Icarus 242 (2014) 329–351

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Icarus

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Landscape formation at the Deuteronilus contact in southern , : Implications for an Isidis Sea? ⇑ G. Erkeling a, , D. Reiss a, H. Hiesinger a, M.A. Ivanov b, E. Hauber c, H. Bernhardt a a Institut für Planetologie (IfP), Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany b Vernadsky Inst. RAS, Moscow, Russia c German Aerospace Center (DLR), Berlin, Germany article info abstract

Article history: Two of the most widely studied landforms that are associated with a putative ocean that filled the north- Received 6 December 2013 ern hemisphere of Mars are (1) the Formation (VBF), plain units that cover a larger por- Revised 19 June 2014 tion of the northern lowlands of Mars, and (2) a candidate paleoshoreline, e.g., the Deuteronilus contact, Accepted 11 August 2014 which represents the outer margin of the VBF. The VBF and the Deuteronilus contact are interpreted to Available online 20 August 2014 result from a short-lived Late ocean that readily froze and sublimated. Similar landforms are also present in the impact basin of Isidis Planitia and suggest formation processes comparable to Keywords: those that formed the VBF and the Deuteronilus contact in the northern lowlands. Mars Our study of the Deuteronilus contact in Isidis revealed geologic evidence that possibly supports the Geological processes Mars, surface existence of a Late Hesperian/Early Isidis Sea. For example, numerous valleys that are incised Mars, climate into the plains of the southern Isidis basin rim between 82°/90°E and 3°/6°N and trend a few tens of kilo- meters to the north following the general topographic gradient toward the center of Isidis Planitia. A few of them reach the Deuteronilus contact and continue as sinuous ridges in the Isidis Interior Plains (IIP). Based on our findings we conclude that the geologic setting along the Deuteronilus contact, including the valleys and ridges is a result of (1) Late Hesperian short-term fluvial activity, (2) a Late Hesperian/ Early Amazonian short-lived Isidis Sea that readily froze, (3) subglacial drainage and esker formation, and (4) subsequent sublimation of the proposed Isidis ice sheet. Although the fluvio-glacial model we introduce in our manuscript cannot fully explain the geologic setting, possible alternative formation models, including relief inversion and fluvio-volcanic scenarios are even less capable in explaining the observed geologic setting along the Deuteronilus contact. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Mars (e.g., Malin and Edgett, 1999; Head et al., 1999, 2002; Tanaka et al., 2000; Kreslavsky and Head, 2002; Carr and Head, The debate about oceans on Mars has now lasted for more than 2003; Ghatan and Zimbelman, 2006; Poulet et al., 2007; Carter two decades and has revealed that there are strong arguments that et al., 2010; Salvatore et al., 2010; Ghent et al., 2012). both support and negate the existence of at least one large standing Two of the most widely studied landforms that are associated body of water that once possibly covered the northern hemisphere, with a hypothesized ocean that filled the northern hemisphere of i.e., up to one third of the ’s surface, respectively. The debate Mars are (1) the Vastitas Borealis Formation (VBF), a plains unit that reflects one of the major unanswered scientific questions in Mars covers parts of the northern lowlands of Mars and possibly repre- research. While the existence of an ocean was repeatedly proposed sents a sublimation residue of frozen and sublimated outflow chan- (e.g., Parker et al., 1989, 1993, 2010; Baker et al., 1991; Head et al., nel effluents (e.g., Parker et al., 1993; Kreslavsky and Head, 2002; 1998, 1999; Clifford and Parker, 2001; Kreslavsky and Head, 2002; Mouginot et al., 2012), and (2) candidate paleoshorelines, e.g., the Carr and Head, 2003; Ghatan and Zimbelman, 2006; Erkeling et al., Arabia contact – the older, Late /Early Hesperian shoreline 2012; Ivanov et al., 2012; Mouginot et al., 2012) there is also – and the younger, Late Hesperian Deuteronilus contact, which rep- evidence that casts doubts on the presence of an ancient ocean of resents the outer margin of the VBF (e.g., Parker et al., 1989, 1993; Clifford and Parker, 2001; Carr and Head, 2003). The VBF and the Deuteronilus contact are interpreted as products of a short-lived ⇑ Corresponding author. E-mail address: [email protected] (G. Erkeling). Late Hesperian ocean that rapidly froze and sublimated. The http://dx.doi.org/10.1016/j.icarus.2014.08.015 0019-1035/Ó 2014 Elsevier Inc. All rights reserved. 330 G. Erkeling et al. / Icarus 242 (2014) 329–351 remnants of this ocean, together with accumulated wind-blown geologic units, which we dated with crater size-frequency distribu- materials, form the VBF (e.g., Kreslavsky and Head, 2002; tion (CSFD) measurements. We analyzed the morphometric Mouginot et al., 2012; Ivanov et al., 2012). parameters of the fluvial landforms in this region to characterize In terms of morphologic appearance, including transgressive the mode and intensity of fluvial erosion, transport and deposition. lobate contacts possibly resembling coastal contacts, widespread We also constrained the aqueous processes that resulted in the for- coned and knobby terrains and esker-like ridges (e.g., Crumpler mation of the Late Hesperian/Early Amazonian valleys and com- and Tanaka, 2003; Erkeling et al., 2012; Ivanov et al., 2012), similar pared them with those responsible for the formation of the landforms are also present in the Isidis impact basin (Kreslavsky significantly older dense and dendritic valley networks in the and Head, 2002; Crumpler and Tanaka, 2003; Erkeling et al., ancient highlands of the Montes. Based on hyperspectral 2012; Ivanov et al., 2012). The geophysical properties, which point image data, we analyzed the mineralogy of the landscape between to widespread aqueous deposition in the past and the the and the Deuteronilus contact. We critically dis- recent presence of mixed with massive ice (Mouginot cuss the valley and ridge geometry along the Deuteronilus contact et al., 2012), suggest formation processes comparable to those that in the context of the fluvio-glacial formation scenario we introduce formed the VBF and the Deuteronilus contact in the northern low- in this study and alternative formation models including relief lands (Kreslavsky and Head, 2002; Crumpler and Tanaka, 2003; inversion. We compare and review our fluvio-glacial formation Erkeling et al., 2012; Ivanov et al., 2012). Also the stratigraphic model with the results of investigations of valleys and ridges else- sequence, for example comparable Late Hesperian model ages where on Mars and Earth (e.g., Pain and Ollier, 1995; Pain et al., and onlap geometries (e.g., Tanaka et al., 2005; Ivanov et al., 2007; Williams et al., 2009, 2013). Finally, we integrated our 2012) is indicative for similarities in the formation time of the Isi- results into a stratigraphic correlation chart that represents a dis plains and the VBF. In the Isidis basin, the origin of the plains self-consistent model of the geologic history and evolution of the units that fill the center of the basin and form the Deuteronilus valleys and ridges along the Deuteronilus contact in southern Isidis contact is also subject to debate. While there is substantial support Planitia. In summary, our investigations of the Deuteronilus con- for the ocean hypothesis (e.g., Lockwood and Kargel, 1994; tact and the fluvial and glacial landforms offer an excellent oppor- Hiesinger and Head, 2000; Kreslavsky and Head, 2002; Webb, tunity to provide significant insights into the late stage water- 2004; DiAchille and Hynek, 2010; Erkeling et al., 2012; Ivanov related geologic record of the southern Isidis basin. et al., 2012; Mouginot et al., 2012), alternative volcanic, eolian or Our work addresses the following questions: (1) Does the geo- tectonic formation scenarios have also been proposed (e.g., logic setting of valleys and ridges support the existence of an Isidis Grizzaffi and Schultz, 1989; Malin and Edgett, 1999; Tanaka Sea? (2) Which processes were responsible for the formation of the et al., 2000; Head et al., 2002; Carr and Head, 2003; Ghatan and valleys and ridges along the Deuteronilus contact in southern Isidis Zimbelman, 2006; McGowan, 2011; Ghent et al., 2012). Although Planitia? Which formation model is most consistent with the new a number of landforms in the Isidis basin, such as the cliffs and ter- observations? (3) What are the time limits for the formation of races of the Arabia contact and the lobate onlap geometry of the the valleys and ridges? In particular, were the valleys and ridges Deuteronilus contact are consistent with the assumption that both formed at the same time? How do they relate to the geologic history contacts represent shorelines of a putative Isidis Sea (Erkeling of the southern Isidis basin rim? For example, how do the younger, et al., 2012), they do not necessarily prove its existence. The rare and isolated valleys relate to the ancient, widespread and source of ambiguity lies in the fact that different processes, i.e., flu- dense valley networks in the Libya Montes toward the ? In vial, volcanic or glacial processes can produce landforms that are addition, is there a temporal relation to standing water in the Isidis morphologically similar or indistinguishable without addi- basin, i.e., the proposed Isidis Sea? (4) Which sources appear plau- tional context data (e.g., Webb, 2004; Ghatan and Zimbelman, sible for the water of the proposed Isidis Sea? Is the formation of the 2006). For example, the origin of the large variety of landforms proposed Isidis Sea comparable to an ocean that possibly filled the in the Isidis basin is particularly complex and puzzling and northern lowlands? (5) What is the origin of the IEP and the IIP? In resulted in interpretations that are partially inconsistent with each particular, are the IIP the result of an ice sheet that possibly filled other (e.g., Grizzaffi and Schultz, 1989; Parker et al., 1989, 1993; the Isidis basin, similar to the one that might have filled the north- Lockwood and Kargel, 1994; Head et al., 1999, 2002; Hiesinger ern lowlands and resulted in the formation of the VBF? and Head, 2000; Kreslavsky and Head, 2002; Carr and Head, 2003; Crumpler and Tanaka, 2003; Mustard et al., 2007, 2009; 2. Data and methods Tornabene et al., 2008; DiAchille and Hynek, 2010; McGowan, 2011; Komatsu et al., 2011; Erkeling et al., 2012; Ghent et al., Our morphologic map of southern Isidis Planitia and the CSFD 2012; Ivanov et al., 2012; Mouginot et al., 2012). measurements are based on High Resolution Stereo Camera (HRSC) To provide additional evidence that may support or contradict (Jaumann et al., 2007), Context Camera (CTX) (Malin et al., 2007) the possible existence of a sea-scale standing body of water in and Thermal Emission Imaging System (THEMIS) VIS and IR-day- the Isidis basin, we systematically investigated the Deuteronilus time and nighttime (Christensen et al., 2004) images. Images from contact in Isidis Planitia, the geologic setting of valleys and ridges the High Resolution Imaging Science Experiment (HiRISE) along the Deuteronilus contact and the two plains units that fill the (McEwen et al., 2007) and the Mars Orbiter Camera (MOC) basin: the Isidis Exterior Plains (hereafter IEP) that appear on the (Malin and Edgett, 2001) were used to investigate detailed surface basin floor circumferential to the Isidis rim and the Isidis Interior morphologies. Our morphologic map of southern Isidis Planitia fol- Plains (hereafter IIP) that fill the center of the basin and may rep- lows the unit definitions of and Guest (1987) and Tanaka resent the equivalent of the VBF within Isidis Planitia (e.g., Ivanov et al. (2005). et al., 2012; Mouginot et al., 2012). The study of these landforms is The Digital Terrain Models (DTMs) of the southern Isidis basin of particular interest, because both plains units are divided by the rim and the perspective views of the surface are based on elevation Deuteronilus contact and are probably closely related to the forma- data from the High Resolution Stereo Camera (HRSC) (Jaumann tion of the possible paleoshoreline and a putative Isidis Sea. et al., 2007; Gwinner et al., 2009, 2010), and the Mars Orbiter Laser We present the results of our morphologic, morphometric, Altimeter (MOLA) (Zuber et al., 1992; et al., 2001). The iden- stratigraphic and mineralogic investigations of landforms in south- tified morphologic-geologic units were dated with CSFD measure- ern Isidis Planitia and the northern parts of the Libya Montes high- ments in order to estimate their absolute model ages. Crater lands. High resolution image data were used to identify the statistics yield important information on the absolute model ages G. Erkeling et al. / Icarus 242 (2014) 329–351 331 of the mapped units ( and Wise, 1976) and the CSFD mea- surements are used to reconstruct the geologic record of the study area, for example to determine the formation time of the Deuter- onilus contact, the valleys, and the ridges. Craters were measured and counted using the crater tool of the GIS ArcMap 10 software (Kneissl et al., 2011). The absolute model ages for the surfaces are based on the crater production function of Ivanov (2001) and were derived from the current Mars cratering chronology model of Hartmann and Neukum (2001). We used the Crater-Stats II soft- ware (Michael and Neukum, 2010) to create the crater count plots shown in this study. Detailed descriptions of the mathematical and theoretical principles of the age dating technique and the CSFD measurements are provided, for example by Neukum (1983), Ivanov (2001), Werner (2005), Michael and Neukum (2010), and Michael (2013). Morphometric measurements were performed in the southern Isidis basin in order to quantitatively characterize the valleys, e.g., to provide insights into the mode of hydrologic activity during their formation, their maturity, and attributed climate conditions. We used the stream order and the valley density index to define Fig. 1. Mars Orbiter Laser Altimeter (MOLA) (Zuber et al., 1992; Smith et al., 2001) the original mode of fluvial activity (Horton, 1945; Strahler, shaded relief of Isidis Planitia and adjacent regions. Location of Fig. 2A–D is outlined 1958, 1964; Gardiner and Gregory, 1982; Schumm, 1997; Hou by white box. et al., 1997). The indices were also used to define morphometric differences between the Late Hesperian/Early Amazonian valleys in the Isidis basin and the Late Noachian valley networks in the 2005, 2010; Erkeling et al., 2010a, 2012), which continue to the Libya Montes. To define the general stream type, which reflects south and represent the northernmost parts of Tyrrhena Terra. the mode of hydrologic activity and the climatic conditions during The Amenthes trough with the plains of Amenthes Planum is cut valley formation, our geomorphic characterization also includes into the eastern rim of the Isidis basin (Tornabene et al., 2008; the calculation of the valleys’ sinuosity. This quantitative index Erkeling et al., 2011), the western rim is embayed and superim- of valley sinuosity is described as the ratio of channel length to val- posed by plains of volcanic origin from Syrtis Major (Crumpler ley length (e.g., Rosgen, 1994). and Tanaka, 2003; Ivanov and Head, 2003; Hiesinger and Head, Compact Reconnaissance Imaging Spectrometer for Mars 2004; Mustard et al., 2007, 2009; Tornabene et al., 2008). The (CRISM) (Murchie et al., 2007) data were used to discriminate northeastern rim of the Isidis basin shows significant degradation the mineralogy of possible fluvial deposits, the Deuteronilus con- by possible inflow from and exchange of water tact and the ridges. The use of CRISM image data is limited due (Carr and Head, 2003; Ivanov et al., 2012). to dust covering the surface. Particularly in low-lying regions such Our study area lies between 82°/90°E and 3°/6°N in the transi- as Isidis Planitia, which is a dusty region on Mars (e.g., Ruff and tion zone from the Libya Montes highlands to the broad plains of Christensen, 2002; Tornabene et al., 2008), possible outcrops are Isidis Planitia (Fig. 2A–C). The morphologic inventory of the south- obscured and do not show visible absorption bands that are for ern Isidis basin rim is based on a threefold division into Late Noa- example diagnostic for hydrous alteration. Therefore, detections chian, Late Hesperian and Early Amazonian terrain (Fig. 2D): (1) of (possible hydrated) minerals in CRISM images are limited to The southern parts of our morphologic map cover a few kilometers the walls and ejecta layers of impact craters, which potentially pro- of the Libya Montes highlands (darker colors, units Nm, Nd, Nc). vide insights into the subsurface mineralogy. We also used NIR The Noachian highland terrain is embayed by (2) the IEP, which observations provided by the Observatoire pour la Minéralogie, appear concentric to the basin floor and represent the Hesperian l’Eau, la Glace et l’Activité (OMEGA) (Bibring et al., 2006; Poulet volcanic filling of Isidis Planitia ( color, unit Hs). The IEP et al., 2007) to derive information about the mineralogy for those extent a few tens of kilometers toward the basin center until they sites that are not covered by CRISM. are superposed by (3) the IIP (light green color, unit Ak), which also Additional information about the surface composition in south- contains the largest occurrence of the knobby thumbprint terrain ern Isidis Planitia is provided by thermal inertia, which represents (TPT) on Mars (e.g., Grizzaffi and Schultz, 1989; Lockwood and the ability of a surface material to conduct and store heat and Kargel, 1994; Hiesinger et al., 2009; Hielscher et al., 2010; Ghent which is used to differentiate between rocky surfaces or soil. The et al., 2012; Ivanov et al., 2012). thermal inertia values shown in this study are available from the The Libya Montes represent a heterogeneous mix of mountain- Thermal Emission Spectrometer (TES) onboard Mars Global ous materials, which are degraded mainly by impact cratering and Surveyor (Jakosky et al., 2000; Mellon et al., 2000) and are based fluvial processes (lower portion of Fig. 2A–C, brown colors in on Bandfield et al. (2011). Thermal inertia is shown in this Fig. 2D). The mountainous terrain is fluvially modified by dense manuscript in thermal inertia units (tiu), which are equivalent to dendritic valley networks (Fig. 3A), which have been formed by JmÀ2 KÀ1 sÀ1/2 (e.g., Putzig and Mellon, 2007). intense and repeated precipitation-induced surface runoff and date from the earliest geologic history, i.e., the Noachian 3. Regional, geologic and morphologic setting (Crumpler and Tanaka, 2003; Erkeling et al., 2010a; Jaumann et al., 2010). Detailed investigations in the Libya Montes (e.g., The Isidis basin is located at the boundary Crumpler and Tanaka, 2003; Jaumann et al., 2005, 2010; Erkeling near the equator and between the southern Noachian highlands et al., 2010a, 2012; Bishop et al., 2013) enable assessing the hydro- and the northern Amazonian lowlands, respectively (Fig. 1). The logic and climatic conditions from the earliest and apparently wet- southern Isidis basin rim complex consists of parts of the plains test history of Mars and led to the selection of the boundary units of Isidis Planitia and of the Libya Montes highlands (Carr between the Libya Montes and the southern Isidis basin as a can- and Chuang, 1997; Crumpler and Tanaka, 2003; Jaumann et al., didate landing site for the European ExoMars 2018/2020 rover 332 G. Erkeling et al. / Icarus 242 (2014) 329–351

Fig. 2. Southern Isidis Planitia and the northern edge of the Libya Montes highlands including the locations of valleys and ridges of the Deuteronilus contact. (A) THEMIS IR- Day 100 m mosaic of the southern Isidis basin rim with the locations of subsequent figures. (B) THEMIS IR-Night 100 m mosaic of the southern Isidis basin rim reveals the division of southern Isidis Planitia into three main units: The Libya Montes highlands, the smooth Isidis Exterior Plains (IEP) and the rough Isidis Interior Plains (IIP). (C) Mars Orbiter Laser Altimeter (MOLA) (Zuber et al., 1992; Smith et al., 2001) map of the southern Isidis basin rim superposed on the THEMIS IR-Night 100 m mosaic. (D) Morphologic map of the southern Isidis basin rim based on CTX images. Our morphologic units are adopted from Greeley and Guest (1987) and Tanaka et al. (2005). mission (Erkeling et al., 2010b, 2012, 2014). The Libya Montes indicate hydrous alteration throughout the Libya Montes highlands experienced multiple cycles of fluvial activity, which (Erkeling et al., 2012; Bishop et al., 2013). Because the terrain resulted in the formation of a series of fluvial and lacustrine land- declines in elevation from the south (Libya Montes highlands) forms, including dendritic valleys, open-basin paleolakes, alluvial toward the north (low-lying Isidis plains) (Fig. 2C), the highlands fans, deltas, candidate shorelines, and hydrated deposits are an important source for material transported by fluvial activity (Crumpler and Tanaka, 2003; Jaumann et al., 2005, 2010; toward the southern Isidis basin. The material eroded from the Erkeling et al., 2010a, 2012; Bishop et al., 2013). Widespread and highlands has been deposited (Fig. 2B, bright white regions) along abundant occurrences of Fe/Mg-rich clays observed with CRISM the boundary between the Libya Montes and the IEP, which is par- G. Erkeling et al. / Icarus 242 (2014) 329–351 333

Fig. 3. Overview of morphologic units along the southern Isidis basin rim. (A) The Libya Montes highland terrain, including mountain summits and ridges that are degraded by impact craters and dissected by valleys (HRSC h_2162_0002). The northernmost section shows a small segment of the Arabia contact that constitutes the boundary between the Libya Montes and the smooth and low-lying IEP of Isidis Planitia. (B) Detailed view of the highland–lowland boundary including the northernmost parts of the Libya Montes, the Arabia contact, and isolated remnants of highland materials that are embayed by the IEP (HRSC h_2162_0002). Local patches of Early Hesperian exhumed terrain crop out within the IEP. (C) Onlap geometry that builds the Deuteronilus contact between the IEP and IIP (CTX B01_010021_1817). (D) Typical knobby appearance of the IIP. Chains of cones that build the TPT occur within the IIP (CTX B19_017234_1859). tially represented by the Arabia contact (Crumpler and Tanaka, Fig. 2B) results in a light-gray appearance of the IEP in THEMIS 2003; Erkeling et al., 2012). The IEP are located immediately north IR-night images (Fig. 2B). Much of the Noachian massif material of the Libya Montes and appear as a smooth band tens of kilome- and the sedimentary material from the Libya Montes has been ters wide and concentric to the Isidis basin (central portion of superposed by the Syrtis-related filling of the Isidis basin Fig. 2A–C, green color in Figs. 2D and 3B). Wrinkle ridges are pres- (Crumpler and Tanaka, 2003; Ivanov and Head, 2003; Hiesinger ent in the IEP and are known to be formed in association with lava and Head, 2004; Mustard et al., 2007, 2009; Tornabene et al., and by horizontal shortening, although they do not necessarily 2008; Ivanov et al., 2012) and appears only in isolated patches of indicate a volcanic composition (e.g., Schultz, 2000). The mixture stratigraphically lower exhumed terrain (Fig. 3B) and in material of volcanic materials from Syrtis Major (darker colors in Fig. 2B) excavated by impact craters (Crumpler and Tanaka, 2003; and fluvial deposits shed off the highlands (brighter colors in Tornabene et al., 2008; Erkeling et al., 2012). The IEP are also dis- 334 G. Erkeling et al. / Icarus 242 (2014) 329–351 sected by northward trending valleys (shown in blue color, Fig. 2A– rare. The valleys originate near the boundary between the IEP D) that appear scattered, fragmented, and isolated throughout and the Libya Montes and exclusively north of the highlands and southern Isidis Planitia. A few of the valleys continue as ridges the Arabia contact (Fig. 4A). In some places and at local scale, how- (shown in red color, Fig. 2A–D) across the Deuteronilus contact, ever, it seems unclear whether the valley heads are located above which separates the IEP from the IIP (significant change from the Arabia cliffs or at the base of the contact (Fig. 4A, white arrows smooth, light-gray colored IEP to the heterogeneous, dark- to with question marks). The sources of some of the valleys are light-gray colored IIP in Fig. 2A) and has been interpreted as a located in a few kilometer-wide band with rougher texture in the paleoshoreline (e.g., Parker et al., 1989, 1993; Clifford and Parker, center of the IEP (Fig. 4B). This setting may be a result of degrada- 2001; Carr and Head, 2003). The Deuteronilus contact is character- tion of the valleys between the Libya Montes and this band, which ized by an onlap of the IIP onto the IEP and represents the outer is located 20–30 km north of the Arabia contact and the Libya margin of the IIP (Fig. 2A–C, light-green color in Fig. 2D, Fig. 3C). Montes highlands. Morphometric investigations reveal a low sinu- These plains that cover the entire, nearly horizontal (1–5 m/km) osity between 1.1 and 1.2 and a slope of 4% for the upper reaches center of the basin appear as a heterogeneous mixture of smooth (40 m/km, based on MOLA topography; Fig. 2C). The middle plains material and chains of cones (Fig. 3D), which are character- reaches of the valleys consist mostly of individual, spatially iso- istic for the TPT (Grizzaffi and Schultz, 1989; Lockwood and Kargel, lated, and elongated sections that trend tens of kilometers toward 1994; Hiesinger et al., 2009; Hielscher et al., 2010; Ghent et al., the center of the basin (Fig. 5A). The valleys do not show significant 2012; Ivanov et al., 2012). changes in plan view along their course following the continuous northward slope of the IEP (Fig. 2A–D). The shallow slope of the 4. Valleys and ridges at the Deuteronilus contact plains varies between 0.5% and 2.5% (5–25 m/km, based on MOLA topography, Fig. 2C). The moderately entrenched middle reaches The focus of our study is on two types of landforms in southern show a sinuosity of 1.2. Associated tributaries and junctions are Isidis Planitia: (1) valleys incised into the IEP located south of the rare (Fig. 5B) and most of the valleys are disrupted by patterns of Deuteronilus contact and (2) ridges that appear in the IIP north secondary craters. A few kilometers south of the Deuteronilus con- of the Deuteronilus contact. Based on our morphologic, morpho- tact, which represents the topographically lower edge of the IEP metric, stratigraphic and mineralogic investigations, we first (Fig. 3C), the general slope toward the center of the Isidis basin flat- describe the valleys in detail, and secondly, the ridges. tens significantly and varies between 0.1% and 1% (1–10 m/km, based on MOLA topography, Fig. 2C). The downstream sections 4.1. Valleys incised into the IEP are characterized by the abrupt termination of the majority of the valleys in the IEP (Fig. 5C), possibly as a result of limited ero- Most of the valleys incised into the smooth IEP occur along the sion on the flat terrain and small amounts of water involved in southern Isidis basin rim and only a few valleys have been identi- the formation of the valleys. In this region, some of the valleys fied on smooth plains elsewhere in the Isidis region (Erkeling et al., show sections with a slightly increased sinuosity of 1.35. 2011; Ivanov et al., 2012). The upper reaches of the valleys are The observed geologic setting and morphology suggest that the characterized by a sparse network of valley segments that are tens valleys are not related to Noachian fluvial activity, which resulted of meters wide (between 50 and 250 m) and a few kilometers in the formation of the dendritic valley networks in the Libya long. Possible main (trunk) valleys with associated tributaries are Montes. Further support for a formation different from that of

Fig. 4. Upstream and source region of the valleys. Downward slope is toward N. (A) The valleys originate exclusively north of the Libya Montes and north of the Arabia contact (CTX B07_12289). However, in some places and at local scale (white arrows with question marks) it is unclear whether the valleys start above the cliffs or at the foot of the Arabia contact. (B) Some of the valleys seem to start (white arrows) significantly farther north of the Arabia contact (CTX P14_006593_1827) in a zone (outlined by dashed lines) that shows rougher morphologies in comparison to the surrounding IEP. G. Erkeling et al. / Icarus 242 (2014) 329–351 335

Fig. 5. Midstream and downstream sections of the valleys. Downward slope is toward N. (A) Most of the valleys appear slightly sinuous and without any tributaries, do not show significant variations in widths, and trend northward to the center of the Isidis basin and the Deuteronilus contact, respectively (CTX P04_002690_1839). (B) Rare junction (arrow) of valleys trending northward (CTX P14_006593_1827). (C) Valleys disappear abruptly (arrow) and terminate in the IEP south of the Deuteronilus contact (CTX B02_010588_1845). the Noachian valley networks comes from our new CSFD measure- images and OMEGA data that cover the basinward parts of the ments. Our model ages show that the valleys were formed between IEP, show that the dust coverage increases significantly toward 3.36 Ga (upper formation limit of IEP) and 3.22 Ga (lower for- the basin center and particularly toward the northeast of Isidis mation limit of IIP) (Fig. 6A–D and A–D⁄, see also Erkeling et al., Planitia (e.g., Ruff and Christensen, 2002, see also Fig. 1B in 2012; Ivanov et al., 2012), thus significantly later than the dendritic Tornabene et al., 2008). Weak spectral signatures confirm that valley networks of the Libya Montes, which ceased to form at the the IEP plains materials contain highly ferric iron, which is consis- Late Noachian/Early Hesperian boundary (3.8 Ga, see also tent with both weathered dust and altered highland material. The Crumpler and Tanaka, 2003; Erkeling et al., 2010a). Additional evi- region along the Deuteronilus contact in southern Isidis Planitia dence against a common origin between the Libya Montes valleys has insufficient CRISM coverage. The only two CRISM images along and those incised into the IEP comes from two morphometric indi- the Deuteronilus contact in southern Isidis show that the area is ces that reflect the mode of hydrologic activity during valley for- covered by large amounts of dust that is spectrally blank. mation: the stream order and the valley density index (Horton, THEMIS thermal inertia nighttime data give insight into the 1945; Strahler, 1958, 1964; Gardiner and Gregory, 1982; physical characteristics of the surface (Fig. 7, see also Bandfield Schumm, 1997; Hou et al., 1997). Our morphometric investigations et al., 2011). The possible source region of the valleys immediately revealed that the stream order of the sparse network of the IEP adjacent to the highlands shows an association of surfaces with ranges between first and third order and is therefore significantly elevated thermal inertia values of 450–650 tiu. These values are lower in comparison to the dendritic valley networks of the Libya indication that the surfaces are not composed of rocky materials, Montes, which can reach stream orders up to seven (Erkeling et al., but have not yet been mechanically processed into coarse grained 2010a). Also the calculated valley densities (0.01–0.03 kmÀ1) are sand (e.g., Bandfield et al., 2011). However, some of the valleys also significantly lower than the valley densities (0.15–0.48 kmÀ1) cal- originate on the IEP with thermal inertia values between 200 and culated for the dendritic valley networks of the Libya Montes high- 350 tiu. The topographically low valley floors (Fig. 7, ‘‘V’’ and lands (Erkeling et al., 2010a). The sinuosity of 1.1–1.35 and a white arrows) show lower thermal inertia values between 150 topographic gradient below 4% are significant differences in com- and 250 tiu, which are consistent with accumulated dust and sand. parison to the steeply sloping (60 m/km, based on MOLA topogra- In summary, the scattered appearance of valleys incised into the phy; Fig. 2C) dense valley networks and the deeply entrenched IEP is clearly different from that of the dense networks found far- longitudinal valleys in the Libya Montes (Crumpler and Tanaka, ther south in the Libya Montes. The valleys in the IEP do not repre- 2003; Erkeling et al., 2010a; Jaumann et al., 2010). sent a continuation of fluvial activity from the highlands toward Ambiguities remain about the possible existence of aqueous the Isidis basin. Our results suggest different fluvial formation pro- alteration and hydrated minerals near the valleys incised into the cesses, possibly under changing Late Hesperian climate conditions IEP. Previous investigations of fluvial landforms and hydrated including decreased and short-term water availability and possibly deposits in the highlands (e.g., Tornabene et al., 2008; Erkeling less-intense fluvial erosion (e.g., Harrison and Grimm, 2005; et al., 2012; Bishop et al., 2013) and the proposal of an ExoMars Erkeling et al., 2010a). The scattered appearance of the valleys on landing site (Erkeling et al., 2010b, 2014) resulted in very good the IEP, absent deposits and the low valley densities and stream CRISM coverage of the margin between the Libya Montes and the orders suggest short-term valley formation in the Late Hesperian. IEP. The margin, which is the possible source region of the valleys, is characterized by a mixture of fluvial and fundamentally mafic 4.2. Ridges near the Deuteronilus contact and the IIP deposits from the Libya Montes highlands, i.e., Fe/Mg- and Al-phyl- losilicates (e.g., Crumpler and Tanaka, 2003; Tornabene et al., A small number of the valleys on the IEP that drain toward the 2008; Erkeling et al., 2012; Bishop et al., 2013). Fe/Mg-rich and basin center of Isidis reach the possible Deuteronilus paleoshore- Al-rich clays are likely results of hydrous alteration of -rich line and appear to continue as ridges in the IIP (Fig. 8A–D). The lavas by liquid water and subsequent fluvial transport from the ridges occur at elevations between À3725 and À3830 m and highlands toward the IEP (Erkeling et al., 2012). A few CRISM have the same orientation (azimuth) as the valleys. They trend 336 G. Erkeling et al. / Icarus 242 (2014) 329–351

Fig. 6. Stratigraphic interpretation of the IEP and IIP along the Deuteronilus contact and associated crater statistics for selected surface units (A–D and A⁄–D⁄). Craters counted and crater count areas are shown in white (A–D). Model ages of the IEP are shown to the right of the chronology curve, the model ages of the IIP are shown to the left of the chronology curve (A⁄–D⁄). Detailed descriptions of the mathematical and theoretical principles of the age dating technique and the CSFD measurements are provided, for example by Neukum (1983), Ivanov (2001), Werner (2005), Michael and Neukum (2010), and Michael (2013). (A/A⁄) IEP and IIP in the western part of the study region. The IEP show a model age of 3.36 Ga, the IIP are slightly younger with a model age of 3.22 Ga. (B/B⁄) CSFD measurements performed in the central part of the study region (see also Fig. 8A). The IEP were formed significantly earlier (3.55 Ga) than the IIP (2.67 Ga). (C/C⁄) Crater counts for the location shown in Fig. 8C. The IEP are formed around 3.45 Ga in the Hesperian and the IIP show an initial Early Amazonian formation age of 3 Ga. However, a number of larger craters in the count areas increase the age of the IIP (3.64 Ga). We interpret this age as a result of underlying Libya Montes deposits or oldest parts of the IEP that were not sufficiently buried by the IIP, indicating that the latter are relatively thin. This is consistent with results of previous authors, e.g., Crumpler and Tanaka (2003) and Ivanov et al. (2012). (D/D⁄) Results of our CSFD measurements in the easternmost part of southern Isidis Planitia. The IEP show an initial Late Hesperian formation age of 3.42 Ga and appear significantly older than the IIP that show an Early Amazonian age of 3.0 Ga. toward the basin center only for a few kilometers, with lengths Tanaka, 2003; McGowan, 2011; Ghent et al., 2012). The ridges also between 800 m and 9 km. The ridges terminate abruptly in the are not related to the chains of cones of the TPT, which appear sev- IIP at elevations between À3730 and À3835 m even though a eral kilometers basinward of the Deuteronilus contact. few possible fragmented parts can be observed more basinward. Close-up HiRISE views of the contact between the valley and the One exception to the ridges that continue the trend of the valleys ridge show that the ridges are topographically higher than the val- (Fig. 8A–C) is the easternmost valley/ridge system, which crosses ley floor (Fig. 9A–F). The widths of the ridges are typically in the the contact obliquely (Fig. 8D). same range as those of the valleys (95 to 175 m, Fig. 9A–F) Deposits that would suggest the exact location of the termini of and do not vary significantly basinward. Variations in heights are the ridges were not observed. The ridges are spatially not con- expected from the morphology observed in HiRISE and CTX, but nected with other ridges in the Isidis basin that have been previ- are only partially confirmed by HRSC DTMs in the southwest and ously interpreted as eskers (e.g., Lockwood and Kargel, 1994; southeast of the Isidis basin. The ridges are typically less than a Ivanov and Head, 2003; Ivanov et al., 2012) or volcanic landforms few tens of meters high and are single crested. All ridges, with (e.g., Grizzaffi and Schultz, 1989; Head et al., 2002; Crumpler and the exception of the easternmost ridge, which has a flat top, display G. Erkeling et al. / Icarus 242 (2014) 329–351 337

Fig. 6 (continued) a rounded shape in cross section. It should be noted that the reso- lead to two important implications: (1) the valleys incised into lution of the HRSC DTMs (50 m/px) possibly has effects on the the IEP are not related to the dense and dendritic valley networks accuracy of our measurements. The ridges are similarly sinuous farther south in the Libya Montes, which likely formed by intense than the IEP valleys and show a sinuosity index between 1.1 and and repeated precipitation-induced surface runoff and (2) the 1.4. As for the valleys, the investigations of the mineralogy are sig- ridges connected with the valleys appear in stratigraphically nificantly limited due to the increased dust cover. THEMIS thermal higher terrain and are thus difficult to explain by relief inversion inertia nighttime data show that both the IIP and the ridges show scenarios commonly used to explain topographic inversion else- decreased thermal inertia values (150–250) in comparison to the where on Mars. IEP and the valley floors (200–350) (Fig. 7). We could not observe In the following, we discuss a number of possible formation sce- any differences in nighttime data between the ridges and the sur- narios, responsible for the Late Hesperian incision of the valleys rounding IIP (Fig. 7, ‘‘R’’ and white arrows). into the IEP, the ridge formation in the IIP and the formation of the Deuteronilus contact. 5. Discussion of formation scenarios for valleys and ridges We first discuss the geologic setting of the valleys, which is flu- vial in origin. Secondly, we discuss the geologic setting of the The observations and investigations of the geologic setting and ridges with respect to relief inversion (fluvial or fluvio-volcanic), the morphologies of valleys and ridges in southern Isidis Planitia because this scenario has been used and discussed for the majority 338 G. Erkeling et al. / Icarus 242 (2014) 329–351

Fig. 7. THEMIS derived thermal inertia values for a valley incised in the IEP of southern Isidis Planitia. (A) The mountain ridges of the Libya Montes show low nighttime values between 100 and 200 tiu, the IEP immediately adjacent to the highlands show the highest values in southern Isidis Planitia (between 450 and 650 tiu), the IEP elsewhere consists of surfaces with decreased nighttime values between 200 and 350 tiu. Location of (B and C) is outlined by white box. (B and C) The valley floors (arrows) represent topographic lows and accumulated sand and dust resulting in low thermal inertia values (200–250 tiu). The thermal inertia values of the IIP and the ridges (150–200 tiu) are on average below the IEP and the valley floors. Thermal inertia values are based on a detailed observation of the Libya Montes and the plains by Bandfield et al. (2011). of topographic inversions identified on Mars and Earth (e.g., Pain It should be noted here that none of the formation scenarios, and Ollier, 1995; Pain et al., 2007; Williams et al., 2009, 2013). including our introduced fluvio-glacial formation scenario, can We briefly describe three sites on Earth and Mars that show evi- fully explain all the geologic observations along the Deuteronilus dence for relief inversion. The terrestrial analog is located in Wes- contact. However, we favor our fluvio-glacial model because the tern Australia. The two relief inversions on Mars are located in valleys and ridges in southern Isidis Planitia are less well explained Crater, the landing site of the NASA MSL Rover Curiosity, and in by relief inversion, the most frequently used formation scenario for eastern Libya Montes and thus immediately south of our study comparable landforms on Mars and Earth (e.g., Pain and Ollier, area. 1995; Pain et al., 2007; Williams et al., 2009, 2013). We also briefly discuss a number of alternative processes and formation scenarios, which result in the formation of ridges or 5.1. Formation of valleys incised in the IEP positive relief features on Earth but are unlikely responsible for the formation of the valley and ridge setting along the Deuteroni- After the emplacement of the IEP in the Hesperian (>3.36 Ga) lus contact. but before the emplacement of the stratigraphically higher IIP Afterwards, we introduce our fluvio-glacial formation model, (<3.22 Ga) (e.g., Crumpler and Tanaka, 2003; Erkeling et al., which assumes the presence of a Late Hesperian Isidis Sea, its tran- 2012; Ivanov et al., 2012), the valleys have been incised by late sition to an ice sheet, and subsequent subglacial or subaqueous stage fluvial activity (Erkeling et al., 2012, this study). As the val- esker formation during melting and retreat of this ice sheet. This for- leys originate exclusively on the smooth IEP, they do not represent mation scenario is similar to the one proposed for the VBF that cov- a continuation of fluvial transport from the highlands to the floor of ers the northern lowlands (e.g., Parker et al., 1993; Kreslavsky and the Isidis basin. Absolute model ages also suggest a formation sig- Head, 2002; Carr and Head, 2003). Finally, our fluvio-glacial forma- nificantly later than the Late Noachian/Early Hesperian Libya Mon- tion model of the valleys and ridges is integrated into a stratigraphic tes fluvial activity and support the hypothesis that there is no clear correlation chart that represents the possible fluvio-glacial forma- between the formation of the Libya Montes valleys and the tion and evolution of the landscapes in southern Isidis Planitia. valleys incised in the IEP. Although we could not identify any con- G. Erkeling et al. / Icarus 242 (2014) 329–351 339

Fig. 8. Examples of valleys cross cutting the Deuteronilus contact and their continuation as a ridge in the IIP. (A) Sinuous valley reaching the Deuteronilus contact. Northward of the valley, a ridge appears in the stratigraphically higher (younger) IIP and trends toward the center of the basin (CTX B17_016179_1866). (B) Similar setting as (A), but the valley has a lower sinuosity and the ridge is better preserved. (C) The valley appears segmented by the and the ridge is indistinctive (white arrows) and possibly fragmented (white arrows with question marks). The very prominent SW–NE trending sinuous ridge is similar to ridges identified elsewhere in the Isidis basin and has been interpreted in previous studies as a possible esker (e.g., Ivanov et al., 2012). The different dimensions of this ridge compared to the dimensions of those in the continuation of the valleys suggest an increased intensity and duration of (glacial) formation processes. The prominent ridge appears both in the IEP and the IIP and was formed possibly at a different time. (D) Valley and ridge setting at the easternmost part of the study region that is slightly different to the settings described in 6A–C. The direction of drainage is from ENE to WSW, the ridge dimensions and ridge morphologies (e.g., flat topped crest) are different. It should be noted that the ridge starts at the Deuteronilus contact (white arrow) even if it seems otherwise due to the illumination of this particular image. tinuation of valleys from the Libya Montes toward the IEP, the Erkeling et al., 2010a). Valley erosion must have been short-term, headwaters of most of the valleys are located at the base of the because the low valley densities, the low stream order, and the mountains and the catchment areas were likely affected by possi- limited branching are typical indicators of a relatively early phase ble short-lived fluvial processes that occurred in the mountainous in valley erosion (Horton, 1945; Strahler, 1958, 1964; Gardiner and terrain. Gregory, 1982; Schumm, 1997; Hou et al., 1997). Lacking tributar- The valleys are, in terms of their morphologies, dimensions and ies and absent deposits at valley termini are also inconsistent with morphometric characteristics, partially comparable to Late Hespe- intense and repeated incision and are an expression of an imma- rian/Early Amazonian valleys identified elsewhere on Mars and are ture, poorly developed valley system. We suggest that the valleys a result of reduced water availability proposed for this martian were formed by surface runoff of liquid water, in a short-term period (e.g., Carr and Chuang, 1997; Harrison and Grimm, 2005; and nonrecurring erosional event as for example proposed for Late 340 G. Erkeling et al. / Icarus 242 (2014) 329–351

Fig. 9. Valley and ridge setting across the Deuteronilus contact. (A) Termini of the sinuous valley, the onlap geometry of the Deuteronilus contact and the shallow ridge that is connected with the valley (HiRISE ESP_033242_1845). (B) Similar to (A) including labels (V = Valley, R = Ridge, D = Deuteronilus contact). White box shows location of (C). (C) Close-up view of the contact between the valley and the ridge shows that the ridge is topographically higher than the valley floor (arrow). (D) Termini of the sinuous valley, the onlap geometry of the Deuteronilus contact and the ridge that is connected with the valley (HiRISE ESP_033453_1845). (E) Similar to (D) including labels (V = Valley, R = Ridge, D = Deuteronilus contact). White box shows location of (F). (F) Close-up view of the contact between the valley and the ridge shows that the ridge is topographically higher than the valley floor (arrow).

Hesperian/Early Amazonian valleys elsewhere on Mars (e.g., water along the entire southern Isidis basin rim. A formation of the Goldspiel and Squyres, 2000; Harrison and Grimm, 2005). Because valleys by water flowing in the subsurface also appears unlikely valleys appear only in southern Isidis Planitia and are absent else- because we could not identify amphitheater-shaped valley heads where in the IEP, the source for the water and its occurrence was that suggest backward erosion or groundwater seepage (sapping). limited to a region along the boundary between the IEP and the However such a sapping scenario has been proposed as the domi- Libya Montes. Short-term melting of ice in the subsurface of the nant process in valley formation during this period in martian his- IEP or melting of glaciated parts of the Libya Montes due to regio- tory (e.g., Goldspiel and Squyres, 2000). We are also confident that nal changes in environmental conditions are possible water lava was not involved in the formation of the valleys, because the sources (Crumpler and Tanaka, 2003; Ivanov et al., 2012; Syrtis Major lava flows that possibly occupied preexisting valleys Erkeling et al., 2010a, 2012). Morphologic evidence for glaciation in the westernmost parts of the Libya Montes are older than the and melting in the Libya Montes is limited scattered appearance valleys incised in the IEP (Jaumann et al., 2010). In addition, the of viscous flow features (see for example Fig. 5G in Erkeling lava flows extend only a few tens of kilometers toward the Libya et al., 2012). Crumpler and Tanaka (2003) suggest that the pres- Montes and their distribution is limited to the western parts of ence of small valleys in the Libya Montes radiating from summits the Libya Montes (Fig. 2D, e.g., Jaumann et al., 2010; Ivanov of massifs implies downward percolation from an overlying mate- et al., 2012). We could not identify lava flows from Syrtis Major rial such as glacial ice. extending to the valleys source regions along the boundary Similar scenarios of short-term and episodic occurrence of sur- between the Libya Montes highlands and the IEP in southern Isidis face water in the Hesperian and Amazonian, perhaps triggered by Planitia. Local volcanic sources such as domes or vents appear iso- cryospheric melting, were inferred from the analysis of deltas else- lated in the eastern Libya Montes and toward Amenthes Fossae and where on Mars (Hauber et al., 2013). While melting of ice due to farther to the east (Erkeling et al., 2011; Brozˇ and Hauber, 2013), late-stage Syrtis-related volcanic influence or impacts is also possi- but are absent along the boundary between the Libya Montes ble, it appears unlikely that this resulted in the occurrence of liquid and the IEP and the upper reaches of the valleys, respectively. G. Erkeling et al. / Icarus 242 (2014) 329–351 341

The evolution of the landscape along the Deuteronilus contact less-resistant valley side slopes. The newly formed valleys on each and in the lower reaches of the valleys is subject of debate. Most side of the deposits can result in twin-lateral erosion. Both valleys of the valleys become faint and terminate on the IEP and are well have a tendency to gradually move away from the erosion- explained by the general slope toward the center of the Isidis basin, resistant floor material (e.g., Ollier and Tuddenham, 1962; Pain which flattens significantly in the region along the Deuteronilus and Ollier, 1995). The adjacent terrain becomes progressively contact (Fig. 2C, Crumpler and Tanaka, 2003; Erkeling et al., eroded until the former, erosion-resistant valley floor remains as 2012). These valleys were not superposed by the younger IIP and a ridge that is higher than the surroundings. Finally, volcanic did not appear and/or continue as ridges. Those valleys that deposits including lava flows and welded tuff that possibly drained extended farther toward the basin center were superposed by into the pre-existing valleys are well-known to cause relief the IIP and occur as ridges within the IIP. The courses of the preex- inversion (e.g., Ollier, 1967, 1988; Pain and Ollier, 1995). The valley isting valleys and the exact location of the termini are buried by floor becomes more erosion-resistant than its vicinity after a the IIP. The ridges likely represent at least the minimum length diverse range of processes, including degassing, cooling and of the preexisting valleys. Also, the existence of deposits possibly sublimation. Again, in the case of volcanic filling, the surrounding obscured by the IIP is difficult to prove. The absence of deposits terrain can be eroded away by lateral erosion to form a ridge. If associated with the valleys that terminate earlier on the IEP give valley formation already had ceased, eolian erosion over a longer reason to presume that deposits also did not form at the termini period of time can also result in the removal of the finer-grained of the valleys buried by the IIP. Materials excavated by impacts surfaces of the surrounding landscape. Most authors suggest that from surface units below the IIP are possibly deposits of fluvial ori- eolian erosion has played and still plays a significant role in the gin, but they are also superposed by the IEP and are likely related formation of relief inversion and might even be the dominant to the Late Noachian valley formation in the Libya Montes factor on Mars (e.g., Pain et al., 2007; Williams et al., 2009). Another (Crumpler and Tanaka, 2003; Tornabene et al., 2008; Erkeling type of terrestrial relief inversion, which is caused by simple accu- et al., 2012). mulation of sand (rather than erosion and exhumation of surfaces) In summary, the valleys incised in the IEP most likely were and the following coalescence of dunes to form ridges (Rinker formed (1) by short-term surface runoff in the Late Hesperian/Early et al., 1991), is based on the presence of vegetation along the valley Amazonian, (2) by different fluvial processes and later than the course and can be excluded as a candidate scenario on Mars. Libya Montes valley networks and presumably under significantly Although the initial events can be different between the land- different environmental and climate conditions, and (3) earlier scapes formed by inversion of relief based on lava filling or accu- than the ridges and the IIP. mulation of coarse-grained fluvial deposits, the morphologic appearance is often very similar. Depending on various geologic 5.2. Relief inversion on Earth and Mars aspects, including for example the type of rock, water availability, intensity of erosion, and age, relief inversion may cause formation Because the ridges occur always in continuation of the valleys of either continuous ridges that correspond to the pre-existing and are absent elsewhere along the Deuteronilus contact at the drainage pattern, or isolated hills and knobs (Fig. 10) (e.g., southern Isidis basin rim, their formation appears to be closely Summerfield, 1991; Pain and Ollier, 1995). Terrestrial landscapes linked to the formation of the valleys. On Earth, a geologic situation formed by relief inversion are mostly found in semiarid regions where a negative relief (a valley) transitions into a positive relief (a such as Australia, Oman, Chile and in the Mid-West of the US ridge), is described as relief inversion (e.g., Tricart, 1972; Büdel, (e.g., Cundari and Ollier, 1970; Barnes and Pitt, 1976; Maizels, 1982; Twidale, 1983; Summerfield, 1991; Pain and Ollier, 1995; 1987, 1990; Ollier and Galloway, 1990; Pain and Ollier, 1995; Pain et al., 2007; Hou et al., 2008; West et al., 2010). This important Hou et al., 2008; West et al., 2010); examples for topographic process in terrestrial landscape evolution occurs when a pre-exist- inversion are also reported for tropical landscapes (Tricart, 1972). ing, erosion-resistant valley floor is exhumed by erosion of the sur- Topographic inversion also occurs on Mars, for example at mul- rounding, less-resistant terrain. The fluvial landform, including tiple places along the inner rim and on the floor of Gale Crater valley floor and valley course, is finally preserved in (Fig. 11A). Simple, slightly branched valleys, which bear evidence and remains as a highstanding ridge or a series of hills (e.g., Büdel, for short-term erosion, are incised into the Gale Crater rim and 1982; Pain and Ollier, 1995). drain toward the floor. The inversion of relief occurs where the Inverted relief examples present on Earth are explained by sev- slope of the rim flattens significantly and the valleys reach the eral processes that cause the valley floor to become more erosion gently sloping floor of the crater. A ridge at the lower part of the resistant than the slopes of the valley and the adjacent terrain. rim represents the inverted valley floor and the downstream con- The majority of geologic settings based on relief inversion are tinuation of the valley. The ridge terminates in a hummocky terrain related to the filling and cementation of the valley floor. These that is interpreted as remnants of exhumed fluvial deposits, which initial processes include, for example, the deposition of are possibly similar to those identified at the landing site of Curios- coarse-grained river sediments such as gravel or boulders during ity (e.g., Anderson and Bell, 2010). Evidence for ancient valley for- the formation of the valley (e.g., Cundari and Ollier, 1970; Pain mation is also preserved in relief inversion at an isolated site in the and Ollier, 1995; Pain et al., 2007; Williams et al., 2009). The easternmost part of the Libya Montes (Fig. 11B). The ridges shown accumulated fluvial deposits cover (‘‘armor’’) the valley floor and in Fig. 11B are present on the floor of an ancient crater and were prevent further erosion of the floor by water. Also, the cementation likely formed after the valleys eroded into the slopes of the inner of the underlying river sediments by the dissolution of minerals crater rim and drained toward the center of the crater. Later ero- can also result in valley floors to be highly resistant to erosion. This sion has caused exhumation of the valley floors. The ridges at this scenario is also associated with the formation of duricrusts (e.g., site represent an ancient network of valleys, which is considerably Summerfield, 1991; Pain and Ollier, 1995), including silcretes more mature than in Gale Crater and may indicate more pro- (e.g., Barnes and Pitt, 1976; Fairbridge and Finkl, 1978; Twidale, nounced valley formation in the Libya Montes region. Although 1983; Van de Graaff, 1983), ferricretes (e.g., Maignien, 1959; the morphologies suggest that the floors of both craters were sub- Folster, 1964; Büdel, 1982; Pain and Ollier, 1992) and calcretes ject to erosion, which resulted in the removal of possibly less-resis- (e.g., Brown, 1960; Mann and Horwitz, 1979; Beydoun, 1980; tant surrounding material, the sources and processes for the Reeves, 1983; Maizels, 1987, 1990). Subsequent erosion is then possible filling of the valleys and further cementation and armor- focused on the areas between the resistant deposits and the ing of the valley floor materials remain unclear. 342 G. Erkeling et al. / Icarus 242 (2014) 329–351

Fig. 10. Relief inversion on Earth. (A) LANDSAT image (LC81140752013213LGN00) shows relief inversion of the Robe River near the mountainous terrain of Hamersley Range in Western Australia. The ridges of the paleoriver (dark red colors) are partially segmented, the channels of the recent Robe River (light green colors) appear subparallel to the ridges as lateral streams and are responsible for exhumation of the former valley floor. (B) Shuttle Radar Topography Mission (SRTM) 100 m/px DEM shows exhumed valley floors that appear as ridges in the exhumed terrain. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Relief inversion Mars. (A) Relief inversion in Gale Crater. Individual valleys are incised in the slopes of the inner crater rim and drain toward the crater floor. The inversion of relief occurs where the slope of the rim flattens significantly (dashed line) and the valleys reach the slightly sloped floor of the crater and continue as ridges (CTX G02_019065_1749). (B) Relief inversion in eastern Libya Montes. A complex drainage pattern has been exhumed on the floor of a crater. Flow direction was from NW to SE (CTX P14_006672_1836). G. Erkeling et al. / Icarus 242 (2014) 329–351 343

Positive relief features that have been interpreted as inverted the Isidis basin continuously decreases and that there is no signif- fluvial valleys have also been observed at numerous sites else- icant drop in the topographic gradient between the IEP and the IIP. where on Mars, indicating that inversion of relief is a widespread A second difference to inverted relief settings is the relative process on Mars: in Aeolis Dorsa, with possibly the highest density stratigraphy, which is unusual for geologic settings characterized of sinuous ridges on Mars (e.g., Burr et al., 2009; Lefort et al., 2012; by topographic inversion (Fig. 12A and B). The ridges appear exclu- Williams et al., 2013), in the vicinity of (Le Deit sively in the IIP, which likely postdate the formation of the IEP. et al., 2010; Weitz et al., 2010), on the floor of Gale Crater Both the onlap of the IIP onto the IEP, and our absolute model ages, (Anderson and Bell, 2010; Le Deit et al., 2013), at several places which show a Hesperian formation age for the IEP (3.6–3.3 Ga) on the low-lying plains of the northern lowlands (e.g., and a Late Hesperian/Early Amazonian formation age for the IIP Zimbelman and Griffin, 2010; Harrison et al., 2013), in alluvial fans (3.2–2.7 Ga), support this stratigraphy. The inverted valleys else- and deltas (e.g., Malin and Edgett, 2003; Moore et al., 2003; Moore where on Mars, for example in Gale Crater, are also commonly and Howard, 2005; Pondrelli et al., 2008; Erkeling et al., 2012), and present in terrains that are younger than the surfaces incised by in highland terrain, for example and the Libya the valleys, although this younger age is attributed to erosion Montes (e.g., Edgett, 2005; this study). and exhumation. The degradation affecting these terrains results in the exhumation of the valley floors, but also in the removal of 5.3. Ridges along the Deuteronilus contact based on relief inversion? impact craters, which leads to younger model ages. In this context, it should be noted that the valleys and ridges appear in two geo- In our study area along the Deuteronilus contact we found little logic units that were likely formed by different processes and at evidence for relief inversion, and there are key differences to different times, which is also inconsistent with relief inversion inverted relief settings identified elsewhere on Mars and Earth. elsewhere on Mars. Typically, valleys and ridges are usually pres- First, reasonable doubts that relief inversion caused the valley/ ent in one geologic unit that is exhumed (removed) in the vicinity ridge setting along the Deuteronilus contact are based on the of the ridges and preserved in the vicinity of the valleys. occurrence of the ridges in the topographically and stratigraphical- Critical is also the question about the formation time of the ly higher terrain of the IIP (Fig. 9A–F). Patches of old, etched terrain ridges, i.e., whether the ridges were formed earlier, contemporane- appear exhumed and crop out locally within the smooth IEP indi- ous or later than the IIP? The temporal resolution based on CSFD cating that these plains represent a thin coverage (Fig. 3B unit measurements is subject to the model-inherent uncertainties He, see also Fig. 9C in Erkeling et al., 2012). The underlying mate- and the relatively coarse resolution for the Hesperian period rial possibly was eroded from the Libya Montes by Noachian fluvial (Hartmann and Neukum, 2001). Our absolute model ages can only activity, transported toward the basin center and finally deposited give the maximum and minimum formation limits, but are insuffi- in southern Isidis Planitia (Crumpler and Tanaka, 2003; Tornabene cient to provide unequivocal evidence for a formation contempora- et al., 2008; Erkeling et al., 2012; Ivanov et al., 2012). These mate- neously to valley incision, during or after IIP formation. Moreover, rials do not appear in the IIP. Either, processes that resulted in the relative stratigraphy is difficult to interpret: On one hand, the exhumation of buried materials were less intense in the IIP or ridges appear exclusively within the IIP and are restricted to the the plains are more resistant to erosion than the older IEP. At some extent of the plains, which suggests that their origin is associated places along the Deuteronilus contact, the IIP also superpose the with a contemporaneous formation of the IIP. On the other hand, exhumed terrain of the IEP and therefore postdate erosional events the location of the ridges in continuation of the valleys might sug- that led to the degradation of the IEP. A few of the valleys incised in gest that both landforms were formed as a result of valley incision the IEP are influenced by the exhumed terrain and change their and subsequent exhumation of the valley floor. courses. This is also indication for fluvial activity postdating pro- In the following, we discuss possible pre-IIP, syn-IIP, and post- cesses of exhumation. However, crater count plots derived for both IIP formation scenarios in the context of our observations the IEP and the IIP show that resurfacing events occurred in the (Fig. 13A–C). One hypothesis may consider that valley formation, Amazonian between 1.6–1.5 Ga and between 1.2–1.0 Ga i.e., incision of the valleys into IEP ceased between 3.3 and (Fig. 6A⁄ and 6B⁄). These erosional events were possibly responsi- 3.2 Ga and the lower reaches of the valleys were exhumed by ble for the formation of some of the exhumed surfaces of the IEP eolian erosion that led to relief inversion. The IIP were formed later and also occurred significantly later than the formation of the val- than 3.2 Ga and thus are superposed on the IEP. The ridges were ley and ridge setting of the Deuteronilus contact has ended. embayed by IIP material, but not completely covered. Support for Because the IEP and IIP have been interpreted to be relatively thin this is provided by previous observations (e.g., Crumpler and (e.g., Crumpler and Tanaka, 2003; Tornabene et al., 2008; Erkeling Tanaka, 2003; Tornabene et al., 2008; Erkeling et al., 2012; et al., 2012; Ivanov et al., 2012) we also exclude that the increasing Ivanov et al., 2012), which suggest that the IIP are relatively thin. thickness of plains materials toward the basin center is responsible Based on our morphologic and stratigraphic analysis, we argue that for the absence of patches of exhumed materials in the IIP. This is the ridges were not formed significantly earlier than the IIP. The also supported by our observations that the elevation of the floor of exclusion of this scenario is based on the observation that the

Fig. 12. Perspective views of a geologic setting based on relief inversion (A) and the geologic setting observed along the Deuteronilus contact in southern Isidis Planitia (B). 344 G. Erkeling et al. / Icarus 242 (2014) 329–351

A the Deuteronilus contact without a significant change in its course (Fig. 9A and D). However, we would expect such a change, in par- ticular where the valleys reach the onlap front of the Deuteronilus contact. In addition, it remains unclear, whether fluvial erosion would have been intense enough to cross cut the Deuteronilus con- tact (Fig. 13C) or whether the water more likely was ponded along the contact. Ponding remains highly speculative because we could not identify any evidence for ponding of water or the deposition of fluvial materials along the Deuteronilus contact. Fig. 9A and B also shows that the contact between the ridge and the surrounding IIP B material is smooth and continuous and that the IIP material is evenly deposited across the ridge. The obscured ridge remnants are indication for superposition of the IIP material on the ridge or a formation of the ridge within the IIP. Although the contact between the ridge and the IIP shown in Fig. 9D–E is relatively prominent and sharp, the setting more likely suggests increased eolian erosion at this site. As wind was the dominant process on Mars throughout the Amazonian to remove less-resistant sur- rounding materials (McCauley, 1973; Thomas et al., 2005), it may C have played a role in the exhumation of the ridges to its present state, but is unlikely responsible for the initial formation and origin of the geologic setting along the Deuteronilus contact. In summary, a possible relief inversion scenario to explain the present-day landscape is subject to significant uncertainties and is not supported by our morphologic and stratigraphic results, nei- ther by a formation of the ridges earlier (Fig. 9A) nor later (Fig. 9C) than the formation of the IIP. Although we do not completely exclude relief inversion, the location of the ridges in continuation of the valleys and the influence of the Deuteronilus contact on their Fig. 13. Possible pre-IIP, syn-IIP, and post-IIP formation scenarios for the valley/ extent more likely suggest a contemporaneous formation of the ridge setting. Ridges shown in solid colors, valleys shown as outlines. (A) The valley/ ridge setting predates IIP formation (i.e., relief inversion). The transition from a ridges and the IIP by alternative processes. valley to a ridge is not associated with the maximum extent of the IIP. Therefore, both the valleys and the ridges have been covered by the IIP. (B) Ridges formed contemporaneous with the IIP. Because the transition from a valley to a ridge occurs 5.4. Alternative formation models of ridges along the Deuteronilus always at the margin of the IIP, the formation of the ridges is controlled by the IIP contact formation. This setting has been observed in southern Isidis Planitia. (C) Valleys postdate IIP formation. Fluvial erosion possibly was influenced by the onlap contact and resulted in (shown from left to right) fan-shaped deposits, ponding of water It should be noted that the relief inversion scenario has fewer along the contact, the continuation of the valley across the contact or flow of water uncertainties in comparison to the following alternative morpho- along the contact and incision of the valley into the IIP at a topographically low logic and geologic processes, which have been proposed for ridges section of the Deuteronilus contact. The post-IIP scenario precludes ridge formation. and positive relief features elsewhere on Mars and Earth:

(1) On Earth, mud volcanism often results in the formation of southward ridge extension is exactly limited by the Deuteronilus mud ridges, which appear in the vicinity of mud volcanoes contact, which has not existed at the time of the proposed exhuma- and represent positive relief features (Snead, 1964; tion (Fig. 13A). The morphologic setting indicates that there is a Freeman, 1968). Mud ridges might be candidate landforms strong association of the ridges with the Deuteronilus contact in the light of the location of the Isidis TPT, which is present (Fig. 13B), which is inconsistent with valley formation and subse- in the IIP in the center of Isidis Planitia. In particular, numer- quent exhumation as inverted relief (ridges) before the formation ous ridge-like landforms are connected with the cones of the of the IIP (Fig. 13A). Furthermore, fluvial landforms, in particular TPT. The latter have been interpreted (among other hypoth- typical twin lateral streams that could have resulted in the erosion eses) as mud volcanoes (e.g., Davis and Tanaka, 1995; of possibly less-resistant material adjacent to the valley floor are McGowan, 2011; Komatsu et al., 2011; Ghent et al., 2012). absent within the IIP (Crumpler and Tanaka, 2003; Erkeling et al., However, the ridges of the TPT are different to the ridges 2012; Ivanov et al., 2012; this study). We also could not observe along the Deuteronilus contact in terms of morphologies a possible armoring or cementation layer on the floors of the val- (chains of cones) and dimensions (more elongated and lar- leys incised into the IEP or a possible source for such material. ger). The absence of cones in a broad (few tens of kilometers) The low thermal inertia values derived for the valley floors possibly band between the Deuteronilus contact and the TPT also represent accumulated wind-blown materials, but not a possible indicates a different origin. We could not identify any fea- original, and erosion-resistant valley floor (Fig. 7A–C). tures in the close vicinity of the ridges along the Deuteroni- A formation of the ridges later than the IIP is also unlikely lus contact that share similarities with mud volcanoes. (Fig. 13C). This hypothesis predicts that the valleys, which repre- (2) We also exclude volcanic, impact-generated or sedimentary sent a prerequisite for the formation of the ridges, were formed dikes, which can be straight or sinuous (Jolly and Lonergan, after the deposition of the IIP and the Deuteronilus contact. The 2002; et al., 2006). However, the ridges observed hypothesis appears unlikely because, the valleys are incised into along the Deuteronilus contact are also too sinuous to repre- the IEP, which are superposed by the IIP. This stratigraphic setting sent (exhumed) dikes. For example, friction dikes form in indicates, together with our CSFD model ages, that the IEP are older association with impacts, which cannot be observed in asso- than the IIP. Furthermore, the valley/ridge setting continues across ciation with the ridges. G. Erkeling et al. / Icarus 242 (2014) 329–351 345

(3) None of the ridges resemble wrinkle ridges or horst and gra- onilus contact. However, possible eskers located on sloped surfaces ben structures, which would have been formed by contrac- also show asymmetric cross profiles (e.g., Golombek et al., 2001), tional or extensional tectonics, respectively. In particular, and Hiesinger and Head (2002) concluded that on the basis of the symmetric cross-sections of the ridges observed along geometry alone it is difficult to distinguish between eskers and the Deuteronilus contact are inconsistent with wrinkle other types of ridges (e.g., wrinkle ridges). The sharp, single- ridges that commonly are asymmetric in cross-section crested ridges at the Deuteronilus contact are also consistent with (e.g., Kargel, 1993). Also small ridges that are commonly observations that sharp-crested ridges appear on nearly level or superposed on top of wrinkle ridges are not present on top gentle descending sloping terrain. of the ridges in continuation of the valleys, although smaller Further support for an esker-based origin of the ridges is pro- ridges have been observed on broader wrinkle ridges only vided by a number of sinuous ridges identified elsewhere in Isidis (e.g., Plescia and Golombek, 1986; Schultz, 2000; Planitia, which have been interpreted to represent eskers (e.g., Golombek et al., 2001; Mueller and Golombek, 2004). Lockwood and Kargel, 1994; Ivanov and Head, 2003; Ivanov (4) Eolian landforms such as patterns of yardangs or linear et al., 2012). Ivanov and Head (2003) observed sinuous narrow dunes (Goudie, 1999; Mashhadi et al., 2007) are inconsistent ridges with lengths between 50 and 60 km and widths between with the geologic setting and are not supported by our 500 m and 1.5 km that are present in narrow depressions. The pre- observations of individual, narrow sinuous ridges. viously observed ridges appear at the periphery of the Isidis basin (5) The isolated appearance of the individual, narrow sinuous floor (e.g., Ivanov and Head, 2003; Ivanov et al., 2012) and are con- ridges is also inconsistent with patterns or series of giant sistent with terrestrial eskers that tend to form near the glacier ripples that form during catastrophic flooding events on margins (e.g., Shreve, 1985a,b; and Walder, 1994; Earth and Mars and are stretched transversely to the flow Brennand, 2000; and Fastook, 2007). However, the ridges (e.g., Baker, 1978, 2009). observed in previous investigations are different in terms of mor- phology, geometry, and stratigraphy. Although the prominent In summary, we exclude the formation scenarios described in ridge shown in Fig. 8C is located near the Deuteronilus contact, it this section as they are even less suitable to explain the ridges is spatially not connected with the ridges in continuation of the along the Deuteronilus contact than relief inversion. valleys and also shows a different orientation (WSW-ENE). Also the dimensions observed for this ridge are different in comparison 5.5. Glacial formation of the ridges along the Deuteronilus contact? to the dimensions of those in the continuation of the valleys and suggest an increased intensity and duration of (glacial) formation Also glacial processes can result in ridges or ridge-like land- processes. The location of the prominent ridge both in the IEP forms. Based on our morphologic, topographic and stratigraphic and the IIP possibly suggests that it is not related to the same for- investigations of the geologic setting along the Deuteronilus con- mation period of the valley and ridge setting along the Deuteroni- tact and based on the investigations of similar ridges elsewhere lus contact. on Mars (e.g., Kargel and Strom, 1991, 1992; Kargel, 1993; The morphologies, the dimensions, the stratigraphic setting and Lockwood and Kargel, 1994; Golombek et al., 2001; Hiesinger the location of the ridges in continuation of the valleys in southern and Head, 2002; Banks et al., 2009; Ivanov et al., 2012; Isidis Planitia are poorly consistent with almost all other terrestrial Bernhardt et al., 2013), we favor an interpretation of the ridges glacial morphologies. Kames often appear as a ridge or crest, but in continuation of the valleys in southern Isidis Planitia as eskers. their plan view is irregular or round (e.g., Bennett and Glasser, Eskers are discussed below as plausible candidate landforms to 1996). Moraines can be linear landforms but often appear as multi- explain the ridges along the Deuteronilus contact and represent a ple parallel ridges (lateral moraines), gently rolling hills (ground significant part of our fluvio-glacial formation scenario we present moraines), tigerstripe pattern (ribbed moraines) or ridges parallel in the following chapter. Eskers are sinuous or serpentine ridges to the glacier termini (terminal moraines) (e.g., Bennett and that consist of the deposits of sub- or intraglacial ice-walled stream Glasser, 1996) and are also not consistent with the isolated ridges channels and tunnels, and were formed by a stream flowing at the Deuteronilus contact. Drumlins represent symmetric, para- beneath or within a stagnant or retreating ice sheet or glacier bolic hills, which often appear as extensive drumlin fields, (e.g., (e.g., Shreve, 1985a,b; Kargel and Strom, 1991, 1992; Clark and Boulton, 1976; Carr and Schaber, 1977; Rossbacher, 1985; Walder, 1994; Brennand, 2000; Hooke and Fastook, 2007). Possible Bennett and Glasser, 1996). We conclude that the studied ridges eskers have been identified at numerous locations on Mars, for on Mars are morphologically different from drumlins, thus render- example on the floor of (e.g., Kargel and Strom, ing this formation process less likely. 1991, 1992; Kargel, 1993; Hiesinger and Head, 2002; Banks et al., 2009; Bernhardt et al., 2013), in the northern lowlands (e.g., 5.6. Fluvio-glacial formation model Kargel et al., 1995), near the south pole (e.g., Ruff and Greeley, 1990; Head, 2000a,b; Head and Pratt, 2001; Ghatan and Head, To address the problems of the various models of the origin of 2004), and in the center of the Isidis basin (Lockwood and Kargel, the studied ridges in Isidis Planitia, we introduce an alternative for- 1994; Ivanov et al., 2012). Martian eskers show lengths up to mation scenario (Fig. 14A–F), which is based on fluvio-glacial pro- 200 km, are 600 m to 2.4 km wide and 40–160 m high (e.g., cesses and a synchronous origin of the ridges and the IIP. This Kargel and Strom, 1991, 1992; Kargel, 1993; Hiesinger and Head, scenario initially starts after emplacement of the IEP and with 2002; Bernhardt et al., 2013) and roughly match the dimensions the fluvial erosion of the IEP (Figs. 14A and 15). This stage is similar of terrestrial eskers, which show maximum length of 400 km and to those previously described and consistent with previous inves- maximum widths of 6 km (e.g., Kargel and Strom, 1991). The ridges tigations in southern Isidis Planitia (e.g., Crumpler and Tanaka, observed in the southern Isidis basin are two orders of magnitude 2003; Erkeling et al., 2012; Ivanov et al., 2012). Valley formation shorter (3–9 km), and have widths of 95–175 m and heights of and incision into the IEP was short-lived due to colder and dryer few tens of meters that are similar to the smallest dimensions of environmental and climate conditions and ceased at 3.2 Ga at martian and terrestrial eskers. Ridges that have been interpreted the latest (Fig. 15). At this time, outflow channels formed at a num- as eskers are mostly symmetric in cross-section (Kargel, 1993; ber of locations on Mars and water was ponding mainly in the Golombek et al., 2001; Hiesinger and Head, 2002), which is consis- northern lowlands (e.g., Baker et al., 1992; Parker et al., 1993; tent with the geometries observed at the ridges along the Deuter- Hiesinger and Head, 2000; Lucchitta, 2001; Ivanov and Head, 346 G. Erkeling et al. / Icarus 242 (2014) 329–351

Fig. 14. Fluvio-glacial formation scenario for valleys and ridges across the Deuteronilus contact in southern Isidis Planitia. (A) Geologic setting after the IEP were emplaced. Crater size–frequency distribution measurements revealed an upper formation limit of the IEP of 3.3 Ga. Late Hesperian fluvial activity led to the formation of valleys that trend from the boundary between the Isidis plains and the Libya Montes highlands toward the center of the basin. The incision of the valleys into the IEP occurred between 3.3 and 3.2 Ga. (B) Fluvial activity in southern Isidis Planitia already had ceased when water possibly ponded in the Isidis basin. The Isidis Sea existed at the same time the proposed ocean existed in the northern lowlands as a result of outflow channel formation elsewhere on Mars. The maximum extent of the Isidis Sea was similar to the location of the recent Deuteronilus contact. The termination of some valleys on the IEP south of the Deuteronilus contact suggests that they were not the source for the proposed Late Hesperian/Early Amazonian Isidis Sea. (C) Climate environmental conditions of the Late Hesperian/Early Amazonian led to rapid freezing of the Isidis Sea and subsequent formation of a stationary ice sheet in a geologically short time (104 years after Kreslavsky and Head, 2002). (D and E) Glacial retreat, melting and sublimation of the stationary ice sheet was possibly prolonged due to high eolian deposition rates and sedimentary veneering in the low-lying Isidis basin (see Fig. 1B in Tornabene et al., 2008). (F) After sublimation of the ice sheet, ridges and the IIP remained as a sedimentary residue.

2001; Kreslavsky and Head, 2002; Carr and Head, 2003; Lasue sedimentary veneer of wind-blown materials (Kreslavsky and et al., 2013). Water may have also been ponding in the Isidis basin Head, 2002). Based on this setting, the ridges may have formed (Fig. 14B), because the IIP show striking similarities such as the in a glacial environment and possibly represent eskers. Subglacial morphologic onlap of the Deuteronilus contact and a similar for- melting resulted in the transport of water and sediments toward mation age to the morphologic inventory and stratigraphic setting the glacier margin, resulting in a southern flow direction that of the VBF, which covers significant parts of the northern lowlands was upslope (Fig. 14C–E). This scenario is based on the hydraulic (Tanaka and Scott, 1987; Hiesinger and Head, 2000; Kreslavsky and head which is responsible for water flow within a glacier Head, 2002; Tanaka et al., 2005). This ponding is possibly related to (Bennett and Glasser, 1996). The basal pressure beneath the glacier the formation of the outflow channels and their debouching into was likely high in the center of the Isidis basin and decreased the northern lowlands (Lucchitta, 2001; Ivanov and Head, 2001; toward the glacier margin. Under the proposed Isidis ice sheet, Kreslavsky and Head, 2002; Head et al., 2002; Carr and Head, water would have flown approximately radial, which is supported 2003). Due to the Late Hesperian/Early Amazonian cold and dry cli- by the orientation of the ridges radial to the center of the Isidis mate, the Isidis Sea possibly froze to form a stationary ice sheet in a basin (Fig. 2A–D). Also the location of the ridges at the margin of geologically relatively short period of time (Figs. 14C and 15; e.g., the proposed ice sheet (the Deuteronilus contact) may be consis- Kreslavsky and Head, 2002). The maximum extent of the proposed tent with terrestrial eskers, which frequently form near the termi- Isidis Sea and the possible Isidis ice sheet that formed subse- nal zone of a glacier, where ice movement is low (e.g., Shreve, quently may correspond to the location of the Deuteronilus con- 1985a,b; Clark and Walder, 1994; Bennett and Glasser, 1996; tact. As the Isidis basin represents a region of high eolian Brennand, 2000; Hooke and Fastook, 2007). The drainage of water deposition (e.g., Ruff and Christensen, 2002, Fig. 1B in Tornabene emanating from beneath the ice sheet might have resulted in the et al., 2008), the proposed glacier may have been covered by a formation of a proglacial lake (Fig. 14D and E), although we could G. Erkeling et al. / Icarus 242 (2014) 329–351 347

P

A

M

G

E

O

E

R

D

I

(

O

G

Libya Montes IEP IIP Geologic events E

L

D

a

)

Craters A on IIP M

A

Z

O Ridges Ac ~ 2.7 on IIP Deuteronilus contact and Isidis Interior Plains (IIP) N

Ak I formed by proposed late-stage and short-lived A

(IIP) -3800 m Isidis sea and stationary ice-sheet N Late Hesperian / Early Amazonian fluvial activity: Valleys Deuteronilus Elongated valleys were incised into on IEP contact the Isidis Exterior Plains (IEP) ~ 3.3

-3700 m Arabia contact formed by sea-level variations H Hs and wave-cut action of proposed Isidis sea E -3600 m S

(IEP) P

Isidis Exterior Plains (IEP) possibly formed by E

Arabia R late-stage fluvial deposition and Syrtis-related contact volcanic flooding of Isidis Planitia I

A

He N

Nd Late Noachian / Early Hesperian fluvial activity: Libya Montes dendritic valley networks formed by ~ 3.7 Patches of precipitation-induced surface runoff

exhumed N Nm

terrain O

Nc A

Dendritic C

valleys H

I

Noachian A

massifs N

Fig. 15. Stratigraphy of the southern Isidis basin rim. The stratigraphic table is subdivided into Libya Montes landforms, the Isidis Exterior Plains (IEP), the Isidis Interior Plains (IIP) and geologic processes (columns). The Noachian, Hesperian and Amazonian periods, as well as the model ages that define the boundaries between them are shown in rows. Morphologic units are adopted from Greeley and Guest (1987) and Tanaka et al. (2005). not identify any remnants of glacial valleys or lacustrine deposits 2012; Ivanov et al., 2012). It has been suggested that Hesperian along the boundary between the IIP and the IEP. Fan-shaped standing bodies of water in the northern lowlands should have deposits observed in many places at the distal margin of the coned formed as an inevitable result of the large amounts of water that chains and the ridges of the TPT (see Fig. 11b in Ivanov et al., 2012) eroded the outflow channels (Clifford and Parker, 2001). The out- are not related to the ridges along the Deuteronilus contact and flow channels predominantly drained into the northern lowlands unlikely represent possible deposits of subglacial drainage. We and likely contributed substantially to their possible filling (e.g., propose that the transport of subglacial water and sedimentary Baker et al., 1992; Ivanov and Head, 2001). Morphologies, such load preferentially appeared along the courses of the pre-existing as those of the VBF, coned and polygonal terrains or possible global valleys because they represented the lowest erosional level on paleoshorelines, i.e., the Deuteronilus contact, bear evidence for a the flat Isidis plains. After the filling of the valleys, the courses of formation by significant amounts of water (e.g., Parker et al., the subglacial streams remained more or less unchanged because 1993; Tanaka, 1997; Kreslavsky and Head, 2002). Specifically, the of the barely moving, stationary ice sheet (e.g., Brennand, 2000; location of the VBF in continuation of the lower reaches of most Kreslavsky and Head, 2002). The continuation of material deposi- of the outflow channels supports a common origin (Tanaka, tion during melting, sublimation and retreat of the glacier led to 1997). Another compelling argument is that the estimates of mate- the formation of the eskers that reflect the course of the pre-exist- rials eroded for example by the circum-Chryse outflow channels ing valleys (Fig. 14F). Finally, the ice sheet completely sublimated coincide with estimates of the VBF volume (Ivanov and Head, and eolian materials that had been previously accumulated on 2001; Kreslavsky and Head, 2002). CSFD measurements and the the glacier surface and within the glacier were deposited as supra- relative stratigraphy also suggest that the VBF formed at the same and intraglacial meltout or sublimation till and now represent the time as a number of outflow channels (e.g., Baker et al., 1992; rough IIP (Fig. 14F). Tanaka, 1997; Ivanov and Head, 2001). Although a number of pre- In particular, this scenario is consistent with the occurrence of vious investigations show that the IIP share morphologic similari- the ridges exclusively in the IIP and in stratigraphically higher ter- ties with the VBF, including the cones of the TPT and that both rain than the valleys, and with the location of the ridges in contin- units were likely formed at the same time and by the same pro- uation of the valleys. However, the model also faces challenges. cesses (e.g., Kreslavsky and Head, 2002; Carr and Head, 2003; First, similar to the relief inversion scenario, the source of water Crumpler and Tanaka, 2003; Ivanov and Head, 2003; Erkeling eroding the valleys and the timespan for the formation of the val- et al., 2011; Ivanov et al., 2012), both geologic units are not spa- leys, ridges and IIP are not well constrained. Second, the presence tially connected with each other (e.g., Kreslavsky and Head, of water and/or an ice sheet in the Isidis basin is – similar to the 2002; Carr and Head, 2003; Ivanov et al., 2012). Therefore, it presence of water and an ice sheet in the northern lowlands – sub- remains unclear if water from the outflow channels that drained ject of debate (e.g., Parker et al., 1989, 1993; Head et al., 1999; into the northern lowlands spilled over from the Utopia basin Kreslavsky and Head, 2002; Carr and Head, 2003; Erkeling et al., (e.g., Carr and Head, 2003). Alternative sources for the possible 348 G. Erkeling et al. / Icarus 242 (2014) 329–351 ponding of water in the Isidis basin, including the Crater out- manuscript cannot fully explain all the landforms observed along flow channel in the Amenthes region east of Isidis Planitia the Deuteronilus contact. However, possible alternative formation (Erkeling et al., 2011) and a outflow channel models, including relief inversion and fluvio-volcanic scenarios are northwest of the Isidis basin (Mangold et al., 2008) can be likely even less capable in explaining the observed geologic setting along excluded due to differences in formation time and morphologic the Deuteronilus contact. observations, which suggest that these channels did not drain into Based on our findings we conclude that the geologic setting the Isidis basin (Erkeling et al., 2011; Mangold et al., 2008; along the Deuteronilus contact, including the valleys and ridges Tornabene et al., 2008). The valleys that trend toward the basin is a result of (1) Late Hesperian short-term fluvial activity, (2) a center but terminate in most cases before they could reach the Late Hesperian/Early Amazonian short-lived Isidis Sea that readily Deuteronilus contact unlikely provided the amount of water neces- froze, (3) subglacial drainage and esker formation, and (4) subse- sary to form a standing body of water in the Isidis basin and to quent sublimation of the proposed Isidis ice sheet and formation form depositional effluents with an extent corresponding to the IIP. of the IIP. Although the knobby morphologies of the TPT have been inter- Although the proposed scenario can better explain the forma- preted to be results of a variety of glacial processes, including tion of the valleys and ridges than alternative relief inversion and dewatering, deicing, brine-flows and kame landforms (e.g., volcanic formation scenarios, in particular when compared with Kreslavsky and Head, 2002; Ivanov et al., 2012), alternative volca- terrestrial analogs, significant aspects remain uncertain. Among nic-based interpretations also exist. In particular, scenarios for the the most important unresolved problems are (1) the source region origin of the TPT that include pseudocraters or phreatomagmatic of the valleys, (2) the timespan between the formation of the val- rootless cones (e.g., Frey and Jarosewich, 1982; Fagents et al., leys, the ridges and the IIP (although we could define the upper and 2002; Bruno et al., 2004), tuff cones (e.g., Bridges et al., 2003) lower formation limits), (3) the source(s) and timing for the water and cinder cones (e.g., Plescia, 1980) might be inconsistent with of the proposed Isidis Sea, (4) the thickness of the possible glacier, a fluvio-glacial scenario. Mud volcanism has been proposed as ori- (5) the direction of drainage of water and transport of materials gin of the TPT, and such a scenario is not completely inconsistent below the stationary ice sheet, (6) no evidence for proglacial lakes, with an Isidis Sea, because it possibly requires standing water e.g., remnants of ice-margin deltas, (7) absent glacial landforms (e.g., Davis and Tanaka, 1995; McGowan, 2011; Komatsu et al., (e.g., kames) in the vicinity of the possible eskers. 2011; Ghent et al., 2012). Our investigations of the Deuteronilus contact and the fluvial Further possible shortcomings of the new hypothesis are and glacial landforms offer an excellent opportunity to provide sig- related to the putative ice sheet in the Isidis basin, in particular nificant insights into the late stage water-related geologic record of the thickness of the possible glacier. Glacial reconstructions based the southern Isidis basin rim. The aqueous history in this region on esker-like ridges in Argyre Planitia with methods derived by was influenced by significantly different climate and environmen- Shreve (1985a,b) resulted in an esker height-ice thickness relation tal conditions compared to the intensive and long-term aqueous of 1/20 (e.g., Hiesinger and Head, 2002; Banks et al., 2009; period in the Noachian that resulted in the formation of dense den- Bernhardt et al., 2013). It should be noted that this ratio is probably dritic valley networks. Finally, our findings provide additional related to gravity and an influence of the lower martian gravity on insights into landscape evolution in southern Isidis Planitia and the height–ice thickness ratio cannot be ruled out. Applied to the may help to reconstruct the origin of the unusual geologic setting ridges on the IIP, the potential ice thickness would be on the order along the Deuteronilus contact. of 300 m, which is possibly not enough for basal melting. How- ever, the ridges in Argyre Planitia are 1–2 orders of magnitude Acknowledgments longer and higher (e.g., Hiesinger and Head, 2002; Banks et al., 2009; Bernhardt et al., 2013). Also the direction of water flow We appreciate the constructive reviews provided by two anon- and material transport in and below the glacier is uncertain. The ymous reviewers that significantly helped to improve the manu- stationary ice sheet possibly was thickest in the center of the basin script. We thank the HRSC Experiment Teams at DLR Berlin and and thinner toward the margins. Hence, the pressure below the ice Freie Universität Berlin as well as the Mars Express Project Teams sheet was likely highest in the center of the basin and decreased at ESTEC and ESOC for their successful planning and acquisition of toward the Deuteronilus contact. Consequently, the direction of data as well as for making the processed data available to the HRSC water drainage and sediment transport was from the center Team. We acknowledge the effort of the HRSC Co-Investigator toward the rim. The only observation, which indicates that the Team members and their associates who have contributed to this transport was toward the upstream direction, is the morphologic investigation in the preparatory phase and in scientific discussions appearance of the Deuteronilus contact. Lobate flow front sections within the team. This research has been partly supported by the of the Deuteronilus contact bear evidence for material transport, German Aerospace Center (DLR) and by the Helmholtz Association which did not follow the topographic gradient (Fig. 3C, see also of German Research Centres (HGF) through the research alliance Crumpler and Tanaka, 2003; Erkeling et al., 2011, 2012; Ivanov ‘‘Planetary Evolution and Life’’. et al., 2012). 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