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.Another piece of the puzzle.
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.
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