Review of Physical Processes and Modeling Approaches

1. 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

2. Most debated questions based on Mercury's observations

3. Observed components of Mercury's exosphere Known Species Mariner 10 Solar Occultation (Broadfoot et al. 1976) At terminator: neutral density < 107 cm-3 Mariner 10 Radio Occultation (Fjelbo et al. 1976) Electronic density around Mercury < 103 cm-3  Which other species?

4. Ground based observations of the Na, K, Ca components • 1985: First Spectroscopic observation (Potter et al. 1985) • 1986: observation of K, Na/K = 80-190 • >> 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?

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

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

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

8. Variation with respect to TAA? 3D model Aphelion TAA Aphelion 0.466 AU Perihelion 0.306 AU The Sun Perihelion Observation Driven by Mercury's rotations? Driven by the solar radiation pressure? Driven by the distance to the Sun? From Potter et al. (2006)

9. 2.0 1.5 1.0 0.5 0 Ca 4226 A Killen et al. (2005) 104 103 102 10 H 1216 Å (Broadfoot et al. 1976) 110 K Emission (Rayleigh) 108 Ca/cm2 420 K T = 12,000 - 20,000 K 2500 3000 3500 4000 4500 -200 0 200 400 600 800 1000 Altitude (km) Altitude (km) Ca meteoroid vaporization and photo-dissociation (+4 up to 6 eV) H and He: thermal desorption and surface accomodation 60 40 20 0 Data Na 5890 A 1500 K 1100 K Na: hotter than surface temperature  Energetic processes (?) ∆λ~6 mA 750 K Intensity (MR/A) Different energy distributions?  Different release mechanisms? Killen et al. (1999) 0.056 0.06 0.065 0.07 λ (5890 A)

10. How could we describe Mercury's exosphere?

11. 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? Exosphere Regolith Boundaries? Crust

12. 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? Photo ionization Neutral loss Exosphere Absorption of neutral and magnetospheric ion Regolith Sinks? Crust

13. 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? Sources? Exosphere Regolith Meteoritic supply + solar wind implantation Meteoroid gardening Diffusion Crust

14. 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? Solar Wind sputtering Micro- Meteoritic Impact Photo Stimulated Desorption Thermal Desorption Regolith Also chemical sputtering, is it negligible? Exospheric production Crust

15. In summary Photo ionization Neutral loss Solar Wind sputtering Micro- Meteoritic Impact Photo Stimulated Desorption Thermal Desorption Regolith Absorption of neutral and magnetospheric ion Meteoritic supply + solar wind implantation Meteoroid gardening Diffusion Crust Adapted from Morgan and Killen (1997)

16. Coordinate observing campaign 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

17. 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