[Previous] | [Session 33] | [Next]
R.M. Killen (University of Maryland), T. Bida (Lowell Observatory)
Observations of Mercury with the High Resolution Echelle Spectrograph (HIRES) at the W.M. Keck I telescope on the evenings of 7 July 1998, 13-19 July 1998 and 21 July 1998 revealed a tenuous calcium atmosphere at Mercury, principally confined to the polar regions (Bida et al., 2000). We now have six years of observations of the calcium exosphere of Mercury. These observations show a persistent but spatially variable blue shift indicating an excess velocity toward the observer of up to 3 km/s, with an average excess velocity of 2.2 km/s above the south pole. In addition, the line profiles reveal a hot corona at about 12,000 K in a thermalized atmosphere, indicating a large range of motion with respect to the observer. We will present spatial maps of calcium column abundance and velocity for all epochs, and a comparison of simultaneous Na and Ca measurements from 2000. We will also show an upper limit for CaII. We show that the likely sources of the calcium are impact vaporization and ion sputtering. A significant fraction of the calcium may be ejected into the atmosphere as CaO and clusters, which are subsequently photodissociated. The excess velocity of the calcium atom results in a hot calcium atmosphere. The total column of calcium in the atmosphere that we have been able to observe has increased from 1998 - 2000 for nearly coincident slit placement. This suggests, but does not prove, a correlation with solar cycle since we cannot observe the entire exosphere on any given night.
These observations were supported by the NASA Planetary Astronomy Program.
[Previous] | [Session 33] | [Next]
Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.