Rim walls and interior ring structures of impact basins produce parabolic-shaded echoes. The inset shows that parabolic-shaped echoes in the radargram project to circular arcs, indicating the presence of a buried impact basin.


 
This image shows the locations of the buried basins detected by the MARSIS sounding radar on board ESA’s Mars Express in July 2005. Locations and inferred diameters from MARSIS echoes are shown in black on a MOLA colour-coded shaded relief.

The locations and diameters of quasi-circular topographic depressions with diameters larger than 200 kilometres are plotted in white. White polygons show the area covered by MARSIS orbits. Where there is left/right ambiguity in the location of a MARSIS basin that coincides with a quasi-circular topographic depression, the plotted location reflects the minimum offset between the centres of the MARSIS basin and the quasi-circular topographic depressions themselves.


 
LEFT: (Top) This radargram obtained thanks to data form the MARSIS sounding radar on board ESA’s Mars Express on 4 July 2005 (orbit 1886), showing parabolic-shaped echoes from the rim walls of a buried impact basin. (Bottom) The parabolic echoes project to circular arcs on the surface and indicate the location of a 210 km-diameter impact basin in Amazonis Planitia buried by young lava flows. The dashed white circles are approximate fits to the arcs (echoes labelled in the radargrams). The echoes are interpreted to be from the top and bottom rim wall of the basin.

RIGHT: (Top) This MARSIS radargram was obtained thanks to data gathered on 7 July 2005 (orbit 1897) and shows a parabolic-shaped echo from the rim wall of a buried impact basin. (Bottom) The surface projection of the parabolic echo indicates a 140 km-diameter basin in Amazonis Planitia buried by young lava flows. The dashed white circles are approximate fits to the arcs (echoes labelled in the radargrams). The echoes are interpreted to be from the far rim wall of the basin.

The best fit to the echo indicates that the orbit track is offset from the basin centre, thus the left/right ambiguity in the data does not allow a unique determination of the basin centre. The two bottom images show the subsurface echoes projected on MOLA colour-coded shaded relief.


 
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’).



Release date: 13 december 2006