The moon about 4 billion years ago, had an atmosphere that lasted for only 70 million years. And this was determined from the gaseous composition of the volcanic moon rock samples brought back from the Apollo mission, and via calculation of how much of these gaseous compositions rose from the atmosphere.
The atmosphere was thickest during the peak in volcanic activity about 3.5 million years ago and, when created, would have persisted for about 70 million years before being lost to space.
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