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Martian Polygons And Deep-Sea Polygons On Earth: More Evidence For Ancient Martian Oceans?

Friday, July 27, 2012 13:57
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Debate over the origin of large-scale polygons (hundreds of meters to kilometers in diameter) on Mars remains active even after several decades of detailed observations. Similarity in geometric patterns on Mars and Earth has long captured the imagination. In this new article from GSA Today, geologists at The University of Texas at Austin examine these large-scale polygons and compare them to similar features on Earth’s seafloor, which they believe may have formed via similar processes.

Understanding these processes may in turn fuel support for the idea of ancient oceans on Mars.

(A) Profile using THEMIS and MOLA imagery showing topographic differences between areas of large-scale Martian peripheral and basinal polygons in Acidalia Planitia. This profile was used to calculate maximum (2.7%) and average (0.8%) slope values. (B) Map showing basinal and peripheral polygons in Acidalia Planitia. 
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Credit: GSA

Through examination of THEMIS, MOLA, Viking, and Mariner data and images, planetary scientists have found that areas on the northern plains of Mars are divided into large polygon-shaped portions and that sets of these polygons span extensive areas of the Martian surface. Smaller polygon-shaped bodies are found elsewhere on Mars, but these are best explained by thermal contraction processes similar to those in terrestrial permafrost environments and not likely to form larger polygons.

In the August 2012 issue of GSA Today, Lorena Moscardelli and her colleagues from The University of Texas at Austin present a detailed comparison of the geometric features of these large Martian polygons and similar features found in deep-sea sediments here on Earth. Moscardelli and colleagues note striking similarities.

On Earth, polygon-shaped areas, with the edges formed by faults, are common in fine-grained deep-sea sediments. Some of the best examples of these polygon-fault areas are found in the North Sea and the Norwegian Sea. These are imaged using detailed, 3-D seismic surveys conducted to search for offshore oil and gas deposits. Images reproduced in this paper show that these deep-water polygons are also 1,000 meters or greater in diameter.

Seismic profile and maps of root mean square of the seismic wave amplitudes showing two levels of a deep-water polygonal fault system within the Eocene-Oligocene Brygge Formation (offshore Norway). Seismic data courtesy of the Norwegian Petroleum Directorate. TWTT—two-way travel time. 
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Credit: GSA

While the details of deep-sea polygon formation on Earth are complex, Moscardelli and her colleagues conclude that the majority of these polygons form in a common environment: sediments made up of fine-grained clays in ocean basins that are deeper than 500 meters, and when these sediments are only shallowly buried by younger sediments. A key observation — also made recently by Michelle Cooke at the University of Massachusetts — is that the physical mechanism of polygon formation requires a thick, wet, and mechanically weak layer of sediment.

Moscardelli and colleagues also conclude that the slope angle of the sea floor plays an important role in both the formation and preservation of these polygons. Where the seafloor slope is very gentle (slopes less than half a degree), the polygons have very regular shapes and sizes. In many locations where polygons have formed on top of buried topographic features on the seafloor, the shapes of the polygons were altered, and in some cases were broken up and disrupted where the slopes were steepest. Both observations are consistent with deformation of the soft marine sediments as they creep or flow downslope in these areas.

(A) Seismic profile, (B) semblance attribute map, and (C) spectral decomposition map on two different deep-water polygonal fault systems within Cenozoic strata of the northern Gulf of Mexico. Semblance attributes measure lateral changes in the seismic response caused by variations in structure and stratigraphy. Spectral decomposition breaks the seismic signal into its dominant frequency components to highlight structural and stratigraphic features. 
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Seismic data courtesy of CGGVeritas.

In the northern plains of Mars, where the surface is basically flat, the polygons have very regular shapes and sizes — remarkably similar to the deep-sea polygons found on Earth. In places where the topography on Mars is more varied, and where there may be evidence for other sediment-transport features on the surface, areas of deformed and disrupted polygons can be found — again similar to the disrupted polygons here on Earth.

On the basis of these striking similarities, the University of Texas at Austin team concludes that these features most likely share a common origin and were formed by similar mechanisms in a similar environment. The team argues that the Martian polygons were formed within a thick, wet, and weak layer of fine-grained sediments that were deposited in a deep-water setting, similar to the Earth polygons. Thus, these interesting geometric features may provide additional evidence for the existence of an ocean in the northern portion of Mars approximately three billion years ago.

Physical model showing transition from (A) normal faulting to (B) separated polygons under multidirectional extension triggered by gravity. The model comprises a brittle layer of dry blue sand overlying a viscous layer of transparent, colorless silicone polymer (polydimethyl-siloxane). Initial fault patterns are controlled by the surface dip of the model and the lateral boundaries of the mobile unit (A). Thickness variations within the mobile unit result in multidirectional extension due to enhanced flow within the thicker mobile unit, separating and rotating the polygons (B). F1 and F2—faults; x and o—arbitrary marker points.
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Credit: GSA

Contacts and sources:
Kea Giles
Geological Society of America

Deep-water polygonal fault systems as terrestrial analogs for large-scale Martian polygonal terrains Lorena Moscardelli, Tim Dooley, Dallas Dunlap, Martin Jackson, and Lesli Wood, Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78713-8924, USA. Pages 1-9; doi: 10.1130/GSATG147A.1, www.geosociety.org/gsatoday/archive/22/8/.

GSA Today is The Geological Society of America’s science and news magazine for members and other earth scientists. Refereed lead science articles present exciting new research or synthesize important issues in a format understandable to all in the earth science community. GSA Today often features a refereed “Groundwork” articles — tightly focused papers on issues of import to earth science policy, planning, funding, or education. All GSA Today articles are open access at www.geosociety.org/pubs/.





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