DETERMINATION OF MERCURY 88 DAY LIBRATION AND FLUID CORE SIZE FROM ORBIT

The requirements for determining the existence and size of the suspect ed fluid outer core of Mercury are discussed. The moment of inertia C- m of Mercury's solid mantle and crust can be calculated from the ampli tude of the 88 day forced libration in longitude and some auxiliary in formation, since the fluid core will not follow this motion unless its viscosity is much higher than expected. The libration amplitude can b e determined from camera observations of the surface near periherm dur ing the Mercury Orbiter mission, provided that the spacecraft location and the camera attitude are measured sufficiently well. The accuracie s required are roughly 15 m in the spacecraft coordinate normal to the (polar) orbit plane and 15 arcsec in the camera attitude for 10% accu racy in C-m. The fluid core radius follows from C-m plus the total mom ent of inertia C and models for the composition and thermal history of the planet. Since maintaining a fluid core appears to require having perhaps 2 or 3% sulfur in the core, determination of the existence and size of the core will provide vital information on mixing of material s between different parts of the solar nebula. In this paper, the accu racy achievable for determining the spacecraft location with an X-band transponder system, as is planned for the mission, is investigated. A very preliminary covariance analysis was carried out based on 65 inde pendent arcs starting every 10 days during the mission, with 6 h of tr acking data around periherm for each arc. Gravity field coefficients u p to degree and order 26 were solved for, and the effects of uncertain ties in the other coefficients up to degree and order 36 were ''consid ered'', along with the effects of uncertainties in some additional par ameters. Six of the eight parameters included in the radiation pressur e model were ''considered'' rather than solved for in order to make su re that model uncertainties were allowed for adequately. The Im assume d range measurement uncertainty was treated as a bias for each arc rat her than a random error. The spacecraft location determination accurac y was found to be consistent with the requirements for measuring C-m t o 10% or better accuracy if C-m/C is about 0.5, as suggested by curren t models. (C) 1997 Published by Elsevier Science Ltd.