Length of day

Measurements of Mars' rotational variations can be conducted via Earth-based radio tracking observations of the Mars Pathfinder lander during an extended mission. Two-way range measurements between an Earth tracking station and the lander will enable precise monitoring of the planet's orientation, allowing details of Mars' internal structure and global surface/atmosphere interactions to be determined. An analysis has been performed to investigate the accuracy with which key physical parameters of Mars can be determined using the Earth-based radio tracking measurements. Acquisition of such measurements over one Martian year should enable determination of Mars' polar moment of inertia to 1% or better, providing a strong constraint on radial density profiles (and hence on the iron content of the core and mantle) and on long-term variations of the obliquity, which influences the climate. Variations in Mars length of day and polar motion should also be detectable, and will yield information on the seasonal cycling of carbon dioxide between the atmosphere and the surface.

Very Long Baseline Interferometry (VLBI) is exclusively sensitive to the complete rotation matrix from the terrestrial to the quasi-inertial frame, while ring lasers being sensitive to the instantaneous Earth rotation vector in an Earth- fixed frame, i.e., to polar motion and length of day variations. Additionally, assuming a perfect knowledge of mass attraction variations, superconducting gravimeters (SG) permit to detect Earth rotation variations from the magnitude of the local gravity. The formula of the Sagnac frequency and gravity variation caused by the motion of the Earth's body w.r.t. the instantaneous Earth rotation vector will be presented and compared to VLBI parameterization. The potential of a combination of the three independent types of observables will be examined for sub-diurnal polar motion and universal time variations.