20 November 2006Boston Mercury Observation Workshop Observational Quests for Mercury’s Exosphere Ann L. Sprague Lunar and Planetary Laboratory University of Arizona Tucson, AZ

20 November 2006Boston Mercury Observation Workshop ConstituentReferenceColumn Abundance (atoms per cm 2 ) Hydrogen (H)Broadfoot et al. 1976~5x10 10 Helium (He)Broadfoot et al. 1976~2x10 13 Oxygen (O)Broadfoot et al. 1976~7x10 12 Sodium (Na)Potter and Morgan 1985 ~2x10 12 Potassium (K)Potter and Morgan 1986 ~1x10 10 Calcium (Ca)Bida et al. 2000~1x10 7 Mercury’s Known Exospheric Species Quest #1 What else?

20 November 2006Boston Mercury Observation Workshop Sprague et al Predict S in Mercury’s Exosphere From: meteorites comets sulfides in regolith Test this hypothesis with MESSENGER

20 November 2006Boston Mercury Observation Workshop 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

20 November 2006Boston Mercury Observation Workshop Sprague Kozlowski Hunten 1990 Potassium

20 November 2006Boston Mercury Observation Workshop 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 silicate pyroclastics? Or…is it something else? Slade et al. 1992, Harmon and Slade 1992, Butler et al Quest #3 Relationship of exosphere to regions of high radar albedo and coherent backscatter

20 November 2006Boston Mercury Observation Workshop

20 November 2006Boston Mercury Observation Workshop A point to be made………..

20 November 2006Boston Mercury Observation Workshop fresh crust might be a source

20 November 2006Boston Mercury Observation Workshop Kuiper-Murasaki Crater Complex with seeing smear

20 November 2006Boston Mercury Observation Workshop 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

20 November 2006Boston Mercury Observation Workshop Image courtesy Frank Melillo, Holtsville, NY. 700nm

20 November 2006Boston Mercury Observation Workshop 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, E 43-73W Filter Imaging and Spectroscopy (Na) Sprague et al. 1998

20 November 2006Boston Mercury Observation Workshop Quest #4 What source for Ca and O ??

20 November 2006Boston Mercury Observation Workshop Not observed over the Surface like Na and K Bida et al Killen et al Radiance of emission ~ 2.5 kRay High radial velocity (-2 km/s) indicates high energy

20 November 2006Boston Mercury Observation Workshop 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  (Wurz et al. 2001, 2003). The similarity of the Ca/O ratio above the limb of Mercury (~ 2.0  ) 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

20 November 2006Boston Mercury Observation Workshop  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 ~10 17 Ca ions  Similar behavior, with more pronounced preference for southern hemisphere impacts is seen for O +6 Mid- latitude enhancements

20 November 2006Boston Mercury Observation Workshop Summary: Quests Quest #1 What other constituents? Quest #4 What is source for Ca and O? Quest #2 Relationship to other physical parameters? Quest #3 Relationship to regions of high radar albedo, coherent backscatter, and high visible albedo?

20 November 2006Boston Mercury Observation Workshop Ann says “Sorry I couldn’t be with you in Boston” Thank You