Simulation of the Ca emission at Mercury and comparison with the observations by PHEBUS/BepiColombo during the first two flybys

Due to the lack of a thick atmosphere, the surface of Mercury is regularly bombarded by micrometeoroids at a rate depending on the position of Mercury around the Sun. One consequence of these impacts is an alteration of its surface (space weathering) and the ejection of its material around Mercury forming a tenuous exosphere. Even if the detail on the origin of the exospheric atomic calcium, observed systematically by MESSENGER is not fully understood, it is mostly associated to such impacts. In order to interpret the MESSENGER observations, we have recently developed a time dependent 3D model of the Ca exosphere of Mercury and successfully reproduced the seasonal variations observed by MESSENGER at dawn during its orbital phase. In this presentation, we will compare the simulated brightness from this model with the observations performed by PHEBUS onboard BepiColombo during the first two flybys of Mercury and discuss the differences.