The LADEE NMS instrument confirms that the moon's exosphere is made up of mostly helium, argon, and neon. Their relative abundance is dependent on the time of day on the moon--argon peaks at sunrise, with neon at 4 a.m. and helium at 1 a.m. The instrument conducted systematic measurements of these gases for seven months, which allowed the team to understand how these gases are supplied to the exosphere, and how they are ultimately lost.
While most of the lunar exosphere comes from the solar wind, NMS showed that some gas comes from lunar rocks. Argon-40 results from the decay of naturally occurring radioactive potassium-40, found in the rocks of all the terrestrial planets as a leftover from their formation.