November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Review of Physical Processes and Modeling Approaches "A summary of uncertain/debated questions from a modeler's point of view" F. Leblanc Service d'Aéronomie du CNRS/IPSL

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Most debated questions based on Mercury's observations

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Observed components of Mercury's exosphere H 3  10 9 ~ 23 (hot) 230 (cold) He 3  ~ 6  10 3 O 3  ~ 4.4  10 4 Mariner 10 Solar Occultation (Broadfoot et al. 1976) At terminator: neutral density < 10 7 cm -3 Mariner 10 Radio Occultation (Fjelbo et al. 1976) Electronic density around Mercury < 10 3 cm -3 SpeciesSubsolar column density (cm -2 ) Near surface subsolar density (cm -3 ) Na  ~ 10 4 K  10 9 ~ 10 2 Ca 1.1  10 8 ? Remarks From Earth Mariner 10 Known Species  Which other species?

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING 1985: First Spectroscopic observation (Potter et al. 1985) 1986: observation of K, Na/K = >> Moon (6), solar (20) (Potter and Morgan 1986) Is the Na/K ratio always so large and why? Suprathermal component in Na line (Potter and Morgan 1987; Killen et al. 1999) Latitudinal, longitudinal, Mercury's position dependencies of this suprathermal component? Sporadic spots of Na emission at high latitudes (Potter and Morgan 1990, 1997; 2006) Due to exospheric recycling and/or to solar wind sputtering? Local enhancement on Caloris of K emission (Sprague 1990) Role of surface topography on the formation of the exosphere? Ground based observations of the Na, K, Ca components

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING West North Occultation of the Solar Na D2 line by Mercury's exosphere (Schleicher et al. 2004) Dayside Terminator Nightside Terminator What spatial distributions? Observations of the Na D lines (Potter and Morgan 1997) Sun N E W S Role of the solar radiation pressure, of Mercury's orbit?

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING What are the origins of the "short" term variation? Potter et al. (1999) South North 220° 214° 229° 223° 217° 236° Is it a CME encounter with Mercury? Is it a solar wind and UV variation inducing this observation? Role of Caloris? Other mechanisms?

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING What drives Mercury's tail formation? 26/05/2001 Potter et al. (2002) morning side 0.42 AU D2 emission TAA = 23° TAA = 83° TAA = 125° TAA = 190° TAA = 261° TAA = 315° Related to the ejection process? To the ionization frequency? To the solar radiation pressure? 3D model

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Driven by Mercury's rotations? Driven by the solar radiation pressure? Driven by the distance to the Sun? From Potter et al. (2006) Aphelion Perihelion Variation with respect to TAA? Perihelion AU Aphelion AU The Sun TAA 3D model Observation

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING H and He: thermal desorption and surface accomodation Ca meteoroid vaporization and photo- dissociation (+4 up to 6 eV)  Ca/cm 2 T = 12, ,000 K Different energy distributions?  Different release mechanisms? Altitude (km) Emission (Rayleigh) H 1216 Å (Broadfoot et al. 1976) 420 K 110 K Ca 4226 A Killen et al. (2005) Altitude (km) λ (5890 A) Intensity (MR/A) Na 5890 A Killen et al. (1999) 1500 K Data 1100 K 750 K Na: hotter than surface temperature  Energetic processes (?) ∆λ~6 mA

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING How could we describe Mercury's exosphere?

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Boundaries? The surface: the first layers of grains or the regolith layer? Is it a finite or infinite reservoir of ambient or/and source particles? The Interplanetary medium: the magnetopause? the bow shock? The orbit of Mercury? Crust Regolith Exosphere

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Surface absorption: is it a sink or/and just a recycling process? Neutral escape: what energy distributions for the ejecta? Ionization and Acceleration through the tail: what ionization cross section and electric field? Sinks? Absorption of neutral and magnetospheric ion Photo ionization Neutral loss Crust Regolith Exosphere

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Diffusion: through the grain and/or the regolith? Meteoroid supply: rate and spatial distribution ? Meteoroid gardening: how to constrain this mechanism? Solar Wind implantation: where, how much, which depth? Meteoroid gardening Diffusion Sources? Meteoritic supply + solar wind implantation Crust Regolith Exosphere

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Photo Stimulated Desorption Solar Wind sputtering Thermal Desorption Micro- Meteoritic Impact Crust Regolith Mechanisms of ejection: - Acting on the same population or on different population? (binding energy distribution, depth of implantation...) - What kind of variability vs heliocentric distance, solar activities (CME), surface temperature, radiative environment? Exospheric production Also chemical sputtering, is it negligible?

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Adapted from Morgan and Killen (1997) Photo Stimulated Desorption Solar Wind sputtering Absorption of neutral and magnetospheric ion Diffusion Photo ionization Neutral loss Thermal Desorption Micro- Meteoritic Impact Meteoritic supply + solar wind implantation Meteoroid gardening Crust Regolith In summary

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Mercury can be observed only the evening or the morning during one hour One hour = less than one Mercury minute One Earth day = 1/176 of one Mercury day ~15 Mercury minutes No possibility to observe simultaneously both evening and morning sides From telescopes located at different longitudes we can observe the exosphere for few hours on the same day  Access to new time scales In particular of the solar wind variability time scale Coordinate observing campaign

November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Conclusions Uncertainties on the real energy, density and composition structure of Mercury's exosphere:  Which mechanisms lead to ejection, with which intensity and with which released energy (related to the boundaries)?  What are the sources and sinks of Mercury's exosphere ? We can partially solve these questions by tracking variations:  from day to night sides (global exospheric recycling)  from perihelion to aphelion (ambient vs source populations)  with respect to latitude (solar wind sputtering or topography)  due to short and long time variations of the solar wind and photon flux (relations with surface and magnetosphere)  Access to new time scales should hightlight other variabilities...  Discovery of new exospheric species