Joint Astrophysics Colloquium

Joint Astrophysics Seminar

Magnetic Field of Mars

Jafar Arkani-Hamed

Earth and Planetary Sciences
McGill University

Mars Global Surveyor (MGS) has measured strong magnetic anomalies largely concentrated over the southern hemisphere of Mars. The immense amount of data allows us to derive a highly accurate spherical harmonic model of the magnetic fied of Mars including harmonics up to degree 65. There is no core dynamo active in Martian core at present. The anomalies arise from lateral variations of remnant magnetization in the crust acquired when when the core dynamo was active. The lack of appreciable anomalies associated with the giant impact basins Hellas, Isidis, and Argyre, and with the northern lowlands implies that the core dynamo has been inactive at least since the impacts occurred about 3.9 Gyr ago, and the crust beneath the basins is demagnetized by impact-induced shock pressure and heating. The cessation of the core dynamo very early in the history of the planet poses strong constraint on the thermal evolution of the core and the mechanism for the generation of the dynamo in the first place. Whether the core dynamo was powered by the thermal convection in the core or by encounter of a large asteroid is not yet clear. The thermal evolution models of Mars suggest that magnetic source bodies are located in the upper about 100, 80, or 45 km of the crust if hematite, magnetite, or pyrrhotite is the major magnetic carrier, respectively. Mars has become a one-plate planet very early in its history and major part of the crust has likely been formed by volcanism on a stagnant lithosphere. The thermal evolution models of a lava flow show that its temperature remains below the magnetic blocking temperatures of magnetite, 480-580C, until it reaches ~30-45 km depth. The highly magnetic single-domain magnetite grains can explain the strong magnetic anomalies of Mars.