Although this atmosphere was extremely thick by lunar standards, it pales in comparison to that of Earth. At its densest (around 3.5 billion years ago), the Moon’s atmosphere would exert a pressure of about 1 kilopascal, whereas at sea level on Earth, we experience around 100 kilopascals of pressure.
In order to calculate how much gas was present in the ancient lunar atmosphere, research scientist Debra H. Needham of NASA Marshall Space Flight Center and senior staff scientist David A. Kring of the Lunar and Planetary Institute (LPI) analyzed ancient lava flows on the Moon’s surface called maria, as well as Moon rocks collected during the Apollo 15 and 17 missions. By examining the rocks, the researchers determined what types of gases were present, while mapping the solidified lava flows allowed them to estimate the total volume of gas that was produced.
In the Apollo rock samples, the researchers found evidence of carbon monoxide, hydrogen and oxygen (the ingredients for water), sulfur, and a number of other volatile gases. Furthermore, the researchers used the rocks to calculate when the most intense periods of lunar volcanism occurred — about 3.8 and 3.5 billion years ago.