Department of Earth Sciences

Polygonal ridge networks are found in numerous locations and geological contexts around Mars. The best-known example for these features is Angustus Labyrinthus near the South Pole. Discovered already in 1972 on Mariner 9 data, the structure exhibits a rectilinear pattern of ridges and internal polygons resembling Incan ruins and therefor it was informally named “Inca City”. The formation of these features is still discussed and ranges from lithified dunes to clastic dikes to magmatic dikes. Later data showed that the ridges trace the part of a large, 86 km in diameter circle, so researchers also supposed an exhumed impact basin structure. Deep faults, created by the impact filled up by rising lava. Later, the softer polar plains material around was eroded leaving back the harder ridges. Another hypothesis is that the ridges are linked to the neighboring Dorsa Argenta formation, which contains many ridge segments interpreted to be eskers, a glacial landform. The left part of the image shows the intriguing Angustus Labyrinthus feature.

The south polar region of Mars displays an excessive abundance of exotic landforms, which mostly occur in regions covered by a seasonal ice cap. This HRSC observation shows a darker region on the left side (southern) side of the image. At closer look, this dark region is composed of many small dark spots which also can be found in all other parts of the image, but in different distributions.

They range in size from 45 m to 1 km and are believed to result from the escape of sand- and dust-laden carbon dioxide gas (CO₂)  from below a condensed slab of CO₂ ice. When CO₂ condenses at the south pole during late fall and winter, it forms a 0.5 to 1 m thick seasonal ice cap. A part of this ice condenses into a slab of solid larger grained CO₂ ice. This ice slab is mostly translucent to the visible light and opaque to the thermal infrared spectrum. This means, that sunlight can pass through the ice layer and warm the underlying base causing sublimation. The pressurized gas is then trapped inside the (dark) substrate. When the pressure rises, the ice cracks and a rapid gas eruption takes place, entraining substrate material from the cavity and depositing the dark material at the surface.

It is assumed that this process represents an important exchange between the polar ice cap and the atmosphere. If one were to zoom in closer to the dark spots with even higher resolution images, one would discover thin lines starting from some of these spots. They are formed when the pressurized  CO₂ escapes along weak zones in the ice and the underlying material is deposited around the dark spot like spider legs. This is why these structures are also called "spiders" in mars research.”

Further towards the center of the image, the landscape changes and eroded layered deposits can be found, displaying large roundish and oval shapes. The center of the image is dominated by some buttes and large mesas raising more than 1500 meters. Interestingly, the dark spots can be found all over the image and at all altitudes even on top of the large mesas and also an some steep flanks. Further to the right (northern) part of the image, the dust coverage of the surface appears much higher. Finally, on the adjoining plateau many dark spots can be found again. There are also some canyons and troughs, presumably they are the witnesses of periodic sublimation.