Effects of Comet Encke's Meteoroid Stream on the Seasonal Variation of Mercury's Ca Exosphere

Abstract Mercury's calcium (Ca) exosphere, observed by NASA's MESSENGER mission, exhibits high temperatures (>50000 K) and pronounced seasonal variability, with its source mainly on the dawn side. Enhanced Ca emission near True Anomaly Angle (TAA) ∼25° and ∼150° has been attributed to Comet 2P/Encke meteoroid streams. Although Micro‐Meteoroid Impact Vaporization (MMIV) is considered the dominant Ca source, previous models underestimate the Ca intensity and energy distribution. We investigate the MMIV process, including the global meteoroid background and the Encke stream contributions. Using the IAPS exospheric Monte Carlo model, we simulate low‐ (10000 K) and high‐energy (>20000 K) Ca components. We consider a non‐homogeneous surface Ca distribution, including Ca‐rich regions, based on MESSENGER data. These inhomogeneities, combined with Mercury's 3:2 spin–orbit resonance, produce distinct exospheric Ca patterns over the planet's 2‐year orbit, differing between odd and even years. The 2P/Encke dust stream dynamics are also considered, showing how impact conditions vary with stream age, TAA, and velocity. Our simulations reproduce the observed background trend and intensity between TAA ∼ 0° and 270°, but the contribution supposed to be due to the Comet 2P/Encke is not well reproduced under literature assumptions. The simulated peaks between TAA ∼ 150° and 190° agree with observations, and the Ca enhancement observed at ∼30° appears driven by mm‐sized meteoroid impacts. In contrast, the simulations predict a secondary peak at 60° TAA that is not evident in the data. The observed intensities are much higher than the simulated ones. The simulations also fail to reproduce the Ca background intensity at TAA ∼ 270°–300°, indicating that additional loss processes may be active. In situ measurements from the ESA/JAXA's BepiColombo mission, with the nominal phase beginning in 2027, will be crucial for refining models and understanding Mercury's exosphere.