The Spectroscopic Detectability of Argon in the Lunar Atmosphere

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Abstract
Direct measurements of the abundance of argon in the lunar atmosphere were made in 1973 by instruments placed on the Moon during the Apollo 17 mission, but the total daytime abundance is unknown because of instrument saturation effects; thus, until we are able to return to the Moon for improved direct measurements, we must use remote sensing to establish the daytime abundance. In this Letter, we present a complete analysis of the potential for measuring argon in the lunar atmosphere via emission-line or absorption-line observations. We come to the surprising conclusion that the lower limit established by the in situ lunar argon measurements implies that any absorption-line measurement of argon in the lower, dayside lunar atmosphere requires analysis in the optically thick regime. In light of this result, we present the results of our extreme-ultraviolet spectrograph sounding rocket observations of the lunar occultation of Spica, which provide a new upper limit on the abundance of argon in the daytime lunar atmosphere. We also reanalyze a recently reported weak detection by the ORFEUS satellite of lunar atmospheric Ar I λ1048 in emission and show that those data are inconsistent with the emission being due to argon over a wide range of temperatures (up to at least 2000 K). This result is primarily due to our use of a more complete curve-of-growth analysis and improved values for the argon fluorescent emission rates from radiation and solar wind interactions. We find that the detection reported by ORFEUS would imply an argon surface density significantly greater than the total surface density of the lunar atmosphere for argon accommodated to typical daytime surface temperatures (~400 K) and also is inconsistent with a high-density transient event. Therefore, we conclude that the reported argon detection is untenable.
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