The meteoroid stream of comet Encke at Mercury: Implications for MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations of the exosphere

Abstract

We test the hypothesis that an annually repeatable Ca emission excess in Mercury's exosphere at a True Anomaly Angle (TAA) of 25° ± 5° is due to particles from comet 2P/Encke impacting the surface. By simulating the dynamical evolution of Encke particles under planetary perturbations and Poynting-Robertson drag, we find that millimeter-sized grains ejected 1-2 ×104 years ago encounter Mercury at TAA = 350°-30°. The timing of the excess emission is consistent with a major dust release episode 20 kyr ago, possibly due to Encke progenitor breakup. The emission mechanism is likely the direct injection of impact-liberated Ca into sunlight rather than nightside surface adsorption for subsequent release at dawn. The timing of dust release from the comet depends on this mechanism; a 10 kyr age is implied by the direct-injection scenario.