Possible ice-wedge polygonisation in Utopia Planitia, Mars and its latitudinal gradient of distribution

On Earth, ice complexes are commonplace landscapes amidst the continuous permafrost of coastal or near-coastal plains in the Arctic. Formed by the freeze-thaw cycling of water, ice complex features include: hummocky (thermokarstic) terrain, inflated or deflated by the presence of absence of excess ice; thermokarst lakes (i.e. excess ice that has thawed and pooled); alases (i.e. thermokarst basins emptied of water); and, ice-wedge polygons, often characterized by raised (ice-aggraded) or lowered (ice-degraded) margins relative to the polygon centres. The origin and development of these complexes is rooted in inter-or intra-glacial pulses of temperature that engender widespread thaw, meltwater distribution and migration through the soil column (sometimes to decametres of depth), and the freeze-thaw cycling of the meltwater. The possible existence of ice-rich terrain on Mars revised by the freeze-thaw cycling of water dates back to the grainy Mariner-mission photographs of the 1960s and 1970s. However, absent of regolith samples from areas where this terrain is hypothesised, attempts to validate the ice-rich hypothesis often have ended abruptly, either with spectrometric inferences of water-equivalent hydrogen to one metre or so of depth or with looks-like, therefore must-be analogies derived of Earth-based ice-complexes. In the case of small-sized Martian polygons with low- and high-centres, the similarities of form between ice and sand-wedge polygons on Earth has equivocated the reach of ice-wedge hypotheses on Mars. Here, we show that: 1) The plains' terrain of our study region in Utopia Planitia (40-50 degrees N; 100-125 degrees E) displays a statistically-significant and positive (linear) correlation between the ratio of low-centred to high-centred polygons (lcps vs hcps) and a poleward latitude of distribution. 2) This linear correlation would be expected, in as much as ground-ice stability increases with latitude, were the shoulders of higher-latitude lcps underlain by (aggraded) ice-wedges and those of lower-latitude hcps underlain by (degraded) ice-wedges. 3) The change of polygon morphology with latitude would not be expected were the lcps and hcps underlain by sand wedges, in as much as ground-ice stability is unrelated to their aggradation or degradation. 4) Crater counts of the polygonised terrain indicate that it is less youthful than previous studies have suggested, perhaps by an order of magnitude. This attenuates the possible inconsistency between the more temperate boundary-conditions required by the formation of ice-wedge polygons and the current constraints of extreme aridity, low temperatures and low atmospheric pressure.

Automated summary

Ice complexes are common landscapes in Arctic coastal plains on Earth, formed by freeze-thaw cycling of water and characterized by features like thermokarst terrain, lakes, basins, and ice-wedge polygons. The hypothesis of ice-rich terrain on Mars remains unvalidated due to lack of regolith samples. The similarities between ice and sand-wedge polygons on Mars have complicated the understanding of ice-wedge hypotheses.