## 2 Background  
### Target Area  
The first target area to test the discontinuity detection is LDA, which is regarded as an evidence of potential glacial processes [28] or ice debris transport in the ancient Martian environment [29]. It can be characterized as an alluvial-fan-like mass of debris, whose thickness varies between tens to hundreds of meters, that flowed from the mountain slope covered by a glacier or periglacial landforms roughly some million years ago [30]. Despite ongoing arguments regarding the formation mechanism of LDA [28; 29; 30; 31], itsomorphic origin is still unclear. Therefore, the subsurface investigation of LDA employing SHARAD data is expected to provide useful clues for LDA structures. It is worth noting that LDAs are mostly distributed along with the dichotomy of the southern hemisphere and northern lowlands [32]. Therefore, the hypothesis that such highland features were presumably involved with glacial and periglacial processes has gained attention. Thus, the scenario established on this clear evidence is that climate change induced the melting of the glaciers, which allowed sediment flow down through the steep sides of LDAs and produced their fan-shaped features (see Figure 1c,d). In this study, two sites, i.e., Euripus Mons LDA & an anonymous LDA (\(44.8^{\circ}S\)\(105.2^{\circ}E\)) close to Euripus Mons, were chosen as the target for SHARAD subsurface feature detection. Based on previous studies, high ice content has already been identified [28] in those target areas. It has also been proposed that at least some of them might be composed of pure ice and, protected from sublimation by a thin debris cover. Recent modeling practices, assuming glacial deformation, have helped to reconstruct some of the inherent structural properties [33]. Moreover, similar modeling studies employing High Resolution Stereo Camera (HRSC) DEM and non-linear flow calculations have shown a potential involvement of dirty ice [34]. Considering the undergoing arguments over these target areas, the identification of subsurface structures by a deep-learning approach may imply essential clues about the origin of LDA and its deposits. Therefore, we choose two LDAs as our first target area to deploy the developed algorithms. Possibly, the Martian deposits in the southern polar zone represent the most interesting history of the evolution of Mars as a solid planet. Several researchers have proposed that the multiple layers of the southern Martian pole represent a sequence of episodic climate changes [35; 36]. Therefore, the Martian south polar layered deposits (SPLD) presumably have involvements with the other footprints of episodic environmental changes, as shown in the potential coastal line of the northern hemisphere [37; 38], layered fluvial deposition [39; 40] and a recently claimed underground lake [41]. The investigation of the subsurface structure of SPLD was proposed as a prime target of SHARAD and MARSIS campaigns, which revealed expected multiple subsurface layering as described in [42]. However, a high density of discontinuity points in SPLD induced severe difficulties in correctly inferring the detailed structures and locations of subsurface layers. Therefore, we choose a part of the SPLD structure as a test area for the constructed algorithm (Figure 1c). The most challenging target area for the application of the developed algorithms is the Phoenix landing site (Figure 1d), where weak discontinuity caused by potential icy bodies are expected. There have been assumptions that the layer of permafrost is extended to the mid-latitude region of Mars [43], which were later confirmed from the excavation during the Phoenix mission [44]. However, such ice layers close to the surface were not the target of SHARAD and MARSIS. Although SHARAD data analysis on the Phoenix landing site identified the discontinuity (which may be an extension of the ice layer very close to the surface), the discontinuity signals are very weak.