UNRESOLVED PROBLEMS IN THE THERMAL EVOLUTION OF MARS

Geophysical observations of Mars combined with surface geology and geochemistry of the surface and from Martian meteorites have enabled significant recent advances in reconstructing Mars' thermal history. But nonetheless many baffling questions remain. Intense, remanent magnetization of the Martian crust is most simply explained by an early, vigorous core dynamo that shut off prior to the end of heavy bombardment. The presumed major plume(s) that formed the Tharsis volcano-tectonic province and/or possible degree-one convection could have been effective means of transporting heat from Mars' deep interior during the ancient Noachian epoch. But despite rapid early cooling, the Martian core may remain molten to the present day. Gravity and topography reveal that the crustal structure of Mars displays a planet-scale variation that is difficult to reconcile with relaxation models. The resurfacing of the northern hemisphere, presumed to be primarily volcanic in nature, dates to the Hesperian epoch, well past the turnoff of the dynamo and the likely period of greatest heat loss from the deep interior. Although present-day measurements of dissipation within Mars from the degree-2 Love number and dissipation of energy from the orbit of Phobos have been interpreted as evidence of a liquid core, other possible internal structure models, including homogeneous dissipation, cannot be excluded. Analogy with terrestrial interior viscosity estimates suggest dissipation within Mars is not limited to the core. Substantial water in the mantle and crust may provide at least a partial explanation for at least some of these enigmas. Current observations from the MRO Radio Science Gravity investigation, with its polar, low-latitude (230 km periapsis) orbit, will be particularly useful in resolving thermal history issues related to crustal structure.