Some scientists have proposed that the gases in Earth’s atmosphere originated within the planet, says Holland. According to those arguments, the atmosphere either seeped out of the Earth as the planet gradually cooled or were expelled from the crust when large numbers of asteroids pummeled the planet and melted its surface around 3.9 billion years ago. But new isotopic evidence gathered by Holland and his colleagues suggests that those scenarios probably aren’t right.
The researchers analyzed samples of gas pulled from a natural reservoir of carbon dioxide that lies several hundred meters below northeastern New Mexico. There, Holland explains, krypton and xenon that originate deep within the Earth — gases that presumably accumulated when the planet coalesced billions of years ago — mix with small amounts of atmospheric krypton and xenon carried downward by rainfall and groundwater.
Ratios of isotopes of krypton and xenon present in the geologic reservoir don’t match the ratios seen in today’s atmosphere. In particular, heavier isotopes of each gas appear in larger proportions in the subterranean samples than they do in the atmosphere. So it’s unlikely that large amounts of these atmospheric gases came from within the Earth, the team argues.
Analyses also show that if the geologic gas samples weren’t tainted by atmospheric krypton and xenon, the isotope ratios measured for those gases would match the ratios seen in meteorites. That’s another sign that neither the planet nor meteorites were the source of the isotopically light xenon and krypton in today’s atmosphere, Holland notes.
Instead, he and his colleagues propose, the krypton and xenon now present in the air — and many other atmospheric components as well — may be remnants of gas clouds swept up by the newly forming Earth. Or, they suggest, the gases may have been delivered to Earth by comets, in which the proportions of light isotopes for xenon and krypton are relatively higher.
“This is an important piece of work, and an extremely interesting contribution to studies of how the atmosphere evolved,” says Robert Pepin, an astrophysicist at the University of Minnesota in Minneapolis. But the team’s results aren’t unambiguous, he notes. The krypton and xenon in today’s atmosphere may, for example, be a mix of isotopically light gases delivered by comets and the heavier versions originating within the Earth.