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.