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He set out to measure the amount of “heavy” oxygen-18 and “light” oxygen-16 in the crystals.

These can arise on land when water interacts with rock to form clays; when the clays get melted down, zircons that form from the magma can inherit the heavy oxygen isotope signature.

In addition to being one of the oldest meteorites ever discovered, it is also the only meteorite that contains cosmic symplectite — or very heavy oxygen isotopes.

Samples from a 3.24-billion-year-old chunk of oceanic crust left on Australia’s mainland were far richer in a heavy oxygen isotope than the present-day oceans.

Because water loses this heavy oxygen when rain reacts with the continental crust to form clays, its abundance in the ancient ocean suggests the continents had barely emerged by that point, Johnson and Wing concluded in a 2020 Nature Geoscience study.