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Martian "spiders" are geological formations thus far unique to the south polar region of Mars. These structures have not been found in the north polar region or any other region of Mars to date. They appear as the carbon dioxide ice cap at the Martian south pole sublimates into carbon dioxide gas during the Martian spring and the surface below is revealed.

The formations, when viewed individually, form a round lobed structure reminiscent of a spider web. They generally radiate outward in lobes from a central point. The central point is sometimes, but not always, a crater. The formation is similar in its branching appearance to diffusion-limited aggregation.

Causes

It is not yet fully understood how the formations occur, or why they appear to only form in the south polar region. Theories include:

1. Dry venting of carbon dioxide (CO₂) gas and dust up joints.
2. Head-ward erosion: Fluid derived from sub-surface layers is expelled up fissures eroding joints to create tributaries capped with mud-like material and/or ice.
3. Modified clathrate hydrate model: Structures form as the outside of the flow chills.
4. Hydrothermal-type sources, and
5. Magma pressurizes overlying fluids, expelling mud-like material, hydrothermal fluids or basalt. Other explanations have been offered by NASA, astrophysicists, astrogeologists and astrobiologists including water erosion in Mars' history , carbon dioxide / sand geysers

It has also previously been suggested that these structures had a volcanic cause or were simply warm patches of bare ground, however recent thermal imaging by NASA has revealed that these structures are generally as cold as the dry ice that covers the area during the Martian southern hemispheric winter.

NASA press release from August 16, 2006 states that an earlier theory proposed that the spots were patches of warm, bare ground exposed as the ice disappeared. However, the camera on Odyssey orbiter, which sees in both infrared and visible light wavelengths, discovered that the spots are nearly as cold as the carbon dioxide ice, suggesting they were just a thin layer of dark material lying on top of the ice and kept chilled by it. The morphology of these formations appears to be controlled by bedding and local jointing of the rocks; implying that expelled fluids are derived from within a few hundred meters of the surface. Some spider ravines modify, some destroy and others create crust in a dynamic near-surface process that extensively reworks the terrain creating and destroying surface layers. This process is rapid, happening in the space of a few days, weeks or months.

• Star burst Channels (Spiders) caused by escaping gas, as seen by HiRISE. Star burst channels, also called spiders, may be about 500 meters in diameter and 1 meter deep

References

1. (Spiders on Earth and Mars. AIG News #85, page 21.)
2. Photographic examples: 1, 2, 3
3. Simulations of Geyser-Type Eruptions in Cryptic Region of Mars' South Polar Cap, Spiders: Water Driven Erosive Structures in the Southern Hemisphere of Mars, Martian Spiders as feasible water-driven erosive structures.
4. NASA Findings Suggest Jets Bursting From Martian Ice Cap, Simulation of Geyser-Type Eruptions in Cryptic Region of Mars' South Pole
5. [http://www.jpl.nasa.gov/news/news.cfm?release=2006-100 NASA Findings Suggest Jets Bursting From Martian Ice Cap ]
6. [http://spsr.utsi.edu/articles/ness.pdf Peter K Ness and Greg M Orme. Spider-Ravine Models and Plant-like Features on Mars - Possible Geophysical and Biogeophysical Modes of Origin and Planet-like Features on Mars JBIS, Vol. 55, pp.85-108.]

See also

• Mars' dark dune spots
• Arachnoid (astrogeology)
• Chaos terrain
• Geology of Mars
• Planetary geology
• Rille
• Swiss cheese features

External links

• Martian "Spiders" photo repository. 1, 2, 3
• Papers on Martian "Spiders" 1, 2, 3, 4, 5, 6, 7,
• Sir Arthur C. Clarke on Martian "Spiders" 1, 2, 3, 4, 5
• Fibonacci Numbers and Nature

• Planetary geology
• Mars