Identification of Gully Debris Flow Deposits in Protonilus Mensae, Mars

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ARTICLE IN PRESS EPSL-09953; No of Pages 10 Earth and Planetary Science Letters xxx (2009) xxx–xxx

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Earth and Planetary Science Letters

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Identiﬁcation of gully debris ﬂow deposits in Protonilus Mensae, Mars: Characterization of a water-bearing, energetic gully-forming process

J.S. Levy ⁎, J.W. Head, J.L. Dickson, C.I. Fassett, G.A. Morgan, S.C. Schon

Department of Geological Sciences, Brown University, Providence, RI 02912, United States article info abstract

Article history: Gullies are a class of geologically recent landform on Mars that show evidence of downslope transport of Accepted 4 August 2009 sedimentary material from recessed alcoves, through incised channels, to distributary fans or aprons. The Available online xxxx mechanisms invoked to account for the formation of gullies on Mars range from completely dry, granular flows or landslides, to debris flows that incorporate some component of liquid water, to fluvial erosion and Editor: T. Spohn alluvial-fan-like deposition. Each of these processes requires different amounts of liquid water, and produces different characteristic morphologies. We report on the identification of unusual lobate structures present in Keywords: Mars proximity to gullies in Protonilus Mensae. The lobes are up to ~3 m thick and terminate in rounded snouts. gully These lobate structures are present mostly downslope of gullies, and can be traced upslope through pasted-on terrain channels, to gully fan termini, and in places, onto gully fan surfaces. Crater dating indicates that the deposits permafrost formed recently—potentially within the past ~500 ka. We use HRSC digital elevation models to constrain debris flow mechanical properties of the lobate deposits, and to compare their formation environment to that of typical climate martian gullies. The Protonilus Mensae lobate deposits are interpreted to indicate local dominance of wet debris flows in the formation of the observed gullies and lobes. These observations are consistent with 1) top-down melting of the ice component of the latitude-dependent mantle terrain in which the gullies form and 2) initiation of debris flows by mobilization of the dusty lithic component of the mantle. The suite of morphological observations diagnostic of wet debris flow processes suggests the identification of an unusual environment in which the wet debris-flow formational end-member is locally the dominant gully forming process; elsewhere on Mars gully morphology may be more consistent with a range of other water- related sediment transport mechanisms including fluvial erosion, hyperconcentrated flow, and low-strength mudflows (that deposit as fans rather than lobes). These results suggest that 1) gully-forming processes involve liquid water, 2) that the water source is associated with the martian latitude-dependent mantle, and 3) that a range of water-related sediment transport processes is involved in gully formation. © 2009 Elsevier B.V. All rights reserved.

1. Introduction formation. Previous studies favoring a debris flow origin for gullies have been based on indirect evidence from the morphology of gully Martian gullies are characterized by evidence for the geologically channels (Costard et al., 2002; Mangold et al., 2008a,b) or gully recent downslope movement of sediment from topographically high depositional fans (Malin and Edgett, 2000; Malin et al., 2006; Pelletier alcoves, through incised channels, to lower-lying depositional fans et al., 2008). Here, we present HiRISE observations of gully-related (Malin and Edgett, 2000)(Fig. 1). Leading hypotheses for the origin of deposits in Protonilus Mensae (44°N, 51°E) (Fig. 2) that meet many of martian gullies range from entirely dry sediment flows (e.g., Treiman, the morphological criteria for identifying terrestrial debris flow 2003; Shinbrot et al., 2004; Pelletier et al., 2008), to debris flows with deposits (Johnson and Rodine, 1984; Coussot and Meunier, 1996). To variable water contents (Malin and Edgett, 2000; Costard et al., 2002; our knowledge, such deposits have not been detected in HiRISE images Hartmann et al., 2003; Mangold et al., 2008a,b; Pelletier et al., 2008), to of gullied terrain elsewhere on Mars (McEwen et al., 2007; Dickson fluvial (water-rich) erosion and alluvial deposition (Heldmann and and Head, 2008), suggesting that the Protonilus Mensae fretted terrain Mellon, 2004; Heldmann et al., 2005; Dickson et al., 2007a; Dickson (Sharp, 1973; Lucchitta, 1984) may be a unique environment in which and Head, 2008; Head et al., 2008; Parsons et al., 2008). Determining debris flows were the dominant gully formation mechanism. gully formation mechanisms is critical for assessing the amount, origin, timing, and climatological significance of liquid water involved in gully 2. Morphological observations

⁎ Corresponding author. Tel.: +1 401 863 3485; fax: +1 401 863 3978. The study site is located on an isolated mesa surrounded by E-mail address: [email protected] (J.S. Levy). lineated valley ﬁll and lobate debris aprons (Carr, 2001; Head et al.,

Please cite this article as: Levy, J.S., et al., Identiﬁcation of gully debris ﬂow deposits in Protonilus Mensae, Mars: Characterization of a water- bearing, energetic gully-forming process, Earth Planet. Sci. Lett. (2009), doi:10.1016/j.epsl.2009.08.002 ARTICLE IN PRESS

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2006a,b, submitted for publication). The mesa itself is incised by gullies, mostly on the southern (equatorwards) slope face (Figs. 3–5). High Resolution Stereo Camera (HRSC) topography data provides context and geometry in this region (Neukum et al., 2004). Gullies (Malin and Edgett, 2000) are present at the margins of the latitude-dependent mantle (Head et al., 2003)in“pasted-on terrain” (Christensen, 2003) occurring along the south ﬂank of the mesa (Figs. 3a and 5). The gullies have small alcoves or lack them entirely (Malin and Edgett, 2000)(Figs. 3a and 5). Over 300 gully channels are present along the ~5 km mesa scarp (Fig. 3). Gully channel widths are typically 5–20 m, and are narrower high on the mesa (Figs. 3 and 5a). Shadow measurements indicate gully channel depths of ~5–6m(Fig. 5a, b). Gully channel lengths are typically ~200 m, ranging from ~100–500 m. Gully channels initiate high on the mesa slope as ﬁne, linear depressions

Fig. 1. A typical martian gully located at 52.1°S, 246.8°E. Note the presence of a large alcove with multiple emanating channels. Channels coalesce from tributaries into a main trunk channel that grades into a fan. The fan is dissected in places by distributary channels. Typical gullies form in thick deposits of latitude-dependent mantle material (Head et al., 2003) (polygonally patterned in this image) and erode through to underlying rocky surfaces. Portion of PSP_002368_1275. North is to image top, indicating that the gully is pole-facing, typical of gullies at this latitude (Dickson et al., 2007a). Illumination is from the lower left.

Fig. 3. The Protonilus Mensae study site. a) Context view of mesas, lobate debris aprons, and lineated valley fill. Box indicates area shown in part b. Portion of HiRISE image PSP_007148_2445 overlain on CTX image P03_002401_2246. North is to image top and illumination is from the lower left. b) View of the Protonilus Mensae mesa scarp. Gullies with well-formed channels and fans dissect the southern flank of the mesa. The locations of subsequent images are marked. c) Sketch map showing the locations of gully channels and the furthest down-slope extents of lobate flow fronts. Basemap is Fig. 2. Regional view of Protonilus Mensae fretted terrain and the martian dichotomy local slope derived from HRSC topography data. The occurrence of gully channels on boundary. The study region is indicated by the white box. The map is composed of steep slopes and lobate flows on shallow slopes is illustrated. Note, however, that MOLA shaded relief overlying MOLA topography—lighter tones are higher in elevation channels and lobes are sub-grid features and that background slopes are measured. than darker tones. HRSC topography from image h1523_000.

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Fig. 4. Lobate deposits in Protonilus Mensae. a) Digitate lobes with steep flow fronts (arrows). b) Lobate flow fronts deflected around underlying topography (arrows). c) Narrow (left) and broad (right) lobe fronts. d) Complexly stacked lobe fronts are highlighted in the white box. All images excerpted from PSP_007148_2245, with north towards image top, down-slope towards image bottom, and illumination from the lower left.

and broaden, either gradually or abruptly, downslope and as the slope aprons (Malin and Edgett, 2000; Schon et al., 2009)(Fig. 5c, d). Gully fan steepens (Fig. 5a). Typically, deeper and wider gully channels are surfaces immediately beneath Protonilus Mensae gully channels have a present in areas of thick pasted-on terrain, while finer channels occur in rubbly texture with scattered boulders present—the source of these thin pasted-on deposits (Figs. 5a and 6). Pasted-on terrain appears large clasts can be traced up-channel to eroding outcrops of bouldery thickest towards the base of the mesa, and thins towards the top of the mesa material present beneath the pasted-on terrain. gullied portion of the mesa, disappearing completely in some instances The most striking geomorphological element in the study site is a (Figs. 5a and 6). Some gully channels have minor sinuosity on meter to collection of lobate features located down-slope of the gully fans (Figs. 4 tens of meter scales (Mangold et al., 2008b), however, the gully channels and 7). These features have not been identified in previous high- are generally extraordinarily straight and parallel (Figs. 3b and 5a) resolution gully observations (e.g., McEwen et al., 2007; Dickson and compared to other gully channels on Mars (Malin and Edgett, 2000; Head, 2008; Levy et al., 2009b). The features are convex-up with steeply McEwen et al., 2007). Protonilus Mensae gully channels are commonly sloped snouts. The features terminate in lobate fronts or in leveed fronts cut-off or cross-cut by other channels, similar to those described (where only a curved band of elevated material is discernable) (Figs. 4 elsewhere by Malin and Edgett (2000) and Schon et al. (2009). and 7). These features will be referred to as “lobes” or “lobate fronts.” Anastomosing between channels is rare, as are dendritic tributaries Lobes are smooth-surfaced at HiRISE resolution and boulder-sized clasts (Fig. 5b). Fans are present at the bases of most gullies, and commonly are exceptionally rare in lobe deposits (Figs. 4 and 7)—most observed have a complex, fluted, and sculpted texture (Malin and Edgett, 2000; boulders can be traced to erosion from upslope scarps, and the presence Schon et al., 2009). As with gully fans elsewhere on Mars, many of the of boulder tracks implies boulder emplacement after lobe formation. No observed gully fans have multiple digitate or triangular depositional meter-scale blocks of coherent pasted-on terrain or mesa material are

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Fig. 5. The morphology of Protonilus Mensae study site gullies. a) Gully channels dissect latitude-dependent mantle related pasted-on terrain (thick exposures of pasted-on terrain are marked with stars). Channels reach their highest point as faint, narrow, and shallow furrows high on the mesa (shown in box). Some gully channels have periodic sinuosity on several meter length-scales (white arrows) while other channels are sinuous on tens of meter length scales (black arrow). b) Multiple cut-off, cross-cutting channels (arrow). c) Gully fan composed of multiple depositional surfaces. Note fresh crater to right. d) Detail of complexly fluted, sculpted, and channeled gully fan texture. Note transitions between fan texture and lobate fronts. All images excerpted from PSP_007148_2245, with north towards image top, down-slope towards image bottom, and illumination from the lower left. present in the lobes, nor are detachment surfaces or crown-scarps Lobe deposits commonly bank up on topographic features (e.g., typical of landslides (Johnson and Rodine, 1984) present upslope of the “brain terrain” knobs) forming marginal deposits that remain lobes. topographically high and that may represent levees (Fig. 7a). Shadow The lobes have a complex stratigraphy. Some lobes are present in measurements indicate that the distal lobes average ~1.5 m in incised channels (Figs. 4 and 7); other lobes emanate from incised thickness at their snouts (range: 0.6–2.8 m, N=20). Lobes average channels (Fig. 8). Lobes commonly overlap and multiple lobe fronts are ~11 m in width (range: 4–50 m, σ=6 m, N=100) and ~50 m in present in some channels (Fig. 7). Lobes commonly have a stacked length (range: 10–200 m, σ=40m, N=100). (lobes cross-cutting other lobes) or pulsed (variable widths in a Lobes commonly have higher blue-green channel I/F values in continuous lobe) appearance (Fig. 7). Channels containing lobes, or HiRISE color data than the surrounding brain terrain and mantle from which lobes emanate, can be traced clearly to the foot of many textures, typical of gully channel deposits (McEwen et al., 2007) and gully fans, where they are lost in the complex fan texture. In some cases, unlike features interpreted as potential dry, granular flows (which are lobe-filled channels can be traced onto fan surfaces. In rare cases typically red-toned in HiRISE color data) (Kolb et al., 2008; Pelletier channels containing lobes can be traced to gully fan apices and into gully et al., 2008). Lobes are also observed on an adjacent mesa, but are not channels (Fig. 8). Small lobes (b10 m wide, b25 m long) are present on observed in HiRISE images surrounding the study site. Lobes cannot some gully fan surfaces (Fig. 5d). be clearly distinguished in HRSC or CTX images of the study site. The lobes are superposed on two pre-existing units, 1) a mounded and pitted “brain terrain” patterned lobate debris apron (Head et al., 3. Slope relationships 2003; Levy et al., 2009c) and 2) a polygonally-patterned mantle surface (Levy et al., 2009a). The lobes are diverted around topographic We use high-resolution (100 m gridded) HRSC stereo topography to highs, and follow local slopes into depressions (Figs. 4 and 7). analyze slopes on which gullies and lobes are observed (Gwinner etal.,

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2009b, McEwen et al., in press). The proximity of the lobes to martian gullies, and their apparent uniqueness in the martian landscape, suggests that they may provide critical insight into the formation mechanisms and hydrological signiﬁcance of martian gullies. First, we assess morphological differences and similarities between the lobe deposits observed in this study and classical martian gullies. Next we compare the deposits to the continuum of mass-movement processes that produce similar morphologies on Earth. Through analysis of terrestrial processes, we are able to determine the signiﬁcance of these unusual deposits by placing them in the spectrum of martian gully-forming mechanisms.

5.1. Similarities and differences with classic martian gullies

The Protonilus Mensae mesa analyzed in this study is broadly typical of martian gullied terrain that has been mapped in several surveys using high-resolution image data (Heldmann and Mellon, 2004; Balme et al., 2006; Dickson et al., 2007a; Heldmann et al., 2007; Dickson and Head, 2008; Levy et al., 2008). Channel and fan structures present on the mesa illustrate two of the three elements considered diagnostic of martian gullies (alcoves, channels, and fans) (Malin and Edgett, 2000). Identiﬁcation of two of these three elements is a criterion commonly used for mapping features as gullies in large-scale

Fig. 6. Gully channels emerging from a section of removed pasted-on terrain (arrows). surveys (Heldmann and Mellon, 2004; Balme et al., 2006; Dickson et Boulders can be seen where pasted-on material has been removed, suggesting that the al., 2007a; Dickson and Head, 2008). The channels dissect pasted-on gullies have eroded down to the underlying mesa surface in places. Portion of terrain (Christensen, 2003), illustrating a typical relationship between PSP_007146_2245, with north towards image top, down-slope towards image bottom, martian gullies and latitude-dependent mantle deposits (Mustard and illumination from the lower left. et al., 2001; Christensen, 2003; Head et al., 2003; Milliken et al., 2003; Berman et al., 2005; Dickson et al., 2007a; Dickson and Head, 2008; Levy et al., 2009b). Like many gullies, some of the observed gully 2005; Oberst et al., 2005; Gwinner et al., 2008)(Fig. 2). Gully channels channels are sinuous; some smaller channels in the study site show observed in the study site initiate on steep slopes, averaging 27° (σ=2°, sinuosity on length scales of meters, while some of the larger channels range: 24–32°) and transition into fans on more gentle slopes, averaging bend on tens of meter scales. Small-wavelength sinuosity may 21° (σ=3°,range:14–24°) (N=300 for gully channels). Lobate flows represent gully channel response to fine scale topography, while are present on still lower inclines, averaging 18° (σ=4°, range: 8–25°, tens of meter wavelength has been interpreted by Mangold et al. N=280). Flows with well-developed lobate snouts are generally (2008a,b) to be strongly indicative of the presence of a liquid water present on even lower slopes, averaging 14° (σ=4°, range: 6–20°, phase in gully channel eroding flows. The gullies are present on the N=20). These slope angles are broadly consistent with previous gullies south (equator-facing) side of the mesa only, a typical orientation for measured using HiRISE stereo analysis (Parsons et al., 2008), although it the relatively few martian gullies observed in the northern hemi- is likely that our slope determinations for sub-grid features (e.g., gully sphere near 45° latitude (Bridges and Lackner, 2006; Heldmann et al., channels) underestimate the slope of steep features (Kreslavsky, 2007). Like other martian gullies, these gullies initiate on extremely 2008b). steep slopes, typically approaching 30°, and deposit fans on lower- slope surfaces (Dickson et al., 2007a; Parsons et al., 2008). The fans of the analyzed gullies commonly have irregular toes or termini, as 4. Surface age identified in MOC images (Malin and Edgett, 2000), in part owing to the complex stacking of fan depositional and erosional surfaces do- Twelve isolated craters, ranging from 1–10 m in diameter, and a cumented on many martian gully fans (e.g., Schon et al., 2009). Some clustered pair of craters reconstructed to pre-fragmentation diameter fans in the study site are composed of multiple depositional fronts, using the technique of Kreslavsky (2008a), were counted on the typical of young gullies interpreted to have formed by episodic flow surfaces of gully fans and lobes (e.g., Fig. 7). These counts on a ~3 km2 (Schon et al., 2009). surface yield a best-fit(Hartmann, 2005) age for the deposits of ~500 ka. Despite their similarities to other martian gullies, the features pre- The N(0.01) production function determined by Kreslavsky (2007) was sent on the Protonilus Mensae mesa exhibit several morphological applied to the count area and produced an identical age based on characteristics that are atypical of gullies forming in classic gully one N10 m impact crater that is clearly present on a gully fan surface, but environments (mesa sides, crater rims, etc.). The observed gullies in which has been modified by lobe-related activity. While small numbers Protonilus Mensae are unusual in that most lack well-defined alcoves— of craters and a small count area make precise age dating challenging, either incised into the mesa (e.g., Malin and Edgett, 2000) or developing these surface age measurements are broadly consistent with youthful in the pasted-on terrain (e.g., Christensen, 2003; Levy et al., 2009a,b,c). ages for typical martian gullies determined from stratigraphic analyses Rather than emanating from alcoves, the observed gully channels by Riess et al. (2004) and Schon et al. (2009). appear continuous with fine, narrow channels that are present higher up on the mesa, and which widen as the channel slope steepens—a 5. Discussion relationship not previously observed for gullies on Mars (Dickson and Head, 2008). The Protonilus gully channels are unusually closely spaced To date, morphological features similar to the lobes identified in (over 300 channels dissect the ~5 km scarp). Typical mesa or butte this study have not been identified in comprehensive HiRISE image gullies on Mars are either singular features, or occur in groups of b~20 surveys of martian gully terrain, despite several thousand gully ob- (Heldmann and Mellon, 2004; Dickson et al., 2007a; Dickson and Head, servations (McEwen et al., 2007; Dickson and Head, 2008; Levy et al., 2008; Head et al., 2008). The channels are unusually straight and parallel

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Fig. 7. Details of lobe morphology. In all frames, white arrows indicate overlapping ﬂow fronts, suggesting stacking of lobes or pulsations in ﬂows. Black arrows indicate topographically high deposits of lobe material on obstacles or leveed lobe margins. All images excerpted from PSP_007148_2245, with north towards image top, down-slope towards image bottom, and illumination from the lower left.

and show few or none of the channel bedform morphologies present in particular, typical gully-related deposits commonly terminate at the typical gullies (e.g., bars, lenticular islands, terracing, channel braiding) toe of the fan. The presence of extended gully-related deposits (the that are interpreted to indicate fluvial gully modification (Dickson et al., lobes) for hundreds of meters from the main fans is a notable feature 2007a; McEwen et al., 2007; Dickson and Head, 2008; Head et al., 2008; of the Protonilus Mensae gullies in this study, and is not typical of Schon et al., 2009). classic gullies. The most striking difference between the Protonilus Mensae In contrast to classic martian gullies, a class of features have been gullies and typical martian gully features is in the depositional observed on martian dunes (“dune gullies”) that commonly lack environment. Protonilus Mensae fans start on slopes that are high alcoves, initiate at dune crests, have closely-spaced and notably (~21°) compared to typical gully fans (Parsons et al., 2008), and are straight channels (although some dramatic sinuosity can be present), unusually fluted (linear, downslope sculpture). Additionally, the small and typically terminate in small leveed and lobate snouts (Mangold lobes observed on Protonilus Mensae gully fans are atypical of martian et al., 2003; Miyamoto et al., 2004). Analysis of martian “dune gullies” gully fans. Lastly, and most strikingly, the Protonilus Mensae mesa suggests that a debris flow—the mobilization of dune sediments, gullies are continuous with the lobate flow features described above, enhanced by interactions with surface frost (CO2 or H2O), and possibly which extend hundreds of meters from the fan termini (Figs. 5 and 6). entraining a liquid or vapor phase—may best explain the origin of these While deposition of gully fans around local topography has been landforms (Mangold et al., 2003; Miyamoto et al., 2004). Given the observed in the form of digitate or crenulated fan termini (Malin and morphological similarities between martian “dune gullies” and the Edgett, 2000; Schon et al., 2009), such long run-out and lobate Protonilus Mensae mesa gully and lobe system, we now outline deposits have not been observed in any classic martian gullies. In the morphological properties of terrestrial debris-flows. Evaluating

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1996). Water-free mass movements rarely display sinuosity in channelized segments (Mangold et al., 2008a). Water-free mass- movements such as dry, granular flows, typically terminate in lobate or digitate snouts on relatively steep slopes (typically N20°), governed by the angle of repose of the material (Felix and Thomas, 2004; Mangold et al., 2008a; Pelletier et al., 2008); in contrast, water- lubricated debris flows terminate in lobate snouts on considerably shallower slopes of ~10° or less, as governed by the critical shear strength of the water-debris fluid (Mangold et al., 2008a). Terrestrial debris flows typically originate from one of three causes: 1) transition from a fluvial or hyperconcentrated flow into a debris flow due to a sudden addition of energy (e.g., a flash flood in a sediment-choked drainage system), 2) transition of a landslide into a debris flow by liquefaction of a coherent mass of sediment in response to a sudden addition of water (e.g., a slump occurring after heavy rains), or 3) mobilization of loose sediments by a sudden pulse of water (the “fire-hose effect”)(Johnson and Rodine, 1984). These debris-flow initiators commonly are associated with particular environments in which debris flows are observed (e.g., flash-floods in arroyos or box canyons, landslides on high-retentivity soil slopes, etc.).

5.3. Identiﬁcation of Protonilus Mensae lobate deposits as debris ﬂow deposits

On the basis of the morphological characteristics outlined above, Fig. 8. Lobate flows (black arrows) emanating from channels that can be traced upslope we interpret the Protonilus Mensae lobate deposits to be debris flow (white arrows). Brackets indicate portions of the channel that appear to have been lobes (Johnson and Rodine, 1984; Coussot and Meunier, 1996). The covered with fan-forming sediment subsequent to lobe and channel emplacement. fi a) The right set of arrows trace lobes through channels over a gully fan surface, to a gully lobes are de ned by abrupt snouts that terminate on low angle slopes, channel, while the left set of arrows traces a channel that is overprinted by the toe of the and which emerge from leveed channels, and display variable-width gully fan. b) Lobes traced over a gully fan surface to the gully channel. Both panels (pulsed) channel deposits and stacked flow fronts. The lack of delicate excerpted from PSP_007148_2245, with north towards image top, down-slope towards fluvial bed and channel structures and the presence of sharply defined image bottom, and illumination from the lower left. snouts strongly implicates debris flows over hyperconcentrated flows in the formation of the lobate deposits. We cannot rule out the possibility that some upslope, channelized sediment may have been the morphological signatures of varying amounts of liquid water, and deposited as subsequent hyperconcentrated flows; however, stream changing degrees of sediment cohesion provides a baseline for deter- terraces, cut banks, and island erosion are not observed in these mining whether the Protonilus Mensae gullies and lobes are strong deposits. The lack of coherent blocks of pasted-on terrain or of candidates for martian debris flow deposits. boulders or layered blocks of mesa materials in the lobes suggests that dry mass wasting (e.g., a landslide) cannot readily account for the 5.2. Characteristics of debris flows and other mass movements formation of the lobes. Further, the lobes terminate on slopes well below the angle of repose of dry sediment. Lastly, the lobes are On Earth, a debris flow is characterized by the transient movement considerably thicker than, and are more rounded than the thin, of a debris-rich water and rock slurry, often displaying periodic pulses cuspate, and digitate termini typical of (potentially) dry martian or surges, that deposits sediments within confined channels, as levee granular flow features (Pelletier et al., 2008). deposits, and in terminal lobes with steep snouts and margins, as flow If the Protonilus Mensae lobate features are debris flows, formed slows and stops (Johnson and Rodine, 1984; Coussot and Meunier, by a moving mixture of water and sediment, how do they compare to 1996). Debris flows are distinguished from fluvial processes, such as terrestrial debris flows in terms of critical shear strength? Shear stream flow and hyperconcentrated flows, in that debris flows are strength can be calculated from debris flow deposits by the approach temporally transient phenomena that form abruptly (Coussot and outlined in Johnson and Rodine (1984): Meunier, 1996). The water content of debris flows is typically 10–50% γ δ ð Þ by mass, in contrast to hyperconcentrated flows or fluvial erosion that k = Tc d sin 1 have water contents of 75–99% by mass (Coussot and Meunier, 1996).

Debris flows in which sediments and water move as a single, coherent where k is the critical shear strength, Tc is the thickness at which the phase, may be followed by a tail of hyperconcentrated flow (in which deposit ceased being able to flow, γd is the “reconstituted” (wet) weight sediment and water move at different speeds), resulting in near- of the sediment (taking 3.69 m/s2 as acceleration due to martian instantaneous modification and erosion of debris-flow deposits gravity), and δ is the angle of the slope upon which the lobe came to rest.

(Coussot and Meunier, 1996). Tc was measured using shadow measurements of lobate snouts visible in Debris flow deposits are distinguished from dry (liquid-water- HiRISE image data, yielding a mean thickness of ~1.6 m (N=20).Atthe free) processes, such as avalanches and landslides, in that sediments shadow-measurement locations, a co-registered HRSC stereo digital in debris flow deposits are well-mixed, poorly sorted, and show no elevation model was analyzed to determine δ, resulting in a mean value 3 3 preservation of primary stratigraphy or bedding (Coussot and of 14°. Lastly, γd was taken to be 2×10 kg/m , a typical value for fine- Meunier, 1996). Additionally, deposits from dry processes such as grained (boulder-free) terrestrial debris flow deposits (Johnson and avalanches and landslides are typically found down-slope of “crown- Rodine, 1984) and broadly similar to the wetted density of martian dust scarps” or a detachment scarp—while wet debris flow deposits are and dust simulants (Allen et al., 1998; Richter et al., 2006). Based on rarely associated with detachment scarps (Coussot and Meunier, these results, the Protonilus Mensae lobes have a shear strength of

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~3×103 N/m2 (with a range of 1–6×103 N/m2). Typical terrestrial resulted in more generalized slope failure or slumping). Melting debris flows (admixtures of water and debris) have a range of shear would be enhanced on the steep slopes from which pasted-on terrain strengths between 2 and 20×103 N/m2—with finer-grained debris is absent due to an increase in direct insolation (Hecht, 2002). Cross- flows lacking boulders populating the lower portion of the range cutting and cut-off relationships between gully channels suggest that (Johnson and Rodine, 1984, Pelletier et al., 2008). melting of pasted-on material was episodic, rather than a singular event, consistent with other gully fans observed on Mars (Head et al., 5.4. Water content, sources, and geological setting 2008; Schon et al., 2009). A debris-flow process resulting from melting of latitude-dependent mantle pasted-on terrain was antici- Beyond the mechanical similarities outlined above, what is the pated by Costard et al. (2002). Although precise comparison of gully morphological evidence for the presence of liquid water as a component channel, fan, and lobe volumes is precluded by the large difference in the Protonilus Mensae mesa debris flows? The distinct thickness and between topography resolution and image data resolution, order of roundness of the distal lobe snouts is strikingly similar to wet debris magnitude estimates suggest that the combined volumes of fan and flow snouts observed in terrestrial environments (Fig. 4)(Johnson and lobe deposits are comparable to the volume of latitude-dependent Rodine, 1984, Coussot and Meunier, 1996). This morphology is atypical mantle material removed by gully channel incision. of thin surface flows on Mars that may or may not contain a liquid phase What does the timing of the flows indicate about the climate (Pelletier et al., 2008). The low-angle slopes on which the flows conditions prevailing during lobe deposition and the potential sources terminate (~14°) are also strongly suggestive of water-lubricated flows. for gully and lobe-forming water? Dating of the gully fans and lobate Lastly, the initiation of gully channels on the high, steep slopes of the deposits to ~500 ka links the gullies and lobes to the waning edge of the latitude-dependent mantle and pasted-on terrain is consistent with a last major (~35°) martian obliquity excursion (Laskar et al., 2004). This lowering of the stability regime of the pasted-on material in developing period within the past ~2 Ma is characterized by the formation of a large gullies—a lowering which is typically accomplished in terrestrial number of gully systems on Mars (e.g., Riess et al., 2004; Head et al., environments by over-pressurization of pores and lubrication by liquid 2008; Schon et al., 2009), and to the degradation of the martian latitude- water (Johnson and Rodine, 1984, Coussot and Meunier, 1996; Lanza dependent mantle (Mustard et al., 2001; Head et al., 2003; Milliken et al., et al., in press). 2003) (manifested in Protonilus Mensae as pasted-on terrain (Chris- What is the relationship between the distal lobes and the gullies tensen, 2003)). located upslope—and to what extent can wet debris-flow processes be While climate conditions over the past ~2 Ma have generally been inferred in the formation of the gullies? The contacts between lobes, exceptionally cold and dry at the Protonilus Mensae study site (Haberle channels containing lobes, and gully fans are complex. Some lobes et al., 2001; Costard et al., 2002), recent work has demonstrated the terminate downslope of the fans and emerge from incised channels possibility of generating liquid water from the melting of surficial ice that can be traced to the toe of a gully fan (Fig. 5), raising the deposits sufficient to produce gully-scale flow in protected micro- possibility that these lobes formed by the remobilization of fan environments (Hecht, 2002; Dickson et al., 2007a,b; Marchant and sediments. In contrast, some channels from which lobes emanate can Head, 2007; Morgan et al., 2007b; Head et al., 2008; Levy et al., 2009b; be traced onto gully fans. Small, lobate structures are present atop Williams et al., 2009). Costard et al. (2002), Christensen (2003), and some gully fans (Fig. 5), suggesting that some lobe-forming flows Williams et al. (2009) have used subsurface thermal models to predict have traveled across a gully fan surface. Finally, in rare cases, channels shallow subsurface melting of ice-rich pasted-on terrain. The range of from which lobate deposits emanate can be traced directly to a gully slope, latitude, surface ice presence, and orientation conditions channel (Fig. 8), implying that the source of debris and water for the sufficient to generate meltwater by the top-down melting of surficial debris flow lobe is located within, or in the source region of, the gully ice outlined by Hecht (2002), Dickson et al. (2007a), Head et al. (2008), channel above the gully fan. and Williams et al. (2009) are all met by the Protonilus Mensae site, Given this complex set of stratigraphic relationships, we suggest suggesting that gully and debris flow water may be sourced by the that the sediments present in both the gully fan deposits and the melting of shallow, debris-rich ice under optimum insolation condi- lobate flows observed in the Protonilus Mensae study site originate as tions. The lack of tributaries feeding many of the Protonilus Mensae sediments mobilized during incision of gully channels and erosion of gully channels, coupled with a lack of large alcoves, suggests that the pasted-on material. On gully fans, the presence of both small lobes, localized accumulation of windblown particulate ice that has been and more complexly fluted surface textures suggests that the fans suggested to be an important factor in other martian gullies (Hecht, may have formed by the deposition of sediments from a repeated 2002; Dickson et al., 2007a; Head et al., 2008; Levy et al., 2009b) was not combination of small, short-run-out debris flows, interspersed with asignificant factor in the formation of these unusual gully deposits. lower energy fluvial or hyperconcentrated flow deposition. In One remaining question raised by this analysis is why debris-flow contrast, the distal lobes are interpreted to represent relatively deposits are present at the Protonilus Mensae site, but are not observed more energetic, high-volume debris flows, with longer run-outs, that in other gully terrains on Mars—including those forming on isolated form in discrete events. The episodic deposition of fan sediments by mesas, or elsewhere in latitude-dependent mantle and pasted-on low-energy flows, and erosion of fan sediments by higher-energy terrain. One possibility may be unusually high deposition of atmo- flows can account for the fluted and sculpted surface texture typical of spheric water ice in Protonilus due to regional climate variation (e.g., the Protonilus Mensae fans, while the repeated activity of water-rich Madeleine et al., 2007, in press), leading to high melt volumes, and fan-forming deposition events can account for the meandering or abnormally energetic flows. This possibility is consistent with the sinuous character of some gully channels (Mangold et al., 2008a,b). presence of a broad range of ice-related features formed over the past What is the origin of water and debris for the observed debris-flow ~100 Ma in the greater Protonilus region, including lineated valley fill deposits? The incision of gully channels into pasted-on terrain, and concentric crater fill (Head et al., 2006a,b; Dickson et al., 2008). coupled with a lack of gully channel tributaries, and thinning of Alternatively, complex wind patterns between mesas in fretted terrain pasted-on terrain towards the tops of the gully channels, suggests that may enhance deposition of wind-blown particulate ice (e.g., Morgan melting of ice present in latitude-dependent mantle (Head et al., et al., 2007a, 2008), or the presence of nearby mesas may increase the 2003) pasted-on deposits (Christensen, 2003) and mobilization of the thermal radiation incident on surface ice, resulting in enhanced melting lithic component may be involved in debris-flow generation. The (e.g., Hecht, 2002; Williams et al., 2009). These deposits could also be presence of closely-spaced, but non-anastomosing gully channels unusually large, well-exposed examples of an end-member process that suggests that source material and melting was concentrated in loci of occurs at a smaller scale in other martian gullies, but does not leave well- debris-flow generation, rather than slope-wide (which could have preserved, coherent deposits there.

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6. Conclusions Gwinner,K.,Scholten,F.,Spiegel,M.,Schmidt,R.,Giese,B.,Oberst,J.,Jaumann,R.,Neukum,G., the HRSC Co-Investigator Team, 2005. Hochaufloesende Digitale Gelaendemodelle auf der Grundlage von Mars Express HRSC-Daten. Photogramm. Fernerkund. Geoinf. 5, Observations of the morphology of Protonilus Mensae gully 387–394. deposits are interpreted to indicate local dominance of water- Gwinner, K., Roatsch, T., Matz, K.-D., Scholten, F., Elgener, S., Preusker, F., Oberst, J., fl Jaumann, R., Heather, D., Neukum, G., 2008. Archival stereo data products of the containing debris ows in the incision of gully channels, the deposition HRSC experiment onboard Mars Express. 39th Lunar and Planetary Science of gully fans, and the formation of unusual lobate features. These Conference, Abstract #2373, League City, TX. March 10–14. observations are consistent with top-down melting of debris-covered Haberle, R.M., McKay, C.P., Schaeffer, J., Cabrol, N.A., Grin, E.A., Zent, A.P., Quinn, R., ice and initiation of debris flows by the interaction of rapidly generated 2001. On the possibility of liquid water on present day Mars. J. Geophys. Res. 106, 23317–23326. liquid water with the lithic component of the latitude-dependent Hartmann, W.K., 2005. Martian cratering 8: isochron refinement and the chronology of mantling deposits and pasted-on terrain in which the gullies form. The Mars. Icarus 174 (2), 294–320. fl Hartmann, W.K., Thorsteinsson, T., Sigurdsson, F., 2003. Martian hillside gullies and observed suite of landforms is diagnostic of wet debris ow processes – fi Icelandic analogs. Icarus 162 (2), 259 277. and suggests that we have identi ed an unusual environment in which Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent the wet debris-flow formational end-member is the dominant gully ice ages on Mars. Nature 426 (6968), 797–802. forming process. Elsewhere on Mars gully morphology may be more Head, J.W., Nahm, A.L., Marchant, D.R., Neukum, G., 2006a. Modification of the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation. Geophys. Res. Lett. consistent with a range of other sediment transport mechanisms 33 (8). doi:10.1029/2005GL024360. including fluvial erosion, hyperconcentrated flow, and low-strength Head, J.W., Marchant, D.R., Agnew, M.C., Fassett, C.I., Kreslavsky, M.A., 2006b. mudflows (that deposit as fans rather than lobes). The Protonilus Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for late Amazonian obliquity-driven climate change. Earth Planet. Sci. 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Observations of martian gullies and constraints on We gratefully acknowledge support from the Mars Exploration potential formation mechanisms. Icarus 168, 285–404. Program Mars Express HRSC grant JPL 1237163, and the Mars Data Heldmann, J.L., Toon, O.B., Pollard, W.H., Mellon, M.T., Pitlick, J., McKay, C.P., Anderson, D.T., 2005. Formation of Martian gullies by the action of liquid water flowing under current Analysis Program grants NNG05GQ46G and NNX07AN95G to JWH. Martian environmental conditions. J. Geophys. Res. 110, E05004. Thanks to the HiRISE and CTX teams for use of high-resolution image Heldmann, J.L., Carlsson, E., Johansson, H., Mellon, M.T., Toon, O.B., 2007. Observations data, and to the Mars Express HRSC team for production of high-quality of martian gullies and constraints on potential formation mechanisms II. The northern hemisphere. Icarus 188, 324–344. digital elevation models. Thanks to John Huffman for assistance in stereo Johnson, A.M., Rodine, J.R., 1984. 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