t so clear. To search for clues, researchers look to noble gases, particularly neon, argon, krypton, and xenon, as tracers.
Now, improved experimental techniques have made it easier to untangle underutilized krypton isotopes in gas trapped in rocks, which may provide new hints about the pasts of Earth and other planets.
Planets’ Shrouded Pasts
Billions of years ago, a swirling cloud of dust and gas started forming our solar system. Out of this cosmic hurricane, the Sun and the planets were born.
Exactly how the rocky planets (including Earth) took shape is still up for debate. In one scenario, planetary building blocks called planetesimals snatched up pebbles in their vicinities, also stashing carbon, nitrogen, water, and noble gases—stuff that planetary scientists call volatiles. In another scenario, planets grew mostly from planetesimals bashing into each other, the force of collisions melting rock and ejecting volatiles, which were replenished later. Or planets may have gained girth through a mix of these two models. Planetary scientists generally agree that there was one major and final impact with early Earth around 4.5 billion years ago that formed the Moon.
To peer into Earth’s past, researchers examine samples from the mantle that contain noble gases, some of which were delivered during Earth’s formation. These samples include basalts that form at ocean ridges and from undersea volcanic eruptions. When the lava cools, forming these rocks, it traps gases from the mantle.
Unlike the stuff of life, noble gases are reticent elements, steering clear of biological processes and chemical reactions on Earth. Researchers can look to ratios of certain noble gas isotopes that have stuck around since their delivery as signposts for volatiles’ sources, such as comets, meteors, the solar nebula, and solar wind.
Researchers have been doing this kind of work for decades. But “krypton has been one of the most underutilized noble gases,” said Michael Broadley, an isotope geochemist at the University of Lorraine in France who was not involved in the new research. “There’s been very little work done using krypton to determine the origin of volatiles.”
Somewhat tricky to track, krypton isotopes occur at low abundances and are difficult to separate from other noble gases. The name krypton means cryptic, said Sandrine Péron, a geochemist at ETH Zürich in Switzerland. “So it is kind of hidden…it was difficult to detect.”
Péron, then at the University of California, Davis, and her colleagues have come up with new techniques to improve the analysis of samples with tiny amounts of krypton.
When “you’re looking at a little bit of rock with a little bit of gas from the mantle in it, the big problem is air contamination,” said Greg Holland, a geochemist at the University of Manchester in the United Kingdom who was not part of this new work. Noble gases from the atmosphere can swamp the subtle signals of mantle gases.
To avoid air contamination, Péron’s team progressively crushed rocks formed from eruptions beneath glaciers in Iceland and from under the ocean in the Galápagos. As the scientists popped bubbles in the basalts, they checked for atmospheric contamination, which is easily detected from the neon isotopic signature. Whenever bubbles’ gases looked contaminated, they disposed of the gas. By keeping the gas when the neon isotopic composition was close to the mantle’s, the researchers enriched the sample’s mantle-derived krypton prior to analysis. Also, by splitting the process that separates the noble gases into two steps, the team upped their ability to isolate krypton isotopes from other noble gases.
Having the ability to measure krypton isotopes precisely is really useful, said Guillaume Avice, a planetary scientist at the Institut de Physique du Globe de Paris in France who was not involved in the new study. “It’s a new tool…that you can use to build this story” of Earth’s formation, he said.
One reason krypton isotope analyses are useful is that the isotopic signatures in different sources are easy to tell apart. Péron and colleagues saw that their mantle samples’ krypton signatures mostly aligned with those of certain meteorites thought to have been incorporated into proto-Earth around the time of the Moon-forming impact. And because the mantle’s krypton isotope signature doesn’t match that of the atmosphere, another source had to have brought some of Earth’s volatiles, the authors reported last year in Nature.
With this combination of techniques, Péron and colleagues have gotten the best insight yet into the deep mantle’s composition, Broadley said. The study has also “opened a lot of questions, especially about the relationship between Earth and the meteorite records.” Because the heaviest krypton isotope didn’t match the meteoric source, it may require another source to explain it, the authors reported. Reanalyzing the old meteorites with the new methods may help explain the mismatch, Avice said, as could sampling more meteorites.
Péron and her colleagues are currently analyzing krypton isotopes in Martian meteorites, which may provide insights into where Mars’s volatiles originated and why Earth and Mars are so different today.
Venus presents another mystery. Similar in size and close in proximity, Earth and Venus are like twins, Broadley said. But scientists don’t know how their paths to planethood and sourcing of volatiles compare. NASA’s DAVINCI+ mission is slated to surveil Venus’s noble gases. An understanding of the noble gases in Venus’s atmosphere and a comparison with Earth will give us really fundamental insight into why Earth became habitable, he said.
With such tiny amounts of krypton isotopes, Péron said, we can learn more about the evolution of Earth and the solar system.
—Carolyn Wilke (@CarolynMWilke), Science Writer