The spectroscopic detectability of argon in the lunar atmosphere

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 saturati on effects; thus, until we are able to return to the Moon for improved dire ct measurements, we must use remote sensing to establish the daytime abunda nce. In this Letter, we present a complete analysis of the potential for me asuring argon in the lunar atmosphere via emission-line or absorption-line observations. We come to the surprising conclusion that the lower limit est ablished by the in situ lunar argon measurements implies that any absorptio n-line measurement of argon in the lower, dayside lunar atmosphere requires analysis in the optically thick regime. In light of this result, we presen t the results of our extreme-ultraviolet spectrograph sounding rocket obser vations of the lunar occultation of Spice, which provide a new upper limit on the abundance of argon in the daytime lunar atmosphere. We also reanalyz e a recently reported weak detection by the ORFEUS satellite of lunar atmos pheric Ar I lambda 1048 in emission and show that those data are inconsiste nt 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 c omplete curve-of-growth analysis and improved values for the argon fluoresc ent emission rates from radiation and solar wind interactions. We find that the detection reported by ORFEUS would imply an argon surface density sign ificantly greater than the total surface density of the lunar atmosphere fo r argon accommodated to typical daytime surface temperatures (similar to 40 0 K) and also is inconsistent with a high-density transient event. Therefor e, we conclude that the reported argon detection is untenable.