This is a radar image of Mars, made with the NASA Goldstone-Very Large Array (VLA) radar system in 1988. Red areas are areas of high radar reflectivity. The south polar ice cap, at the bottom of the image, is the area of highest reflectivity. The other areas of high reflectivity are associated with the giant shield volcanoes of the Tharsis ridge. The dark area to the West of the Tharsis ridge showed no detectable radar echoes. The investigators call this region the “Stealth” region and believe it is a huge deposit of low density, volcanic ash with an absence of volume scatterers (rocks) to depths of many meters.
Credit: D. Muhleman, B. Butler, A. Grossman, and M. Slade, NRAO/AUI/NSF