Carbon dioxide is Mars’ primary atmospheric constituent and is an active driver of Martian surface evolution. CO2 ice sublimation mechanisms have been proposed for a host of features that form in the contemporary Martian climate. However, there has been very little experimental work or quantitative modelling to test the validity of these hypotheses. Here we present the results of the first laboratory experiments undertaken to investigate if the interaction between sublimating CO2 ice blocks and a warm, porous, mobile regolith can generate features similar in morphology to those forming on Martian dunes today. We find that CO2 sublimation can mobilise grains to form (i) pits and (ii) furrows. We have documented new detached pits at the termini of linear gullies on Martian dunes. Based on their geomorphic similarity to the features observed in our laboratory experiments, and on scaling arguments, we propose a new hypothesis that detached pits are formed by the impact of granular jets generated by sublimating CO2. We also study the erosion patterns formed underneath a sublimating block of CO2 ice and demonstrate that these resemble furrow patterns on Mars, suggesting similar formation mechanisms.

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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 (iceaggraded) 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 waterequivalent 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 o N; 100-125 o 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.

Araneiforms are some of the most intriguing surface expressions found on another planet in that they are active today, have no direct Terrestrial analog, and represent a formation process that is unlike anything seen on Earth. They are defined as converging systems of branching troughs exhibiting fractal properties and comprise a variety of different types based on observed activity and location. These include (a) the original "spiders" (e.g., C. J. Hansen et al., 2010;Piqueux et al., 2003), which are large (up to 1 km in diameter), dendritic, often radial negative topography features restricted to the high south polar latitudes; (b) sand furrows (M. C. Bourke, 2013; M. C. Bourke & Cranford, 2011), which are up to 50 m in length and extend along dune slopes in both hemispheres over a season (then disappearing), and (c) dendritic troughs (G. Portyankina et al., 2019), which have been observed to form and grow interannually on loosely consolidated inter-dune material. All of these features are proposed to be formed by some variant of a "geyser" (non-geothermal) process known as the Kieffer model (Kieffer et al., 2006). This model postulates that insolation penetrating through translucent CO 2 ice during spring yields basal sublimation and produces energetic jets. The process can leave fans and spots of dark albedo fines strewn across the ice surface, having scoured the substrate to form dendritic patterns (Kieffer et al., 2006;Piqueux et al., 2003). Fans and spots are identified in the locales of all of these features in spring , but only both furrows (M. C. Bourke, 2013) and dendritic troughs (G. Portyankina et al., 2017) have been observed to form today.