Semiannual Oscillation of the Lunar Exosphere: Implications for Water and Polar Ice

A time-dependent simulation of the argon-40 exosphere of the Moon shows that the semiannual oscillation of argon detected by the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft is consistent with adsorptive respiration in seasonal cold traps near the lunar poles. The magnitude of the oscillation requires that high-energy adsorption sites on soil grain surfaces at polar latitudes be as free of water contamination as soils at low latitudes. This requirement is met by the combination of two generally ignored water removal mechanisms: solar wind bombardment of exposed adsorption sites and the serpentinization reaction of water with olivine. The significance of these processes is supported by the lack of evidence of water in Lunar Atmosphere and Dust Environment Explorer data, which, in turn, establishes an upper bound for exospheric transport of water to polar traps at less than 10(14) molecules/Ga. Plain Language Summary The neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft recorded a gradual rise and then fall of atmospheric argon-40 that is consistent with a 140-day segment of a semidraconic oscillation. The semidraconic oscillation of argon is important because its existence has harsh implications for the accumulation of water in polar cold traps. Simulations show that the existence of the oscillation implies respiration of argon atoms in seasonal cold traps near both poles, which in turn requires that polar soil grain surfaces have significant areas of high-energy adsorption sites that are not contaminated by water molecules. The paper argues that such extreme cleanliness can be explained by two previously ignored processes. One is surface scouring by solar wind bombardment of the lunar surface, which leads to escape or scatter to lower latitudes. The other is sequestration of water by the reaction of olivine with water, a process that is known as serpentinization. Coupled with meteoritic gardening, these process are capable of removing water from the lunar atmosphere faster than current estimates of water sources. This conclusion does not preclude the assimilation of water in permanent traps, but it severely reduces the amount of water available for assimilation.