Impact demagnetization by phase transition on Mars



Recent solar system exploration has revealed the presence of a complex pattern of strong crustal magnetization on Mars [Acuña et al., 1999; Figure l]. The north-south dichotomy of the magnetization might be interpreted in terms of shutdown of the Martian dynamo before the formation or reworking of the younger and/or thinner northern crust, as well as the Tharsis dome. On the other hand, there is no apparent difference in the older, southern “Noachian” heavily-cratered terrane between the magnetized and non-magnetized crust. One could invoke the proximity of the Hellas and Argyre giant impact craters (Figure 1) if impacts occurred after dynamo shutdown, and if a mechanism is available to demagnetize the crust at distances of several crater radii. This mechanism cannot be thermal, as the thermal effect is very limited at such distance; thus, the mechanism must be pressure-related.