Mercury Atmospheric and Surface Composition Spectrometer Observations of Sodium and Calcium in Mercury's Exosphere During the Second MESSENGER Flyby

MESSENGER will make its second flyby of Mercury on October 6, 2008, with exospheric sodium and calcium being among several species that will be observed. The observations of sodium begin in the extended tail region ~ 100,000 km in the anti-sunward direction and continue up to ~ 5000 km from the planet. The spacecraft will oscillate back-and-forth about the spacecraft-Sun vector, resulting in scanning the field of view up and down in the Z direction (i.e., north/south) in a Mercury-fixed coordinate system. The extent of the scan in the Z direction will be five Mercury radii above and below the anti-sunward vector at the beginning of the tail sweep and decrease to 3 Mercury radii as the spacecraft nears the planet. Calcium observations begin in the extended tail region ~ 35,000 km behind the planet and continue along with the sodium observations. After the extended tail-region observations, MESSENGER will enter into the shadow of the planet where sodium and calcium nightside observations begin. The spacecraft will be rotated ~ 180° around the spacecraft-Sun vector, resulting in a fan-tail pattern that begins with the field of view pointing in the equatorial plane toward the dawn-side hemisphere of the planet, rotates to the northern polar region, and then continues rotating until pointing in the equatorial plane toward the dusk-side hemisphere of the planet. The field of view will then begin to intersect the planet on the nightside where the observations continue up to the dawn-side terminator region. As the line-of-sight moves out in front of the sub-solar point during the outbound leg of the flyby, dayside observations of sodium begin above a 1000-km tangent height and continue up to several thousand kilometers. A portion of these observations will provide measurements similar to those during the first Mercury flyby and will provide a basis to compare morphological similarities and differences in sodium and calcium between the two sets of observations. Observations performed during this flyby that were not performed previously include measurements of the sodium tail at a much greater distance from the planet, providing additional data on the extent of the sodium tail. Moreover, because calcium was observed on the nightside hemisphere of the planet during the first flyby, this flyby will include a search for calcium in the extended tail region that will provide information concerning the release energy and transport for calcium. Finally, this flyby will search for extended sodium on the dayside exosphere and possibly help to constrain the release processes for sodium on the dayside.