The Galápagos Islands’ extraordinary biodiversity famously helped inspire Charles Darwin to formulate his theory of the evolution of life on Earth. But the islands also offer a window into our planet’s even deeper past. The islands’ volcanoes—like the one shown here, and like those in Iceland and a few other parts of the world—sit on top of a mantle plume that draws on a region of Earth’s interior that’s been relatively undisturbed since at least the first 100 million years of the planet’s history. The minerals and gases they bring to the surface are thus a measure of Earth’s original composition.

Early Earth was a hot place. The young planet was constantly enduring energetic collisions with planetesimals that it hadn’t yet cleared out of its orbit. It wouldn’t seem like volatile substances such as water, nitrogen, and carbon compounds could stick around long in such an environment. Because Earth clearly does contain those substances, one theory is that they must have been delivered later, perhaps by a comet.

To test that idea, Sandrine Péron (a postdoc at the University of California, Davis, at the time she did the work, now at ETH Zürich) and colleagues sampled primordial mantle material from the Galápagos and Iceland and analyzed their krypton isotopes.

Why krypton? Like the elemental building blocks of life, it’s volatile. Its six naturally occurring isotopes are present in different ratios in air, the Sun, and meteorites. As a noble gas, it doesn’t participate in chemical reactions that can change its isotopic composition over time.

But there’s not much krypton in volcanic rock—and the little that there is could just as easily have come from air that infiltrated the lava after it erupted as from the original mantle gas. Previous efforts to analyze primordial mantle krypton have been stymied by the possibility of atmospheric contamination.

Péron developed a way around that problem. She’d crush the sample a bit at a time, and at each step she’d look for clues in the other gases released—particularly neon—to see whether they’d been contaminated by air. If they were, she’d discard all the krypton she collected at that step; if not, she’d keep it.

The figure shows the results of that technique applied to the Galápagos and Iceland samples. The krypton compositions closely match each other, and in five of the six isotopes they match phase Q, a substance found in a class of meteorites called carbonaceous chondrites. (The line marked ‘AVCC’ shows the average of all carbonaceous chondrite materials.) Carbonaceous chondrites are thought to represent the original composition of the outer solar system, where volatiles could more easily condense (see the article by Bernard Wood, Physics Today, December 2011, page 40).

The researchers conclude that early Earth had at least some volatiles, perhaps from a source similar to the carbonaceous chondrites. But because mantle krypton doesn’t match the krypton in present-day air, the early volatiles couldn’t have been the only ones the planet ever received. More had to be delivered later, from some as-yet-undetermined source. (S. Péron et al., Nature 600, 462, 2021.)