## 3 Results
### Radar Data  
The aptitude of radar signals to differentiate among distinct surface roughnesses is particularly advantageous in low contrasting sandy desert regions that show analogous spectral behaviors on multispectral data, as is the case of the present study area. Radar data clearly show an isolated crater-like structure of nearly spherical shape with a dark radar backscatter delimited by bright radar signals (Figure 3b). This can be interpreted as a smooth-sand floored basin (crater's interior) delimited by a rough bedrock surface (crater's outer rim). At the center of the structure, a near-circular ring feature of a bright radar backscatter, surrounded by a dark annular trough, is clearly visible.  
In the Sentinel-1 image (Figure 4), the overall circularity of the Rimaal structure, however, was concealed, to some extent, along its eastern flank. This could be due to the presence of a coarse-grained material that covers a vast part of the structure and, thus, acts as a rough surface to the shorter radar wavelength of the Sentinel-1 C-band, causing a specular backscatter of bright radar signals. The Sentinel-1 C-band VV like-polarized image is, however, substantially superior in portraying the gullies that incised the interior wall of the structure's outer rim. A close-up look at the structure divulges traces of concentric faults on the inner wall of the northern flank of the structure (Figure 4). Considering a penetration depth of 0.5 m for the Sentinel-1 C-band signals in a dry sand surface [18] leads to the proposition that the western and northern flanks of the Rimaal structure are covered by, at most, 50 cm of dry desert sand.  
Figure 3: Multispectral and radar images of the Rimaal structure. (**a**) Sentinel-2 using bands 432 as RGB, (**b**) Landsat-8 using bands 432 as RGB and (**c**) ALOS PALSAR image of the structure. While the structure is barely noticeable as a bright circular feature in a homogeneous low-contrasted sandy terrain in both Sentinel-2 an Landsat-8 images, it is clearly visible in the microwave imagery due to the penetration capability of the radar signals to desert sand.  
The longer wavelength of the PALSAR L-band (Figure 4) was capable of penetrating deeper (\(\sim\)3 m in dry fine sand) and reveal larger parts of the hidden eastern flank of the inferred crater when compared to that of Sentinel-1 C-band (with max penetration of 0.5 m in dry fine sand) [15; 27]. Coarse grained material are smooth to the radar L-band and, thus, radar waves are capable of penetrating deeper and reflect less energy back to the radar's receiving antenna. This allows key morphometric elements, like the structure's annual trough, to appear dark in the radar L-band imagery. As a consequence, it provides greater contrast between the structure's trough and its bright outer rim and central topographic ring and peak, emphasizing the overall circularity of the Rimaal structure. The co-polarized L-HH imagery shows a significant improvement in exposing the structure rim when compared to the cross-polarized imager (L-HV). In the L-HH imagery, the rim shows a stronger backscatter than the surroundings, therefore becoming easier to detect.