Krypton isotopes provide new clues to planets’ past

To support life, our planet needed key ingredients: carbon, nitrogen and water. Information about the origin and date of these crucial chemical entities is not so clear. To search for clues, researchers turn to noble gases, particularly neon, argon, krypton and xenon, as tracers.

Now, improved experimental techniques have made it easier to disentangle underutilized krypton isotopes from gas trapped in rocks, which may provide new insights into the past of Earth and other planets.

Hidden pasts of the planets

Billions of years ago, a swirling cloud of dust and gas began to form our solar system. From this cosmic hurricane, the Sun and the planets were born.

This cross-section shows gas bubbles in the basalts in an image taken by a technique called X-ray microtomography. Credit: Sandrine Péron

Exactly how the rocky planets (including Earth) took shape is still under debate. In one scenario, planetary building blocks called planetesimals grabbed pebbles from their surroundings, also storing carbon, nitrogen, water and noble gases – things planetary scientists call volatiles. In another scenario, the planets grew primarily from planetesimals colliding with each other, the force of the collisions melting rock and ejecting volatiles, which were later replenished. Or the planets may have gained in circumference from a mixture of these two patterns. Planetary scientists generally agree that there was a major and final impact with early Earth about 4.5 billion years ago that formed the Moon.

To peer into Earth’s past, researchers examine samples of the mantle that contain noble gases, some of which were released during Earth’s formation. These samples include basalts that form on ocean ridges and from underwater volcanic eruptions. As the lava cools, forming these rocks, it traps gases in the mantle.

Unlike the stuff of life, the noble gases are reluctant elements, avoiding biological processes and chemical reactions on Earth. Researchers can consider the ratios of certain noble gas isotopes that have remained since delivery as warning signs for volatile sources, such as comets, meteors, the solar nebula, and the solar wind.

The name krypton means cryptic. “So it’s kind of hidden…it was hard to detect.”

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 has been very little work done using krypton to determine the origin of volatiles.”

Somewhat difficult to track, krypton isotopes occur in low abundance 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’s kind of hidden…it was hard to detect.”

glowing rocks

Péron, then at the University of California at Davis, and his colleagues developed new techniques to improve the analysis of samples containing small amounts of krypton.

When “you look at a bit of rock with a bit of mantle gas, the big issue is contamination in the air,” said Greg Holland, a geochemist at the University of Manchester in the UK who wasn’t a party. of this new job. The noble gases of the atmosphere can overwhelm the subtle signals of mantle gases.

To avoid air contamination, Péron’s team gradually crushed rocks formed by eruptions under glaciers in Iceland and under the ocean in Galapagos. When the scientists burst bubbles in the basalts, they checked for atmospheric contamination, which is easily detected from the isotopic signature of neon. Whenever the gases in the bubbles seemed contaminated, they got rid of the gas. By retaining the gas when the isotopic composition of neon was close to that of the mantle, the researchers enriched the mantle-derived krypton of the sample before analysis. Additionally, by splitting the process that separates noble gases into two steps, the team improved its ability to isolate krypton isotopes from other noble gases.

The new research has given the best insight yet into the composition of the deep mantle.

Having the ability to accurately measure krypton isotopes 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 of the reasons krypton isotope analyzes are useful is that isotopic signatures from different sources are easy to distinguish. Péron and his colleagues found that the krypton signatures of their mantle samples mostly aligned with those of some meteorites that would have been embedded in proto-Earth at the time of the Moon’s formation impact. And because the isotopic signature of mantle krypton does not match that of the atmosphere, another source must have brought in some of Earth’s volatiles, the authors reported last year in Nature.

With this combination of techniques, Péron and his colleagues got the best insight into the composition of the deep mantle, Broadley said. The study also “opened up many questions, particularly about the relationship between Earth and meteorite records.” Because the heaviest krypton isotope did not match the meteoric source, it may require another source to explain it, the authors reported. Reanalyzing old meteorites with the new methods can help explain the lag, Avice said, as can sampling more meteorites.

Beyond Earth

Péron and his colleagues are currently analyzing krypton isotopes in Martian meteorites, which could provide insight into the origin of Mars’ volatiles and why Earth and Mars look so different today.

Venus presents another mystery. Similar in size and close to each other, Earth and Venus are like twins, Broadley said. But scientists don’t know how their paths to the planet and volatile supply compare. NASA’s DAVINCI+ mission is scheduled to monitor Venus’ noble gases. An understanding of the noble gases in Venus’ atmosphere and a comparison with Earth will give us really fundamental insight into why Earth became habitable, he said.

With such small amounts of krypton isotopes, Péron said, we can learn more about the evolution of Earth and the solar system.

—Caroline Wilke (@CarolynMWilke), science writer

Quote: Wilke, C. (2022), Krypton isotopes provide new clues to planetary pasts, Eos, 103, https://doi.org/10.1029/2022EO220226. Posted May 4, 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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