Identification of a New Outflow Channel on Mars in Syrtis Major Planum

Home , Dao Vallis, Harmakhis Vallis, Syrtis Major Planum

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Planetary and Space Science 56 (2008) 1030–1042 www.elsevier.com/locate/pss

Identiﬁcation of a new outﬂow channel on Mars in Syrtis Major Planum using HRSC/MEx data

N. Mangolda,Ã, V. Ansana, D. Baratouxb, F. Costarda, L. Dupeyrata, H. Hiesingerc, Ph. Massona, G. Neukumd, P. Pinetb

aIDES, UMR8148, CNRS, and Universite´ Paris-Sud, Batiment 509, 91405 Orsay, France bDTP-OMP, UMR5562, CNRS, Toulouse, France cInstitut fu¨r Planetologie, 48149 Mu¨nster, Germany dFreie Universita¨t, Berlin, Germany Received 20 August 2007; received in revised form 24 January 2008; accepted 28 January 2008 Available online 16 February 2008

Abstract

Syrtis Major Planum is a volcanic plain dominated by lava flows. High resolution stereo camera (HRSC) images of the northern Syrtis Major region display erosional features such as grooves, teardrop-shaped islands and valleys. These landforms are characteristics of outflow channels seen on Mars, therefore implying that a flood event took place in this region. The flow of 100 km long and a few kilometer wide followed the local slopes in most locations. Maximum flood discharges estimated from images and topography vary from about 0.3 106 to 8 106 m3/s, and therefore are in the range of terrestrial mega-floods in the Scablands or Lake Bonneville. In North Syrtis Major, the relationships with surrounding lava flows and the timing of the flood coeval to Syrtis Major volcanic activity suggest that it could be related to the subsurface water discharge mobilized by the volcanic activity. The proximity of Noachian age basement rocks 20 km away from the flood and below lava flows might have played a role in its formation and water presence. r 2008 Elsevier Ltd. All rights reserved.

Keywords: Mars; Outﬂow; Water; Volcanism

1. Introduction Mangala Valles (Southwest of the Tharsis region) or Athabasca Valles (southeast of Elysium region) have Very energetic floods suggesting catastrophic episodes of channel characteristics similar to those of Xanthe Terra concentrated water flows formed outflow channels on Mars but their source areas consist of a single fracture, probably in a similar way as the release of water by glacial surges on related to volcano-tectonic activity (Burr et al., 2002; Head Earth. However, the exact origin and conditions under et al., 2003) or tectonic processes (Hanna and Phillips, which the outflow channels formed on Mars are still 2006). The identification of new areas of similar floods is controversial (e.g., Carr, 2006). Outflow channels of the important to complete the classification and geographic Xanthe–Margaritifer region, east of Valles Marineris, date distribution of Martian outflows and to understand their back to the Hesperian period (43 Gy ago) and chaotic relationships with magmatic activity, groundwater distri- terrains of unknown origin characterize their source bution and climatic events. regions. Most models suggest a formation related to The high resolution and spatial coverage of the high groundwater release either from aquifers, or permafrost resolution stereo camera (HRSC) allows us to study melting resulting from increased geothermal flux, or outflow channels in more detail or discover new locations magmatic activity (Carr, 1979; Baker et al., 1992; of similar flood activity. The studied region is located at the Rodriguez et al., 2005; Coleman, 2005). Outflows such as transition from Syrtis Major Planum and the Nili Fossae regions (161N, 75–761E), in the eastern hemisphere, west of ÃCorresponding author. the Isidis plain (Fig. 1). No landform indicative of volcano- E-mail address: [email protected] (N. Mangold). ice interactions (pseudocraters, table mountains, etc.) or

Fig. 2. (a) Overall image of the studied area (HRSC mosaic 1347-988). At this low resolution the deep valley (DV) in the bottom center of the image is the only landform visible suggesting a lava tube associated to the lava ﬂows. Details shown in Figs. 3–5 show other erosional features not consistent with volcanic processes.

of interest. The Digital Elevation Model (DEM) of each orbit has been computed using tools developed for the HRSC instrument (Gwinner et al., 2005). Nevertheless, the accuracy is very low over smooth plains which lack roughness, limiting the number of correlation points (e.g. Ansan et al., 2008). We therefore use MOLA data for topographic measurements rather than HRSC DEMs on these smooth plains. The Nili Fossae region belongs to Noachian highlands Fig. 1. (a) MOC wide-angle mosaic of the Syrtis Major region. (b) Close- up over the studied region (centered 171N, 761E). that can be easily identified by their rough and hilly landscape in contrast to the dark smooth lava plains of Syrtis Major (Fig. 2). A north–south lineation, visible in fluvial/glacial erosion (channels, valleys) has been pre- the top center of Fig. 2, is a fracture parallel to the main viously observed on these Syrtis Major volcanic plains. Nili Fossae system. This fracture is probably due to the Only a few studies suggest a possible interaction of same tectonic stress as Nili Fossae and it is buried by lava volcanism with volatile-rich sediments in the Isidis–Syrtis flows to the south, showing that it pre-existed lavas. The area (Ivanov and Head, 2003), as well as possible subsur- highlands locally display valleys with alluvial fans face water storage hypothesized by the presence of lobate and possible associated lakes (Fassett and Head, 2005; ejecta craters (e.g. Baratoux et al., 2005). In our study, we Mangold et al., 2006, 2007). OMEGA/MEx spectral data focus on erosional features such as grooves, teardrop- show that the highlands outcrops contain hydrated mine- shaped islands, and a deep valley located close to the rals such as clays, suggesting an alteration of the ancient boundary between the Syrtis Major smooth plains and the crust by liquid water (Poulet et al., 2005). In contrast, northern Noachian highland. On the basis of our Hesperian aged lava flows of Syrtis Major do not show any investigations, we interpret these features as being the sign of hydrous minerals. This implies a history devoid of result of an aqueous flood event because these landforms long-term liquid water activity. The presence of minerals are typical of energetic outflows. We estimate discharge such as pyroxenes and olivine confirm the volcanic origin rates and discuss the age and origin of these landforms. of the material (Bandfield, 2002; Mustard et al., 2005; Poulet et al., 2007; Baratoux et al., 2007; Pinet et al., 2007). 2. Data and geologic context 3. Observation of erosional landforms on volcanic plains The studied HRSC images are orbits 988, 1347 and 1593 with nadir resolution from 15 to 25 m. A mosaic of the At regional scale (Fig. 2), the most interesting erosional three images has been completed at 25 m/pixel in the region landform is a valley that crosses the northern part of the ARTICLE IN PRESS 1032 N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 lava plain close to the contact with the Noachian landforms are typical of erosion into a resistant bedrock. highlands. This narrow and deep valley is 60 km long with Grooves and valley might have been generated by the same an EW orientation; we will refer to it as deep valley (DV). process, as they show same directions. The DV, as observed on wide-angle images (Fig. 2), could Another type of erosional landform is the teardrop- be interpreted as a volcanic feature such as a large shaped islands A and B (Fig. 4). The most obvious example collapsed lava tube. However, in the following section, (A in Fig. 4a) is a small topographic feature with a we will present observational facts that render a volcanic teardrop shape that asymmetrically extends away from a interpretation less likely (Figs. 3–8). 600 m diameter impact crater. The second small island (B in Fig. 4d) is less developed and is observed to the south, in 3.1. Erosional landforms on HRSC images a region where grooves are also less dense. Typical lava flow features include lobes, dikes, and rilles. 3.1.1. Observations at the deep valley source area Flow lobes of Syrtis Major lavas are identified by typical The western part of DV shows some branching patterns lobate shape in the southwest corner of the image only a few kilometers long and a low sinuosity suggesting a (Figs. 3, 4c). In contrast, no lobate shapes are observed in more complex formation than expected at regional scale association with the DV, the grooves or the teardrop- (Fig. 3). The valley head (Fig. 4b) is characterized by a shaped islands. Wind streaks are the only clear eolian theater-head shape, but a young crater modified the source landforms visible (Fig. 4a). They form by differential dust area complicating the interpretation. We see a sinuous deposition and erosion behind the relief created by impact valley continuing on the northwestern side of the crater craters. that branches at the valley head, showing that the valley The observed erosional landforms are elongated and head has not just a single source. their orientation is variable. Grooves have a N–S direction Some striations, or grooves, cover a large part of the in the northern part of Fig. 3 (Fig. 4a) and change to area in the west and northwest of the valley head (Figs. 3 NW–SE in the southern part (Fig. 4d). The direction of the and 4a, b). These grooves are relatively narrow (few tens of two teardrop-shaped islands are exactly similar to that of meters) compared to their length of several kilometers. the grooves direction. In addition, these islands show a These grooves are present all around the valley head and sense of erosion from the north to the south. These follow locally the same direction (Fig. 4b). South of the directions are consistent with the valley system in the deep valley, other grooves can be observed, with appar- western part, but grooves extending south of the valley ently smaller size, and a NW–SE direction (Fig. 4d). Such show a different orientation than the valley. The wind streaks indicate the orientation of the current wind direction from east to west and the lava lobes indicate an orientation of lava flows from the southwest to the northeast. Thus, these two last types of landforms, volcanic lobes and current eolian landforms, are, nonetheless distinct in shape, but also distinct in orientation of the elongated grooves and teardrop-shaped islands.

3.1.2. Observations of the deep valley eastern part In the eastern part (Fig. 5), the DV is sinuous with locally straight parts and is interrupted by a 10 km diameter impact crater that formed after the valley. Overall, DV has a nearly constant width of 300–800 m and few tributaries with apparent theater-heads (Fig. 5b). These tributaries branch together and connect to the main valley. These tributaries do not exist everywhere along the main valley, and do not show a dendritic geometry. The eastern termination of DV is enigmatic (Fig. 5c) because it stops abruptly after the connections with tributaries. Due to image resolution and quality, it is difﬁcult to know what exactly occurred at this point. It seems that the valley disappears on the plain but a few lineations are still visible east of that point in Fig. 5c and more to the east. Indeed, a few other grooves and valley- like features can be observed further east. These lineations Fig. 3. (a) Portion of HRSC image 1347 over the lava plain showing a may represent the continuation of the ﬂow to the eastern deep valley and many grooves (G) from top left edge through bottom right part of the studied area but they remain poorly visible edge. features that should be interpreted with caution. In ARTICLE IN PRESS N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 1033

Fig. 4. Detailed views of landforms on HRSC image 1347: (a) A: teardrop-shaped island indicating an N to S erosion together with grooves (G) elongated in the same direction over a 10 km wide area (only grooves only is pointed by arrow). (b) Grooves (G) at the head of the deep valley. A sinuous valley (SV) is observed with same main direction as grooves. The wind streak indicates an E to W dominant wind. (c) Lobate features typical of lava ﬂows lobes (LL). (d) Grooves south of the deep valley with a second smaller teardrop shaped island (B).

addition to this deep valley a few sinuous landforms seen as MOLA gridded data (Fig. 7). In Fig. 7, arrows (in blue in negative relief are also identified in diverse locations of the the online version) indicate directions of erosional land- smooth plain, especially to the north, northwest, and east forms (valleys, grooves and teardrop-shaped landforms) of the central area of Fig. 3; they are mapped in the that can be compared to lines with measured slope geomorphic map as valley-like lineations (dotted lines in azimuths (in yellow in the online version). The lava flows Fig. 6). lobes are present in the southwest part of the grooved plain (in red in the online version). The orientations of the 3.2. Comparison between erosional landforms directions and erosional landforms seem consistent with the overall slopes from topographic data current topography. A more detailed comparison is done in Fig. 7c. In the region of the teardrop-shaped island A, The previous observations show the presence of a 60 km the regional slope (in yellow) is of 0.11 to the south and long DV with sinuous shape and few branching tributaries. increases to 0.51 where we observe the most developed Close to the source area, DV is surrounded by elongated grooves. To the east, other grooves also developed grooves and tear-dropped islands both typical of erosion. on a 0.51 slope to the south. At the head of the DV, there The directions of flow indicated by these features show a is a gentle step in the topography with a southern azimuth NS direction in the western part of the study area changing and a slope reaching here 0.71. South to the DV, grooves into a WE direction in the eastern part (Fig. 6). These are still present on a 0.51 slope. East of the DV end, morphologically derived flow directions can be further the slope is nearly constant at 0.61. Thus, from A to the investigated with topographic data. eastern end, directions given by landforms are consistent The landforms observed are located at MOLA elevations with the direction of the main regional slopes. Never- of 1000 m at the west to 2000 m at the eastern end of theless, notice a small saddle (mapped with S) that DV. Regional slope azimuths have been derived from indicates a small relief here (100 m of elevation difference ARTICLE IN PRESS 1034 N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042

Fig. 5. (a) Eastern part of the deep valley (DV). (b) Streams connecting (CS) to the deep valley with small theater-shaped head. (c) The deep valley ends suddenly with poorly visible lineations to the right of the image.

Fig. 6. Interpretative geomorphic map of the studied area. Grooves (gray lines) and valleys (black) indicate a NS direction at left changing to a WE direction at the bottom central part of the image. Putative erosional landforms (dotted lines) indicate a continuation of the ﬂow to the east and possible indication for the ﬂow source area in the northwest. ARTICLE IN PRESS N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 1035

Fig. 7. (a) MOLA color map of the area with elevation contours every 200 m. (b) Regional slopes azimuths (yellow arrows), lava flows direction (red arrows) and fluvial landforms direction (blue arrows). Main slopes are consistent with directions observed for aqueous landforms all over the area. S is the saddle north of the dense grooves of Fig. 4a. The lava front (in red) is determined from the difference in shape of the lava plain (rougher inside red lines). (c) Close-up of MOLA topography contours and HRSC image on the location of Fig. 3. at maximum). North of A, the plain is very flat with the scales depending on the intensity of the flood, thus slope lower than 0.11; this area is devoid of any obvious suggesting a smaller flood at Syrtis Major than in Ares erosional landform. Vallis. By comparison, the teardrop-shaped island observed on Fig. 8d in Athabasca Valles, a more limited but 4. Interpretation of erosional landforms recent flood (Burr et al., 2002), is 1 km wide, only two times larger than the similar landforms of the Syrtis Major 4.1. Teardrop-shaped islands origin region. The length/width ratio (L/W) has been widely used to The two teardrop-shaped landforms A and B are similar compare these landforms to other processes (e.g. Baker and in shape to the teardrop-shaped islands observed in Kochel, 1979). Values of L/W ratio are known to be of 3.15 classical outflow channels (e.g. Baker et al., 1992). The for Scablands (Washington State) and 3.25 for the Martian crater creates a zone of non-erosion, or at least of limited islands in average (Baker and Kochel, 1979; Komar, 1984). erosion, in the ‘‘shadow’’ behind the crater (Fig. 8a, b). They are generally slightly lower than fluvial islands in Compared to Ares Vallis, a major flood event, the observed classical river streams with 4.3 (Komar, 1984). The L/W teardrop-shaped landforms are more than ten times smaller ratio of the first one (Fig. 8a) is approximately 3.0 fitting to (Fig. 8c). The same shadow effect can occur at different the range of L/W ratio on earth and for the Martian ARTICLE IN PRESS 1036 N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042

Fig. 8. Comparison of landforms on Syrtis Major lava plain and other ouflow channels on Mars. (a,b): Close-ups of the two teardrop-shaped islands of North Syrtis area (see Fig. 4 for localization). (c,d): Teardrop-shaped islands in Ares Vallis floor (HRSC 1607) and Athabasca Vallis (HRSC 2121). (e) Close-up on grooved area in Syrtis area. (f) Grooves in Ares Valles (HRSC 1000). channels. The second one (Fig. 8b) has slightly lower ratio, plains might explain difference of erosion but this should with 1.9, and is less similar as noticed in the description. indicate a contrary trend, because highlands are likely Could other processes explain these landforms? The more erodable than lava flows. From the difference of scale plains material is of volcanic origin and volcanic landforms and shape, it is therefore unlikely that wind erosion can can locally be erosional. However, the presence of explain the observed teardrop-shaped landforms in Fig. 4. streamlined islands is not possible from lava flows themselves. These two craters obviously formed after the 4.2. Grooves origin plains formed, or they would not have shown erosional patterns associated. Thus, teardrop-shaped landforms are Some eroding forces such as wind, glacier abrasion or not volcanic in origin. any aqueous flows often form grooves. Energetic aqueous Current eolian activity follows E–W direction different floods can create grooves, similar to those frequently from the grooves and islands, but a past eolian erosion observed in other outflow channels (Fig. 8e, f). Here, the might have followed other pathways. Teardrop-shaped example of Ares Vallis is not larger than the grooves of islands and wind streaks are both due to the ‘‘shadow’’ Syrtis, in contrast to the teardrop-shaped islands. These created by the crater topography but they do not show grooves are located on the external edge of the valley, at a similar morphologies, suggesting the teardrop-shaped location where the flood was probably not as much islands are not wind related features. Small tear-dropped developed as in the main course channel, therefore shapes behind craters can locally be due to eolian processes suggesting a discharge rate of lower amplitude than the in dusty regions (see for example MOC E01-01738). Ares Vallis main channel system. Grooves follow the However, these landforms are very small compared to the topographic slope as would aqueous flows too. observed case, as they affect craters of typically 50 m wide Wind action forms erosional features such as yardangs, or less. In addition, the outcrops of Noachian highlands do i.e. elongated hills formed in weak material parallel to the not show any sign of any NS direction of erosion, wind direction. However, wind streaks indicate E–W suggesting the observed erosion was limited to the plains prevailing winds, which are not consistent with the N–S material. This characteristic is more difficult to explain if orientation of the grooves. One can argue that ancient wind this was created by wind. Eventually, a difference due to directions could have been different, but yardangs are the difference in material property between highlands and typically formed by erosion into weak material and are ARTICLE IN PRESS N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 1037 much more difficult to create on a lava plain. As explained subsurface sapping. Fig. 4b indicates that grooves and for tear-dropped islands, no wind erosion of a past N–S DV have approximately same directions at the valley head. direction is seen in the relics if highlands visible inside the In order to reconcile these apparently contradicting plain, an argument not favorable to explain grooves by this observations, an interpretation is proposed: DV might be a process too. The grooves follow the main slopes and local incision of a catastrophic flood that predominantly change of direction sharply with it, despite the slope is very created grooves. Comparison can be made with the low, a characteristic also difficult to explain with a wind Holocene sub-glacial volcanic flood of the Jo¨kulsa a erosion origin. Fjo¨llum in Iceland (Waitt, 2002a, b). Therefore, the surface Volcanic grooving is not common, but might occur when erosion was followed by the incision of a nearly 1 km large lavas are sequestered along a given topography, leading to and 100 m deep canyon about 100 km away from the flood more erosion than deposition. In Fig. 4c, in the south- source region. Such change of flow mode might occur, for western part of the studied area, lava flows are clearly example, when the bedrock strength or the slope changes, visible with a lobate shape and individual flows can be or when a preexisting landform such as a lava tube exist followed. However, where plains are grooved and cut by and is enlarged and re-incised. The increase of regional valleys, no lobes are visible, and no individual flows can be slopes to 0.71 at the valley head is consistent with a flow followed. Moreover, the presence of streamlined islands mode change (Fig. 7). These interpretations might also associated to the grooves, showing the same direction, does explain the direction difference between DV and grooves not favor a volcanic origin due to the gap of time necessary south of the valley (Fig. 4d). The flood was not fully to form these two craters after the plains. Thus, the overall sequestered in the valley but part of the flood continued as lack of any volcanic features due to the lava flows visible at a sheet flow south to the DV. the surface, and associated to the erosional landforms In summary, the presence of widespread grooves and supports the idea that the grooves are not volcanic in associated tear-dropped islands that are consistent with the origin, and formed after the emplacement of the lava regional slopes is evidence for a high-energy aqueous flow bedrock. having carved these landforms. We interpret the deep Glacial abrasion forms grooves at different scales with valley as formed by progressive surface erosion, and/or associated features such as drumlins and moraines (e.g. infiltration of volatiles inside the subsurface and backward Allen, 1997). This hypothesis is possible from the geometry sapping. In this interpretation, (1) aqueous flows formed of grooves, but no residual moraines are observed in this first as a sheet flow forming grooves, and tear-dropped region whereas moraines should be present when ice islands, over the smooth plains, and (2) aqueous flows sublimates or melts. Given the slope of o0.51, any glacier carved the DV and its tributaries in specific location would be very thick: Given the basal strength of glacier of controlled by slope or local geology. This interpretation 50 kPa, such a slope would invoke a 1.6 km thick glacier might also explain the abrupt end of the DV. Indeed, which should have left other landforms such as moraines. catastrophic floods often disappear progressively because Despite glacial grooves should not be fully excluded, the flow discharge decreases progressively in intensity from erosional grooves from an aqueous erosion is easier to the source area and fluids infiltrate in the ground or explain these landforms. evaporate (e.g. Baker et al., 1992).

5. Putative source areas 4.3. Geometry of the valley system The teardrop-shaped island A and erosional grooves The valley by itself resembles lava tubes at low indicate that the flood originated north of them. Following resolution. However, the presence of a few branching the erosional features to the north, we observe a gentle tributaries indicates a geometry different from usual lava topographic saddle less of 80 m high over 20 km (S in Fig. tubes, generally isolated and not connected. DV has a 7). Strong floods might climb low slopes (here only 0.1–0.21 geometry and a width similar from beginning to end, with over 20 km) when over pressurized at the source area. An only a few tributaries, indicating an origin different from alternative, and probably more realistic explanation, is that usual overland flows which are generally characterized by this topographic saddle formed after the flood, thus an increasing width and more dendritic patterns. The explaining the apparent local discrepancy between the flow apparent theater-heads of several tributaries and the nearly directions and the slopes. In fact, the saddle is located at constant width could suggest a formation by subsurface backward slope of a wrinkle ridge (compressive structure) sapping, i.e. the seepage of water and regressive erosion (Fig. 3). Wrinkle ridges have typically elevation differences forming large and poorly hierarchized valleys (Laity and on the order of several hundred meters that could explain Malin, 1985). A similar landform is visible in the Kasei the existence of this relief (e.g. Watters, 1993; Mangold et Valles outflow channels and is interpreted mainly as a late al., 1998). They have also longer wavelength relief such as episode of subsurface sapping (Williams et al., 2000). broad arches that create small bulge inside volcanic plains. However, the observed grooves point towards an erosion Moreover, these lava plains are formed by lava flows that by surface flows, hence a mechanism different from follows main slopes. As there is no apparent vent or dikes ARTICLE IN PRESS 1038 N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 feeding lavas at the saddle top, there is no reason that the that this terrain might be a possible source area. Never- lavas would have shown this topography during their theless, we do not observe any flow features right at the formation. Wrinkle ridges in Syrtis Major Planum might opening of this chaotic region. have formed at the Late Hesperian epoch (Mangold et al., Several outflows such as Mangala or Athabasca Valles 2000), so after the lava flows, and perhaps after the are born on large fractures; i.e. Cerberus Fossae in the case erosional landforms formed. of Athabasca Valles. Similar fractures cross the highlands In addition, the plains north of A are very flat (typically (Fig. 2). However, none is seen crossing the lava plains that 0.11) while we observe grooves occurring on slopes of 0.51. could be a potential source area. Northwest of A, there are It is possible that the very flat relief could have impeded the only few signs of lineations possibly corresponding to formation of erosional landforms because of the relative fluvial flows in three areas, especially close to the contact lower discharge involved by this flat topography, therefore with Noachian highlands. Here, the grooves indicate a rendering difficult the identification of the flow origin. flow, perhaps coming from beneath the lava layers An interesting landform is the chaotic terrain (Fig. 9a) (Fig. 9b), thus suggesting the presence of aquifers beneath observed 50 km north of A. Here, the smooth plains are no the uppermost lava layers, but this interpretation remains more flat and continuous, but are broken in many pieces speculative because of the limitation in the image resolu- that appear to be uplifted from a previously flat surface. A tion. In summary, the source region of the aqueous flow is 100 m deep depression formed in these lava blocks. Chaotic very difficult to identify. Signs of flows exist, but they are terrains are found in many source areas of other outflow not continuous to the region of abundant grooves. channel on Mars. Chaotic terrains at the source of outflows in Xanthe Terra are much larger and deeper than the one 6. Estimation of discharge rates of Fig. 9b, with usually more than 2 km in depth and more than 50 km in width, but the resulting outflow channels are Discharge rates are important for our understanding of much wider too. By analogy with Xanthe Terra, we suggest the magnitude of outflow events. A main question for this calculation is: do we observe the effect of several episodes of flows or just a single one? DV might have formed as a second stage in the evolution of the outflow, but this stage can be part of the same episode of flow. Grooves visible at the scale of HRSC images required an energetic flow, but a suite of medium-size floods is not excluded. We estimate discharge rates as produced by a single episode of flow, but do not exclude the possibility of multiple episodes. As for other Martian outflow channels, we assume aqueous flows to be liquid water mixed with rocks, but this does not exclude other more exotic volatiles (e.g. Baker et al., 1992; Coleman, 2005).

6.1. Maximum discharge rate in the deep valley

We can calculate the maximum channel discharge with the well-known Manning equation assuming a bankfull discharge in the channel. This must be taken as a rough maximum that might overestimate the true discharge rate. We use the equation modiﬁed for Mars by Komar (1979) which corrects for Martian gravity: 4=3 2 1=2 Q ¼ AðgmsR =gen Þ

where A is the flow cross-sectional area, gm and ge are gravity on Mars and Earth, respectively, s is the local slope, n is the Manning roughness coefficient and R is the hydraulic radius, defined as the ratio of flow cross-sectional area to wetted perimeter. Use of this equation to determine flow discharges has been used extensively for Martian outflow channels (e.g. Carr, 1996). A and R are calculated assuming a rectangular shape of the valley, using the depth Fig. 9. Close-up on possible flow-related features north of the main of 50 m measured with MOLA data, and an average width landforms. (a) Chaotic zones as a possible origin for the flow. (b) Flow of 500 m. At this location, the slope s is of 0.71. The main lineations at the highlands–lavas boundary. approximation in this calculation is the coefficient n. ARTICLE IN PRESS N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 1039

Taking n ¼ 0.04 as commonly used for a rough non-vege- and similar to the overflow of Lake Bonneville with tated bedrock, the result gives a value of Q ¼ 0.5 106 m3/ 1 106 m3/s (O’Connor, 1993) and to the largest reported s. Taking extreme values of n ¼ 0.01 and 0.07 would result sub-glacial eruption in Iceland with 0.7 106 m3/s (Waitt, in values from 0.3 to 2 106 m3/s. The value of QE0.5 2002a, b). These two latter comparisons show that the 106 m3/s should thus be taken as a rough estimate of the North Syrtis outflow, despite being not as large as most of maximum discharge rate possible through the valley. the Martian outflows, is an important flood.

6.2. Discharge rate on the flood plain 7. Discussion: Stratigraphic relationships, age and origin the flood On the grooved plains, the flood appears as a sheet flow and might not have been as deep as in the valley. The 7.1. Age relationships largest teardrop island (A) and grooves are erosion markers that can be used to estimate the flow depth. The Early Hesperian epoch is defined by a crater density However, they are topographically subtle structures not of N(5)=125–200, i.e. 125–200 craters larger than 5 km visible on HRSC or MOLA DEM. The teardrop-shaped in diameter per million square kilometers, or N(2)= island is also invisible on MOLA profiles because it is 1200–750, i.e. 1200–750 craters larger than 2 km per located in a gap in between individual profiles. Grooves million square kilometers (Tanaka, 1986). Crater densities can be compared to the surrounding landforms in Fig. 7c. found for Syrtis Major Planum are of N(5) ¼ 15476 Here, the 100 and 200 m high highland residual hills are not (Hiesinger and Head, 2004), or N(5) ¼ 124716 (Mangold affected by the flow limiting the flow thickness to smaller et al., 2000), or N(5) ¼ 100 for the main activity episode depth. On HRSC close-ups, grooves are apparently of (Greeley and Guest, 1987), which all put Syrtis Major similar height as the rim of the 500 m diameter crater in volcanic plains in the Early Hesperian. Using the new their surroundings (Fig. 4a). Estimating the rim height of HRSC images, the area of 4300 km2 is too small to derive a this crater size would give an order of magnitude for the N(5) age, whereas our crater count gives an N(2) ¼ grooves height, so for the flow depth too. The height of 11607520, which also indicates an Hesperian age, but crater rim (Hr) is related to the crater diameter (D) seems to be slightly older than previous estimates. according to Hr ¼ 0.036D1.01 on the moon (Melosh, 1989) The age of the volcanic plains does not give the age of and Hr ¼ 0.03D0.96 for small Martian craters (Garvin et the flood itself but a maximum age. Most craters are fresh al., 1999). On the basis of the diameter of the impact crater and not affected by the aqueous flows, but the presence of of 500 m, the height would be about 18 m using the first the two teardrop-shaped islands shows that two impact relation or 12 m using the second. Thus, we use a value of craters, at least, were present before the flood, thus 1575 m for the flow thickness. We assume for the implying a hiatus of time between the lava plain formation calculation a flow width of 10 km, which is the lateral and the catastrophic flood. It is therefore unlikely that the extent of the grooves, and a slope of 0.51 measured locally. floods formed as a direct consequence of the underlying The calculation gives a value of about 1.3 106 m3/s for lava emplacement. Nevertheless, the presence of fresh lava n ¼ 0.04 and a depth of 15 m, with extreme values of a lobes in the SW and North of the studied region (Fig. 6) minimum of 0.4 106 m3/s using n ¼ 0.07 and 10 m of suggests that the volcanic activity continued in the region depth to a maximum of 8 106 m3/s using n ¼ 0.01 and after the flood occurred. In that case, the flood might have 20 m of depth. This estimation of about QE1 106 m3/s is formed in between two distinct episodes of activity of Syrtis consistent with the value obtained for the valley of Major lavas, in the Hesperian epoch. QE0.5 106 m3/s: discharge rates are consistent with an initial sheet flow that was later partially sequestered inside the valley to the east while it was still flooding the plains to 7.2. Origin of the flood the southeast. The origin of this flood over Syrtis lava flows is difficult 6.3. Comparison with other floods to understand as it is for many other outflow channels, but the interpretation is complicated by the poor knowledge of The calculated peak discharge rates in the range of the source area. Here, different hypotheses can be magnitude of 1 106 m3/s for the flood system are proposed such as (1) a subsurface origin due to ground 100–1000 times lower than those calculated for large ice melting or groundwater release by either increasing Martian outflows such as Kasei or Ares Vallis with values geothermal activity or heating by lava flows; (2) a of up 109 m3/s (Komar, 1979), but they are similar to the subsurface origin due to tectonic pressurization by seismic values found for late episodes of floods in Kasei Vallis activity; (3) a regional rise of the water table; or (4) a (Williams et al., 2000). These discharge rates are also in the melting of glaciers present at the surface by incoming lava range of calculated peak discharges of terrestrial mega- flows. Hypotheses involving a lake overflow are not floods, smaller than the glacial surge of Lake Missoula discussed hereafter because of the lack of evidence in favor in the Scabland region with 20 106 m3/s (Baker, 1973), of such a lake in the surroundings. ARTICLE IN PRESS 1040 N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042

The magmatic activity and presence of dikes driving eskers that could support this hypothesis either locally at lavas to the surface might be the best candidate to trigger the north of DV source area, or at any locations of the Nili outflows on Mars (Wilson and Head, 2004). The presence Fossae-Syrtis Major region. This hypothesis is therefore of chaotic terrain (Fig. 9b), or putative aqueous flows much more speculative than the subsurface triggered coming from underneath lavas (Fig. 9a) may favor a floods. subsurface origin. These putative source areas suggest that From the present set of observations, we conclude that water was stored in the ground of Syrtis either as liquid the exact formation mode of the flood incising Syrtis Major water, or as water ice that subsequently melted due to the lava flows remains uncertain. We favor the first hypothesis, magmatic activity. This questions if such water was stored the role of the volcanic heating, involving water ice melting inside Syrtis lavas or in the underlying bedrock. An and/or liquid water pressurization below lavas, as sug- important parameter is that the lava plains on which the gested by the presence of the chaotic terrain, the restriction outflow formed are located in the vicinity, only 20 km, of of the aqueous flows to lava plains and the presence of lava the Noachian highlands–lavas boundary. Clay minerals are flows unaffected by fluvial features that might form later. known to exist in the crust of the Nili Fossae region (Poulet The presence of this outflow close to the region of Nili et al., 2005; Mangold et al., 2007) suggesting that water Fossae at a location where the lava is likely thin is certainly might have existed in this region before the Syrtis Major not a coincidence: the source of water might be found in lavas were emplaced. Clay minerals present below the lava the basement below the lava flows. This hypothesis could flows could have contributed to increase the liquid water also explain why the inferred flood features are only storage ability of the subsurface. Thus, at this location, observed in the Syrtis region. Indeed, the overall volcanic lava could have buried a Noachian crust in which region of Syrtis Major is devoid of any other fluvial abundant water, as liquid or ground ice, was stored. landforms, as observed with new datasets. The flood might A recent work (Hanna and Phillips, 2006) shows that sign an activity at the periphery of the bulge due to outflows might form when the tectonic pressurization due interactions with the basement. to fault movements would progressively create the subsur- At this point, it should be noted that most outflow face drainage under pressure (Hanna and Phillips, 2006). channels on Mars are observed in connection with volcanic This hypothesis requires water to be liquid to occur. A regions. Late Amazonian outflows have been identified in large fracture (Fig. 2) appears to cross the highlands north the Cerberus plain (Athabasca Valles, Marte Valles) in of the study region and might have been a source of seismic close relation with recent volcanic activity (e.g., Burr et al., activity. However, this fracture does not cross the surface 2002). Other examples were found on HRSC images at the of lava plain. Whether this fracture played as a blind fault southeastern margin of Olympus Mons (Basilevsky et al., after its burial is impossible to demonstrate, and the source 2006). Many outflow channels are connected to Hesperian of fluid is difficult to explain from a blind fault. Thus, the volcanism, such as Dao Vallis and Harmakhis Vallis tectonic pressurization hypothesis is here not as convincing around Hadriarca Patera, or Hebrus Vallis around as it is for outflows clearly controlled by major fractures Elysium Mons. Recently, a paleoflood activity has been such as for Mangala or Athabasca Valles. identified using HRSC images over the Hesperia Planum. A third hypothesis is that the remobilization at surface of Hesperian plain (Ivanov et al., 2005) and volcanic cones subsurface liquid water might have occurred due to a have been identified in the chaotic region of Margaritifer regional rise of the groundwater table. The exact reason of Terra (Meresse et al., 2008). All these outflows seem to be a water table rise is uncertain, and the result of rovers study genetically related to the formation of volcanic plains or on the Terra Meridiani area suggests that modifications of their thermal consequences. Nevertheless, Syrtis Major the water table on Mars might have occurred (McLennan Planum, as well as Hesperia Planum, are some of the oldest and 31 colleagues, 2005). Sapping valleys exist in the Nili locations on Mars where water activity seems to have been Fossae region showing the possibility of groundwater negligible. According to the identification of new outflows activity (e.g. Mangold et al., 2006, 2007). Nevertheless, we in Hesperia Planum (Ivanov et al., 2005) and now in Syrtis do not observe other flood signatures connected to these Major Planum (this study), it appears that some locations sapping valleys, so this hypothesis does not well explain the previously identified as among the driest on Mars once had occurrence of the single outflow channels on the volcanic local hydrological activity characterized by quick fluid terrain. discharge and flood at the surface. This activity might An alternative explanation to the three subsurface outline the interactions between the volcanic activity and processes proposed is that surface ice stored as glaciers the water rich basement presently buried beneath the lavas. over lava flows might have triggered floods. Glacial surges are the most frequent processes to trigger catastrophic 8. Conclusion floods on Earth (e.g. O’Connor, 1993). This hypothesis could explain the sudden presence of flood features north HRSC images of the northern Syrtis Major region at of the DV: if a glacier was present at this location, the first the contact with the Nili Fossae highlands display grooves would be located at the glacial front. However, we erosional features such as grooves, teardrop-shaped islands do not observe any glacial landforms such as moraines or and valleys. These landforms display similar directions ARTICLE IN PRESS N. Mangold et al. / Planetary and Space Science 56 (2008) 1030–1042 1041 indicating a likely coeval activity different from lobate lava Olympus Mons as seen in Mex Hrsc and Mgs Moc images of Mars. flows where observed. We interpret these landforms as due Lunar Planet. Sci. Conf. 37th, Houston Clear Lake, abstract #1179. to a flood event that likely took place in the Hesperian Burr, D.M., McEwen, A.S., Sakimoto, S.E.H., 2002. Recent aqueous floods from the Cerberus Fossae, Mars. Geophys. Res. Lett. 29 (1), period. Flow landforms of 410 km long and few km wide 1013. are consistent with main topographic slopes in most Carr, M.H., 1979. Formation of Martian flood features by release of water 6 3 locations. Peak discharge estimates of about 1 10 m /s from confined aquifers. J. Geophys. Res. 84, 2995–3007. are in the range of terrestrial mega-floods. To date, the Carr, M.H., 1996. Water on Mars. Oxford University Press, New York, most likely origin invokes subsurface water mobilization 229pp. Carr, M.H., 2006. The Surface of Mars. Cambridge University Press, and discharge due to volcanic activity and subsequent Cambridge, 308pp. pressurization. The observation of an aqueous flood is Coleman, N.M., 2005. Martian megaflood-triggered chaos formation, unusual in the Syrtis Major region, which is a generally dry revealing groundwater depth, cryosphere thickness, and crustal heat region dominated by volcanic landforms and mafic flux. J. Geophys. Res. 110, E12S20. minerals. This identification is comparable to the recent Fassett, C. I., Head III, J. W., 2005. Fluvial sedimentary deposits on identification of outflow channels by HRSC/MEx data in Mars: ancient deltas in a crater lake in the Nili Fossae region. Geophys. Res. Lett. 32, L14201, doi:10.1029/2005GL023456. different volcanic plains in the Hesperia Planum, Elysium Garvin, J.B., Sakimoto, S.E.H., Schnetzler, C., Frawley, J.J., 1999. Global Mons or Tharsis regions. It shows how frequent is this geometric properties of Martian impact craters: a preliminary process even in regions previously supposed to be very dry assessment using Mars orbiter laser altimeter (Mola). Lunar Planet. and devoid of aqueous flows. This outflow occurred in the Sci. Conf. 30th, #6163. periphery of the region, possibly as an expression of water Greeley, R., Guest, J.E., 1987. Geologic map of the eastern equatorial region of Mars, USGS map I-1802B, 1802. storage buried in the underlying basement rocks. 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