ARTICLE IN PRESS Planetary and Space Science 57 (2009) 982–999 Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Fluvial morphology of Naktong Vallis, Mars: A late activity with multiple processes S. Bouley a,b,Ã, V. Ansan a,b, N. Mangold a,b, Ph. Masson a, G. Neukum c a IDES UMR8148 CNRS, University Paris Sud, 91405 Orsay, France b LPG Nantes UMR6112 CNRS 44322, University Nantes, France c FU Berlin, Germany article info abstract Article history: The morphology of fluvial valleys on Mars provides insight into surface and subsurface hydrology, as Received 17 July 2008 well as to Mars’ past climate. In this study, Naktong Vallis and its tributaries were examined from high- Received in revised form resolution stereoscopic camera (HRSC) images, thermal emission imaging system (THEMIS) daytime IR 26 January 2009 images, and mars orbiter laser altimeter (MOLA) data. Naktong Vallis is the southern part of a very large Accepted 29 January 2009 fluvial basin composed by Mamers, Scamander, and Naktong Vallis with a total length of 4700 km, and is Available online 10 February 2009 one of the largest fluvial system on Mars. Naktong Vallis incised along its path a series of smooth Keywords: intercrater plains. Naktong’s main valley cut smooth plains during the Early Hesperian period, estimated Mars 3.6–3.7 Gyr, implying a young age for the valley when compared to usual Noachian-aged valley Valley network networks. Branching valleys located in degraded terrains south of the main Naktong valley have sources Sapping inside a large plateau located at more than 2000 m elevation. Connections between these valleys and Runoff Hesperian period Naktong Vallis have been erased by the superimposition of late intercrater plains of Early to Late Hesperian age, but it is likely that this plateau represents the main source of water. Small re-incisions of these late plains show that there was at least one local reactivation. In addition, valley heads are often amphitheatre-shaped. Despite the possibility of subsurface flows, the occurrence of many branching valleys upstream of Naktong’s main valley indicate that runoff may have played an important role in Naktong Vallis network formation. The importance of erosional landforms in the Naktong Vallis network indicates that fluvial activity was important and not necessarily lower in the Early Hesperian epoch than during the Noachian period. The relationships between overland flows and sapping features suggest a strong link between the two processes, rather than a progressive shift from surface to subsurface flow. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction terrains, suggesting that they were formed at the end of the Noachian period and or at the beginning of the Hesperian (Hr) Since the beginning of Mars orbital explorations, valley period (Tanaka, 1986; Scott and Tanaka, 1986; Greeley and Guest, networks have been identified in the heavily cratered terrain 1987; Carr, 1996; Hartmann and Neukum, 2001; Irwin et al., located in the southern hemisphere of Mars (e.g., Milton, 1973; 2005a; Fassett and Head, 2008). Valley networks have also incised Schultz and Ingerson, 1973; Sharp and Malin, 1975; Carr and Clow, preferentially into large impact crater rims (Craddock and 1981; Mars Channel Working Group, 1983; Carr, 1996). Valleys are Maxwell, 1993; Craddock et al., 1997; Craddock and Howard, usually arranged in a branching pattern similar to that of fluvial 2002; Forsberg-Taylor et al., 2004) and upland regions where the valley networks on Earth, suggesting that they were formed regional slope is sufficiently high, slope 411 (e.g., Baker, 1982; mainly by liquid water flows when the Martian climate may have Brakenridge, 1988; Craddock and Howard, 2002; Ansan and been warmer than today (Carr, 1981; Gulick, 2001; Craddock and Mangold, 2006; Ansan et al., 2008). The presence of valleys on Howard, 2002). Valley networks were incised into Noachian, flatter terrains is still debatable, since valley networks often 43.7 Ga, heavily cratered terrain without affecting younger disappear into smooth, ridged, and flat regions located between large degraded impact craters (Baker, 1982). These regions, called ridged intercrater plains, have been mapped as Late Noachian plains (e.g., Nplr) or Early Hesperian plains (Scott and Tanaka, Ã Corresponding author at: IDES UMR8148 CNRS, University Paris Sud, 91405 Orsay, France. Tel.: +33169 15 63 47. 1986; Greeley and Guest, 1987) depending on crater density. The E-mail address: [email protected] (S. Bouley). nature and origin of these plains remains uncertain. Plains may be 0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2009.01.015 ARTICLE IN PRESS S. Bouley et al. / Planetary and Space Science 57 (2009) 982–999 983 composed of a mixture of impact debris and melt products Our study focused on the source area of the Naktong Vallis (Mouginis-Mark et al., 1981); volcanic flows (Greeley and Spudis, basin, centered at 5.241N–32.91E. Questions still remain regarding 1978; Tanaka, 1986; Brakenridge, 1988; Greeley and Guest, 1987); the origin of the flows that created such a large and long valley or sedimentary rocks (Malin, 1976; Scott and Carr, 1978) including system with putative lacustrine systems. The size of the valley aeolian (Moore, 1990), fluvial (Scott and Tanaka, 1986; Greeley and system suggests a terrestrial-like hydrologic flow to sustain liquid Guest, 1987), and lacustrine deposits (Carr, 1981; Greeley, 1985; water. However, the valley network geometry presented in our Irwin and Howard, 2002). study indicates a low number of tributaries in the source region Understanding hydrologic and chronological relationships, that question the role of overland flows in creating Naktong Vallis. between dissected heavily cratered terrain and non-dissected When compared to previous studies completed using mars orbiter intercrater plains are important for determining the early Martian laser altimeter (MOLA) topographic data (Smith et al., 1999) and water cycle, and many questions remain. For example, we still do thermal emission imaging system (THEMIS) infrared (IR) datasets not understand when intercrater plains formed relative to fluvial at 100 m/pixel (Christensen et al., 2003), our study contains valleys, or how their formation interacted with the presence of improved data from high resolution visible images, 10–20 m/pixel, liquid water. Questions of this nature were examined in the study acquired by the high-resolution stereoscopic camera (HRSC) area of Terra Sabaea, 301S–401N, 101E–751E, which contains many onboard the Mars Express orbiter (Neukum et al., 2004). Using highlands, intercrater plains, and large craters, Fig. 1. Terra Sabaea this data, we focused on relationships between different geologi- is located south of the Martian dichotomy, dipping 0.21 north- cal units, especially, intercrater plains and fluvial landforms inside westward with a maximum elevation of 5 km, north of the Hellas the Naktong basin, to determine when valley networks formed basin boundary. The largest impact craters of Terra Sabaea are and which processes controlled their formation. Huygens, Schiaparelli, and Tikhonravov, which are characterized by a high degree of degradation. Terra Sabaea has several 4300 km valleys (e.g., Indus Vallis, Mamers Vallis, Scamander 2. Data Vallis, and Naktong Vallis) whose downstream direction is controlled by the regional slope to the northwest, Fig. 1. In this We used nine panchromatic nadir images acquired by the high region, Naktong, Scamander, and Mamers form a single-valley resolution stereo camera of Mars Express (Neukum et al., 2004; system, if we assume that the three valleys were once connected, Jaumann et al., 2007) that have a spatial resolution ranging from and that intercrater plains formed later than valleys or were 10 to 15 m/pixel at the image center (orbits #h1917, h1928, h1950, intermediary lakes (Irwin et al., 2005a), Fig. 1. The Naktong, h1961, h1972, h1994, h2005, h2027, and h3183). We mosaicked Scamander, and Mamers valley system is the longest organized images at a spatial resolution of 20 m/pixel to obtain a regional valley on Mars, with a total length of 4700 km. The valley system, map, Fig. 2. Fig. 1, starts close to the summit of Terra Sabaea , 51S–401E, 3km HRSC images do not cover the whole region of Naktong Vallis. in elevation, crosscuts the whole Arabia Terra, and debouches Therefore, we completed the study area with a mosaic of the northward into Deuteronilus Colles, 47.61N–7.21E, 4km in thermal emission imaging system (Christensen et al., 2003) elevation. The valley crosses different geologic units from a daytime infrared images, Fig. 2, having a spatial resolution of Noachian age (Hoke and Hynek, 2008) to an Early Hesperian age 230 m/pixel. These datasets allowed us to map the whole Naktong (McGill, 2000; Fassett and Head, 2008), that may have involved Vallis network and the different geologic/geomorphic terrains different formation processes—from predominant runoff in with classical photo-interpretation methods. Naktong Vallis (Irwin et al., 2005a) to more sapping controlled valleys in Mamers Vallis (McGill, 2000; Carr, 2001). Fig. 1. (a) MOLA topographic map. The black box corresponds to Terra Sabaea, and (b) MOLA topographic map for which most geographic features (e.g., terrae, impact craters, and valley) are labeled. Terra Sabaea is located at the northwestern side of Hellas Basin and the southeastern side of Arabia Terra. White lines correspond to the location of the longest discontinuous Martian valley network including Naktong Vallis, Scamander Vallis, and Mamers Vallis from head to outlet (Deuteronilus Colles). Note that a widespread depression, bounded by black Fig. 2. Mosaic of nine HRSC images (h1917, h1928, h1950, h1961, h1972, h1994, arrows, is located to the southeast of the Naktong Vallis, reaching the outer bulge h2005, h2027, h3183) with a spatial resolution of 20 m/pixel, and THEMIS daytime of Hellas basin.