Marsis penetrating radar measurement of the total electron content and of the south polar layered deposits
Résumé
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), aboard Mars Express began routine science observations in early July, 2005. The objective of the subsurface experiment is to detect and characterize subsurface material discontinuities in the upper few km of crust. The Martian polar caps correspond to one of the principal targets for the ongoing radar sounding experiment due to the generally low electrical loss (water ice) and the importance of characterizing their internal structure. MARSIS is a synthetic aperture radar working at 4 frequency bands 1.8, 2.5, 3.5, 4.5 MHz. The vertical free space resolution is about 150 m. Horizontally, the cross-track and along-track footprint range is respectively 10-30 km and 5-10 km. As of June, 2005 MARSIS has acquired over 300 successful sounding observations. The radar waves travel through the ionosphere before and after reflection on the surface. This ionosphere's effect depends on the radar frequency. The bandwidth of MARSIS is relatively wide, about 1 MHz, compared to the central frequency used. In consequence, the signal is distorted and delayed, that why it must be corrected before data interpretation. Our third order correction based on amplitude maximization of the surface echo correct the distortion of the signal significantly. We take advantage of MOLA topography to locate the theoretic position of surface echo and use it also to constraint our correction. This correction is directly linked to the Total Electron Content (TEC) of the ionosphere and Chapman model. We present TEC measurements organised as function of the solar zenith angle. The average behaviour is discussed. The radar echoes from the surface of south polar cap are typically comparable in strength than those from the south polar bedrock. The signals clearly penetrate deep into the polar layers at all frequency bands. The time delay between surface and bedrock yields an estimation of ice depth at a given latitude-longitude. With a computation of all successful sounding orbits of the south polar cap, it's now possible to build map of ice depth and to describe the three dimensional structure of the south polar cap. We present the map of the depth over the whole polar region.