Formation of Ares Vallis (Mars) by Effusions of Low-Viscosity Lava Within Multiple Regions of Chaotic Terrain

Geomorphology 345 (2019) 106828

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Geomorphology

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Formation of Ares Vallis (Mars) by effusions of low-viscosity lava within multiple regions of chaotic terrain

David W. Leverington

Department of Geosciences, Texas Tech University, Lubbock, TX 79409, United States of America article info abstract

Article history: Ares Vallis is one of the largest outflow channels on Mars, extending northward N1500 km from the highlands of Received 3 February 2019 Margaritifer Terra into the Chryse impact basin. This outflow system developed primarily during the Hesperian Received in revised form 5 July 2019 and Amazonian as a result of voluminous effusions from the subsurface that took place within Iani Chaos, Accepted 24 July 2019 Aram Chaos, Margaritifer Chaos, and Hydaspis Chaos. Though Ares Vallis is widely interpreted as a product of Available online 30 July 2019 catastrophic outbursts from aquifers, its basic attributes do not appear to support aqueous origins of any kind: fl Keywords: aquifer outburst mechanisms lack meaningful solar system analogs, clear examples of uvial or diluvial sedimen- Mars tary deposits are apparently absent at Ares Vallis, and there is no mineralogical evidence along component chan- Outflow channels nels and within terminal basins for extensive aqueous alteration or for deposition of thick evaporite units. Volcanism Instead, as is the case at hundreds of ancient channels of the inner solar system, the nature of Ares Vallis is aligned Lava with dry volcanic origins. Such origins are broadly consistent with the system's relatively pristine mineralogy, the widespread mantling of component channels by lava flows, and the apparent presence of voluminous mare-style flood lavas within terminal basins. With assumed lava viscosities of 1 Pa s and temperatures of 1350 °C, discharge rates of ~97 × 106 m3/s are estimated at Ares Vallis for 100-m-deep flows along channel reaches with widths of 25 km and slopes of only 0.2°. Mechanical and thermal incision are respectively estimated to be ~12.2 m/day and 2.3 m/day for 100-m-deep flows on these slopes. Formation of the main Ares Vallis channel and associated regions of chaos is likely to have required a minimum effused lava volume of ~1.8 × 106 km3, and still greater lava volumes would have been necessary to form associated channel reaches including those located south of Margaritifer Chaos. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Barker and Bhattacharya, 2018; Citron et al., 2018; Rickman et al., 2018). Aqueous interpretations have also been used to help identify The outflow channels of Mars are mainly interpreted as products of sites of possible astrobiological interest (e.g., Masursky et al., 1979; catastrophic outbursts of water from aquifers confined by frozen ground McKay, 1992, 1996; Oehler and Allen, 2012; Cabrol, 2018). (e.g., Carr, 1979; Baker et al., 1991; Clifford, 1993, 2017; Head et al., Problematically, aqueous interpretations of the Martian outflow 2003, 2018; Rodriguez et al., 2005; Leask et al., 2006a; Komatsu, 2007; channels do not appear to be consistent with the basic character of Carr and Head, 2010b, 2019; Grotzinger et al., 2011; Warner et al., these systems (e.g., Leverington, 2004, 2011). For example, despite 2011; Rodriguez et al., 2012; Baker et al., 2015; Larsen and Lamb, assumed sediment loads of up to 40% (e.g., Komar, 1980; Carr, 1996; 2016; Durrant et al., 2017; Lapotre et al., 2016; Hamilton et al., 2018; Leask et al., 2006b, 2007; Carr and Head, 2015), deposits of obvious Baker, 2018a; Zuber, 2018). Aqueous interpretations of the Martian out- fluvial origin remain elusive along the outflow channels and within flow channels have led to inferences of enormous ancient and modern their terminal basins (e.g., Greeley et al., 1977; Burr and Parker, 2006; stores of water in the near subsurface, the past existence of large lakes Ghatan and Zimbelman, 2006; Leverington, 2007, 2009a, 2018, 2019; and oceans within the terminal basins of outflow systems, and the Carling et al., 2009a; Hobbs et al., 2011; Leone, 2014, 2017; Rice and past occurrence of major oscillations in global climate (e.g., Parker Baker, 2015). Aqueous minerals such as sulfates, carbonates, and clays et al., 1989; Baker et al., 1991; Rotto and Tanaka, 1992; Moore et al., are widespread on Mars, but exposures of these minerals are in many 1995; Gulick et al., 1997; Williams et al., 2000; Lucchitta, 2001; Parker cases geographically limited and are spatially separated by apparently and Currey, 2001; Jaumann et al., 2002; Carr and Head, 2010a, 2010b, unaltered materials, and overall, the surface of Mars has been subjected 2019; Warner et al., 2013; Salvatore and Christensen, 2014a, 2014b; to little alteration by water (e.g., Bibring et al., 2005, 2006; Hurowitz and McLennan, 2007; Christensen et al., 2008; Bibring and Langevin, 2008; E-mail address: [email protected]. Leverington, 2009a, 2011; Ehlmann, 2014; Leone, 2014, 2018). There

https://doi.org/10.1016/j.geomorph.2019.07.015 0169-555X/© 2019 Elsevier B.V. All rights reserved. 2 D.W. Leverington / Geomorphology 345 (2019) 106828 is little spatial correlation between hydrated minerals and Martian out- associations with other highland features including Morava Valles and flow channels (e.g., Bibring et al., 2006; Mangold et al., 2008; Carter Ladon Valles, additional sources have previously been inferred for the et al., 2013; Wilson and Mustard, 2013; Ehlmann, 2014), despite the hy- Ares Vallis system as far south as the Argyre impact basin (e.g., Parker, pothesized occurrence of up to dozens to thousands of individual flood 1989; Clifford and Parker, 2001; Coleman, 2003; Grant and Parker, events for aqueous development of each outflow system (e.g., Manga, 2002; Pondrelli et al., 2005; Baker, 2007; Coleman and Baker, 2009; 2004; Andrews-Hanna and Phillips, 2007; Harrison and Grimm, 2008) Salvatore et al., 2016; Wilson et al., 2018)(Figs. 1 and 2). The total and despite reliance of aquifer outburst models on saturation of the eroded volume of the main Ares Vallis channel, not including related upper ~10 km of megaregolith by liquid and solid water over geological chaotic terrain, is estimated to be ~105 km3 (Carr, 2012). The timescales (e.g., Baker et al., 1991; Clifford, 1993; Baker, 2001; Clifford kilometer-scale longitudinal slopes typical of component channels are and Parker, 2001; Wilson et al., 2009). Similarly lacking are well under 1° (Fig. 3). the widespread aqueous alteration and thick evaporitic sequences As with the other circum-Chryse outflow channels (e.g., Tanaka, that some workers expect within the terminal basins of outflow 1986; Warner et al., 2009; Chapman et al., 2010a, 2010b), the Ares Vallis channels (e.g., Fairén et al., 2011; Mouginot et al., 2012; Pan et al., system is expected to have developed as a result of multiple flow 2017). The aquifer outburst processes most commonly hypothesized episodes that took place primarily in the Hesperian but that also to have driven formation of the outflow channels do not appear to extended well into the Amazonian (Rotto and Tanaka, 1995; Robinson have meaningful solar system analogs (Leverington, 2011) and may et al., 1996; Nelson and Greeley, 1999; Warner et al., 2009, 2010a; be inconsistent with the permeabilities expected of the Martian Neukum et al., 2010; Rodriguez et al., 2014, 2015). Early system megaregolith (Marra et al., 2014). development likely involved flow from sources associated with Iani The properties of the Martian outflow channels instead appear to be Chaos and Margaritifer Chaos in the Late Noachian to Early Hesperian, closely aligned with those of hundreds of ancient channel systems that ~3.6 Ga before present, but additional substantial development of the developed on large rocky bodies of the inner solar system and that are system is inferred to have involved these and other sources for time interpreted as having developed as a result of volcanic processes frames as recent as the Early Amazonian, ~2.5 Ga before present (e.g., Leverington, 2004, 2011, 2014, 2019; Byrne et al., 2013). As with (Warner et al., 2009, 2010a; Roda et al., 2014). Less major periods of numerous volcanic channels preserved on the Moon, Venus, Mercury, system development are likely to have occurred over intervals that and Earth, the Martian outflow systems show evidence for having collectively span much of the Amazonian Period (e.g., Neukum et al., acted as conduits for volcanic flows erupted at or near their heads, 2010; Rodriguez et al., 2014, 2015). and their terminal basins are apparently mantled by extensive volcanic The main Ares Vallis channel extends northward from Iani Chaos and units including mare-style flood basalts (e.g., Leverington, 2007, 2009a, has an approximate average width of 25 km, but the system's overall 2011, 2018, 2019; Hopper and Leverington, 2014; Leone, 2014, 2017). width exceeds 100 km where the main channel divides into multiple Volcanic processes can realistically account for the existence and basic conduits and enters Chryse Planitia (Masursky et al., 1977; Carr, 1979, properties of the Martian outflow channels without the need to appeal 1980; Mars Channel Working Group, 1983; Komatsu and Baker, 1997; to the past operation of exotic aqueous processes that, according to Costard and Baker, 2001; Baker, 2002)(Figs. 5 to 7). Within this basin, some workers, lack theoretical support and have apparently left no Ares Vallis is truncated by channels of the Tiu and Simud systems, obvious traces (e.g., Schonfeld, 1977a, 1977b, 1979; Leverington, 2004, which continue northward into Acidalia Planitia. Along the main 2011, 2014, 2018, 2019; Hopper and Leverington, 2014; Leone, 2014, Ares Vallis channel, floors are generally ~150 m to 2 km below adjacent 2017, 2018; Baumgartner et al., 2015, 2017). uplands (e.g., Warner et al., 2009). Some of the most shallow channel Ares Vallis is a prominent outflow system on Mars that extends reaches are located at higher elevations and are likely to have formed northwestward from Margaritifer Terra into the northern lowlands. in earlier stages of system development, prior to the incision of narrower This outflow system is one of the largest on Mars, with typical channel and deeper conduits (e.g., Nelson and Greeley, 1999; Warner et al., widths of ~25 to 100 km and a total length in excess of 1500 km 2009, 2010a). (Fig. 1). Ares Vallis has numerous attributes that seem to be inconsistent As with other large Martian outflow systems (e.g., Baker, 1982; Carr, with development by aqueous processes, and instead shows substantial 1996), the component channels of Ares Vallis are variously characterized evidence for dry igneous origins. This paper presents a volcanic inter- by the presence of streamlined erosional residuals, longitudinal grooves, pretation of Ares Vallis that involves the past effusion of large volumes inner channels, hanging valleys, cataracts, and patches of chaotic terrain of low-viscosity magmas at several major igneous sources associated (e.g., Masursky et al., 1977; Baker and Kochel, 1978; Lucchitta, 1982, with large regions of chaos. Estimates of possible flow conditions and 2001; Komar, 1983; Mars Channel Working Group, 1983; Komatsu and total eruption volumes are presented, and potential implications for Baker, 1997; Parker and Rice Jr., 1997; Nelson and Greeley, 1999; the nature of other channel systems on Mars are identified. Costard and Baker, 2001; Costard et al., 2007; Warner et al., 2009, 2010b; Pacifici et al., 2009; Moscardelli and Wood, 2011; Rodriguez 2. Overview of the Ares Vallis outflow system et al., 2011, 2015)(Figs. 5 to 10). On the basis of widespread associations with maficreflectance properties, high thermal inertias, and the The main Ares Vallis outflow system extends N1500 km from the presence of geomorphic features such as wrinkle ridges and flow fronts, highlands of Margaritifer Terra into the lowlands of Chryse Planitia volcanic mantling units are interpreted to exist along much of the Ares (e.g., Baker and Milton, 1974; Mutch and Saunders, 1976; Masursky Vallis system (e.g., Robinson et al., 1996; Salvatore et al., 2010, 2016; et al., 1977; Carr, 1979, 1996; Baker, 1982; Lucchitta, 1982; Mars Ody et al., 2011, 2012a, 2012b; Wilson and Mustard, 2013; Leone, Channel Working Group, 1983; Rotto and Tanaka, 1995; Komatsu and 2014)(Fig. 10). Fluvial sedimentary features such as depositional bars Baker, 1997; Marchenko et al., 1998; Pacifici et al., 2009)(Figs. 1 to 4). and giant current ripples have been proposed, but clear examples have The system heads at multiple sites of voluminous effusions from the proven difficult to identify (e.g., Komatsu and Baker, 1997; Rice and subsurface, including those of Iani Chaos, Aram Chaos, Margaritifer Baker, 2015). Regions of chaos show the signs of disturbance and subsi- Chaos, and Hydaspis Chaos (e.g., Carr, 1979; Komatsu and Baker, dence typical of Martian chaotic terrain located elsewhere, and contain 1997; Tanaka, 1999; Glotch and Christensen, 2005; Rodriguez et al., features such as flat-topped plateaus, knobby remnants of highland 2005, 2011; Andrews-Hanna and Phillips, 2007; Harrison and Grimm, terrain, and mantling units including interior layered deposits (ILDs) 2008; Massé et al., 2008; Coleman and Baker, 2009; Pacifici et al., (e.g., Glotch and Christensen, 2005; Massé et al., 2008; Guallini et al., 2009; Warner et al., 2009, 2010a, 2011; Neukum et al., 2010; Guallini 2011; Warner et al., 2011; Sefton-Nash et al., 2012; Roda et al., 2014; et al., 2011; Sefton-Nash et al., 2012; Roda et al., 2014; Baioni and Baioni and Tramontana, 2015)(Fig. 9). South of the main Ares Vallis Tramontana, 2015; Rodriguez et al., 2015). On the basis of spatial system, the Uzboi-Ladon region contains fan deposits, including the D.W. Leverington / Geomorphology 345 (2019) 106828 3

Fig. 1. The main Ares Vallis outflow system extends northward into Chryse Planitia from several source regions including Hydaspis Chaos, Aram Chaos, Iani Chaos, and Margaritifer Chaos. Tiu Valles truncates Ares Vallis and extends further north into Acidalia Planitia. The Ares Vallis system is associated with sources located further to the south such as those that formed Uzboi Vallis and Ladon Valles. The areas depicted in Figs. 2, 3, and 4 are indicated. Equirectangular projection. Mars Orbiter Laser Altimeter (MOLA) data after Smith et al. (2003).

Eberswalde deposit, that are interpreted by many workers as possible Rotto and Tanaka, 1995; Head III et al., 2002; Pan et al., 2017). Trunca- lacustrine deltas that formed in association with fluvial activity during tion of Ares Vallis channels by those of the Tiu and Simud systems the Noachian Period and possibly as recently as the Amazonian Period suggests that major development of Ares Vallis occurred prior to that (e.g., Grant and Parker, 2002; Malin and Edgett, 2003; Bhattacharya of these systems (e.g., Costard and Baker, 2001; Ivanov and Head, et al., 2005; Moore and Howard, 2005; Wood, 2006; Mustard et al., 2001). Locations of ancient oceanic shorelines have been proposed for 2008; Pondrelli et al., 2008; Grant and Wilson, 2011; Irwin et al., 2015). Chryse Planitia (e.g., Parker et al., 1993; Clifford and Parker, 2001; The main Ares Vallis channel terminates within the Chryse impact Ivanov and Head, 2001; Rodriguez et al., 2016; Citron et al., 2018), but basin, though related channels continue northward into Acidalia clear evidence for the past ponding of water and development of related Planitia (e.g., Masursky et al., 1977; Rotto and Tanaka, 1995; Komatsu sedimentary deposits is absent here and within the other basins that and Baker, 1997; Tanaka, 1997; Golombek et al., 1997a, 1999; Tanaka form the northern lowlands (e.g., Masursky et al., 1977; Malin and et al., 2005)(Fig. 1). The Chryse basin is widely mantled by thick Edgett, 1999; Ghatan and Zimbelman, 2006; Leverington, 2007, 2018; wrinkle-ridged volcanic units that have properties similar to those of Pretlow, 2013; Leone, 2014, 2017; Pan et al., 2017). Nevertheless, the lunar maria (e.g., Greeley et al., 1977; Scott and Tanaka, 1986; surface units of the Chryse and Acidalia basins have previously been 4 D.W. Leverington / Geomorphology 345 (2019) 106828

Fig. 2. Topography of the Ares Vallis region. The areas depicted in Figs.5,6,7,and11are indicated. The location of the depicted area is given in Fig. 1. Equirectangular projection. MOLA data after Smith et al. (2003). interpreted as possible sedimentary materials deposited during forma- The Mars Pathfinder landing site, located at the mouth of Ares Vallis tion of Ares Vallis and the other circum-Chryse outflow channels in Chryse Planitia (Fig. 7), was chosen in part so that the diverse range of (e.g., Rotto and Tanaka, 1995; Komatsu and Baker, 1997; Rice Jr. and highland materials transported during hypothesized aqueous flooding Edgett, 1997; Tanaka, 1997; Marchenko et al., 1998; Wilson et al., at Ares Vallis could be examined at a single location (e.g., Crumpler, 2004; Kleinhans, 2005; Tanaka et al., 2005). 1997; Golombek et al., 1997a, 1997b, 1999; Mars Pathfinder Rover D.W. Leverington / Geomorphology 345 (2019) 106828 5

Fig. 3. Elevations and absolute slopes for one of many possible paths along the main Ares Vallis outflow channel and associated channels of Chryse Planitia. Along this flow path (red), the system drops a total of ~800 m over a channel distance of 2350 km. At kilometer scale, most longitudinal slopes are well under one degree. Reaches with much greater slopes are mainly associated with relatively young mantling units or superposed features such as impact craters. The location of the depicted area is given in Fig. 1. Equirectangular projection. MOLA data after Smith et al. (2003).

Team, 1997; Parker and Rice Jr., 1997; Rice Jr. and Edgett, 1997; Smith rich igneous units (consisting of up to ~25% olivine or more) exist et al., 1997; Treiman, 1997; Golombek, 1998; Basilevsky et al., 1999; along the floors and inner margins of parts of Ares Vallis, along parts Hobbs et al., 2011). Topographic relief near the Pathfinder landing of the upland plateaus adjacent to the channel system, within Aram site is not uniquely diagnostic of fluvial landforms, but some local de- Chaos and Iani Chaos, and at and beyond the mouth of Ares Vallis posits have previously been interpreted as possible flood-related (e.g., Rogers et al., 2005; Koeppen and Hamilton, 2008; Salvatore et al., sediments, including materials in the lees of topographic features, 2010, 2016; Leverington, 2011; Ody et al., 2011, 2012a, 2012b, 2013; various conglomerate and gravel units, and the extensive boulder Wilson and Mustard, 2013)(Fig. 11). The youngest olivine-rich units fields that mantle the area (e.g., Mars Pathfinder Rover Team, 1997; here are of Hesperian and Amazonian age, and the oldest such units Smith et al., 1997; Hobbs et al., 2011; Rodriguez et al., 2019). form the Noachian uplands into which Ares Vallis was incised Water and carbon were not detected in the soils of the Pathfinder (e.g., Rogers et al., 2005; Wilson and Mustard, 2013). Within the landing site (e.g., Foley et al., 2003), and rocks examined by the Margaritifer Terra region, localized exposures of hydrated minerals in- Alpha Proton X-ray Spectrometer of the Sojourner rover consist of cluding Fe-Mg-smectite clays have been identified, as have olivine- minimally-altered igneous materials (e.g., Smith et al., 1997; Wänke bearing basalts (e.g., Salvatore et al., 2016; Thomas et al., 2017). Expo- et al., 2001; Foley et al., 2003, 2008), including basalts and possibly sures of hematite, phyllosilicates, sulfates, and carbonates exist within basaltic andesites (e.g., Brückner et al., 2003; Farrand et al., 2008). Over- chaotic terrain associated with Ares Vallis, including at Aram Chaos all, weathering and erosion rates at the Pathfinder landing site have and Iani Chaos, confirming the past aqueous alteration of geological been extraordinarily low since the time of deposition of local units, up materials here (e.g., Newsom et al., 2001; Glotch and Christensen, to ~3.5 Ga before present, implying the long-term persistence of dry 2005; Glotch and Rogers, 2007; Massé et al., 2008; Sefton-Nash et al., and desiccating conditions here (e.g., Golombek and Bridges, 2000; 2012; Baioni and Tramontana, 2015; Thomas et al., 2017; Amador Farrand et al., 2008). et al., 2018). However, despite the presence of hydrated minerals Lithological and mineralogical mapping has been conducted for the along isolated parts of Ares Vallis, there is little correlation between Ares Vallis region using data generated from Mars orbit by instruments the spatial distribution of hydrated minerals and the spatial extent of including the Thermal Emission Imaging System (THEMIS), Thermal this system or other Martian outflow channels, and the overall level of Emission Spectrometer (TES), Observatoire pour la Minéralogie, l'Eau, aqueous alteration here and at the other outflow channels is remarkably les Glaces et l'Activité (OMEGA), and Compact Reconnaissance Imaging low (e.g., Rogers et al., 2005; Bibring et al., 2006; Mangold et al., 2008; Spectrometer for Mars (CRISM). Crustal units exposed in the walls of Salvatore et al., 2010; Carter et al., 2013; Wilson and Mustard, 2013; Ares Vallis and along the surfaces of component channels, including Ehlmann, 2014; Amador et al., 2018). As in other regions of Mars, mantling units that extend well into the northern lowlands, are pre- there is no special geographic correlation between hydrous mineraliza- dominantly mafic in composition (e.g., Rogers et al., 2005; Salvatore tion and valley networks in the Ares Vallis region (e.g., Bibring et al., et al., 2010; Ody et al., 2011, 2012a, 2012b; Pan et al., 2017). Olivine- 2006; Ehlmann, 2014; Kaufman et al., 2019). 6 D.W. Leverington / Geomorphology 345 (2019) 106828

interaction of lava with surface snow and ice have been proposed for other Martian outflow systems (e.g., Cassanelli and Head, 2018, 2019). Among aqueous hypotheses, groundwater outbursts have been favored for development of Ares Vallis, as well as for development of many other large Martian outflow systems, since alternative aqueous processes (involving e.g. the pooling and sudden release of meltwaters at system heads) are incapable of generating the floodwater volumes and discharge rates necessary for channel development (e.g., Carr, 1979, 2012; Mars Channel Working Group, 1983; Clifford and Parker, 2001; Andrews-Hanna and Phillips, 2007). Channel development at Ares Vallis is hypothesized by some workers to have possibly taken place beneath local or regional ice cover (e.g., Pacifici et al., 2009; Roda et al., 2014), and the most distal reaches of the system are widely expected to have been periodically subjected to submarine conditions related to the accumulation of floodwaters in terminal lakes or oceans (e.g., Parker et al., 1993; Ivanov and Head, 2001; Moscardelli and Wood, 2011). Channel inci- sion and the formation of features such as cataracts are predicted to have been products of processes similar to those of terrestrial megafloods including those that formed the Channeled Scabland of Washington (e.g., Baker and Milton, 1974; Baker, 1982, 2009; Carling et al., 2009b; Warner et al., 2010a; Lamb et al., 2014; Larsen and Lamb, 2016; Lapotre et al., 2016). The large areas of chaos at which Ares Vallis commences have been interpreted as the primary sites from which groundwater was catastrophically released during channel formation (e.g., Glotch and Christensen, 2005; Andrews-Hanna and Phillips, 2007; Harrison and Grimm, 2008; Warner et al., 2011), with some workers previously hypothesizing that these types of outbursts might have occurred in conjunction with explosive processes such as those related to the dissociation of gas hydrate (e.g., Komatsu et al., 2000; Max and Clifford, 2001). Some landforms at Ares Vallis are thought to have been formed by aqueous processes that may have been directly or indirectly related to hypothesized outbursts. For example, areas of irregular topography along Ares Vallis and within associated regions of chaos have been attributed to the possi- ble development of thermokarst terrain during or after channel for- mation (e.g., Costard and Baker, 2001; Costard et al., 2007; Warner Fig. 4. Geological map of the Ares Vallis region (Tanaka et al., 2014). Unit names: eNh, et al., 2009, 2010b; Pacifici et al., 2009; Baioni and Tramontana, Early Noachian highland unit; mNh, Middle Noachian highland unit; lNh, Late Noachian 2015). Some topographic basins of the Ares Vallis system, including highland unit; Nhu, Noachian highland undivided unit; HNt, Hesperian and Noachian transition unit; Hto, Hesperian transition outflow unit; Ht, Hesperian transition unit; lHt, impact craters and regions of chaos, are inferred to have been the Late Hesperian transition unit; AHi, Amazonian and Hesperian impact unit. Robinson sites of ancient lakes (e.g., Cabrol and Grin, 2001, 2002; Newsom projection. The location of the depicted area is given in Fig. 1. et al., 2001; Glotch and Christensen, 2005; Guallini et al., 2011; Roda et al., 2014). The hydrous alteration of some chaos materials, including particular sections of interior layered deposits, has led to 3. Aqueous interpretations of Ares Vallis inferences of their past interaction with groundwater or with pooled surface waters (e.g., Glotch and Christensen, 2005; Sefton-Nash The Ares Vallis system is widely interpreted as having been formed by et al., 2012; Baioni and Tramontana, 2015). numerous catastrophic aqueous floods in the Hesperian and Amazonian Estimates of maximum discharge rates at Ares Vallis range between (e.g., Carr, 1974; Masursky et al., 1977; Komar, 1983; Komatsu and ~106 and 5 × 109 m3/s, with assumed water depths of ~20 to 100 m Baker, 1997; Marchenko et al., 1998; Golombek et al., 1997a, 1999; (e.g., Masursky et al., 1977; Carr, 1979; Komatsu and Baker, 1997; Pacifici et al., 2009; Warner et al., 2009; Wilson et al., 2018). These floods Smith et al., 1997, 1998). Each source region is expected to have are predicted to have primarily involved outbursts of groundwater had its own discharge history driven by local igneous or impact resulting from the breaching of a globally-continuous cryosphere by processes. For example, a total of ~6 to 49 floods, with discharges of igneous or impact processes (e.g., Carr, 1979; Baker et al., 1991; Clifford, ~106 to 107 m3/s, are predicted to have developed as a result of aquifer 1993; Baker, 2001; Clifford and Parker, 2001; Glotch and Christensen, effusions at Iani Chaos, with individual flood volumes of up to 2005; Andrews-Hanna and Phillips, 2007; Harrison and Grimm, 2008; ~5000 km3 and with at least ~44 years between these floods Carr and Head, 2010a, 2010b; Warner et al., 2010a, 2011; Head et al., (Andrews-Hanna and Phillips, 2007). Under assumptions of average 2018). Some models of channel formation have also involved near-surface permeabilities of ~10−9 m2, the modeling of outburst the hypothesized formation and later release of meltwater lakes mechanisms has suggested the past occurrence of a minimum of (e.g., Masursky et al., 1977; Roda et al., 2014), the emplacement of ~160 to 660 separate aqueous flood events at Ares Vallis (Harrison debris flows (e.g., Chapman and Kargel, 1999; Tanaka et al., 2001), and Grimm, 2008). More generally, large outflow systems on Mars and/or the flow of glaciers (e.g., Lucchitta, 1982, 2001; Robinson et al., are expected to have possibly required up to thousands of flood events 1996; Chapman and Kargel, 1999; Costard and Baker, 2001; Pacifici for development of each system (e.g., Andrews-Hanna and Phillips, et al., 2009). Flood models involving the production of melt by the 2007; Harrison and Grimm, 2008). D.W. Leverington / Geomorphology 345 (2019) 106828 7

Fig. 5. Effusions at Aram Chaos, Iani Chaos, and Hydaspis Chaos directly contributed to development of the main reaches of the Ares Vallis outflow system. The location of the depicted area is given in Fig. 2. Associated areas depicted in Figs. 8 and 9 are indicated. Thermal Emission Imaging System (THEMIS) daytime infrared mosaic courtesy of Arizona State University.

4. A volcanic interpretation of the Ares Vallis system 2010, 2016; Ody et al., 2011, 2012a, 2012b, 2013; Wilson and Mustard, 2013), is arguably inconsistent with the past occurrence The basic attributes of the Ares Vallis system are arguably not consis- of dozens to thousands of aqueous outburst floods, and with the pur- tent with its development by large aqueous outbursts. Though fluvial, ported long-term existence of water in highly porous and permeable glacial, lacustrine, and oceanic sedimentary deposits continue to be pro- aquifers later exposed by channel incision (e.g., Leverington, 2009a, posed for component channels and basins (e.g., Komatsu and Baker, 2011, 2019). South of the main Ares Vallis system, the level of aqueous 1997; Costard and Baker, 2001; Lucchitta, 2001; Cabrol and Grin, alteration along reaches such as the Uzboi-Ladon segment (e.g., Thomas 2002; Costard et al., 2007; Pacifici et al., 2009; Warner et al., 2009; et al., 2017) is similarly very low for channels purportedly formed Moscardelli and Wood, 2011; Rodriguez et al., 2014; Salvatore and by rainfall and/or outbursts from extensive aquifers. Depending on Christensen, 2014a, 2014b), clear examples of such features have environmental conditions, Martian basalts subjected to wet conditions proven difficult to identify at this or any other Martian outflow system would become substantially altered over timescales of years to (e.g., Greeley et al., 1977; Burr and Parker, 2006; Ghatan and millennia (e.g., Oze and Sharma, 2007; Hausrath et al., 2008), producing Zimbelman, 2006; Leverington, 2007, 2009a, 2018; Carling et al., materials characterized by distinct aqueous-absorption features in 2009a; Hobbs et al., 2011; Leone, 2014, 2017; Rice and Baker, 2015). reflectance spectra (e.g., Noe Dobrea et al., 2003; Pommerol and There is much yet to be learned about the detailed mineralogy of the Schmitt, 2008; Leverington, 2009b; Leverington and Moon, 2012; Martian surface and subsurface, in part because of the paucity of ground Leverington and Schindler, 2018; Wang et al., 2018). Thus, the existence truth data collected by landers and rovers on the surface. Nevertheless, at the outflow channels of vast tracts of little-altered materials is as is the case at all other outflow channels on Mars, there is no evidence very difficult to reconcile with hypothesized channel development by for extensive aqueous alteration of Ares Vallis, and the system is instead aqueous floodwaters effused from enormous perennial aquifers widely characterized by pristine igneous mineralogy (Bibring et al., (e.g., Leverington, 2009a, 2011, 2019; Leone, 2014, 2018). 2006; Mangold et al., 2008; Carter et al., 2013; Wilson and Mustard, Though alternative interpretations have previously been suggested 2013; Ehlmann, 2014). The upper ~10 km of Martian regolith have (e.g., Salvatore and Christensen, 2014a), clear evidence for the pervasive been hypothesized to be highly porous and permeable and to have hydrous alteration of geological materials exposed within the Chryse hosted large volumes of water over geological timescales, dynamically basin and other northern basins hypothesized to have held large exchanging water with the surface at numerous points in the history Martian water bodies (e.g., Leverington, 2011), and for the thick of the planet (e.g., Baker et al., 1991; Clifford, 1993; Baker, 2001; sequences of evaporite minerals that some workers expect of the sites Clifford and Parker, 2001; Wilson et al., 2009). Yet, the apparent preser- of former lakes and oceans (e.g., Leverington, 2011; see also e.g. Hills, vation at Ares Vallis of large volumes of pristine olivine-rich 1984; Krijgsman et al., 1999; Zavialov et al., 2003), is missing in materials, including those of the Noachian units that form the inner mineralogical maps generated using images acquired from orbit walls of component channels (e.g., Rogers et al., 2005; Salvatore et al., (e.g., Bibring et al., 2006; Christensen et al., 2008; Pan et al., 2017). 8 D.W. Leverington / Geomorphology 345 (2019) 106828

Fig. 6. Toward its northern reaches, the main channel of the Ares Vallis outflow system generally shallows and increases in width. The location of the depicted area is given in Fig. 2. Associated areas depicted in Fig. 10 are indicated. THEMIS daytime infrared mosaic courtesy of Arizona State University.

Consistent with this orbital view, the mineralogy of the Pathfinder consistent with expected ice volumes at depth. Both the formation landing site at the mouth of Ares Vallis is known to have been little and melting of ice in the subsurface would presumably have involved altered by water (Smith et al., 1997; Golombek and Bridges, 2000; the subsurface existence of water in the liquid state, and this water Wänke et al., 2001; Foley et al., 2003, 2008; Brückner et al., 2003; should have had the capacity to chemically alter the geological materials Farrand et al., 2008). with which it was in contact prior to freezing and after melting; further- Beyond inconsistencies between the expected and actual more, the adsorbed and other water that can coexist with frozen geolog- mineralogy of the Ares Vallis system, the catastrophic release of ical units (e.g., Patterson and Smith, 1981; Williams and Smith, 1989) groundwater pressurized by the downward propagation of a freezing might have similarly allowed for the alteration of materials with front (e.g., Carr, 1979; Baker, 2001; Clifford and Parker, 2001; Carr which it was in contact. The thermokarst-like subsidence of hundreds and Head, 2010a, 2010b; Head et al., 2018)isaverydifferentprocess of vertical meters would arguably be expected to involve massive ice from the dam failures that drove the largest known terrestrial floods rather than pore ice, the latter of which generally does not involve (e.g., Bretz, 1969; Leverington et al., 2000, 2002; Leverington and substantial subsidence during melting (e.g., Smith, 1984; Williams and Teller, 2003), and is yet to be validated as a realistic mechanism for Smith, 1989). This massive ice could conceivably occur in the form of development of the Martian outflow channels (e.g., Hanna and relict ice (i.e., buried ice; e.g., Zegers et al., 2010) or segregated ice Phillips, 2005; Marra et al., 2014). Indeed, though alternative perspec- (which would be expected to form relatively small ice bodies in geolog- tives exist (e.g., Manga, 2004; Andrews-Hanna and Phillips, 2007), ical materials located very near the surface where associated heave can recent work suggests that expected megaregolith porosities are insuffi- be accommodated during freezing; e.g., Murton et al., 2006). There is no cient for the formation of large outflow channels without the genera- special reason to expect that substantial volumes of buried glacial ice tion of extraordinarily large chambers in the subsurface through have existed in regions of chaos, nor is there reason to expect that crustal flexure (Marra et al., 2014), a process normally associated with enormous bodies of segregated ice formed at considerable depth within the high lava pressures of igneous plumbing systems (Leverington, bedrock units such as the Noachian-aged materials widely exposed 2011, 2019). along the margins of many Martian outflow channels, chasmata, and The subsidence of regions of chaos at Martian outflow channels has chaos regions; formation of such segregated ice would have required previously been attributed to processes such as excavation by aqueous heave of up to hundreds of meters during formation, for which there outbursts, depressurization of aquifers, and the melting of subsurface is no apparent evidence. If ice were present at all within the bedrock ice (e.g., Carr, 1996; Rodriguez et al., 2005; Leask et al., 2006a). units of chaos regions, it would instead be expected to predominantly Processes involving outbursts from pressurized aquifers are arguably take the form of pore ice, and its melting would not generally be not consistent with the areas of anhydrous mineralogy in chaos regions. expected to produce the large amounts of subsidence seen at chaos The melting of large volumes of ground ice appears to similarly be regions since its original development would not have produced sub- incongruous with exposed mineralogy and, in addition, may not be stantial amounts of ground heave. D.W. Leverington / Geomorphology 345 (2019) 106828 9

Fig. 7. Effusions that formed the Ares Vallis system ultimately flowed into Chryse basin. The location of the depicted area is given in Fig. 2. Associated areas depicted in Figs. 8 and 10 are indicated. The landing site of the Mars Pathfinder mission (MPF) is given. THEMIS daytime infrared mosaic courtesy of Arizona State University.

In contrast to aqueous mechanisms, dry volcanic processes can loss of volatiles from lava, contraction during lava solidification, readily account for the formation and basic attributes of the Ares and drainback into magmatic plumbing systems (e.g., Leverington, Vallis outflow system. Though exotic by modern terrestrial standards 2009a). (e.g., Leverington, 2014, 2018), such processes resulted in the de- The capacity of silicate lavas for the erosional development of velopment of hundreds of volcanicchannelsonrockybodiesofthe channels is substantially a function of viscosity (e.g., Hulme, 1982; inner solar system, including bodies with anhydrous surfaces such Williams et al., 2001, 2011; Jaeger et al., 2010; Leverington, 2014, astheMoon,Mercury,andVenus(e.g.,Greeley, 1971; Gornitz, 2018, 2019; Hopper and Leverington, 2014; Dundas and Keszthelyi, 1973; Carr, 1974; Wilhelms, 1987; Baker et al., 1992, 1997; Head III 2014), which varies with factors including silica content, temperature, et al., 1992; Head et al., 2011; Komatsu et al., 1993; Komatsu, 2007; and the abundances of crystals, bubbles, and xenoliths (e.g., Williams Hurwitz et al., 2013a; Byrne et al., 2013; Williams et al., 2001, et al., 1998, 2000; Hurwitz et al., 2012; Chevrel et al., 2013; Robert 2011; Baker et al., 2015; Roberts and Gregg, 2019). Under appro- et al., 2014; Cataldo et al., 2015; Takeuchi, 2015; Vetere et al., 2017, priate eruptive conditions, low-viscosity lavas have a clear capacity 2019; Pistone et al., 2016; Vona et al., 2016; Lesher, 2017; Klein et al., for thermomechanical erosion that can result in the incision of 2018). In contrast with the ~50–5000 Pa s typical of the minimum large channel systems (e.g., Peterson and Swanson, 1974; Hulme viscosities of modern silicate lavas on Earth (e.g., Shaw et al., 1968; and Fielder, 1977; Hulme, 1982; Huppertetal.,1984; Lesher and Murase and McBirney, 1973; Self et al., 1997), mafic or ultramafic Campbell, 1993; Greeley et al., 1998; Williams et al., 2000, 2011; lavas involved in the formation of ancient volcanic plains and large Barnes, 2006; Houlé et al., 2008, 2012; Jaeger et al., 2010; Hurwitz channel systems had minimum viscosities at least as low as ~0.5 Pa s et al., 2012, 2013b; Stockstill-Cahill et al., 2012; Gole et al., 2013; on the Moon (e.g., Murase and McBirney, 1970, 1973; Weill et al., Baumgartner et al., 2015, 2017; Staude et al., 2016, 2017; Lesher, 1971), ~4.5–7.5 Pa s on Venus (e.g., Kargel et al., 1993), ~0.02– 2017; Vetere et al., 2019), including the development of scabland 14.2 Pa s on Mercury (e.g., Stockstill-Cahill et al., 2012; Byrne et al., features such as cataracts (e.g., Dundas and Keszthelyi, 2014)and 2013; Vetere et al., 2017), ~0.1–1 Pa s on the Earth (e.g., Williams streamlined erosional residuals (e.g., Bakeretal.,1992,1997; et al., 2001, 2011; Lesher, 2017), and ~0.5 Pa s on Mars (e.g., Chevrel Leverington, 2004). Large volumes of lava erupted at high rates of et al., 2014). Since lava temperatures decrease with distance from effusion and with low viscosities should be emplaced rapidly and eruptive centers as a result of radiative and conductive cooling as with high levels of turbulence (e.g., Jaeger et al., 2010; Dundas and well as substrate assimilation, the overall capacity for channel inci- Keszthelyi, 2014; Cataldo et al., 2015; Baumgartner et al., 2017; sion should tend to be greatest along reaches that are relatively Vetere et al., 2019), allowing for the development of channel systems close to eruptive centers and least along the most distal reaches of in part through the filling and overflow of local topographic basins. channel systems (e.g., Williams et al., 2001, 2011; Jaeger et al., Lava levels within individual basins could conceivably become 2010; Hurwitz et al., 2012; Cataldo et al., 2015; Baumgartner et al., reduced over time as a result of processes such as outlet incision, 2017; Leverington, 2018). 10 D.W. Leverington / Geomorphology 345 (2019) 106828

Fig. 8. Examples of landforms present along the main Ares Vallis channel: A: streamlined erosional residual (S) surrounded by lava flows (F); B: longitudinal ridges (R) along the exposed periphery of a channel that is otherwise mantled by lava flows (F); C: longitudinal ridges on the floor of a channel that extends northward from Hydaspis Chaos; D: inner channel (C) that extends northward from the outlet channel of Aram Chaos. Context Camera (CTX) images: A: B03_010711_1952_XI_15N030W; B: B11_013994_1870_XN_07N020W; C: B07_012280_1824_XN_02N025W; D: P15_006887_1839_XN_03N017W. The locations of the depicted areas are given in Figs. 5 and 7.

As is evident at the volcanic plains that form the floors of some lunar existence of fluvial deposits along Ares Vallis (e.g., Salvatore et al., craters (e.g., Schultz, 1976; Jozwiak et al., 2012; Wilson and Head, 2016), and does not preclude formation of the main channel system 2018), igneous intrusions can fracture and undermine geological units by the past operation of aqueous processes. Nevertheless, as at other in a manner superficially similar to that seen in Martian regions of Martian outflow channels (e.g., Leverington, 2004, 2011, 2014), the chaos (Leverington, 2019), and the subsidence that helps to form overall mineralogical and geomorphological characteristics of the Ares chaotic terrain on Mars is hypothesized here to be a product of igneous Vallis system are consistent with dry volcanic origins. processes including: 1) the fracturing and melting of country rock by igneous intrusions; and 2) magma drainback into plumbing systems 5. Constraints on the effusive volcanic eruptions that formed during the latter stages of eruptions. Chaos regions on Mars are in Ares Vallis numerous cases associated with features such as cones and flows (e.g., Meresse et al., 2008; Berman and Rodriguez, 2016; Berman et al., If the Ares Vallis system was formed by the eruption of voluminous 2017, 2018; Leverington, 2019), landform types that are not uniquely low-viscosity lava flows, what flow conditions, incision rates, and total the product of volcanic processes (e.g., Farrand et al., 2005; Skinner lava volumes might have been involved? The precise manner in which and Mazzini, 2009; Komatsu et al., 2016) but are consistent with volca- low-viscosity lava flows are emplaced and incise into substrates is not nic interpretations. The Aram Chaos region contains candidate igneous yet well understood (e.g., Dundas and Keszthelyi, 2014; Cataldo et al., source features (Fig. 9d) that are similar in character to the large 2015; Baumgartner et al., 2017), partly because the most voluminous updomed plates of lava present near the head of Kasei Valles, the largest examples of these types of flows were erupted in much earlier stages outflow channel on Mars (Chapman et al., 2010a, 2010b; Leverington, of solar system history when the internal temperatures of large rocky 2018). Component channels of Ares Vallis are widely mantled by units bodies were considerably greater than at present (Leverington, 2014). interpreted by many workers as lava flows (e.g., Robinson et al., 1996; Nevertheless, useful quantitative models are available for constraining Salvatore et al., 2010; Ody et al., 2011; Wilson and Mustard, 2013; basic flow parameters including rates of mechanical incision (involving Salvatore et al., 2016) (e.g., Fig. 10), and the system terminates in the the removal of substrates by kinetic energy) and thermal incision Chryse impact basin, which many workers interpret as being covered (involving the melting of substrates). This study utilized flow and by wrinkle-ridged mare-style flood basalts (e.g., Carr et al., 1976; incision equations previously applied to the study of lunar and Martian Greeley et al., 1977; Scott and Tanaka, 1986; Rotto and Tanaka, 1995; volcanic channels (Hurwitz et al., 2010, 2012) and derived from quanti- Frey et al., 2002; Head III et al., 2002; Salvatore et al., 2010; Ody et al., tative relationships described in numerous earlier volcanic and fluvial 2012a, 2012b; Leone, 2014; Pan et al., 2017). The existence of what studies (e.g., Hulme, 1973; Huppert and Sparks, 1985; Williams et al., must be relatively young volcanic mantles along parts of Ares Vallis 1998, 2000; Keszthelyi and Self, 1998; Sklar and Dietrich, 1998). The and within its terminal basin does not itself preclude the possible specific software implementation of the flow model was previously D.W. Leverington / Geomorphology 345 (2019) 106828 11

Fig. 9. Examples of landforms present within chaos regions associated with the Ares Vallis system: A: erosional residual mesas (e.g. at M) at Aram Chaos; B: large mounds (e.g. at arrows) at Iani Chaos; C: interior layered deposits (e.g. at L) in Iani Chaos; D: updomed plates (e.g. at P)offloor materials at Aram Chaos, similar to those present at and near the head region of the Kasei Valles outflow system (e.g., Chapman et al., 2010b; Leverington, 2018) and in southern Elysium Planitia (e.g., Keszthelyi et al., 2008). These plates are interpreted here as possibly marking the sites of volcanic effusions, but could more simply be related to lava inflation without the presence of local igneous sources (Keszthelyi et al., 2008). Context Camera (CTX) images: A: D20_035092_1826_XI_02N022W; B: P04_002562_1795_XN_00S017W; C: P02_001995_1794_XN_00S018W; D: P08_004039_1827_XN_02N021W. The locations of the depicted areas are given in Fig. 5.

applied in several recent studies of terrestrial and Martian volcanic where g is gravitational acceleration, dlava is lava depth, α is the channel channels (Leverington, 2014, 2018, 2019; Hopper and Leverington, slope in degrees, and Cf is the friction factor, which is given by 2014). (Keszthelyi and Self, 1998): Mechanical incision associated with the flow of lava is given by "#! !− (Sklar and Dietrich, 1998; Hurwitz et al., 2010, 2012): 0:92 2 1 2 Re þ 800 C ¼ log 6:15 ð3Þ f 32 10 41 ðÞ d dchan ¼ K ρ gQw sinα ð1Þ dt mechanical The Reynolds number (Re) is given by: where K designates the erodibility of substrates in Pa−1, ρ is lava ρ vd density, g is gravitational acceleration, Qw is the discharge of lava Re ¼ lava ð4Þ per meter of channel width (given in m2/s), and α is the channel μ slope in degrees. The physical processes involved in mechanical lava erosion are not well understood at present, and therefore the re- where ρ is the density of magma, υ is the velocity of magma, dmagma is sults generated using Eq. (1) should be treated with caution (Dundas the lava depth, α is the channel slope in degrees, and μ is the dynamic and Keszthelyi, 2014). The range that defines appropriate values of viscosity of lava. the erodibility constant (K) is poorly constrained for lava, and it's Thermal incision associated with the flow of lava is given by (Hulme, not yet clear that the plucking and abrasion processes of lava are suf- 1973; Huppert and Sparks, 1985; Williams et al., 1998, 2000; Hurwitz ficiently similar to those of water to allow for the use of this form of et al., 2012): mechanical erosion law in the study of channelized lavas (Dundas and Keszthelyi, 2014). ° dðÞd h T−T For slopes of b10 , the velocity of lava is given by (Hurwitz et al., chan ¼ T mg ð5Þ 2010, 2012): dt thermal Emg

α 2 gdlava sin fi ðÞv ¼ ð2Þ where hT is the coef cient of heat transfer, T is the lava temperature, Tmg lava C f is the temperature at which the substrate materials melt, and Emg is the 12 D.W. Leverington / Geomorphology 345 (2019) 106828

Fig. 10. Mantling units present along much of Ares Vallis are widely characterized by maficreflectance properties, high thermal inertias, and the presence of features including wrinkle ridges and flow fronts; and on this basis these units are interpreted by many workers as volcanic in origin (e.g., Robinson et al., 1996; Salvatore et al., 2010; Ody et al., 2011, 2012a, 2012b; Wilson and Mustard, 2013; Leone, 2014; Salvatore et al., 2016). Examples of mantling units present along the main Ares Vallis channel are depicted above. Context Camera (CTX) images: A: P14_006518_1903_XN_10N023W; B: B02_010302_1884_XI_08N023W; C: P18_008008_1929_XN_12N028W; D: P18_007929_1974_XI_17N032W. The locations of the depicted areas are given in Figs. 6 and 7. energy required to melt the substrate materials (Hulme, 1973; Huppert Byrne et al., 2013; Dundas and Keszthelyi, 2014; Leverington, 2014, and Sparks, 1985; Williams et al., 1998, 2000): 2018, 2019; Hopper and Leverington, 2014). Water depths of up to ~100 m have been previously hypothesized for Ares Vallis hi ¼ ρ − þ ð Þ (e.g., Masursky et al., 1977), and lava flow depths of 50, 75, and 100 m Emg g cg Tmg Tg f mg Lg 6 were therefore considered in flow modeling in the present study (extending the 25 and 50 m results presented in Leverington, 2019). where ρ is the density of the substrate materials, c is the specificheat g g Formation of large volcanic channels on rocky bodies of the inner solar of the substrate materials, T is the temperature at which the substrate mg system is expected to have involved minimum lava viscosities at least materials melt, T is the initial temperature of the substrate materials, g as low as ~0.01 to 10 Pa (e.g., Hulme, 1982; Kargel et al., 1993; Lesher f is the fraction of the substrate materials that must be melted prior mg and Campbell, 1993; Williams et al., 2000, 2011; Barnes, 2006; Houlé to mobilization, L is the latent heat of fusion of the substrate materials, g et al., 2008, 2012; Jaeger et al., 2010; Hurwitz et al., 2012, 2013b; and h is the coefficient of heat transfer, which is given by (Hulme, T Stockstill-Cahill et al., 2012; Gole et al., 2013; Baumgartner et al., 2015, 1973): 2017; Staude et al., 2016, 2017; Lesher, 2017), and this study utilized 4 2 an intermediate lava viscosity of 1 Pa s that is close to the 0.5 Pa s vis- 0:017 kRe5 Pr5 cosities known for lunar basalts recovered by the Apollo program hT ¼ ð7Þ dlava (e.g., Murase and McBirney, 1970, 1973; Weill et al., 1971)andfor Martian basalts examined geochemically by the Spirit rover at the where k is the thermal conductivity of lava [2.16 – (0.0013 T)] (Williams mouth of the Ma'adim Vallis channel system (Chevrel et al., 2014). et al., 1998), Re is the Reynolds number, Pr is the Prandtl number [(cgμ)/k], The temperatures of ancient maficandultramafic lavas of relevance and dlava is the lava depth. to the past development of large volcanic channels are likely to have The above equations are functions of each other and are solved iter- approached or exceeded 1400 °C at the time of eruption (e.g., Burns atively until variables converge upon specific values. Except with regard and Fisher, 1990; Nisbet et al., 1993; Grove and Parman, 2004; to certain flow depths and channel widths, the parameters used to Filiberto et al., 2008; Williams et al., 2011; Baumgartner et al., model flow conditions are the same as those utilized in recent studies 2015, 2017; Cataldo et al., 2015; Vetere et al., 2019), and a magma (Leverington, 2018, 2019), and are given in Table 1. Maximum lava temperature of 1350 °C was used here. The insulating effects of flow depths at large volcanic channels of the inner solar system are ex- lava crusts can limit cooling effects to as little as 1° per 30 km for pected to have typically been at least tens of meters and are likely to flows with depths of 20 to 30 m (e.g., Keszthelyi et al., 2006), poten- have been as great as hundreds of meters (e.g., Jaeger et al., 2010; tially allowing voluminous lavas to flow for distances of up to D.W. Leverington / Geomorphology 345 (2019) 106828 13 thousands of kilometers (e.g., Leverington, 2018). Kilometer-scale multiple individual eruptive episodes separated by extended periods slopes along Ares Vallis are predominantly well under 1° (Fig. 3), of little to no volcanic activity. with much higher slopes locally associated with relatively young Based on thermal principals that conservatively ignore the indepen- features including superposed impact craters and late-stage lava dent action of mechanical processes, the minimum volume of erupted flows. This study considered slopes of 0.0 to 1.0°, recognizing that lava required for removal of a particular volume of substrate can be slopes closer to ~0.2° or less are likely to be especially relevant to crudely estimated as roughly ~8 to 25 times the volume removed the study of relatively mature Martian volcanic channels that have (e.g., Leverington, 2007, 2018, 2019; Hopper and Leverington, 2014; incised hundreds of meters into their substrates (e.g., Dundas and Cataldo et al., 2015). Thus, for the approximate system volume of Keszthelyi, 2014; Leverington, 2014, 2018, 2019; Hopper and ~105 km3 (Carr, 2012), a value that ignores chaos regions and applies Leverington, 2014; Baumgartner et al., 2017; Dundas et al., 2019). only to the main channel of Ares Vallis north and immediately south Extending the 25 and 50 m results of Leverington (2019) that were of Iani Chaos, a total minimum erupted lava volume of ~8 × 105 to determined for Ravi Vallis, the flow conditions and incision rates pre- 2.5 × 106 km3 is predicted. This range is broadly consistent with dicted for channelized lavas of the type expected to have formed Ares the ~1.35 × 106 km3 volume predictions made above on the basis Vallis are summarized in Fig. 12 for flow depths of 50, 75, and 100 m. of discharge estimates and conservative incision rates. Formation of Flow velocities predicted for lava depths of 50 m and 100 m respectively the full Ares Vallis system, including regions of chaos and related reach as great as ~61 m/s and 92 m/s for slopes of up to 1°. On more channel reaches, is expected to have involved a greater lava volume. relevant slopes of 0.2°, flow velocities of 25 m/s and 39 m/s are respec- The volumes of missing materials at the four key chaos regions asso- tively predicted. For a channel width of 25 km, which is typical of much ciated with Ares Vallis are not easily estimated, since the original of the main Ares Vallis system north of Iani Chaos, discharge rates as form of local terrain prior to disruption and subsidence is unknown; great as ~75 × 106 m3/s to 230 × 106 m3/s are respectively estimated nevertheless, the approximate volume of missing materials at these for 50-m-deep and 100-m-deep flows on slopes of up to 1°. On slopes chaos regions is estimated on the basis of MOLA data to be of 0.2°, these discharges are estimated to be ~31 × 106 m3/s to 97 ~130,000 km3, which exceeds the assumed volume of the main ×106 m3/s. For 50-m-deep and 100-m-deep lavas, flow is predicted to Ares Vallis channel by ~30% (individual estimated volumes of be fully turbulent on all slopes, with Reynolds numbers of ~7.1 × 105 missing materials are as follows: Iani, ~60,000 km3;Margaritifer, and 2.1 × 106 respectively estimated on slopes of only 0.01°, and ~45,000 km3;Aram,~14,000km3; and ~Hydaspis: 12,000 km3). For much larger Reynolds numbers of ~8.5 × 106 and 25 × 106 respectively the combined volumes of the main Ares Vallis channel and the four predicted for flowsonslopesof1°. related regions of chaos, a total minimum erupted lava volume of Mechanical incision for 50-m-deep and 100-m-deep flows on ~1.8 × 106 to 5.8 × 106 km3 is predicted under the simplifying as- slopes of 0.2° is estimated to be ~4 m/day and 12.2 m/day, respec- sumption that formation of chaos volumes involved mechanical tively, and for slopes of 1° is estimated to be a remarkable and thermal processes related to those that operated at the channels. 47 m/day and 144 m/day (Fig. 12). Consistent with earlier findings A still greater minimum erupted lava volume would be required if (e.g., Hurwitz et al., 2012), thermal incision for 50-m-deep and the reaches near Margaritifer Chaos and north of the Argyre impact 100-m-deep flows on slopes of 0.2° is estimated to be more modest basin, outside of the main Ares Vallis channel and chaos regions, than mechanical incision, at ~1.9 m/day and 2.3 m/day, respectively. were also considered. Such reaches are interpreted here as having For slopes of 1°, thermal incision is respectively estimated to be developed volcanically, and include, for example, the Uzboi, Ladon, 3.8 m/day and 4.7 m/day for 50-m-deep and 100-m-deep flows. and Nirgal components of the broader Ares Vallis system. The capacity of lava for incision will diminish with distance from an eruptive center, as lava temperatures decrease and as viscosities 6. Discussion increase (e.g., Williams et al., 2001, 2011; Jaeger et al., 2010; Hurwitz et al., 2012; Cataldo et al., 2015; Baumgartner et al., 2017; Formation of the main Ares Vallis channel and the four largest asso- Leverington, 2018). A decrease of ~1° per 30 km would enable volu- ciated regions of chaos is conservatively estimated to have required minous lavas to maintain their capacity for incision for distances eruption of a minimum lava volume of ~1.8 × 106 km3. In comparison, of up to thousands of kilometers, and more voluminous flows will formation of Kasei Valles, the largest outflow system on Mars, is pre- generally maintain their temperatures better than less voluminous dicted to have required eruption of a lava volume of at least ~5 flows (e.g., Keszthelyi et al., 2006). ×106 km3 (Leverington, 2018). Smaller Martian outflow systems are Total effused lava volumes can be crudely estimated for large expected to have involved eruption of proportionately lower total Martian volcanic channel systems on the basis of parameters such as lava volumes. For example, formation of the Hrad Vallis system, a channel widths, incision depths, estimated discharge rates, and esti- 1450-km-long system that heads on the distal flanks of Elysium Mons mated incision rates (e.g., Leverington, 2007, 2009a, 2018, 2019; and which is characterized by widths of only several kilometers, is Hopper and Leverington, 2014). For example, respective total effused predicted to have required eruption of as little as ~11,000 km3 of lava lava volumes of ~1.35 × 106 km3 are predicted for lavas with depths of (Hopper and Leverington, 2014). The Ravi Vallis outflow system, either 50 m or 100 m on the basis of: 1) the above predicted discharge located north of Valles Marineris, is deeply incised into its substrate rates of 31 × 106 m3/s and 97 × 106 m3/s, involving reaches of 25 km despite its preserved channel length of only ~215 km, and is predicted width and 0.2° slope; 2) assumed incision depths below adjacent to have required a minimum erupted volume of ~64,000 km3 in order uplands of 1000 m; and 3) conservative overall combined mechanical to form its main channel and adjacent Aromatum Chaos (Leverington, and thermal incision rates of 2 m/day and 6 m/day (roughly half those 2019). The Athabasca Valles outflow system, located in the Cerberus predicted above for mechanical incision alone). These lava effusions Plains region, is very shallowly incised into its substrate and is involve total event lengths of ~500 days and 167 days, respectively, if associated with only ~7500 km3 of basalt flows (Jaeger et al., 2010; system development is simplistically assumed to have only involved Keszthelyi et al., 2017). periods of maximum discharge. More realistically, channel formation The outflow systems of Mars are characterized by a wide range of is likely to have involved lava effusions of a wide range of volumes sizes (e.g., Baker, 1982; Carr, 1996; Leverington, 2011, 2014). In the and rates, including some lower-volume effusions that are more likely present study and in recent work (Leverington, 2018), the largest of to have constructively mantled the system rather than incised into these outflow systems are individually predicted to have formed as a re- substrates, reducing the overall efficiency of channel incision. The Ares sult of minimum total lava effusions in the millions of cubic kilometers. Vallis system is known to have formed over an extended period of On the basis of thermal principles applied above, a global outflow- geological history, and thus channel development must have involved channel and chasmata volume of ~6 × 106 km3 (Carr, 1996) implies 14 D.W. Leverington / Geomorphology 345 (2019) 106828 associated lava effusions on Mars of at least ~50 × 106 to 150 × 106 km3, thousands to tens of millions of cubic kilometers (e.g., Coffinand under the simplifying (and unlikely) condition that all chasmata were Eldholm, 1994). The largest known LIP on Earth is the ~120 Ma formed volcanically. On Earth, individual large igneous provinces Ontong-Java plateau, which may have a total volume (including both (LIPs) of the Phanerozoic had original volumes of hundreds of intrusive and extrusive units) of 50 × 106 km3 or more (e.g., Coffin D.W. Leverington / Geomorphology 345 (2019) 106828 15

Table 1 2011). Aqueous interpretations of the outflow channels have frequently List of variables and constants. led to the characterization of Mars as a “water world” that has, in Symbol Name Units Value contrast to the cold and dry conditions of the present, repeatedly expe- − − rienced periods of wet surface conditions and correspondingly high at- K Proportionality constant of substrate Pa 1 10 9 (bedrock) erodibility mospheric pressures (e.g., Baker et al., 1991; Baker, 2001, 2018b). A ρ Density of lava kg/m3 2800 volcanic origin for Ares Vallis and the other Martian outflow channels g Gravitational acceleration m/s2 3.7 would bring the total predicted volatile contents of Mars back in line 2 Qw Discharge per meter width m /s calculated with the lower estimates derived from surface observations and α Longitudinal slope degrees 0 to 1.0 v Velocity of lava m/s calculated geochemical inferences, by greatly reducing the need to hypothesize

dlava Depth of lava m 50, 75, 100 the current or past existence of enormous long-lived aquifers of global Cf Friction factor none calculated extent, the early or periodic existence of thick atmospheres capable of Re Reynolds number none calculated supporting the widespread presence of wet surface conditions that μ Dynamic viscosity Pa s 1 2 could have led to the development of giant aquifers, and the past exis- hT Heat transfer coefficient (J m ) / (s K) calculated T Temperature of lava °C 1350 tence of large bodies of standing water including geographically exten- ° Tmg Substrate melting temperature C 1150 sive oceans (e.g., Leverington, 2011, 2014). 3 Emg Energy needed to melt substrate J / m calculated The aqueous paradigm that has dominated the past half century of ρ 3 g Density of solid substrate kg/m 2900 Mars exploration is based largely on aqueous interpretations of the fi ° cg Speci c heat of substrate J/(kg C) 1500 fl T Initial temperature of ground °C -23 Martian out ow channels. Yet, the mineralogical and geomorphological g fl fmg Fraction of substrate melting required none 0.4 (40%) properties of the Martian surface near out ow channels, including those for mobilization of the landscapes that variously comprise the Ares Vallis outflow Lg Latent heat of fusion of substrate J/kg 587,000 system, strongly suggest dry channel origins and the predominance of k Thermal conductivity of lava W/(m K) calculated relatively cold and dry surface conditions over most or all of the history of the planet. Although Mars possesses significant confirmed water reservoirs in the form of perennial ice caps and near-surface stores of and Eldholm, 1994). Thus, formation of all Martian outflow channels ice at middle and high latitudes (e.g., Smith et al., 1999; Christensen, and associated chasmata might have involved a total minimum lava vol- 2006), corresponding water volumes are far smaller than those pre- ume approaching, and possibly exceeding by several times, the viously hypothesized by some workers to have been required for forma- entire volume of this especially large terrestrial LIP. Notably, the largest tion of Martian channel systems, and are modest relative to the size of known terrestrial volcanic channels did not form in association with the planet (Leverington, 2011). Incongruities between the assumptions LIPs of Phanerozoic age, and instead formed during the Archean and of the prevailing aqueous paradigm and the surface properties of Mars Proterozoic, when the voluminous effusion of turbulently-flowing suggest that this paradigm should be replaced by a more realistic re- lavas was more common (e.g., Leverington, 2014). Correspondingly, it search framework that better recognizes the importance of dry surface is unlikely that the Martian outflow channels could have been formed processes in the development of Martian landscapes. by the mere effusion of voluminous lavas. Such lavas had to also be characterized by the remarkably low viscosities and high effusion 7. Conclusions rates most typical of the initial ~1 to 2 billion years of solar system history (Leverington, 2011, 2014). Ares Vallis is widely interpreted as having formed as a result of The origins of the Martian outflow channels are pertinent to our catastrophic outbursts of groundwater at several associated regions broader understanding of the basic characteristics and history of Mars. of chaos, but aquifer outburst mechanisms do not appear to have Aqueous interpretations of the Martian outflow channels have meaningful solar system analogs, and aqueous origins are arguably previously suggested near-surface water volumes that represent at not consistent with the sedimentological and mineralogical characteris- least several hundred meters of globally-averaged water (even while tics of this system. Obvious examples of fluvial or shoreline deposits, or assuming remarkably high sediment loads of ~40% by volume), which of extensive areas of hydrous alteration or the deposition of evaporites, far exceeds the several tens of meters commonly estimated on the are not known for component channels and basins. The preservation at basis of ground observations or from geochemical considerations Ares Vallis of large exposures of olivine-rich materials, including those (e.g., Carr, 1986; Scambos and Jakosky, 1990; Carr and Wänke, 1992; of the Noachian units that form the substrates into which channels Wänke and Dreibus, 1994; Beaty et al., 2005; Christensen, 2006; Carr were incised, does not appear to be compatible with the hypothesized and Head, 2015; Breuer et al., 2016). Carr and Head (2015) brought past occurrence of dozens to thousands of aqueous outburst floods estimates of near-surface volatiles closer to agreement by assuming here, and with the long-term existence of highly porous and permeable that the canyons of Valles Marineris are mainly tectonic features with aquifers later exposed by channel development. Consistent with orbital origins that did not involve substantial erosion by effused fluids, and perspectives, the Pathfinder landing site at the mouth of Ares Vallis is that the ice contents of hypothesized global cryospheric seals were dominated by volcanic lithologies that have been little altered by water. less voluminous than previously hypothesized; and by not including The basic properties of the Ares Vallis outflow system suggest dry the groundwater volumes that must necessarily have been left behind volcanic origins analogous to those of hundreds of other ancient chan- in aquifers during and after effusions to the surface (e.g., Leverington, nel systems on bodies including the Moon, Earth, Venus, and Mercury.

Fig. 11. As with all other Martian outflow channels, the surface mineralogy of the Ares Vallis system is predominantly anhydrous in character (e.g., Bibring et al., 2006; Mangoldetal.,2008; Carter et al., 2013; Wilson and Mustard, 2013; Ehlmann, 2014). A large segment of the main Ares Vallis channel is depicted above with regard to A: hydrous mineral exposures (orange) overlain on THEMIS daytime infrared imagery; B: OMEGA pyroxene content (blue = lower; green and yellow = higher) overlain on OMEGA near-infrared albedo; and C: OMEGA high olivine content (red) overlain on OMEGA near-infrared albedo [mineralogy in A is modified after Carter et al., 2013, and mineralogy in B and C is modified after Ody et al., 2012a, 2012b, 2013]. Regions of low albedo here are dominated by well-preserved mafic materials of volcanic origin, whereas regions of high albedo are mantled by dust that is dominated by the pres- ence of anhydrous nanophase ferric oxides (Ody et al., 2012a, 2012b) that were produced by weathering processes that did not involve liquid water (Bibring et al., 2006). The generally anhydrous character of these surface materials, including the preservation of kilometer-scale exposures of pristine ancient materials with high olivine content (~10% to N25% olivine), does not appear to be consistent with aqueous development of Ares Vallis or with the long-term hosting of water in an aquifer of regional or global extent. At the higher spatial resolutions of THEMIS data, many additional exposures of preserved high-olivine materials are recognized along the floors and margins of Ares Vallis and across parts of adjacent uplands (e.g., Rogers et al., 2005). The location of the depicted area is given in Fig. 2. 16 D.W. Leverington / Geomorphology 345 (2019) 106828

Fig. 12. Flow parameters calculated for Ares Vallis lavas with viscosities of 1 Pa s and temperatures of 1350 °C, for assumed flow widths of 25 km and for depths of 50 m (solid), 75 m (short dashes), and 100 m (long dashes).

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