Abstract | The sodium component is the most observed element of Mercury’s exosphere. First detected from ground based observatories, it appears peaking in emission intensity at high latitudes and to be variable on hour time scale. The large set of ground based observations obtained during many consecutive years also leads to the characterization of the dependency of the emission sodium intensity with respect to its position along Mercury’s orbit around the Sun. Such picture was significantly changed by MASCS/MESSENGER 4 years of observations around Mercury. Focusing on the equatorial regions, MASCS provided a different view of Mercury’s sodium exosphere, highlighting a surprisingly steady radial subsolar profile of the emission intensity in contradiction with the high latitudinal variable component observed from the Earth. Moreover, MASCS observed an equatorial dawn/dusk asymmetry opposite between Mercury’s inbound portion and outbound portion of its orbit. Such variation of the dawn/dusk asymmetry contradicts somehow the idea that Mercury’s sodium exosphere should be partly driven by a global day to nigh sides circulation induced by the solar radiation pressure. Indeed, such circulation should lead to a systematically more intense emission at dawn than at dusk, even if variable along Mercury’s orbital position. To reconcile observations and modelling, each potential driver of Mercury’s sodium exosphere needs to be reviewed in order to find out what controls the fate of the sodium around Mercury. |
