Potential relations between Caloris basin and Mercuryʹs sodium exosphere

We have developed a 1D thermal model of Mercuryʹs regolith, in order to simulate the heat diffusion in the upper subsurface (first 10 m). We assume in our model that the thermophysical properties of the Hermean regolith are similar to those of the lunar regolith. We apply our thermal model to the Caloris basin which slopes induce distortions of the surface temperature compared to results obtained for a perfect spherical planet. This thermal model is then coupled with a 3D Monte Carlo model of Mercuryʹs sodium exosphere [Leblanc, F., Johnson, R.E., 2003. Icarus 164, 261–281; Leblanc, F., Delcourt, D., Johnson, R.E., 2003b. J. Geophys. Res. 108 (E12), doi:10.1029/2003JE002151/.5136], in order to describe the signatures of Caloris basin on Mercuryʹs sodium exosphere in term of temporal and spatial variabilities. In particular, we find a motion of the maxima of sodium density in the exosphere towards the Northern hemisphere similar to the one observed by Potter et al. [Potter, A.E., Morgan, T.H., Killen, R.M., 1999. Planet. Space Sci., 47, 1441–1449] but did not reproduce the observed change of the emission brightness. The main conclusion of this study is that the Caloris basin–exosphere relations might be observable from the Earth which we hope will motivate new observations of Mercuryʹs exosphere.