The Size of Mars' Fluid Core From Mars k2 Love Number Obtained From Analysis of MGS Doppler Tracking.

The solar tidal deformation of Mars, measured by its k₂ potential Love number, has been obtained from analysis of MGS radio tracking. The observed k₂ =0.164+-0.016 is large enough to rule out a solid iron core. The inferred core radius R_c (1600km<R_c<1900km) is remarkably independent of interior properties such as temperature, composition (as measured by the molar ratio Mg/(M+Fe)) and crustal thickness, even after correcting for atmospheric thermal tides, mantle anelasticity, spin pole nutations and seasonal changes in shape from ice cap ablation/accretion. One critical model feature is the ability to isolate the second degree and m'th order harmonic components: semidiurnal (m=2), diurnal (m=1) and long period (m=0) and solve for independent k_2m parameters. Detection of tides depends on finding reliable, long period signatures since short period changes are too small. A crucial tidal signature is a secular drift in spacecraft orbit inclination related to the sun-synchronous spacecraft orbit and which is seen only in the m=2 tide. In order to minimize the effect of along-track changes on a solution for k₂₂, the drag model solves for a daily coefficient and thus effectively minimizes the influence of the along-track residual signature on this solution parameter. The k₂₁ and k₂₂ coefficients primarily affect the orbit node (k₂₁ has an annual variation) where it is strongly mixed with seasonal changes in Mars' even zonal gravity harmonics (J_2, J₄). The odd gravity harmonics (J₃, J₅) are detected through seasonal changes in orbit eccentricity where the influences of tides are weak. The observed J₃, J₅ amplitudes are consistent with estimates of ice cap mass only if the cap thickness increases with latitude and the south cap is significantly larger than the north cap.