1. Seasonal Variability in Mercury’s Calcium Exosphere Matthew Burger Morgan State University/GESTAR Rosemary Killen (NASA/GSFC) Bill McClintock (U. Colorado/LASP) Ronald Vervack, Jr. (JHU APL) Menelaos Sarantos (UMBC) Tim Cassidy (U. Colorado/LASP) Aimee Merkel (U. Colorado/LASP)

2. Part 1 Burger et al. (2012), Modeling MESSENGER observations of calcium in Mercury’s exosphere, JGR, E00L11, doi:10.1029/2012JE004158.

3. Ca Data - Flybys Fan tail observations S/C roll while in Mercury’s shadow

4. Flyby Models Isotropic Ejection from the Surface

5. Dawn Source Exponential Dropoff from Dawn, Equatorial Point

6. M3M2

8. Evidence for a ~5-10º shift northward during M3 Improved fits to Fantail and Pole data No effect on fits to the tail This shift has no effect on the fits to the M1 or M2 data

9. Orbit 22 Dawn Source σ = 25º

10. Orbits 22 - 28

11. New results: Burger et al., Seasonal Variability in Mercury’s Calcium Exosphere, in prep. Part 2

13. Calcium Limb Scans Sun Dawn 6 (dawn) 8 1012 (noon)14 16 18 (dusk) Equatorial Plane

14. Sun Dawn 8 Years of Limb Scans >162 orbits with at least 5 limb scans

15. Sun Dawn 8 Years of Limb Scans >162 orbits with at least 5 limb scans

16. Fits to Limb Scans Fit data from each hour angle bin with I = I 0 exp(-z/H) [kR] N = I×10 9 /g [cm -2 ] I 0 = intercept [kR] H = scale height [km] z = altitude [km] g = g-value [phot s -1 ]

17. Variations vs. TAA I 0 vs TAAH vs TAA TAA = 0° at perihelion TAA = 180° at aphelion Fits at 6 am (Dawn)

18. Variations vs. Distance I 0 vs Distance from SunH vs Distance from Sun Fits at 6 am (Dawn)

19. We Know What Is Happening All the calcium comes from a small region on the surface near the dawn, equatorial point Comes off very hot (T>20,000 K) Source size and temperature don’t change much over 8 Mercury Years Source strength varies with Mercury’s distance from the sun Burger et al. (2012) Dawn SourceIsotropic Source

20. We Don’t Know Why Not related to the surface geology Source is fixed in local time and does not rotate with Mercury Not related to the magnetosphere (ion sputtering or electron stimulated desorption) Magnetosphere is highly variable Wouldn’t produce a source at dawn Possibly related to asymmetric impact vaporization But not clear why only Ca shows this dawn source Not related to Ca freezing on the nightside and vaporizing as it moves into sunlight

21. Terminator Motion I 0 vs Terminator SpeedH vs Terminator Speed Fits at 6 am (dawn) Sun moves backwards in sky

22. Summary Calcium comes off the surface very hot from a small region near dawn The source has been stable for eight Mercury years The source mechanism is still unknown We are using numerical models of the physics that we do understand to figure out what we don’t understand.