The protoplanets that collided to make the Earth may themselves have had atmospheres and oceans. Venus has vastly more argon and neon than Earth: fossil evidence, perhaps, of protoplanetary atmospheres? Noble gases are the flotsam of the Solar System.They seem simple: they shun chemistry, they are difficult to freeze, and they accumulate in atmospheres. We see them as passive tracers of our cosmogonic theories, especially theories that address the origin and evolution of planetary volatiles. Perhapsit is a measure of our theories that every newly probed atmosphere has surprised us. Yet the temptation to read profound meaning into noble-gas abundances remains strong. On page 842 of this issue, Genda and Abe ask whether atmospheres can survive a series of giant impacts, such as the collision with Earth that formed our Moon. The answers are ambiguous, with argon agreeable but neon cryptic.
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