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How spiders are formed

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.”