Towards magnetic sounding of Mars using diurnal variations

Abstract:

InSight landed on Mars in November 2018, equipped with a suite of geophysical instruments and the goal of characterizing the martian interior. Among the auxiliary instruments that monitor environmental conditions, is the InSight Fluxgate Magnetometer (IFG). The IFG is the first magnetometer on the martian surface and measures the vector magnetic field that includes contributions from the static crustal magnetic field and time-varying fields. The dominant contribution to the time-varying field are diurnal variations and their harmonics resulting from ionospheric currents and the draped interplanetary magnetic field. These fields induce currents in the planetary subsurface that depend on interior electrical conductivity, in particular of the upper mantle. Electrical conductivity in turn depends on temperature, mineralogy and volatile content. We analyze the first 479 sols on Mars using a classical geomagnetic depth sounding approach, calculating the transfer functions between the vertical and horizontal components. The C-response and apparent conductivity require knowledge of the inducing field geometry, which, for diurnal variations and harmonics, corresponds to terms of spherical harmonic order m=1 (1 sol), m=2 (0.5 sols), etc., in the Mars Solar Orbit frame. Magnetic field data from MAVEN allow estimation of the latitudinal contributions for each m. We calculate apparent conductivity values for the diurnal harmonics and compare these estimates with forward model predictions from a suite of conductivity models. Our preliminary results indicate relatively high conductivity and are consistent with a warm upper mantle model or perhaps higher Fe3+ and/or water content.