Mercury's Sodium Exosphere Dawn Dusk Asymmetry

It is known that the overall pattern of sodium emission in Mercury's exosphere is a repeatable function of Mercury's true anomaly angle, and that the variation with true anomaly angle of the dawn/dusk asymmetry is related to Mercury's 3/2 spin-orbit resonance. The dawn/dusk asymmetry in Mercury's exosphere has been shown be a repeatable function of Mercury's true anomaly angle (Cassidy et al., 2016). We further elucidate the causes of these asymmetries at the particular true anomaly angles where they occur. Because Mercury's orbital eccentricity is large, 0.2, its orbital speed is not constant, being maximum at perihelion and minimum at aphelion. At the same time, the angular rotation rate is constant. The limb speed relative to the sun angle reaches its maximum at aphelion when the orbital speed is minimum, and it reaches a minimum and becomes negative at TAA 340°. As a result, the longitude of the terminator not only does not advance uniformly but moves backward near perihelion. Mercury's limb speed is slightly negative between true anomaly angles 340° and 20° , resulting in the dawn limb moving backward by 1.1°. Although the terminator is moving forward starting at 20°TAA, the longitude of the dawn terminator is almost unchanged until the planet reaches TAA 54° . Therefore the longitude of the dawn terminator does not return to the position when the rock-back commenced until TAA 54°. This is not the position where the rock forward begins but where the longitude of the terminator catches up. We propose that the underlying cause of the dawn/dusk asymmetries revealed by the MESSENGER MASCS data is due to nighttime sequestration of adsorbed sodium, and not to longitudinal differences in composition or to thermal differences.