Ascraeus Mons South Flank
Located in the Tharsis region of Mars, Ascraeus Mons is the northernmost and tallest of the three shield volcanoes known as the Tharsis Montes, which are aligned along a northeast-southwest oriented topographic high. With a diameter of roughly 480 km and a height of 18 km, Ascraeus Mons is the second highest mountain on Mars, only surpassed by the giant Olympus Mons. The volcano has a very low profile with an average flank slope of only 7°. The image shows a portion of the southern flank of Ascraeus Mons. The height difference from the left side (south) and the right side (north) of the image is about 10 km! The dramatic features shown here are named Ascraeus Chasmata, an enormous collapsed terrain, extending more than 70 km at the lower southern flank of the volcano.
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.
Sinuous rilles (see annotated image) are another common surface feature found at the flanks of volcanos. They are commonly smaller than the pit structures and describe rimless, sinuous channels. Volcanic processes such as ash flow erosion or lava flow emplacement, but also surface water flow and a combination of volcanic and water activity have been suggested as formation mechanisms for sinuous rilles.
The left side of the image is dominated by several large fissures, reaching length of up to 40 km. Braided channel networks emerge from these fissures. Along the channels, streamlined islands and terraces in the channel walls can be spotted (see annotated image). This suggests a formation by water and not by lava. Presumably snow/ice deposits accumulated on the flanks of the volcano and were buried under subsequent volcanic airfall deposits. They were then melted in a later active episode of Ascraeus Mons.