Long‐Term Observations and Physical Processes in the Moon's Extended Sodium Tail

Abstract

The history of astronomy book by Arthur Berry (1898) set the foundation for the explosion of knowledge about the cosmos that would come in the twentieth century. Towards the end of his narrative, he describes possible new lunar science dealing with the surface, and then concludes: "The question of a lunar atmosphere is more difficult: if there is air its density must be very small, some hundredfold less than that of our atmosphere at the surface of the earth; but with this restriction there seems to be no bar to the existence of a lunar atmosphere of considerable extent, and it is difficult to explain certain observations without assuming the existence of some atmosphere." A half-century later, at the dawn of the Space Age, a new history of astronomy by Pannekoek (1961) described how past stellar occultation-by-moon experiments indicated that the lunar atmosphere had to be less than 1/2000th of the density of the Earth's. He then asked:

Comparative Na and K Mercury and Moon Exospheres

“…The absence could indicate that vapour produced by showers is relatively cold or that meteoroid showers do not augment the overall bulk exosphere except on rare occasions. Baumgardner et al (2021) found significant correlations between annual variations in the sporadic influx and the Na tail spot brightness, however, signifying that the sporadic meteor influx is more influential than transient meteoroid showers in driving atmospheric escape.…”

“…Colored dots highlight data within two days of a meteor shower's peak, accounting for the Moon-Earth transit time. The pink plus sign marks the initial discovery of the feature, following the 1998 Leonids shower (Baumgardner et al 2021) At least in part, it can likely explain the 20% decrease in sodium surface abundance between 1st and 3rd Quarter that was reported from measurements by the SELENE (Kaguya) orbiter (Kagitani et al 2010). By the time atoms have Doppler shifted into the continuum due to their increasing heliocentric radial velocity when moving away from the Moon, they also scatter up to 20 times more solar photons.…”

“…Line et al (2012) observed a mean gas velocity of 12.5 km/s in the spot with measurable emissions of up to 30 km/s originating from down-tail distances of 1.5 million km. Baumgardner et al (2021) archive of all-sky imaging data to study the spot's apparitions 2006-2019. Figure 7 shows the duration and brightness of the apparition.…”

The 2022 Encounter of the Outburst Material from Comet 73P/Schwassmann--Wachmann~3

“…However, the dominance of < 100 µm meteoroids as well as the low arrival speed seem to suggest that few of these events can be detected as lunar flashes from the ground. The dominance of < 100 µm meteoroids, on the other hand, may provide insight into the driver of the lunar sodium tail which appears to be correlated with the rate of sporadic meteors that are of similar sizes (Baumgardner et al 2021).…”

Sodium Distribution on the Moon

“…Since these measurements are highly localized and limited in spatial extent, we do not have a complete understanding of how sodium and such minor elements vary across the Moon. Though Na is a minor component of the lunar regolith, it is well observed in the lunar exosphere which extends beyond one lunar radius, with a tail (Smith et al 1999;Baumgardner et al 2021) that extends much further. Ground (Potter & Morgan 1988;Killen et al 2019Killen et al , 2021 and in-orbit (Szalay et al 2016;Colaprete et al 2016) measurements of the sodium exosphere have shown its temporal and spatial variability.…”