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| '''Martian "spiders"''' <ref name=2006-100 > {{cite news | first= | last= | coauthors= |authorlink= | title=NASA Findings Suggest Jets Bursting From Martian Ice Cap | date=August 16, 2006 | publisher=NASA | url =http://www.jpl.nasa.gov/news/news.cfm?release=2006-100 | work =Jet Propulsion Laboratory | pages = | accessdate = 2009-08-11 | language = }}</ref> are geological formations thus far unique to the ] of ]. These structures have not been found in the ] or any other region of Mars to date. They appear as the ] ice cap at the Martian south pole ] 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.<ref name=AIG > {{cite web|url=http://www.aig.asn.au/pdf/AIGnews_Aug06.pdf |title=Spiders on Earth and Mars (pp. 21)|accessdate=2009-08-11 |date=August 2006 |format=PDF |publisher=Australian Institute of Geoscientists }}</ref> The central point is sometimes, but not always, a crater.<ref> Photographic examples: , , </ref> The formation is similar in its branching appearance to ]. | |||
| ==Causes== | |||
| The formations were discovered on images acquired by the ] during 1998-1999.<ref>{{cite journal|title=Mars Global Surveyor Mission: Overview and Status |journal=Science |year=1998 |first=A. L. |last=Albee |coauthors=F. D. Palluconi, R. E. Arvidson |volume=279 |issue=5357 |pages=1681–1685 |doi=10.1126/science.279.5357.1681 |url= |format= |accessdate= }}</ref> Soon after the first detection, they were discovered to be negative topographical features - i.e. radial troughs or channels.<ref name=Portyankina > {{Citation| first= | last=| coauthors=| contribution=SIMULATIONS OF GEYSER-TYPE ERUPTIONS IN CRYPTIC REGION OF MARTIAN SOUTH| title=Fourth Mars Polar Science Conference| editor-first=G. Portyankina| editor-last=| coeditors=| publisher=| place=| pages=| date=| year=2006| id= | contribution-url=http://www.lpi.usra.edu/meetings/polar2006/pdf/8040.pdf| format=PDF| accessdate=2009-08-11 }}</ref> This unusual shape and appearance of "spiders" caused a lot of speculation about their origin. The reasons for their geographic distribution is still unknown. Their boundary does not correlate with any other properties of the surface such as elevation, geological structure, chemical composition or ] properties. <ref name=Portyankina /> | |||
| Some investigators have suggested that the "spiders" are dendritic drainage channels. But one characteristic that makes the formations so difficult to explain is the way they "work against" gravity. In fact, the branching occurs radially from a center, positively excluding a drainage function. Moreover, the spiders form in identical shapes irrespective of the terrain. Often, a single ravine is seen moving both uphill and down, and many of these radial patterns occur on a consistent incline.<ref name=AIG /> | |||
| ]. This one may be about 500 m in diameter and 1 m deep.]] | |||
| Some theories include: | |||
| *Dry venting of carbon dioxide (CO<sub>2</sub>) gas and sand. As the southern spring CO<sub>2</sub> ice receives enough Sun energy, it starts subliming the CO<sub>2</sub> ice from the bottom. This vapor accumulates under the slab rapidly increasing pressure and erupting.<ref name=2006-100 /><ref name=Portyankina /> High-pressure gas roars through at speeds of 161 km/h (100 m/h) or more; under the slab, the gas erodes ground as it rushes toward the vents, snatching up loose particles of sand and carving the spidery network of grooves. The dark material falls back to the surface, creating dark patches and patterns on the ice cap. | |||
| *Active water-driven erosive structures. Water derived from sub-surface layers is expelled up fissures eroding joints to create tributaries capped with mud-like material and/or ice.<ref>Spiders: . Prieto-Ballesteros, Olga ''et al.'' '''Astrobiology'''. August 2006, Vol. 6, No. 4. pp: 651-667</ref><ref> {{cite journal|title=Martian Spiders as feasible water-driven erosive structures|journal=Centro de Astrobiología-INTA-CSIC|date=2005|first=Olga|last=Prieto-Ballesteros|coauthors=|volume=|issue=|pages=|id= |url=http://nai.nasa.gov/nai2005/abstracts/653%20-%20NAI2005-spiders.doc.pdf|format=|accessdate=2009-08-11 }}</ref> | |||
| It has also previously been suggested that these structures had a volcanic cause {{Citation needed|date=February 2007}} 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.<ref name=2006-100 /> | |||
| 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 ] orbiter, which sees in both ] and ] 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.{{Citation needed|date=November 2008}} 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.<ref> [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.] </ref> | |||
| ==See also== | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| ==References== | |||
| {{reflist}} | |||
| == External links == | |||
| * photo repository. , , | |||
| * Papers on Martian "Spiders" , , , , , , , | |||
| * Sir Arthur C. Clarke on Martian "Spiders" , , , , | |||
| * | |||
| {{Mars}} | |||
| {{DEFAULTSORT:Martian Spiders}} | |||
| ] | |||
| ] | |||
| ] | |||
| ] | |||
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