Evidence for a Localized Source of the Argon in the Lunar Exosphere

Abstract

We perform the first tests of various proposed explanations for observed features of the Moon's argon exosphere, including models of the following: spatially varying surface interactions; a source that reflects the lunar near‐surface potassium distribution; and temporally varying cold trap areas. Measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE) are used to test whether these models can reproduce the data. The spatially varying surface interactions hypothesized in previous work cannot reproduce the persistent argon enhancement observed over the western maria. They also fail to match the observed local time of the near‐sunrise peak in argon density, which is the same for the highland and mare regions and is well reproduced by simple surface interactions with a ubiquitous desorption energy of 28 kJ mol−1. A localized source can explain the observations, with a trade‐off between an unexpectedly localized source or an unexpectedly brief lifetime of argon atoms in the exosphere. To match the observations, a point‐like source requires source and loss rates of ∼1.9 × 1021 atoms s−1. A more diffuse source, weighted by the near‐surface potassium, requires much higher rates of ∼1.1 × 1022 atoms s−1, corresponding to a mean lifetime of just 1.4 lunar days. We do not address the mechanism for producing a localized source, but demonstrate that this appears to be the only model that can reproduce the observations. Large, seasonally varying cold traps could explain the long‐term fluctuation in the global argon density observed by LADEE, but not that by LACE.

Seasonal Polar Temperatures on the Moon

“…Modeling by Prem et al () shows that surface roughness can have this effect by providing cold surfaces at high latitudes that can act as temporary reservoirs for molecules near the poles. A semiannual oscillation in argon observed by the Lunar Atmosphere and Dust Environment Explorer spacecraft has been linked to sequestration in transitory seasonal cold traps (Hodges & Mahaffy, ; Kegerreis et al, ). Hodges () notes that the magnitude of seasonal oscillation implies that significant areas of high‐energy adsorption sites on soil grains in the seasonal cold traps are free of water molecules, placing a restrictive upper bound on the exospheric transport of water.…”

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

“…It should be noted that an alternative scenario is proposed by Kegerreis et al () that argon‐40 originates in, and leaks out of, a near‐surface deposit of K‐rich material that underlies the western maria. Since there is no evidence of seismic wave attenuation that would signal a semimolten state in this region, and since Kegerreis et al do not discuss why argon should escape from subsolidus rock, their theory will be ignored here.…”

Volatiles and Refractories in Surface-Bounded Exospheres in the Inner Solar System

“…Some of them are temporarily sequestered at depth (where they arrived after diffusing downwards during the lunar night) and are released much later (mid-day). This mechanism, proposed by Kegerreis et al (2017), could explain the slight time delay from dawn of the peak 40 Ar exospheric density recorded by LACE and LADEE without requiring the high activation energy all over the lunar surface proposed by Hodges and Mahaffy (2016). Interestingly, a similar mechanism (the "thermal pump") has been proposed for other species -most notably waterat the Moon (Schörghofer and Taylor 2007;Schörghofer and Aharonson 2014), Mercury (Vasavada et al 1999), and Mars (Mellon and Jakosky 1993).…”

“…As discussed in Sect. 2.2, Kegerreis et al (2017) showed that this process, by which argon atoms migrate downwards during the night and are released during the day later than dawn, can explain the half-an-hour delay in the sunrise exospheric density bulge measured by both LACE and LADEE.…”

“…Hodges and Mahaffy (2016) found that the argon-40 bulge can be explained by a lower activation energy in that region and a very high activation energy (∼24,000 cal mol −1 ) everywhere else. On the other hand, Kegerreis et al (2017) found that the bulge can be explained by an enhanced outgassing rate in that region (the western maria). Modeling LADEE/NMS data, they found that, in general, 40 Ar has higher exospheric densities above maria, compared to highlands.…”