Complementarity of seismological and electromagnetic sounding methods for constraining the structure of the Martian mantle

The complementarity of seismological and electromagnetic sounding methods for the thermodynamical characterization of the Martian mantle is discussed by illustrating the observational constraints and limitations of both methods. The increase of temperature within a few hundreds of kilometers thick Martian outer lid with conductive heat transfer should induce the presence of a seismic low-velocity zone, due to the relatively small increase of pressure within Mars. The depth of minimum velocity will help to constrain the thickness and mean thermal gradient of the lid. These parameters will be strongly constrained by electromagnetic sounding methods. At greater depths, temperature Variations of the order of 400 K will be detectable if seismic velocities can be determined with an accuracy better than 2%. An extrapolation of presently available laboratory data to the pressure range of Mars' mantle predicts that the deep mantle electrical conductivity will be accessible if Mars' mantle is cold, and mineralogically similar to the Earth's. On the other hand, the high temperature and/or the high conductivity of garnet might impede an interpretation of electromagnetic sounding data at depths greater than 300 km for a nominal duration of the NetLander mission of the order of one Martian year. If the mantle is olivine-rich, the phase transitions of olivine should translate into first-order seismic and electromagnetic discontinuities, eventually smoothed if the iron content of Mars' mantle is about twice the Earth's one. The depth of occurrence of the exothermic olivine to waldsleyite and ringwoodite transitions will provide information on the temperature of the mantle. A pyroxene-rich mantle should instead be characterized by a constant increase of seismic velocities in the depth range 800-1200 km. This seismic gradient would be generated by the progressive increase of the garnet content at the expense of pyroxenes in solid solutions of non-olivine minerals. (C) 2000 Elsevier Science Ltd. All rights reserved.