The MARSIS radar sounder instrument on the European Space Agency’s Mars Express spacecraft is comprised of a 40 meter (about 131 feet) dipole antenna. MARSIS transmits long wavelength, low frequency radio waves that can penetrate certain geologic materials and are reflected back where the radio waves encounter a change in bulk density or composition. The radargram, a time delay rendering of the reflected radar pulses, shows a subsurface feature or reflector (upper pointing arrows) separated from the surface reflection (downward pointing arrows) in Meridiani Planum. The subsurface reflector is interpreted to be the base the Meridiani Planum deposits, about 1 kilometer (about 0.62 miles) beneath the surface.
Image courtesy ESA/NASA/JPL/KU/Smithsonian