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1 Planetary and Space Science 57 (2009) Contents lists available at ScienceDirect Planetary and Space Science journal homepage: Episodes of floods in Mangala Valles, Mars, from the analysis of HRSC, MOC and THEMIS images Alexander T. Basilevsky a,b,, Gerhard Neukum a, Stephanie C. Werner b,1, Alexander Dumke b, Stephan van Gasselt b, Thomas Kneissl b, Wilhelm Zuschneid b, Daniela Rommel b, Lorenz Wendt b, Mary Chapman c, James W. Head d, Ronald Greeley e a Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin Str., 19, Moscow, Russian Federation b Institute of Geosciences, Freie Universitaet Berlin, Germany c US Geological Survey, Flagstaff, AZ, USA d Department of Geological Sciences, Brown University, Providence, RI, USA e Arizona State University, Tempe, AZ, USA article info Article history: Received 15 April 2008 Received in revised form 18 July 2008 Accepted 24 July 2008 Available online 3 August 2008 Keywords: Mars Mangala Valles Fluvial processes Crater counts Age estimates abstract The Mangala Valles is a 900-km long outflow channel system in the highlands adjacent to the southeastern flank of the Tharsis bulge. This work was intended to answer the following two questions unresolved in previous studies: (1) Was there only one source of water (Mangala Fossa at the valley head which is one of the Medusae Fossae troughs or graben) or were other sources also involved in the valley-carving water supply, and (2) Was there only one episode of flooding (maybe with phases) or were there several episodes significantly separated in time. The geologic analysis of HRSC image 0286 and mapping supported by analysis of MOC and THEMIS images show that Mangala Valles was carved by water released from several sources. The major source was Mangala Fossa, which probably formed in response to magmatic dike intrusion. The graben cracked the cryosphere and permitted the release of groundwater held under hydrostatic pressure. This major source was augmented by a few smaller-scale sources at localities in (1) two mapped heads of magmatic dikes, (2) heads of two clusters of sinuous channels, and (3) probably several large knob terrain locals. The analysis of results of crater counts at more than 60 localities showed that the first episode of formation of Mangala Valles occurred 3.5 Ga ago and was followed by three more episodes, one occurred 1 Ga ago, another one 0.5 Ga ago, and the last one 0.2 Ga ago. East of the mapped area there are extended and thick lava flows whose source may be the eastern continuation of the Mangala source graben. Crater counts in 10 localities on these lava flows correlate with those taken on the Mangala valley elements supporting the idea that the valley head graben was caused by dike intrusions. Our observations suggest that the waning stage of the latest flooding episode (0.2 Ga ago) led to the formation at the valley head of meander-like features sharing some characteristics with meanders of terrestrial rivers. If this analogy is correct this could suggest a short episode of global warming in Late Amazonian time. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Mangala Valles 900 km long, north south trending, outflow channel system in the southern highland region adjacent to the south-eastern flank of the Tharsis topographic bulge. The Valles was discovered during the Mariner 9 Mission (e.g., McCauley et al., 1972), and in the Viking era it was studied in more detail and Corresponding author at: Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin Str., 19, Moscow, Russian Federation. Tel.: address: Alexander_Basilevsky@Brown.edu (A.T. Basilevsky). 1 Now at Geological Survey of Norway, Trondheim, Norway. geologically mapped (e.g., Tanaka and Chapman, 1990; Chapman and Tanaka, 1993; Craddock and Greeley, 1994; Zimbelman et al., 1994). The Viking-based studies suggested that the source of water which carved the Mangala Valles channels was one of Memnonia Fossae grabens at the valley head (Carr, 1981; Tanaka and Chapman, 1990). Recently Wilson and Head (2002) suggested that formation of the graben and water release were due to emplacement of a magmatic dike. In our early study of this area we found evidence that some of the water that carved Mangala Valles could be released from more than one source (Basilevsky et al., 2006a, 2007). Our goal was to map the Mangala Valles system in detail using new data and to investigate whether there was only one source of water or other sources were also involved in the water supply /$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi: /j.pss

2 918 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Viking imagery analysis for the lower reaches of Mangala Valles showed cross-cutting relations interpreted as signatures of older and younger periods of catastrophic flooding (Tanaka and Chapman, 1990). Crater counts on the units, one of which predated the valley incision while another one was considered as emplaced in between the two flooding periods, suggested a significant (tens to hundreds millions of years) time interval between them (Tanaka and Chapman, 1990). Zimbelman et al. (1994) supported the conclusion of the two flooding periods, although their age estimates differ from those of Tanaka and Chapman (1990). Some researchers, however, questioned the need for more than one period of flooding to explain the observed geology (e.g., Craddock and Greeley, 1994). Recently Ghatan et al. (2005) revisited this problem, analyzing the Mars Odyssey THEMIS and MGS MOLA data for this area. They interpreted their observations as evidence of a single period of catastrophic flooding, although consisting of early and late phases. The validity of crater counts used by previous researchers to support the multiple periods of flooding were thought to be uncertain because of the small areal extent (and thus the poor statistics) of the dated units. Even more recently, Leask et al. (2007) favored the idea of two episodes of the Mangala-forming floods and suggested additional pieces of evidence for that (see below). So the second goal of this work is to consider if our new observations provide morphologic evidence of multiple episodes of water release. We assess this question through numerous crater counts made by us using high-resolution HRSC, MOC, and THEMIS images, and thus improving on the problem of poor statistics. Our study is mostly based on the photogeologic analysis and mapping of the southern half of nadir channel HRSC image H0286 (12.5 m/p resolution), which covers the upper and middle reaches of Mangala Valles, although in some cases we expand our studies to an area km east. Analysis of the HRSC-based DTM was an essential part of this study. All altitudes mentioned in this paper refer to the IAU 2000 ellipsoid (A ¼ B ¼ km, C ¼ km). The maps projection is sinusoidal. The MGS MOC images (3 6 m/px) and Mars Odyssey THEMIS (daytime, 18 m/px) and, in a few cases, HRSC images 0299, 4095, and 4150 (12.5 m/px), have also been analyzed in this study. Impact crater counts on the mapped units using the routine described elsewhere (see below) have been completed and allowed us to estimate the ages of a number of morphologic units and thus to date the geologic events. The area mapped lies between 121 and 19.61S and and W (Fig. 1). The two major physiographic features within the map area are: Mangala Fossa and an adjacent 400-km long stretch of the channel floor (Figs. 1 and 2). The fossa is 220 km long and 3 13 km wide, and the floor altitudes vary from 500 to 700 m (depending on altitudes of the surrounding terrain the trough depth varies from about 700 m to more than 1.5 km). In agreement with earlier studies Mangala Valles consist of two major components: a broad high-standing valley floor mapped by us as smooth plains (ps) and a set of more deeply incised valley elements (e.g., Tanaka and Chapman, 1990; Chapman and Tanaka, 1993; Craddock and Greeley, 1994; Zimbelman et al., 1994; Ghatan et al., 2005). Fig. 1. (A) A locator map for the study area. (B) Study area (white box 1) shown on the background of HRSC image H0644. Boundaries of Mangala Valles as summarized by Ghatan et al. (2005) as well as outlines of areas covered by Fig. 5 (white box 2) and Fig. 21 (white box 3) are also shown. (C) HRSC-based DTM for this region (right). Numbers on the DTM are altitudes relative the IAU 2000 elipsoid. (D) Location of the paper figures on the mapped part of HRSC image H0286.

3 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 2. Study area: (1) HRSC image H0286; (2) Color-coded DTM; (3) Geomorphic map. Legend for the morphologic units: gd glacial deposits; htf head trough floor; vfp valley floor pitted, vfs valley floor smooth; vfm valley floor mottled; vfg valley floor grooved; vff valley floor fluted; vsg valley slope grooved; vst valley slope terraced; klt large knob terrain; bpt polygonal block terrain; kst small knob terrain; pm plains with mesas; ps smooth plains; pmc post-mangala (large) crater; phc post-hummocky-plains (large) craters; ph hummocky plains; hp highland plateau; hm highland mountains.

4 920 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Near the opening of Mangala Fossa, the valley floor is about 500 m in elevation. To the north (downstream) where the valley becomes wider, most of the floor is close to 400 m elevation, although in local depressions deepen to nearly 500 m. Further north, the floor ranges between 400 and 600 m elevations, and within the northernmost section of the map area the floor reaches 700 m. Thus, the average downslope altitude gradient of the map area is about typical of large rivers on Earth that originate in mountains. For example, the Amazon river has a similar gradient. However, gradient is an order of magnitude greater than for large terrestrial rivers originating in plains, like the Volga or the Mississippi. With a source trough floor being at a lower elevation than the floor of its downstream valley, the Mangala Valles system resembles the Siberian river Angara, whose source is Lake Baikal filling the tectonic basin. The Baikal lake surface level is 455 above sea level and the deepest part of its floor is about 1200 m, much lower than the floor of the Angara river valley along all its length. 2. Photogeologic analysis In total, 19 morphologic units have been mapped (Fig. 2). Thirteen units represent terrains of different parts of Mangala Valles, the floor and slopes having different morphologies. We added another unit (gd), interpreted to represents deposits from local glacial activity in the vicinity of the water source graben (Head et al., 2004). Four mapped units represent terrains of Noachian highlands of the area (phc, ph, hp, and hm). One unit (pmc) is composed of the relatively large (D ¼ 7 km) post- Mangala crater. In reconnaissance examination of the regions surrounding the mapped area, we found that km east there are large (up to 100 km wide) lava flows. The source of at least part of these flows is a graben that is a continuation of the Mangala Fossa source graben. We did not map that area in detail, but briefly describe it following the description of the mapped units and we also performed crater counts on several of those lava fields Highland and large crater units In mapping these units we generally followed the nomenclature of Ghatan et al. (2005), who summarized previous regional studies (Chapman and Tanaka, 1993; Craddock and Greeley, 1994; Zimbelman et al., 1994), to arrive at units designated as highland mountains (hm), highland plateau (hp), and hummocky plains (ph) materials. In order to draw the boundaries of these units, we relied on the morphologies seen in HRSC image 286 and on its derived DTM. We also mapped as a separate units two relatively large craters (8 and 25 km in diameter) with their ejecta clearly superposed on the hummocky plains: post-hummocky-plains craters (phc) and another 7-km crater superposed on Mangala Valles Highland mountains (hm) unit This unit was mapped along the south-eastern boundary of the study area (Fig. 2). It represents the western flank of the NNE trending mountain range that is about 400 km long and 50 km wide (Fig. 1), one of several ranges distributed tangential to the southern and western margins of Tharsis (Scott and Tanaka, 1986). The highest part of the range (outside of the study area) reaches +4.2 km in elevation. Within the map area elevations of this unit average between +0.8 and +1.2 km and its morphology is characterized by the presence of gently sloped ridges, scarps and craters from a few hundred meters to a few kilometers in diameter. It may be correlated with the ridged unit of the Noachian Plateau and High Plains Assemblage of Tanaka et al. (1992) Highland plateau (hp) unit Within the study area this unit borders Mangala Valles from the west and also forms several islands up to a few tens of kilometers across among the Mangala Valles units (Fig. 2). Most of its surface reaches heights between +0.3 and +1 km in the southern part of the study area and from 0.1 to +0.1 km in the north. The highland plateau surface is complicated by numerous low hills from a few hundred meters to a few kilometers across and with noticeably fewer craters of the same size range. Remnants of five larger craters form the islands of the hp unit among the Mangala Valles units. In their preserved parts, ejecta of these craters show the fluidized appearance of the multiple-layer type. The concentration of craters with this ejecta type in the Mangala Valles region was described by Barlow and Perez (2003) who interpreted this as an indication of enrichment of the target in liquid water at the time of crater formation. Floors of these craters reach elevations of 900 to 1400 m, deeper than the valley floor within all of the study area. The deepest parts of these crater floors are very smooth horizontal locales appearing whitish in the HRSC image (Fig. 2). We correlate unit hp with the hilly unit of the Noachian Plateau and High Plains Assemblage of Tanaka et al. (1992) Hummocky plains (ph) unit Within the study area, this unit is observed only in the south (Fig. 2). The hummocky plains surface here is between 0 and +0.3 km. The plains surface is peppered with numerous low gently sloping hills typically a few hundred meters across, and has relatively few craters. North of the Mangala Valley source trough, the surface of this unit looks homogeneous in brightness, while south of the trough it is mottled. We correlate this unit with smooth and mottled plains unit of the Noachian Plateau and High Plains Assemblage of Tanaka et al. (1992) Post-hummocky-plains craters (phc) unit This unit is represented by two craters and their ejecta (Fig. 2). One crater centered at 18.81S, W, is 25 km in diameter; its depth is about 1.5 km. It has a relatively small elongated (1 3 km) central peak standing 0.2 km above the flat crater floor. Its ejecta deposit extends up to km from the crater rim. Another crater, centered at 17.71S, W, is 8 km in diameter with a depth of about 0.7 km. It has a small isometric (in plan view) central mound standing o0.1 km above the undulating crater floor, and an ejecta deposit that extends 5 7 km from the crater rim. The ejecta deposits of both craters have fluidized appearances typical of the multiple-layer type, suggesting the presence of liquid water in the target subsurface at the time of crater formation (Kuzmin et al., 1988; Barlow and Perez, 2003). Floors of these two craters are hypsometrically higher than the floor of the neighboring parts of the valley and the source trough. No whitish smooth subareas are seen within them (Fig. 2) Post-Mangala crater (pmc) unit This unit is represented by the crater centered at 13.51S, W (Fig. 2). It is 7 km in diameter, its depth is about 0.5 km. The crater is superposed on the smooth plains terrace of Mangala Valles. It has a isometric (in plan view) central peak standing o0.1 km above the crater flat floor. Its ejecta deposit also has a fluidized appearance (multilayer type), and extends up to 3 7 km from the crater rim. The floor of this crater reaches 900 m deep (also deeper than the valley floor within the study area). Within

5 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) the deepest parts of the crater floor, whitish smooth subareas are observed Mangala Valles units Twelve units comprise the Mangala Valles broad high-standing valley floor (smooth plains (ps)) and deeply incised valley elements Plains smooth (ps) unit This unit is observed mostly in the eastern part of the mapped area, forming a high ( m) terrace above the deeper parts of the valley floor and locally present as relatively small islands within the more deeply incised part of the valley. The hypsometric elevation level of the smooth plains surface is close to 200 m in the southern half of the mapped area, and 300 m in the northern half. The terrace slopes on the more deeply incised part of the valley are often grooved downslope. The groove orientation generally follows the trend of the implied channel flow. However, in some cases the orientation varies, which suggests subsurface water discharge from the slope (see below in description of the Valley slope grooved unit). In the central-eastern part of the study area, and the grooved slope in the deeply incised part of the valley lies a flat, sparsely cratered surface of smooth plains (Fig. 3a). The surface of the ps unit locally shows obvious traces of erosion in the form of the NNW oriented system of scour-like featuress with m spacing (Fig. 3b). In this area it is sometimes seen in MOC images that small ( m in diameter) impact craters superposed on unit ps have blocky ejecta, suggesting mechanically strong target material within tens of meters depth. At the northernmost end of the study area is a terrace of smooth plains and the more deeply incised part of the valley (Fig. 4a). The flat, sparsely cratered surface of smooth plains here shows diffuse lineations trending NW, which is in agreement with orientation of grooves on the terrace slope and fluted islands on the valley floor. Fig. 4b shows details of the ps surface: NE trending dark small dunes, small craters and the 1-km highly degraded crater (probably buried and then partly exhumed). Seventeen km east of the mapped area is a 3-km crater on the surface of smooth plains (Fig. 5), that has obviously been streamlined by powerful water flow. The orientation of the bar behind the crater suggests a NNW direction of water flow, in agreement with the dominant orientation of flow features in the mapped area. However, the surface of smooth plains near this streamlined crater shows no other traces of water erosion, even in MOC images with resolution 3 6 m/px (Fig. 6a). This is typical for most other localities of smooth plains studied, and thus the presence of supposedly water-carved scours shown in Fig. 3 is the exception rather than the rule. More typical for the detailed textures of the smooth plains surface are traces of eolian activity in the form of dunes, wind streaks and dust devil tracks (Fig. 6a and b). The widespread distribution of these features suggests significant resurfacing of the smooth plains. This unit generally corresponds to unit AHpl of Tanaka and Chapman (1990), who considered it to be lavas emplaced in the valley in between two distinct periods of water floods. In our analysis we did not find conclusive evidence of the volcanic origin of the smooth plains material, but cannot rule out this interpretation. The previously mentioned blocky ejecta from some impact craters, superposed on unit ps, indirectly support the presence of lavas in its materials, but these may be pre-flood lavas eroded by the ps-carving floods or even competent sediments. Fig. 3. Smooth plains (ps) in the eastern and central part the study area: (a) part of HRSC image H0286 and (b) fragment of MOC E

6 922 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 4. Smooth plains (ps) in the northern part of the study area: (a) part of HRSC image H0286 and (b) part of MOC R Fig. 5. Streamlined crater on the surface of smooth plains east of the study area. HRSC images 0286 (a) and 0644 (b), nadir channel, and THEMIS image V (c) Head trough floor (htf) unit The 220-km long head (source) trough of Mangala Fossa trends N701E. The fossa is 3 7 km wide along most of its length, but near its opening to the valley, its width reaches km (Figs. 2 and 7a). The fossa is composed of three graben arranged en echelon and the maximum width is where these three graben join together. The fossa cuts the Noachian units. Its general linearity and closer relation to the system of Medusae Fossae troughs radiating to the southwest from the Tharsis rise suggest its tectonic origin (graben) (Tanaka and Chapman, 1990), probably related to radial dike emplacement events (Wilson and Head, 2002). As noted, most of the trough floor lies between 500 and 700 m in elevation, where it cuts through units ph and phc, the depth reaches m. Where the trough cuts through topographically higher units hp and hm in its eastern section, the depth increases reaching 1500 m. The trough slopes are ,

7 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 6. (a) Surface of smooth plains south of the 3-km streamlined crater; part of mosaic of MOC images R , R , and R (b) Surface of smooth plains in the central part of the mapped area forming an island within the deeply incised part of the valley; fragment of mosaic of MOC images S and S but a few km away distally the trough walls have scalloped outlines, indicating modification by downslope movement of wall material. In the HRSC image the trough floor is seen as flat, with separated small and large knobs and larger massifs, whose morphology, heights and placement suggest that they are blocks of the material composing the trough walls (Fig. 7a). The linear chain of elongated knobs to the east of its opening to the valley is probably a remnant of the wall between two adjacent graben. Leask et al. (2007) interpret this chain as parts of the head of the first of the two dikes, whose subsequent emplacement is responsible for the formation of Mangala Valles. The trough floor appears smooth, with narrow ridges that are typically parallel to the trough walls. MOC images (Fig. 7b) show two types of ridges, both resembling wrinkle ridges seen in other areas of Mars as well as on the Moon and Venus; these ridges are considered to have resulted from moderate compressional deformation (e.g., Plescia and Golombek, 1986; Watters, 1988; McGill, 1993; Mueller and Golombek, 2004). Ridges of the first type are also seen in HRSC images. They are km long, m wide, and generally parallel to the trough walls, forming a train of en echelon-arranged features. Ridges of the second type are typically shorter ( m), narrower (10 30 m) and form a polygonal network. Craters typically smaller than m are seen on this unit, as are fields of small dunes close to the trough walls. Fig. 7. The Mangala Valles source trough; fragment of HRSC image H0286 (a). White box shows position of part of MOC image S , which illustrates details of the trough floor morphology (b) Valley floor smooth (vfs) unit This unit is observed in several places in the mapped area of Mangala Valles: close to the valley head and in several other areas. Close to the valley head, the vfs surface is at altitudes close to 500 m; further downstream, at 500 m with local depressions down to 600 m, and as deep as 700 to 800 m at the northern margin of the mapping area. At the NE corner of the study area, we have mapped a small subarea of the vsf unit incised in the smooth plains unit at much higher altitudes: about 400 m. Typically the vfs unit is hypsometrically lower than other neighboring valley

8 924 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Valley floor mottled (vfm) unit This unit was mapped in the upper part of the valley (Fig. 2). Its surface elevation is typically close to 500 m, but in places varies from 400 to 600 m high, with locally higher subareas close to the western valley slope. In the HRSC image, the surface of this unit looks mottled, with slightly darker and brighter patches with diffuse boundaries between them (Fig. 10a). It also contains linear features: some are curvilinear, probably representing meanderlike channels. Others are straighter and locally intersecting in a polygonal manner. Fig. 10b shows details of the valley floor mottled unit with a curvilinear meander-like feature and a straighter lineament. In the right part of the image, a sinuous feature trending N S, and resembling the front of a lava flow, is seen. The surface of the supposed flow to the left (west) of its front is cut by the curvilinear and straight lineaments. So, if lava emplacement did happen here, it was postdated by the meanderforming fluvial activity. In the southern part of the vfm unit area, close to the valley head trough, a number of possible obstacle marks are seen, similar to those observed within the valley floor smooth unit. Fig. 11a shows that they are concentrated downstream of where the opening from the head trough started to widen but is still much narrower than in places downstream. Obstacle marks in this locale are next to the meander-like channels. Fig. 11b shows that obstacle marks do not overlap the channels while channels locally partly overlap moats of the obstacle marks (arrows). Fig. 8. Valley floor smooth (vfs) unit in the opening of the valley trough head: (a) part of HRSC image H0286 and (b) part of MOC R floor units. The exception is unit vfp (see below), which has the same altitude as vfs. In HRSC images, the surface of this unit appears almost featureless (Fig. 8a). In MOC images, however, it is observed that close to the valley head this unit has a network of small ridges similar to that seen on the head trough floor (Fig. 8b). In contrast to the morphology of unit htf, the surface of unit vfs here has elongated loops of channels a few kilometers long, each about m wide. Channels are not cut by ridges, so they probably postdated the ridges. Channels and channel-like features are also seen in other localities of the valley smooth floor (Fig. 9, arrows). In the plan view the channel loops resemble meanders of mature terrestrial river development (Knighton, 1998). Locally possible obstacle marks, large blocks surrounded by moats washed by violent water flow (Russell, 1993; Burr and Parker, 2006), are seen within unit vfs. The blocks are from less than 100 m to almost 1 km across, and moats surrounding them are typically m wide. The obstacle marks are equidimensional without downstream tails, and thus resemble terrestrial marks formed by rotary or multidirectional mean bed stress (Russell, 1993, his Fig. 16). Small craters and dunes are also seen on the surface of this unit. The boundary between units htf and vfs is gradual. Unit vfs is mapped, in the transition zone between the head trough floor and the uppermost part of the valley, where the meander-like channels are located Valley floor fluted (vff) unit This unit is observed in the central and northern parts of the mapped area (Fig. 2), and characteristically contains streamlined islands typically 1 2to310 km (Fig. 12a). The surface elevation varies from 500 to 600 m up to 200 to 300 m in the top parts of small islands locally mapped as pieces of unit ps. The background surface, above which the streamlined islands stand, is generally similar to valley floor smooth and valley floor mottled units. On the HRSC image, it is seen that the vff background surface contains linear features, some of which are sinuous and are resolved in MOC images into m wide, meander-like channels (Fig. 12a and b). Impact craters of tens of meters to 1 2 km in diameter are also seen here. Streamlined islands have orientations consistent with the general orientation of the deeply incised part of the valley. The presence of streamlined islands is typical of terrestrial and Martian valleys formed by catastrophic floods (Baker, 1973, 1978; Parker and Rice, 1997) Valley floor grooved (vfg) unit This unit is observed in the valley in several parts of the mapped area (Fig. 2). It is distinct from other units of the valley floor due to the presence of groove-and-ridge subparallel systems with m ridge-to-ridge spacing (Fig. 13). Planimetrically, grooves and ridges are straight to slightly arcuate and their orientations are consistent with the general orientation of the deeply incised part of the valley. The length of ridges is typically 2 5 km. Locally, the groove-and-ridge systems become chains of knobs. Typically the valley floor grooved unit is hypsometrically slightly higher than the valley floor smooth unit (vfs) and appears embayed by material of vfs (Fig. 13a, c, and d). In MOC images, the ridge and groove outlines are slightly sinuous (Fig. 13b). These systems are also described as scoured units (e.g., Levy and Head, 2005). They are considered to be the result of erosion by powerful water flows (e.g., Baker, 1979). An alternative hypothesis suggested that they were formed by the movement of ice, that could have filled the Martian outflow channels (Lucchitta, 2001). Our observations are more consistent with the water erosion

9 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 9. Valley floor smooth (vfs) unit in the central (a) and northern (c and d) parts of the study area and obstacle marks in the catastrophic flood valley, Eastern Iceland (b). Parts of MOC images S , S , R , and aerial photo by Mary Chapman, USGS. hypothesis. For example, the streamlined crater and the bar behind it, stand m above the grooved floor, and are not scoured. The meaning that the scouring agent did not reach that level is consistent with the water erosion (right side of Fig. 13d). Lucchitta (2001), in describing her hypothesis, mentioned in the cases she documented, that the moving ice body responsible for scouring its substrate, was likely a kilometer to a few kilometers thick Valley floor pitted (vfp) unit In the northern part of the mapped area (Fig. 2, the generally smooth surface of unit vfp is roughened by numerous pits of tens meters to m across (Fig. 14). The pits are planimetrically circular, ovoidal and more irregular, locally coalescing; however, they have no raised rims and this distinguishes them from impact craters that are also present here. This unit was described by Levy and Head (2005) in this area as part of their smooth unit. They interpreted the pits to have formed by the sublimation of subsurface ice. Ice-rich sediments ponding in local lows at the end of the flooding event froze, and then underwent sublimation; in the areas richer in ice, sublimation pits were formed (Levy and Head, 2005). The presence in this area of dark wind streaks (Fig. 14c) suggests winds blowing to the north. The topographic slope in this region in general, and in the valley in particular, is to the north and thus northward, slope-generated winds could be

10 926 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) important in the valley and thus could have assisted in the sublimation of ice and deflation of material, perhaps adding to the elongation of some of the pits. Locally the valley floor pitted (vfp) unit is in contact with the valley floor smooth (vfs) unit. From analysis of HRSC and MOC images, we interpret the pitted surface of unit vfp as a surface, which underwent sublimation of locally concentrated, residual ice from the Mangala Valles flood events, enhanced and oriented by potentially intense downslope winds. This process is analogous to the orientation of terrestrial thermokarst pits by local wind conditions Valley slope terraced (vst) unit This unit was mapped mostly along the western edge of the valley and in a few other places. It forms bands 1 3 km wide and tens of kilometers long (Fig. 2). The altitude range from the top to the base of the terraced slope is usually m. Terraces are generally perpendicular to the trend of the slope (Fig. 15). There are typically not more than five terraces on a given slope segment. Terraces are not perfectly formed, and probably do not represent distinct stages in the valley development, but rather record the decreasing level of eroding flow with time. They probably also do not indicate layering in the eroded material because we observed Fig. 10. Valley floor mottled (vfm) unit: (a) part of HRSC image H0286 and (b) part of MOC image FHA Fig. 12. Valley floor fluted (vff) unit in the central part of the mapped area: (a) fragment of HRSC image H0286 and (b) fragment of MOC image E Fig. 11. Valley floor mottled (vfm) unit close to the head trough: (a) part of HRSC image H0286 and (b) part of mosaics of MOC images R and R

11 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) the release of the artesian water discussed above: it obviously did not lead to significant change in the structure of the system of erosional grooves and flutes beneath the slope produced earlier Plains with mesas (pm) unit This unit was mapped in a few small (1 4 to 7 15 km) localities km north of the source trough (Fig. 2). Their altitude is close to 300 m. This unit is generally plains-like, with numerous small mesas and knobs (Fig. 17). The plains with mesas are in close spatial association with smooth plains and appear to be formed due to partial removal of the surface layer of smooth plains. Dunes of a few meters to m wide and up to 100 m long are locally seen within this unit (Fig. 17c). Fig. 13. Valley floor grooved (vfg) unit in the northern (a and b), central (c), and southern (d) parts of the mapped area: (a, c, and d) Portions of HRSC image H0286 and (b) portion of MOC image R Arrows in image c show linear ridge that could be the top of a magmatic dike (see text). similar local terracing in eroded ejecta from large impact craters. Terrestrial impact ejecta is typically not layered (e.g., Basilevsky et al., 1983; Melosh, 1989), so steps and terraces in this material are interpreted as an indication of a hydrologic regime, rather than layering Valley slope grooved (vsg) unit In the central part of the study area along the 80-km long segment at the boundary between the deeply incised valley floor and the higher-standing terrace of smooth plains lies the grooved unit (Fig. 2). The width of the slope here varies from 2 to 12 km. The slope height is several tens of meters and thus slope is rather gentle and its surface is grooved (Fig. 16). Grooves trending downslope are m wide and a few kilometers long. Fig. 16a shows that the orientation of grooves is variable on the adjacent part of the fluted valley floor below. This is opposite to what was observed in the northern part of the mapped area (Fig. 4). It is seen in Fig. 16a that the major change in groove orientation is obviously related to the linear ridge similar to linear ridges observed in several areas of Mars (see also Fig. 13c) and interpreted to be tops of magmatic dikes (Shean et al., 2006; Head et al., 2005; Basilevsky et al., 2006b). The groove orientation here is perpendicular to the dike. The dike acting as a mechanical obstacle should cause parallel adjacent groove orientation, but it is perpendicular; and therefore a possible indication of water release from the subsurface along the dike boundaries. Evidence of the release of subsurface water in this part of the valley is observed in the form of two clusters of sinuous channels crossing the grooves of the vsg unit (Fig. 16b). To form these channels, water release from areas of their heads seems to be necessary. To carve the channels, and not following preexisting grooves but crossing them, is possible only if the flow was not channelized by the grooves. But this could occur only if the slope was covered with ice and the channel-carving flow was first on the ice and only then carved into sub-ice material. This release was evidently later in time and of much smaller scale compared to Small knob terrain (kst) unit This unit was mapped in a few small (2 5to10 40 km) localities km north of the source trough (Fig. 2). In all observed cases it is adjacent to smooth plains (ps). In HRSC and MOC images, it is seen that unit kst is hypsometrically lower than unit ps (Fig. 18a and b). The DTM confirms this finding and also shows that the surface of unit kst is slightly inclined from the kst/ps boundary toward units of the valley floor, as well as toward the units bpt and klt described below. The presence of numerous knobs of a few tens to a few hundred meters across is typical for unit kst. Sometimes, linear chains of knobs are seen on generally chaotic assemblages of knobs. Fig. 18c shows numerous dunes several meters wide and several tens of meters long covering the surface in between knobs and locally the knob surfaces. The characteristics described suggest that small knob terrain was formed at the expense of smooth plains due to partial removal of their surface layer and subsequent eolian resurfacing Polygonal block terrain (bpt) unit This unit was mapped in five small (2 3 to km) localities in the area where small knob terrain (kst) is observed (Fig. 2). In four localities, unit bpt is adjacent to unit kst and in one locality, to unit pm. The available images and the DTM show that polygonal block terrain is slightly lower in elevation than units kst and pm. Unit bpt is characterized by the presence of polygonal blocks ranging from a few hundred meters to 1 2 km across (Fig. 18a and c). Polygons mostly have four and sometimes three or five rather straight sides. Their top surfaces appear flat and rather horizontal, not inclined. In between polygonal blocks, there are troughs typically tens of meters wide and U-shaped in profile. Quite often, one trough continues along the rims of several blocks suggesting that some flat surface was dissected by troughs in larger polygons that were in turn dissected into smaller ones. The flat polygon surfaces, and locally the troughs in between them, are often covered with dunes of several meters width and several tens of meters long Large knob terrain (klt) unit This unit was mapped in eleven small (5 5to10 15 km) localities in the area where small knob terrain (kst) and polygonal block terrain (bpt) are observed (Fig. 2). Unit klt is typically in contact with units kst and bpt, locally with smooth plains, valley slope grooved, valley floor grooved and valley floor fluted units. Large knob terrain is always hypsometrically lower than these units. In all 11 localities unit klt is also in contact with the valley floor smooth unit and the latter unit is hypsometrically lower than unit klt. Unit klt is characterized by the presence of numerous knobs several hundred meters to 1 km across (Fig. 19a c). The larger knobs planimetrically resemble polygons of unit bpt. But in contrast to the latter, their upper flat surfaces are often not horizontal but inclined. The smaller knobs have rounded outlines,

12 928 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 14. Valley floor pitted (vfp) unit: (a) and (c) parts of HRSC image H0286 and (b) part of MOC image R White box in image c is southern part of image a. and for knobs of m in diameter, circular outlines are typical. Surfaces in between knobs, and locally on the knobs, are covered by numerous dunes of a few meters to 20 m width and up to m long. The surfaces of some knobs, and locally surfaces in between them, are in places cut by grooves with sharp outlines. The grooves are m wide and m long. Typically the grooves cut the dune patterns obliquely and probably represent eolian scours Glacial deposits (gd) Glacial deposits were first found in this area by Head et al. (2004) through analysis of THEMIS daytime images. These authors observed lobate deposits with numerous concentric ridge-like features along the northern and southern rims of the source trough (Fig. 20a). They considered several possible mechanisms for lobate deposit formation, including lava flows, landslides, and glacier deposits. The lava flow option was rejected because of the lack of a thick deposit interior to the outer ridges and the draping of ridges over relatively high subjacent topography. The landslide option was rejected because of the lack of obvious high-standing source regions. The option of moraines formed by the movement of ice that accumulated on the rims of the trough when it was filled with water during the Mangala Valles outflow event is consistent with the morphologic characteristics of the deposits and was preferred by Head et al. (2004). The deposits they described are located a few kilometers to the east of our mapping area and these authors pointed out that similar deposits are seen

13 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 15. Valley slope terraced (vst) unit: (a) fragment of HRSC image H0286 and (b) part of MOC image M Arrows show the terraced slopes. Fig. 16. Valley slope grooved (vsg) unit: (a) portion of HRSC image H0286, white arrows show variations in the groove orientation (third from the left arrow has noticeably different orientation comparing to other arrows), black arrows show linear ridge which could be the top of a magmatic dike (see text), (b) portion of HRSC image H0286, white arrow shows area of ground water release that formed a few small sinuous channels. Scale bar refers to both parts of the figure. on THEMIS images nearby to the west. We have identified these deposits in HRSC image 286 and MOC image M and mapped them as unit gd (see Fig. 2). The unit gd was mapped in seven localities at the northern and southern flanks of the eastern part of the Mangala Fossa (Fig. 2). The smallest localities are about 3 4 km. The largest one consisting of four radiating tongues is km across. Typically these deposits are observed at altitudes of +500 m and up to +800 m, but in places they descend along the local lows down to 100 m. These concentric sinuous ridge-like features described by Head et al. (2004) is an attribute of their surface morphology (Fig. 20a and c). The features are m wide and up to 2 km long. These assemblages have lobate planimetric geometry. In one locality the gd unit ends downslope in the m feature

14 930 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 17. Plains with mesas (pm) unit: (a and b) parts of HRSC image H0286 and (b) part of MOC image R resembling a frog (Fig. 20b, arrow). The length of individual tongues of unit gd varies from 4 to10 km Lava flows east of the mapped area As discussed previously, extensive lava flows are observed east of the mapped area (Fig. 21). They are lobate in planimetric morphology, km wide and km long. MOLA altimetry measurements show that marginal front scarps of the most prominent flows are m high. In the central part of the area in Fig. 21 are seen sections of a graben a few kilometer wide, striking NE 701, with a total length of about 1500 km. It is an obvious continuation of the Mangala source-region graben (see Figs. 1 and 21). The close spatial location of the graben and at least three lava flows suggest that source of the latter could be this graben (Fig. 21) Summary of geologic observations Based on our observations, we generally agree with the results of earlier studies (e.g., Tanaka and Chapman, 1990; Chapman and Tanaka, 1993; Craddock and Greeley, 1994; Zimbelman et al., 1994; Head et al., 2004; Levy and Head, 2005; Ghatan et al., 2005). However, the complete coverage of the study area by the 12.5 m/px HRSC image, highly informative by itself and representing valuable context for MOC images as well as the HRSC-based DTM (Fig. 2), provided new insights into the analysis and new conclusions. In a manner similar to other researchers who worked in this area, we propose that the head trough cut in the Noachian unit was a source of release of artesian water that carved Mangala Valles. This hypothesis is consistent with the fluidized appearance of crater ejecta deposit of the multilayer type in this area, suggesting enrichment of the subsurface here in liquid water (Barlow and Perez, 2003). We see this type of ejecta around craters predating the formation of the head trough (units hp and phc) and around the crater postdating units ps and vff of the Mangala Valles assemblage (unit pmc). This suggests an enrichment in subsurface liquid water in the study area during an extended period. We have found that the floors of these craters are deeper than the valley floor within the study area and that in the deepest places of the floors there are whitish smooth subareas. These may be sediments of lakes hydrologically connected to the aquifer which later fed the Mangala floods. Alternatively, water of the Mangala-carving floods could laterally infiltrated these craters and after evaporated leaving the whitish deposits. In agreement with the results of earlier studies we consider the surface of the smooth plains (ps), as a terrace of the Mangala valley. It bears streamlined relatively large craters and scours/ grooves suggesting effective erosion by a powerful flow of water (Figs. 3 5). The orientation of these erosional features on the smooth plains is consistent with a water flow that originated in the head trough. Typically we do not see small-scale signatures of water erosion on the surface of unit ps. Instead we see dunes there, sometimes densely packed into dune fields, and dust devil tracks (Fig. 6). Locally we see the blocky texture of ejecta from the relatively small impact craters superposed on unit ps, and this may be an indirect indication of the presence of lavas. All this implies that following erosional episode(s) that formed this terrace, its surface was significantly modified by eolian processes and possibly locally by volcanism. This means that in the analysis of results of crater counts on unit ps, this significant resurfacing should be taken into account. The suite of units representing the more deeply incised part of Mangala Valles (vfs, vfm, vff, vfg, and vst) bears streamlined islands, longitudinal grooves, obstacle marks, and meandering

15 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 18. Small knob terrain (kst) unit and polygonal block terrain (bpt) unit: (a) partr of HRSC image H0286 and (b and c) are parts of MOC image S White boxes in HRSC image show the positions of the figure parts (b and c). channel-like features suggesting water erosion (Figs and 15). The spatial association of the grooved valley floor with the low standing non-grooved streamlined crater and the bar behind it (Fig. 13d; see described above), is evidence that grooves on this section of the valley floor were carved by water, not by ice. Streamlined islands, longitudinal grooves and obstacle marks are indicative of powerful high-flux and high-volume water flow, while meandering channels (Fig. 22), suggest more shallow-water, lower velocity flow acting during the waning stage(s) of the floods. Published estimates suggest that the duration of the catastrophic flood episodes were within a range from a few days to a few months (e.g., Tanaka and Chapman, 1990; Zimbelman et al., 1994; Carr, 1996; Ghatan et al., 2005). This obviously relates to the high-flux, high-volume, more violent phases of the floods. The phase when the meanders formed probably had to occur over a significantly longer period. Observations of meanders of terrestrial rivers show that their formation is a much slower process. For example, measurements made by us on the map representing the 78-year long record of Willamette River, Oregon, USA (Dykaar and Wigington, 2000, their Fig. 9), showed that during the observation time, different segments of the meanders were shifting laterally with a mean rate of up to 5 10 m/year. The meandering Willamette channels are from a few tens of meters to m wide, comparable to meanders of the Mangala Valles floor. Brooks (2005) reported on an even lower rate of the lateral shift of the m wide meandering channels of Red River, Manitoba, Canada: about 4 cm/year. The meandering Rainbow Creek Channel, Thomson River, Victoria, Australia, increased its mean meander wavelength from 430 m in 1957 to 560 m in 1982; this corresponds to a meander lateral shift of 5 m/year (Brizga and Finlayson, 1990). To form the overlapping loops shown in Fig. 22, the Mangala meanders need to shift laterally by a few kilometers. We do not see successive stages of the shift that may question the direct analogy with meanders of terrestrial rivers. If we apply the terrestrial rates to the Mangala meander lateral shifts, this suggests a duration from a hundred years to about 100,000 years. The applicability of terrestrial rates to the Mangala meanders is questionable, but it provides a useful approximation. This may be justified by comparisons of the meanders within Mangala Valles with terrestrial meanders using the plot of meander wavelength versus flow width (Garde and Raju, 1977; da Silva, 2005, her Fig. 2). If we plot on this diagram (Fig. 23) a box representing the Mangala meanders (meander wavelength 1 2 km, flow width m), its location is within the terrestrial river meander trend, suggesting some similarity in the formational process. Considering a possibility that in evolution of Mangala Valles the flowing lavas could be involved (see also below the hypothesis of Leverington (2007)), we put in Fig. 23 boxes representing

16 932 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 19. Large knob terrain (klt) unit: (a) part of HRSC image H0286 and (b) portion of MOC image R Arrow in image a shows cluster of sinuous channels on the valley groove slope. meandering characteristics for lava channels of Venus and the Moon. Venusian Baltis Vallis meandering characteristics are noticeably outside the trend for terrestrial meanders. Characteristics for Venusian and lunar rilles are within the terrestrial meander trend, but typical of these rilles is a presence of well visible source of the lavas (crater or pit) and this is not typical for the Mangala meanders. So we think that the best (although not totally convincing) explanation of the formation of the Mangala meander-like features is carving them by the flowing water which source was the valley head graben. If the formation of the Mangala meander system took hundreds of years or more this requires a climate warm enough to allow water in channels to flow and build meanders at least during the summer seasons. This suggestion contradicts the present paradigm that since the early Hesperian, the climate of Mars was as cold as it is now (e.g., Scott et al., 1995; Carr, 1996; Jakosky and Phillips, 2001; Bibring et al., 2006) although some evidence of later surface fluvial activity is also discussed (e.g., Baker, 2001). This contradiction could be resolved if around 0.2 Ga ago there was a short-time period of thicker atmosphere and thus

17 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 20. Glacial deposits (gd) unit close to the source trough: (a) subarea east of mapped area described by Head et al. (2004) and (b) part of THEMIS image V ; subarea north of the head trough, part of HRSC image 246; (c) subarea shown by white outlines in (b), part of MOC image M warmer climate possibly due to massive release of volcanic gases. Volcanic gas release was suggested by Bibring et al. (2006) to support the appearance of the sulfate era in their sequence of three mineralogic eras in the history of Mars. The possible correlation of catastrophic floods and volcanic episodes on Mars was suggested by Neukum et al. (2007) and much earlier by Baker et al. (1991). The grooves/scours on the valley slope grooved (vsg) unit and in some other situations, as well as flutes on the valley floor neighboring these grooves, show two types of groove and flute orientation. One, the dominant orientation, follows the suggested direction of water flow that formed the smooth plains terrace (Fig. 4), and this flow was probably the major agent in formation of these grooves. Another type of orientation is nearly normal to the first. It occurs near the ridge that may have originated due to emplacement of a magmatic dike (see above, Fig. 16a) suggesting possible release of subsurface (artesian) water from the dikeassociated subsurface discontinuity implying the second source of Mangala Valles-carving water (the first being the Mangala Fossa). The dike ridge crosses the lower parts of some normally oriented grooves with no evidence of ponding or changing flow orientation at these places (see second left black arrow in Fig. 16a), and this suggests that the dike postdated these grooves. If this is the case, then this dike had to be emplaced, and the associated water release should occur, either in the waning stage of the formation of grooves with the first type of orientation or even later. A similar feature is observed about 50 km north of the suggested magmatic dike (Fig. 24, black arrows). In the eastern part of the mapped area, and further to the east in association with this feature, are plains with mesas and smooth plains cut with irregular troughs coalescing into a 2 3 km wide channel which opens into the deeper-incised part of Mangala Valles. This assemblage could suggest water release from one additional subsurface source. Two clusters of sinuous channels seen on the valley slope grooved unit (Figs. 16b and 19a) also imply water release from the subsurface. Evidence described above suggests the water that carved them was initially flowing over an ice cover implying that this water release happened when the water flow that formed the slope grooves had terminated. So this was one more episode of water release in the Mangala Valles area. This water release was obviously of rather small scale because it did not influence at all the orientation of flutes in the adjacent area of valley floor fluted unit. Four units (pm, kst, pbt, and klt) record the destruction of the units formed by the Mangala Valles-carving water erosion. Plains with mesas (pm) and knob small terrain (kst) formed at the expense of smooth plains due to removal of the material of their surface layer. It is not clear which process was responsible for this removal. The source of one of the clusters of sinuous channels resembles a small area of plains with mesas (compare Figs. 16b and 17). One possibility is that in order to form units pm and kst, the ps surface layer material was removed by sapping due to

18 934 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 21. Lava flows (white arrows), the source of which are interpreted to be the graben (dark arrows); this graben is a continuation of Mangala source graben; portion of the 128 px/degree MOLA-based shaded relief map. release of subsurface water. Perhaps this happened simultaneously with formation of the sinuous channel clusters. However, contrary to the case of the channel clusters, we do not see any obvious signatures of water erosion downslope of units pm and kst. Polygonal block terrain appears to represent the next stage of the process which formed units pm and kst. Although units pm, kst, and pbt are hypsometrically lower than smooth plains (ps), at the expense of which they formed, they probably should not be considered as varieties of chaotic terrain. These units became lower than unit ps due not to the type of collapse that is an attribute of chaotic terrain (e.g., Sharp, 1973; Carr and Schaber, 1977; Carr, 1979; Squyres et al., 1992), but rather due to the removal of surface material. Large knob terrain, however, probably can be considered as a variety of chaotic terrain. Blocks within it do not maintain horizontal top surfaces, but are inclined; thus is an indication of collapse, probably due to removal of subsurface water or ice. If so, the large knob terrain (klt), which is always in contact with the hypsometrically lowest valley floor smooth (vfs), could be a source of some water release. Fig. 19a shows that unit klt postdated the valley floor fluted and valley slope grooved units formed by the major water erosion, and thus can record one of the latest episodes of water release within Mangala Valles. The presence of glacial deposits on the flanks of the eastern part of the source (head) trough was probably caused by vaporization of water ponded in the graben and accumulation of ice in the cold traps of the trough rims (Head et al., 2004). Glacial deposits are not observed, however, in the flanks of the western part of the trough although ponded water should be there too and accumulation of ice should also happen there. Probably deposition of ice occurred all around the trough, but in the mountainous flanks in the east the accumulated ice flowed downslope forming glacier deposits while in the flat terrain in the west ice did not flow and no glacial deposits formed. Ice could also occur not only at the trough flanks but also downstream at the valley banks too. This could explain the formation of an ice cover on the valley grooved slope where the clusters of sinuous channels (described above) are observed (Figs. 16b and 19a), and the pitted terrain in the lowest parts of the channels (e.g., Levy and Head, 2005). Recently Leverington (2007) considered an alternative hypothesis, that is, that Mangala Valles system was formed by volcanic lava erosion. He pointed out that although aqueous mechanisms for formation of Mangala Valles are broadly congruous with known characteristics of the channel system, an alternative volcanic hypothesis for formation of the system appears to be worthy of consideration on the basis of (1) its consistency with the volcanotectonic nature of the system and (2) commonalities between the basic nature of the system and that of large volcanic channels of the inner solar system. On the basis of previous analyses, the formation of Mangala Valles is indeed linked to magmatism and associated tectonics. Long graben radial to central Tharsis have been interpreted to be formed by dike emplacement events (e.g., Wilson and Head, 2002). This interpretation is supported by the observation of lava flows emerging from the eastern continuation of the Mangala head graben (Fig. 21). The graben in turn cracked the cryosphere and permitted the release of groundwater held under hydrostatic pressure (e.g., Head et al., 2006). Initial penetration of the dike to the surface in the Mangala Valles source region was accompanied by a phreatomagmatic eruption (Wilson and Head, 2004) caused by the explosive interaction of the rising magma with groundwater and melted ground ice. Dike emplacement events were accompanied by collapse and enlargement of the graben related to aqueous flow (e.g., Leask et al., 2007). Little evidence has been found in this study that would support the lava flow origin for the Mangala Valles themselves. As pointed out by Leverington (2007) the aqueous-flood hypotheses can account for the basic nature of the Mangala Valles system and has as a result been appropriately used as a foundation for recent studies of the system. In outlining the concept of volcanic processes as alternative mechanisms of channel formation at Mangala Valles, Leverington (2007) points to the following lines of evidence: (1) location of the head of the system at a large structural feature known to have been a source for volcanic eruptions; (2) distribution of landforms with volcanic affinities along and beyond the Mangala Valles system, suggesting past conveyance and pooling of large volcanic flows by the system; and (3) the potential for lunar and Venusian landforms to act as both morphological and process-based analogs for formation and evolution of the system. Although we searched for evidence of possible extrusive volcanism in the interior of Mangala Valles, we found no evidence to support a volcanic origin for channels or for flow-related features, nor did we find evidence for volcanic vents. Those features that appear to initiate within the Valles are much more plausibly interpreted as late stage drainage of water in the waning stages of aqueous floods. Smooth deposits on the channel floors appear to be plausibly interpreted as sediment-ice residues from the aqueous flooding event (e.g., Levy and Head, 2005). Moreover, as it was mentioned above, the deeply incised part of Mangala valley locally has depressions whose floors are hypsometrically lower than the valley floor downstream. From analysis of the HRSC-based DTM of the mapped area, we see that these lows do not show any correlative lows in the topography of neighboring terrains. This excludes a tectonic origin for the depressions and suggests that they were carved by the valleyforming liquid. The floors of some of these depressions bear obstacle marks and meandering channel-like features (Figs. 8, 9, 11, and 22). If these features were carved by lava, the latter should

19 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 22. Meanders on the floor of the uppermost part of Mangala Valles: (a) fragment of highpass-filtered image HRSC H0286 and (b) meanders drawn on unfiltered image. Fig. 23. Diagram showing dependence of the meander wavelength on the flow width for terrestrial meanders (dots, data from Garde and Raju, 1977; da Silva, 2005). Boxes show the ranges of these characteristics for the meanders of Mangala Valles, Mars, Schroeter Vallis and Hadley rille, the Moon (box 1), Samuda Vallis and Lo Sheh Valles, Venus (box 2) and Baltis Vallis, Venus (box 3). be ponded in the depressions and hide the features. Water as the valley-carving liquid could and obviously did infiltrate into the subsurface and/or evaporated, leaving the features visible. So while it is always important to consider alternative hypotheses, on the basis of our analysis, we agree with Leverington (2007) that the aqueous-flood hypotheses can account for the basic nature of the Mangala Valles system. In summary, geologic analysis of HRSC image 286, the DTM derived from it, available MOC images and some THEMIS images, showed that Mangala Valles was carved by water and that there was more than one sources of valley-forming water: The major source was the head trough graben, and it may have been supplemented by a few smaller-scale sources, including the localities of the two magmatic dikes, sources of two clusters of sinuous channels and probably the eleven localities of large knob terrain. In the Mangala Valles elements we could see some age sequence: the terrace of smooth plains is the most ancient, units of the deeper-incised part of the valley being partly contemporaneous with smooth plains and partly postdating them, and finally four units formed at the expense of the water-carved units. Glacial deposits mapped on the flanks of the head trough were found to be superposed on the smooth plains terrace but their age relations with the younger elements of Mangala Valles could not be derived from the morphologic record. Geologic analysis, however, showing the sequence of events, could not resolve if there were several episodes of valley formation, significantly separated in time and evolution, or whether it was one relatively short episode in time with a few phases. To resolve this question we have made crater counts on the HRSC, MOC, and THEMIS images covering the different mapped geologic units.

20 936 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 24. Suggested magmatic dike in 280 km NNE of the head trough; mosaic of HRSC images H0286 and H0644 and THEMIS images V , V , and V Crater counts 3.1. Crater counting methodology On the basis of our geologic map described above, we choose areas to perform crater counts. Except for a few cases, we counted craters separately on different units even if the units were neighboring each other, and from geologic analysis appeared to be of the same age. For crater counts we used HRSC image 286 (nadir channel, 12.5 m/px resolution), as well as MOC narrowangle and THEMIS daytime images. Film transparencies with the areas selected for counts were produced and examined using a Zeiss PSK-2 stereo comparator. Crater measurements were recorded and partitioned into bins of increasing crater diameter based on standard practices, and these binned data were used to produce cumulative crater size-frequency distribution plots with corresponding statistical errors (see e.g., Hartmann, 1966; Neukum and Wise, 1976; Crater Analysis Techniques Working Group, 1979; Neukum and Hiller, 1981; Neukum, 1983). The cumulative size-frequency distributions were analyzed to determine crater densities at specific reference diameters, and cratering model ages, following Hartmann and Neukum (2001). Cumulative crater densities for 1, 2, 5, 10, and 16-km diameter craters were used by Hartmann and Neukum (2001) to assess the relative ages for Martian geologic units and to place units into the Martian chronologic system using key units as reference (e.g., Tanaka et al., 1992). A cratering model age (in Ga) was calculated from the cumulative crater density at a reference crater diameter of 1 km, using an established cratering chronology model for Mars (Neukum, 1983; Hartmann and Neukum, 2001; Ivanov, 2001).The lunar cratering chronology was extrapolated from the lunar model, in which crater frequencies were correlated with radiometric ages from Apollo samples. The model has been adjusted for Mars, relative to the Moon, for different orbital mechanics, crater scaling, and impact flux. The transfer of the lunar cratering chronology model to Mars could introduce a systematic error of up to a factor of 2 (Hartmann and Neukum, 2001; Ivanov, 2001). This means that the typical uncertainty in cratering model ages could vary by a factor of 2 for ages o3.5 Ga (in the constant flux range), whereas the uncertainty is about 7100 Ma for ages 43.5 Ga (Hartmann and Neukum, 2001). Extensive testing and application of these techniques, however, have shown that the applied Martian cratering chronology model results in ages for basin formation and volcanic surfaces that are in good agreement with Martian meteorite crystallization ages with respect to peak activity periods (Neukum et al., 2007). This suggests that the chronology model is correct within an uncertainty of less than 20% (Werner, 2005). Specific error bars for each cratering model age are included in Table 1. We did not observe a deviation in the size-frequency curve steepness between measured and predicted crater size-frequency distributions, except for kinks (which are attributed to resurfacing processes) even down to crater sizes of about m

21 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Table 1 Results of crater counts for the study area No Unit Image Bas. age Res. age 1 Res. age 2 Area (km 2 ) No Unit Image Bas. age Res. age 1 Res. age 2 Area (km 2 ) 1 hp HRSC vff HRSC hp E vff HRSC hp E vff HRSC phc HRSC vff E ps HRSC vff E ps HRSC vff E ps HRSC vff E o ps R vff vfg HRSC 286? ps HRSC vfg vfs HRSC ps HRSC vfg HRSC ? ps HRSC vfg FHA01232? ps R vfg E ps FHA vfg R ps HRSC vfg HRSC ps E vfp HRSC ps R vfp R ps R vst HRSC ps R ? vsg HRSC ps HRSC vsg HRSC ps HRSC vsg HRSC ps HRSC vsg E ps HRSC vsg R ps V vsg THM mos ps THM mos vsg S ps MOC mos.? pm E ps MOC mos bpt S ps MOC mos gd M ps vfm HRSC gd M vfs HRSC gd V vfs HRSC gd THM mos vfs M gd THM mos vfs HRSC lf HRSC vfs MOC mos lf HRSC vfs S lf HRSC vfs S ? lf HRSC vfs S lf HRSC vfm HRSC lf HRSC vfm HRSC lf HRSC vfm MOC mos lf HRSC vfm HRSC lf V vfm HRSC lf V vfm HRSC lf V Numbers starting with E, F, M, R, and S are MOC images, and numbers starting with V are THEMIS images. in diameter, when counting on MOC images, and m, when counting on HRSC and THEMIS images. We note this because of the ongoing controversy regarding the minimum crater size that is reasonable to include in crater counts of Mars (e.g., McEwen et al., 2005; Hartmann, 2005), due to the concern that many craters p300 m diameter are distant secondary craters, rather than primary craters. Recently Malin et al. (2006) estimated the current Martian cratering rate based on changes observed in MOC images over a 7-year period. Their work showed that the models that scale lunar cratering rates to Mars are consistent with the observed Martian cratering rate (Hartmann, 2007). For surfaces with complex geologic histories, the crater sizefrequency data require multiple curve fits because of one or more kinks in the plotted data (e.g., Neukum and Hiller, 1981; Werner, 2005). These kinks are interpreted to represent resurfacing events, in which the crater distribution has been affected by either the emplacement of new material (e.g., covering by new volcanic flows, see Hiesinger et al. (2002), or aeolian deposition) or the degradation of existing material (e.g., by fluvial or aeolian erosion), such that the model age date marks the end of a period of significant erosion. A cratering model age is obtained from the crater production function (Hartmann and Neukum, 2001) using a non-linear least-squares fitting procedure (following Marquardt, 1963; Levenberg, 1944). Any deviation from the crater production function indicates a modification of the geological unit as explained above. New piece-wise fitting methods yield a refined cratering model age to identify the end of the resurfacing processes. The procedure involves fitting the well-known crater production function to the crater size range smaller than a certain diameter d* and predicting the expected number N(d*) of craters larger than that diameter. Because the production function is cumulative towards lower sizes, N(d*) influences the position of the curve in the smaller diameter range. There are two approaches to finding the expected N(d*): the first (Werner, 2005) is to calculate the excess in the measured N(d*) by fitting the production function to the larger diameter range. That fit can be used to obtain the excess over the smaller diameter fit at the cut-off diameter d*. The other approach, also used here, is to obtain N(d*) by an iterative extrapolation process from the smaller diameter measurements alone (Michael and Neukum, 2007). The corrected cumulative crater size-frequency distribution can finally be used to derive the resurfacing age. In cases where additional resurfacing processes took place, the correction has to be repeated at the next cut-off diameter (Werner, 2005).

22 938 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Crater count results The results of crater counts (Figs and Table 1) show the estimated ages for different localities where the craters were counted. In Fig. 30, as well as in the text, the age estimates were mostly rounded to one decimal place. In Figs , we did not round the age estimates, in order to give the reader a sense of the accuracy of the method. Fig. 31 shows a histogram that summarizes all of the age estimates. It is necessary to emphasize that crater counts on erosional surfaces (the case for the valley units) represent the age of erosion (and subsequent resurfacing), but not the age of the eroded material. In some cases, however, when we have kinks on the crater count diagram, the oldest estimate may record the pre-erosional geologic event. One of the first things that is observed in Figs is the wide range of age estimates even for the same unit, or for units of supposedly the same, or similar, geologic age. This, however, does not discredit the results because as it was shown in the geologic analysis and mapping discussions, most units have been repeatedly resurfaced after their formation. Therefore, in attempting to estimate the age of the units, we should pay special attention to the oldest ages. In the following discussion, we consider the results by sorting them into five assemblages: (1) Noachian highland units; (2) smooth plains forming a terrace of Mangala Valles; (3) assemblage of units representing different elements of the deeply incised part of the valley; (4) assemblage of units pm, kst, pbt, and klt, representing various types of degradation of the valley floor, slope, and terrace units, plus glacial deposit unit; and (5) lavas east of the Mangala source graben Noachian highland units Two units of this assemblage have been dated: highland plateau (hp) and post-hummocky-plains (large) craters (phc). Dating of these units was undertaken although their morphology leaves no doubt about their belonging to the Noachian highland assemblage. Unit hp has three age estimates in generally one locality in the north-central part of the mapped area, one based on counts on an HRSC image (3.8 Ga with an episode of resurfacing at 2.5 Ga), and two based on counts on MOC images (3.5 and 3.5 Ga) (Figs. 25 and 30a). This confirms that they belong to the Noachian highlands assemblage (Hartmann and Neukum, 2001). The signature of resurfacing at 2.5 Ga ago probably does not reflect any specific event but represents the cumulative effect of the neighboring fluvial and accompanied lacustrine and glacial activities during the Mangala Valley floods (see below). Dating of a locality of unit phc at the southern flank of the head trough led to the age of 1.5 Ga. This is probably not the true age of the unit but again the effect of later resurfacing. We conclude this because formation of this crater obviously predated formation of the head trough (see Figs. 2 and 7) which, being the source for Mangala Valles floods, had to have formed around 3.5 Ga ago (see below). The count area is neighboring two localities of glacial deposits (see Figs. 2 and 30a) and probably was covered by ice and glacial deposits in some part. Fig. 25. Crater count plot for unit hp. Upper Noachian age of the unit (3.8) and 2.7 Ga age of resurfacing are seen. Counts were done on HRSC image H Smooth plains We have done crater counts in 24 localities on this unit. As seen from Table 1 and Figs. 25a, b and 30a, among them there is a group of five dates that show ages around 3.5 Ga with episodes of subsequent resurfacing. For one locality at the very beginning of Fig. 26. Crater count plots for unit ps showing ages close to 3.5 Ga, which we consider as the age of the unit, and younger ages due to subsequent resurfacing. Counts were done on HRSC image H0286.

23 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 27. Crater count plots for unit vsf (a and b), htf (c), and vfm (d and e) showing ages close to 1 and 0.2 Ga, which we consider as ages of floods contributing to the formation of the deeply incised part of the valley. Counts were done on HRSC image H0286 (a and d) and MOC images M (b), S S (c), and R R (d). Fig. 28. Crater count plots for unit bpt (a) and gd (b and c). Counts were done on MOC images S (a) and M (b), and THEMIS image V (c). Fig. 29. Crater count plots for lava flows east of the Mangala source graben. Counts were done on THEMIS image V (a) and HRSC H4095 image (b and c).

24 940 A.T. Basilevsky et al. / Planetary and Space Science 57 (2009) Fig. 30. Results of age estimates for different unit assemblages: (a) highland units (white numbers) and smooth plains (black numbers), (b) units of the deeply incised part of the valley, and (c) units representing degradation of the valley elements and glacial deposits. Counts on HRSC image are shown by a regular font, counts on MOC and THEMIS images, by italics. Positions of numbers on the figure correspond approximately to locations where crater counts have been done.