THE INTERIOR STRUCTURE OF MARS - IMPLICATIONS FROM SNC METEORITES

Two end-member models of Mars' present interior structure are presente d: the first model is optimized to satisfy the geochemical data derive d from the SNC meteorites in terms of the bulk chondritic ratio Fe/Si = 1.71, while the second model is optimized to satisfy the most probab le maximum value C = 0.366 x M(p)r(p)(2) of the polar moment of inerti a factor. Hydrostatic equilibrium and stationary heat transfer are ass umed, and the basic differential equations for the mechanical and ther mal structure are solved numerically together with an isothermal Murna ghan-Birch type equation of state truncated in Eulerian strain at fort h order. We obtain the radial distribution of mass, hydrostatic pressu re, gravity, temperature, and heat flow density along with the corresp onding density stratification, viscosity profiles, and the global seis mic velocity structure of model Mars. The first model being consistent with the geochemical requirement produces C = 0.357 x M(p)r(p)(2), wh ereas the second model commensurate with the geophysical constraint gi ves Fe/Si = 1.35. The calculated central pressure is about 40 GPa in b oth models, and the central temperature is in the 2000 to 2200 K range . The model calculations suggest a Fe-Ni-FeS core a little less than o ne half of the planetary radius in size surrounded by a silicate mantl e subdivided into lower spinel and upper olivine layers and overlain b y a 100- to 250-km thick basaltic crust and a surface heatflow density of 25 to 30 mW m(-2); In both models the pressure in the mantle is no t sufficient for the spinel to perovskite transition to occur. The pre sent thermal lithosphere is estimated to be about 500 km thick and to be subdivided into a 300-km-thick outermost rheological lithosphere an d an underlying thermal boundary layer of mantle convection. Given the core sulfur content of 14 wt% as derived from SNC meteorites, the Mar tian core is found to be entirely molten, implying the nonoperation of a self-sustained dynamo due tb the absence of sufficiently vigorous c onvection.