seasonal variability in mercury’s calcium exosphere matthew burger morgan state university/gestar...
TRANSCRIPT
Seasonal Variability in Mercury’s Calcium
ExosphereMatthew 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)
Part 1• Burger et al. (2012), Modeling MESSENGER
observations of calcium in Mercury’s exosphere, JGR, E00L11, doi:10.1029/2012JE004158.
•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
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Sun
Dawn
8 Years of Limb Scans•>162 orbits with at least 5 limb scans
Fits to Limb ScansFit data from each hour angle bin with I = I0 exp(-z/H) [kR] N = I×109/g [cm-2]
I0 = intercept [kR]H = scale height [km]z = altitude [km]g = g-value [phot s-1]
Variations vs. TAAI0 vs TAA H vs TAA
TAA = 0° at perihelionTAA = 180° at aphelion
Fits at 6 am (Dawn)
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
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
Terminator Motion
I0 vs Terminator Speed
H vs Terminator Speed
Fits at 6 am (dawn)
Sun moves backwards in sky