The seasonal polar caps of Mars are mainly composed of carbon dioxide (CO₂) ice. This ice partially sublimates in spring and releases large amounts of gas into Mars’ thin atmosphere, whereas in fall, the carbon dioxide condenses again, causing the polar caps to grow and to reach as far as 55 degrees latitude by late winter. These processes form a surface that Mars researches named cryptic terrain.

It is spring in the southern hemisphere of Mars, and the region shown here appears to have recently emerged from beneath a thick winter blanket of frost. The left side of the image is dominated by a thick stack of layered deposits, usually composed of a mixture of CO₂ ice and dust. The right side of the image also shows the smooth surface of the polar layered deposits. The center of the image shows the lower-lying areas of the region where darker tones dominate. A closer look reveals, that the surface is covered with a pattern of polygons with different scales. On Earth, this kind of polygons are common periglacial features in arctic and antarctic regions and usually indicate the presence of water ice in the ground. Periglacial is a term that refers to regions and processes where cold climate contributes to the evolution of landforms and landscapes. Polygons form from freeze/thaw cycles of ground ice over the course of several years and even centuries. Landforms like this are also called “patterned ground”. The patterns often take the form of large polygons, each bounded by either troughs or ridges made up of rock particles different in size from those seen in the interior of the polygon.

In many locations throughout the image, fan-shaped deposits cover a large area with fans oriented in the direction of prevailing winds. The lengths of the fans range from tens of meters to several hundreds of meters. Two types of fans can be observed in the HRSC image: Dark fans that are result of transported fine-grained dark surface material and bright fans that indicate frost condensing from the CO₂ gas. These seasonal polar fans are deposits from jets that ejected dust from the subsurface. Their formation is associated with “spiders” (araneiforms) [see HRSC PR April 2024 – Angustus Labyrinthus] and their activity is governed by solar radiation. When the sunlight penetrates through the translucent CO₂ ice layer in early spring and warms up the underlying substrate, the sublimation temperature of the ice is quickly reached and pockets of pressurized gas start to form. Finally, the ice slab cracks releasing the gas and forming CO₂ jets. The escaping gas entrains some of the loose material of the substrate and brings it up to the atmosphere where it is deposited on the upper surface of the seasonal ice. The results of this process can be observed as dark fans. After this first phase of activity where dark material is deposited onto the translucent top ice surface, a second phase kicks in, when the ice layer interacts with the dark material on it. Mars researchers believe, that the bright fans form either by recondensation of ice or that bright fans may also be produced from dark fans. When dark grains sink through the ice layer they absorb sunlight and warm the CO₂ ice, which sublimates and leaves a hole. New CO₂ frost recondensates on top of the sinking dark grains. This process results in a bright fan at the place of the dark. Another hypothesis is that the warmer dark dust at the surface creates a quick thermal jump in the ice-slab crystal which greatly decreases the transparency of the ice which then leads to a strong light reflection compared to the surrounding material. In general, the fans only develop during local spring until the complete (translucent) seasonal ice layer is sublimed from the surface and the fans become indistinguishable from the underlying surface.