The deficiency of small craters within cratered terrain is attributed to obliteration by volcanism. Many of the compressional and extensional features on Mars have orientations consistent with formation by fracturing in response to loading by the Tharsis plateau. Lithospheric thicknesses from 25 to 50 km under volcanoes in the Tharsis and Elysium provinces to >150 km under olympus Mons have been obtained from consideration of the effects of mass loading by volcanic constructs. Mean crustal thicknesses from 23 to 40 km have been obtained from modeling of Bouguer gravity data. Substantial stresses must be supported, either statically by a thick, rigid lithosphere, or dynamically. The Tharsis plateau, which dominates the low-degree harmonics of the gravity field, appears to be only partially compensated Olympus Mons appears to be completely uncompensated. A major geologic dichotomy exists between the complex northern plains and the ancient southern cratered terrain. Martian surface more ยป materials probably consist of variable proportions of mafic igneous minerals and weathering products, the latter primarily oxides and carbonates. The dominant Martian lavas are probably mafic or ultramafic. The Martian mantle is probably denser than the terrestrial mantle. Possible extremes for the zero-pressure density of the Martian mantle could be as high as 3.6 g/cm/sup 3/ or as low as 3.3 g/cm/sup 3/. 2200 km, with the core constituting from approx. For plausible core density, core radii can range from approx. The mean density, 3.393 g/cm/sup 3/, and the estimated moment of inertia factor constrain the density distribution within Mars but do not define it uniquely.
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