...shows various geomorphological features.
Numerous geomorphologic structures of different age and origin can be found within the image. Starting at the right side of the image, still 60 km away from the summit of Ascraeus Mons, numerous lava flows can be spotted (see annotated image) partly better visible in the color-coded terrain image.
Pit structures, also called pit crater chains, can be found all over the HRSC scene (see annotated image for an example). Aligned circular to near circular depressions form chains and can combine to form a trough-like structure called coalesced pit crater troughs. It is consensus, that these structures are formed by collapse into a subsurface void. However various processes have been suggested as origin of these voids. They range from dyke-swarm emplacement and karst dissolution to tectonic and volcanic origin. Many researchers believe that these structures represent "lava tubes", which form when the lava flow cools quickly at the surface over a subsurface lava flow. The flow then eventually ceases and drains the tube, leaving back a void space several meters below the surface.