ARTICLE IN PRESS Planetary and Space Science 54 (2006) 219–242 www.elsevier.com/locate/pss New observations of Warrego Valles, Mars: Evidence for precipitation and surface runoff V. AnsanÃ, N. Mangold Laboratoire IDES - UMR 8148-CNRS, Universite´ Paris-Sud, baˆt. 509, 91405 Orsay Cedex, France Received 25 August 2004; received in revised form 11 July 2005; accepted 12 December 2005 Abstract Most valley networks have been identified primarily in the heavily cratered uplands which are Noachian in age (43.5 Gyr). A striking exception to this general observation is Warrego Valles located on the southeastern part of the Tharsis bulge. Recent data obtained by the Mars Orbiter Laser Altimeter, the Thermal Emission Imaging System (THEMIS) spectrometer and the Mars Orbiter Camera give new insight into the formation of valley networks and the early Mars climate. We focus our study on the southern Thaumasia region especially on Warrego Valles and determine the organisation of valleys in relation to regional topography and structural geology. Warrego Valles is the most mature valley network that incised the southern side of Thaumasia highlands. It developed in a rectangular- shaped, concave-up drainage basin. Four times more valleys are identified in THEMIS infrared images than in Viking images. Valleys exist on both sides of the main tributary contrary to what was visible in Viking images. Their distribution is highly controlled by topographic slope, e.g. there is a parallel pattern on the sides and dendritic pattern on the central part of Warrego Valles. We quantitatively analyse valley morphology and morphometry to determine the processes responsible for valley network formation. Warrego Valles displays morphometric properties similar to those of a terrestrial fluvial valley network. This valley network is characterised by seven Strahler’s orders, a bifurcation ratio of 3, a length ratio of 1.7, a drainage density of 0.53 kmÀ1 and a ruggedness number of 3.3. The hypsometric curve and integral (0.46) indicate that Warrego Valles reached the mature Davis’ stage. Valleys have undergone external degradation since their incision, which masks their main morphological characteristics. Our study supports the assertion that valley networks formed by fluvial processes controlled by an atmospheric water cycle. Further, they seem to develop by successive stages of erosion that occurred during Noachian through the late Hesperian. r 2006 Elsevier Ltd. All rights reserved. Keywords: Warrego Valles; Mars; Valley network; Water; Fluvial process; Noachian–Hesperian 1. Introduction combination of surface runoff and groundwater sapping (Milton, 1973; Baker and Kochel, 1979; Gulick and Baker, Since the Mariner missions, valley networks are found in 1989, 1990; Baker et al., 1992; Carr, 1995, 1996; Grant, the heavy cratered uplands south of the Martian dichot- 2000; Malin and Edgett, 2000; Ansan and Mangold, 2003); omy (Schultz and Ingerson, 1973; Carr and Clow, 1981; (2) groundwater sapping resulting from geothermal or Mars Channel Working Group, 1983; Carr, 1995). Valley hydrothermal heating (Sharp and Malin, 1975; Pieri, 1976, networks are defined as a set of valleys typically arranged 1980; Howard, 1988; Squyres, 1989; Baker, 1990; Goldspiel in a dendritic pattern. Because of their similarity with that et al., 1993; Gulick, 1998, 2001; Goldspiel and Squyres, of terrestrial fluvial valley networks, a variety of processes 2000; Luo, 2002); (3) groundwater flow associated with involving liquid water have been proposed to explain their chemical and mechanical erosion (Malin and Carr, 1999; formation. Such processes include: (1) fluvial erosion by a Carr and Malin, 2000); and (4) water-lubricated mass wasting (Carr, 1995). Because valley networks are found on ÃCorresponding author. Fax: +33 1 60 19 14 46. the heavily cratered terrains, they probably formed during E-mail addresses: [email protected] (V. Ansan), the late Noachian (43.5 Gyr). Their formation by liquid [email protected] (N. Mangold). water would imply that Mars had a warmer and wetter 0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.12.009 ARTICLE IN PRESS 220 V. Ansan, N. Mangold / Planetary and Space Science 54 (2006) 219–242 climate early in its history (Sagan et al., 1973; Carr, 1981; Emission Imaging System (THEMIS) which covers visible Craddock and Howard, 2002) because liquid water is and thermal infrared (IR) wavelengths (Christensen et al., unstable on the Martian surface under the present-day 2003). By combining these data sets (altimetry, Viking, climatic conditions (Leighton et al., 1965; Farmer and MOC and THEMIS images) and using standard geospatial Doms, 1979). The debate continues about climatic condi- analytical techniques that are available in many Geo- tions on early Mars because numerous climate models graphic Information Systems (GIS), we derived a geologic argue in favour of a cold and dry climate since 4 Gyr map of this area in which valleys networks, craters and (Kasting, 1991; Clifford, 1993). faults were identified. Our objectives are to: (1) determine In order to discriminate between the two opposite the organisation of valleys in relation with the regional Martian climatic scenarios, we analysed valley networks topography and structural geology; (2) estimate the age located on the southern boundary of the Tharsis bulge in and the duration of the formation of the valley networks; the Thaumasia region (Fig. 1a). This region is characterised (3) analyse the morphology and the morphometry of by a 4 km high plateau surrounded by highlands rising to Warrego Valles; (4) determine the process(es) for valley an elevation 45km(Fig. 1b). Numerous folds, faults, rift network formation; and (5) deduce the climatic conditions valleys and volcanoes have been identified (Schultz and in which Warrego Valles and surrounding valley networks Tanaka, 1994; Tanaka et al., 1998; Dohm and Tanaka, formed within the Thaumasia highlands. 1999; Dohm et al., 2001). These investigations indicated that Thaumasia is 3.5 Gyr old corresponding to the 2. Morphology and age of valley networks Noachian epoch (Tanaka, 1986; Dohm and Tanaka, 1999). It is bounded by cratered plains dated from the late The southern boundary of Thaumasia region has been Noachian to the early Hesperian in the south (Dohm et al., dissected by numerous valley networks (Fig. 1). The most 2001). The Thaumasia highlands have been dissected by well-defined network, Warrego Valles, developed in the numerous valley networks (Fig. 1a and 1b) which debouch area between 401 and 441 south, and between 911 and 961 onto the southern plains (Fig. 1a). The valley networks west. We studied the characteristics of the valley network formed from the late Noachian through the early system at different scales, using both MOLA altimetry, and Hesperian ( 3.0 Gyr) based on their relative chronology o available visible and mid-IR images acquired during the and crater counts from Viking images (Tanaka, 1986; Viking, MGS and Mars Odyssey missions. Each data set Tanaka et al., 1998; Dohm and Tanaka, 1999). Valley was projected on the IAU2000 Mars datum (Duxbury networks display an usually low degree of organisation et al., 2002; Seidelmann et al., 2002) with a Lambert with few branches in this area, but some of them appear to conformal projection in GIS (ARCVIEW and Er-mapper). be more mature, such as Warrego Valles, which is located From these data, we mapped the valley networks, impact (421S–931W) on a regional south-facing slope on the craters and faults present in the Warrego Valles area in southern boundary of the Thaumasia region (WR in order to define the geologic context in which valley Fig. 1). Viking images show that Warrego Valles is networks formed. characterised by a well-formed parallel valley system in which secondary valleys join the main one (Pieri, 1980). It is interpreted to have been formed by surface runoff 2.1. Organisation of valleys in Warrego Valles from imagery because of its resemblance to terrestrial fluvial valley systems at different scales (Carr, 1981; Baker, 1982, 1985). However, surface runoff is often rejected because of the low drainage density and the Warrego Valles was first mapped using Viking image low degree of valley organisation of Warrego system (Cabrol mosaics (MDIM.2, images extracted from PDS web site; and Grin, 2001; Stepinski and Collier, 2003). Alternatively, it Kirk et al., 2001) with a resolution of 200 m/pixel at 421S has been suggested that groundwater sapping triggered by of latitude (Fig. 2). In this mosaic, the Martian surface was geothermal or hydrothermal heating (Tanaka et al., 1998; illuminated by low sun angles, which enhanced the local Dohm and Tanaka, 1999; Gulick, 2001) supported by the ground morphology. We superposed this mosaic on to regional volcanic and tectonic context of Thaumasia high- MOLA altimetry data, interpolated with a spatial resolu- lands, carved the Warrego system. tion of 463 m/pixel (Zuber et al., 1992; Smith et al., 1999, New data from Mars Global Surveyor and Mars 2001), to help the geomorphic analysis. Odyssey allow us to study Warrego Valles in much more The southern part of the Viking mosaic contains the detail. These data allow us to test hypothesis regarding the Hesperian-age plain of Sirenum Terra (Tanaka, 1986; origin of valley networks in this area. Unlike previous Dohm and Tanaka, 1999) standing at an elevation of studies, we also have topography from the Mars Orbiter 2.4 km. It borders the southern Thaumasia highlands that Laser Altimeter (MOLA) (Smith, 1998) with a height reach a maximum elevation of 8.8 km (92.71W–40.51S) accuracy of 50 cm. In addition, the surface of Mars has with a 21 south-facing slope. The Thaumasia highlands been imaged with spatial resolution ranging from 100 to are cut by three sets of normal faults oriented W–E, few m/pixel by different instruments including the Mars NNE–SSW and NNW–SSE, all of which contain long and Orbiter Camera (MOC) (Malin et al., 1998) and Thermal narrow grabens that crosscut the southern Hesperian plain.