These two radargrams from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) show echoes from an approximately 250-kilometer (155-mile) diameter circular structure below the surface of Mars. The circular structure is interpreted to be a buried impact basin.
In two orbits spaced about 50 kilometers (31 miles) apart, MARSIS detected a series of arc-shaped reflectors that have no apparent source in the surface topography or geology. In the lower image, a linear reflector nearly parallel to the surface is seen embedded in the arcs. This reflection may be coming from the floor of the basin. The time delay to the linear reflector suggests a depth of 1.5 to 2.5 kilometers (0.9 to 1.6 miles).
MARSIS is an instrument on the European Space Agency's Mars Express orbiter. NASA and the Italian Space Agency jointly funded the instrument.