Enhanced sodium abundance in Mercury's north polar region revealed by the MESSENGER Gamma-Ray Spectrometer

MESSENGER Gamma-Ray Spectrometer measurements demonstrate that the abundance of Na varies across the surface of Mercury. The maximum Na/Si abundance ratio of 0.20. ±. 0.03 by weight (~5. wt% Na) is observed at high northern latitudes and is significantly larger than the equatorial Na/Si ratio of 0.11. ±. 0.01 (~2.6. wt% Na). Comparisons of forward-modeled surface distributions with the gamma-ray measurements suggest that the observed distribution of Na can be explained by differences in elemental composition between the volcanic smooth plains units and heavily cratered terrain. The comparison improves when thermally driven depletion of Na from areas near Mercury's hot poles is included. When combined with other MESSENGER data sets, these results indicate that the smooth plains units include substantial abundances of alkali feldspars. Thermal depletion of Na from the hot poles without an assumed underlying compositional variability can also reproduce the measured Na/Si distribution, but that mechanism fails to account for other MESSENGER observations that support the presence of higher abundances of feldspars in the smooth plains units.