Observational Quests for Mercury’s Exosphere

1. Observational Questsfor Mercury’sExosphere Ann L. Sprague Lunar and Planetary Laboratory University of Arizona Tucson, AZ 85721 sprague@lpl.arizona.edu Boston Mercury Observation Workshop

2. Mercury’s Known Exospheric Species Quest #1 What else? Boston Mercury Observation Workshop

3. Predict S in Mercury’s Exosphere From: meteorites comets sulfides in regolith Test this hypothesis with MESSENGER Sprague et al. 1995 Boston Mercury Observation Workshop

4. Na and K • Association with surface features • Diurnal wind • Distribution with true anomaly • Multiple release mechanisms • Relationship to the magnetosphere Quest #2 Relationships to other physical parameters Boston Mercury Observation Workshop

5. Potassium Sprague Kozlowski Hunten 1990 Boston Mercury Observation Workshop

6. Quest #3 Relationship of exosphere to regions of high radar albedo and coherent backscatter The radar image that shook the world of planetary science!!! Ice at the poles Of Mercury!!! Or…is it Sulfur? Or…is it cold iron and titanium free silicatepyroclastics? Or…is it something else? Slade et al. 1992, Harmon and Slade 1992, Butler et al. 1993 Boston Mercury Observation Workshop

7. Boston Mercury Observation Workshop

8. A point to be made……….. Boston Mercury Observation Workshop

9. fresh crust might be a source Boston Mercury Observation Workshop

10. Kuiper-Murasaki Crater Complex with seeing smear Boston Mercury Observation Workshop

11. Radar and Visible bright Spot B Geometry not quite right to show Spot A rotating around from the west. Amateurs are imaging the surface of Mercury and finding high albedo regions Na and K observations over these regions to look for sources would be useful Image courtesy of Steve Massey Boston Mercury Observation Workshop

12. 700nm Image courtesy Frank Melillo, Holtsville, NY. Boston Mercury Observation Workshop

13. Filter Imaging and Spectroscopy (Na) a. 55N, 27S; 6E, 354W Spot B and Spot A b. Same c. Same, also Kuiper-Murasaki d. Spots B and A e. Same f. Same g. Caloris Basin h. 65N, 205E; 155W 0, 235E,125W 9S, 255E, 105W i. 35S, 287-317E 43-73W Sprague et al. 1998 Boston Mercury Observation Workshop

14. Quest #4 What source for Ca and O ?? Boston Mercury Observation Workshop

15. Not observed over the Surface like Na and K Radiance of emission ~ 2.5 kRay High radial velocity (-2 km/s) indicates high energy Bida et al. 2000 Killen et al. 2004 Boston Mercury Observation Workshop

16. Test the hypothesis that exospheric O and Ca are of solar wind origin. At Mercury the Ca/O ~ 2.0 10-3 with a factor of 2 uncertainty in the O measurement (Broadfoot et al. 1976); there is small variability in the Ca measurements. The ratio of Ca/O in the solar wind is ~ 1-4  10-3 (Wurz et al. 2001, 2003). The similarity of the Ca/O ratio above the limb of Mercury (~ 2.0  10-3) to that of the solar wind, compels serious consideration that the source of Ca and O in Mercury's exosphere is originally the Sun via the delivery of O+6 and Ca+11 in the solar wind. Koehn and Sprague, In Press, PSS, 2006 Boston Mercury Observation Workshop

17. Mid- latitude enhancements • Model impact distribution of Ca+11 on the surface of Mercury for specific solar wind conditions • Southern hemisphere impact sites dominate • Color bar--the number of parcels impacting per site • Each parcel represents ~1017 Ca ions • Similar behavior, with more pronounced preference for southern hemisphere impacts is seen for O+6 Boston Mercury Observation Workshop

18. Summary: Quests Quest #1 What other constituents? Quest #2 Relationship to other physical parameters? Quest #3 Relationship to regions of high radar albedo, coherent backscatter, and high visible albedo? Quest #4 What is source for Ca and O? Boston Mercury Observation Workshop

19. Ann says “Sorry I couldn’t be with you in Boston” Thank You Boston Mercury Observation Workshop