Mars: a magnetic field due to thermoremanence?

Presently available magnetic field data suggest a dipole moment of Mars of less than 10(-4) times the Earth's dipole moment. Presumably, Mars does not have an active dynamo at present which could give rise to a significant magnetic field. Nevertheless, the presently available data do not rule out a minor intrinsic field which may originate from a magnetized lithosphere. The objective of this study is to examine how much magnetization the Martian lithosphere could have acquired in the presence of an internal dynamo in early Martian history. Two models are presented the first one assumes a laterally homogeneous lithosphere with a globally uniform thickness; the second model takes a possible dichotomy of the Martian lithosphere into account, which is assumed to be two to three times thicker in the southern hemisphere than in the northern hemisphere. The gradual cooling of the lithosphere below the Curie temperature is reproduced by a multi-layer model. Curie isotherm motion with time and the dipole moment of the dynamo field are calculated from a thermal evolution model. For simplicity, the magnetic properties of the lithosphere are parameterized by its iron content. In the homogeneous case, the magnetostatic boundary value problem is solved exactly; in addition, an approximate solution is presented which is applicable with the dichotomy model. Using either method, the demagnetizing field in the separate layers mainly leads to the resulting remanent dipole moment. The remanent dipole moment is found to depend approximately linearly on the Curie isotherm depth and approximately quadratically on the iron content the lithosphere. To reproduce the upper limit of the dipole moment derived from the Phobos mission (similar to 10(18) Am-2), a mean depth of 150 to 200 km for the Curie isotherm and a free iron content ranging from 5 to 6 vol.% are needed. The mean surface field may be less than 10 nT for both the homogeneous and the dichotomy model. The latter model yields a marked difference between the northern and southern hemisphere, with a pronounced anomaly at the dichotomy boundary. (C) 1997 Elsevier Science Ltd. All rights reserved.