Geophysical constraints on the evolution of Mars

The evolution of Mars is discussed using results from the recent Mars Globa l Surveyor (MGS) and Mars Pathfinder missions together with results from ma ntle convection and thermal history models and the chemistry of Martian met eorites. The new MGS topography and gravity data and the data on the rotati on of Mars from Mars Pathfinder constrain models of the present interior st ructure and allow estimates of present crust thickness and thickness variat ions. The data also allow estimates of lithosphere thickness variation and heat flow assuming that the base of the lithosphere is an isotherm. Althoug h the interpretation is not unambiguous, it can be concluded that Mars has a substantial crust. It may be about 50 km thick on average with thickness variations of another +/- 50 km. Alternatively, the crust may be substantia lly thicker with smaller thickness variations. The former estimate of crust thickness can be shown to be in agreement with estimates of volcanic produ ction rates from geologic mapping using data from the camera on MGS and pre vious missions. According to these estimates most of the crust was produced in the Noachian, roughly the first Gyr of evolution. A substantial part of the lava generated during this time apparently poured onto the surface to produce the Tharsis bulge, the largest tectonic unit in the solar system an d the major volcanic center of Mars. Models of crust growth that couple cru st growth to mantle convection and thermal evolution are consistent with an early 1 Gyr long phase of vigorous volcanic activity. The simplest explana tion for the remnant magnetization of crustal units of mostly the southern hemisphere calls for an active dynamo in the Noachian, again consistent wit h thermal history calculations that predict the core to become stably strat ified after some hundred Myr of convective cooling and dynamo action. The i sotope record of the Martian meteorites suggest that the core formed early and rapidly within a few tens of Myr. These data also suggest that the sili cate rock component of the planet was partially molten during that time. Th e isotope data suggest that heterogeneity resulted from core formation and early differentiation and persisted to the recent past. This is often taken as evidence against vigorous mantle convection and early plate tectonics o n Mars although the latter assumption can most easily explain the early mag netic field. The physics of mantle convection suggests that there may be a few hundred km thick stagnant, near surface layer in the mantle that would have formed rapidly and may have provided the reservoirs required to explai n the isotope data. The relation between the planform of mantle convection and the tectonic features on the surface is difficult to entangle. Models c all for long wavelength forms of flow and possibly a few strong plumes in t he very early evolution. These plumes may have dissolved with time as the c ore cooled and may have died off by the end of the Noachian.