Planetary and Space Science 52 (2004) 1169–1175 First observations of the Na exosphere of Mercury with the high- resolution spectrograph of the 3.5M Telescopio Nazionale Galileo Cesare Barbieri a,Ã , Stefano Verani a , Gabriele Cremonese b , Ann Sprague c , Michael Mendillo d , Rosario Cosentino e , Donald Hunten c a Department of Astronomy, University of Padova, I-35122 Padova, Italy b Astronomical Observatory of Padova, INAF, Italy c Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA d Center for Space Physics, Boston University, MA 02215, USA e Telescopio Nazionale Galileo, INAF, Italy Received 16 September 2003; received in revised form 14 June 2004; accepted 6 July 2004 Abstract Analysis of three spectra of the exosphere of Mercury in the Na-D lines are presented. Spectra were secured with the high- resolution spectrograph (SARG) of the 3.5M Telescopio Nazionale Galileo (TNG, located on the Roque de los Muchachos, Canaries)ontheeveningsofAugust23and24,2002.Spectrahaveresolution R ¼ 115000; theslitlengthwas26.7’’.TheNacolumn abundances range from 4.3 to 19:7  10 10 atoms=cm 2 with the highest abundances being close to the illuminated limb. Our observations at true anomaly angles (TAA) from 1711to 1741show the traces of the emission lines to be strongly peaked at the illuminatedlimb,supportiveofrecentmodelingthatshowsthermaldesorptiontobeastrongfactorindeterminingthedistribution of Na about the planet. r 2004 Elsevier Ltd. All rights reserved. Keywords: Mercury; Planetary exospheres; Na emission atmospheres; High-resolution planetary spectroscopy 1. Introduction TheexistenceoftheatmospherearoundMercurywas discoveredbytheMariner10spacecraft,whichrevealed UV emissions of three atomic elements: H, He and O (Broadfoot et al., 1976). Three other elements (Na, K, and Ca) were later discovered with ground-based observations (Potter and Morgan, 1985; Potter and Morgan, 1986; Bida et al., 2000, respectively). Due to thelowdensity(ofabout n ¼ 10 5 atoms=cm 3 ; P ¼ 10 12 bar,onthedayside),theatmosphereiscollisionless,i.e., themean-freepathoftheatomsislongerthanthevalue of the scale height H of the atmosphere (cf. Chamber- lain and Hunten, 1987). Therefore, the whole atmo- sphere is comparable with an exosphere having the exobase coincident with the planet’s surface. The lifetime for the species in the exosphere is ruled by the interaction with the interplanetary medium. Photoionization is the fastest loss mechanism of the neutral atmosphere: the ionized atoms are driven away from the planet by the solar wind, or aimed back to the surface along the lines of Mercury’s magnetic field. To maintain the exosphere, the lost atoms must be replaced by some source mechanisms. Processes of endogenic and exogenic origin are supposed to act in repopulating the exosphere. The expected sources of Na include thermal, photon, and electron-stimulated desorption, ion and chemical sputtering, vaporization of regolith and of projectiles following micro- meteoritic impacts (cf. McGrath et al., 1986, Hunten and Sprague, 1997). ARTICLE IN PRESS www.elsevier.com/locate/pss 0032-0633/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2004.07.013 Ã Corresponding author. Fax: +39-498278245. E-mail address: barbieri@pd.astro.it (C. Barbieri).