Effects of Comet Encke’s meteoroid stream on the seasonal variation of Mercury’s Ca exosphere

The NASA/MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission provided measurements of Mercury’s Ca exosphere, allowing the study of its morphology and its seasonal variations. Observations of Ca column densities exhibit a scale height consistent with a temperature > 50,000 K, with a source located mainly on the dawn-side of the planet [1]. Although the Micro-Meteoroid Impact Vaporization (MMIV) process is considered as the main source of exospheric Ca, previous estimates have not yet been able to justify the observed intensity and energy. In addition, the observed seasonal Ca dependence shows an excess emission shortly after Mercury’s perihelion at TAA ∼25° and at TAA ∼ 150°. These enhancements have been attributed to the vaporization of surface material induced by meteor stream impacts, possibly resulting from Comet Encke [2,4,9].

In this work, we investigate the role of the MMIV process in shaping the Mercury’s Ca exosphere using the exospheric Monte Carlo model developed at the Institute for Space Astrophysics and Planetology (IAPS) [7]. We extend the model by including simulations of both meteoroid background flux [10] and the Encke stream flux [2], considering both low (< 10000 K) and high-energy (> 20000 K) Ca components released by meteoroid impacts [5,6]. We assume a non-homogeneous surface Ca abundance, with Ca-rich regions based on MESSENGER’s surface composition data [8]. This assumption, combined with Mercury’s 3:2 spin-orbit resonance, results in periodic exposure of Ca-rich regions to varying meteoroid fluxes, modulating the exospheric Ca structure over its 2-year orbit. As a consequence, the exosphere evolves differently in even and odd Mercury years (Fig. 1). Furthermore, dust dynamics studies [3] indicate that Encke stream particles of different ages follow slightly different orbits and impact Mercury at distinct TAAs, velocities, and arrival directions. 

By combining the effects of background meteoroid fluxes and the Encke stream, we can perform more accurate modelization of the exospheric Ca variability across Mercury's 2-year.

We compare our results with MESSENGER observations of the dayside Ca exosphere and show that the simulated Ca distribution from the background meteoroid flux is in good agreement with data between TAA 0° and 270°. However, the contribution from the Encke stream results in lower than observed Ca abundances (Fig. 2). This can be attributed to the underestimation of the total stream mass influx and/or the uncondensed Ca fraction in the model. To better reproduce the observed Ca peaks, both parameters likely need to be significantly higher than the values currently adopted from the literature.

We will show that by considering a higher relative contribution of the energetic Ca component (~ 50,000 K) the observed exospheric intensities are better reproduced. This is consistent with observations, which show no direct evidence of a low-energy Ca population. Despite the model’s success in reproducing key features of the Ca exosphere, it fails to reproduce the observed Ca decrease between TAA 270° and 300°, suggesting that additional mechanisms may influence exospheric dynamics.

Overall, this study shows the complexity of the processes governing the refilling of the exosphere and underline the need for accurate calibration of model parameters. Future high-quality data provided by the ESA/JAXA BepiColombo, starting its nominal phase in 2026, will be crucial for providing deeper insights into the dynamic of the Ca exosphere and refining the accuracy of our model.

Figure 1: Seasonal Ca content in Mercury's exosphere due to the meteoroid background flux over the course of Mercury's 2-year orbital cycle. Comparison of simulation with ones of Burger et al. (2014) obtained through a best fit to the UVVS observations

Figure 2: Seasonal Ca content in Mercury's exosphere due to the meteoroid background flux (black line), and the background including the cometary contribution (orange line), averaged over the course of Mercury's 2-year orbital cycle, compared with the observations (Burger et al., 2014; blue line) along the orbit, and, hence, TAA

References

 [1] Burger, M.H., Killen, R.M., McClintock, W.E., et al., 2014. “Seasonal variations in Mercury’s dayside calcium exosphere”, Icarus 238, 51–58

[2] Christou, A. A., R. M. Killen, and M. H. Burger, 2015, “The meteoroid stream of comet Encke at Mercury: Implications for MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations of the exosphere”, Geophys. Res. Lett., 42, 7311–7318

[3] Christou, A. A., Egal, A., Georgakarakos, N., 2024, “The Taurid resonant swarm at Mercury”, Monthly Notices of the Royal Astronomical Society, 527(3), 4834-4846.

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

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

[6] 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

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

[8] Nittler, L. R., Frank, E. A., Weider, S. Z., Crapster-Pregont, E., Vorburger, A., Starr, R. D., Solomon, S. C., 2020, “Global major-element maps of Mercury from four years of MESSENGER X-Ray Spectrometer observations”, Icarus, 345, 113716.

[9] Plainaki, C., Mura, A., Milillo, A., Orsini, S., Livi, S., Mangano, V., Massetti, S., Rispoli, R., De Angelis, E., 2017, “Investigation of the possible effects of comet Encke’s meteoroid stream on the Ca exosphere of Mercury”, J. Geophys. Res. Planets 122, 1217–1226.

[10] Pokorný, P., Sarantos, M., & Janches, D., 2018, “A comprehensive model of the meteoroid environment around Mercury”, The Astrophysical Journal, 863(1), 31