Martian thermal-contraction polygons as sounders of subsurface properties in Utopia Planitia
Authors: Conway Susan , Philippe Meven , Soare Richard , McKeown Lauren 
Published: March 2024

On Earth, temperature decreases can cause the thermal contraction of ice-cemented ground. This forms polygonal networks of surficial fractures &#8211; called &#8216;thermal-contraction polygons&#8217; (Washburn, 1956). Polygons exhibit different morphologies with time (Black, 1954): initially showing no relief (&#8216;flat-centred polygons&#8217;, FCPs), their margins uplift with the growth of ice or sand wedges (&#8216;low-centred polygons, LCPs); subsequently to wedge degradation, polygon margins then collapse into the wedge casts (&#8216;high-centred polygons, HCPs).On Mars, polygons of similar dimensions (~&#160;5-25&#160;m in diameter) and morphologies (FCP/LCP/HCP) to those on Earth are commonly observed in the mid-latitudes. They are inferred to form by thermal contraction of ice-cemented ground (Mellon, 1997). Further, polygons in Utopia Planitia (UP) have been identified as ice-wedge polygons (Soare et al., 2021). This indicates a potential role of liquid water in UP during the Amazonian, at a period where the martian climate is thought to be non-conducive to the stability of surface liquid water.Here, we seek to understand whether the characteristics of these ice-wedge polygons could be used to understand the subsurface properties of their substrate. Hence, we investigate the density and type (FCP/LCP/HCP) of polygons for three morphological units in UP, in the area (44-52&#176;N 100-130&#176;E) where polygons were identified as ice-wedge polygons by Soare et al. (2021).In UP, we mapped two morphological units: the &#8220;sinuous unit&#8221; (elongated, sinuous features) and the &#8220;boulder unit&#8221; (covered in decametre-scale boulders). We then mapped polygons over the two units using a grid-based technique (Ramsdale et al., 2017).We developed three parameters, that we infer reflect various properties of the ground: &#961;pol, reflecting the cementation of the substrate by ice; &#961;wf, reflecting the capacity of the substrate to form wedge ice; &#961;wp, reflecting capacity of the substrate to preserve ground ice.The boulder unit has no polygons. Therefore, it must be a massive material, non-conducive to ice cementation. Its surface is an extensive field of boulders, and shows blocks shattered in place. It points toward a volcanic origin for the boulder unit. This result is consistent with studies that concluded to the presence of volcanic units in UP (e.g. Tanaka et al., 2005).Our parameters show that the sinuous unit was an initially porous material that became cemented by ice, and underwent wedge formation. Therefore, the sinuous unit was deposited on top of the boulder unit, either as water-rich deposits from a large aqueous flow, which subsequently froze; or by condensation of water vapour from the atmosphere within porous sediment. Those two emplacement modes were suggested to have occurred in UP (e.g. Costard and Kargel, 1995; S&#233;journ&#233; et al., 2012). The sinuous unit was then degraded, exposing the underlying boulder unit.These interpretations show that polygon characteristics can be used to unveil properties of their substrate. In our study zone in UP, it allowed us to link geomorphological units with specific geological processes, that were suggested to have occurred in UP. Therefore, the parameters we developed can be considered as additional tools to study the martian geology at the sub-regional scale.