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2 Planetary and Space Science 79 8 (213) 1 38 Contents lists available at SciVerse ScienceDirect Planetary and Space Science journal homepage: Lunar sinuous rilles: Distribution, characteristics, and implications for their origin Debra M. Hurwitz a,n, James W. Head a,1, Harald Hiesinger b,2 a Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 2912, United States b Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Munster, Germany article info Article history: Received 28 May 212 Received in revised form 18 October 212 Accepted 31 October 212 Available online 1 January 213 Keywords: Lunar sinuous rilles Lunar rille morphologies Lunar sinuous rille formation Lunar sinuous rille ages Lunar volcanism abstract Lunar sinuous rilles (SRs) are enigmatic features interpreted to have formed as the result of lava erosion into the lunar surface. While specific SRs have been studied in detail over the past few decades, the most recent general survey of these features was conducted in 1969 using Lunar Orbiter IV and V photographs. The current global study updates the catalog of SRs, using higher resolution SELENE and LRO image and topography data to provide detailed observations and measurements of the rilles observed across the lunar surface. The new survey catalogs more than 2 SRs that vary in length from 2 km to 566 km (median length 33.2 km), in width from 16 m to 4.3 km (median width 48 m), in depth from 4.8 m to 534 m (median depth 49 m), in slope from 1.41 to.51 (median slope.21), and in sinuosity index from 1.2 to 2.1 (median sinuosity 1.19). Oceanus Procellarum contains 48% of the rilles mapped in this survey, and these rilles are typically associated with the known centers of volcanism within the Procellarum-KREEP Terrain, the Aristarchus Plateau and the Marius Hills. The current study also constrains the timing of the formation of lunar SRs, using the assumptions that the incised unit represents an upper age limit and the terminal or embaying unit represents a lower age limit. Results indicate that the distribution of ages of rille formation is highly correlated with the emplacement ages of mare units, where the majority of rilles are observed to have formed between 3. Ga and 3.8 Ga ago, though some of the features associated with the Aristarchus Plateau may have formed as recently as 1. Ga to 1.5 Ga ago. The documented observations can be used to better understand how SRs formed; for example, the range of slopes observed for all rilles ( 1.41 to.51) indicates that thermal erosion is likely dominant during the formation of the 78% of rilles that are observed to have formed in solidified mare basalt material, though mechanical erosion is likely to have been a more significant process during the formation of the 25% of observed rilles that originate in the highlands (2% of the mapped rilles crossed from the highlands into the mare), where a thicker regolith is expected to have been more easily eroded. & 212 Elsevier Ltd. All rights reserved. 1. Introduction Lunar sinuous rilles (SR) have been considered enigmatic features since they were first studied in detail with Apollo and Lunar Orbiter data collected during the 196s and 197s. Sinuous rilles are commonly characterized by channels of varying depths and widths with parallel-striking, laterally continuous walls. These features are often observed with associated depressions of various morphologies that represent potential source vents, n Corresponding author. Present address: Lunar and Planetary Institute, Universities Space Research Association, 36 Bay Area Boulevard, Houston, TX 7758, United States. Tel.: þ ; fax: þ addresses: hurwitz@lpi.usra.edu (D.M. Hurwitz), james_head@brown.edu (J.W. Head), hiesinger@uni-muenster.de (H. Hiesinger). 1 Tel.: þ ; fax: þ Tel.: þ ; fax: þ and the channels typically terminate in mare regions, either gradually fading into the mare or abruptly truncating at a contact with a younger, embaying mare unit. Initial theories for the formation of these channel-like features included surface and subsurface erosion by flowing water (e.g., Peale et al., 1968; Burke et al., 197; Schubert et al., 197), erosion by ash flows (e.g., Cameron, 1964), lava channel formation (e.g., Gornitz, 1973; Hulme, 1973), lava tube collapse (e.g., Greeley, 1971; Gornitz, 1973), gas venting along subsurface fractures (e.g., Schumm et al., 1969; Schumm, 197; McCall, 197), and intersection of fracture patterns (e.g., Oberbeck et al., 1971). As the lunar environment and surface geology are better understood, the favored hypotheses for the formation of the lunar SRs have been consolidated to include formation by lava flow and erosion, either in subsurface lava tubes that subsequently collapsed (e.g., Greeley, 1971) or in surface lava channels (e.g., Hulme, 1973; Carr, 1974; Williams et al., 2; Hurwitz et al., 212) /$ - see front matter & 212 Elsevier Ltd. All rights reserved.

3 2 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 While formation by lava flow has been generally accepted, the details of SR formation are still debated, particularly in regards to whether these features formed as the result of levees that developed as a lava flow cooled, channelizing flow in a constructed channel (e.g.,; Spudis et al., 1988; Komatsu and Baker, 1992; Gregg and Greeley, 1993), or as the result of lava eroding into the substrate via mechanical erosion (e.g., Siewert and Ferlito, 28), thermal erosion (e.g., Hulme, 1973, 1982; Carr, 1974; Head and Wilson, 1981; Wilson and Head, 1981; Williams et al., 1998, 2; Kerr, 29; Hurwitz et al., 212), or a combination of the two erosion regimes (Williams et al., 1998, 2; Fagents and Greeley, 21). A recent analysis of the formation of lunar SRs with an interpreted erosion origin (e.g., Rima Prinz, Hurwitz et al., 212) supports the theory that thermal erosion was likely to have dominated the formation of SRs when lava flowed over a gradually sloping (i.e., o3.51) surface of bedrock or cooled lava, though mechanical erosion is expected to have played a significant role in the initial stages of SR formation as lava contacted a less consolidated surface regolith layer. The implications of this analysis can be applied to any lunar SR observed to form on shallow slopes of consolidated bedrock or mare material, and thus the distribution of lunar SRs that meet these criteria must be established. Analyses of the distribution and morphology of lunar SRs have not been conducted since the initial lunar surveys that employed Lunar Orbiter images and topography (i.e., Oberbeck et al., 1969, 1971). These early analyses categorized SRs into two classes, one with 91 simple channels, or isolated channels that usually contain no tributaries, and the other with 29 complex rille systems, or channels characterized by many intersecting or branching sections, identified. Most of the complex rille systems identified are observed on floors of impact craters and are interpreted to have formed as the result of tectonic uplift and/or volcanic modification during episodes of mare flooding onto the crater floors (i.e., Schultz, 1976). Many of the remaining simple channels may represent SRs of an eroded origin, but more detailed analyses of channel dimensions were not conducted due to limitations in Lunar Orbiter data coverage. Fortunately, the recent SELenological and ENgineering Explorer Kaguya (SELENE) and Lunar Reconnaissance Orbiter (LRO) missions to the Moon have vastly improved the data resolution and coverage available for conducting lunar surveys. The current study endeavors to update the catalog of lunar SRs, specifically identifying features that are most likely to have formed as the result of lava erosion (Fig. 1). This updated catalog will facilitate analyses of the range of observed lunar SR morphologies and analyses of trends in SR locations, dimensions, and ages. Identification of the locations and ages of these features can facilitate a better understanding of how lunar SRs formed and how these enigmatic features fit into the volcanic history of the Moon. 2. Methodology 2.1. Identifying an incised lunar sinuous rille Many rilles of varied morphologies have been identified on the lunar surface, including linear rilles, arcuate rilles, and sinuous rilles. Linear and arcuate rilles are commonly considered to be graben that formed from localized tensional stress fields associated with basin formation and subsequent mascon-related lithospheric loading (e.g., Melosh, 1968, 1976; McGill, 1971; Golombek, 1979; Solomon and Head, 1979, 198) or from nearsurface dike emplacement (e.g., Head and Wilson, 1993). These tectonic features tend to have consistently straight or arcuate, parallel-striking walls bounded by steep, inward-dipping normal faults (e.g., McGill, 1971), and these features typically ignore topographic obstructions, cutting across lunar mare and highland terrains and typically exhibiting little change in propagation direction (Fig. 2a) but often exhibiting an increase in width as the tectonic rille cuts into highland materials (e.g., McGill, 1971). These linear and arcuate tectonic features are not included in the current survey. By contrast, sinuous rilles typically exhibit varying degrees of sinuosity while maintaining the parallel-striking, laterally continuous walls typical of rilles in general. In addition, SRs tend to avoid topographic obstructions, forming along topographic margins rather than cutting across topographic boundaries. However, these characteristics are consistent with any lava unit flowing on or in the lunar surface, and thus other morphological traits must be identified to distinguish between a leveed channel, a subsurface lava tube, and a surface incised SR. For example, a leveed channel typically forms within a surface lava flow unit that cools, forming levees that bound the fastest-flowing portion of the lava flow (e.g., Hulme, 1974). The resulting leveed channel is expected to be relatively shallow, and the marginal levees are expected to remain preserved even after the lava flow has solidified (Fig. 2b). Alternatively, a subsurface lava tube is characterized by a structurally stable roof that remains intact after lava has evacuated the tube. Surface expressions of lava tubes can be observed when the roof collapses, leaving behind mirrored arc segments on opposing sides of a pit or tube skylight. Multiple collapses along a tube may result in the formation of aligned skylights, but each skylight is expected to be bounded by these mirrored arc segments, resulting in non-parallel-striking walls (Fig. 2c). Fig. 1. Global map of sinuous rilles. Global map of SRs identified across the Moon, shown with a LROC WAC global mosaic.

4 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) km 1 N 1 km 2 5 km 1 2 km 4 Fig. 2. Examples of various types of rilles observed on the Moon. (a) Tectonic graben (e.g., features in this WAC image centered at 18.31S, E) typically have parallel-striking, steep walls that cut across topographic obstructions as they propagate in generally straight or arcuate paths across the surface. (b) Constructed channels (e.g., features in this LO image centered at 241N, E) tend to form within thin surface lava floods, forming levees that channelize the fastest-moving lava (white arrows). These channels tend to be shallow, and they lack associated source depressions. (c) Surface expressions of lava tubes (e.g., feature in this WAC image centered at 34.71N, E) are observed as aligned pits that represent potential skylights where the roof of the lava tube has collapsed. (d) Eroded surface lava channels called sinuous rilles (e.g., features in this WAC image centered at 151S, E) tend to have parallel-striking, laterally continuous walls that lack marginal levees and bound a relatively deep channel that incises the substrate. Eroded surface channels tend to avoid topographic obstructions, flowing along mountain bases rather than cutting through them. With these characteristics of constructed lava channels and subsurface lava tubes in mind, SRs interpreted to be eroded surface lava channels are defined as relatively deep channels that incise into the substrate but avoid topographic obstructions, with parallel-striking, laterally continuous walls that lack marginal levees and that meander to some degree across the substrate (Fig. 2d). Eroded surface lava channels are expected to have associated source vents, though sources that originate in mare terrain may have been embayed by subsequent mare emplacement. These distinguishing characteristics are used to identify the SRs observed on the lunar surface. For consistency, future references to sinuous rilles (SRs) in this article refer to the entire feature in question, including the potential sources, channels, and channel walls that may be associated with the feature, whereas references to channels, for example, refer specifically to the channel segment of the SR in question Mapping method The purpose of the current study is to map and analyze the SRs observed on the lunar surface with the most recently acquired image and topography data. The initial phase of the survey, during which the locations of potential SRs are identified, was conducted using the LRO Wide Angle Camera (WAC) global mosaic with a resolution of 1 m/pix (Robinson et al., 21). The mapping phase of the survey was conducted using Kaguya Lunar Imager and SpectroMeter (LISM) Terrain Camera (TC) images with a resolution of 6 m/pix (Ohtake et al., 2), using LRO Lunar Orbiter Laser Altimeter (LOLA) track and gridded data (Riris et al., 28; Smith et al., 21), and using Clementine UltraViolet/Visual (UVVIS) composition data (Nozette et al., 1994). The LOLA gridded data was geoid-corrected using a 4 ppd geoid map resampled to 64 ppd (Goossens et al., 211). The LISM TC images provide high-resolution context that is optimal for mapping, measuring, and analyzing each SR in detail. All mapping was completed in the geographic mapping software program ArcMap, and measurements were calculated using a geodesic toolkit to account for the latitudinal variations of the observed SRs. The specific quantitative measurements and qualitative observations collected for each identified feature are described below; measurements are summarized in Table 1, and observations are summarized in Table A Measurements collected Sinuous rilles observed on the lunar surface vary widely in morphology. Measurements of channel length, width, depth, and sinuosity as well as source volume and regional slope were collected for each SR to characterize the range of SR dimensions and to identify potential trends in morphology. Channel length is defined as the average length of the two parallel-striking walls of each SR; each wall was mapped individually, and the calculated lengths for the walls were averaged. This average length and the deviation between the two wall lengths are noted in Table 1. Channel width is defined as the distance between the tops of the channel walls, with the true channel width oriented

5 4 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 perpendicular to the direction of channel propagation. Channel width was measured at several points along the length of each SR, and the widths were then averaged to get a characteristic width of each SR. These average widths and the standard deviations of width measurements are documented for each SR in Table 1, though the individual widths along each channel were retained for further analysis. Channel depth is defined as the difference in elevation between the terrain surrounding the SR and the bottom or thalwag of the channel. The observed elevations of each channel wall were averaged in order to remove regional topographic influences from the depth measurements. Elevations of channel walls and thalwags were measured at several points along the length of each SR using LOLA track data, with track points spaced at 15-m increments along the lunar surface. Depths calculated along the length of the channel were averaged to get a characteristic depth of each SR. These average depths and the standard deviations of depth measurements are documented for each SR in Table 1, though, like the width measurements, the individual depths along each channel were retained for further analysis. Channel sinuosity is defined for terrestrial fluvial channels as the ratio of the channel length, as measured above, and the meander-belt axis length (Brice, 1964; Le Roux, 1992). Sinuosity values for terrestrial fluvial channels typically range from 1 to 5, with a theoretical maximum sinuosity value of 5.24 (Le Roux, 1992). Classically, the meander-belt axis is a straight or arcuate line that dissects the zone of sedimentary structures such as scroll bars and ox-bow lakes that form as a sinuous channel develops (consult figures in Le Roux, 1992). Lunar SRs typically lack this zone of sedimentary structures, and thus the meander-belt axis was mapped approximately to dissect the meanders for each channel. Additional deference was paid to topographic obstructions, so that the meander-belt axis was mapped to avoid topographic obstructions in the same manner as the SR is expected to avoid the same obstruction (for example, Fig. 3). Sinuous rille sinuosity was then calculated as the ratio of the measured channel length and the meander-belt length; sinuosity values are documented in Table 1. Source depression volume was determined by mapping the area bounded by the rim of the observed depression and by measuring the depth of the depression. The depth was measured in a similar method to the depth of the channel, by averaging the elevations of the bounding depression rims as observed using LOLA track data and then comparing the average rim elevation with the elevation at the bottom of the depression. Several depth measurements were averaged to determine the characteristic depth of the depression, and this depth was multiplied by the source area to determine the approximate volume of the deposit (Table 1). The regional slope in this study is defined as the gradient of the material surrounding the SR. This gradient is considered to represent the slope of the uneroded, pre-sr terrain, on which the SR formed. The regional slope was measured as the change in topography between the source and the terminus of the SR as measured using LOLA gridded data, along a horizontal distance similar to the meander-belt axis length defined above. This nondimensional slope was converted to degrees, and the regional slope associated with each SR is documented in Table Observations made The measurements described above represent the quantitative observations documented for each SR identified in the survey. Additional qualitative observations were made for each SR in order to more completely analyze the range of morphologies observed. These observations can be separated into several classes, including the general coordinates, age, and surface composition, the source morphology and location, the channel morphology and location, channel preservation such as associations with tectonic and impact features, and relative changes and trends in channel morphometry. Observations are summarized in Table A1. The coordinates of the source or observed head of each feature were documented for each SR, and the impact basin or topographic basin most closely associated with each feature was documented to provide global context for the mapped SRs. In addition, upper and lower age limits for each SR were documented using mare unit crater-count ages (Hiesinger et al., 2, 21, 211; Whitten et al., 211) in an attempt to constrain the timing of formation of lunar SRs. The age of the unit incised by each SR is considered to represent an upper age limit for the formation of a given feature, and the age of the unit that embays 3 6km Fig. 3. Example of channel length vs. meander-belt axis. (a) A sketch of the highly sinuous Murrumbidgee River of New South Wales. The active channel is represented by the narrow double solid line in the center of a plain of sedimentary features such as ox-bow lakes, noted by dotted lines. The straight solid line that approximately dissects the plain of sedimentary features represents the meander-belt axis used to determine channel sinuosity by Le Roux (1992). The Murrumbidgee River was observed to have a sinuosity index of 2.1. This panel is from Fig. 2 in Le Roux, (b) A segment of Rima Marius in central Oceanus Procellarum with an example of a meander-belt axis as mapped in the current study; the solid lines trace the channel of the SR, and the dashed line represents the meander-belt axis. This rille has a sinuosity index of 1.2.

6 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) or lies at the terminus of each SR is considered to represent a lower age limit for the formation of the given feature. These observations were made to facilitate analyses of spatial and temporal trends in SR formation, to identify, for example, which basin contains the most SRs, the longest SRs, and the youngest SRs. The composition of both the incised unit and the unit at the terminus of each SR were noted using the Clementine data set. It should be noted that only general compositions are described (i.e., red highlands material vs. orange/yellow lower-ti mare material vs. blue/green higher-ti mare material), and more detailed analyses of specific spectra of each unit are necessary to gain further insight into the complex compositions of surface materials associated with the observed SRs (e.g., Lucey et al., 2). The morphology of the source depression associated with a given SR was characterized as either circular, elongate, or irregular, and the location of the source was documented as either in mare material, in the highlands, or on the ejecta or rim of a specified basin or crater. Observations of source morphology and location can be used to identify trends in locations that preferentially facilitated eruptions required to form SRs and that preferentially preserved the sites of those eruptions. The morphology of a channel associated with a given SR was documented by noting, for example, whether the channel has a nested channel or an associated chain of pit craters aligned with the mapped channel. The location of the channel was also documented as either in the center of the mare, at the edge of a mare deposit, or in the highlands. The degree of channel preservation of each SR was also considered, specifically noting whether the source and/or terminus of the SR is superposed or altered by subsequent lava emplacement, ejecta emplacement, or tectonic deformation. Observations of channel morphology, location, and preservation can be used to analyze, for example, trends in the distribution of the freshest and longest SRs, and whether SRs that form on positive gradients are typically deformed by wrinkle ridges. Trends in SR morphometry that might be considered include significant changes in width, depth, and slope along the length of the feature. Significant changes in width and depth might correlate with significant changes in slope, for example in cases where the SR originates in highland material and terminates in mare material. In these cases, a SR that originated in highland material might have been expected to erode more efficiently into a substrate characterized by thicker layers of poorly-consolidated regolith material. If this SR then continued onto a more consolidated mare subsurface, erosion efficiency might have been expected to decrease, resulting in the formation of a more narrow and shallow channel. A significant change in width and depth is considered to have occurred if the standard deviation of a given measurement exceeds 5% of the average measurement, and a significant change in slope is considered to have occurred if the channel is observed to cross from the highlands into mare material. Additional qualitative observations are also documented in Table A1, including whether a SR is considered wide (width4 1 km), highly sinuous (sinuosity41.5), or formed as the result of construction or as the result of impact melt rather than lava erosion. The observations made during the survey of lunar SRs are presented in Section 3, and implications of these observations are investigated in Section Observations and results 3.1. Global results This survey of LROC WAC and SELENE LISM images resulted in the observation of 194 distinct SRs of a variety of morphometries and morphologies (Tables 1 and A1). These 194 features represent a significant increase in the number of SRs identified previously (i.e., 91 SRs identified by Oberbeck et al. (1969, 1971)), an improvement that is due primarily to the increase in available image resolution. The SRs observed in the current study tend to lie within the Procellarum-KREEP Terrain on the lunar nearside between longitudes of 311E and 31E and at latitudes equatorward of 551N and 31S, with 75% of SRs located between 311E and 31E (Fig. 4a) and 65% of SRs located in the lower northern latitudes between 1 and 351N (Fig. 4b). These concentrations of SRs correspond to the mare regions observed on the Moon with a particular abundance in volcanic centers in Oceanus Procellarum (e.g., Whitford-Stark and Head, 198). The age of a SR s formation (Fig. 5) is estimated based on the age of the unit the SR incises (the upper age limit), and on the unit age at the terminus of the SR (the lower age limit). The majority of identified SRs (8%) is observed to incise into material that was emplaced between 3 Ga and 3.75 Ga ago, with only 8% of incised material dating to the most recent 2 Ga of lunar history. The oldest incised surface was identified to have been emplaced 3.75 Ga ago (Imbrian epoch) and the youngest incised surface was identified to have been emplaced 1.14 Ga ago (Eratosthenian epoch). While the majority of identified SRs is observed to have terminated in material emplaced between 3 Ga and 3.75 Ga ago (56%), a significant percentage of SRs (24%) is observed to terminate in material emplaced within the most recent 2 Ga of lunar history. The oldest unit at a SR terminus was identified to have been emplaced 3.75 Ga ago (Imbrian epoch) and the youngest unit at a SR terminus was identified to have been emplaced.8 Ga ago (Copernican epoch), with the youngest material being concentrated in the central Oceanus Procellarum (Hiesinger et al., 2, 21, 211) and Lacus Autumni in northeastern Orientale basin (Whitten et al., 211). These age distributions indicate that the majority of SRs was most likely to have been emplaced during the Imbrian and early Eratosthenian epochs of lunar history, though 1 25% of SRs observed predominantly in central and eastern Oceanus Procellarum were likely to have been emplaced more recently, in the late Eratosthenian epoch of lunar history Ranges in channel morphometry The identified SRs are observed to have a wide array of morphometries, varying in length, width, depth, slope, and sinuosity. Sinuous rilles are observed to range in length from 2.1 km to 566 km (average length, 67.7 km; median length, 33.2 km), with more than 85% of observed SRs characterized by lengths less than 1 km and 64% of observed SRs characterized by lengths less than 5 km (Fig. 6a). Sinuous rille widths are observed to range from 16 m to 4.3 km (average width, 64 m, median width, 48 m), with 88% of observed SRs characterized by widths less than 1 km and 51% of observed SRs characterized by widths less than 5 m (Fig. 6b). Channel depths are observed to range from 4.8 m to 534 m (average depth, 72 m, median depth, 49 m), with 79% of observed SRs characterized by depths less than 1 m, and 5% of observed SRs characterized by depths less than 5 m (Fig. 6c). Sinuous rille depths have a linear relationship with observed SR widths (Fig. 6d), a trend that is characterized by a line with a more gradual slope of 6.3 than observed in previous analyses of a more select population of six SRs (i.e., 13.3; Gornitz, 1973). This trend is consistent with erosion efficiency increasing proportionally in both vertical and horizontal orientations during the higher-flux eruption that is expected to have been required in the formation a larger SR. The regional slopes of the terrain adjacent to the observed SRs are observed to range from 1.41 to þ.51 (average slope,.31, median slope,.21; Fig. 7a), with 96% of observed SRs currently

7 Table 1 Sinuous rille locations and measurements. 6 Rille Basin Segment Lat Lon Length (km) unc (km) Width (km) stdev (km) Depth (m) stdev (m) Slope (deg.) Sinuosity Source Area (km 2 ) Source Depth (m) Source Volume (km 3 ) Description of Rille 1 Procellarum Total Rille east of southern extent of Rima Herigonius in S Procellarum, east of mountains east of rille 5 2 Procellarum Total Long rille that may have source disrupted by wrinkle ridge east of Herigonius in SE Procellarum/NW Nubium 3 Procellarum Total Small rille west of Bulliardus crater in southeastern Procellarum/NW Nubium 4 Procellarum Total Long skinny rille that originated in SE Procellarum into NW Mare Nubium 5 Procellarum Total Rille that formed east of and parallel to southern Rima Herigonius in S Procellarum 6 Procellarum Total Rille east of northern part of Rima Herigonius that appears to have originated in ejecta of Herigonius crater 7 Procellarum W W branch of Rima Herigonius branch 7 Procellarum E/C E/central branch of Rima Herigonius that originates in a wrinkle ridge branch 7 Procellarum E branch E most branch of Rima Herigonius 8 Procellarum Total Rille that formed along a mound NW of Mare Cognitum, very small. 9 Procellarum Total Potential rille in raised terrain east of an embayed crater in SW Procellarum 1 Procellarum Total Strange inverted potentially leveed channel southeast of Lambert crater, near Luna 5 landing site; varies from 1þ m tall, to 6 m in depth 11 Procellarum Total Channel east of Lambert crater, with a circular source, starts as fairly narrow channel but then, after a disruption by a secondary crater chain, the channel widens; this channel formed just N of the inverted feature 1 12 Procellarum Total Channel south of Kepler crater that is embayed on both ends 13 Procellarum Total Small sinuous rille west of Rima Seuss that lies between two wrinkle ridges, one at the apparent head of the channel is more prominent than the other to the S 14 Procellarum Total Small rille west of Rima Seuss that might be a continuation of rille 13 on the neast side of a wrinkle ridge 15 Procellarum Total Small rille west of Rima Seuss that might be a continuation of rille 13 and rille 14 that is superposed at its terminus by ejecta associated with an impact crater 16 Procellarum Total Small sinuous rille west of Rima Seuss that curves around a small impact crater and fades into the mare, formed north of previous rilles Procellarum Total Skinny channel southwest of Kepler crater apparently embayed on both ends 18 Procellarum Total Channelized feature north of an impact crater SW of Procellarum basin, potentially formed from impact melt 19 Procellarum Total Small rille in central Procellarum basin that is deformed by a wrinkle ridge 2 Procellarum Total Wide rille that potentially pools west of a wrinkle ridge before incising across wr, south of Marius crater 21 Procellarum Total Small rille near the south end of the Marius Hills, near a crater that superposes the rille, covering the central portion with ejecta 22 Procellarum Total Small narrow rille southeast ofsource of Rima Seuss 23 Procellarum Total Rima Seuss in central Procellarum 24 Procellarum Channel Highly sinuous rille that extends from a series of potential tube skylights that originate in same source as Rima Seuss and extend north and east into central Procellarum; potentially has same source as Rima Seuss, rille Procellarum Total Small rille south of previous rille 27; appears to be embayed on either end, and it might have formed along the southeastern edge of a cone associated with the Marius Hills D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38

8 26 Procellarum Total Skinny rille east of the Kepler crater, Rima Milichius 27 Procellarum Total na na Very long skinny rille south and west of Rima Galilei in central Procellarum, on SW edge of Marius rise; superposed by many secondaries, but part of the middle section may be either erased by secondaries or extended in substrate as lava tube 28 Procellarum Total Small rille that originates in a cone within Marius Hills in central Procellarum 29 Procellarum Western Western branch of Rima Galilei in central Procellarum west of Marius Hills 29 Procellarum Eastern Eastern branch of Rima Galilei in central Procellarum west of Marius Hills 3 Procellarum Total Large but abruptly embayed rille on the SE margin of Marius crater 31 Procellarum Channel Rille that forms along a cone in Marius Hills, flows north and fades into impact ejecta 32 Procellarum Total Small degraded rille that originates in the W ejecta of Marius crater 33 Procellarum Total Rille west of Marius rise that changes in width along the rille length, is clearly embayed on both ends, and is superposed by ejecta from Kepler to the southeast 34 Procellarum Total Small rille that may begin as a lava tube in the NE ejecta of Marius crater 35 Procellarum Total Possible rille northeast of rille 31 on the southwest flank of the Marius rise 36 Procellarum Channel Skinny rille west of Marius Hills that ends abruptly at a superposed crater and that may begin as a series of tubes 37 Procellarum Total Short rille in Marius Hills that ends in a potential tube, only one depth measurement made that is unaffected by superposed impact crater ejecta 38 Procellarum Total Skinny rille west of the Marius Hills, potentially terminus of rille 36 that is interrupted by a superposed impact crater 39 Procellarum Channel Potential very short rille west of Marius Hills that begins as a channel and ends potentially as a tube, only one depth measurement available for each source, channel, and potential skylight 4 Procellarum Total Rille just south of previous rille 43 that originates in Marius Hills, starts out deep, gets shallower, then deep again 41 Procellarum Total prominent rille with large source depression that is embayed by mare 42 Procellarum Total potential huge very sinuous rille that formed on the eastern flank of the Marius rise that is partially obscured by mare 43 Procellarum Total Rille that originates in Marius Hills 44 Procellarum Total Short rille east of the Marius rise embayed by mare 45 Procellarum Total Rima Marius in central Procellarum, with several very narrow channels that branch off of the main channel 46 Procellarum Total na na Long prominent rille north of Rima Marius that both cuts through a wrinkle ridge and is cut by a different wrinkle ridge, superposed by secondaries from Aristarchus crater 47 Procellarum Total Narrow small rille west of Marius rise south of small cone 48 Procellarum Total na na Small rille in SW rim of Eddington crater in W Procellarum 49 Procellarum Total Rille heavily deformed by Aristarchus secondaries, E of Aristarchus crater 5 Procellarum Total Small rille in southern Aristarchus Plateau, south of Schroters Valles 51 Procellarum Total Small rille in southern Aristarchus Plateau, appears to be deformed by wr 52 Procellarum Total Rille on Aristarchus Plateau south of Schroters Valles 53 Procellarum N branch N branch of branching sinuous rille in SW Aristarchus plateau 53 Procellarum S branch S branch of branching sinuous rille in SW Aristarchus plateau 54 Procellarum Total na na Small rille in SW of Aristarchus plateau 55 Procellarum Outer na na na Schroters Valles on Aristarchus Plateau; ends apparently still on plateau; very wide valley, not terribly sinuous but with large kink, turning from SE/ NW trend to NE/SW trend 55 Procellarum Inner na na na Inner channel of Schroters Valles; highly sinuous, often hugs valley walls, continues outside of valley where valley ends, cutting into and beyond south valley wall to enter mare 56 Procellarum Total Short rille on SW Aristarchus plateau with small source depression 57 Procellarum Total Potential rille just north of Aristarchus crater, heavily deformed by Aristarchus secondaries 58 Procellarum Total Long but subdued rille just east of Aristarchus-Harbinger area, no evidence source depression and altered by impact, but not necessarily flooded? D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213)

9 Table 1 (continued ) 8 Rille Basin Segment Lat Lon Length (km) unc (km) Width (km) stdev (km) Depth (m) stdev (m) Slope (deg.) Sinuosity Source Area (km 2 ) Source Depth (m) Source Volume (km 3 ) Description of Rille 59 Procellarum Total Prominent but short rille on southern edge of Prinz Rise 6 Procellarum Central Rima Aristarchus, central branch; circular source depression, terminus cuts across mare plains that embay Rimae Prinz, Beethoven; separate branch width is 1315 m 61 Procellarum Total Smaller rille on southern edge of Prinz Rise 62 Procellarum Eastern Rima Aristarchus, eastern branch; independent circular source depression, merges with the central branch; relationships with central branch is not clear, could be younger than central branch because of sharp morphology at merger; separate branch width is 922 m 63 Procellarum Outer Rima Prinz with outer and inner channel (dimensions noted for outer channel); circular source depression; inner channel apparently only extends partway along observed channel. 64 Procellarum Total Rima Aristarchus, northwestern branch; independent potential circular source though not as large as previous two, merges with other two branches lower in central branch than southern branch; appears to be youngest branch because it blocks off other branches at merger; separate branch width is 613 m 65 Procellarum Total Rima Beethoven cuts through Harbinger Mts 66 Procellarum Total Rima Handel on Harbinger Prinz rise, circular source depression and clearly embayed at terminal end 67 Procellarum Total Truncated rille north of Rima Beethoven, may merge with Rima Beethoven. 68 Procellarum Total Small narrow rille that feeds into Rima Handel from the east 69 Procellarum Total Small channel northwest of NW branch of Rima Aristarchus 7 Procellarum Total Rima Telemann on Harbinger Prinz rise, no visible source depression, likely embayed, and clearly embayed at distal/terminal end of channel 71 Procellarum Total Short rille in eastern Harbinger area, starts very straight, likely as a lava tube or tectonically influenced, then when it hits topography it becomes very sinuous; at that point the slope decreases from.73 to.51 deg 72 Procellarum Total Small channel north of rille Procellarum Total Rille NE of rille 72 that is likely longer than mapped, but it is heavily covered/ modified by secondaries/impact ejecta from a crater to the north; channel has irregular source depression 74 Procellarum Western W branch of rille north of Schroters Valles, superposed by secondaries from Aristarchus, and ends abruptly when it gets off the Aristarchus plateau and encounters the mare; circular source depression that might have two aligned depressions 74 Procellarum Eastern E branch of rille north of Schroters Valles, superposed by secondaries from Aristarchus, seems to lack a source depression 75 Procellarum Total Long skinny rille NW of Rima Aristarchus that flows north into Oceanus Procellarum, west of Rima Krieger; part of it appears to be concealed 76 Procellarum Total Rille west of the previous rille, north of Aristarchus; widest where rille cuts into Aristarchus Plateau (ave depth 194 m), narrows when intersects the mare (ave depth 44 m), and then abruptly appears to shallow and narrow further (ave depth 2 m)ypotentially indicating point at which erosion no longer is efficient/occurring 77 Procellarum Total Rille with continuous wall west of rille 75; clearly overlain by secondaries, likely from Aristarchus 78 Procellarum Total Rima Krieger, that begins in Krieger crater and continues into Oceanus Procellarum 79 Procellarum Total Highly braided, short channel west of rille 8 8 Procellarum Total Wide potential rille north of Schroters Valles, embayed on both sides, exposed only on an embayed portion of Aristarchus plateau 81 Procellarum Total Long sinuous rille in mare NW of Aristarchus Plateau; appears to be deformed by a wrinkle ridge D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38

10 82 Procellarum Total Sinuous rille west of embayed bar of highland material off of northwest end of Aristarchus plateau 83 Procellarum Total Rille with somewhat elongate source depression; most likely a collapsed lava tube with many aligned pit craters, some areas have clear levees or potential overflow/constructed lava bridges 84 Procellarum Total Small short rille in NE Procellarum with little erosion but potentially no embayment; near Gruthuisen Domes 85 Procellarum Main branch 85 Procellarum Main branch Prominent rille in NE Procellarum, unclear whether rille has a source depression, but there appear to be potential volcanic edifices in the potential source depression unlike any depression seen before; should be targeted with NAC; several branches; at one point appears deformed by a wrinkle ridge, but branch might not be Source depression from 85 could be a terminus of a channel; source could be in north and feed two channels; this one to the west and south, and a second one below to the east. The description for this channel is the same as 85 but with only one branch and with the potential wrinkle ridge deformation; potential depression in the north rather than in the south? Rima Sharp Same as 85; this is the smaller channel that flows to the east from the potential source, branching significantly as it rounds a bend of topography. 85 Procellarum Main branch 86 Procellarum Total Small rille in NE Procellarum with potential elongate source depression unconnected to short channel that hugs topography; appears to flow from NW to SE; near Gruthuisen Domes 87 Procellarum Total Short but prominent rille on W edge of W Imbrium ejecta; could be embaying ejecta of nearby crater or could be cut off by that same crater, unclear stratigraphic relationships 88 Procellarum Total Small rille in N/central Procellarum with streamlined island near visible channel terminus 89 Procellarum Total Skinny, shallow rille with circular source near N/E Procellarum, just west of a large prominent rille in which this rille terminates; rille is superposed by impact craters that hide some of the walls. 9 Procellarum Total Skinny rille in N/central Procellarum that is likely at least partially a collapsed lava tube 91 Procellarum Total Small rille NW of Imbrium that might hav been partially covered by a crater 92 Imbrium Total E wall of prominent rille S of Imbrium, Rima T. Mayer 93 Imbrium Total Long rille in ejecta of Copernicus, possible source; clearly predates ejecta 94 Imbrium Total Long skinny rille in ejecta of Eratosthenes in S Imbrium; possibly an impact melt channel? May predate Eratosthenes ejecta; depth represents upchannel deeper depth, downchannel shallower depth (23þ 5.6) and full channel depth (49.4þ 31.2) 95 Imbrium Total Small rille in ejecta of Copernicus, might extend farther north but unclear in ejecta 96 Imbrium Total Long skinny rille in SE Imbrium basin that is superposed by a 5.7 km diam crater, rille has long skinny source depression in S 97 Imbrium Total Small but relatively wide rille in middle of mare unit, beginning of rille is near topo rise but appears to start off the rise, flow up the rise, then down the rise, suggesting the rise formed after the rille 98 Imbrium Total Possible rille southeast of SE rim of Imbrium 99 Imbrium Total Very small but wide rille at bottom of topo high N of rille 18, possible continuation of rille 18? Appears embayed on both ends, maybe evidence of collapse at N endylava tube? 1 Imbrium Total Small rille south of Rima Euler, completely embayed, no sign of source or deposits 11 Imbrium Total Small rille south of Rima Euler, completely embayed, no sign of source or deposits, NE of previous rille Imbrium Total Small rille west of Euler crater and east of Rima Brayley; channel may end before mapped terminus, lose S wall of channel near end; no source 13 Imbrium Total Rille that splits at topogrpahy and flows NW, SW of Mozart Rille, source possible D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213)

11 1 Table 1 (continued ) Rille Basin Segment Lat Lon Length (km) unc (km) Width (km) stdev (km) Depth (m) stdev (m) Slope (deg.) Sinuosity Source Area (km 2 ) Source Depth (m) Source Volume (km 3 ) Description of Rille 13 Imbrium Total Rille that splits with 13 at topography and flows SE 14 Imbrium Total Very long skinny rille south of and older than Euler crater: Euler Rille 15 Imbrium Branch a Main branch of very long rille in SW Imbrium, Rima Brayley? Interesting part of rille, possible cataract as lava flowed over tectonic feature? Rille intersects a wr later as well, appears to be deformed by wr in this case.: Embayed at both ends? 15 Imbrium Branch b Secondary branch of Rima Brayley, consistently narrower (segments are 229 m vs. 359 m) and shallower, likely formed before a since more erosion occurred since; main branch is the same for both segments, equalizes averages: Embayed at both ends? 16 Imbrium Total Very small rille N of rille 13 that ends abruptly in a steep drop to a valley 17 Imbrium Total Long skinny rille that originates near a wrinkle ridge, no obvious source depression, ends in mare to north 18 Imbrium Total Short rille SE of the middle of the main branch of Rima Brayley 19 Imbrium Total too subdued to measure Too subdued for measurements: constructed? 19 Imbrium Total too subdued to measure Too subdued for measurements: constructed? 11 Imbrium Branch a Eastern branch of channel NW of Euler crater, very sinuous with tributaries 11 Imbrium Branch b Western branch of channel NW of Euler crater 111 Imbrium Total too subdued to measure Very subdued, sinuous channel just west of rille 11 and Euler crater; too subdued for measurements: constructed? 112 Imbrium Total Hadley Rille, begins at edge of highlands and flows along edge of mare in eastern Imbrium 113 Imbrium Total Rima Vladimir (Rima Mozart?), begins and ends in pits that appear aligned with tectonic graben 114 Imbrium Branch a Main branch of very long sinuous rille south of Diophantus Crater, Rima Diophantus; both ends appear to be embayed, unclear which direction flow is but most likely into Imbrium 114 Imbrium Branch b Secondary branch of Rima Diophantus, very minor feature, not measured independently 115 Imbrium Total Relatively large sinuous rille in W Imbrium N of Rima Diophantus: Rima Delisle 116 Imbrium Total Short rille north of Rima Delisle, rille 35 in W Imbrium 117 Imbrium Total Short rille that begins at a wrinkle ridge north of Rima Delisle, rille Imbrium Total Substantial rille north of rille 116, may cut/flood a crater? Interesting texture outside of crater, should check with NAC 119 Imbrium Total Relatively small rille that originates in a crater just north of rille 12 Imbrium Total Relatively prominent rille with several potential source areas; possibly very interesting textures in the channel floor and outside the channel, worth looking at in NAC 121 Imbrium Total Relatively wide channel with no defining start or end in NW Imbrium interior; might flow around a small impact crater in path 122 Imbrium Total Relatively wide channel with no defining start or end, E of wrinkle ridge in NW Imbrium 123 Imbrium Total Small rille along NW margin of Imbrium, may originate from tectonic graben 124 Imbrium Total Small rille SW of Sinus Iridum 125 Imbrium Total Narrow channel with no obvious source in N. Imbrium basin, might have been deformed and disconnected by wrinkle ridges 126 Imbrium Total Small rille near terminus of rille 13 with independent source, tectonics influence 127 Imbrium Total Narrow chan with no obvious source in N. Imbrium basin, hugs edge of mare lava 128 Imbrium Total Narrow chan with no obvious source in N. Imbrium basin, may connect previous two channels, deformed by wrinkle ridges D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38

12 129 Imbrium Total Long rille in large tectonic(?) valley NE of Imbrium, not completely continuous, might be example of partial tube, partial rille 13 Imbrium Total Large rille with large source SE of Plato in NE Imbrium 13 Imbrium Total Second rille near source of rille 13, no source 13 Imbrium Total Third rille with independent small source that intersects rille Imbrium Total Small channel in highlands north of Imbrium just east of rille Imbrium Inner Inner channel of rille west of Plato in N Imbrium, inner chan flows from highlands into mare 132 Imbrium Outer Outer channel of rille Imbrium Total Channel in highlands north of Imbrium, meanders south near highlands/ mare boundary 134 Imbrium Total Small rille in highlands north of Imbrium 135 Imbrium Total Small rille that flows down from highlands N of Imbrium, splits in a valley, then possible ends in a pond still above the mare 135 Imbrium Branch b W branch of rille that branches in a valley N of Imbirum 136 Imbrium Branch a Short rille east of rille 135 that flows out of a crater into same pond as Imbrium Branch b Second branch of rille Imbrium Total Channel in highlands north of Imbrium, narrows substantially near terminus 138 Imbrium Total Wide rille north of N Imbrium 139 Imbrium total Rille with large source depression east of Plato in N Imbrium 14 Imbrium Total Small channel NE of Plato in N Imbrium 141 Orientale Total Sinuous rille in southern Orientale rim that intersects a graben and then narrows substantially as it encounters mare 142 Orientale Total Wide rille in small mare pond in eastern Orientale 143 Orientale Total na na Rille in central Orientale with a circular source 144 Orientale Total Small rille in central Orientale basin east of rille Orientale Total Small subdued rille in east central Orientale that cuts through rough terrain 146 Orientale Total Long skinny heavily sinuous rille in eastern Orientale 147 Orientale Total Skinny rille in Lacus Veris in northeastern Orientale basin, superposed by secondaries from Maunder crater to the southwest 148 Orientale Total Short wide rille in highlands off of Lacus Autumni in northeastern Orientale 149 Orientale Total Very faint rille in northeastern Orientale basin 15 Orientale Total Skinny rille in Lacus Autumni in northeastern Orientale 151 Orientale Total Short skinny rille in Lacus Autumni in northeastern Orientale 152 Orientale Total Very long very skinny sinuous rille in northern Orientale that originates in an embayed crater 153 Orientale Total Sinuous rille that begins in rim of northern Orientale basin and forms down the wall and changes greatly in width after it pooled and crossed topographic boundary 154 Serenitatis Total Long rille connecting Mare Tranquilitatis and Mare Serenitatis 155 Serenitatis Total Potential rille on rim of crater between Tranquilitatis and Serenitatis 156 Serenitatis Total Potential rille west of crater west of rilles 154 and Serenitatis Total Small rille south of rille Serenitatis Total Very small rille south of rille 157 that may originate in cone 159 Serenitatis Total Small rille in southeast mare Serenitatis 16 Serenitatis Total Small rille in southern mare Serenitatis embayed at terminus 161 Serenitatis Total Small rille southwest of Posidonius crater in Mare Serenitatis 162 Serenitatis Total Very sinuous rille in Posidonius crater, Rima Posidonius 163 Serenitatis Total Possible rille southeast of Eudoxus crater in northern Serenitatis 164 Serenitatis Total Possible rille in northern Serenitatis 165 Tranquillitatis Outer Outer rille of of long rille in southern Tranquillitatis 165 Tranquillitatis Inner Inner rille of of long rille in southern Tranquillitatis, not necessarily visible for the entirety of the rille but clearly cuts across tectonic rille 166 Tranquillitatis Total Small rille west of head of rille 165, possibly with a circular source 167 Tranquillitatis Total Rille southeast of Plinius crater in northern Tranquillitatis 168 Jules Verne Total Potential rille parallel to an embayed portion of rim of a small crater in Jules Verne 169 Mare Smythii Total Potential rille in northwest Mare Smythii 17 Nectaris Total Potential rille with a circular depression in northeastern Nectaris basin 171 Nubium Total Possible rille in Wurzelbauer crater south of mare Nubium D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213)

13 12 Table 1 (continued ) Rille Basin Segment Lat Lon Length (km) unc (km) Width (km) stdev (km) Depth (m) stdev (m) Slope (deg.) Sinuosity Source Area (km 2 ) Source Depth (m) Source Volume (km 3 ) Description of Rille 172 Nubium Total Small rille on rim of Mercator crater in southwest mare Nubium 173 Nubium Total Small rille southwest of Lubiniezky crater northwest of Bullialdus crater in northwest Nubium 174 Nubium Total Very small potential rille southwest of Lubiniezky northwest of Bullialdus in northwest Nubium 175 Fecunditatis Channel Rille in western Mare Fecunditatis with a circular source depression in an embayed crater 175 Fecunditatis Tube Aligned pits that may represent a collapsed lava tube that continues from rille Humboldt Total Rille with an elongate source on northeast rim of Lyot crater 177 Sinus Total Potential rille south of Schroter crater south of Sinus 178 Sinus Total Potential impact melt channel on southeast rim of Pallas basin near Rima Bode 179 Sinus Total Potential impact melt channel on southeast rim of Pallas basin near Rima Bode 18 Sinus Total Potential impact melt channel on southeast rim of Pallas basin near Rima Bode 181 Sinus Total Potential impact melt channel on southeast rim of Pallas basin near Rima Bode 182 Sinus Total Potential impact melt channel on southeast rim of Pallas basin near Rima Bode 183 Sinus Total Narrow channel in western Sinus 184 Sinus Total Potential rille south of Ukert crater near Rima Bode 185 Sinus Total Potential rille south of Ukert crater near Rima Bode 186 Sinus Total Prominent rille northwest of Bode crater that terminates in a pool of mare that might feed Rima Bode farther north 187 Sinus Total Rima Bode that starts from a mare pool that may have been fed by previous rille Sinus Total Small potential rille downslope of Rima Bode 189 Sinus Total Small narrow channel that originates in an elongate source northeast of Rima Bode 19 Sinus Total na na na na Potential small rille north of Rima Bode in SE Sinus 191 Thomson Total Potential rille with circular depression in crater west of Jules Verne 192 Thomson Total Potential rille perpendicular to ejecta material in west Thomson crater 193 Thomson Total Small rille in northwest Thomson crater that terminates in Mare Ingenii 194 Thomson Total More prominent rille northeast of rille 193 on north rim of Thomson crater D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38

14 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) Number of Features Number of Features Sinuous Rilles KREEP Units Latitude (degrees) PKT Region Longitude (degrees East) Mare Units Fig. 4. Global distributions of lunar sinuous rilles as a function of (a) longitude and (b) latitude. The gray area represents the latitude and longitude distribution of mare units, the lighter speckled area represents the latitude and longitude distributions of the Procellarum-KREEP Terrain (PKT, e.g., Jolliff et al., 2), and the dark gray area represents the locations of the highest concentrations of KREEP materials. Each black bar represents the frequency of sinuous rilles observed within the corresponding 51 latitude or 11 longitude range. The locations of SRs closely mirror the distribution of mare materials on the nearside of the Moon, and SR longitudes closely correlate with the longitude distributions of the PKT and KREEP materials % Mare along given Latitude % Mare along given Longitude lying on slopes less than 11, 61% lying on slopes of less than.31, and 22% lying on slopes of less than.11. A negative slope in this study represents a channel that is currently observed to extend down-gradient, while a positive slope represents a channel that is currently observed to extend up-gradient. Seven percent of the observed SRs reside on positive slopes (Fig. 8), suggesting that surface modification was likely to have occurred after SR formation in these cases, resulting in a change in observed regional slope. Of these positive-sloping SRs, 75% lack a visible source depression, and thus the direction of regional slope is determined based on the trend in channel width and depth, with width and depth expected to decrease in the original down-gradient direction as erosion efficiency decreased with increasing distance from the source. Approximately 3% of the SRs observed to have a positive slope are located at the edge of basins loaded with mare, and the others tend to be associated with wrinkle ridges, observations that are consistent with the idea that subsequent deformation changed the apparent regional slope of these positively-sloped features. Sinuous rille sinuosity ratios are observed to range from 1.3 to 2.9 (average sinuosity ratio, 1.22, median sinuosity ratio, 1.19; Fig. 7b), with 2% of observed channels characterized by high sinuosity ratios greater than 1.5, 69% of SRs characterized by intermediate sinuosity ratios between 1.3 and 1.5, and 11% of SRs characterized by low sinuosity ratios less than 1.1. Channel sinuosity has a complex relationship with regional slope, where terrestrial fluvial channels are expected to become sinuous at slopes of.26 and continue developing meanders until a maximum sinuosity of 1.25 is reached at a corresponding slope of.15, at which point a braided channel is expected to develop (e.g., Schumm and Khan, 1972). This relationship is made more complex by a significant dependence on sediment load, a factor that both alters slope and is influenced by slope, as well as by a dependence on pre-existing faults in the substrate (e.g., Timár, 23). A relationship between sinuosity and slope is not observed in the data collected in this survey, but because sediment load, or equivalently, crystallinity of the flowing lava, is not currently observable on the Moon, further investigation of this relationship is not conducted in the current analysis Morphology of channel extremes The various ranges of channel morphometry correspond to varied channel morphologies that are observed in the extremes of 7 6 Frequency of Sinuous Rilles as a Function of Age Mare Units Incised Unit Embaying Unit Number of Features NA Age (Ga) Fig. 5. Age distribution of lunar sinuous rilles. Age distributions of lunar SRs, determined using mare unit ages defined by Hiesinger et al. (2, 21, 211). Each bar represents the frequency of sinuous rilles observed within the corresponding.1 Ga range. The age of the incised units (black bars) represents an upper age limit for the SRs, and the age of the terminal unit (i.e., the unit observed at the terminus of the SR, shown with gray bars) represents a lower age limit for the SRs. On the basis of these data, the majority of observed SRs in this survey formed between 3 Ga and 3.8 Ga ago, though some SRs may have formed in the most recent 1.8 Ga of lunar history; these young SRs typically coincide with the Aristarchus Plateau, Marius Hills, and southwestern Imbrium basin regions. The frequency distribution of mare units mapped by Hiesinger et al. (2, 21, 211) is shown by the gray envelope.

15 14 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 Frequency Sinuous Rille Length Sinuous Rille Width Frequency Length (km) Sinuous Rille Depth Depth (m) Width (m) Width (km) Sinuous Rille Width vs. Depth Gornitz, 1973 This study R 2 = Depth (m) 6 Fig. 6. Trends in observed sinuous rille lengths (a), widths (b), depths (c), and width vs. depth relationships (d). Each bar in parts (a c) represents the frequency of sinuous rilles observed within the corresponding range of length (2 km), width (.1 km) and depth (1 m), respectively. Observed SRs have an average length of 67.7 km and a median length of 33.2 km, and the longest SR identified is located in northeastern Oceanus Procellarum (Rima Sharp). Similarly, SRs have an average width of 637 m and a median width of 48 m, and an average depth of 72 m and a median depth of 49 m. The widest and deepest SR is located on the Aristarchus Plateau (outer valley of Valles Schröteri). Channel widths and depths are directly correlated (R 2 ¼.7433), and the linear trend identified as the result of all mapped SRs is slightly shallower than the linear trend identified for six lunar SRs as determined by Gornitz (1973); gray line in (d). Number of Features Slope (degrees) Sinuosity Index Fig. 7. Trends in observed regional slopes (a) and sinuosity indices (b). Each bar represents the frequency of sinuous rilles observed within the corresponding.11 slope or.1 sinuosity index range. All SRs identified formed on a slope that is currently observed to be less than Sinuous rilles are expected to form on negative (downward) slopes as lava is expected to flow down-gradient, and the majority of observed SRs follow that expectation. However, several SRs are currently observed to reside on positive (upward) slopes, suggesting that surface modification occurred after the SR formed. Surface modifications could occur, for example, as the result of isostatic compensation from mare loading or from more localized tectonic activity such as the formation of a wrinkle ridge. Lunar SRs typically have a sinuosity index (1.2) that is similar to the sinuosity index on Venus (1.6) and smaller than the sinuosity index observed on Earth (2.4), possibly indicating that lateral erosion and channel meandering is less common in lunar lava channels than in terrestrial fluvial channels. channel length, width, depth, slope and sinuosity. The longest observed SR, Rima Sharp (Fig. 9), has an average length of 566 km and is located at 36.71N and E in northeastern Oceanus Procellarum near the western extent of exposed highland terrain associated with the rim of Imbrium. This rille has an average width of 84 m, an average depth of 76 m, an average regional slope of.81, and a sinuosity of The channel of this long SR is observed to cut predominantly into high-ti mare material (blue in Clementine data, Fig. 9c) with an age of 2.14 Ga, though low-ti mare material is also incised at both the source end of the rille as well as at the terminus (orange in Clementine data), with an age of 1.33 Ga. In contrast, the shortest observed SR has an average length of 3 km and is located at 191N, 27.51E, originating in a cone in southeastern Serenitatis basin (Fig. 1). This rille has an average width of 283 m, an average depth of 25 m, and average slope of.91, and a sinuosity of The channel of this short SR is observed to incise into a high-ti mare material (blue, Fig. 1c) with an age of 3.55 Ga, the same material that is also observed at the terminus of the channel. The morphology of this short rille, specifically its location within what may be a larger flow unit, may be consistent with a leveed channel rather than an incised SR, but the apparent connection between the channel of

16 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) E 33 E 3 E 4 N 4 N Oceanus Procellarum Imbrium 3 N 3 N Serenitatis 2 N 2 N 1 N 1 N Tranquillitatis 1 S 1 S 2 S 2 S Humorum Nubium 3 S 3 S 4 S 4 S 3 E 33 E 3 E Fig. 8. Distribution of sinuous rilles identified to have a positive slope. Approximately 3% of these positive-sloping SRs are located along the rim of a mare-loaded basin, and the remaining features tend to be associated with a wrinkle ridge. These locations are consistent with subsequent surface modification that alters the original slope of the terrain surrounding the SRs. High : m Low : -25 m 5 1km 5 1km 5 1km Fig. 9. The longest sinuous rille observed. Rima Sharp (SR #85; length 566 km), as observed in a mosaic of LROC WAC images (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where red indicates highland material, orange/yellow represents low-ti mare material, and blue/green represents high-ti mare and fresh highland material (i.e., impact ejecta). Rima Sharp formed in northeastern Oceanus Procellarum along the western edge of highland material associated with the Imbrium basin. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

17 16 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 High: -26 m Low: -29 m.5 1km.5 1km.5 1km Fig. 1. The shortest sinuous rille observed. Sinuous rille #158 (length 3. km), as observed in SELENE LISM image TC_EVE_2_N21E27N18E3SC (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where blue/green represents high-ti mare. This SR appears to have originated in a small cone before flowing into southeastern Serenitatis, and it may be consistent with a leveed channel that formed within a broader flow. However, the intersection between the channel and the interpreted source depression has led to the interpretation that this feature formed as lava was released from the depression, incising into the pre-existing terrain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) the feature and the potential source depression leads to an interpreted origin of incision by lava to form a SR. The widest, and also deepest, SR observed in the survey is the outer valley of Vallis Schröteri (Fig. 11). This SR is located at 24.51N, 31.71E on the Aristarchus Plateau in central Oceanus Procellarum. The outer valley of this SR has an average length of 176 km, an average width of 4.3 km, an average depth of 534 m, an average slope of.311, and a sinuosity of Vallis Schröteri also contains a smaller, nested rille that begins at the same source and continues beyond the outer valley into the mare material west of the Aristarchus Plateau. This inner SR is characterized by a longer average length of 261 km, a narrower average width of 638 m, a shallower average depth of 121 m, a similar slope of.321 as measured along the valley floor, and a higher sinuosity of Both the valley and the nested SR originate in material that is completely superposed by ejecta from Aristarchus crater, and both outer and inner SRs continue down-gradient to incise into the mafic mare material associated with the western portion of the Aristarchus Plateau (i.e., Le Mouélic et al., 2; Mustard et al., 211). The outer valley terminates in this material with an age of 3.48 Ga, while the inner SR continues until it reaches a contact between the lower-ti (orange) mare material of the Aristarchus Plateau and the higher- Ti (bluer) mare material with an age of 1.14 Ga west of the plateau. The narrowest SR is also among the shallowest SRs observed in this survey (Fig. 12), and this SR is located at 41N, E in central Oceanus Procellarum. This narrowest and shallowest SR has an average length of 14 km, an average width of 164 m, an average depth of 7 m, an average slope of.171, and a sinuosity of The channel is observed to incise into high-ti mare material (blue in Clementine data, Fig. 12c) with an age of 3 Ga, and the SR is embayed at both ends with material that is 1.33 Ga old. The lighter color component is associated with rays of secondary craters from Kepler crater to the northeast. The steepest SR observed in this survey is located at 29.21S, E on the rim of Mercator crater in southwestern Nubium basin (Fig. 13). This rille has an average length of 12.8 km, an average width of 666 m, an average depth of 79.4 m, an average slope of 1.41, and a sinuosity of The channel of this SR appears to incise into highlands material (reddish in Clementine data, Fig. 13c), and the channel ends near a contact between the red highlands material and a pool of higher-ti mare material (blue in Clementine data). The mare pool was likely to have been emplaced 3.63 Ga ago, but the age of the incised material has not been identified specifically. At the other extreme of slopes observed, the SR with the lowest slope identified in this survey is located at 19.81N, E in Imbrium basin (Fig. 14). This rille has an average length of 1.5 km, an average width of 647 m, an average depth of 17.6 m, an average slope of.71, and a sinuosity of This SR is observed to have incised into lower-ti (orange) mare material, and it was embayed at both ends by similar material that has an approximate age of 3.33 Ga. Because the channel has been embayed at both ends, the noted slope represents only a small portion of the channel rather than the regional slope present during the formation of the original, unembayed SR. The SR with the greatest sinuosity observed in the current survey is located at 18.31S, E in northwestern Nubium basin (Fig. 15). This rille has an average length of 5.4 km, an average width of 28 m, an average depth of 14.1 m, an average slope of

18 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) A A High : -15 m Low : -25 m 1 2km 1 2km Elevation (m) -8-9 A A Main Valley Inner Channel Horizontal Distance (km) 1 2km Fig. 11. The widest and deepest sinuous rille observed. Vallis Schröteri (SR #55; width 4.3 km, depth 534 m), as observed in a mosaic of LROC WAC images (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where red indicates highland material, orange/yellow represents low-ti mare material, and blue/green represents fresh highland material (i.e., impact ejecta in the eastern half of the image) and high-ti mare in the southwest corner of the image. Valles Schröteri originates in a circular source depression on the Aristarchus Plateau and flows north, west, then southwest, incising into the Aristarchus plateau. Valles Schröteri has an inner channel that is highly sinuous and continues beyond the walls of the outer valley into the mare southwest of the Aristarchus Plateau. The inner channel can be seen in the topographic profile (d) that plots LOLA track data; the inner channel has an average depth of 121 m. The profile was selected where LOLA track data was available and oriented as close to perpendicular to the channel as possible. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).31, and a sinuosity of 2.9. This SR is observed to incise into mafic material that has an age of 3.53 Ga, the same age as the material that embays both ends of the SR. The observed sinuosity is high for this feature because: (1) the slope varies in only one direction, meaning the meander-belt axis is represented by a straight line parallel to the gradient; and (2) the SR is relatively short, making the difference between the meander-belt axis and the observed channel length more significant than for a longer SR. For example, the SR with the second-highest sinuosity ratio observed in this study, located north of Rima Seuss at 8.71N, E, has an average length of 76.4 km, an average width of 26 m, an average depth of 15.9 m, an average slope of.5, and a sinuosity of 1.73 (Fig. 16). The channel of this rille is characterized by significant meanders, terraces, and potential oxbows (Fig. 16c), features that are associated with a highly sinuous channel. The meander-belt of this second-most sinuous SR is also drawn to approximately dissect the observed zone of erosion, including, for example, the observed ox-bow cut-offs, as well as with deference to the observed regional slope. This sinuous SR is observed to incise into mafic material with a higher Ti content (blue in Clementine data, Fig. 16d) with an age of 3.72 Ga, and the channel appears to be embayed by material that is 1.73 Ga old. In contrast, the SR with the smallest sinuosity observed in the current survey is located at 13.71N, 321E near the Marius Hills in central Oceanus Procellarum (Fig. 17). This SR has an average length of 2.6 km, an average width of 59 m, an average depth of 11 m, an average slope of.171, and a sinuosity of 1.7. This channel is observed to incise into mafic material with a higher-ti content (blue, Fig. 17c), though the age of the incised material is not determined. The SR begins as a channel that incised into the underlying terrain, but the feature is likely to have continued to form down-gradient as a lava tube, as evident in the aligned pits that are observed beyond the observed terminus of the channel. The channel of this rille is very straight, resulting in the low sinuosity ratio associated with this feature Other observed trends and sinuous rille characteristics Additional trends and characteristics identified in this survey include details of SR sources, locations of observed SRs, and identification of significant changes in channel morphology. Most of the identified SRs (57%) lacked an associated source depression, with 41% of those 119 SRs forming without an associated depression and the remaining 59% of them exhibiting morphologies consistent with embayment in the source regions subsequent to SR formation. The remaining 84 SRs or SR branches have source depressions that vary in volume from.2 km 3 to 65 km 3 and range in morphology, with 58% beginning in circular sources (e.g., rim of Marius crater, Fig. 18a, volume 11.6 km 3 ), 35% beginning in elongate sources (e.g., Rima Delisle in southeastern Imbrium basin, Fig. 18b, volume 4.1 km 3 ), and 7% beginning in irregular sources (e.g., eastern Imbrium basin, Fig. 18c, volume.96 km 3 ). Approximately 2% of the identified SRs have a source on the rim or in the ejecta of an impact crater or basin, with host craters including but not limited to Marius crater (e.g., Fig. 18a),

19 18 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 A A 2 4km 2 4km High: -13 m Low: -14 m 2 4km Elevation (m) A A Channel Impact crater Horizontal Distance (m) Fig. 12. The narrowest and shallowest sinuous rille observed. Sinuous rille #15 (width noted by black arrows in (a) of 164 m, depth of 7 m), as observed in SELENE LISM image TC_EVE_2_N6E312N3E315SC (a), with LOLA gridded topography overlaid (b), (white arrow designates interpreted flow direction), and with Clementine UVVIS color map (c), where blue represents high-ti mare and green represents secondary ejecta material associated with Kepler crater in central Oceanus Procellarum. The topographic profile of LOLA track data displays the shallow property of the channel as compared to an adjacent small impact crater. The profile was selected where LOLA track data was available and oriented as close to perpendicular to the channel as possible. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) High: m Low: -2 m 3 6km 3 6km 3 6km Fig. 13. The steepest sinuous rille observed. Sinuous rille #172 (slope 1.41), as observed in SELENE image TC_EVE_2_S27E333S3E336SC (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where red indicates highland material, and blue/green represents high-ti mare and relatively fresh highland material (i.e., impact ejecta). This SR formed on the rim of Mercator crater in southwestern Nubium basin. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

20 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) High : -19 m Low : -2 m 1 2km 1 2km 1 2km Fig. 14. The shallowest sloped sinuous rille observed. Sinuous rille #99 (slope.71), as observed in SELENE image TC_EVE_2_N21E351N18E354SC (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where red indicates highland material, and green represents fresh material ejected from nearby small craters. This channel is located in southwestern Imbrium basin, and it is observed to be embayed on either end of the channel, indicating that the documented slope measurement represents only the unembayed segment of this SR. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Prinz crater, Bode crater, Pallas crater, and Plato crater (e.g., Fig. 19). Approximately 1% of the identified SRs are observed to originate in the lunar highlands and continue onto the mare, such as the SR observed at the margin of the northwestern Aristarchus Plateau (e.g., Fig. 2), with 1% of these 2 SRs exhibiting a significant decrease in width and 7% exhibiting a significant decrease in depth associated with this crossing. This decrease in width and depth is expected to occur as erosion efficiency decreases when the channelized lava transitions from eroding into the steeply-sloped, unconsolidated material characteristic of the highlands materials to eroding into the more gradually-sloped, rigid material characteristic of the mare materials (e.g., Hurwitz et al., 212). This composition change is observed in 35% of these 2 SRs. The other SRs that exhibit a significant change in depth but do not cross from highlands to mare may indicate embayment near the channel terminus (e.g., Hurwitz et al., 212) or a decrease in erosion efficiency as the lava that formed the channel cooled and decreased in velocity with increased distance from the source (Fig. 2; Hulme, 1973, 1982; Carr, 1974; Williams et al., 2). Additional channels not associated with a change in incised material composition are observed to change significantly in width and depth, with a significant change considered when the standard deviation of the observed width or depth exceeds 5% of the average width or depth. The channels of the identified SRs are observed to change significantly in width in 3% of the identified rilles and are observed to change significantly in depth in 27% of identified SRs. This trend is consistent with gradual embayment of the SR terminus, which is observed to occur in 67% of the observed rilles. The embayed SRs that do not exhibit the significant change in width or depth were likely to have terminated abruptly, resulting in a less gradual change in width and depth that contributed less to the noted standard deviation. Other considered characteristics of identified SRs include the location of the SR, the composition of the incised substrate, modifications of SRs, and associations with potential lava tubes or impact melt. Sinuous rilles were observed to form in the highlands, along the edge of the mare, within the center of the mare, and in impact melt. The majority (51%) of identified SRs formed within the mare, 27% formed along or near the edge of the mare, 25% formed in the highlands (2% of the SRs crossed from the highlands into the mare), and 6% formed in impact melt, with 4% of the channels identified as potential impact melt channels (e.g., Fig. 21). Similarly, 2% of identified SRs appear to incise into highlands material (red in Clementine data), 63% incise into lower-ti mare material (orange/yellow in Clementine data), 34% incise into higher-ti mare material (green/blue in Clementine data), and 6% incise into material that has subsequently been concealed by ejecta-related materials (bright green/blue in Clementine data). It should be noted that 17% of the identified rilles are observed to embay a substrate that changes composition, with 63% of these 35 SRs crossing from red highlands to orange low-ti mare material, 23% changing from orange low-ti mare to blue high-ti mare, 11% changing from green high-ti material to orange low-ti mare material, and 3% changing from orange low-ti mare material to yellow, potentially ejecta-related material. These compositions are only first-order approximations based on the Clementine data, and it should be noted that the observed colors can also vary as a function of surface maturity, effects of space

21 2 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 High: -21 m Low: -23 m.5 1km.5 1km.5 1km Fig. 15. The most sinuous channel observed. Sinuous rille #174 (sinuosity index of 2.1), as observed in SELENE image TC_EVE_2_S18E336S21E339SC (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where red indicates highland material, and blue/green represents high-ti mare and relatively fresh material (i.e., impact ejecta). The sinuosity index is measured to be high for this sinuous rille because the topography changes very little, and this change in slope occurs along the straight length of the channel. Because the channel is so short, the mapped meander-belt axis is significantly less than the mapped channel length, resulting in the high sinuosity value. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) b 3 6km 1 2 km High: -11 m Low: -15 m 3 6 km 3 6 km Fig. 16. The second-most sinuous channel observed. Sinuous rille #24 (sinuosity index of 1.73), as observed at two scales in a mosaic of SELENE images TC_EVE_2_N12E39N9E312SC and TC_EVE_2_N12E312N9E315SC (a and b), with LOLA gridded topography overlaid (c), and with Clementine UVVIS color map (d), where blue represents high-ti mare and green represents relatively fresh material ejected from nearby impact craters. This SR is located north of Rima Seuss in central Oceanus Procellarum and represents a more classically sinuous channel, with wide meanders, terraces, and ox-bow cutoffs evident in the close-up of the channel (b). Arrows in (a) and (b) designate interpreted flow direction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

22 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) High: -11 m Low: -15 m 2 4km 2 4km 2 4km Fig. 17. The least sinuous channel observed. Sinuous rille #39 (sinuosity index of 1.7), as observed in SELENE image TC_EVE_2_N15E3N12E33SC (a), (arrow designates interpreted flow direction), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where blue represents high-ti mare and green represents relatively fresh material ejected from nearby impact craters. This channel is observed near the Marius Hills in central Oceanus Procellarum and appears to end as a lava tube with several coalesced skylights aligned with the initial channel. The skylights apparently formed in response to a wrinkle ridge that cuts into the mare on either side of the observed pits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) weathering, and grain size; therefore more detailed studies that focus on the spectra of the observed SRs are necessary to complete the analysis of effects of surface composition on SR morphology. The age analysis conducted on the surveyed SRs indicates that the majority of these features formed in the Imbrian epoch, and thus these rilles are likely to have been modified over the more recent 3 Ga of lunar history. Analysis of the location of SRs relative to wrinkle ridges indicates that 6% of identified rilles originate at wrinkle ridges (e.g., Fig. 22a), potentially forming contemporaneously, 5% are modified by or cut by a subsequent wrinkle ridge (e.g., Fig. 22b), and 7% of identified rilles incise or modify a pre-existing wrinkle ridge (e.g., Fig. 22c). This distribution suggests that the location of SRs is not necessarily related to regions that experienced significant tectonic activity. Finally, a small number of identified SRs have unique characteristics. Nested channels are observed to occur in 2% of SRs, including Vallis Schröteri (Fig. 11), Rima Prinz, a SR in southern Tranquillitatis basin, and a rille that originates in the ejecta of Plato crater northeast of Imbrium basin (Fig. 19). It is interesting to note that three of these examples originate near the rim of an impact crater (i.e., Aristarchus crater, Prinz crater, and Plato crater), though nested channels do not necessarily occur in all SRs that originate on crater rims (e.g., Fig. 18a). Another unique characteristic includes aligned pits. These pits are observed in association with 3% of identified rilles (e.g., Fig. 2c, Fig. 17), suggesting that some SRs may continue in the substrate beyond what is currently observed at the surface. More detailed analyses of a pit associated with rilles in the Marius Hills region using Kaguya LISM TC and LRO Narrow Angle Camera (NAC) images indicate that the walls of the pits expose thin layering within the surrounding mare deposits (Haruyama et al., 29; Robinson et al., 212). Measurements of the thickness of these layers can provide constraints for the thickness of mare basalt flows (e.g., Robinson et al., 212). A separate class of sinuous rilles includes constructed channels (e.g., Fig. 2b) that may represent 2% of identified rilles. These channels are observed mostly in southern Imbrium basin and are characterized by narrow, shallow channels that tend to be associated with effusive surface flows that have been identified previously (e.g., Schaber, 1973). One unique feature that may represent an additional constructed channel is located at 25.71S, 17.51E in southeastern Oceanus Procellarum. This channel appears to be very sinuous (sinuosity ratio of 1.44) and inverted (Fig. 23), rising as much as 1 m above the surrounding material. The inverted feature appears to transition to a channel with prominent marginal levees, indicating that this feature may have formed when substantial bounding levees merged to conceal the negative relief typical of a channel. These unique features represent a minority of the SRs identified in this survey, as the majority of features observed coincide with the morphologic properties that represent an eroded surface lava channel as defined initially, with parallel-striking, laterally continuous walls that incise into a substrate and avoid topographic barriers Basin trends The descriptions in Section 3.1 summarize the wide variety of SR morphologies observed across the Moon. The following section investigates trends in the locations (Fig. 24) and ages (Fig. 25) of SRs within specific lunar basins in order to identify concentrations of SRs across the Moon.

23 22 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) km 5 1km 12km Fig. 18. Examples of source depressions with various morphologies. Observed morphologies include circular (a), (SELENE LISM image TC_EVE_2_N12E39N9E312SC), elongate (b), (a mosaic of SELENE LISM images TC_EVE_2_N33S24N3E327SC and TC_EVE_2_N33E327N3E33SC), and irregular (c), (a mosaic of SELENE LISM images TC_EVE_2_N33E321N3E324SC and TC_EVE_2_N36E321N33E324SC). Arrows designate interpreted flow directions. The circular source is associated with a short but wide SR and formed on the rim of Marius crater in central Procellarum (SR #3). This channel is clearly embayed by subsequent mare emplacement. (b) The elongate source depression is associated with Rima Delisle (SR #115) east of Diophantus crater in southwestern Imbrium basin. The source depression and initial segment of this SR appear to be influenced by the direction of the source dike before the channel begins to meander as the SR extends into Imbrium basin. (c) The irregular source depression (SR #12) originates near the mare-highland boundary in western Imbrium basin and feeds a wide SR that continues into the interior of Imbrium basin to the east Oceanus Procellarum Oceanus Procellarum (Fig. 26) contains 48% of the SRs identified in this survey. The rilles observed within Oceanus Procellarum tend to be concentrated in the center of the basin, with 61% forming entirely within the mare and with 19% forming along the margins of mare deposits (Fig. 24). An additional 2% of the SRs observed in association with Oceanus Procellarum occur in the highlands, with 5% of those SRs crossing from the highlands into the mare terrain. The most prominent SRs observed in Oceanus Procellarum include the longest feature identified, Rima Sharp, as well as Vallis Schröteri and Rimae Aristarchus, Marius, Galileo, and Seuss. These features represent some of the longest SRs identified on the Moon. Many of the rilles identified, particularly features associated with the Aristarchus Plateau (Fig. 26a) and the Marius Hills (Fig. 26b) regions in the center of the basin, are observed to be associated with the youngest mare material identified on the Moon, with these SRs potentially forming within the most recent 2.2 Ga of lunar history (e.g., Hiesinger et al., 2, 21, 211; Fig. 25a). In addition to the 95 SRs identified, five chains of pits potentially related to subsurface lava tubes and two potential impact melt channels were identified within Oceanus Procellarum Imbrium basin Imbrium basin (Fig. 27) contains 28% deposits of the SRs identified in this survey. Imbrium SRs are observed more frequently in the highlands materials associated with the rim of the basin (35%) than was observed in Oceanus Procellarum, with 33% of these highland channels continuing onto the mare (e.g., Fig. 24). The SRs that formed in the highlands (35%) and along the mare margins (2%) are generally concentrated in the northern and southeastern rims of the basin and constitute approximately half of the SRs observed in Imbrium. Prominent features in these regions include Rima Plato, Vallis Alpes, and several unnamed SRs in the northeast and Rima Hadley in the eastern basin. In contrast, the remaining 45% of SRs observed within the center of the mare are mostly concentrated in the southwest of the basin. Prominent features in this region include Rimae Euler, Brayley, Diophantus, and Delisle. The majority of SRs observed in Imbrium formed between 3.8 Ga and 2.8 Ga ago, but 17% may have formed within the most recent 1.5 Ga of lunar history, and these young features are associated with the mare deposits in southwestern Imbrium basin (Fig. 25b) Orientale basin Orientale basin (Fig. 28) contains 7% of the SRs identified in this survey. Orientale SRs are generally observed in the small mare ponds along the margins of the impact basin, and 44% of the features observed appear to have formed entirely within these mare patches while 25% formed along the margins of these pools. The remaining 31% of the observed SRs originated in the highlands, and 4% of those channels continued to flow into the mare while the remaining 6% terminated at the boundary between the highlands and the mare. The longest SR identified in the Orientale basin is located in Lacus Veris in the northeastern region of the basin, and other notable features include a SR that intersects tectonic graben before continuing into the mare of southern Orientale, and a very wide SR that appears to incise into the entire width of a very small mare pool outside the Outer Rook

24 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) b 1 2km 2 4km High: m Low: -3 m 1 2km 1 2km Fig. 19. An example of a sinuous rille that contains a nested rille. Sinuous rille #132, as observed in a mosaic of SELENE images TC_EVE_2_N54E345N51E348SC, TC_EVE_2_N51E345N48E348SC, and TC_EVE_2_N51E348N48E351SC (a), at a smaller scale in SELENE image TC_EVE_2_N54E345N51E348SC (b), with LOLA gridded topography overlaid (c), and with Clementine UVVIS color map (d), where red represents highland material, orange/yellow represents low-ti mare and blue/green represents relatively fresh material ejected from nearby impact craters. Arrows in (a) and (b) designate interpreted flow direction. This SR originates in a source depression on the rim of Plato crater northeast of Imbrium basin and incises into highland terrain. The outer channel terminates at the highland-mare boundary, but the inner channel, evident in the close-up image in (b), continues to incise into mare material within the Imbrium basin. Nested rilles (also see Fig. 11) potentially represent thalweg channels that formed during the waning stages of lava emplacement and erosion or separate channels that formed during a subsequent but independent eruption from the same source depression. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) km Width (m) Depth (m) embayed or slowed erosion Incises plateau 2 1 Incises plateau Incises mare Incises mare embayed or slowed erosion Horizontal Distance (km) Fig. 2. An example of a sinuous rille that exhibits a significant change in width and depth. Sinuous rille #76 changes significantly in width and depth as it crosses from Aristarchus plateau onto mare, shown in a mosaic of SELENE LISM images TC_EVE_2_N3E36N27E39SC and TC_EVE_2_N3E39N27E312SC (a). White arrow denotes flow direction, and dashed white lines indicate locations of changes in channel morphometry as indicated in the accompanying profiles of changes in channel width and depth along the length of the SR (b). The segment of this channel that is observed on the Aristarchus Plateau is characterized by a width of41 km and a depth of 42 m, but when the channel initially encounters mare material off the northwestern Aristarchus Plateau the width decreases to 5 m and the depth decreases to 1 m. The width and depth decrease again, to 35 m in width and 2 m in depth, after the lava had flowed 15 km across mare. The first abrupt change in width and depth may correspond to a change in erosion efficiency as the lava encountered a more consolidated substrate in the mare than in the highlands (e.g., Hurwitz et al., 212), and the second abrupt change in depth and width may correspond to a shift in flow from turbulent to laminar, at which point erosion efficiency would decrease significantly (e.g., Hulme, 1973; Carr, 1974).

25 24 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) km 3 6km Fig. 21. An example of an impact melt channel. Feature#18isobservednorthofa 6 km diameter crater west of the Oceanus Procellarum, in SELENE LISM image TC_EVE_2_N9E291N6E294SC. While the morphology of impact melt channels might be similar to that of sinuous rilles, they are formed from an exogenic process rather than an endogenic process and are thus not identified as SRs in the current survey. Mountains in eastern Orientale. The SRs observed within Orientale tend to be shorter, and the majority of these features are considered to have formed between 3.2 Ga and 3.6 Ga ago (Fig. 25c). The features identified in this study represent a subset of the SRs mapped previously by Whitten et al. (211), as the other previously mapped SRs were not confirmed in the higherresolution SELENE LISM images used in this survey. 3 6km Serenitatis, Tranquillitatis, and other basins Serenitatis and Tranquillitatis basins (Fig. 29) contain 8% of the SRs identified in this survey. All of the features identified in these basins are observed within the mare or along the mare margins (Fig. 24), and the features are substantially shorter than the SRs observed in Procellarum and Imbrium basins. The most prominent features in this region include Rima Posidonius in eastern Serenitatis basin and a long unnamed SR in southern Tranquillitatis basin, and these SRs are likely to have been emplaced between 3.5 Ga and 3.8 Ga ago (Fig. 25d). Sinuous rilles are also observed in other basins (9%), including Nectarus basin, Mare Fecunditatis (Fig. 29), Humorum basin, Nubium basin (Fig. 3), as well as several SRs observed in Mare Smythii, Jules Verne crater, and Thomson crater on the lunar farside (Fig. 31). The SRs within Nectaris basin and Mare Fecunditatis are very short and are located along the edge of the basin mare deposits. The SR observed in Mare Fecunditatis, Rima Messier, crosses a wrinkle ridge before ending in a series of pits that is likely associated with a subsurface lava tube. In contrast, the SRs associated with Humorum and Nubium basins are associated with highlands material and basin deposits, and the most prominent features in this region include Rimae Herigonius and several unnamed SRs northwest of Bullialdus crater in northwestern Nubium basin. These longer SRs apparently originated in southeastern Oceanus Procellarum, forming channels that extended southward into Mare Humorum and Nubium basin. The channels within the farside lunar craters tend to originate either in highlands along crater rims before forming into the associated crater, or in impact eject or melt within the crater interior. These SRs tend to be heavily degraded by secondary craters from subsequent 3 6km Fig. 22. Interactions between sinuous rilles and wrinkle ridges. Interactions between SRs and wrinkle ridges occur when rilles either (a) originate at a wrinkle ridge, such as Rima Herigonius (SR #7) in northern Mare Humorum (a mosaic of SELENE LISM images TC_EVE_2_S9E324S12E327, TC_EVE_2_- S9E321SS12E324, TC_EVE_2_S12E324S15E327, and TC_EVE_2_S12- E321S15E327), (b) cut a wrinkle ridge, such as the prominent SR north of Rima Marius (SR #46) in central Procellarum (SELENE LISM image TC_EVE_2_- N18E312N15E315SC), or (c) are modified by a wrinkle ridge (SELENE LISM image TC_EVE_2_N18E39N15E312SC), such as the same prominent SR as shown in (b). These observations of the SR north of Rima Marius thus indicate that several episodes of tectonic deformation occurred in central Procellarum.

26 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) High: -7 m Low: -12 m 2 4 km 2 4 km 2 4 km Fig. 23. An example of a potentially inverted channel. Feature #1 is located in southeastern Oceanus Procellarum, as observed in SELENE image TC_EVE_2_- NE333S3E336SC (a), with LOLA gridded topography overlaid (b), and with Clementine UVVIS color map (c), where red represents a component of highland material and blue/green represents high-ti mare. The inverted channel (white arrows) may represent a continuation of an incised channel seen in the northeast. The inverted channel rises 1 m above the adjacent surface, and eventually the channel appears to develop potential levees characteristic of a constructed channel. We interpret that this inverted sinuous feature may represent a constructed channel that formed substantial levees that merged to conceal the negative topographic signature typically associated with a channel, allowing this feature to rise above the adjacent terrain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) impact events. Sinuous rilles are not typically observed in the isolated mare patches present on the lunar farside. 4. Discussion and closing remarks Highlands Edge of Mare Center of Mare Procellarum Imbrium Serenitatis/ Orientale Other Tranquillitatis Fig. 24. Distribution of sinuous rille locations by basin. Procellarum contains 48% of observed SRs, Imbrium contains 28%, Orientale contains 7%, Serenitatis and Tranquillitatis contain 8%, and the remaining 9% are observed within other basins including Humorum, Nubium, Nectaris, Mare Fecunditatis, and farside craters. Most SRs are observed in the center of the mare or along the edge of the mare, though 25% of observed SRs originate in the highlands of Procellarum, Imbrium, Orientale, and other basins. The survey conducted in this study has more than doubled the number of sinuous rilles (SRs) cataloged on the Moon (n¼194, compared to Oberbeck et al., 1969, 1971, n¼91), using high resolution image and topography data collected by the SELENE and LRO missions to document detailed morphometric and morphologic characteristics of lunar SRs. The vast majority of the cataloged SRs are observed in the western part of the lunar nearside, where nearly 8% of the documented SRs are observed in the area corresponding to the Procellarum-KREEP Terrain (PKT, e.g., Jolliff et al., 2; Lucey et al., 26). More than 6% of the mare basalts by area occur within the boundaries of the PKT, which makes up only 16% of the surface of the Moon (e.g., Head, 1975). This observation was one of the major factors that led Wieczorek and Phillips (2) to propose that there was a cause and effect between the abundance of KREEP materials and the concentration of mare basalts in the region. Specifically, the heatproducing elements that make up the KREEP terrain may have caused the underlying mantle to partially melt over much of lunar history to generate the observed basaltic volcanic sequence. The thermal model of Wieczorek and Phillips (2) predicts that partial melting begins immediately after the model is started at 4.5 Ga ago and continues to a lesser degree to the present. Melting initiates immediately beneath the KREEP basalt layer and becomes deeper with time, with the maximum depth of melting being 6 km, and the KREEP layer is kept above its liquidus for most of lunar history. Does this hypothesis account for the origin of mare basalts and the distribution of SRs in terms of the timing, duration, areal distribution, volumes, and changes in depth with time? Hess and Parmentier (21) pointed out several difficulties with this model in terms of petrogenetic evolution, geophysical evidence against the long-term duration of a near-liquid KREEP

27 26 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) Incised Unit Embaying Unit Age (Ga) Age (Ga) Age (Ga) Age (Ga) Fig. 25. Distribution of sinuous rille ages by basin. (a) Procellarum, (b) Imbrium, (c) Orientale and (d) Serenitatis/Tranquillitatis. Each bar represents the frequency of sinuous rilles observed within the corresponding.1 Ga range. Sinuous rilles typically formed between 3. Ga and 3.8 Ga ago, but SR are interpreted to have formed as recently as 1. Ga to 1.8 Ga ago in central Procellarum and southwestern Imbrium basin, coinciding with identified lunar volcanic centers Fig. 26. Mapped locations of sinuous rilles in Northern (a) and Southern (b) Oceanus Procellarum. Locations are shown using LROC WAC global mosaics; contours are displayed in 5 m increments. Features identified as SRs are shown in black, features identified as examples of collapsed lava tubes (as discussed in the text) are shown in green, and features identified as examples of impact melt channels (as discussed in the text) are shown in orange. Examples of collapsed lava tubes and impact melt channels are included to distinguish them from features interpreted to represent sinuous rilles, but it should be noted that the examples shown do not represent a comprehensive survey of these features. Oceanus Procellarum contains 48% of observed SRs, and these features are clearly clustered around Aristarchus Plateau and Marius Hills regions in the center of Oceanus Procellarum. Prominent SRs observed in Oceanus Procellarum include Rimae Sharp, Aristarchus, Marius, Galilei, Seuss, and Vallis Schröteri. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) layer, and the fact that such a layer might form an impenetrable barrier to the eruption of mare basalts. To date, no one has undertaken an analysis of the nature of volcanism in the PKT to address the predictions of the Wieczorek and Phillips (2) model and the Hess and Parmentier (21) caveats. The concentration of SRs in this region (Figs. 4, 26 and 27) adds another factor that needs to be explained. The SRs observed in the PKT are primarily located in regions associated with known volcanic centers in central and eastern Oceanus Procellarum (i.e., Aristarchus Plateau, Marius Hills) as well as in the southwestern Imbrium basin, areas that have been identified to have been volcanically active within the last Ga of lunar history (e.g., Hiesinger et al., 211). Sinuous rilles are not only observed in association with these volcanic centers, but they are also observed along the margins of relatively young mare-filled basins such as Imbrium and Serenitatis. These SRs formed in older marginal parts of the basin fill, while later, younger mare emplacement filled the interior of the basin and buried the termini of the older SRs. These older SRs typically formed within the Imbrian epoch of lunar history. A similar

28 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) Posidonius crater Serenitatis Crisium Tranquillitatis collapsed lava tubes Mare Fecunditatis Nectaris 15 3km Fig. 27. Mapped locations of sinuous rilles in Imbrium basin. Locations are shown using LROC WAC global mosaics. Features identified as SRs are shown in black, and features identified as potential leveed flows are shown in blue. Sinuous rilles within Imbrium basin are observed in the highlands associated with the northern basin rim and along the boundary between mare and highland along the southeastern basin rim. A cluster of younger SRs are observed in the southeastern portion of the basin, and these features may be related to the more recent volcanic activity associated with the Aristarchus Plateau to the west. Prominent SRs observed in Imbrium basin include Rima Plato, Vallis Alpes, and Rima Hadley in the north and east, and Rimae Euler, Brayley, Diophantus, and Delisle in the southwest. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 29. Mapped locations of sinuous rilles in Serenitatis, Tranquillitatis, Nectaris, and Mare Fecunditatis. Locations are shown using LROC WAC global mosaics. Features identified as SRs are shown in black, and features identified as potential collapsed lava tubes are shown in green. Sinuous rilles in these basins tend to be much smaller and less frequent than the SRs observed in Procellarum and Imbrium basins. Prominent SRs in these basins include Rima Posidonius in eastern Serenitatis basin and a long unnamed SR in southern Tranquillitatis basin. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Procellarum Rimae Herigonius Lacus Autumni Maunder crater Bullialdus crater Orientale Lacus Veris Mare Humorum Mare Nubium 1 2km 1km 2 Fig. 28. Mapped locations of sinuous rilles in Orientale basin. Locations are shown using LROC WAC global mosaics. Features identified as SRs are shown in black and are similar to SRs mapped previously by Whitten et al. (211). Sinuous rilles within Orientale tend to originate in highlands material and continue into the relatively low-albedo mare pools Lacus Veris and Lacus Autumni that are located between the Inner and Outer Rook Mountains of northeastern Orientale. Prominent SRs in Orientale include several longer SRs observed in Lacus Veris in northeastern Orientale and a SR that intersects a tectonic graben before continuing into mare in southern Orientale. spatial distribution is also observed within Orientale basin, which is generally only slightly filled in the interior (e.g., Head, 1974; Whitten et al., 211). Had flooding continued to fill Orientale, Fig. 3. Mapped locations of sinuous rilles in Humorum, Nubium basins. Locations are shown using LROC WAC global mosaics. Features identified as SRs are shown in black. Sinuous rilles in these basins tend to form near basin rims and ejecta, originating in extreme southern Oceanus Procellarum and extending south into Humorum and Nubium basins. Prominent SRs observed in this region include Rimae Herigonius and several unnamed SRs northwest of Bullialdus crater in northwestern Nubium basin. patterns of mare and SR distribution would have been expected to be similar to those observed in Imbrium and Serenitatis basins. In general, the age frequency distribution of the observed SRs closely correlates with that of the mare basalt units (e.g., Hiesinger et al., 2, 21, 211), with the highest frequency of SR formation occurring during the peak of mare unit emplacement, during the Imbrian epoch. These older SRs, as noted

29 28 D.M. Hurwitz et al. / Planetary and Space Science 79 8 (213) 1 38 Jules Verne crater Thomson crater 1 2km Fig. 31. Mapped locations of sinuous rilles in the lunar farside craters Thomson and Jules Verne. Locations are shown using LROC WAC global mosaics. Features identified as SRs are shown in black. Sinuous rilles in lunar farside craters are not prevalent, and they are rarely observed in association with the isolated pools of lunar farside mare. The SRs that are observed tend to originate in the walls of impact craters or within ejecta deposits, and they appear to be heavily deformed by younger ejecta. above, typically are observed along the margin of relatively young, mare-flooded basins. However, a concentration of SRs that are observed to incise into younger mare deposits are observed near the Aristarchus and Marius volcanic centers, and these younger SRs are interpreted to have formed within the Eratosthenian epoch of lunar history. This distribution of SR ages suggests that SR formation is not related to impact basin formation, though the correlation of SR location to the Procellarum-KREEP Terrain suggests that SR formation may be related to the distribution of these potentially melt-inducing elements. More than half of the mapped SRs lack an initial depression at their source, but many of these sources are likely to have been covered by subsequent mare emplacement. Of the 84 features that have source depressions, 58% begin in circular depressions and 42% begin in elongate or irregular depressions. Elongate depressions are attributed to the surface manifestation of blade-like dikes that breach the surface from depth in highly effusive eruptions. These surface eruptions are propelled by large overburden pressures that result from large mantle-crust density differences on the Moon (Head and Wilson, 1992) and are expected to be required to form the observed SRs (e.g., Hurwitz et al., 212). By contrast, circular depressions are interpreted to be due to pyroclastic activity during the initial stages of the eruption, thermally eroding the substrate below the fountain and forming a lava lake (e.g., Wilson and Head, 1981; Head and Wilson, 1981). This lake eventually overflowed, feeding a flood of lava along the surface that resulted in the formation of the observed SR (e.g., Hurwitz et al., 212). Channel morphometries of SRs are observed to vary widely, with SRs ranging in length from 2 km to more than 55 km (median length, 33.2 km), in width from 15 m to 4.3 km (median width, 48 m), in depth from 5 m to 53 m (median depth, 49 m), in slope from 1.41 to.51 (median slope,.21), and in sinuosity index from 1.2 to 2.1 (median sinuosity, 1.19). The longest SRs tend to be located within northern and central Procellarum and in southwestern Imbrium, which is consistent with identified volcanic centers at the Aristarchus Plateau and Marius Hills. The observed width and depth tend to decrease along the length of a SR, a trend interpreted to indicate the presence of a turbulent flow that facilitated thermal erosion near the source of the SR, and a transition to laminar flow that resulted in a decline in thermal erosion efficiency down-channel. The average SR width (637 m) and depth (72 m) are typically greater than observed for typical leveed lava channels on both the Moon and on Earth, providing additional evidence in support of the role of thermal and mechanical erosion in the formation of these lunar features. Observations of incision into both highlands terrain and pre-existing lava flows further support this hypothesis. The cataloged SRs almost universally formed on terrains that are currently observed to have very shallow slopes (96% formed on slopes o11), and these low slopes are consistent with SR formation by thermal erosion (Hurwitz et al., 212). However, some of the SRs that formed on higher slopes originated in the highlands and then crossed onto mare material. These SRs tend to have greater widths while in the highland terrain than when they transition to the mare terrain, suggesting that the highland megaregolith material might be more susceptible to lateral incision by mechanical erosion. By contrast, the solidified mare basalt material might be more susceptible to vertical incision by thermal erosion. A small percentage (7%) of cataloged SRs is observed to have formed on terrains that are currently observed to have a positive upward slope. This positive slope is attributed to post-emplacement deformation, potentially by flexural bulges resulting from interior basin loading (e.g., Rima Hadley SR #112, Table A1; Solomon and Head, 1979, 198), or by subsequent formation of wrinkle ridges (e.g., SR #47, Table A1). The sinuosity of lunar SRs falls in the lower range of sinuosity indices for fluvial rivers on Earth, which can theoretically range from 1. to 5.2 (Le Roux, 1992). However, terrestrial channels also tend to have sinuosity indices that range from 1.1 (Ohio River) to 2.4 (Arkansas River) for segments of familiar rivers in the United States (Schumm et al., 1972; Gornitz, 1973), and from 2.1 (Murrumbidgee River in Australia, Fig. 3a) to 2.8 (unidentified river in Malaysia) for segments of more sinuous rivers observed globally (Le Roux, 1992). Lunar SRs can also be compared to similar features that have been identified and analyzed on Venus. Sinuous rilles on Venus have sinuosity indices that tend to be lower as well, ranging from 1. to 1.3, though larger and longer channels with distributary channels on Venus called canali (Baker et al., 1992) tend to have a higher sinuosity index of 1.6 (Komatsu and Baker, 1994). The factors leading to the development of channel sinuosity are not yet well understood even in terrestrial fluvial channels (e.g., da Silva, 25), but influential factors on the Moon potentially include substrate stratigraphy and structure. For example, a substrate layer of unconsolidated regolith material is likely to be more susceptible to erosion than an underlying layer of solidified basalt, and lava that encounters this contact might be more likely to erode laterally into the regolith layer than to erode vertically into the solidified basalt. This lateral erosion may lead to the development of sinuosity. Alternatively, a lava flow that encounters a pre-existing structure such as a crater or fissure is likely to change course, potentially resulting in the development of sinuosity. As indicated earlier, SRs tend to lack identifiable terminal deposits related to the lava that flowed through and formed the observed features. This lack of observed deposits has been attributed to either concealment by subsequent mare emplacement or degradation of thin deposits of low-viscosity lava due to regolith formation. However, analytical models have been used to estimate the lava volume required to form a SR (i.e., Rima Prinz; Hurwitz et al., 212), and results indicate that a volume of 5 km 3 of a lava with a low viscosity, such as a lava similar to a komatiite, and a volume of 25 km 3 of a lava with an intermediate viscosity, such as a high-ti basalt, would have been