Constraints on the depth of magnetized crust on Mars from impact craters

Large (diameter greater than similar to 500 km) Martian impact basins are a ssociated with observed magnetic fields which are statistically distinct fr om, and smaller than, fields associated with smaller craters. We suggest th at this effect arises because impacts cause shock, heating, and excavation, reducing the magnetization of previously magnetized crust. For a simple, u niformly magnetized model the magnetic field at 100 km altitude is reduced by similar to 50% when a crater-shaped demagnetization zone reaches the bas e of the magnetized layer. By analogy with terrestrial data, we assume that in Martian craters the zone of demagnetization extends to a depth of 0.04- 0.15 crater diameters. On the basis of this assumption, the data suggest th at the depth to the base of the magnetized layer on Mars, if uniform, is si milar to 35 km, with lower and upper bounds of 10 and 100 km, respectively. These bounds imply magnetizations of 5-40 A m(-1) and are consistent with likely Mars geotherms at 4 Gyr B.P.