The Rotation Axis of Mercury: A New Determination from Radar Observations
Session 43 -- SL-9, Comets, Solar System
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

## [43.13] The Rotation Axis of Mercury: A New Determination from Radar Observations

C.~H.~De~Vries (Cornell University), J.~K.~Harmon (NAIC)

Ground based S-band radar observations of Mercury show a group of features in the northern and southern polar regions with highly depolarized radar echoes. These features are coincident with the positions of known craters and thought to be water ice deposits on the permanently shadowed floors of the craters (Harmon, et al., 1994, Nature, 369, 213). A byproduct of the discovery of these small features is an improved determination of the direction of Mercury's rotation axis. Twenty-seven separate radar images taken from the Arecibo Observatory in the summers of 1991 and 1992 are in a correct viewing geometry to resolve the northern polar region. The direction of the rotation axis of Mercury has been determined to higher precision than possible from previous observations by minimizing the motion of these features relative to the latitude longitude coordinate system through perturbations of the axis direction. One source of error is the apparent motion of these features caused by changing crater floor illumination due to the different viewing geometries of the observations. This has been modeled and a correction has been applied to the results of the axis perturbation technique, resulting in a rotation axis direction of RA(1950) $281.7\deg\pm0.7\deg$ , Declination $61.6\deg\pm0.3\deg$.