“…Some easily vaporized elements (in particular Na and K) have been detected and studied from such ground observations Morgan, 1985, 1986). Subsequent observing campaigns from Earth-based telescopes Morgan, 1987, 1990;Sprague, 1990;Sprague et al, 1997Sprague et al, , 1998 have found the abundance of Na and K undergoes strong changes from 10 6 to 10 12 atoms cm À2 for Na and 10 4 to 10 9 atoms cm À2 for K. Such observations revealed in particular the close relations with Mercury's magnetosphere through solar wind penetration along open magnetospheric field lines, subsequent sputtering of Mercury's surface leading to remarkable features in Mercury's exosphere (Potter and Morgan, 1990;Potter et al, 1999;Killen et al, 2001;Leblanc et al, 2008Leblanc et al, , 2009. Part of these observations have also been interpreted through exospheric volatile particles recycling from hot to cold surfaces (Leblanc and Johnson, 2003), the later being high latitude and nightside regions Sprague et al, 1997;Barbieri et al, 2004;Schleicher et al, 2004), Caloris basin and observed radar bright spots at high latitude (Harmon and Slade, 1992), most probably corresponding to small craters partly permanently shadowed .…”