A post‐Viking view of Martian geologic evolution

Abstract

The mean density, 3.933 g/cm³, and the estimated moment of inertia factor, 0.365, constrain the density distribution within Mars but do not define it uniquely. For plausible core densities, core radii can range from ∼1350 to ∼2200 km, with the core constituting from ∼13 to ∼35% of the planet's mass. Possible extremes for the zero‐pressure density of the Martian mantle could be as high as ∼3.6 g/cm³ or as low as ∼3.3 g/cm³. The Martian mantle is probably denser than the terrestrial mantle; however, the actual density and composition of the Martian mantle are not well constrained by present data. The dominant Martian lavas are probably mafic or ultramafic. Viking lander analyses suggest that soils are hydrated, Fe3+‐bearing weathering products of mafic rocks. Earth‐based reflectance spectra indicate olivine (or basaltic glass) and pyroxene in dark areas and several percent Fe3+ oxides in bright areas; integral disk spectra indicate the presence of H2O ice and mineral hydrates. Stable weathering products under current surface conditions are primarily oxides and carbonates. Martian surface materials probably consist of variable proportions of mafic igneous minerals and weathering products; the actual mineralogy is not well constrained by present data. A major geologic dichotomy exists between the complex northern plains and the ancient southern cratered terrain. 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. Substantial stresses must be supported, either statically by a thick, rigid lithosphere, or dynamically. Mean crustal thicknesses ranging from 23 to 40 km have been obtained from modeling of Bouguer gravity data. Lithospheric thicknesses ranging 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. Many of the compressional and extensional features on Mars have orientations consistent with formation by fracturing in response to loading by the Tharsis plateau. The deficiency of small craters within cratered terrain is attributed to obliteration by volcanism which formed the intercrater plains in cratered terrain. These intercrater plains, which appear to be the first units formed after the ancient cratered terrain, overlap in relative ages with the ridged plains and the fretted regions; remaining plains units are younger. The maximum resurfacing rate due to volcanism occurred between 1.0 and 1.5 b.y. ago if a constant cratering flux is assumed and between 3.5 and 4.0 b.y. ago if the lunar cratering flux (scaled to Mars) is assumed. Thermal evolution models have considered the formation of initial crust, core formation, mantle differentiation, and planetary radius changes, but not the major geologic asymmetries of Mars. The time scales of thermal evolution models can be lengthened or shortened by making various assumptions about initial temperatures and heat sources. Models in which the core formed in the first billion years and in which the peaks of mantle differentiation, volcanism, and planetary radius occur between 1.5 and 3.5 b.y. ago are consistent with a Martian cratering flux intermediate between the constant flux model and the scaled lunar flux. The high 15N/14N ratio of the Martian atmosphere, 1.7 times the terrestrial value, is ascribed to mass‐dependent loss of 10–150 times the present amount of atmospheric 14N. The absence of observable isotopic effects in C and O suggests that atmospheric CO2 and H2O must exchange periodically with a larger, normally nonatmospheric reservoir. The Martian atmosphere exhibits a ‘planetary’ type pattern of noble gas abundances, with xenon depleted in relation to the other noble gases. Estimates of the whole planet column abundances of CO2 and H2O range from 290 to 8000 g/cm² and from 600 to 1600 g/cm², respectively. Amounts of H2O and CO2 which are comparable to or perhaps greatly in excess of the whole planet estimates made on the basis of atmospheric noble gas abundances can be stored in plausible reservoirs: the residual polar caps; hydration, oxidation, and carbonation of surface materials; adsorption and absorption into the regolith; and as subsurface ices. A number of surface features have morphologies which appear to require tens of meters of water, and perhaps more, for their formation: fretted terrain, channels, patterned or polygonal ground, rampart ejecta deposits, and possible table mountains.