GEODYNAMICS OF EARLY MARS

Following fractionation of a primordial crust, the geodynamical evolution of early (Noachian/pre-Noachian) Mars was dominated by the formation of the Tharsis rise and of the global dichotomy between the northern lowlands and southern highlands. The debate on the origin of the dichotomy has focused on exogenic vs. endogenic processes. The basement of the northern lowlands dates from the earliest Noachian, but geophysical analyses using Mars Global Surveyor (MGS) topography and gravity field data yield no evidence in modeled crustal structure for an impact origin for the northern lowlands. Moreover, recent geodynamical modeling demonstrates the plausibility of a convective origin for the dichotomy. However, neither argument eliminates completely an exogenic origin. Sometime after the development of the dichotomy, the Tharsis rise began to form from the magmatic products of a superplume, and its mass was largely in place by the end of the Noachian epoch. Tharsis loaded and faulted the global lithosphere and, along with the pole-to-pole slope, continues to control the long-wavelength shape of the planet, with the mass load creating the circumferential Tharsis trough and the antipodal Arabia bulge. Valley network orientations are largely controlled by this long-wavelength shape, so much of the observed population must have postdated much of the formation of Tharsis.

The Noachian climate on Mars may have approached clement conditions, allowing liquid water at or near the surface to facilitate widespread erosion. Favorable surface temperatures may have been enabled through volatile release to the atmosphere by Tharsis volcanism, and specific climate models are able to produce regional temperatures above freezing by incorporating the volatiles associated with the estimated mass of Tharsis magmas. This scenario provides a link between Tharsis growth and valley network formation.

The magnetometer on MGS mapped remanent magnetic fields, confined mostly to the Noachian southern highlands. The nature of the global inducing field, and when it ceased, is strongly tied to both the thermal history of the planet and the evolution of the atmosphere. The apparent absence of magnetic anomalies at the giant impact basins of Hellas and Argyre has been used to argue that the global field had vanished by the time of earliest Noachian.