Modelling of Mercury’s Ca exosphere observed by PHEBUS during the First Three Flybys

Meteoroid bombardment of Mercury’s surface causes seasonal variability in its calcium (Ca) exosphere, as observed by the MESSENGER mission [1, 2]. The observed high-energy Ca source exhibits a dawn enhancement and a distinct dawn–dusk asymmetry, consistent with the directionality of meteoroid impacts. The most likely mechanism generating Mercury’s Ca exosphere involve a combination of processes, including the release of both atomic and molecular surface materials [3].

In this study, we use the exosphere generation model developed at the Institute for Space Astrophysics and Planetology (IAPS) [6] to simulate the 3D spatial distribution of atomic Ca and Ca-bearing molecules produced by the Meteoroid Impact Vaporization (MIV) process. The model includes two distinct populations: a high-energy Ca component, originating from shock-induced, non-equilibrium dissociative ionization and neutralization of Ca+ during the vapor cloud expansion, and a lower energy Ca component, generated through the photo-dissociation of Ca-bearing molecules released by MIV [4,5].

Model parameters, such as the photolysis lifetime of the molecules and the relative abundances of atomic and molecular Ca components involved in the different released processes, play a crucial role in shaping Mercury’s Ca exosphere, but they are still not well constrained by observations.

In order to better understand the mechanisms governing the release and distribution of the Ca exosphere, we compare model simulations with data acquired by the PHEBUS spectrometer (Probing of Hermean Exosphere By Ultraviolet Spectroscopy) onboard ESA-JAXA’s BepiColombo spacecraft, currently in the cruise phase towards its destination. During the first three Mercury flybys, performed in 2021, 2022 and 2023, PHEBUS was able to observe the Ca emissions near closest approach using the two visible channels dedicated to the Ca measurements [7]. The three flybys had similar trajectories: the spacecraft approached from the planet’s nightside, crossing the shadow and reaching the closest approach about 200 km above the surface, then moved to the planet’s dayside. However, during the third flyby, the closest approach was slightly further at 235 km of altitude, and PHEBUS’s lines of sight was pointing southward, in the opposite direction with respect to the first two flybys where it was pointing northward (Fig. 1).

We reconstructed the geometry of the observations for each flyby and simulated the Ca exosphere under those conditions. Then, we derived Ca altitude profiles along the PHEBUS line of sight and compared them with the observational ones. Preliminary results suggest that a high-energy Ca component with a temperature of approximately 50,000 K is consistent with the observed intensities, while the contribution from a low-energy component seems to be negligible. The absence of lower energy component in the PHEBUS Ca observations is consistent with the MESSENGER data. This does not necessarily imply that this component, expected by the surface release modeling after micro-meteoroid impact, does not exist. It could be due to limitations in PHEBUS’s observations.

It should be noted that the constrained set of model parameters offers the opportunity to also investigate the seasonal variability of the Ca source rate and to reconstruct the exospheric seasonal Ca profile along Mercury’s orbit. Our theoretical calculations (Fig. 2) show good agreement with MESSENGER/UVVS data, supporting the interpretation of seasonal effects driven by meteoroid flux variations. During nominal mission, PHEBUS will collect extensive data along Mercury’s orbit, that will be complemented by SERENA-Strofio in situ measurements, so we’ll be monitoring seasonal variations too and these data could be used to compare with the model. This study allows evaluation of the consistency between the modeled and observed Ca distributions, and helps constrain the physical parameters governing exospheric source processes. Our results advance our understanding of the MIV process on Mercury, providing a valuable tool for interpreting data and guiding observational strategies for the ESA/JAXA BepiColombo mission, which will insert into Mercury’s orbit at the end of 2026.

Figure 1: Geometry of PHEBUS observations during the first three flybys of Mercury: spacecraft arrived on the planet's nightside, crossed its shadow and then moved to its dayside

Figure 2: Simulation of seasonal Ca content in Mercury's exosphere due to the meteoroid flux (red line) compared with the observations (blue line) [2] along the orbit, and, hence, True Anomaly Angle (TAA)

References

[1] Burger, M.H., Killen, R.M., McClintock, W.E., Vervack, R.J., Merkel, A.W., Sprague, A.L., Sarantos, M., 2012. “Modeling MESSENGER observations of calcium in Mercury’s exosphere”. J. Geophys J. Res. 117

 [2] Burger, M.H., Killen, R.M., McClintock, W.E., Merkel, A.W., Vervack, R.J., Cassidy, T.A., and Sarantos, M., 2014. “Seasonal variations in Mercury’s dayside calcium exosphere”, Icarus 238, 51–58

[3] Killen, R.M., and Hahn, J.M., 2015. “Impact vaporization as a possible source of Mercury’s calcium exosphere”, Icarus 250, 230-237

[4] Killen, R.M., 2016. “Pathways for energization of Ca in Mercury’s exosphere”, Icarus 268, 32–36.

[5] Moroni, M., Mura, A., Milillo, A., Plainaki, C., Mangano, V., et al., 2023. “Micro-meteoroids impact vaporization as source for Ca and CaO exosphere along Mercury's orbit”, Icarus, 401, 115616

[6] Mura, A., Milillo, A., Orsini, S., and Massetti, S., 2007. “Numerical and analytical model of Mercury’s exosphere: dependence on surface and external conditions”, Planet. Space Sci. 55, 1569–1583

[7] Robidel, R., Quémerais, E., Chaufray, J. Y., Koutroumpa, D., Leblanc, F., Reberac, A., et al., 2023. “Mercury's exosphere as seen by BepiColombo/PHEBUS visible channels during the first two flybys”, Journal of Geophysical Research: Planets, 128(12)