TMC image suggests crater 1 to be a fresh crater, ~7200 m in diameter and ~1700 m deep, centered at 72°12′S, 133°12′E (Figure 2). The crater was emplaced on an irregular surface of the peak ring. As a result, the rim in between the eastern and southern side is at about 300 m higher elevation than the rim between the western and northern side. The diameter of the crater varies from 6800 m to 7500 m. The crater rim between the western and northern side fits perfectly to a 6860 m diameter circle, while the rest of the rim receded as much as 600 m away from the circle, indicating greater wall collapse in those regions and the effect of pre-impact topography on the crater shape and the slopes (Figure 2a). Interestingly, the slope of the interior wall between eastern and southern sides is gentler (~30°) than that on the western and northern wall, where the slope varies from 34° to 36° (Figure 2d). The interior wall between the eastern and southern rim shows significant degradation leading to formation of bedrock spurs, gullies, landslides, and slope streaks.
 When the shadow on the floor and the crater wall is enhanced by an image processing dynamic range enhancement technique, which reveals greater details in the images where pixels have nearly similar values, we observe a prominent arcuate ridge from the western to northern boundary of the crater floor (Figure 2b). A pond (~800 m × ~2000 m) that is composed of smooth deposits overlies the floor and embays the ridge materials (Figure 2b), indicating that the pond material postdates the ridge material. A 200 m-diameter impact crater was emplaced on the pond material. The ridge crest is ~125 m above the pond surface. The arcuate ridge is absent from the eastern to southern boundary of the floor, where the corresponding crater wall suffered greater wall collapse followed by the formation of gullies and landslides. The debris from the wall collapse has given rise to the arcuate ridge. The pond is oriented toward the landslide site near the southern wall, indicating the source of ponded material. The crater rim is also characterized by the presence of concentric faults at several places (Figure 2c), which strike parallel to the rim and dip toward the crater floor. The hanging wall of the fault is characterized by displacement in the downdip direction, followed by slumping on the interior wall. The concentric faults significantly contribute to the landslide formation on the interior wall. Similarly, the rim of craters 2 and 3 contain concentric faults leading to gullies and landslides (Figures S1 and S2).
 The interior wall between the eastern and southern rim shows a spectacular development of gullies, landslides, and slope streaks (Figures 3 and 4). The gullies are abundant between the eastern and southeastern walls, while the landslides dominate the southern and southeastern walls. The gullies have a typical alcove-channel-fan morphology; the alcove is a concave, elongated, triangular depression located at the head of the gully and is a zone of erosion of crater wall materials. Channels emanate from the downslope apex of the alcove. They are straight, curved, and sinuous, depending upon the slope and the presence of bedrock spurs. The channels also lengthen toward the alcoves due to progressive headward erosion. The eroded materials in the alcoves were transported along the channels and deposited at the fans or debris aprons. Fans are characterized by triangular, oval, and eye shapes. The fan deposits also contain large fragments or boulders and fine sediments derived from the alcoves or channel walls (Figures 3b and 3c). Erosion exposes the bedrock on the walls and floors of the alcoves and channels. At places, talus cones are also present (Figure 3b).
Figure 3. The LROC NAC image M118979214L showing the detailed illustrations of the gullies [image credit: NASA/Goddard Space Flight Center (GSFC)/Arizona State University]. (a) The LROC NAC context image showing the locations of Figures 3b–3e and Figures 4a–4e. (b) A set of gullies originating from the upper wall have typical alcove-channel-fan morphology; the materials eroded at the alcove regions are transported along the channels and deposited in the fan. Fan deposits also contain large fragments of rocks either removed from the alcoves or the channel walls. (c) A distinct example of gullies, one with bright alcove and channel materials with a poorly developed fan because of larger dispersion of fan sediments; the upper gully shows a spectacular broad alcove with four orders of channels and fan deposits, while the lower one is less complicated. Some of the head alcoves are bright, indicating the youthful activity. The lower bright gully has narrow, sinuous, irregularly shaped alcoves. The angular fragments in the fan may indicate a nonviolent, short-lived, transport mechanism. (d and e) Lobate fans formed at the base of bedrock-incised straight channels that may have exploited the intensely fractured bedrock; note the fan deposits containing large rock fragments, probably derived from the straight channel walls; the fragments do not show the trails as they may have been transported much before the surrounding sediments. The maximum size of the boulder is about 35 m.
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Figure 4. The LROC NAC image M118979214L showing the detailed illustrations of gullies [image credit: NASA/GSFC/Arizona State University]. (a) Large slope streaks forming from the bright gullies. The bright mantle deposits that coat the crater wall are colluvial sediments partly derived from the gully region. The bright gullies do not have prominent fan deposits suggesting a greater mobility of the fan deposits toward the crater floor. (b) Episodic gully activity. The first and second order alcoves (a1 and a2) predate the third-order bright alcoves (a3). The brightness increases from lower to higher order alcoves. The bright gullies represent the youthful sedimentary activity. (c) Examples of dark slope streaks. The slope streaks predate the bright mantle deposits; the individual dark streaks have finger-like distal ends. (d and e) Examples of gullies with narrow alcoves and channels.
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 Figure 3b shows a set of straight and narrow alcoves and channels that are closely arranged in a parallel manner, incising into the bedrock, indicating the presence of closely spaced weakness zones such as faults and fractures in the bedrock. During the impact process, a wide variety of fractures (radial, concentric, and conical) and faults (radial and listric) are produced, which are exposed on the rim and interior wall after the crater formation [Kumar, 2005; Kumar and Kring, 2008]. As seen in terrestrial impact sites, the fractures/faults are composed of fault breccia that can easily be dismantled and removed by mass movement [e.g., Kumar et al., 2010]. Sediments in the fans were also derived either from a single or multiple alcoves. Talus cones were formed as a single debris apron without the channel and alcove, indicating their derivation from the loosely packed sediments that coat the crater wall. The deposits in the channels and fans are usually brighter than the underlying or surrounding sediments, which were formed by either an earlier gully-forming activity, talus sediments, impact breccia, or fall back deposits, as seen in the terrestrial impact sites [e.g., Kumar et al., 2010]. Figure 3c shows an interesting gully morphology in which a shorter fan was formed from a longer channel and alcove; the alcove shows multiple sites of erosion leading to three or more orders of channels. In contrast, the adjoining gully has a longer fan with abundant fragments, formed from shorter channels and narrow alcoves. This morphologic variation indicates the changes in the dynamics of fan formation, as the fragment-loaded fan mobilized longer distance compared to the other fan. Interestingly, these gullies also depict variation in the brightness of their surfaces, which may indicate different timings of gully formation or changes in the bedrock composition or both. Figures 3d and 3e illustrate fan deposits, which are loaded with large angular rock fragments as big as 30–50 m, derived from the bedrock exposures in either the channel or alcove region. Interestingly, none of these large rock fragments (or boulders) left their tracks on the fans or channels, suggesting the burial of the tracks by the sediments formed subsequently from the alcove regions.
 Figure 4 shows a distinct example of complex gullies that have bright alcoves and channels with poorly developed fans because of larger dispersion of fan sediments. A broad triangular alcove as wide as ~500 m contains at least four orders of much smaller compound alcoves; the fourth-order alcove gives rise to a broad channel and a compound fan (one of the fans is labeled in Figure 4a). The superposition relationship between the compound alcoves suggests an episodic gully activity. In Figure 4b, a bright alcove (labeled as a3) truncates a dark alcove (labeled as a2). The alcoves that have smooth and darker surface (labeled as a1) appear to have formed before the other two sets of bright alcoves (a2 and a3). An increase in the brightness from older to younger alcoves would indicate the successive generation of newer surfaces or reactivation of older surfaces, leading to an increase in the optical immaturity. Individual gullies without compound morphology also occur near or on downslope of the compound gullies (Figures 4d and 4e). They have small alcoves but have much longer channels and fans, approximately twice the length of the alcoves.
 The crater wall down the slope of the compound gullies contains widely spread bright and dark mantle deposits (Figures 4a and 4c). The bright mantle deposits (BMD) are likely to have derived from the bright youthful gullies, while the dark mantle deposits (DMD) underlie them. The DMD also contain coarse fragments that are likely to be derived either from the old darker gullies or colluvial sediments mixed with impact or fallback breccia. Flow of a smaller amount of DMD over the BMD as subtle channels gave rise to finger-like, 1–5 m-wide dark slope streaks that extend several tens of meters in the down slope direction (Figure 4c). Alternatively, removal of BMD because of flow of colluvial sediments on the crater wall may expose the underlying DMD, producing the slope streak structures. The small impact craters (labeled as IC 1 and IC 2 in Figure 4c) were fully emplaced in the BMD and provide its minimum thickness to be ~3 m, assuming a diameter to depth ratio of 5. On the other hand, the 7 m-diameter crater (IC 3) that exposes the DMD on its floor suggests the thickness of BMD to be slightly above 1 m. The slope streaks cause mixing of BMD and DMD materials. The streaks represent small-scale dry-granular flows that formed during or after the gully-forming event. Absence of boulders in the distal end of these streaks rules out the possibility of being the boulder tracks.
 We measured morphometric parameters (length, width, depth, and slope) of 15 prominent gullies (Figure 5) using the LROC NAC images and a 30 m-per pixel digital terrain model (DTM), which was generated from the Lunar Orbiter Laser Altimeter (LOLA) data (http://ode.rsl.wustl.edu/moon/indexDatasets.aspx). The gullies have a wide range of sizes. Length of alcoves varies from 15 m to 560 m, width from 10 m to 360 m, and apparent depth ranges from 10 m to 50 m. These values are typical for the lengthened alcoves, as the length is greater than the width (Figure 5a). Width of the alcoves increases with increasing length, in the observed scale range. The channels are narrower (Figure 5a), width varies from 5 m to 125 m, length from 40 m to 225 m, and depth ranges from 5 m to 30 m, which are slightly less than the depth of the alcoves. The fans are more prominent than the alcoves, in terms of length; their length varies from 50 m to 750 m, width from 15 m to 200 m, and their thickness (elevation difference between the center and boundary of the fan) ranges from 5 m to 40 m. Interestingly, the alcove width increases linearly with the increase of its length in the observed scale range (Figure 5a). On the other hand, width of fans increases linearly up to a length scale of 250 m, beyond which their width remains more or less the same, irrespective of further increase of its length (Figure 5a). When the alcove length is compared with the corresponding fan length (Figure 5b), the fan is longer than the corresponding alcove except for a few cases. It indicates greater sediment mobility in the fans in the downslope direction. The DTM also provides a new information about the slope of the gullies (Figure 5d). The slope of alcoves varies from 24° to 52°, channels from 31° to 53°, and fans from 23° to 52°. Although the slope values are in a comparable range, some fans are either gentler or steeper than the corresponding alcoves (Figure 5c). Accumulation of sediments in the fans decreases their slopes, while dispersion or collapse of fans may have led to steepening of their surfaces. Boulders are also abundant in the fan deposits. There are about 75 boulders above a diameter of 4 m (Figure 5c) with a maximum diameter of 55 m. The ratio of short axis to the long axis is less than 1, indicating that the boulders are poorly sorted rectangular fragments. The sizes of boulders do not exceed the width of channels, which would imply that the provenance of the boulders might be either the walls of channels or alcoves.
Figure 5. Morphometric properties of the gullies and landslides. (a) The relationship between the length and width of alcoves (filled diamond), channels (plus) and fans (open circle). While the alcoves and channels show an increase of width with increasing lengths, some of the fans do not show increase of width approximately beyond 300 m length. (b) Length of the fans depends on the length of the alcoves, the longer fans are associated with longer alcoves. (c) Length and width of the boulders measured on the eastern crater interior wall. Most of the boulders are elongated in nature. (d) The slopes of the alcoves and fans are related to each other. The steeper fans are associated with the steeper alcoves, except for a few exceptions.
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Landslides and Pond Deposits
 Landslides are present on the southern and southeastern interior walls (Figures 2 and 6). Landslides are landforms produced by mass wasting processes without forming a typical alcove-channel-fan morphology. They are formed by slumping of crater wall materials along preexisting concentric faults, which are parallel to the interior wall (Figures 2c and 6). Figure 6b illustrates a fault-controlled landslide producing the morphology similar to the experimentally produced dry-granular flows [e.g., Shinbrot et al., 2004]. These structures are produced preferentially on the upper crater wall. The landslide also contains isolated channels without typical alcoves, in the middle to lower crater walls. The fan deposits have lobate margins with abundant coarse-grained fragments (Figure 6b). A 700 m-wide NNW-SSE oriented debris mound occurs along the western periphery of the crater floor and is oriented toward the landslide surface, which is associated with the concentric fault, where enhanced crater wall slumping occurred. Arguably, the landslides can also form during the modification stage of the crater formation and undergo subsequent degradation in the form of young slides. Terrestrial impact sites show clear evidence of wall slumping along the concentric faults [Kumar, 2005; Kumar and Kring, 2008].
Figure 6. Landslides on the southern interior wall, as in the LROC NAC image mosaic M126057860LR [image credit: NASA/GSFC/Arizona State University]. (a) A 2200 m long and 400 m wide landslide originating from a concentric fault scarp. The landslide is brighter than the surrounding. A 700 m-wide NNW-SSE oriented debris mound occurs along the western periphery of the crater floor and is oriented toward the landslide surface, which is associated with the enhanced crater wall slumping along the fault. (b) A close-up view of the landslide. (c) An example of concentric fault-controlled crater wall slumping that led to “spur-and-gully” morphology, similar to the experimentally produced dry-granular flows. Width of the image is 95 m. (d) Formation of alcoves and fans (also called debris aprons) without developing channels. These pictures display features dissimilar to the gullies with alcove-channel-fan morphology. (e) Examples of channels associated with the landslide. Width of the image is 215 m.
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 Feature extraction using a shadow removal algorithm and image enhancement in the shadow region of LROC NAC image shows the morphology of the pond deposits in the crater floor (Figure 7). The pond has a smooth and flat surface with sediments derived from the landslides covering its upper part. The pond surface is overlain by coarse-to-fine debris derived from the landslide producing a hummocky surface along the southern periphery of the pond. In other areas, the pond material shows embayment relationship with the surrounding ridge and floor materials (Figure 7b) and is oriented in the north-south direction toward the landslide wall. The embayment relationship suggests that the pond material formed after the ridge and floor materials. If we consider the ridge materials as the products of young landslide activity, the pond formation could be younger than the landslide activity or at least formed during the late-stage landslide activity. At a few places, a number of ~ E-W oriented sinuous cracks cut across the pond materials (Figure 7a and 7c). The cracks are approximately perpendicular to the flow direction of the sediments that were derived from the landslide surface. The flattening of the pond surface is intriguing. Impact-melt ponds usually tend to have a flat surface, show embayment relationships with the surrounding materials and develop cooling cracks. Although the pond materials show these morphologic characteristics, there are no melt channels connected to the pond and no melt channels in other places as well. If we consider the pond materials as the impact-melt deposits, the landslide debris should now cover them. Ponds of fine-grained sediments in asteroid surfaces have been observed by several workers, who have interpreted them to have formed by seismic shaking due to micro-meteoroid impacts [e.g., Robinson et al., 2001]. We suggest that the pond materials are finer sediments derived from the landslide activity and the flattened surface is due to the seismic shaking of the host crater. However, presence of an impact-melt deposit beneath the pond materials cannot be ruled out. If the pond materials were the impact-melt deposits, the arcuate ridge materials would have formed by the crater collapse following the impact event and the impact melt ponded around them.
Figure 7. The LROC NAC image mosaic M126057860LR showing the morphology of the pond deposits, which are interpreted to be sediments derived from the landslides as shown in Figure 6a. (a) Approximately 2000 m long and 700 m-wide pond deposits on the crater floor. It is oriented in the N-S direction. A number of ~E-W oriented sinuous cracks cut across the pond materials. The cracks are approximately perpendicular to the flow direction of the sediments. A 200 m-diameter impact crater is superimposed on the pond, predating the cracks. (b) A close-up view of the pond deposits that show embayment relationship with the surrounding ridge and floor materials. Note the presence of curvilinear cracks at the lobate front of the pond. (c) Examples of cracks developed in the pond materials, these are straight to curved in nature. Boulders are partially buried in the pond materials. The ejecta of the impact crater at the bottom of the figure partially cover some of the cracks [image credit: NASA/GSFC/Arizona State University].
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Small Impacts on the Crater Wall
 Careful observation of the LROC NAC data indicates that the interior wall of crater 1 contains many small impact craters (Figure 8). Many of them are associated with landslides in the downslope direction of the host crater interior wall (Figure 8). Some of the craters emplaced in the rim of the host crater have lost a portion of their rims by collapse and contributed to the landslide materials (e.g., Figures 8a, 8h, and 8i). Some craters were emplaced on the crater wall and show bright runout deposits emanating from the exterior wall in the downslope direction of the host crater interior wall (e.g., Figure 8g). A few impact craters do not show any landslide activity in the host crater wall. These either predate or postdate the talus deposits on the crater wall (e.g., Figures 8c, 8e, and 8f). It is interesting to note that, although many small impact craters produced landslide activity, none of the large-scale gullies and landslides in Figures 3, 4, and 6 show presence of impact craters in their alcove regions. We also noticed a few impact craters superimposed on the concentric faults near the crater rim (Figure 6c) that caused minor slumping in the downdip direction at the place of impact, but the craters do not produce displacement or slumping in other parts of the fault.
Figure 8. LROC NAC image mosaics M118979214LR and M126057860LR showing the relationship between the small-sized impact craters and the interior wall mass wasting features. (a, b, d, f, g, h, and i) The impact craters on the crater interior wall show significant mass wasting activities. Note the well-developed landslide features emanating from the crater rims. The arrow indicates the direction of the slope and the material transport. (c) An example of impact crater that postdates the colluvial sediments. (e) Colluvial sediments cover the impact crater, suggesting that the sediments flowed down the slope after the impact event. Location and diameter of the impact craters are as follows: (a) 134°1′13″E, 72°3′12″S; 63 m; (b) 134°11′14″E, 72°8′1″S; 77 m; (c) 134°2′3″E, 72°4′52″S; 32 m; (d) 134°4′21″E, 72°13′47″S; 83 m; (e) 134°4′37″E, 72°4′34″S; 60 m; (f) 134°1′16″E, 72°10′42″S; 15 m; (g) 134°4′4″E, 72°6′44″S; 9 m; (h) 133°25′59″E, 72°6′23″S; 50 m; (i) 133°28′28″E, 72°4′42″S; 17 m.
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 Another possible mechanism triggering the mass wasting process can be rolling and bouncing of boulders on the interior wall. We noticed a few boulder tracks of 700 m length on the crater wall (Figure 9a). Bouncing of boulders has also produced a chain of depressions along the track (Figure 9b). However, none of the boulder tracks produced gullies and landslides on the crater wall. Bouncing of boulders, in some cases, has been attributed to the seismic activity. For example, on Mars, several boulder tracks have been observed on steep slopes in and around faults and are considered to be recent Marsquake activity [e.g., Roberts et al., 2012]. The boulder tracks observed in this study are fresh and are not covered by any other wall materials. Therefore, the rolling and bouncing of the boulders can be a recent activity and might be related to seismic shaking from recent impact events.
Figure 9. LROC NAC image mosaic M118979214LR illustrating the boulder tracks (~700 m in length) on the eastern interior wall. The tracks originate from the upper crater wall at S1 and S2. (b) The boulder track contains a chain of near-circular depressions formed by bouncing of boulders likely to have originated from S2. The white arrows show the individual depressions (4–6 m diameter), while the black arrow indicates the boulders (3–4 m diameter) that are likely to have produced the depressions.
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