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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.

Additional theories

To be valid any theory on how Martian spiders form must explain: 1) the clear-cut geological relationships between spiders with joints and faults (spiders always form in joint intersections and avoid certain faults) 2)the link between seasonal and temperature relationships to CO2 sublimation (spiders start to form as CO2 is sublimed and stop once the temperature approaches zero Celcius) 3) why spiders only form in certain regions of the south pole 4) why spiders convert to ravines in summer months 5) how the spiders form such intricate patterns, and 6) whether jet/geyser structures found on Encleadus and Ganymede form the same way or not.

Spider formations appear only in certain regions of Mars' south pole, but not in corresponding regions of Mars' north pole.

Mars' orbit around the sun is not symmetrical, nor circular. Mars has an elliptic orbit. At aphelion (the point in its orbit furthest from the sun), Mars is 249,228,730 km (154,863,553 mi) away from the sun. At perihelion (the point in its orbit closest to the sun), Mars is 206,644,545 km (128,402,967 mi) away from the sun. Mars reaches perihelion during the south polar summer and north polar winter. Mars reaches aphelion during the north polar summer and south polar winter. For this reason, the south pole receives considerably more direct energy from the sun during its summer than the north pole receives during its summer. The difference in energy received is due to the difference in intensity of radiated energy per square unit of surface area at the considerably different distances. One theory as to why the south pole develops "spiders" but the north does not relates to the difference in heating patterns of the carbon dioxide ice in the polar CO2 ice caps during the respective south and north polar springs and/or summers.

Recent imaging of auroras on Mars have led to a correlation between auroras and remnant magnetization of susceptible minerals in Mars' crust that create miniature magnetospheres. From observations, it appears that the majority of remaining magnetization of Mars' crustal minerals occurs in the southern hemisphere , with little to no remnant magnetization in the northern hemisphere.

Another theory as to why the "spiders" formed in the southern region and not in the northern relates to how charged particles from solar radiation interact with magnetic field lines. Charged particles tend to flow along magnetic field lines. Due to the south polar remnant magnetic fields being significantly stronger than the small or non-existent northern magnetic fields, charged particles would interact different with the south pole than the north. The fact that the south pole is aligned closer to the sun at perihelion, thus receiving more solar radiation and charged particles could factor in as well.

Never-the-less the known facts (see below reference list) are as follows: 1. Spiders start to form as the CO2 springtime sublimation commences, but by mid-summer they have ceased. 2. Fans form as CO2 and other material is ejected up degassing joints and small fault lines. 3. Spiders form in decreasing numbers down dip from major fault lines with highest concentration just above and below scarps. 4. Spiders are confined to the 80-86 S latitude. However, spiders have been found in other parts of Mars, most notably within 10km of Viking 2 (P.K Ness & Greg Orme 2006, unpublished report). 5. Spiders form in fibonacci-like structures 6. If the local air pressure due to spring degassing were to rise above 6 mbars directly above the spiders and a polar cap replenished saline brine fed existed in the subsurface then the spiders could be fully explained by a cold-emplacement hydrothermal model.

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