First Evidence of Large-Scale Englacial Folding in the South Polar Layered Deposits (Ultimi Scopuli, Mars) Unveiled by MARSIS

Introduction:  The Radio Echo Sounding (RES) technique is commonly used on Earth to study the internal structure of ice sheets and understand ice flow (e.g., [1], [2]). Although large ice caps also exist at the Martian poles, their internal deformation and flow dynamics remain largely unknown. This study presents radar data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS; [3], [4]) at Ultimi Scopuli on Mars (Fig. 1A, B), revealing the radar first evidence of large-scale englacial folds within the South Polar Layered Deposits (SPLD). These features are similar to those found in Earth's ice sheets, indicating that the Martian ice sheet has experienced movement under either dry or wet basal conditions.

Results:  The radar sections collected at US (Fig. 1C) exhibit a general stratified ice structure, typically ~1500-2500 m thick, overlying a flat/slightly rough sub-horizontal bedrock (at MARSIS resolution). The ice stratigraphy is generally characterized by sub-horizontal and continuous or gently undulated and disrupted reflection horizons that are roughly equally divided in two principals radar units by a semi-continuous and sub-horizontal sharp and thick radar reflector (labelled “H1” in figures). In some cases, the radar horizons are locally slightly inclined and truncated by the H1, suggesting an unconformable discontinuity surface. The lower unit (labelled “U1” in figures) is ~700-1200 m thick and is mostly characterized by low radar reflectivity with few relatively thick and fuzzy internal reflection horizons. On the contrary, the upper unit (labelled “U2” in figures) is ~800-1200 m thick and mainly composed by a thinly stratified radar sequence with bright reflectors.

Observing the MARSIS radargrams (i.e., Fig. 2), in its interior the ice sheet layering is locally interrupted and disrupted by clear radar dark-areas of non-reflective ice (from now on “Non-Reflective Areas” or “NRA”). However, in their inner core, the NRAs sometimes show bright-toned and rounded/elliptical layering (“eye-folds” like structures”) and “anti-form” like folding. Some other folding can partially envelop the dark NRAs or are located near such structures. In general, the NRAs have an irregular rounded to elliptical shape and usually develop from the interface between the bedrock and the base of the icy mass. On average, surveyed NRAs are ~16 km long and several hundreds of meters thick (up to ~1500 m), thus they are markedly elongated along the horizontal x-axis. All the above-described radar features are mainly present in the basal unit (U1), below the depth of ~700-1000 meters (i.e., below the H1 horizon); nevertheless, in some cases (e.g., Figs. 3, S1 and S3) deformation structures appear to interrupt or to be upon the H1 horizon, partially involving the upper sequence (U2), however always lying below ~500 meters in depth from surface.

Discussion and Conclusions:  In terrestrial ice sheets, highly deformed layers linked to areas of high dynamic ice flow driven by variation in basal shear stress typically cause strong power loss in internal reflectors of ice-penetrating radars (e.g., [5], [6]). In RES images, these zones tend to be dark (i.e., “layer free”) and interrupt the lateral continuity of the horizons from the bedrock up to some depth since internal layering slopes are most extreme near the ice bed (it is exhibited strong deformation) and thus power loss intensify at depth [5]. In such circumstances, the lossy/diminished radar power areas can be bordered by bright folded reflectors/undefined areas [1], [7], [8] and can include folded bright reflectors in their core as well. All these radar structures are the signature of broad asymmetric/complex geological shear folds, characterized by one limb steeper than the other [5] and/or completely tilted and classified as overturned/recumbent and sheath folds (e.g., [1], [7, [8]).

Net of the unavoidable approximations due to the different frequency and resolution of the radar instruments, we compared the main NRAs radar features detected by MARSIS at US with some examples of the above described englacial structures observed by RES in Greenland and Antarctica polar ice sheets (e.g., Fig. 3 after [1], [7], [8]). The data matching points out their consistency in terms of radar morphology and morphometry, and the most plausible interpretation is that large-scale geologic folds are affecting the SPLD stratigraphic architecture of US ice-sheet. In particular, considering the overall setting of the main surveyed structures, the MARSIS dark NRAs and the deformed radar patterns in U1 unit seem to be consistent with the englacial sheath folds known on Earth. The existence of sheath folds states that large displacements have occurred in the US region due to ice-flow/basal sliding, similarly to what observed in the SPLD in Promethei Lingula by previous studies [12]. On Earth, the origin of large englacial folds is still debated, with various dry or wet mechanisms proposed. Several mechanisms (dry or wet) have been proposed since now, often depending on case to case (e.g., [1], [9], [13]). Understanding similar Martian structures is harder due to limited data, environmental knowledge, and structural analysis. Ongoing studies aim to shed light on all these aspects.

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Acknowledgements: This research was supported by the Next Generation EU program, Mission 4, Component 1, through project “Combining mAchine Learning and optImization for Planetary remote Sensing missiOns” (CALIPSO), Unique Project Code C53D23010010001.

Fig. 1. MOLA location and MARSIS radargrams

Fig. 2. Examples of “NRAs” and englacial folding (“eye-folds” -EF- and “anti-form” -SF- folding like structures) in MARSIS radargrams.

Fig. 3. Example of folding in RES (Greenland) compared with MARSIS structures. Image after Figure 13 in [1] © AGU-Wiley.