Observations of Mercury's Exosphere during the First MESSENGER Flyby

Details Observations of Mercury's Exosphere during the First MESSENGER Flyby Observations of Mercury's Exosphere during the First MESSENGER Flyby

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..406k&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

Mathematics

Scientific paper

Abstract Observations of Mercury's tenuous exosphere were taken with the UltraViolet and Visible Spectrometer (UVVS) of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) during MESSENGER's first Mercury flyby on January 14, 2008 [1]. Concurrent ground-based observations of the sodium exospheric emission were taken at the McMath-Pierce Solar Telescope at Kitt Peak, Arizona [2] (Fig. 1). We make some interesting comparisons. In particular, observations of neutral sodium in the antisunward tail region during the flyby [2] are compared with tail observations taken at the McMath- Pierce solar telescope by Potter, Killen, and Mouawad. Only one Na observation was obtained by UVVS on Mercury's dayside, making our groundbased observations important for understanding the Na distribution at the time of the first MESSENGER flyby. In addition, potassium was detected faintly at the McMath-Pierce solar telescope on Kitt Peak during the flyby timeframe but was not detected with the UVVS. This indicates that K emission may have been patchy and or weak during that time period. Of particular interest is the morphology of the sodium exosphere. A possible north/south asymmetry was seen in the sodium tail in both UVVS (Fig. 2) and ground-based observations (not shown here); however, no definitive asymmetry was seen either on the dayside or nightside at that time. Fig. 2 The sodium tail observed by the MESSENGER MASCS instrument on January 14, 2008. A possible north/south asymmetry is present. The appearance of a possible north/south asymmetry in the sodium tail observed with the MASCS instrument may indicate that the portion of exosphere escaping down the tail is a minor part of the dayside exosphere with a different morphology from that of the bulk of the dayside exosphere. Calcium emission observations by UVVS in the near-tail region exhibited a marked dawn/dusk asymmetry (Fig. 3). Ground-based observations of calcium taken with the cross-dispersed echelle spectrograph, HiRES, on the Keck I telescope by Bida and Killen on the evenings of May 13 and 14, 2008, showed very strong antisunward Ca emission at about the same distance from the planetary surface as observed by MASCS. These and previous Ca measurements [4] demonstrate that Ca appears several Mercury radii from the surface and has an extended distribution. Fig. 3 Calcium emission observed by the MESSENGER MASCS instrument, looking outward from the shadow on the nightside, is maximum looking east and decreases monotonically as the field of view rolls north and then west. References [1] McClintock W. E. and Lankton M. R. (2007) Space Sci. Rev., 131, 481-522. [2] Killen R.M. et al. (2008) Lunar Planet. Sci., 39, 1452. [3] McClintock W. E. et al. (2008) Science, in press. [4] Killen R. M. et al. (2005) Icarus, 173, 300-311.

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