Radargrams and ground-range projections of MARSIS data obtained in July 2005 when Mars Express was flying over the Chryse Planitia region of Mars. The two bottom images show the subsurface echoes projected on MOLA colour-coded shaded relief. Echoes are plotted in time-delay versus position along the orbit track. The peak surface return is corrected to agree with the MOLA topography.
The top left radargram was obtained thanks to MARSIS data on 9 July (orbit 1903), while the top right one was obtained thanks to data gathered on 6 July (orbit 1892). Parabolic echoes that project as arcs on the surface are interpreted to be from the near and far rim walls (relative to the orbit track) of buried impact basins.
The dashed white circles are approximate fits to the arcs (echoes labelled in the radargrams). Echoes are interpreted to be from the far rim wall of an approximately 220-km-diameter basin (corresponding echoes in a. and b. labelled as ‘Far wall’), superimposed on a larger, 310-km-diameter basin that may have echoes from the top and bottom of the near rim wall (corresponding echoes in a. and b. labelled as ‘Near wall’).