Physics of the earth and planetary interiors vol:117 issue:1-4 pages:397-405
Mars transfer functions for nutations, tides and surface loading are computed for different models in order to study the influence of the physics of the interior on the planet's response to external forcing. If the core is liquid, there is a resonance at the Free Core Nutation (FCN) period which must be taken into account. The FCN period depends on the density of the core (and consequently on the density jump at the core-mantle boundary (CMB)) and on the dimensions of the core. The responses of Mars forced by the solar gravitational attraction involve frequencies which could be very close to the FCN. The observation of their amplitudes can, in that case, give us information about whether the core is liquid or not, about its dimensions, as well as about the density jump at the CMB. We therefore consider several models for the interior structure of Mars involving different normal mode frequencies and compute the transfer functions for nutations, tides, and loading as a Function of the forcing frequency. We show that the amplitudes of the semi-annual, tri-annual and quadri-annual nutations could be amplified by several ten percent or even several hundred percent by the FCN resonance, depending on the FCN eigenfrequency value (here, annual means one Martian year). The same holds for the second-degree diurnal tides and loading effects. The impact of the core radius, density and state on the body and load Love numbers is analysed. Body and low-degree load Love numbers h and k can change by up to 25% for variations in the core radius of 200 km or for a change in the core state (liquid/solid). The corresponding variations in the tidal gravimetric factor and the loading Green's function are about 10 times smaller. High-degree load Love numbers are more sensitive to the upper mantle rheology. (C) 2000 Elsevier Science B.V. All rights reserved.