Mercury exosphere I. Global circulation model of its sodium component

Abstract

Our understanding of Mercury’s sodium exosphere has improved considerably in the last 5 years thanks to new observations (Schleicher, H., Wiedemann, G., Wöhl, H., Berkefeld, T., Soltau, D. [2004]. Astron. Astrophys. 425, 1119–1124) and to the publication of a summary of the large set of ground based observations (Potter, A.E., Killen, R.M., Morgan, T.H. [2007]. Icarus 186, 571–580; Potter, A.E., Killen, R.M. [2008]. Icarus 194, 1–12; Potter et al., 2009). In particular, the non-uniformity in longitude of the dayside sodium distribution (the dawn/dusk asymmetry) has now been clearly observed. This suggests that Mercury’s sodium exosphere is partly driven by a global day to nightside migration of the volatiles. One of the key questions remaining is the nature of the prevailing sodium ejection mechanisms. Because of the uncertain parameters for each ejection mechanisms, solving this problem has been difficult as indicated by the numerous papers over the last 15 years with very different conclusions. In addition, the variation of the size and of the spatial distribution of the surface reservoir (Leblanc, F., Johnson, R.E. [2003]. Icarus 164, 261–281) varies with distance from the Sun affecting the importance of each ejection mechanism on Mercury’s orbital position. We here present an updated version of the Leblanc and Johnson (Leblanc, F., Johnson, R.E. [2003]. Icarus 164, 261–281) model. We take into account the two populations of sodium in the surface reservoir (Hunten, D.M., Morgan, T.M., Shemansky, D.M. [1988]. The Mercury atmosphere. In: Vilas, F., Chapman, C., Matthews, M. (Eds.), Mercury. University of Arizona Press, Tucson, pp. 562–612), one ambient population (physisorbed in the regolith with low binding energy) and one source population (chemisorbed coming from grain interior or from fresh dust brought to the surface and characterized by a higher binding energy). We also incorporate a better description of the solar wind sputtering variation with solar conditions. The results of a large number of simulations of the sodium exosphere are compared with the measured annual cycle of Mercury sodium emission brightness. These measurements were obtained from the published data by Potter et al. (Potter, A.E., Killen, R.M., Morgan, T.H. [2007]. Icarus 186, 571–580) as well as from our own data obtained during the last 2 years using THEMIS solar telescope. These data show that: the annual cycle in the emission brightness is roughly the same from 1 year to another; there are significant discrepancies between what would be observed if the exospheric content were constant; and the annual cycle of Mercury’s sodium exosphere strongly depends on its position in its orbit so that there are seasons in Mercury’s exosphere. Based on these comparisons we derived the principal signatures for each ejection mechanism during a Mercury year and show that none of the ejection mechanisms dominates over the whole year. Rather, particular features of the annual cycle of the sodium intensity appear to be induced by one, temporarily dominant, ejection mechanism. Based on this analysis, we are able to roughly explain the annual cycle of Mercury’s exospheric sodium emission brightness. We also derive a set of parameters defining those ejection mechanisms which best reproduce this cycle. For our best case, Mercury’s exosphere content varies from ∼1.6 ± 0.1 × 10 28 Na atoms at TAA = 140° and 70° respectively to ∼4.5 ± 0.3 × 10 28 Na atoms at TAA = 180° and 0°. In addition, Mercury’s exospheric surface reservoir contains ∼1 × 10 31 Na atoms at TAA = 300° and at TAA = 170° with up to three times more sodium atoms trapped in Mercury’s nightside than in its dayside surface.