Martian precession and rotation from Viking lander range data

Abstract

The spatial orientation of the Martian pole of rotation and axial rotation parameters have been determined at a midpoint in the Viking epoch (January 1, 1980). The mean obliquity ε and node ψ angles are ε=25.1894°±0.0001, ψ=35.4796°±0.0002(2σ). The mean precession rate is ψo=−7.83±0.30 arc sec yr−1. The corresponding moment estimate is C/MRe2=0.355±0.015 and covers two extreme theoretical estimates. Seasonal (annual and semiannual) variations in Mars' rotation angle have also been observed and are 279±100 and 311±100 milliarc‐second of angle (mas) respectively, after correcting for a general relativity effect. A simple model has been constructed for the main contribution involving seasonal mass exchange between the ice caps and atmosphere where the air pressure at the Viking 1 lander site is a proxy for the global ice cap/air pressure histories. The annual term is about 190 mas smaller and differs in phase by ∼30° compared to the model prediction. The semiannual term is 100 mas larger and also differs in phase by ∼30° compared to the model. A combination of tidally driven rotation changes, zonal winds, polar motion and ice cap model deficiencies may account for any discrepancy. The upcoming Pathfinder lander and Mars Global Surveyor missions carry ranging transponders which can dramatically improve our knowledge of Mars' interior through precise determination of the precession rate and detection of core and tidal effects. We advocate that plans be developed for a coordinated campaign of simultaneous ranging and Doppler tracking to each transponder.