NMS also revealed an unexpected source of some of the helium in the lunar exosphere. "About 20 percent of the helium is coming from the moon itself, most likely as the result from the decay of radioactive thorium and uranium, also found in lunar rocks," said Benna. This helium is being produced at a rate equivalent to about seven liters per second at standard atmospheric pressure.
"The data collected by the NMS addresses the long-standing questions related to the sources and sinks of exospheric helium and argon that have remained unanswered for four decades," said Benna. "These discoveries highlight the limitations of current exospheric models, and the need for more sophisticated ones in the future.