Enigmatic Clastic Polygons on Mars

Post by Laura Brooker, Open University, Milton Keynes, UK.

Polygonal ground of centimetre- to decametre-scale is one of the most common features found in cold-climate regions on Earth and on Mars. Polygonal shapes on Earth can form through a number of different processes including the thermal contraction of ice-cemented soils, forming fracture patterns known as thermal contraction polygons, through the freezing and thawing of ground ice moving clasts, in the case of sorted patterned ground, or through the dehydration of volatile-rich material, termed desiccation polygons. Around a large crater found in the northern latitudes of Mars, named Lyot, we observe stunning and unusually large clastic polygons (Image 1), but how do they form? To understand landforms on Mars we turn to analogues on Earth and compare morphological data to look for similarities and differences.

Image 1

Image 1: HiRISE (ESP_016985_2315) image of clastic polygonal ground observed to the north east of Lyot crater, Mars. These enigmatic polygons are demarcated by clastic material in their borders and are averagely 130 metres in diameter. Image credit: NASA/JPL/University of Arizona.

The clastic polygons around Lyot crater are far larger than sorted patterned ground on Earth, and possible examples of patterned ground on Mars, which are typically only up to metres in diameter (Image 2). Such polygons require cold climates and a pre-existing layer of clastic material (Goldthwait, 1976; Wilson and Sellier, 1995). They transition from circular shapes and polygons to ellipses and then to stripes with increasing slope gradient (Washburn, 1956; Goldthwait, 1976). This is a relationship that is not observed in the clastic polygons around Lyot crater (Brooker et al., 2018).

Image 2

Image 2: A) HiRISE (PSP_004072_1845) image of possible sorted patterned ground located in the Elysium Planitia region of Mars (Balme et al., 2009). Clasts and polygon dimensions are significantly smaller than the clastic polygons around Lyot crater. Image credit: NASA/JPL/University of Arizona. B) Sorted stone circles from Brøgger Peninsular on the west coast of Spitsbergen, Svalbard, Norway. The pole is approximately 1.25 metres tall. Sorted stone circles occur on slopes of approximately 2° to 4° (Goldthwait, 1976). Image credit: Matt Balme. C) Sorted stone polygons from the western side of Hafnarfjalt, W. Iceland. Sorted stone polygons and nets form on slopes of 2° to 4°. Image credit: Susan Conway. D) Sorted stripes form the western side of Hafnarfjalt, W. Iceland. Stripes occur on slopes of 4° to 11°. Image credit: Susan Conway.

The Lyot clastic polygons are however similar in size and network morphology to thermal contraction polygons, which have diameters ranging from 10 to 40 metres with maximum diameters of over 100 metres (Washburn, 1956; Black, 1976; Maloof et al., 2002). A possible Martian analogue has been observed in Utopia Planitia (Image 3; Yoshikawa, 2003; Lefort et al., 2009), demonstrating that polygons of such scale can be produced on Mars. However, the clastic borders of the Lyot polygons remain a mystery; it is difficult to explain how boulders of up to 15 metres in diameter could cluster just at the margins of these features. It is suggested that they could be the result of the infill of thermal contraction polygons with wind-blown material, which becomes cemented or indurated. Differential erosion then exposes the network followed by fracturing of the fill material to form the unusual large angular clasts we observe today. These beautiful landforms reveal a unique process occurring in one location on Mars and provide evidence of a possible ice-rich layer deposited during the Lyot impact event (Brooker et al., 2018).

Image 3

Image 3: HiRISE (PSP_002070_2250) image of possible thermal contraction crack polygons observed in Utopia Planitia, Mars. The polygons have diameters of 30 to 170 metres (Yoshikawa, 2003; Lefort et al., 2009) and similar network morphologies to the clastic polygons around Lyot crater. Image credit: NASA/JPL/University of Arizona.

Further Reading:

Balme, M.R. et al. (2009) Sorted stone circles in Elysium Planitia, Mars: Implications for recent martian climate, Icarus, 200, (1), 30 – 38.

Black, R. F. (1976) Periglacial features indicative of permafrost: Ice and soil wedges, Quaternary Research, 6, (1), 3 – 26.

Brooker, L. M. et al. (2018) Clastic Polygonal Networks Around Lyot Crater, Mars: Possible Formation Mechanisms from Morphometric Analysis, Icarus, 302, 386 – 406.

Goldthwait, R. P. (1976), Frost Sorted Patterned Ground: A Review, Quaternary Research, 6, 27 – 35.

Lefort, A. et al. (2009) Observations of periglacial landforms in Utopia Planitia with the high resolution imaging science experiment (HiRISE), Journal of Geophysical Research, 114, (E04005), doi: 10.1029/2008JE003264.

Maloof, A. C. et al. (2002) Neoproterozoic sand wedges: crack formation in frozen soils under diurnal forcing during a snowball Earth, Earth and Planetary Science Letters, 204, 1 –15.

Washburn, A. L. (1956), Classification of patterned ground and review of suggested origins, Bulletin of the Geological Society of America, 67, 823 – 866.

Wilson, P. and Sellier, D. (1995), Active Patterned Ground and Cryoturbation on Muckish Mountain, Co. Donegal, Ireland, Permafrost and Periglacial Processes, 6, 15 – 25.

Yoshikawa, K. (2003) Origin of the polygons and the thickness of the Vastitas Borealis Formation in Western Utopia Planitia on Mars, Geophysical Research Letters, 30, (12), doi: 10.1029/2003GL017165.

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