SHAPE AND ORIENTATION OF MERCURY FROM RADAR RANGING DATA

If Mercury's radius is expanded in Legendre functions to the second de gree and order, the systematic error in radar ranging can be reduced s ignificantly. We interpret the expansion coefficients in terms of a be st-fit ellipsoid displaced with respect to the center of mass in the e quatorial plane, The ellipsoid's principal axes are rotated in the equ atorial plane such that the long axis is aligned with cartographic lon gitude 15.3 degrees +/- 2.9 degrees (west). The pole location is consi stent with the IAU pole, normal to Mercury's orbital plane. There is a significant equatorial ellipticity (a - b)/a = (540 +/- 54) x 10(-6). The center of figure is offset from the center of mass (C.F.-C.M.) by 640 +/- 78 m in the equatorial plane in the direction of cartographic longitude 319.5 degrees +/- 6.9 degrees. The magnitude of the equator ial center of figure offset implies an excess crustal thickness of 12 km or less, comparable to the Moon's excess. By comparing the equatori al ellipticity with the Mariner 10 gravity coefficient <(C)over bar (2 2)> and assuming Airy isostatic compensation, we conclude that Mercury 's crustal thickness is in the range from 100 to 300 km. (C) 1996 Acad emic Press, Inc.