Structure of the ionized lunar sodium and potassium exosphere: Dawn‐dusk asymmetry

Abstract

We present latitude and longitude distributions of Na+ and K+ fluxes from the Moon derived from Kaguya low‐energy ion data. Although the latitude distribution agrees with previous ground‐based telescope observations, dawn‐dusk asymmetry has been determined in the longitude distribution. Our model of the lunar surface abundance and yield of Na and K demonstrates that the abundance decreases to approximately 50% at dusk compared with that at dawn due to the emission of the exospheric particles assuming the ion fluxes observed by Kaguya are proportional to the yield. It is also implied that the surface abundance of Na and K need to be supplied during the night to explain the observed lunar exosphere with dawn‐dusk asymmetry. We argue that the interplanetary dust as well as grain diffusion and migration/recycling of the exospheric particles may be major suppliers.

High‐Resolution Potassium Observations of the Lunar Exosphere

“…We interpret this finding as the likely consequence of meteoroid impacts. The light gray and gray lines in Figure 3 simulate PSD processes, with the light gray line given a 20 • peak shift from noon toward dawn (similar to Yokota et al, 2014, who reported a dawn-dusk density asymmetry of Na + and K + measured by the Kaguya lunar orbiter) and the gray line centered at the subsolar point. The version of the model used here includes the effect of radiation pressure on potassium trajectories.…”

The lunar surface-exosphere connection: Measurement of secondary-ions from Apollo soils

“…These ions can be observed locally by mass spectrometers that allow the identification of charged exospheric components. Measurements from AMPTE, WIND, and SELENE have identified Ar + , Na + , K + , Si + , Al + , C + , and O + plus solar wind ions (Hilchenbach et al, 1993;Stern, 1999;Yokota et al, 2009Yokota et al, , 2014 Secondary ion/atom emission by sputtering is a physical process which occurs when energetic (>20 eV) primary ions impact atoms on a surface, transferring energy and momentum such that one (or more) of the surface atoms or molecules are ejected. In the laboratory, mass analysis of secondary ions (positive or negative) ejected by incident energetic ($keV) ions is termed secondary ion mass spectroscopy (SIMS) and is often used to infer surface specific composition.…”

Structure and composition of the distant lunar exosphere: Constraints from ARTEMIS observations of ion acceleration in time‐varying fields

“…3). We do note that Kaguya observations indicate a high flux of K + ions compared to Al + and Na + , even at a fixed 100 km altitude [Yokota et al, 2014], so we take this possibility seriously. Another plausible contribution to a hot heavy exospheric population could come from photodissociation of oxides such as CaO and FeO, which could lead to a hot population of dissociation byproducts, as recently observed at Mercury [Killen and Hahn, 2015].…”

“…Potassium (K + ) seems to provide the most likely candidate given our present knowledge, but this would require higher than expected production of K + relative to Na + and Al + , even when accounting for the fact that the probes effectively sample higher altitudes for these lighter species (see Figure 3). We do note that Kaguya observations indicate a high flux of K + ions compared to Al + and Na + , even at a fixed 100 km altitude [Yokota et al, 2014], so we take this possibility seriously. Another plausible contribution to a hot heavy exospheric population could come from photodissociation of oxides such as CaO and FeO, which could lead to a hot population of dissociation byproducts, as recently observed at Mercury [Killen and Hahn, 2015].…”