Space Sci Rev 2007 131 161 186 DOI 10 1007 s11214 007 9260 9 Mercury s Atmosphere A Surface Bounded Exosphere Deborah L Domingue Patrick L Koehn Rosemary M Killen Ann L Sprague Menelaos Sarantos Andrew F Cheng Eric T Bradley William E McClintock Received 28 August 2006 Accepted 7 August 2007 Published online 24 October 2007 Springer Science Business Media B V 2007 Abstract The existence of a surface bounded exosphere about Mercury was discovered through the Mariner 10 airglow and occultation experiments Most of what is currently known or understood about this very tenuous atmosphere however comes from groundbased telescopic observations It is likely that only a subset of the exospheric constituents have been identified but their variable abundance with location time and space weather events demonstrate that Mercury s exosphere is part of a complex system involving the planet s surface magnetosphere and the surrounding space environment the solar wind and interplanetary magnetic field This paper reviews the current hypotheses and supporting observations concerning the processes that form and support the exosphere The outstanding questions and issues regarding Mercury s exosphere stem from our current lack of knowledge concerning the surface composition the magnetic field behavior within the local space environment and the character of the local space environment Keywords Atmospheres Exosphere Mercury Space physics Space weathering MESSENGER D L Domingue A F Cheng The Johns Hopkins University Applied Physics Laboratory Laurel MD 20723 USA e mail deborah domingue jhuapl edu P L Koehn Department of Physics and Astronomy Eastern Michigan University Ypsilanti MI 48197 USA R M Killen M Sarantos University of Maryland College Park MD 20742 USA A L Sprague Lunar and Planetary Laboratory University of Arizona Tucson AZ 86721 USA E T Bradley W E McClintock Laboratory for Atmospheric and Space Physics University of Colorado Boulder CO 80303 USA 162 D L Domingue et al 1 Introduction The discovery of an atmosphere or more accurately an exosphere around Mercury was made through the ultraviolet airglow and occultation experiments on the Mariner 10 spacecraft during its three flybys of the planet in 1974 and 1975 The Mariner 10 occultation experiment set an upper limit on Mercury s atmospheric density of approximately 105 atoms cm3 corresponding to a pressure of about 10 12 bar Broadfoot et al 1976 Hunten et al 1988 thus defining it as a collisionless exosphere with its exobase coincident with Mercury s surface a surface bounded exosphere Ultraviolet UV emissions of the three atomic elements hydrogen H helium He and oxygen O were detected with the UV airglow spectrometer Broadfoot et al 1976 Kumar 1976 Since the Mariner 10 flybys exploration of Mercury s exosphere has been conducted by means of ground based telescopic observations Three additional elements sodium Na potassium K and calcium Ca have been detected through their resonance scattering emission lines Potter and Morgan 1985 1986 Bida et al 2000 Because the combined pressures of the known species are much less than the total exospheric pressure measured by the Mariner 10 occultation experiment other species are expected to exist in this tenuous atmosphere Additional constituents such as carbon C carbon monoxide CO carbon dioxide CO2 lithium Li argon Ar neon Ne and xenon Xe have been sought but not detected Broadfoot et al 1976 Fink et al 1974 Hunten et al 1988 Sprague et al 1996 Other species such as hydroxyl OH and sulfur S have been suggested Slade et al 1992 Butler et al 1993 Sprague et al 1995 and modeled Killen et al 1997 Koehn 2002 Koehn et al 2002 as related to the radar bright deposits near Mercury s poles Harmon and Slade 1992 Slade et al 1992 Telescopic observations from the mid 1980s to today have shown that there is temporal and spatial variability in Mercury s exosphere The elements have both high and lowvelocity components and are influenced by the thermal and radiative environments in addition to the interstellar medium For example the variability in exospheric Na has been mapped to variability in the solar wind Killen et al 1999 2004a 2004b and its effects on Mercury s magnetosphere Mariner 10 also made the first in situ measurements of the planet s magnetic field Ness et al 1974 Simpson et al 1974 and the space environment around Mercury Ogilvie et al 1977 During the first flyby the spacecraft passed through the magnetotail of the planet and provided the first hint that Mercury may have a magnetic field similar to though of lower amplitude than the Earth s The second flyby passed across the dayside of the planet and the third again crossed the tail this time closer to the planet s surface Analysis of these data showed that the planetary magnetic field was probably a dipole with a moment of 350 to 400 nT R3M oriented within 10 of the rotational axis Connerney and Ness 1988 Additional details concerning Mercury s magnetosphere can be found in a companion paper Slavin et al 2007 The fundamental observation however was that the magnetic field of Mercury is able to stand off the solar wind at least under nominal solar wind conditions This implies a dynamical coupling to the planet that is mediated by magnetospheric current systems that must close near or within the planet At Earth the corresponding current systems close in Earth s ionosphere but Mercury has no ionosphere How do the required current systems close at Mercury One hypothesis is that Mercury s exosphere provides a so called pickup conductance derived from the ionization and electric field acceleration of atmospheric species that enables the formation of an Earth like magnetosphere despite the absence of an ionosphere Cheng et al 1987 Ip 1993 Another proposed mechanism for closing current systems is based on the assumption that the surface of Mercury is itself conducting Mercury s Atmosphere A Surface Bounded Exosphere 163 Janhunen and Kallio 2004 As will be discussed the exosphere magnetosphere and surface of Mercury form a complex interacting system whose properties and dynamics are still incompletely understood Exosphere surface interactions for many of the exospheric constituents are not well understood since many properties of Mercury s regolith such as porosity and composition are still poorly known Head et al 2007 Boynton et al 2007 Given the variability of solar wind conditions at Mercury s orbit and the relative weakness of Mercury s magnetic field the