The Moon has been through some rough times.
Just a few hundred million years after its formation, it experienced what astronomers like to call the period of Late Heavy Bombardment. During this time — lasting from about 4.1 to 3.8 billion years ago — the Moon (and indeed, the entire inner solar system) endured endless artillery strikes from interplanetary debris. On the Moon, this triggered a series of volcanic eruptions that left its surface a hellish landscape filled with lava flows that stretched on for hundreds of miles.
In a recent study published in Earth and Planetary Science Letters, a pair of researchers showed that between 3 and 4 billion years ago, when the primordial Moon underwent this extremely violent period of volcanic activity, it spewed out enough gas to produce a relatively thick atmosphere that persisted for about 70 million years.