Icarus 337 (2020) 113464

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Icarus

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The Tsiolkovskiy crater landslide, the moon: An LROC view

Joseph M. Boyce a,*, Peter Mouginis-Mark a, Mark Robinson b

a Hawaii Institute for Geophysics and Planetology, University of Hawaii, Honolulu 96822, USA b School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA

ARTICLE INFO ABSTRACT

Keywords: Evidence suggests that the lobate flow feature that extends ~72 km outward from the western rim of Tsiol­ Moon surface kovskiy crater is a long runout landslide. This landslide exhibits three (possibly four) morphologically different Landslides parts, likely caused by local conditions. All of these, plus the ejecta of Tsiolkovskiy crater, and its mare fill are Impact processes approximately of the same crater model age, i.e., ~3.55 � 0.1 Ga. The enormous size of this landslide is unique Geological processes on the Moon and is a result of a combination of several geometric factors (e.g., its location relative to Fermi Terrestrial planets crater), and that Tsiolkovskiy crater was an oblique impact that produced an ejecta forbidden zone on its western side (Schultz, 1976). The landslide formed in this ejecta free zone as the rim of Tsiolkovskiy collapsed and its debris flowedacross the relatively smooth, flatfloor of Fermi crater. In this location, it could be easily identified as a landslide and not ejecta. Its mobility and coefficientof friction are similar to landslides in Valles Marineris on Mars, but less than wet or even dry terrestrial natural flows.This suggests that the Mars landslides may have been emplaced dry. The high density of small craters on the landslide is likely an illusion caused by the effects of age related differences in regolith thickness on crater morphology, and the presence of the abundant young, circular secondary craters produce by debris ejected from distant fresh craters.

1. Introduction rocky, airless body. This makes the Tsiolkovskiy landslide an excellent end-member for the study of the mechanics and conditions that control The Tsiolkovskiy landslide (Fig. 1) is a huge (~72 km long, with a the emplacement of such rapid natural mass flows and their runout volume of ~3745 km3) runout landslide on the Moon, located on the distance. � western rim of the 185 km diameter Tsiolkovskiy crater (20.1 S, Here we use the new data from recent lunar missions to characterize � 128.6 E). It formed when a portion of the western rim of the crater the physical nature of the slide, as well as present evidence of the origin collapsed, causing material from that rim to slide on to and across the of this feature and its relationship with other parts of the crater. We floor of the adjacent Fermi crater (El Baz and Worden, 1972; El Baz, present reasons for why it is the only large long runout landslide on the 1973; Howard, 1973; Boyce et al., 2016). Previously, this lobate feature Moon, how its mobility relates to those of other long runout landslides, has also been proposed to be an unusual type of fluidized ejecta facies and the reasons that it appears to have a high density of small craters unique on the Moon (Guest and Murray, 1969; Guest, 1971; Howard, superposed on its surface. 1973; Schultz, 1976; Wilhelms, 1987; Morse et al., 2018), but morphologic evidence predominantly indicates that this feature is an 2. Background enormous, long runout landslide (see Section 4.2). Recently acquired data from the Lunar Reconnaissance Orbiter Tsiolkovskiy crater was first photographed by Luna 3 in 1959, and (LRO) (Robinson et al., 2010) and other lunar missions (e.g., Kaguya) later at higher resolution by Lunar Orbiter and Apollo missions (Fig. 2). have provided a wealth of new information about this landslide, offering These missions provided images of sufficient quality to discover that a an opportunity for its in-depth study not possible before now. This huge lobate flowfeature, the Tsiolkovskiy landslide, originated from the landslide is the only large, long runout landslide on the Moon, and while western side of this crater. other long runout landslides are found on other solar system bodies, the It is evident that the Tsiolkovskiy landslide has morphological sim­ Tsiolkovskiy landslide is also the largest example formed on a dry, ilarities to the March 27, 1964 Sherman Glacier long runout rockslide on

* Corresponding author. E-mail address: [email protected] (J.M. Boyce).

https://doi.org/10.1016/j.icarus.2019.113464 Received 22 July 2018; Received in revised form 18 September 2019; Accepted 2 October 2019 Available online 25 October 2019 0019-1035/© 2019 Elsevier Inc. All rights reserved. J.M. Boyce et al. Icarus 337 (2020) 113464

Earth (Shreve, 1966, 1968), as well as certain landslides within Valles of the northwestern portion of Fermi crater are in the ejecta forbidden Marineris on Mars (McEwen, 1989; Harrison and Grimm, 2003), zone resulting in the absence of any type of ejecta facies, including including sets of parallel grooves formed in the direction of flow( Fig. 3). chains and clusters of secondary craters, outward of the landslide or on Lunar Orbiter and Apollo images showed that the landslide flowed the plains of Fermi crater in the ejecta forbidden zone as would be ex­ across the Pre-Nectarian age plains in the floor of Fermi crater, a pected if the landslide was impact ejecta. The southern part of the floor ~230 km diameter Pre-Nectarian age impact crater overlapped by of Fermi crater is outside the forbidden zone and contains deposits of Tsiolkovskiy crater (Wilhelms and El-Baz, 1977; Wilhelms, 1987). continuous ejecta from Tsiolkovskiy crater (Fig. 4). The southern part of Geologic mapping indicates that Tsiolkovskiy crater is most likely an the Tsiolkovskiy landslide appears to have overridden these ejecta de­ Upper Imbrium age crater located in Pre-Nectarian age heavily cratered posits near the crater rim (Boyce et al., 2016). highlands in the interior of the Tsiolkovskiy-Stark basin (Wilhelms and Greenhagen et al. (2016) determined the crater size frequency dis­ El Baz, 1977, Wilhelms, 1987). This basin is on the southeastern edges of tribution on the northwest rim of Tsiolkovskiy crater, and measured a a relatively elevated region of the lunar farside highlands. Based on model age of 3.8 � 0.1 Ga. Boyce et al. (2016) also counted craters on crater counts, Boyce et al. (2016) first estimated the age of both Tsiol­ the ejecta located on northwest rim of Tsiolkovskiy crater just inside the kovskiy crater and its landslide at ~3.6 Ga and the surround Pre- ejecta forbidden zone boundary, and found evidence of a resurfacing Nectarian age highlands terrain at ~4.05 Ga. event that erased craters <~350 m diameter at ~3.6 � 0.1 Ga, but the Wu et al. (1972) derived a moderate scale (1:250,000) 100 m con­ model age for the crater population with diameters >~1 km was esti­ tour topographic map of the “landslide” from-high resolution Apollo 15 mated as ~4.05 � 0.1 Ga, likely the model age of the pre-impact terrain. stereo images, from which El Baz (1973) observed that the landslide Boyce et al. (2016) found evidence of resurfacing (3.6 � 0.1 Ga) on the terminated in a low, rampart ridge similar to those on many terrestrial Fermi plains that this is most likely to be an older terrain ~4.05 � 0.1 Ga landslides. Howard (1973) also used this topographic map to compare in age. Boyce et al. (2016) also counted crater on the continuous ejecta the Tsiolkovskiy landslide with the long run-out distance of landslides southwest of the rim of Tsiolkovskiy crater and found a model crater age on Earth and the Moon, finding that lunar avalanches such as the of ~3.6 � 0.1 Ga for the entire diameter range counted. They concluded Tsiolkovskiy landslide are as mobile and efficient as their terrestrial that Tsiolkovskiy landslide formed soon after formation of the Tsiol­ counterparts. He proposed that this may have been because of lubrica­ kovskiy crater at ~3.6 Ga on ~4.05 � 0.1 Ga highlands terrain. tion by impact generated fluids,although he did not specify the fluidor From Diviner Lunar Radiometer thermal measurements Greenhagen its origin. El Baz and Worden (1972) and El Baz (1973) noted that the et al. (2016) found anomalously high surface rock abundance and low Tsiolkovskiy landslide exhibited a higher number of small, fresh craters regolith thickness on Tsiolkovskiy crater ejecta to the southeast of the (some appeared rimless) than the nearby older, cratered plains in Fermi crater. These two properties are consistent with a massive well- crater. It was proposed that these craters could be the result of drainage preserved impact melt deposit in this area. Morse et al. (2018) also of fine-grain material into void spaces initially sealed over by large mapped relatively large patches of impact melt with the largest on the blocks in the landslide, or were gas vents, or that the absence of a thick southern side of the crater in its continuous ejecta deposits. None of this regolith on the comparatively young landslide caused the small craters material is found in the forbidden zone. We propose that the distribution to appear morphologically fresh. In addition, Boyce et al. (2016) of rock abundance is produced as a result of the oblique impact. examined high-resolution LROC images and, consistent with the idea of El Baz and Worden (1972), found that the density of small crater appears 3. Tsiolkovskiy landslide greater than the older, adjacent terrain. Boyce et al. (2016) also favored the idea of El Baz and Worden (1972) that the anomalous density is The main parts of the Tsiolkovskiy landslide extend outward from probably due to a lower rate of morphologic maturing. the western rim of Tsiolkovskiy crater for ~72 km (and a part may even Guest and Murray (1969); Schultz (1976); and Morse et al. (2018) extend to ~90 km). The landslide fans to a width of ~250 km at its outer observed that the ejecta blanket of Tsiolkovskiy is asymmetrical and edge, but can be traced back toward Tsiolkovskiy crater where it in­ attributed this ejecta asymmetry to the effects of an oblique impact that tersects the western rim on both ends of a ~90 km long, low, narrow produced Tsiolkovskiy crater (Fig. 4). This oblique impact resulted in a section of the rim (Fig. 5). This low section of the rim averages ~18 km missing section in the ejecta blanket, i.e., an ejecta forbidden zone, on wide, stands an average of ~2.4 km above the plains in Fermi crater and the northwest side of the crater. The edges of this ejecta forbidden zone ~1.6 km below the adjacent rim crests (at ~4.0 km above the floor of � are centered at ~315 azimuth and extend outward along lines that Fermi crater) on either side. This geometry is consistent with this low � diverge by an angle of ~75 on either side of that line (Guest and section of the rim being the scar left after collapse of the rim of Tsiol­ Murray, 1969; Wilhelms, 1987; Morse et al., 2018). As a result of this kovskiy crater that produce the landslide (also see Boyce et al., 2016). ejecta asymmetry, most of the landslide as well as the plains in the floor A knob or pinnacle is located (at the 3.5 km point) near the middle of

Fig. 1. Location of the Tsiolkovskiy crater and landslide. (a) Mosaic of the eastern hemisphere. White box shows location of Fig. 1b. (b) Location of landslide on the western rim of Tsiolkovskiy crater. (Both images are LROC WAC mosaics). North is at the top of both images.

2 J.M. Boyce et al. Icarus 337 (2020) 113464 this low section of the rim. It stands ~1500 m about the average height result of their age and lack of steep slopes (Ghent et al., 2014). of the rest of this section of the rim (Fig. 5). Because of the presence of this knob, the collapse of the rim would likely have been most dramatic 3.1. North slide on either side of it, resulting in greater volumes of landslide debris furnished to the landslide from those areas. This also means that while North slide is the largest of the morphologically different parts of the some rim collapse would have occurred in the area of the pinnacle, the Tsiolkovskiy landslides that make up the main slide (Fig. 7). It has an amount of landslide debris produced from this area would likely have area of ~2540 km2, average thickness is ~800 m, and volume of been considerable less than from the areas on either side of it. As a result, 3 ~2032 km . It originates from the northern most part of the low section the parts of the landslide outward from the pinnacle (i.e., middle and of Tsiolkovskiy rim, and extends ~65 km outward. It has a triangular west slides) would be expected to contain comparatively less debris than shape in map view with its widest dimension at the base of the rim the parts of the Tsiolkovskiy landslide on either side of it (i.e., north and (~50 km). south slides). As mentioned above, parallel sets of slump blocks with long axes Although the Tsiolkovskiy landslide likely formed as a single event, transverse to flow direction of north slide are developed on the slopes we have identified at least three, and tentatively four (i.e., north, mid­ extending down from the rim. In addition, several low, broad ridges and dle, west south, and possibly a west slide), morphologically different troughs also trending transverse to landslide flow and parallel to the parts (Fig. 6). These parts probably owe their differences to local con­ crater rim are found on the relatively flat,hummocky, horizontal surface ditions at the time of formation of the landslide (Boyce et al., 2016). For of north slide. Several large nearly circular pits (4 to 6 km diameter) are example, north, middle and the north part of west slides are superposed found in the western part of the slide that could be buried impact craters on the relatively flatPre-Nectarian plains in the floorof Fermi that are in or unusual flow features. the ejecta forbidden zone of Tsiolkovskiy crater (Fig. 6). This super­ The surface of north slide is crossed outward by closely-spaced fur­ position relationship, combined with the topographic data from LROC rows that formed longitudinal to the direction of flow,and radial to the and Kaguya, allows calculation of thickness and volume of each part of crater rim (Fig. 7). These furrows are similar to grooves common on the landslide superposed on these plains (Table 1). However, calculation terrestrial (Shreve, 1966, 1968; Bull and Marangunic, 1968) and of these parameters for south slide that sits outside the ejecta forbidden Martian landslides (Lucchitta, 1979), as well as some types of layered zone require an additional step that takes into account the presence of a ejecta deposits (Dufresne and Davies, 2009; De Blasio, 2014; Boyce layer of continuous ejecta from Tsiolkovskiy crater deposited on the et al., 2014) (Fig. 3). These features are typically <~50 m deep, up to a Fermi plains and overran by the landslide. The surface of the continuous 300 m wide, and as long as ~40 km. They tend to widen and curve ejecta deposits from Tsiolkovskiy crater in Fermi crater outward of south outward near the outer edge of the slide, most likely indicating exten­ slide is ~500 m higher than the surface of the nearby plains in Fermi sion and pulling apart of the flowingdebris in that area of the slide as it crater. This suggests that the thickness of these ejecta deposits outward spreads. The presence of these features on this slide indicates that the of the slide, and likely buried by south slide, is ~500 m. In addition, the development of such furrows did not require ice, water, or clay minerals average surface elevation of south slide is ~500 m higher than the at their base to act as a lubricant to enable flowbecause the Tsiolkovskiy surface of the continuous ejecta deposits of Tsiolkovskiy crater and landslide was dry when emplaced, and formed in a historically dry ~1000 m higher that the Fermi plains in the area. This also suggests that environment (e.g., Dufresne and Davies, 2009; De Blasio, 2014; Watkins the continuous ejecta deposits from Tsiolkovskiy crater ejecta is ~500 m et al., 2015). thick and that south slide is ~500 m thick, as well. North slide terminates in a raised ridge (i.e., a rampart ridge) at the The slide generally has the characteristics that, starting at the rim of contact with Fermi Crater plains along its outer edge. It connects Tsiolkovskiy crater, the surface of each part of the slide slope relatively continuously with the distal edge of middle slide, and it is therefore steeply away from the rim. This slope likely mimics the slopes of the likely that they formed at the same time. This rampart ridge is typically topography of the rim uplift buried beneath. About 15 km outward from ~1.5 to 3 km wide, and has ~10% higher relief than the body of the slide the rim, the surfaces of these slides dramatically change slopes to behind it (measured from the terrain outward of the slide). Remarkably, become relatively level and flat,out to a distal rampart ridge where they this ridge also bifurcates were it meets middle slide’s distal ridge and terminate in a scarp. This rampart ridge varies in height along the edges extends back toward the pinnacle on the crater rim. This ridge borders of the slides, but is typically ~10% higher than the adjacent body of the north slide and middle slide and is likely a lateral rampart ridge pro­ slide (Boyce and Mouginis-Mark, 2018). Large blocks are uncommon on duced as the thicker north slide expands against the thinner middle slide the surface of the landslide, as well as on the rampart ridge, probably a during their formation. Along the edge facing Fermi crater plains, this

Fig. 2. Progression of image quality before LROC. Image on the left is a low resolution Zond 3 image taken in 1959 of the farside of the Moon including Tsiolkovskiy crater (arrowed). Picture in the middle was acquired in 1967 by Lunar Orbiter III (frame III 121 H 1/3) with a resolution of ~40 m. The image on the right is an Apollo 15 metric image (frame AS15-0892) acquired in 1971 with a resolution of a <10 m. Newly acquired images and other types of data by LROC and other missions (e.g., Kaguya) offer a significant improvement in pixel scale and data quality. North is at the top of each image.

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Fig. 3. Examples of landslides on the Earth (left) and Mars (right), both of which display radial grooves on their surfaces potentially comparable to the grooves on the Tsiolkovskiy landslide. Terrestrial example is the 1964 Sherman Glacier landslide in Alaska, which covered ~8.25 km2 with an average thickness of ~1.5 m (U.S. � 0 Geological Survey photo by Austin Post). The Martian landslide is located on the foot of the southern wall of Melas Chasma within Valles Marineris at 12 50 S, � 0 290 29 E, and has a maximum width of ~15 km (oblique view projected on a digital elevation model derived from Context Camera (CTX) images D04_028857_1684 and D04_028923_1684).

Fig. 4. Sketch map of the distribution of ejecta around Tsiolkovskiy crater and the Tsiolkovskiy landslide. Ejecta boundaries and secondary crater chains derived from Morse et al. (2018). North is at the top. ridge is not a long smooth continuous feature, but is a series of small has accumulated numerous impact craters. These craters are generally lobate sections commonly 2 km to 5 km wide. This gives the outer edge circular, in close proximity to one another but generally do not overlap, of north slide a scalloped appearance. The individual lobate sections are and have a small range in sizes and a morphology that indicates similar separated by relatively wide (i.e., ~1 km) radial grooves that extend all stages of freshness. Judging from their interior shadows that suggest that the way through the rampart. De Blasio (2014) proposed that such they are relatively deep for their diameter, these craters appear to have morphology indicates a dry, thick landslide, consistent with the Tsiol­ nearly fresh crater depth-to-diameter ratios, and hence are relatively kovskiy landslide. Such rampart ridges are also found on distal edges of young. We believe that these small craters are secondary craters pro­ terrestrial and Martian landslides, as well as some types of layered ejecta duced by a relatively young large crater (e.g., Necho) and that these deposits. These ridges are thought to be due to particle separation pro­ craters were produced by impact of high-velocity, high-ejection angle cesses and shear in the flows that result in the accumulation of coarse ejecta thrown a great distance (see Preblich et al., 2007). In addition to particles at their leading edges (e.g., see Savage and Hutter, 1989; these small secondary craters, many of the small craters on north slide Iverson, 1997; Major and Iverson, 1999; Pouliquen and Vallance, 1999; (and the other slides) also appear to be fresher than expected for the age McSaveney and Davies, 2007; Barnouin-Jha et al., 2005; Wada and of the surface as noted by El Baz and Worden (1972) and El Baz (1973). Barnouin-Jha, 2006; Campbell, 2006; Boyce and Mouginis-Mark, 2006, This age relationship is discussed in more detail in Section 4.4. Iverson et al., 2010; Johnson et al., 2012). Like the rest of the Tsiolkovskiy landslide, the surface of north slide

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Fig. 5. Contour map of the slide, derived from Kaguya DTMTCOs01_03240S197E1243SC. Contour interval is 500 m with numbers indicating elevations (in km) relative to local datum, i.e., the lowest elevation portion of the floorof Fermi crater. Color rainbow displays low elevations in blue and high points in yellow. North is at the top.

3.2. Middle slide same size as those found on north slide, but are less well developed and less frequent. Similar to north slide, middle slide terminates in a rampart Compared with north slide, middle slide (Fig. 8) has a substantially ridge composed of lobate sections typically 2 km to 5 km wide separated smaller surface area (~1290 km), is thinner (~350 m), and has a smaller by radial furrows that extend all the way across the rampart. This volume (~452 km3). Middle slide extends to ~72 km from the western rampart is continuous with that of north slide, consistent with the idea rim of Tsiolkovskiy crater. The surface of this slide gently slopes away that they formed at the same time and likely as part of the same flow. from its distal rampart back to the break in slope below the crater’s rim. The crater population of middle slide is similar to that on north slide. In plain view, the middle slide is triangular shaped with its widest Crater counts indicate that middle slide is the same model age as north dimension at its distal edge. It narrows toward the crater to below the slide, or ~3.55 � 0.1 Ga (Michael and Neukum, 2010). Middle slide also pinnacle in the middle of the low section of the crater’s rim. exhibits numerous small fresh impact craters that have the same char­ The presence of this pinnacle probably indicates that collapse in this acteristics as those on north slide. Like on north slide, some of these are area of the rim was less extensive compared with other parts of the low clearly secondary impact craters similar to those formed by high ve­ section of the rim, and most likely accounts for the relatively small locity, high ejection angle debris from a distant crater. volume of middle slide compared with north slide. Middle slide and north slide are separated by a ~5 km wide trough and a ridge 3.3. South slide (mentioned earlier). The close proximity of these two features supports the idea that they may be related in some way. The long axis of the South slide (Fig. 9) has a run-out distance of ~50 km, i.e., two-thirds trough is on a line that points toward the pinnacle on the rim. The border of that of north or middle slides. It has a surface area of ~2415 km2, and between middle and south slides, on middle slide’s south side, is a its surface is ~500 m higher than the continuous ejecta from Tsiolkov­ topographic step up (i.e., subdued scarp) of ~500 m, unlike a terminal skiy crater outward from it, and averages ~1000 m higher than the rampart ridge that slopes downward from its summit in both directions. nearby plains in Fermi on which these ejecta deposits are superposed. The surface of middle slide is morphologically similar to north slide. Assuming that south slide was deposited on top of the continuous ejecta It is only moderately hummocky and exhibits a pattern of furrows that deposits of Tsiolkovskiy crater and that these ejecta were deposited on formed longitudinal to the direction of flow. The furrows are about the the Pre-Nectarian plains in Fermi crater, then the thickness of south slide

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continuously crater-ward to the break in slope below the rim. This ge­ ometry suggests that the flow that produced this rampart likely came from that area. However, on its north side, the boundary with middle slide is not a rampart, but a ~600 m high east-west trending subdued scarp that steps-down to separate the lower elevation middle slide from the higher elevation south (Fig. 5). The trend of this scarp points in the direction of the pinnacle on the rim and is coincidental with the boundary of the ejecta forbidden zone (Morse et al., 2018). Assuming that middle slide is ~350 m thick (see Section 3.2) and that the Tsiol­ kovskiy continuous ejecta deposit and south slide together average ~1000 m thick, then this difference in elevation is ~650 m. However, the topographic step up from middle slide to south slide at the boundary between the two slides is <650 m, meaning that either the buried Tsiolkovskiy ejecta deposit or south slide (or both) thin along their edges, a reasonable expectation. This is most likely an indication that south slide is superposed on these ejecta deposits as well as on its west side (see Section 3.0). Furthermore, the hummocky nature of south slide is likely due to the effects of the landslide debris flowing over buried hummocky ejecta deposits. This is in contrast to north and middle slides that were emplaced over the relatively smooth Pre-Nectarian plains in Fermi crater and are much less hummocky. South slide also lacks radial fur­ rows found on the other parts of the Tsiolkovskiy landslide. The rough surface of the hummocky ejecta most likely disrupted the flowprocesses that produced the furrows, and hence prevented their production (e.g., see Dufresne and Davies, 2009; De Blasio, 2014). Furthermore, the lobate, digitate segments, like those along the distal ramparts of north and middle, are absent along the distal ramparts of south slide, probably because the furrows are not present on south slide. The crater population on south slide is similar to that on north and middle slides. Based on crater counts the model age of south slide is 3.55 � 0.1 Ga (Michael and Neukum, 2010). Within error limits, this is the same age as north slide and middle slide. South slide also exhibits Fig. 6. Map showing the different morphologic parts of the Tsiolkovskiy numerous small fresh impact craters. Some occur in clusters that can be landslide and surrounding terrain. The dashed and dotted line extending out­ traced off the slide. The small craters in these clusters have the same ward from west slide marks the southern boundary between the ejecta characteristics as those on the rest of the slide and are likely secondary forbidden zone to the north and Tsiolkovskiy ejecta to the south. “NS” is North Slide, “MS” is Middle Slide, “SS” is South Slide and “WS” is West Slide. Base impact craters formed by high velocity, high ejection angle debris from a image is a LROC WAC mosaic. distant young crater. is ~500 m with a volume of ~1208 km3 for south slide. 3.4. West slide The surface of south slide is morphologically different than north or 2 middle slides; it is hummocky and lacks furrows. On the slopes of south West slide is a small area (546 km ) located outward from the slide leading up to the rim of Tsiolkovskiy crater, relatively large sets of western edge of the distal rampart of middle slide (Fig. 10). West slide, parallel ridges and troughs concentric with the rim crest have formed. starts abruptly at the edge of the distal ramparts of middle and south These ridges and troughs are probably large slump blocks composed of slides. It exhibits many of the same morphologic characteristic as north rim material and landslide debris. Outward from these features the and middle slides, such radial furrows, and high density of small fresh surface of the slide becomes nearly flat, but quite hummocky. Some craters. Its average thickness is ~100 m, and hence its volume is only 3 hummocks have as much as ~500 m relief. A rampart ridge has devel­ ~55 km . oped on the western and southern edges (up to near the crater rim) of The boundaries between west slide and the surrounding terrain are south slide (Fig. 9). On its west side this rampart ridge connects different than those of the other parts of Tsiolkovskiy landslide. For continuously with the distal rampart of middle slide suggesting they example, the exact location of the western and northern edges of west formed as part of the same flow,and at the same time, i.e., 3.55 � 0.1 Ga slide are ill-defined because they thin continuously outward and (Michael and Neukum, 2010). disappear, instead of ending in a distal rampart ridge as do the other On the south side of south slide, the rampart ridge appears to extend parts of the Tsiolkovskiy landslides. The northern third of this slide thins from an average thickness of ~100 m to zero, i.e., to the elevation of the

Table 1 Tsiolkovskiy landslide geometric (i.e., maximum runout distance, average thickness, area, and volume) and mobility (i.e., L/H and H/L and L/V) parameters.

Slide Max run-out from rim, Avg, thickness, Volume, km Approx. surface area, km Height of drop, Coefficient of friction, Mobility L/ L/V km km cu. sq. km H/L H

North slide 65 0.8 2032 2540 3.5 0.054 18.6 0.032 Middle 72 0.35 452 1290 3.5 0.049 20.6 0.159 slide South slide 50 0.5 1208 2415 3 0.060 16.7 0.041 West Slide 90 0.1 55 546 3.5 0.039 25.7 1.648 Total slide 71 0.49 3746 6791 3.5 0.049 20.3 0.019

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Fig. 7. North slide. (a) LROC WAC mosaic, with the perimeter of the slide delineated by a dashed line. North is at the top. The field of view in bottom image is marked by a solid line. (b) Low sun-angle oblique view generated from Kaguya image and DEM DTMTCOs01_03240S197E1243SC. View is looking northwest, and illustrates the numerous radial grooves which characterize this part of the slide.

Fermi plains. This part of west slide is separated from the southern two- over 5 km from these craters. On its south side, west slide terminates thirds of the slide by a subdued ~300 m high scarp that runs east-west. against a northeast trending chain of relatively large (also averaging This scarp is on the same trend line as the boundary between middle and ~5 km diameter) craters that separates west slide from south slide. south slides and may have been caused by overriding and burial of West slide is crossed by shallow furrows that approximately parallel continuous ejecta deposits from Tsiolkovskiy by west slide. The surface those on middle slide (Fig. 11). These furrows start abruptly at the outer of the southern two-thirds of west slide is more hummocky than middle edge of middle slide’s rampart, so that either the west slide was over­ slide, but less hummocky than south slide. The surface of this part of ridden by middle slide or that west slide may be a facies of middle slide west slide is inclined, rising westward from ~200 m (above the eleva­ and originated from it. The furrows can be traced westward across the tion of the nearby plains in Fermi crater) at the base of the rampart of southern part of the chain of large secondary craters on the western side middle slide to ~400 m where it intersects, and in places (its southern of west slide. These craters do not appear to be appreciably filled with half) overrides, the north trending chain of large (averaging ~5 km slide debris, indicating that the distal part of the western side of this slide diameter) craters to the west. In the places where west slide overrides is quite thin and overrode these craters without fillingthem. Like on the these craters it appears to be quite thin, <100 m and may extend out for north side of this slide, a discernable rampart is not apparent on the west

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Fig. 8. Middle slide with the perimeter of the slide delineated by a dashed line. North is at the top. Image is an LROC WAC mosaic.

Fig. 9. South slide with the perimeter of the slide delineated by a dashed line. The radial furrows common on north and middle slides are absent on south slide. North is at the top. Image is a LROC WAC mosaic. margin of the slide although the hummocky nature of the southern part addition, like the other parts of the Tsiolkovskiy landslide, west slide of west slide may have made a rampart in such a thin deposit difficultto appears to have a high density of small (less than a few hundred meters identify. diameter) fresh craters superposed on its surface. These are even found The crater population of west slide is similar to that on the other on the surface of the relatively large elongate craters on the southwest slides. Crater counts indicate that west slide is approximately the same side of the slide that are crossed by the grooves. Many of these small age as other parts of Tsiolkovskiy landslide, or 3.55 � 0.1 Ga. In craters are circular, but in places occur in clusters and are likely to be

8 J.M. Boyce et al. Icarus 337 (2020) 113464

Fig. 10. West slide with the perimeter of the slide delineated by a dashed line. The portion of west slide in Location of Fig. 11 is also indicated. LROC WAC mosaic of west slide. North is at the top.

Fig. 11. Portion of west slide (WS) showing shallow furrows (arrows point to examples). See Fig. 10 for location. Note that some furrows start at the edge of middle slide (MS) while some extend a short distance beyond the north trending crater chain on the left. LROC WAC mosaic of west slide. secondary craters from a distant source. et al., 2015a, 2015b; Tornabene et al., 2019) produced along the edge of West slide is the only segment of the Tsiolkovskiy slide that does not the forbidden zone that was emplaced before the slides. However, there extend back to the rim. The western edge of this slide is ~90 km from the is no morphologic evidence that provides clues to which alternative is rim of Tsiolkovskiy crater, but its eastern edge contacts the distal ram­ valid. While all are possible, no other similar thin deposit is found part of western edges of middle slide and south slide 71 km from the rim outward of north or south slides. We therefore conclude that the origin of Tsiolkovskiy crater. This abrupt contact with the distal edge of middle of west slide remains speculative. and south slides suggests that, if west slide is, indeed, a part of the Tsiolkovskiy landslide, it is (1) an outer facies of middle slide emplaced 4. Discussion at the same time as middle slide, (2) an earlier slide that was overridden by middle slide, (3) a later thin, highly fluidslide that overrode parts of Recently acquired high-resolution data (especially LROC images, as south and middle slides, or (4) possibly a thin ejecta facies (see Boyce well as Kaguya and LOLA topographic data) are used here to support a

9 J.M. Boyce et al. Icarus 337 (2020) 113464

detailed analysis of the morphology and morphometry of the Tsiolkov­ skiy landslide and its geologic setting to better understand its origin and emplacement, as well as its relationship with other terrain in the im­ mediate vicinity. In this section, we discuss the age relationships be­ tween the different morphologic parts of the landslide and their age relationship to nearby terrain, as well as address some of the open questions, including (1) the origin of this feature, (2) why it formed where it did, (3) how its mobility compares with other natural geophysical flows, and what does that tell us, and (4) what causes the apparent high density of small craters on its surface reported by El Baz and Worden (1972) and El Baz (1973).

4.1. Age relationships from crater counts

Understanding age relationships between units are key for placing constraints on the origin of this feature as well as placing it in the context of the geologic history of its surrounding. A variety of techniques are used here to establish age relationships between geologic units, such as superposition relationships, intersection relationships, and the density of superposed impact craters on a surface (from crater counts). Of these techniques, crater counts, provide a reasonably reliable means of determining relative age, and with certain assumptions provide a means of estimating the feature’s absolute (model) age (see Michael and Neu­ Fig. 12. Location of crater counts on the Tsiolkovskiy landslide and vicinity. “FP” Fermi Plains, “NS” is North Slide, “MS” is Middle Slide, “SS” is South Slide, kum, 2010). For this reason, in this study, crater counts are the primary “CE” is continuous ejecta, “WS” is West Slide, WM is western mare, and SM is tool used as a way of determining the chronology of events at Tsiol­ south mare. Other areas counted but not shown here include Tsiolkovskiy � � kovskiy crater. continuous ejecta near the southeast rim (23.0 S, 132.7 E) and one area 110 km � � LROC NAC images were used as the primary data base for mea­ from the rim on the continuous ejecta east of the crater rim (24.2 S, 133.9 E), � surement of crater diameter for performing the crater counts. Cumula­ and, an area in the highlands 483 km northwest of Tsiolkovskiy (1.8 S, � tive size-frequency distribution (CSFD) curves were constructed from 123.3 E) in its ejecta forbidden zone. Base image is a LROC WAC mosaic. the crater count data collected for each sample area. Crater model ages were calculated based on the CSFD curve using the craterstats2 program slides, craters >~100 m diameter were counted to avoid the effects of of Michael and Neukum (2010). Error bars, based on the statistical error crater size saturation that, for this age surface, should occur at ~80 m inherent to the number of craters counted (assuming a Poisson distri­ diameter (see Moore et al. 1980, p. B6). However, on north slide, craters bution of values), were calculated for the number of craters in each were counted down to ~19 m diameter as a test to investigate the diameter bin (N) (calculation included in craterstats2 program). The population of small craters on these slides. We found, as predicted, that lunar production function and lunar chronology of Neukum et al. (2001) below the crater saturation size, the CSFD of craters on this slide exhibits were used to estimate model crater age for each CSFD curve. Care was a bend to a shallower negative slopes compared to the lunar impact taken to ensure that each sample area contained enough area to be production function. We interpret this shallowing of slope to be due to confident that their impact crater production function could be confi­ the effects of the crater saturation size for this age surface (e.g., see dently determined, and that these areas were flatand horizontal enough Hartmann et al., 1981). Alternatively, the roll-over of this curve at to insure that the effects of slope and mass wasting on the crater pop­ ~80 m diameter could be partially a result of the properties of the sur­ ulation were not significant(e.g., see Ghent et al., 2014, van der Bogert face materials, such as a thick regolith, that effects small craters more et al., 2015). While generally all impact craters greater than a particular than large craters in these areas. diameter (which is dependent on area size and image resolution) were To determine the temporal relationships between the formation of included in the counts, rimless and irregular-shaped craters and craters the landslide and the formation of Tsiolkovskiy crater, craters were occurring in chains or tight clusters were excluded because they may be counted on the continuous ejecta deposits of Tsiolkovskiy crater that either endogenic (i.e., volcanic) or are secondary impact craters. How­ occur on the floor of Fermi crater west of south slide, the ejecta on the ever, even with their exclusion, their effects on the CSFDs are generally southeast rim of Tsiolkovskiy crater, the ejecta 110 km east of craters predictable, which itself could provide valuable information about the rim, and the ejecta deposits on the northwest rim of Tsiolkovskiy crater history of the surface (Schultz and Singer, 1980; Hartmann et al., 1981; located immediately outside the ejecta forbidden zone boundary van der Bogert et al., 2015). (Fig. 12). The CSFD of these counts are shown in Fig. 13b. The CSFD for Craters were counted in the areas shown in Fig. 12 (i.e., the four crater in the first three of these areas approximately follow the lunar morphologic parts of the Tsiolkovskiy landslide discussed above, the production function with a model age of ~3.55 � 0.1 Ga (see Neukum cratered plains in Fermi crater, continuous ejecta deposits in an area on et al., 2001; Michael and Neukum, 2010). However, while the CSFD of the northwest rim of Tsiolkovskiy crater on the boundary with the ejecta craters <~300 m diameter in the area on the northwest rim of Tsiol­ forbidden zone, and the continuous ejecta in southwest Fermi crater). In kovskiy crater also follows the 3.55 � 0.1 Ga production function, cra­ addition, craters were also counted in an area near the southeast rim of ters >~1 km diameter in this area follow the lunar production function the crater, as well as a highlands region 483 km northwest of Tsiol­ with a model age of ~4.10 � 0.1 Ga. This CSFD curve indicates that the kovskiy crater in its ejecta forbidden zone. The intent of these counts is small crater population was partially reset at ~3.55 � 0.1 Ga (from to determine the model age of Tsiolkovskiy crater, its relationship to the ~4.10 � 0.1 Ga), most likely by the Tsiolkovskiy formation event. formation of the landslide as well as the volcanism that has occurred in In an effort to put the landslide and formation of the crater into a its interior, and its effects on the older surrounding terrain. temporal context with nearby units, craters were also counted on the The CSFD of craters on north, south, middle and west slides are Pre-Nectarian plains in Fermi crater in the ejecta forbidden zone, and the plotted in Fig. 13a and show that all approximately follow the lunar Pre-Nectarian age highlands terrain in the ejecta forbidden zone � production function with a model age of ~3.55 0.1 Ga (see Neukum northwest of Tsiolkovskiy crater (Fig. 14a). In addition, preliminary et al., 2001; Michael and Neukum, 2010). On south, middle and west

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Fig. 13. (a.) CSFD of craters on the four different parts of Tsiolkovskiy landslide. (b.) CSFD of craters on continuous ejecta deposits of Tsiolkovskiy crater within approximately a crater radius of the rim on the northwestern (two areas), southeastern and eastern sides of Tsiolkovskiy crater, and the floorof Fermi crater outward of south slide. The crater population on the northwestern crater rim is near the boundary with the ejecta forbidden zone and shows evidence (i.e., knee in the CSFD curve) of partial resurfacing of a ~4.10 Ga surface at ~3.55 Ga (presumably the Tsiolkovskiy formation event). The counts are based on the following data where N ¼ number of crater counted in each area, A ¼ the area counted in km2, and R ¼ range of diameters in km. Graph on left (a.), north slide (low Resolution), N ¼ 32, A ¼ 118, R ¼ 0.440–0.786; north slide (Medium Resolution) N ¼ 164, A ¼ 4.2, R ¼ 0.060–0.262; north slide (high resolution), N ¼ 119, A ¼ 0.32, R ¼ 0.020–0.095; south slide, N ¼ 101, A ¼ 27, R ¼ 0.150–0.600; middle slide, N ¼ 112, A ¼ 29, R ¼ 0.150–0.500; west slide, N ¼ 81, A ¼ 16, R ¼ 0.130–0.470. Graph on right (b.), Tsiolkovskiy ejecta in Fermi Floor, N ¼ 115, A ¼ 39, R ¼ 0.180–0.720; SE rim of Tsiolkovskiy, N ¼ 31, A ¼ 1346, R ¼ 0.85–2.3; NW rim of Tsiolkovskiy (large area) N ¼ 100, A ¼ 2752, R ¼ 0.550–7.4; NW rim of Tsiolkovskiy (high-resolution, small area), N ¼ 63, A ¼ 52, R ¼ 0.216–1.1; near rim east of Tsiolkovskiy, N ¼ 19, A ¼ 334, R ¼ 0.630–1.4. crater counts were done for test areas in the north, south, east and west curve instead of the effects of crater saturation of small craters on an old parts of the mare in Tsiolkovskiy in an effort to place volcanism in the surface because both parallel segments of the CSFD curve follow pro­ crater in a context to crater formation. The CSFD that resulted from duction functions. In addition, crater saturation of a ~3.55 Ga lunar these counts are plotted in Fig. 14b. surface occurs at ~80 m diameter (Moore et al. 1980, p. B6), well below Craters were counted on the floorof Fermi in two places (Fig. 12); a the offset diameter at 350 m. The cause of the small craters erasure at relatively flat, small area between large (>~10 km diameter) craters ~3.55 � 0.1 Ga is likely the formation of Tsiolkovskiy crater, and that and a larger area (where large craters were counted and a sub area the reason that not all craters in these areas were obliterated is they are where smaller craters were counted) including large circular craters. located in or near the ejecta forbidden zone. These CSFD curves suggest The CSFD for these craters are plotted in Fig. 14a. The CSFD curve for that some process, such as seismic shaking or possibly erosion by small craters in the small area (in a spot between large craters) follow the secondary particles, operated in this zone. production function with a model age of ~3.55 � 0.1 Ga. However, the In an effort to understand the events that unfolded around the CSFD of craters in the large area exhibit similar structure to the craters development of the Tsiolkovskiy landslide, we have counted craters on on the northwest rim of Tsiolkovskiy crater. In this large area, craters the west and south sides of the mare fill in the Tsiolkovskiy crater. In <~350 m diameter follow a crater production function for a model age addition to placing these in the context of the formation of the landslide, of ~3.55 � 0.1 Ga, while craters with diameters > ~1 km follow a the timing of flooding by basalt in the floor of Tsiolkovskiy crater may production function with a model age of ~4.10 � 0.1 Ga (see Neukum provide deeper insight into whether these volcanic deposits are related et al., 2001; Michael and Neukum, 2010). Like with the data for the to the formation of the crater. There is some debate about the age of the craters on the northwest rim of Tsiolkovskiy crater, this also indicates mare inside of Tsiolkovskiy crater. Wilhelms and El-Baz (1977) and partial resurfacing of 4.10 � 0.1 Ga terrain by the formation of Tsiol­ Wilhelms (1987) proposed that the age of this mare fillis Upper to Later kovskiy crater. We favor this interpretation for the structure of these Imbrium age or >3.2 Ga. While crater model ages from impact crater

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Fig. 14. (a) CSFD of two areas on the Pre-Nectarian plains of Fermi Crater. The small area marked FP is located between several large (>10 km diameter) craters (N ¼ 163 craters, A ¼ 45, R ¼ 0.140–0.512), while the curve marked medium (N ¼ 144, A ¼ 53, R ¼ 0.153–1.190) is an area within the large area (also marked FP) (N ¼ 9, A ¼ 695, R ¼ 2.40–4.60) southwest of the small area. A smaller range of craters is counted in the medium location compared with the entire large FP area. CSFD for the sample areas in the mare on the floor of Tsiolkovskiy is shown in (b) (North, N ¼ 20, A ¼ 114, R ¼ 0.370–0.742; South, N ¼ 45, A ¼ 183, R ¼ 0.410–0.700; East, N ¼ 54, A ¼ 129, R ¼ 0.208–1.012, West, N ¼ 41, A ¼ 68, R ¼ 0.306–0.762). Plotted in (c) are the CSFD shown earlier as well as the Pre- Nectarian cratered terrain northwest of Tsiolkovskiy crater (NW highlands, N ¼ 34, A ¼ 100,654, R ¼ 22.0–100.0). This figure shows that the Tsiolkovskiy crater formed in the lunar highlands on 4.05 Ga terrain and that the landslide and mare fillingoccurred soon after crater formation. Note: As in Fig. 13, the counts are based on the following data where N ¼ number of crater counted in each area and A ¼ the area counted in km2. counts are generally consistent with this idea, they range from ~3.19 Ga measured for the larger craters on the Pre-Nectarian plains in Fermi to ~3.8 Ga (Gornitz, 1973; Wilber (1978); Walker and El-Baz, 1982; crater and the northwest rim of Tsiolkovskiy. Wilhelms, 1987; Tyrie, 1988; Greenhagen et al., 2016; Mouginis-Mark The crater count data presented here (Fig. 14c) indicate that Tsiol­ and Boyce, 2017), although Pasckert et al. (2015) found different areas kovskiy crater formed at a model age of ~3.55 � 0.1 Ga on ranging from 2.22 and 3.69 Ga. In addition, based on crater erosion ~4.10 � 0.1 Ga Pre-Nectarian farside highlands. It should also be noted morphology, Boyce et al. (1975) estimated the age to be ~3.35 � 0.2 Ga that none of the CSFD collected near the crater show a positive bump in for this mare fill.The crater counts we collected for this study are plotted their curves at smaller diameters (or any diameter for that matter) ex­ in Fig. 14b and indicates an average model age of ~3.55 � 0.2 Ga. This pected for contamination by self secondaries as proposed by Zanetti age and that of the landslide suggest that within the limits of error, the et al. (2017) and Plescia and Robinson (2019). Self secondaries were Tsiolkovskiy landslide and mare emplacement occurred at approxi­ most likely produced by Tsiolkovskiy crater, they are smaller than the mately the same time, i.e., soon after the formation of Tsiolkovskiy lower limits of our counts, and hence would not affect results from the crater. In addition, it has been proposed by Wilber (1978) and Pasckert counts. In addition, because Tsiolkovskiy crater was formed by an et al. (2015) that the scatter of model age values for the mare fillin these oblique impact an ejecta forbidden zone was produced on the west side studies may indicate the presence of several mare units of nearly the of the crater. Small scale surface features, such as small craters, were same ages. The differences in our two counts could be interpreted as two obliterated in this forbidden zone, while larger features were preserved. mare units of slightly, though not statistically significant,different ages. This obliteration could have been done by seismic shaking. The Tsiol­ More work is needed to resolve the issue of timing of mare emplacement kovskiy landslide (and all its parts) formed soon after formation of the in Tsiolkovskiy crater. crater, also at ~ 3.55 � 0.1 Ga. At nearly the same time, the mare basalts Tsiolkovskiy crater formed in Pre-Nectarian cratered terrain on the were emplaced in the interior of Tsiolkovskiy crater. However, the errors lunar farside (Wilhelms, 1987). To determine the model age of this associated with crater counts are so large that the exact timing of the terrain in the general vicinity of Tsiolkovskiy crater, we counted craters slides cannot be determined with respect to the formation of Tsiolkov­ in an area ~380 km northwest of Tsiolkovskiy crater. We took special skiy crater, but our crater counts demonstrate that there likely was only care to ensure that the sample area in this terrain is in the Tsiolkovskiy a relatively short period of time between crater formation and the ejecta forbidden zone in order to avoid contamination by secondary landslides (i.e., a few seconds/minutes or thousands of years, but not craters from Tsiolkovskiy crater. We counted craters > ~22 km diameter tens of millions of years). in order to be confident that secondary craters from the nearby young 30 km diameter crater, Necho, were not included in the counts. The CSFD curve for these craters is plotted in Fig. 14c and follows a 4.2. Origin: long runout landslide or fluidized ejecta? ~4.10 � 0.1 Ga production function (Neukum et al., 2001). We assume this is approximately the age of the Pre-Nectarian farside highlands in Since its discovery, the origin of the Tsiolkovskiy landslide has been the area of Tsiolkovskiy crater, which is consistent with the model age controversial, with some researchers proposing that it is a fluidized ejecta deposit (e.g., Guest and Murray, 1969; Guest, 1971; Howard,

12 J.M. Boyce et al. Icarus 337 (2020) 113464

1973; Schultz, 1976; Wilhelms, 1987; Morse et al., 2018) and others older flatfloored craters similar to the intersection of Tsiolkovskiy crater contending that it is a long runout landslide (e.g., El Baz and Worden, with Fermi crater. On these Mercury craters, flow features that 1972; El Baz, 1973; Howard, 1973; Boyce et al., 2016). The arguments morphologically resemble the Tsiolkovskiy landslide originate from the for both alternatives were mainly based on its proximity to Tsiolkovskiy rims of the younger craters and extend partly across the plains in the crater and its surface morphology (Barnouin-Jha et al., 2005), and floors of the older craters (Fig. 15). Xiao and Komatsu (2013) interpret infered it was emplaced by a ground-hugging flow similar to the for­ the flowfeatures associated with Mercury craters to be fluidizedejecta. mation of the layered ejecta of Martian craters and long runout land­ However, we believe that the same lines of reasoning outlined above slides. Considering the implications of these two alternatives to our apply to the Mercury features and that they are likely landslides because understanding of mechanics of long runout landslides or lunar ejecta, it they are topographically-controlled and form only where the wall of a is important to resolve this controversy. relatively young crater formed on the rim of an older crater. In considering the layered ejecta alternative, besides some morpho­ Finally, while layered ejecta deposits are not observed anywhere on logic similarities with Martian layered ejecta, most lines of evidence the Moon, landslides are common (Brunetti et al., 2015; Scaioni et al., indicate that this feature is not ejecta. For example, previous researchers 2018) (Fig. 16). This observation supports the idea that the processes (Guest and Murray, 1969; Guest, 1971; Schultz, 1976; Wilhelms, 1987; and conditions responsible for producing landslides commonly allow Morse et al., 2018) found that the ejecta deposits of Tsiolkovskiy crater landslides, but not fluidizedejecta. We believe that for this reason, it is are asymmetrically distributed around the crater, and attribute this unlikely, though not impossible, that fluidizedejecta could occur in one asymmetry to the effects of an oblique impact that produced Tsiolkov­ place on the Moon, and nowhere else. Furthermore, no convincing skiy crater. This asymmetry of ejecta deposits around the crater results quantitative model for the development of Martian-like fluidizedejecta in an ejecta forbidden zone. This ejecta forbidden zone, mapped by on the Moon has been proposed, but in the case of landslides, no unique Wilhelms (1987) and Morse et al. (2018), is centered at an azimuth of model for their formation need be developed to explain their presence � � ~315 , with boundaries diverging outward at ~60 angle from this because they are common on the Moon. center line (Fig. 4). This distribution of ejecta puts the Tsiolkovskiy flow Considering that (1) the morphologic attributes of this feature fitsthe feature mainly inside this ejecta forbidden zone, suggesting that it is not criteria for a long runout landslide, (2) it formed in the ejecta forbidden ejecta. zone where there is no evidence that substantial ejecta was emplaced, Furthermore, if this feature was ejecta, no matter whether Martian- (3) it shows none of the features outward of it generally associated with type layered ejecta or ballistically-emplaced ejecta like all other lunar impact ejecta, such as secondary craters, and discontinuous ejecta (4) it craters, then ejecta facies, such as secondary craters and thin discon­ shows no lateral continuity with the rest of the ejecta of Tsiolkovskiy, tinuous ejecta deposits should occur beyond its outer edge. No such and (5) there is no evidence for the volatiles in the area of Tsiolkovskiy ejecta features are observed beyond this flow in the ejecta forbidden crater required to produce the degree of fluidization observed, we zone. Their absence would be surprising if this flowfeature was ejecta, conclude that the evidence is overwhelming that this feature is not especially considering that such ejecta facies are observed around fresh ejecta of any type, but most likely a long runout landslide. This is in fluidized ejecta craters on other planets (e.g., see Schultz and Singer, agreement with the origin proposed by El Baz and Worden (1972), El 1980; Barlow et al., 2014; Boyce et al., 2015a, 2015b; Tornabene et al., Baz, 1973), Howard (1973), Boyce et al. (2016), and many others. 2019), and because such ejecta facies are produced as a result of hy­ pervelocity impact excavation of consolidated materials whether the 4.3. Why only one giant landslide on the Moon? continuous ejecta deposits were emplaced ballistically or as a surface hugging flow(e.g., Schultz and Singer, 1980; Melosh, 1989; Housen and The Tsiolkovskiy landslide is one of the best preserved examples of a Holsapple, 2011). Moreover, these ejecta facies are found beyond the long run-out landslides in the Solar System, but questions remain as to ballistically emplaced continuous ejecta deposits around the rest of why this huge landslide formed, why it is the only one on Moon, and Tsiolkovskiy crater. over what time frame did it form. It seems most likely that the answer to In addition, if this feature is a layered ejecta deposit, then like the these questions is a combination of size and geometry, and that the ejecta of the Martian crater Zunil whose ejecta blanket exhibits both landslide components (i.e., north, south, west and middle slides) formed fluidized and ballistic ejecta morphologies (Mouginis-Mark and Sharp­ almost simultaneously. Specifically, the formation of the Tsiolkovskiy ton, 2016), this deposit should be laterally continuous with the rest of landslide required that Tsiolkovskiy and Fermi craters both be relatively the Tsiolkovskiy continuous ejecta deposits surrounding Tsiolkovskiy large so that (1) the segment of the rim of Tsiolkovskiy that crosses the crater, in spite of their differences in emplacement style. This is clearly floor of Fermi crater was sufficient voluminous when it collapsed to not the case, particularly on the north side where there is a gap between provided enough debris to produce a huge landslide; and (2) this rim this feature and the ballistically emplaced continuous ejecta deposits segment collapsed in a place on the flat floor of Fermi crater that pro­ (Figs. 4 and 5). Furthermore, the deposit can be traced from its lobate vided adequate space for its full development. In addition, the formation distal edges on the plains in Fermi crater directly back and intersects of this landslide in the relatively broad ejecta forbidden zone of Tsiol­ with a ~90 km long, ~18 km wide, low (~2.4 km lower than the rest of kovskiy crater enhanced its identification. rim) relatively flatsection of the rim of Tsiolkovskiy. Boyce et al. (2016) It is proposed that the large diameter of Tsiolkovskiy and Fermi proposed that this nearly missing section of the rim is likely a landslide craters was a key factor in development of the Tsiolkovskiy landslide. scar, and the source of material for the Tsiolkovskiy flowfeature (Fig. 5). This is because their size provided both a relatively large flat,horizontal Moreover, there is considerable evidence that the degree of fluid­ runout space on the floor of Fermi for development of such a huge ization of ejecta required to produce the long runout distance of the landslide, and a guarantee of enough landslide debris from the source (i. Tsiolkovskiy features requires the presence of substantial volatiles e., rim of Tsiolkovskiy crater) to produce such a huge landslide. In (Gault and Greeley, 1978; Schultz, 1992; Barnouin-Jha et al., 2005; particular, relatively large, old craters such as Fermi commonly have Barlow, 2006). The surface of the Moon is generally regarded as broad flatplains on their floors,and in Fermi, the distance from the rim including only small amounts of any volatile, especially in the central of Tsiolkovskiy crater (the probable source of the Tsiolkovskiy landslide) latitudes where Tsiolkovskiy is located. Furthermore, there is no to the edge of the flatfloor of Fermi opposite that location is ~ 160 km. morphologic (e.g., sublimation pits) or remote sensing evidence of This is enough distance for the landslide to develop unrestricted until its abundant volatiles in the area containing Tsiolkovskiy required for kinetic energy dropped below the frictional resistance with the surface ejecta fluidization to the degree suggested by the dimensions of the and it stopped. Tsiolkovskiy feature. A similar situation is found on Mercury where Furthermore, the source of the Tsiolkovskiy landslide is likely the ~ there are a number of large craters whose rims intersect the floors of 90 km low relief section of the ~140 km long segment of the rim of

13 J.M. Boyce et al. Icarus 337 (2020) 113464

Fig. 15. Large landslide-like flowfeatures associated with Mercury impact craters. These features are morphologically similar to the Tsiolkovskiy landslide and occur in similar geologic settings (originating from the rim of a younger relatively large crater where it crosses the flat,horizontal plains in the floorof an older large crater (MESSENGER Images Left: EW0212285623G; Right: EW0223659303G). North is at the top of both images.

� 0 � 0 Fig. 16. Landslides within craters close to Tsiolkovskiy. Left: Dobrovolskiy crater at 12 47 S, 129 41 E, ~130 km northwest of the rim of Tsiolkovskiy crater. This � 0 � 0 slide formed when the rim between this crater and the 45 km Shirakatsi crater collapsed. Right: Litihe Lutke crater (16 50 S, 123 04 E) ~80 km from the rim of Tsiolkovskiy crater on the northwestern rim of Fermi crater. North is at the top in both images, which are LROC WAC mosaics from the LROC Quickmap site.

Tsiolkovskiy that crosses the floor of Fermi crater. Assuming that this unstable, over-steepened outer rim slopes caused by the difference in low area initially had approximately the same dimensions as the rest of relief between the relatively high, narrow rim and the low elevation of the rim of Tsiolkovskiy crater (minus the ejecta), we calculate the vol­ the adjacent plains in Fermi. In addition, it is also possible that added to ume of the material missing from this low area to be ~2000 km3. This this instability was the near lack of continuous ejecta deposit in the area estimate is based on the width at the base of the rim in this area to be of the forbidden zone that would have buttressed the rim slopes from ~18 km, an initial height of ~4.0 km (i.e., approximately the average failure. elevation of the rim elsewhere on Tsiolkovskiy crater) and ~2.4 km as its In addition, the collapse of the rim along the low section left a present average height above the Fermi plains. If the landslide origi­ pinnacle near the middle of this section. This pinnacle is coincidentally nated from this location, then the volume of material missing from the located where (1) the trough between north slide, and middle slide low section of the rim should approximately equal the volume of the intersected the rim, as well as (2) the southern boundary of the landslide (i.e., ~3700 km3, see Table 1). While our calculated volume of continuous ejecta deposits and ejecta forbidden zone. Although it is not ~2000 km3 is nearly a factor of two less than the volume of the landslide clear why the pinnacle occurs at this location, it is clear that all the deposit, considering a reasonable amount of bulking of material in the landslide debris originating from the section north of the pinnacle slid slide caused by fragmentation (i.e., ~50%–60%), the volume of the onto the flat, relatively smooth, Pre-Nectarian plains on the floor of collapsed rim material most likely roughly equals the volume of the Fermi crater that contained little to no high relief topography to disrupt landslide. While this suggests that no unusually high, voluminous rim flow.Whereas, landslide debris originating from the rim section south of segment needed to be produced to provide enough material to form the the pinnacle slid onto both the flat,relatively smooth plains on the floor landslide, it also leaves open the question of what caused the landslide. of Fermi crater, producing middle slide, and across the relatively rough, We propose that the landslide formed by collapse due to gravitationally hummocky continuous ejecta of Tsiolkovskiy to produce south slide. The

14 J.M. Boyce et al. Icarus 337 (2020) 113464 rough hummocky nature of the ejecta surface should result in compar­ measure of their efficiencyof movement, while the inverse (i.e., H/L) is atively higher friction and drag between the surface of the substrate and the coefficientof friction (μ) that quantifiesthe macroscopic dissipative landslide. Hence, this should reduce the runout distance of the landslide properties of the flow(e.g., see Hsu, 1975; Voight, 1978; Middleton and on the continuous ejecta deposits, consistent with the comparatively Wilcock, 1994; Iverson, 1997; Harrison and Grimm, 2003). These two shorter runout distance of south slide (see Table 1). The higher rough­ parameters are traditionally used to describe the ability of such natural ness (and increased friction) of the ejecta substrate should also inhibit flowsto flow.Furthermore, the runout distance of these natural flowsis formation of such surface features as radial furrows (De Blasio, 2014), generally thought to be dependent on the volume V of rock mobilized by while the formation of rampart ridges, like that at the edge of south the slide (e.g., see Heim, 1932; Dade and Huppert, 1998; Legros, 2002). slide, as well as the other slides, is likely unaffected by roughness of the Heim (1932) originally advanced the hypothesis, prompted by an substrate (Iverson et al., 2010). As a result, the morphology of south analogy with solid friction, that the whole of the initial potential energy slide is different from that of the other parts of the Tsiolkovskiy land­ of the mass is dissipated by the work of frictional forces along the slides, lacking the radial furrows and more hummocky (see Section 3.3 topography. He proposed that an additional mechanism (e.g., some kind for summary of evidence for it being a landslide). We believe that such of enhanced lubrication) is required to explain the long runout distance landslides may be common in places on the Moon with similar large of some landslides. Since his observations, numerous mechanisms have crater overlap geometry as with Tsiolkovskiy and Fermi craters, but been proposed that would act to lubricate the movement of these flows. because their landslides run over rough hummocky ejecta, they are However, recently it has been proposed that the long runout distance of usually misidentified as ejecta and not landslides. some natural flows can be explained without the need for a lubricating As a result, a number of factors combine in order for the Tsiolkovskiy mechanism and that volume of the flow is irrelevant (Lajeunesse et al., landslide to develop in the form we observe, in particular the geometry 2006; Soukhovitskaya and Manga, 2006; Staron and Lajeunesse, 2009). of the overlap relationships of Tsiolkovskiy and Fermi craters and their We plotted the dimensions of Tsiolkovskiy landslide (Table 1) on the large size. A contributing factor also may have been the lack (or nearly traditional L/H (i.e., M) and H/L (i.e., μ) verse V plots (Fig. 17), and so) of thick ejecta deposits outward of the uplifted rim of Tsiolkovskiy included for comparison data from other types of terrestrial mass crater in the ejecta forbidden zone. This may have facilitated the ready movements as well as the landslides in Valles Marineris (VM) on Mars collapse of the uplifted rim because such ejecta deposits may have help (from data previously published). As a result of concerns raised about buttressed the weight of the rim. Although, if this were a factor then it the physical meaning of M (i.e. L/H) because L varies over a much wider contribution was only minor, judging by the landslide-like flowfeatures range than H, so that the H could add nothing more than scattering to the in Mercury craters with symmetrical ejecta deposits. Alternatively, it is dependence of L on V, we have also plotted L versus V for these data also possible that the oblique nature of the impact could have produced shown in Fig. 18 (Davies, 1982; Staron and Lajeunesse, 2009). As a anomalously high rim uplift in the area of the forbidden zone. This could result, Fig. 18 shows a modified measure of mobility. The morphologic have resulted in a higher rim, substantially steepening the rim slopes parameters of the natural flowsin these plots are all measured the same and causing the ready collapse in that section of rim. However, there is way, where L is the maximum runout distance, H is the maximum fall evidence that the greatest rim uplift from an oblique impact is normal to height from the where the slide began to where it ends, and V is the final the direction of the impactors path, indicating this was most likely also volume (see Soukhovitskaya and Manga, 2006, page 349 for discussion). not a factor (Melosh, 1989; Schultz and Anderson, 1996; Pierazzo and In addition, in these figures, parameters for the entire Tsiolkovskiy Melosh, 2000). Considering the unlikelihood of these two possibilities, landslide are plotted (Table 1), and not for the different morphologic we conclude that the most likely reason for rim collapse to produce parts discussed earlier. This is because of the likelihood that the entire Tsiolkovskiy landslide was its location at the highest aspect ratio of the mass of the landslides was emplaced at roughly the same time, and that rim (rim relief to its base width), i.e., where the rim stood highest above morphologic differences within the slide were likely caused by local surrounding topography. This is most likely where over-steepening of conditions, such as roughness of the subsurface (see Iverson and Den­ the rim slopes would occur that would result its failure under its own linger, 2001) or thickness variation caused by incomplete rim collapse weight, probably immediately after crater formation. Most likely, the that affected the thickness of middle slide. debris produced by collapse of the rim both spread onto a flat, hori­ These plots shows that the V, M, μ and L of the Tsiolkovskiy landslide zontal, relatively smooth surface on the floorof Fermi crater as well as in are comparable to those of long run-out landslides on Mars in Valles places over Tsiolkovskiy ejecta (i.e., west slide). We also infer that if the Marineris (McEwen, 1989; Quantin et al., 2004; Soukhovitskaya and ejecta forbidden zone had developed elsewhere on the crater, it is likely Manga, 2006), but different than any type of the long runout mass that this landslide would resemble south slide and be easily mis­ movement on Earth. Like the landslides in Valles Marineris, the Tsiol­ identified as part of the continuous ejecta deposit. kovskiy landslide exhibits lower flow mobility (~29.6), and higher co­ efficient of friction (~0.0034) than any terrestrial mass flows. 4.4. Landslide mobility/geometry Considering that the Tsiolkovskiy landslide was emplaced demonstrably dry, this plot is consistent with the contention of McEwen (1989) that Understanding what controls the horizontal runout distance (L) and the landslides in Valles Marineris were also emplaced dry. being able to predict L of rapid natural mass movements, such as of Furthermore, as noted by earlier researchers, these plots show trends landslides, and rockslides, is an important issues because of the obvious that suggest there may be different functions for the relationship for a concern caused by their destructive power and the great distance over given V of M, L and μ of Martian landslides and terrestrial natural flows which they can travel, i.e., they can travel as much as 30 times their fall (McEwen, 1989; Quantin et al., 2004; Soukhovitskaya and Manga, 2006; height (H) (Dade and Huppert, 1998). While this goal is beyond the Staron and Lajeunesse, 2009). Fig. 18 also shows that there may be three scope of this paper, we recognize that because the Tsiolkovskiy landslide distinct and different functions of L verses V describing (1) terrestrial is a large, long run-out landslide emplaced under completely dry con­ natural flowsthat contain substantial water, (2) relatively dry terrestrial ditions (owing to its location on the Moon), its geometry provides a natural flows, and (3) the landslides in Valles Marineris on Mars. The valuable end-member for testing hypothesis of the mechanisms of Tsiolkovskiy landslide on the Moon appears to be included in the emplacement of such rapid natural mass flows. Our objective in this Martian function. The differences in these functions have been attrib­ section is to use our new measurements of the dimensions of the Tsiol­ uted to differences in rheology of the different types of flows(e.g., yield kovskiy landslide to estimate its flow efficiency (i.e., mobility and co­ strength), difference in gravity of Earth and Mars, and/or the dominate efficient of friction) during emplacement and how they compares to flow regime governing flow (McEwen, 1989; Harrison and Grimm, other natural rapid geophysical mass movements. 2003; Quantin et al., 2004; Lajeunesse et al. (2006); Soukhovitskaya and The ratio L/H of natural flowsis termed their “mobility” (M) and is a Manga, 2006; Staron and Lajeunesse, 2009, Brunetti et al., 2015).

15 J.M. Boyce et al. Icarus 337 (2020) 113464

Fig. 17. (Left) Relationship between the measured mass movement volume and mobility for extraterrestrial, non-volcanic, volcanic and submarine events (plot derived from Pudasaini and Miller, 2013, with data from Howard, 1973; Lucchitta, 1978, 1979; Voight, 1978; Crandell et al., 1984; Francis et al., 1985; Siebert, 1984; McEwen, 1989; Dade and Huppert, 1998; Capra et al., 2002; Deganutti, 2008). (Right). Plot of coefficientof friction for terrestrial and Martian landslides (data from McEwen, 1989; Scheidegger, 1973; and Hsu, 1975).

eliminates the hypotheses that the small craters in question are either drainage or degassing pits. These images clearly show that most rela­ tively fresh craters on the slide components have raised rims, hence are not drainage craters that would be rimless. In addition, these images also show that crater chains are not found along the furrows (higher porosity pathway for gas to escape) as expected if the small craters were caused by degassing, and, hence the small craters are likely not degassing pits either. However, the proposal of El Baz and Worden (1972) that the apparent crater density differences may actually be an illusion caused by the effects of differences in the thickness of the regolith on the landslide compared with that on the adjacent plains may be at least part of the answer, but not all of it. This is because the differences in the age of these terrain could be due to differences in the thickness of their regolith, which in turn may have had an effect on crater morphology. Considering the landslide is ~3.55 Ga, its regolith should average ~5 m thick, while the model age of the Pre-Nectarian plains in Fermi crater (craters > ~700 m diameter) is ~4.10 Ga implying an average regolith thickness as much as ~25 m (e.g., see Quaide and Oberbeck, 1968; Moore et al., 1980; Fa et al., 2015). We also note that even though the CSFD of craters on the Fermi plains indicate that the population of small craters (i.e., Fig. 18. Runout distance, L, verses volume of geophysical flows. Data for <~200 m diameter) was reset by the Tsiolkovskiy crater formation terrestrial dry and volcanic landslides comes from Hayashi and Self (1992), event at ~3.55 Ga, there is no evidence that the process that obliterated Evans and DeGraff (2002), Legros (2002); for Earth’s wet debris flows from the small crater on the Fermi plains would also remove the regolith (i.e., Iverson (1997), Wieczorek and Naeser (2000), Bulmer et al. (2002), Evans and no evidence of erosion, scouring, or mantling). Consequently, we as­ DeGraff (2002), and Legros (2002); and for Martian slides in VM from Quantin sume that the average regolith thickness on the Fermi Pre-Nectarian et al. (2004). plains is at least ~25 m. This factor of five difference in regolith thickness can have a sub­ 4.5. Anomalously high density of small craters stantial effect on the morphology of craters in the size range considered to be anomalous. A regolith (or ash mantle) can affect the morphology of Previous investigators noted that there may be an anomalously high craters whose depths are as much as five to ten time its thickness, < density of small (i.e., ~300 m diameter) fresh craters on the Tsiol­ causing them to be initially shallower, and appearing more degraded kovskiy landslide, and that these craters may be caused by drainage of than if they formed on a surface with no regolith or a substantially fine-grained ejecta into voids (El Baz, 1973), degassing out of the slide thinner one (Quaide and Oberbeck, 1968; Melosh, 1989; Senft and (Guest, 1971), or that the absence of a thick regolith on the compara­ Stewart, 2007). In the case of the Tsiolkovskiy landslide, where the tively young landslide caused the small craters to appear to be fresh regolith is relatively thin (i.e., ~5 m), fresh craters >~50 m diameter compared with a thick regolith on the surrounding terrain (El Baz and should form with a higher depth to diameter (d/D) ratio (i.e., see Pike, Worden, 1972). However, because no high resolution images were 1980) than those <~50 m diameters. On the Fermi Plains, where the available and no crater counts were done on these different surfaces to regolith is five times as thick, craters >~250 m diameter should also test these hypotheses, the issue was left unexplored until now. Toward form with a high d/D ratio while craters <~250 m diameter should this end, we examined high-resolution LROC images and counted craters initially form with a more subdued shape (i.e., lower d/D ratio). This to search for evidence of these processes, or ones not considered. means that relatively young craters in the diameter range of 50 m to Our crater counts (see Section 4.1) indicate that, while the small 250 m would appear sharper and morphologically fresher on the land­ crater population on the landslide may visually appear to be higher slide compared with craters <250 m diameter of the same age on the density than the surrounding terrain, it is not. The CSFD curves for small plains in Fermi crater. Hence, as proposed by El Baz and Worden (1972), craters on all parts of the landslide, Tsiolkovskiy ejecta, and those on the this would make the small crater population of the landslide to appear to Fermi Plains are essentially the same (Fig. 13). Furthermore, high- have more small fresh craters than on the Fermis plains. Since fresh resolution LROC NAC images provide observational evidence that

16 J.M. Boyce et al. Icarus 337 (2020) 113464 craters stand out visually, this could easily result in the illusion that small craters on the landslide are more abundant than those on the Fermi plains. However, differences in regolith are likely not the only cause of the apparent anomalously high number of small crater on the landslide. The high population of secondary craters also adds to this perception of a high density of small craters. This is because there is a large number of secondary crater clusters occurring in this area, whose individual craters exhibit characteristics of secondary craters produced by high velocity debris thrown great distance from the primary crater (Preblich et al., 2007), i.e., small, individual, closely-spaced, fresh, and circular craters (Fig. 19). These secondary craters are typically <200 m diameter and appear fresh on the landslide, as well as on Tsiolkovskiy ejecta deposits where the regolith is only ~5 m deep. However, the crater clusters on old terrain (e.g., the plains in Fermi crater) commonly appear morphologically more subdued and degraded compared with those on the slide (Fig. 19). The long axes of most of the clusters are in the di­ rection of several distant, relatively large, young impact craters north­ west of Tsiolkovskiy crater (i.e., Necho, King and Giordano Bruno). We believe that these secondary craters likely originate from one of these distant craters, most likely Necho because it is closest and quite fresh (Fig. 20). Consequently, we conclude that the apparent high-density of small craters on the Tsiolkovskiy landslide noted by the Apollo astronauts and previous workers is an illusion as suggested by El Baz and Worden (1972). It is the result of (1) the presence of the abundant young, sec­ ondary craters produced by high impact-angle, high-velocity debris, and (2) the effects of differences in regolith thickness on crater morphology. Fig. 20. High sun angle LROC WAC mosaic. Note the ray crater Necho located ~450 km northwest of Tsiolkovskiy. Rays from Giordano Bruno located to the 5. Summary and conclusion northwest of Necho also extend toward Tsiolkovskiy crater. North is at the top. Images are LROC WAC mosaics. While this feature has been proposed to be layered (fluidized)ejecta, (1) its similarity in morphology to huge, long runout landslides found on is unique in its enormous size. Its size is likely the result of the combi­ Earth and other planets, (2) its location in the ejecta forbidden zone of nation of several critical factors that coincidently came together in one Tsiolkovskiy crater, (3) the lack of other ejecta facies such as secondary place. These include (1) Tsiolkovskiy and Fermi craters are both huge craters and discontinuous ejecta deposits outward from it, (4) its lack of craters; (2) the segment of the rim of Tsiolkovskiy that crosses the flat lateral continuity with the rest of the continuous ejecta deposits of floorof Fermi crater was sufficientvoluminous that when it collapsed it Tsiolkovskiy crater that occur at the same distance from the rim, and (5) provided enough debris to produce an easily identified enormous the lack of evidence that the lunar target materials include the quantity landslide; (3) this rim segment formed in a place on the floor of Fermi of volatiles required to produce the degree of fluidizationdemonstrated crater where there was enough space for the landslide to flow outward by this feature are conclusive evidence that it is a long runout landslide. unrestricted and fully formed, and (4) a relatively broad ejecta Furthermore, we believe that, except for features resulting from the forbidden zone occurred in Fermi crater in a place where a substantial coincidental formation of this landslide in an ejecta forbidden zone, part of this landslide developed, indicating it is not ejecta and making it these criteria apply to landslides on other solar system bodies, such as easier to identify. Mercury (see Fig. 15), that originate from the rims of younger, relatively We have identified three major parts (i.e., north, middle, and south large impact crater where they cross the flatplains in the floorsof older, slides), with a possible fourth (i.e., west slide) of the Tsiolkovskiy large craters (Xiao and Komatsu, 2013). landslide that exhibit morphologic differences that are likely caused by Although landslides are relatively common on the Moon, this feature local conditions. All of these parts of the slide originate from a 90 km

Fig. 19. Examples (outlined by dashed lines) of clusters of small (typically <300 m diameter) craters in three areas on Tsiolkovskiy landslide (NS is north slide, MS is middle slide and WS is west slide) and nearby Pre-Nectarian Fermi plains (FP), likely to be secondary craters from a distant large, young primary crater. These craters commonly appear fresh, with high d/D ratios and raised rims, but rarely overlap or form chains with herringbone ejecta patterns typical of low velocity secondary craters (Oberbeck and Morrison, 1973). Tsiolkovskiy ejecta is TE and the crater rim is TR. North is at the top. Images are LROC WAC mosaics.

17 J.M. Boyce et al. Icarus 337 (2020) 113464 long, 18 km wide, low section (~2.4 km above the plains in Fermi crater, primary impact craters, but are frequently found in elongate clusters and and ~1.6 km below the surrounding rim of Tsiolkovskiy crater) in the have the morphology of secondary craters produced by high impact- west rim of Tsiolkovskiy crater. This low section is probably the scar left angle, high-velocity debris from distance sources. In the case of the after the rim collapsed under its own weight and slid outward as the small craters on the slide, many occur in elongate clusters whose long landslide. In addition, these parts (except for west slide) of the Tsiol­ axis points to Necho crater, which may be their source. kovskiy landslide also terminate outward in a low, rampart ridge that We can infer that at least the northern, middle and southern slide extends continuously along their distal edges, consistent with the idea segments formed at one time because they share a distal rampart. Had they were all emplaced at the same time. This also suggests that their any segment formed prior to the other parts, then we would expect to see individual morphology and size is likely due to local conditions (e.g., breaks in the distal rampart. The west slide, which may not be a land­ roughness of the terrain). slide, may have formed first because it is over-ridden by the south and The individual parts of this slide have essentially identical impact middle segments. The exact timing of the slides cannot be determined crater model ages of ~3.55 � 0.1 Ga, so that all parts of the Tsiolkovskiy with respect to the formation of Tsiolkovskiy crater, but our crater landslide either formed at one time, or alternatively, in sections very counts (Section 4.1) demonstrate that there was only a relatively short close in time. Crater counts on the ejecta of Tsiolkovskiy have a model period of time between crater formation and the landslides. It is there­ crater age of ~3.55 � 0.1 Ga, and hence, the landslide likely occurred fore possible that these events were separated by seconds/minutes or by soon after the crater formed. Crater counts in the ejecta forbidden zone thousands of years, but not tens of millions of years. However, we of Tsiolkovskiy crater in the Pre-Nectarian Fermi plains on the northwest postulate that the most likely time interval was at most a few days. rim of Tsiolkovskiy crater (in the ejecta forbidden zone) show a model age of ~4.10 � 0.1 Ga for craters >~400 m diameter, and Acknowledgements ~3.55 � 01 Ga for the population of crater <~400 m diameter. Partial resurfacing probably occurred (i.e., erasure of craters <~400 m dia.), as Thanks to Nadine Barlow for her thoughtful and insightful evalua­ indicated by the CSFD and their resultant model ages for the Fermi tion of this manuscript. Her efforts are greatly appreciated and have plains and Tsiolkovskiy rim. Because there is no morphologic evidence made this a better contribution. This work was supported under a NASA for processes such as impact erosion, scouring or blanketing to have GSFC/ASU LROC contract. PMM was supported by NASA Grant affected the smaller craters, we conclude that the partial resurfacing 80NSSC17K0307. may have been a result of seismic shaking from the formation of Tsiol­ kovskiy crater. Furthermore, the ~4.10 Ga � 0.1 Ga model age is the References same model age as a sample area in the Pre-Nectarian crater terrain in the ejecta forbidden zone found ~380 km northwest of Tsiolkovskiy. In Barlow, N.G., 2006. Impact craters in the northern hemisphere of Mars: layered ejecta and central pit characteristics. Meteoritics Planet. Sci. 41, 1425–1436. addition, none of the CSFD collected near the crater show a positive Barlow, N.G., Boyce, J.M., Cornwall, C., 2014. Martian low-aspect ratio layered ejecta bump in their curves at smaller diameters expected for contamination by (LARLE) craters: distribution, characteristics, and relationship to pedestal craters. self secondaries as proposed by Zanetti et al. (2017) and Plescia and Icarus 239, 186–200. Barnouin-Jha, O., Baloga, S., Glaze, L., 2005. Comparing landslides to fluidized crater Robinson (2019). It therefore seems likely that self secondaries either ejecta on Mars. J. Geophys. Res. Planets 110 (4), 1–22. were not produced by Tsiolkovskiy crater, or that they are all smaller van der Bogert, C.H., Michael, G., Kneissl, T., Hiesinger, H., Pasckert, J., 2015. than the lower limits of our counts, and hence would not affect results Development of guidelines for recommended lunar CSFD count area sizes via from the counts. analysis of random CSFDs. In: Workshop on Issues in Crater Studies and the Dating of Planetary Surfaces, Laurel, MD (Abstract 9023). The dimensions of the Tsiolkovskiy landslide indicate that its Boyce, J.M., Mouginis-Mark, P.M., 2006. Martian craters viewed by the THEMIS mobility is ~20.3 and coefficientof friction is ~0.049. These values are instrument: double-layered craters. J. Geophy. Res. 111, E10005 https://doi.org/ similar to landslides found in Valles Marineris on Mars, but somewhat 10.1029/2005JE2638. Boyce, J.M., Mouginis-Mark, P.J., 2018. Post-emplacement deflation of Martian layered less than those of terrestrial long runout natural mass flows emplaced ejecta from flow body thickness/rampart thickness ratio: concept study and under relatively dry conditions, and even less than wet terrestrial nat­ preliminary findings. Lunar Plant Sci. XXXXIX (Abstract 1116). ural flows(e.g., debris flows).In addition, the functions describing these Boyce, J.M., Dial, A.L., Soderblom, L.A., 1975. Summary of Relative Ages of Lunar Nearside and Farside Plains: U.S. Geol. Survey Interagency Report: Astrogeology, 66, three different situations may be different. It would not be surprising p. 26. that wet and dry mass flows behave differently, but it leaves open the Boyce, J.M., Mouginis-Mark, P.J., Tornabene, L.L., 2014. Morphology of radial grooves question of why the landslides in Valles Marineris and the demonstrably on the inner ejecta layer of Martian double layered ejecta craters. Lunar Plant Sci XXXXIV (Abstract 1589). dry Tsiolkovskiy landslide on the Moon would be similar to one another, Boyce, J.M., Mouginis-Mark, P.J., Wilson, L., 2015. Flow features on Martian layered but different than others. ejecta. Lunar Plant Sci. XXXXV (Abst. # 1043). The Tsiolkovskiy landslides appear to exhibit a high density of fresh Boyce, J.M., Wilson, L., Barlow, N.G., 2015. Origin of the outer layer of Martian low- aspect ratio layered ejecta craters. Icarus 245, 263–272. small impact craters (El Baz and Worden, 1972). However, we believe Boyce, J.M., Peter Mouginis-Mark, P.J., Robinson, M., 2016. An LROC update: the that this is an illusion caused by the effects of age related differences in Tsiolkovsky landslide. Lunar Plant Sci XXXXVII (Abstract 2471). regolith thickness on crater morphology, and on the presence of the Brunetti, M.T., Xiao, Z., Komatsu, G., Peruccacci, S., Guzzetti, F., 2015. Large landslides abundant young, secondary craters. The regolith on the Tsiolkovskiy in lunar and Mercurian impact crates. Lunar Plant Sci. XXXXIV (Abstract 1066). Bull, W., Marangunic, M., 1968. The great Alaskan earthquake 1964. Nat. Acad. Sci. Proc. landslide is ~5 m thick. This should cause craters <~50 m diameter to 1603, 309–317. appear more subdued than predicted for fresh craters of the same size Bulmer, M., Barnouin-Jha, O.S., Peitersen, M.N., Bourke, M., 2002. An empirical that form in competent material. Larger fresh craters are not affected by approach to studying debris flows: implications for planetary modeling studies. J. Geophys. Res. 107 https://doi.org/10.1029/2001JE001531. the regolith and form with the predicted shapes. However, the regolith Campbell, C., 2006. Granular material flows—an overview. Powder Technol. 162, on the surrounding old terrain is nearly 5 times as thick causing craters 208–229. <~250 m diameter to form with more subdued morphologies for the Capra, L., Macias, J.L., Scott, K.M., Abrams, M., Garduno-Monroy, V.H., 2002. Debris avalanches and debris flows transformed from collapses in the Trans-Mexican same reason. As a result, the number of fresh or relatively fresh Volcanic Belt, Mexico-behavior, and implications for hazard assessment. appearing craters between ~25 m and ~250 m diameter would be J. Volcanology and Geothermal Res. 113, 81–110. different on these terrains, and because fresh craters tend to visually Crandell, D.R., Miller, C.D., Glicken, H.X., Christiansen, R.L., Newhall, C.G., 1984. Catastrophic debris avalanche from ancestral Mount Shasta Volcano, California. stand out more than subdued craters, the surface with the thinner Geology 12 (3), 143–146. regolith appears to have more craters in this size range. 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