|Topography of the south polar region of Mars from MGS MOLA data, with locations of MARSIS measurements of the SPLD thickness shown as open circles. SPLD unit as mapped by (15) is outlined in black. Red lines indicate ground tracks of orbits. Apparent gaps in coverage are due to the lack of a discernible basal interface, and not to gaps in observations. No MARSIS data are available poleward of 87Â° S (dark circle in upper center).