While planetary researchers have speculated on the presence of the noble gas neon in the Moon’s atmosphere (called a surface boundary exosphere) for years, the Neutral Mass Spectrometer onboard NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft has confirmed its existence for the first time.
Moon. Image credit: Gregory H. Revera / CC BY-SA 3.0.
“The presence of neon in the exosphere of the Moon has been a subject of speculation since the Apollo missions, but no credible detections were made. We were very pleased to not only finally confirm its presence, but to show that it is relatively abundant,” said Dr Mehdi Benna from the University of Maryland and NASA’s Goddard Space Flight Center.
Since the lunar exosphere is extremely tenuous, about 100 trillion times less dense than Earth’s atmosphere at sea level, rocket exhaust and outgassing from spacecraft could easily change its composition.
Most of the Moon’s exosphere comes from the solar wind, a thin stream of electrically conducting gas blown from the surface of the Sun into space at around a million miles per hour. Most of the solar wind is hydrogen and helium, but it contains many other elements in small amounts, including neon.
All these elements impact the Moon, but only helium, neon, and argon are volatile enough to be returned back to space. The rest of the elements will stick indefinitely to the lunar surface.
The LADEE Neutral Mass Spectrometer (NMS) confirms that the lunar 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 NMS conducted systematic measurements of these gases for seven months, which allowed the scientists 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, the instrument 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.
“We were also surprised to find that argon-40 creates a local bulge above an unusual part of the lunar surface, the region containing Mare Imbrium and Oceanus Procellarum,” said Dr Benna, lead author on the study published in the journal Geophysical Research Letters.
“Although the reason for this local enhancement is not yet understood, one could not help to notice that this region happens to be the place where potassium-40 is most abundant on the surface. So there may be a connection between the atmospheric argon, the surface potassium and deep interior sources.
A second surprising behavior of argon was that the overall amount in the lunar exosphere was not constant over time.
Instead, it increased and then decreased by about 25 percent during the course of the LADEE mission.
“This transient source of argon may be the result of enhanced outgassing from the surface that is triggered by tidal stress on the Moon,” Dr Benna said.
NMS also revealed an unexpected source of some of the helium in the lunar exosphere.
“About 20% 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. This helium is being produced at a rate equivalent to about 7 liters per second at standard atmospheric pressure,” Dr Benna said.
“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.”
“These discoveries highlight the limitations of current exospheric models, and the need for more sophisticated ones in the future.”
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M. Benna et al. 2015. Variability of helium, neon, and argon in the lunar exosphere as observed by the LADEE NMS instrument. Geophysical Research Letters, vol. 42, no. 10, pp. 3723-3729; doi: 10.1002/2015GL064120
