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Home , Adirondack (Mars), Kasei Valles

Icarus 301 (2018) 37–57

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Icarus

journal homepage: www.elsevier.com/locate/icarus

Is () the largest volcanic channel in the solar system?

David W. Leverington

Department of Geosciences, Texas Tech University, Lubbock, TX 79409, United States

a r t i c l e i n f o a b s t r a c t

Article history: With a length of more than 20 0 0 km and widths of up to several hundred kilometers, Kasei Valles is Received 31 January 2017 the largest outflow system on Mars. Superficially, the scabland-like character of Kasei Valles is evocative

Revised 2 October 2017 of terrestrial systems carved by catastrophic aqueous floods, and the system is widely interpreted as a Accepted 3 October 2017 product of outbursts from aquifers. However, as at other outflow channels, clear examples of Available online 5 October 2017 fluvial sedimentary deposits have proven difficult to identify here. Though Kasei Valles lacks several key Keywords: properties expected of aqueous systems, its basic morphological and contextual properties are aligned Mars with those of ancient volcanic channels on Venus, the Moon, Mercury, and Earth. There is abundant Surface evidence that voluminous effusions of low-viscosity magmas occurred at the head of Kasei Valles, the Mars channel system acted as a conduit for associated flows, and mare-style volcanic plains developed within

Interior its terminal basin. Combined mechanical and thermal incision rates of at least several meters per day are Volcanism estimated to have been readily achieved at Kasei Valles by 20-m-deep magmas flowing with viscosities of 1 Pa s across low topographic slopes underlain by bedrock. If Kasei Valles formed through incision by magma, it would be the largest known volcanic channel in the solar system. The total volume of

magma erupted at Kasei Valles is estimated here to have possibly reached or exceeded ∼5 × 10 6 km 3, a volume comparable in magnitude to those that characterize individual Large Igneous Provinces on Earth. Development of other large outflow systems on Mars is expected to have similarly involved eruption of up to millions of cubic kilometers of magma. ©2017 Elsevier Inc. All rights reserved.

1. Introduction bacher, 1985; Baker, 2001, 2009; Andrews-Hanna and Phillips, 2007; Coleman and Baker, 2009; Warner et al., 2010a; Lamb The outflow channels of Mars formed mainly in the et al., 2014; Larsen and Lamb, 2016; Lepotre et al., 2016 ). The and as a result of voluminous fluid effusions from limited volumes of topographic basins at most system heads have the subsurface (e.g., Baker, 1982, Carr, 1996; Rodriguez et al., implied that outflow channel development was primarily driven 2015a , 2015b ). Channels head at topographic depressions, chaotic by catastrophic releases of groundwater from aquifers confined by terrain, graben-like landforms, and/or ridged plains, and extend frozen ground (e.g., Baker and Milton, 1974; et al., 1977; downslope for distances of up to thousands of kilometers (e.g., Theilig and Greeley, 1979; Carr, 1979, 1996; Mars Channel Working Milton, 1973; Sharp and Malin, 1975; Carr and Clow, 1981; Baker, Group, 1983; Clifford, 1993; Clifford and Parker, 2001; Keszthelyi 1982; Carr, 1996, 2012 ). Outflow systems are characterized by et al., 2007, 2014a; Wilson et al., 2009; Carr and Head, 2010; the presence of features such as anastamosing reaches, stream- Warner et al., 2010a; Rodriguez et al., 2012; Coleman, 2013; Lasue lined erosional residuals, longitudinal ridges and grooves, channel et al., 2013; Mangold and Howard, 2013; Morgan et al., 2013; cataracts, and inner channels (e.g., Baker and Kochel, 1979; Mars Marra et al., 2015; Rodriguez et al., 2015a; Cassanelli and Head, Channel Working Group, 1983; Baker, 20 01; Burr 20 05; Pacifici 2016 ), possibly in conjunction with glacial or other processes (e.g., et al., 2009; Chapman et al., 2010a,b ; Coleman, 2013; Morgan Lucchitta et al., 1981; Lucchitta, 1982, 2001; Chapman and Scott, et al., 2013 ). Similarities between these features and those of 1989; Costard and Baker, 2001; Head et al., 2004; Chapman et al., terrestrial rivers and diluvial systems have suggested common 2010a,b ; Bargery and Wilson, 2011; Cassanelli and Head, 2016 ). aqueous origins (e.g., Baker and Milton, 1974; Carr, 1974; Sharp Candidate fluvial and shoreline deposits continue to be iden- and Malin, 1975; Masursky et al., 1977; Baker et al., 1978a,b,c; tified at the Martian outflow channels and associated terminal Trevena and Picard, 1978; Mars Channel Working Group, 1983; basins (e.g., Chapman et al., 2010a,b ; Harrison and Chapman, 2010; Squyres, 1984; Mouginis-Mark et al., 1984; Elfström and - Balme et al., 2012; Mouginot et al., 2012; Erkeling et al., 2012; Mangold and Howard, 2013; Ivanov et al., 2014; Rodriguez et al., 2014, 2015b; Keske et al., 2015 ), but recognition of clear examples E-mail address: [email protected] https://doi.org/10.1016/j.icarus.2017.10.007 0019-1035/© 2017 Elsevier Inc. All rights reserved. 38 D.W. Leverington / Icarus 301 (2018) 37–57 of such landforms has proven challenging (e.g., Greeley et al., 1977; on Mars suggests extraordinarily dry surface conditions during Baker and Kochel, 1979; Tanaka, 1997; Wilson and Mouginis-Mark, the timeframe of outflow channel development (e.g., Hoefen et al., 2003; Burr and Parker, 2006; Ghatan and Zimbelman, 2006; Lever- 2003; Goetz et al., 2005; Rogers et al., 2005; Bibring et al., 2005, ington, 20 07, 20 09; Carling et al., 20 09; Hobbs et al., 2011; Hopper 2006; Koeppen and Hamilton, 2008; Carr and Head, 2010; Hand, and Leverington, 2014; Rice and Baker, 2015 ). Channel-mantling 2012; Ehlmann, 2014; Salvatore et al., 2014 ), and there is little units with clear non-aqueous origins (e.g., lava flows) are generally spatial correlation on Mars between channel systems and hydrated interpreted by the research community as products of resurfacing (e.g., Bibring et al., 2006; Mangold et al., 20 07, 20 08; events that followed initial development of channel systems by Carter et al., 2013; Ehlmann, 2014 ). Contrary to some accounts aqueous floods (e.g., Plescia, 2003; Kezthelyi et al., 2007, 2014; (e.g., Craddock et al., 1997; Crumpler, 1997; Golombek et al., 1997 ), Coleman and Baker, 2009; Dundas and Keszthelyi, 2014; Morgan outflow sites visited by three landers (, Pathfinder, and et al., 2013; Salvatore and Christensen, 2014; Keske et al., 2015 ). rover) are clearly dominated by the effects of flood vol- Aqueous interpretations of the Martian outflow channels have canism and physical weathering (e.g., Binder et al., 1977; Greeley shaped modern perspectives regarding the geological history of et al., 1977, 2005; Leverington, 2011 ). Mars, and have influenced estimates of near-surface water vol- Though the Martian outflow channels lack many of the prop- umes (e.g., Baker, 1982; Carr, 1996; Carr and Head, 2015; Baker erties expected of aqueous systems, their properties are closely et al., 2015 ). Such interpretations have implied past environmental aligned with those of ancient volcanic channels including those conditions conducive to the flow of water along surface conduits of Venus, the Moon, Mercury, and Earth ( Leverington, 2004 , and to the ponding of water in lakes or oceans (e.g., Sharp and 2011 , 2014; Byrne et al., 2013; Hopper and Leverington, 2014 ). Malin, 1975; Baker, 1979; Baker et al., 1991; Clifford, 1993; Parker As expected of volcanic origins, the outflow channels of Mars et al., 1993; Head et al., 1999; Clifford and Parker, 2001; Ivanov commence at or near sites of eruption of low-viscosity lava, are and Head, 2001; Baker, 2001; Harrison and Chapman, 2008; Dohm extensively mantled by volcanic flows along component channels, et al., 2009; Moscardelli and Wood, 2011; Mouginot et al., 2012; and terminate at volcanic plains (e.g., Leverington, 20 04, 20 07, Lasue et al., 2013; Warner et al., 2013; De Blasio, 2014; Iijima 2009, 2011; Jaeger et al., 2010; Hopper and Leverington, 2014; et al., 2014; Ivanov et al., 2014; Moscardelli, 2014; Roda et al., Leone, 2014, 2016, 2017 ). Igneous processes have a demonstrated 2014; Salvatore and Christensen, 2014; Rodriguez et al., 2016; Carr capacity to drive enormous volumes of magma to the surfaces of and Head, 2016 ). Aqueous interpretations have suggested the past rocky bodies over short periods of time (e.g., Schaber et al., 1976; operation of a vigorous hydrological cycle (e.g., Carr, 1979, 1996; Wilson and Head, 1981, 2017; Head and Wilson, 1991, 1992, 2017; Risner, 1989; Baker et al., 1991; Clifford, 1993; Moore et al., 1995; Coffin and Eldholm, 1994; Baker et al., 1997; Williams et al., 2001, Williams et al., 20 0 0a; Clifford and Parker, 20 01; Baker, 20 01; 2011a; Stockstill-Cahill et al., 2012; Byrne et al., 2013; Hurwitz Harrison and Grimm, 2004, 2008; Rodriguez et al., 2007, 2015a; et al., 2013a b), and such processes, involving voluminous flow of and Head, 2007; Lasue et al., 2013 ), and have been used lava sourced from large upper-mantle reservoirs, are correspond- to identify regions of possible astrobiological significance (e.g., ingly realistic mechanisms by which large outflow systems could Cabrol and Grin, 1995; Carr, 1996; Farmer and Des Marais, 1999; have developed on Mars ( McGetchin and , 1978; Jaeger Burr et al., 2002; Dohm et al., 2004; Levy and Head, 2005; Murray et al., 2010; Leverington, 2011 ; 2014; Hopper and Leverington, et al., 2005; Warner et al., 2010b; Salvatore and Christensen, 2014; 2014; Baumgartner et al., 2015 , 2017 ). The potential importance of Pajola et al., 2016 ). volcanic processes in the development of Martian outflow systems Aqueous interpretations of the Martian outflow channels has previously been recognized for systems such as are characterized by numerous serious weaknesses. Such inter- ( Leverington, 2007; Leone, 2017 ), ( Jaeger et al., pretations are based on the hypothesized operation of exotic 2010 ), ( Hopper and Leverington, 2014 ), and the circum- hydrological processes that lack meaningful solar system analogs Chryse systems ( Schonfeld, 1979; Leverington, 2009 , 2011; Leone, ( Leverington, 2004 , 2011 ). Maximum estimated rates of flow 2014; Dundas and Keszthelyi, 2014; Baumgartner et al., 2015 , 2017 ) are comparable to those of terrestrial ocean currents ( ∼10 8 to ( Table 1 ). If valid, igneous interpretations have major implications 10 9 m 3 /s; Baker, 2001 ) and, in the absence of special conditions regarding our understanding of near-surface volatile contents on (e.g., Marra et al., 2014 ), aqueous interpretations require regional- Mars, the aqueous and volcanic history of the , and the past scale subsurface permeabilities as great as ∼10 −7 m 2 (orders of potential for the development of ( Leverington, 2011 ). magnitude greater than expected values of ∼10 −14 m 2 ) to allow With widths of up to hundreds of kilometers and a total length for the occurrence of localized effusions from enormous aquifers of more than 20 0 0 km, Kasei Valles is the largest outflow channel (e.g., Head et al., 2003; Wilson et al., 2009 ). Recent models sug- on Mars (e.g., Carr, 1996 ) ( Fig. 1 ). This system, located east of Thar- gest that aquifer depressurization would have necessitated the sis and north of , is one of several major outflow occurrence of dozens to thousands of separate flood events for channels that extend onto ( Milton, 1973; Sharp development of individual outflow systems (e.g., Manga, 2004; and Malin, 1975; Greeley et al., 1977 ). The head of Kasei Valles Andrews-Hanna and Phillips, 2007; Harrison and Grimm, 2008 ), is located at Echus , an elongate topographic depression each involving groundwater recharge driven by uncertain pro- that partly lies on the Martian equator. The path of Kasei Valles cesses ( Cassanelli et al., 2015 ). Aqueous interpretations of the extends northward from but turns sharply eastward Martian outflow channels imply much greater near-surface volatile near northern midlatitudes, ultimately crossing the dichotomy contents than are otherwise suggested by independent geochem- boundary and terminating within the northern lowlands. As with ical and mineralogical considerations (e.g., Carr, 1986, 1987, 1996; other Martian outflow systems, Kasei Valles is widely believed Squyres, 1989; Carr and Wänke, 1992; Wänke and Dreibus, 1994; to have formed primarily as a result of catastrophic outbursts of Beaty et al., 2005; Leverington, 2011; Carr and Head, 2015; Breuer water from the subsurface (e.g., Baker and Milton, 1974; Baker and et al., 2016 ). Sedimentary deposits of clear fluvial origin are not Kochel, 1979; Masursky et al., 1979; Robinson and Tanaka, 1990; recognized along the outflow channels and within their terminal Williams et al., 20 0 0a; Chapman et al., 2010a,b ). basins (e.g., Greeley et al., 1977; Baker and Kochel, 1979; Tanaka, This paper presents an alternative igneous hypothesis for 1997; Wilson and Mouginis-Mark, 2003; Burr and Parker, 2006; development of Kasei Valles, outlining the consistency of basic Ghatan and Zimbelman, 2006; Leverington, 20 07, 20 09; Carling attributes of this system with volcanic origins, and utilizing the et al., 2009; Hobbs et al., 2011; Hopper and Leverington, 2014; overall morphology of this system to help constrain quantitative Leone, 2014, 2017; Rice and Baker, 2015 ). Mineralogical evidence estimates of flow conditions associated with hypothesized for- D.W. Leverington / Icarus 301 (2018) 37–57 39

Table 1 Martian outflow channels investigated regarding the possible influence of magma flow on overall system development.

System length Estimated volume of channel system Measured volume of erupted magma Estimated volume of erupted magma ( ∗including large head depressions)

∗ Allegheny Vallis a ∼200 km 280 km 3 –6400 km 3

Athabasca Valles b ∼300 km > 100 km 3 50 0 0–750 0 km 3 – ∗ Elaver Vallis a ∼150 km 2640 km 3 – 62,0 0 0 km 3

Hrad Vallis c ∼1450 km 435 km 3 – 10,900 km 3

Mangala Valles d ∼900 km 8600 km 3 –2 × 10 5 km 3

Kasei Valles e > 20 0 0 km 7 × 10 5 km 3 –5 × 10 6 km 3

a Leverington (2009) .

b Jaeger et al. (2010) , Cataldo et al. (2015) .

c Hopper and Leverington (2015).

d Leverington (2007) .

e This study.

Fig. 1. Mars Orbiter Laser Altimeter (MOLA) topographic map of the Kasei Valles region (after Smith et al., 2003 ). The Kasei Valles outflow channel extends northward from Echus Chasma, divides near mid-latitudes into two major east-sloping branches, and terminates within Chryse Planitia and ( Sharp and Malin, 1975; Baker and Kochel, 1979; Baker, 1982; Robinson and Tanaka, 1990; Chapman et al., 2010a,b ). East of the Kasei Valles system are numerous other outflow channels, including the Maja, Shalbatana, Simud, and Tiu systems ( Greeley et al., 1977; Carr, 1996 ). Uranius Dorsum is labeled U.D. , and the Viking 1 and Pathfinder landing sites in Chryse Planitia are indicated. mative eruptions. The results of this study are consistent with 1–3 ). As with other outflow channels on Mars, this system is char- those of several other recent investigations of the basic capacity acterized by anastamosing reaches, high width-to-depth ratios, of magmas for channel incision, suggesting that, even for strong and low channel sinuosities; and by the presence of streamlined bedrock substrates, total incision rates of at least several meters erosional residuals, cataracts, chaotic terrain, channel terraces, per day are readily achieved under realistic flow conditions. The inner channels, and longitudinal ridges and grooves ( Baker and total volume of magma needed to form Kasei Valles is estimated Milton, 1974; Sharp and Malin, 1975; Baker and Kochel, 1979; here to have possibly reached or exceeded ∼5 × 10 6 km 3 , which Robinson and Tanaka, 1990; Tanaka and Chapman, 1992; Scott, is comparable to the ∼1 × 10 6 km 3 to 50 × 10 6 km 3 volumes 1993; Chapman and Tanaka, 1996; Crumpler, 1997; Tanaka, 1997; that characterize individual Large Igneous Provinces on Earth (e.g., Williams et al., 20 0 0a; Williams and Malin, 20 04 ). The volume of Coffin and Eldholm, 1994 ). Kasei Valles is ∼7 × 10 5 km 3 ( Carr and Head, 2015 ). Along numerous reaches, the floors of Kasei Valles are 2. Overview of the Kasei Valles outflow system ∼1–3 km below adjacent upland plateaus (e.g., Sharp and Malin, 1975; Tanaka and Chapman, 1992; Tanaka, 1997 ), and associated The Kasei Valles outflow system is more than 20 0 0 km channel margins are in many cases defined by steep cliffs and as- long, extending from its head at Echus Chasma to the lowlands sociated talus deposits ( Sharp and Malin, 1975; Baker and Kochel, of Chryse Planitia, and descending ∼3 km over this distance 1979 ). Channel walls, including those of some inner channels, are ( Robinson and Tanaka, 1990; Tanaka and Chapman, 1992 ) ( Figs. crossed in places by low-order tributary canyons that extend into 40 D.W. Leverington / Icarus 301 (2018) 37–57

Fig. 2. A : Hill-shaded depiction of MOLA topographic data for Kasei Valles (after Smith et al., 2003 ). Areas depicted in other figures are identified and labeled by fig- Fig. 3. Geological map of Kasei Valles, simplified after Chapman et al., (2010a,b) . ure number. B : Elevation profile along one of numerous possible flow routes along Depicted units include: highland plateau sequence ( Nplh ), undivided the Kasei Valles system (red path in A ). C : Semi-log plot of absolute kilometer-scale Hesperian–Noachian material ( HNu ), Hesperian ridged plains material ( Hr ), Hespe- topographic slopes along the elevation profile depicted in B . Kasei Vallis is widely rian fractured material ( Hf ), Hesperian channel unit 1 ( Hch1 and streamlined is- characterized by slopes of less than 0.5 °. Along the depicted path, the median slope land subunit Hch1i ), Hesperian channel unit 2 ( Hch2 and streamlined island sub- is 0.13 ° and the average slope is 0.43 °. Some of the steepest slopes here are asso- unit Hch2i ), members of the Formation (lower - Hsl , middle - Hsm , ciated with impact features that postdate channel development. (For interpretation and upper - Hsu ), second and fourth members of the Montes Formation of the references to color in this figure legend, the reader is referred to the web ( Ht2, Ht4 ), Amazonian channel floor material ( Ach and streamlined island subunit version of this article.) Achi ), Amazonian chaotic terrain material ( Act ), fifth member of the Formation ( At5 ), Amazonian platy-flow unit ( Apf ), Amazonian theater-headed chan- nel floor unit ( Acht ), Amazonian Echus Chasma plain material ( Apec ), superposed impact craters ( C ), partly buried impact craters ( Cpb ), and degraded impact craters uplands and typically possess blunt cirque-like heads ( Baker and ( Cd ). The Apec materials in the southwest corner of this map mark the southern- Kochel, 1979 ) ( Fig. 4 a–c). Areas of chaotic terrain are located along most part of the head region of this channel system. parts of the floors and channel margins of several reaches of Kasei Valles; such areas are generally characterized by irregular arrangements of mesas or mounds that are separated by complex 1977; Baker and Kochel, 1979; Crumpler, 1997 ). Many parts of Ka- systems of valleys, and collectively delimit zones of past disrup- sei Valles, including most of the head region at Echus Chasma, tion of terrain (Sharp, 1979; Baker and Kochel, 1979 ) ( Fig. 4 d–f). are mantled by flows with a platy and ridged character consistent An especially deep and labyrinthine inner channel complex is with volcanic origins ( Chapman et al., 2009, 2010b; Mangold et al., present along the central part of Kasei Valles, ultimately opening 2010; et al., 2010 ) ( Figs. 3, 6 , and 7 ). In places, some of eastward toward the lower reaches of the system (e.g., Baker and these platy-ridged materials appear to have been worn by later Kochel, 1979; Dundas and Keszthelyi, 2014 ) ( Fig. 4 b). A prominent fluid flows and are associated with terrace-like erosional residual ridge, Uranius Dorsum, is located on the northwestern edge of landforms ( Chapman et al., 2010b ). Some sections of channel floors Kasei Valles and is associated with dozens of mound-like features mantled by platy-ridged materials have been fractured, tilted, and ( Chapman and Scott, 1989; Chapman et al., 2007, 2010b ) ( Fig. 1 ). partly buried by later volcanic flows ( Chapman et al., 2010b ) Local elongate and ridge-like landforms, with attributes suggestive ( Fig. 8 ). Exposed channel floors are in many cases characterized by of development as inverted topography, are present along some the presence of longitudinal ridges ( Fig. 9 ). Parts of Kasei Valles channel reaches ( Chapman et al., 2010b ). are extensively mantled by distinct lobate-margined volcanic flows Wrinkle-ridged plains are present along the floors of numerous that were not erupted within the channel system itself, but instead channel segments (e.g., Masursky et al., 1977; Baker and Kochel, flowed downslope from adjacent east-facing flanks of Tharsis (e.g., 1979; Tanaka and Chapman, 1992 ), and are especially extensive Tanaka and Chapman, 1992; Chapman et al., 2010a,b ) ( Fig. 10 ). along the terminal reaches of Kasei Valles ( Fig. 5 ). Along the most Impact craters provide a basis for the relative and absolute distal parts of Kasei Valles, the topographic relief of channels be- dating of several landscape components of Kasei Valles ( Baker and comes markedly reduced ( Ivanov and Head, 2001 ) and the chan- Kochel, 1979; Chapman and Scott, 1989; Chapman et al., 2010a,b ; nels progressively fade into the northern plains ( Greeley et al., Neukum et al., 2010 ) ( Fig. 3 ). Crater statistics suggest that the up- D.W. Leverington / Icarus 301 (2018) 37–57 41

Fig. 4. Examples of valley networks associated with Kasei Valles. A : Valleys ( V ) that extend westward into the Tharsis highlands ( TH ) adjacent to Echus Chasma ( EC ) (HRSC image H2204_0 0 0 0_ND3). B : ( V ) and associated cataract-like features (e.g., at arrow) located along the main floor of Kasei Valles ( K ) (HRSC image H3217_0 0 0 0_ND3) ( Dundas and Keszthelyi, 2014 ). C : Valley network ( V ) and chaotic terrain ( C ) located along the main floor of Kasei Valles ( K ), near the mid-latitude transition from northward to eastward flow directions (HRSC image H3217_0 0 0 0_ND3). D : Chaotic terrain ( C ) located along the eastern margin of the main –north reach of Kasei Valles ( K ) (HRSC image H6241_0 0 0 0_ND3). E : Fracture-like network of valleys (e.g., at V ) developed within chaotic-like terrain associated with channel floor materials characterized in part by longitudinal ridges. F : Valley networks (e.g., at V ) located within chaotic-like terrain along the floor of an east-west reach of Kasei Valles ( K ) (HRSC image H1147_0 0 0 0_ND3). The locations of depicted areas are given in Fig. 2 a. per parts of the ridged plains of Lunae Planum, , and catastrophic aqueous floods ( Baker and Milton, 1974; Sharp and Syria Planum, into which Kasei Valles was incised, were volcani- Malin, 1975; Baker and Kochel, 1979 ). Hypothesized sources for cally emplaced during a time frame that extends across the bound- such floods include those believed to have existed in the Tharsis ary between the Noachian and Hesperian ( Neukum et al., 2010; uplands and those associated with the head depression of the Chapman et al., 2010a,b ). Crater statistics further suggest that the system, Echus Chasma ( Robinson and Tanaka, 1990; Tanaka and overall form of Kasei Valles as it exists today may have been the Chapman, 1992; Chapman et al., 2010a,b ). Under this perspec- product of initial eastward flow of fluids from the Tharsis region tive, the branching valley networks that extend from uplands during the Early Hesperian, followed by later northward flow from adjacent to channel margins (e.g., Fig. 4 a) are considered to be a Echus Chasma in the Hesperian and in the Amazonian ( Neukum possible fluvial overprint formed by sapping or surface runoff pro- et al., 2010; Chapman et al., 2010a,b ). The Uranius Dorsum ridge cesses that may not have been directly connected to the outflow is estimated to have formed ∼3.6 Ga before present, during the processes that formed Kasei Valles (e.g., Chapman et al., 2010a ). Hesperian Period ( Neukum et al., 2010; Chapman et al., 2010a ). Aqueous interpretations of Kasei Valles acknowledge the pos- The lobate-margined flows that extend eastward from the Tharsis sible secondary roles of tectonic, glacial, periglacial, or volcanic region were emplaced in the Late Hesperian, as were the flood processes in system development. For example, some channel lavas that infill the troughs on Tempe Terra and mantle the floor walls are hypothesized to coincide with the bounding faults of of Kasei Valles north of Uranius Dorsum ( Neukum et al., 2010 ). down-dropped blocks of plateau materials ( Chapman et al., 2009, Kasei Valles consists of channels that in places complexly 2010b ). The effects of floodwaters are hypothesized by some anastamose about erosional residuals, producing an overall visual workers to have been supplemented by those related to the flow impression that is reminiscent of terrestrial scablands carved by of glacial ice, with the longitudinal ridges and grooves of channel 42 D.W. Leverington / Icarus 301 (2018) 37–57

Fig. 6. An example of platy-ridged lava flows in Echus Chasma. Flows such as these are widely exposed across the head depression of Kasei Valles and within many of

Fig. 5. The Kasei Valles system terminates at extensive mare-style ridged flows in the topographic lows that are present along component channels. Context Camera

Chryse Planitia. Thermal Emission Imaging System (THEMIS) daytime infrared mo- (CTX) image P03_002050_1813_ XN_01N079W. The location of the depicted area is saic courtesy of Arizona State University. The location of the depicted area is given given in Fig. 2a. in Fig. 2 a.

emplacement of extensive lowland units such as those of the floors (e.g., Fig. 9 ) considered the possible products of glacial Formation (e.g., Kreslavsky and Head, 2002; overprinting of aqueously-incised conduits ( Lucchitta, 1982; Chap- Kleinhans, 2005; Salvatore and Christensen, 2014, 2015 ). man et al., 2010b ). Several classes of elongate and narrow ridge landforms are present at Kasei Valles, and have been interpreted 3. The volcanic hypothesis for formation of Kasei Valles as possible glacial landforms including eskers ( Zealey, 2009; Chap- man et al., 2010a ). Channel widening is believed to have possibly Superficially, the scabland-like character of Kasei Valles is taken place as a result of thermal disturbance and melting of evocative of terrestrial systems carved by catastrophic aqueous ice-rich plateau materials (e.g., Tanaka and Chapman, 1992 ). Some floods. However, as at other outflow systems on Mars ( Rice and inner channels of Kasei Valles are inferred to have formed during Baker, 2015 ), clear examples of fluvial sedimentary deposits have the operation of late-stage mudflow, glacial, or sapping processes proven difficult to identify here (e.g., Greeley et al., 1977; Baker capable of driving the retreat of cataracts ( Tanaka, 1997; Williams and Kochel, 1979; Tanaka, 1997; Leone, 2014 ). This is problematic, et al., 20 0 0a; Williams and Malin, 2004; Chapman et al., 2010b ). particularly since aqueous models of channel incision are based on Aqueous interpretations acknowledge the necessity of periods extraordinarily high sediment loads of ∼20–50% by volume (e.g., of widespread volcanic resurfacing between hypothesized aqueous Komar, 1980; Carr, 1996; Williams et al., 20 0 0a; Leask et al., 20 06, flood events, resulting in emplacement of flow units such as those 2007; Carr and Head, 2015 ). The possible absence of aqueous sed- that extend from Tharsis across the central part of Kasei Valles. imentary deposits at Kasei Valles and other Martian outflow sys- The voluminous platy-ridged flows that were erupted at Echus tems ( Leverington, 2011 ), in combination with the poor spatial cor- Chasma are hypothesized by some workers to have utilized the relation on Mars between hydrous minerals and outflow channels same near-surface structural conduits as those believed to have ( Bibring et al., 2006; Mangold et al., 2007 , 2008; Carter et al., 2013; fed earlier aqueous floods, and the tilting of some lava-covered Ehlmann, 2014 ), brings into question the notion that water plates in locales such as Echus Chasma is interpreted as a possible once flowed through these systems. The absence of terrestrial result of the local rise of magma or, alternatively, the melting of scablands carved by catastrophic discharges from aquifers implies subsurface ice ( Chapman et al., 2010b ). Aspects of inner channel that the analog value of systems such as the Channeled Scabland development and cataract recession within Kasei Valles are be- of Washington has previously been overstated ( Leverington, 2011 ). lieved by some workers to have been driven by late-stage eruption Of much greater potential analog value in the study of Mar- of low-viscosity magmas ( Dundas and Keszthelyi, 2014 ). tian outflow systems such as Kasei Valles are ancient volcanic Formation of Kasei Valles by floodwaters discharged from a channel systems of Venus, the Moon, Mercury, and Earth ( Baker cryosphere-confined aquifer is estimated to have possibly required et al., 1997; Leverington, 2004 , 2011 , 2014; Williams et al., 2011a; ∼2900 flooding events, each lasting ∼20 days and involving Byrne et al., 2013; Hopper and Leverington, 2014 ). The more than ∼190 km 3 of water ( Harrison and Grimm, 2008 ). Recognition of 200 recognized lunar channels typically extend downslope from the large volumes of water involved in the formation of Kasei topographic depressions that correspond to sites of voluminous Valles and other outflow systems has led to inferences of the past effusion of magma to the surface, and terminate at extensive existence of a northern ocean on Mars ( Carr, 1986, 1987; Parker volcanic plains (e.g., Greeley, 1971a b; Cruikshank and Wood, 1972; et al., 1993; Carr, 1996; Williams et al., 20 0 0a; Clifford and Parker, Carr, 1974; Guest and Murray, 1976; Head, 1976; Wilhelms, 1987; 20 01; Baker, 20 01; Kreslavsky and Head, 2002; Fairén et al., 2003 ), Hurwitz et al., 2013a ). These systems have widths of up to ∼5 km with Chryse Planitia acting as a bay into which the circum-Chryse and lengths of up to hundreds of kilometers, and can have features systems flowed ( Chapman et al., 2010b ). Reaches of Kasei Valles such as inner channels and terraces, anastomosing reaches, and that extend into Chryse Planitia are hypothesized as former sub- streamlined islands (e.g., Gornitz, 1973; Wilhelms, 1987; Levering- marine channels within which density currents might have flowed ton, 2004; Garry and Bleacher, 2011; Hurwitz et al., 2013a ). Except ( Ivanov and Head, 2001 ). Factors such as oceanic convection cur- along topographic lows where the ponding of lava flows could rents and sediment hyperconcentrations are hypothesized to have occur, the floors of large lunar channels in many cases became allowed suspended sediments from the circum-Chryse channels well exposed after the termination of eruptive events (e.g., Swann to become distributed over relatively broad areas of the northern et al., 1972; Wilhelms, 1987 ). Incision of lunar channels involved plains without forming delta-like features, possibly resulting in flows with mafic or ultramafic compositions that had minimum D.W. Leverington / Icarus 301 (2018) 37–57 43

Fig. 7. Examples of platy-ridged ( PR ) volcanic flows at Kasei Valles. Such flows are exposed across the head of the system at Echus Chasma ( A –C ) and are pooled within topographic lows located along component channels ( D –F ). The margins of flow deposits are indicated by arrows. A : CTX image P14_0 06586_180 0_XN_0 0N079W. B : CTX image F18_042835_180 0_XN_0 0N080W. C : CTX image G23_027077_1811_XI_ 01N080W. D : CTX image D17_033828_20 0 0_XN_20N073W. E : CTX image D20_035186_2026_XN_22N069W. F : CTX image G22_026905_2070_XN_27N068W. The locations of depicted areas are given in Fig. 2 a. viscosities at least as low as ∼0.5 Pa s and that were erupted with mantling volcanic units of mafic composition are widespread maximum sustained flow rates in excess of 40 0 0 m 3 /s ( Murase ( Surkov, 1983; Surkov et al., 1987 ), and channel incision on Venus and McBirney, 1970, 1973; Greeley, 1971a; Weill et al., 1971; likely involved magmas that had viscosities at least as low as Hulme, 1973; Hulme and Fielder, 1977; Williams et al., 20 0 0b; ∼4.5–7.5 Pa s and that flowed with rates of up to ∼5 × 10 7 m 3 /s Hurwitz et al., 2012 ). More generally, effusion rates expected to ( Kargel et al., 1993; Baker et al., 1997 ). have been typical of lunar sourced from magma reservoirs At least 10 channel systems have been identified on Mercury, at sub-crustal depths range from ∼10 4 to 10 6 m 3 /s ( Wilson and with maximum widths in excess of 30 km and with maximum Head, 2017 ). lengths of up to ∼102 km ( Head et al., 2011; Byrne et al., 2013; The more than 200 channels on Venus have lengths of up to Hurwitz et al., 2013b ). These Mercurian systems, the floors of thousands of kilometers and widths of up to tens of kilometers, which are covered by lava flows, acted as relatively short volcanic and typically head at topographic depressions and terminate at conduits between large topographic basins that are also mantled volcanic plains ( Baker et al., 1992; Head et al., 1992 ). Though only by extensive volcanic units ( Byrne et al., 2013 ). Some of these a few kilometers wide, Venusian channel Baltis Vallis has a length channel systems are associated with elongate topographic features of ∼6800 km and is considered to be the longest known channel that may have been streamlined by channel flows ( Head et al., of any kind in the solar system ( Komatsu, 2007 ). Venusian systems 2011 ). Incision of the Mercurian channels is believed to have in- have an especially wide range in morphologies, from simple volved Mg-rich basalts that had viscosities of ∼0.02–14.2 Pa s and rille-like channels to systems with anastomosing reaches that are that were erupted with maximum effusion rates of ∼10 6 –10 8 m 3 /s characterized by complexities that rival those of Martian outflow ( Stockstill-Cahill et al., 2012; Byrne et al., 2013; Hurwitz et al., systems ( Komatsu et al., 1993; Komatsu and Baker, 1994; Baker 2013b ). et al., 1997 ). The full range of lava compositions exposed at the Landforms that developed early in the history of Venus are not surface of Venus remains uncertain ( Filiberto, 2014 ), but plains- preserved at the surface, and the volcanic channels recognized on 44 D.W. Leverington / Icarus 301 (2018) 37–57

Fig. 8. Examples of upwardly-deformed sections of volcanic flows associated with Kasei Valles. Parts of tilted volcanic flow units ( TU ) became buried by later lavas (the mar- gins of these lavas are indicated by arrows). Features such as these may be the surface expressions of large magmatic plumbing systems. All of the above examples are located along the floor of Kasei Valles except Fig. 8e, which is located on the eastern flanks of Tharsis. A : CTX image B07__012203_180 0_XN_0 0S080W. B : CTX image P08_004173_ 1812_XI_01N080W. C : CTX image P11_005162_1811_XN_ 01N080W. D : CTX image F04_0037270_1860_XI_06N080W. E : CTX image B11_013904_1968_XN_16N084W. F : CTX image P13_006230_1863_XN_06N080W. The locations of depicted areas are given in Fig. 2 a.

that body are thus likely to have formed relatively late in solar there is no obvious sedimentary or mineralogical evidence for the system history ( Strom et al., 1994; Basilevsky and Head, 1995; past flow of water along the Kasei Valles system, there is abundant Price et al., 1996 ). Preserved volcanic channels of the Moon and evidence that voluminous low-viscosity magmas were erupted at Mercury mainly formed relatively early in solar system history ( > the head of the system in Echus Chasma (e.g., Chapman et al., 3 Ga B.P.) ( Head et al., 2011; Hurwitz et al., 2013a ). Development 2009, 2010b; Mangold et al., 2010; Neukum et al., 2010 ), and that of large rille-like channels, with widths of up to several kilometers Kasei Valles acted as a conduit for volcanic flows that flowed tur- and lengths of up to hundreds of kilometers, took place on Earth bulently and had a capacity for substrate erosion (e.g., Leverington, during the Archean and Paleoproterozoic as a result of voluminous 2011; Dundas and Keszthelyi, 2014 ). Lava flow features present eruptions of ultramafic magmas that flowed turbulently and that along various reaches of Kasei Valles include platy-ridged flows, in- incised into substrates (e.g., Groves et al., 1986; Barnes et al., 1987, flated flows, and lobate-margined overflow units ( Chapman et al., 2011; Lesher and Campbell, 1993; Hill et al., 1995; Lesher and 2010b; Leverington, 2011; Dundas and Keszthelyi, 2014 ). Kasei Arndt, 1995; Williams et al., 2001, 2011a; Barnes, 2006; Houlé Valles terminates in a region that is thickly mantled by mare-style et al., 2008, 2012; Gole et al., 2013; Staude et al., 2016 ). Much wrinkle-ridged flood lavas (e.g., Carr et al., 1976; Greeley et al., larger terrestrial volcanic systems may have developed during the 1977 ), and the Viking 1 revealed a volcanic landscape Hadean and Archean ( Leverington, 2014 ). here with no recognizable fluvial landforms ( Binder et al., 1977 ). The very existence of large volcanic channel systems on bodies Though some large volcanic channel systems of the inner solar of the inner solar system confirms that igneous plumbing systems system maintain relatively uniform channel widths or gradually have had a capacity to deliver magmas at volumes and rates narrow along their distal reaches (e.g., systems associated with the sufficient to form large channels ( Leverington, 2011 , 2014 ). Though Marius Hills region of the Moon; Greeley, 1971b ), others widen D.W. Leverington / Icarus 301 (2018) 37–57 45

Fig. 9. Examples of sets of longitudinal ridges ( LR ) associated with selected channel reaches of Kasei Valles. Topographic lows along such reaches tend to be partly or com- pletely mantled by volcanic flows ( F ). A : CTX image P16_007390_1953_XN_15N073W. B : CTX image B17_016264_2026_XN_22N074W. C : CTX image F20_043797_2068_XN_ 26N069W. D : CTX image P17_007561_2064_XN__26N064W. E : CTX image P18_007943_2077_ XN_27N055W. F : CTX image P06_003262_2081_XN_28N052W. The locations of depicted areas are given in Fig. 2 a.

into terminal basins that are mantled with flood lavas (e.g., Angkor ton, 2006 ) are more consistent with surface mineralogy and are Vallis on Mercury; Byrne et al., 2013 ) in a manner similar to that worthy of future consideration. seen at Kasei Valles. Mechanisms of development of longitudinal ridges at the Mar- tian outflow channels remain poorly understood, but the complex 4. Estimates of possible volcanic flow conditions involved in character of some longitudinal ridges, including some at Kasei development of Kasei Valles Valles, suggest the possible operation of processes beyond the mere scour of horizontally-bedded units ( Fig. 11 ). Many of the Despite relatively high minimum viscosities of > = 10 2 Pa s volcanic landforms typical of the lava-mantled Athabasca Valles and low maximum effusion rates on the order of ∼10 1–10 3 m 3/s system (e.g., Keszthelyi et al., 2004; Jaeger et al., 2007, 2010; (e.g., Thordarson and Self, 1993; Hon et al., 1994; Calvari et al., Leverington, 2009; Ryan and Christensen, 2012 ) have apparent 2002 ), historical mafic eruptions on Earth have in some cases counterparts along the floor of Kasei Valles ( Fig. 12 ). possessed the capacity to drive relatively modest amounts of ero- Under the volcanic hypothesis, development of the branching sion and channel incision (e.g., Peterson et al., 1994; Kauahikaua valley networks that extend from uplands adjacent to channel et al., 1998; Greeley et al., 1998; Calvari and Pinkerton, 1999; margins (e.g., Fig. 4 a) is considered to be independent of the Kerr, 20 01; Williams et al., 20 04; Ferlito and Siewert, 2006 ). outflow processes that formed Kasei Valles itself. Though the However, development of ancient channel systems on Venus, the existence of valleys such as these is most commonly attributed Moon, Mercury, and Earth is expected to have involved eruptive to past aqueous processes related to sapping or surface runoff conditions very different from those of most modern terrestrial (e.g., Chapman et al., 2010a ), dry mechanisms related to highland environments, with minimum magma viscosities at least as low volcanic processes (e.g., Leverington and Maxwell, 2004; Levering- as ∼1–10 Pa s and effusion rates as high as ∼10 3–10 8 m 3/s (e.g., 46 D.W. Leverington / Icarus 301 (2018) 37–57

for thermal and mechanical erosion ( Hurwitz et al., 2010; Jaeger et al., 2010; Cataldo et al., 2015 ). The viscosities of silicate magmas generally decrease with

lower SiO2 contents, higher volatile contents, and higher temper- atures (e.g., Takeuchi, 2015 ). Higher interior temperatures in the earlier histories of rocky solar system bodies should have favored deeper and more voluminous melting in association with sites of decompression such as those related to mantle plumes, resulting in development of partial melts with relatively low viscosities and relatively high potential temperatures (e.g., Nisbet et al., 1993; Walter, 1998; Arndt, 2003; Grove and Parmon, 2004; Baratoux et al., 2011 ). On Earth, early melting at pressures of ∼3–15 GPa led at times to voluminous generation of magmas of picritic to komatiitic composition (e.g., Herzberg and O’ Hara, 1998; Walter, 1998 ). The character of such terrestrial melts suggests an early prevalence of hot spot magmatism, and the changing chemical and petrological character of related magmatism over Earth history should reflect progressive cooling of the planet’s interior (e.g., Takahashi, 1990; Walter, 1998 ). The lava flows hypothesized here to have formed Kasei Valles are basalts or komatiitic basalts such as those previously deter- mined to be associated with Martian outflow channels including Fig. 10. Numerous lava flows with lobate margins and with topographic re- Kasei Valles (e.g., Greeley et al., 1977; Gellert et al., 2004; Mangold lief of tens of meters extend eastward from the Tharsis region onto parts of et al., 2010; Jaeger et al., 2010 ). Under superliquidus conditions, the central reaches of Kasei Valles. The flows depicted here are located on Martian basaltic magmas are expected to have had minimum the easternmost flanks of Tharsis, immediately west of Kasei Valles. CTX im- ∼ age P21_009381_2010_XN_21N080W. The location of the depicted area is given in viscosities at least as low as the 0.5 Pa s values known for Fig. 2 a. Fe–Ti-rich lunar basalts, and associated flow conditions would certainly have been highly turbulent ( Chevrel et al., 2014 ). Such low viscosities could have led to Martian magma rise speeds in Hulme, 1973, 1974, 1982; Hulme and Fielder, 1977; Huppert et al., excess of 10 m/s for dike widths of only 3 m ( Chevrel et al., 2014 ) 1984; Huppert and Sparks, 1985; Baker et al., 1997; Keszthelyi and upon eruption to the surface are very likely to have promoted and Self, 1998; Williams et al., 20 0 0b, 20 01, 20 05, 2011a; Fagents thermal and mechanical processes of erosion and channel incision and Greeley, 2001, Jaeger et al., 2010; Hurwitz et al., 2012; Byrne (e.g., Hurwitz et al., 2010; Jaeger et al., 2010; Hopper and Lever- et al., 2013; Dundas and Keszthelyi, 2014; Leverington, 2011, 2014; ington, 2014, Baumgartner et al., 2017 ). The lower gravitational Hopper and Leverington, 2014; Baumgartner et al., 2015, 2017; acceleration at the surface of Mars is expected to have favored de- Staude et al., 2016 ). velopment of much wider dikes than typical of Earth under similar Development of Kasei Valles clearly involved multiple flow conditions ( Heap et al., 2017 ), further increasing the capacity for episodes, as is illustrated by e.g. wrinkle ridges that are cut low-viscosity magmas to have been erupted at high effusion rates by channels and channels that are cut by wrinkle ridges (e.g., on Mars. Though the volumes of the largest individual eruptions Binder et al., 1977 ), and the existence of tilted blocks of flow at Kasei Valles are expected to have been extraordinary compared materials that became inundated along their margins by later to those typical of modern Earth, the system is known to have volcanic flows ( Chapman et al., 2010b ) ( Fig. 8 ). Crater statistics developed over geological time, and is thus hypothesized here to indicate that the channels that comprise Kasei Valles formed over have been a product of numerous separate eruptive events, each geological timescales that extended from the Noachian to the far less voluminous than the channel system is today. Amazonian, possibly involving sources within both Kasei Valles Much remains to be learned regarding even the most basic itself and the Tharsis region to the west (e.g., Chapman et al., aspects of magma flow and incision (e.g., Dundas and Keszthelyi, 2010a,b ; Neukum et al., 2010 ). In this study, the Echus Chasma 2014; Cataldo et al., 2015 ). Nevertheless, useful numerical tools region is hypothesized to have been the site of repeated eruptive are presently available for exploratory investigation of possible events that, over geological time, gradually formed the main past flow conditions at large volcanic channel systems. This study Kasei Valles system. Candidate secondary volcanic sources include utilized equations that consider both the effects of mechanical regions of chaotic or upwardly-deformed terrain along various and thermal processes, and that have been found to be useful in channel reaches (e.g., Fig. 4 d and Fig. 8 a–c), as well as volcanic the preliminary study of flow conditions at volcanic channels on sources within the Tharsis uplands (e.g., Fig. 8 e). bodies such as the Moon and Mars (e.g., Hurwitz et al., 2010, 2012; Some of the volcanic flows that extend from Tharsis onto Leverington, 2014; Hopper and Leverington, 2014 ). For low slopes, the floor of Kasei Valles have distinct lobate margins defined by the rate of mechanical incision of a flow ( Sklar and Dietrich, 1998; outward-facing scarps with typical heights of ∼40 m, which is Hurwitz et al., 2010 , 2012 ) is given by: suggestive of alkaline-rich mafic lavas emplaced with relatively high flow viscosities of ∼10 5 –10 8 Pa s (e.g., Zimbelman, 1985;  

Chevrel et al., 2014 ) that would likely have inhibited substantial d( dchan ) = K ρ g Q w sinα (1) erosion of underlying units. In contrast, numerous flow units dt mech preserved at the surface of Echus Chasma and along various reaches of the Kasei Valles system (e.g., Mangold et al., 2010; Dundas and Keszthelyi, 2014 ) lack prominent flow margins and where K is a constant that parameterizes substrate erodibility in have the textural properties of platy-ridged flows such as those Pa −1 , ρ is the density of lava, g is the acceleration due to gravity, of Athabasca Valles ( Keszthelyi et al., 20 0 0, 20 04 , 20 06; Jaeger Q w is the average discharge of lava per meter of width (expressed et al., 2007 ), which flowed turbulently and possessed a capacity in m 2 /s), and α is the longitudinal slope in degrees. For slopes D.W. Leverington / Icarus 301 (2018) 37–57 47

Fig. 11. The longitudinal ridges that characterize the floors of Martian outflow channels in many cases show complex morphologies that suggest origins that are not exclu- sively related to the scour of flat-lying units. For example, some ridges at Athabasca Valles ( A ), Elaver Vallis ( B ), and Allegheny Vallis ( C ) are defined in part by the upturned edges of tilted layers ( Jaeger et al., 2007, 2010; D.W. Leverington, 2009 ). Longitudinal ridges formed by similarly tilted layers may be present along the floors of some reaches of Kasei Valles, producing cuestas ( D ) and ring-like features ( E and F ). A : High Resolution Imaging Science Experiment (HiRISE) image PSP_006762_1840. B : HiRISE image PSP_0 05662_170 0. C : CTX image P13_006150_1711_XN_08S053W. D : HiRISE image ESP_027234_2050. E : HiRISE image ESP_014391_2045. F : HiRISE image ESP_035449_2100. The locations of depicted parts of Kasei Valles are given in Fig. 2 a. of < 10 °, flow velocity ( Hurwitz et al., 2010 , 2012 ) is given by: by:

α   2 g dla v a sin ( v ) = (2) d ( d ) h ( T − T mg ) lava chan = T Cf (5) dt E mg therm α where g is the acceleration due to gravity, dlava is lava depth, is the longitudinal slope, and Cf is the friction factor (Keszthelyi and where h T is the coefficient of heat transfer (given below), T is the

Self, 1998): lava temperature, T mg is the substrate melting temperature, and         . −2 Emg is the energy required to melt the substrate (Hulme, 1973; + 0 92 1 2 Re 800 Huppert and Sparks, 1985; Williams et al., 1998; 20 0 0b ): C = log 6 . 15 (3) f 32 10 41 E = ρ [c ( T − T ) + f L ] (6) with the nu mber ( Re ) given by: mg g g mg g mg g

ρ v d where ρg is the substrate density, c g is the specific heat of the sub- Re = lava (4) μ strate, T mg is the substrate melting temperature, T g is the initial temperature of the ground, f mg is the fraction of the substrate that where ρ is the density of lava, υ is the velocity of lava, d is lava lava is melted prior to mobilization, L g the latent heat of fusion of the depth, α is the longitudinal slope, and μ is the dynamic viscosity substrate, and h T is the coefficient of heat transfer ( Hulme, 1973 ): of lava. The rate of thermal incision ( Hulme, 1973; Huppert and Sparks, 4 2 0 . 017 k R e 5 P r 5 1985; Williams et al., 1998, 20 0 0b; Hurwitz et al., 2012 ) is given h = (7) T dla v a 48 D.W. Leverington / Icarus 301 (2018) 37–57

Fig. 12. Numerous features characteristic of the volcanic flows that mantle Athabasca Valles (images at left; e.g., Keszthelyi et al., 2004; Jaeger et al., 2007, 2010; D.W. Leverington, 2009; Mangold et al., 2010 ) are also present at Kasei Valles (images at right). These features include overflow units ( A and B ; flow margins are indicated by arrows and nearby inner channels are labeled C ), sinuous blocky ridges within flows ( C and D ; selected examples are indicated by arrows), blocky mounds within flows ( E and F ), and ring mounds within flows and arranged in linear or arcuate groups that are collectively oriented parallel to flow direction ( G and H ; flow direction is southwest in G and northeast in H ). HiRISE images: A : ESP_030576_1885. B : ESP_035463_2010. C : ESP_045266_1870. D : ESP_047569_1795. E : ESP_030 0 09_1895. F : ESP_032193_1990. G : PSP_0 02226_190 0. H: ESP_035595_2020. The locations of depicted parts of Kasei Valles are given in Fig. 2 a. where k is the thermal conductivity of lava [2.16 –(0.0013 T )] 10 0 0 Pa s, values that span the range from those known to have ( Williams et al., 1998 ), Re is the Reynolds number, Pr is the Prandtl been involved in emplacement of lunar basalts and terrestrial ko- number [(cg μ)/k], and dlava is the depth of lava. matiites and that are expected to have characterized some Martian The above equations are functions of each other and thus lava flows (e.g., Murase and McBirney, 1970, 1973; Williams et al., must be solved iteratively until variables converge upon particular 2001, 2011a; Chevrel et al., 2014 ), to the lowest viscosities typical solutions. Equation parameters related to these equations are sum- of mafic flows on modern Earth ( ∼100–1000 Pa s; e.g., Shaw et al., marized in Table 2 . In the calculation of mechanical and thermal 1968; Heslop et al., 1989; Williams et al., 2011a ). The proportion- incision rates, this study considered viscosities of 1, 10, 100, and ality constant ( K ) was conservatively assigned a value of 10 −9 , D.W. Leverington / Icarus 301 (2018) 37–57 49

Table 2 List of constants and variables.

Symbol Name Units Value

− K Proportionality constant of substrate erodibility Pa −1 10 9 (bedrock)

ρ Density of lava kg/m 3 2800

g Gravitational acceleration m/s 2 3.7 2 Q w Discharge per meter width m /s Calculated α Longitudinal slope Degrees 0–0.4 v Lava velocity m/s Calculated

d lava Depth of lava m 5 and 20 C f Friction factor None Calculated Re Reynolds number None Calculated μ Dynamic viscosity Pa s 1, 10, 100, 10 0 0 2 h T Heat transfer coefficient (J m )/(s K) Calculated T lava temperature °C 1250, 1300, 1350

T mg Substrate melting temperature °C 1150 3 E mg Energy needed to melt substrate J/m Calculated 3 ρg Density of solid substrate kg/m 2900 c g Specific heat of substrate J/(kg °C) 1500 T g Initial temperature of ground °C −23 f mg Fraction of substrate melting required for mobilization None 0.4 (40%) L g Latent heat of fusion of substrate J/kg 587,0 0 0 k Thermal conductivity of lava W/(m K) Calculated

which defines a strong bedrock substrate. Individual flow depths incision rates of 11.6 and 2.1 m/day, respectively, for magmas flow- of 5 and 20 m were specified, based on the average flow depth ing on slopes of 0.4 ° and characterized by viscosities of 1 Pa s and of ∼20 m estimated for the Athabasca Valles system ( Jaeger et al., temperatures of 1300 °C. Much higher incision rates are expected 2010 ), though local magma depths of up to ∼100 m may not be for all of the above flow conditions if substrates are significantly unrealistic for large volcanic channel systems ( Byrne et al., 2013 ) weaker than bedrock (e.g., Williams et al., 1998; Hurwitz et al., and still greater depths of hundreds of meters are expected of 2012; Leverington, 2014; Hopper and Leverington, 2014 ). individual flows where ponded within topographic lows along For 20-m-deep magmas that have viscosities of 1 Pa s and that channel reaches ( Dundas and Keszthelyi, 2014 ). This study consid- flow on slopes of 0.2–0.4 °, velocities are predicted to range from ered magma temperatures of 1250 °, 1300 °, and 1350 °C, which are 14 to 21 m/s. Under these conditions, Reynolds numbers range realistic values for basalts and komatiitic basalts, though eruption from 8 × 0 5 to 1.2 × 10 6 , indicating fully turbulent flow. Indeed, temperatures of > = 1400 °C were likely for ancient terrestrial ko- as expected, turbulent flow is predicted to exist under almost all matiites (e.g., Burns and Fisher, 1990; Nisbet et al., 1993; Grove and flow conditions explored here that involve magma viscosities of Parman, 2004; Filiberto et al., 2008; Williams et al., 2011a; Baum- 1 Pa s. For example, Reynolds numbers of ∼15,0 0 0 are predicted gartner et al., 2015, 2017 ). The insulating effects of free-floating or for 5-m-deep flows with viscosities of 1 Pa s flowing on a surface fixed surface crusts, which can limit cooling to as little as 1 °C per slope of only 0.01 °. 30 km for flow depths of 20–30 m ( Keszthelyi et al., 2006 ), are Kasei Valles is among the most complex channel systems expected to have allowed the most voluminous magma flows to on Mars ( Fig. 1 ). The main Kasei Valles system ranges in width remain liquid over distances of > 20 0 0 km. This study considered from ∼40 km at Echus Chasma to a maximum of almost 500 km kilometer-scale longitudinal slopes of under 0.4 °, which are typical where the northern reaches of channels extend eastward toward of large volcanic channels (e.g., Williams et al., 2001; Leverington, Chryse Planitia. Streamlined erosional residuals of various sizes 2007; Hurwitz et al., 2013b; Hopper and Leverington, 2014 ), and are present along numerous channel reaches (e.g., Fig. 9 f), further which characterize much of the Kasei Valles system ( Fig. 2 ). complicating channel morphologies. The widest channel reaches of Kasei Valles are not uniformly deep, and instead are generally 5. Results comprised of extensive and relatively shallow parts that are pe- ripheral to much narrower and deeper inner channels; the shallow Mechanical and thermal incision rates predicted for strong peripheries are expected to have gradually been abandoned as bedrock substrates with slopes of < 0.4 °, for flows with depths of inner channels became more deeply incised over time. 5 m and 20 m, are given in Figs. 13 and 14 . Under the investigated A rudimentary indication of the magnitudes of volcanic dis- conditions, predicted mechanical incision rates are greatest for charges that might have been required to form Kasei Valles 20-m-deep flows with viscosities of 1 Pa s, with incision rates can be determined by disregarding the various morphological reaching as high as ∼2.5 m/day. In comparison, mechanical inci- complexities of the channel system, and instead simplifying the sion rates are less than 0.3 m/day for 5-m-deep flows, including system’s history as involving a series of individual voluminous those with viscosities of 1 Pa s. Thermal incision rates reach up to flows of 50–200 km width across slopes of less than 0.4 °. Dis- ∼2 m/day for 20-m-deep flows with relatively high magma tem- charge rates predicted for such flows are given in Fig. 15 for peratures of 1350 °C and viscosities of 1 Pa s, whereas 5-m-deep depths of 5 m and 20 m and for viscosities of 1 Pa s. Rates of flows reach maximum rates of ∼1.3 m/day for the same magma millions of cubic meters per second are typical of flows with these temperature. Estimates of thermal incision rates are low for characteristics. For example, a flow with a depth of 20 m and magmas with relatively high viscosities. Consistent with previous width of 100 km on a slope of 0.2–0.4 ° is predicted to have been results (e.g., Hurwitz et al., 2012; Leverington, 2014; Hopper and associated with discharge rates of 28.8 × 10 6 to 42.0 × 10 6 m 3/s Leverington, 2014 ), the relative magnitudes of thermal incision ( Fig. 15 ). Though several orders of magnitude greater than the rates become significant at especially low slopes. As expected, flow ∼8.7 × 10 3 m 3/s maximum estimated to have been involved in depths greater than 20 m are associated with greater capacities for emplacement of flows during the terrestrial Laki eruption incision than those depicted in Figs. 13 and 14 . For example, flow of 1783–1784 ( Thordarson and Self, 1993 ), and also larger than depths of 50 m are expected to produce mechanical and thermal the ∼1 × 10 4 m 3/s to > 1 × 10 6 m 3/s rates likely involved in 50 D.W. Leverington / Icarus 301 (2018) 37–57

Fig. 13. Mechanical incision rates predicted for a rigid bedrock substrate and for lava flow depths of 5 m ( A ) and 20 m ( B ). Incision rates are given for dynamic viscosities of 1, 10, 100, and 10 0 0 Pa s. For a depth of 5 m, flow is fully turbulent ( Re > 15,0 0 0) on all but the lowest slopes for a viscosity of 1 Pa s, and is relatively laminar ( Re < 15,0 0 0) for the remaining three viscosities. For a depth of 20 m, flow is fully turbulent on all but the lowest slopes for viscosities of 1 and 10 Pa s, and is relatively laminar for the other two viscosities. These flow conditions also apply to the incision plots of Fig. 14 .

Fig. 14. Thermal incision rates predicted for a rigid bedrock substrate and for lava flow depths of 5 m ( A ) and 20 m ( B ). Incision rates are given for four viscosities: 1 Pa s (top; dash-dot curves), 10 Pa s (long-dash curves), 100 Pa s (dotted curves), and 10 0 0 Pa s (bottom; solid curves). For each viscosity value, estimates with initial lava temperatures of 1350 °C (top), 1300 °C (middle), and 1250 °C (bottom) are given. emplacement of individual units of the Columbia River Basalt a total channel volume of 6.9 × 10 5 km 3 that approximates that Group as well as channelized of the Cape Smith Belt of the modern system. With a very conservative vertical incision ( Reidel, 1998; Williams et al., 2011a ), these rates are comparable rate of 1.5 m per day (more likely, greater incision rates would to those previously predicted for large volcanic channels on Venus be associated with channel reaches closer to volcanic sources, and and Mercury ( Baker et al., 1997; Byrne et al., 2013 ). The above lower incision rates would be associated with more distal reaches), discharge rates are consistent with realistic eruption conditions development of such a simplified system would involve only 10 0 0 involving magmas that had viscosities of 1 Pa s and that were fed days of flow. If the history of Kasei Valles is crudely simplified by dikes rooted in deep and voluminous magma chambers. For as having involved individual 5-day eruptive events spaced across example, a single dike with the ∼120 km length of the southern- geological time from the Late Noachian to the Amazonian, there most source region of Kasei Valles, with an assumed width of only would be 200 such events, each involving a total erupted magma 5 m and a magma rise speed of 10 m/s (e.g., Chevrel et al., 2014 ), volume of ∼25,0 0 0 km 3 if flows were 200-km-wide and 20 m could alone produce a magma effusion rate of ∼6 million m 3/s if it high on slopes of 0.2 °. Each such flow volume would be roughly were associated with one or more magma reservoirs of sufficient 3.3 times larger than the ∼7500 km 3 flow that mantles Athabasca volume and at sufficient depths to drive such flows to the surface Valles ( Jaeger et al., 2010 ). The highly simplified scenario outlined (e.g., Leverington, 2011 ). Wider dikes would have proportionately above would require a total magma volume of ∼5 × 10 6 km 3 for greater capacities for rapid and voluminous effusion to the Martian development of Kasei Valles. surface. Could a magma volume of 5 × 10 6 km 3 have realistically The geometry of Kasei Valles can be greatly simplified as formed a system as large as Kasei Valles? This estimate is ∼7.1 rectangular in plan view, with a width of 200 km and a depth times larger than the modern volume of the Kasei Valles system. of 1.5 km along a longitudinal distance of 2300 km, producing For a substrate at -23 °C and a magma erupted at ∼1350 °C, the D.W. Leverington / Icarus 301 (2018) 37–57 51

least several meters per day should be readily achieved on low topographic slopes and under realistic flow conditions. These results are broadly consistent with expectations, as is the greater significance of mechanical erosion relative to thermal erosion for all but the lowest slopes. The total volume of magma involved in development of the Kasei Valles system is estimated to have reached or exceeded ∼5 × 10 6 km 3 . This is a crude estimate that could be lower if e.g. incision rates were significantly higher than the conservatively- assumed rate of 1.5 m/day, and could be higher if e.g. magma eruption temperatures or magma depths were far lower than expected. Determination of a minimum lava volume for incision of Kasei Valles is complicated by several factors. For example, possible sources of volcanic effusions at Kasei Valles, including regions of chaotic terrain ( Fig. 4 d) and upturned flows ( Fig. 8 ), are not restricted to the head of the system in the southernmost part of Echus Chasma. Also, though highly voluminous eruptions should be particularly efficient at incising channels (e.g. a 50 m deep magma body flowing with a viscosity of 1 Pa s on a slope Fig. 15. Discharge rates predicted for lava flows with viscosities of 1 Pa s and with of 0.4 ° is predicted to have a capacity to mechanically incise more depths of 5 m (solid curves) and 20 m (dashed curves). For each flow depth, dis- than 11 m/day into rigid bedrock substrates), few constraints are charge rates are given for flow widths of 200 km (top), 100 km (middle), and 50 km

(bottom). presently available regarding the flow depths typical of Martian outflow systems and regarding the effusion rates and volumes typical of associated igneous plumbing systems. Even relatively substrate volume mobilized or melted during channel develop- thin flows of only 5 or 10 m depth should flow turbulently ment (if complete substrate assimilation were to take place, if the on low slopes and are expected to have a capacity for gradual densities and specific heat capacities of solid and liquid phases development of large channel systems if enough such flows are are very similar, and if the energy involved in phase changes is erupted over time (though incision would correspondingly be neglected) should be less than ∼13% of the volume of magma more restricted to reaches that are closer to sites of effusion, as erupted, in order to allow magma temperatures to remain above a result of higher rates of magma cooling relative to those of ∼1125 °C, below which rheological changes are especially likely more voluminous flows). The crude thermal considerations used to inhibit turbulence and incision (e.g., Keszthelyi and Self, 1998; in the previous section to evaluate the realism of the estimate of Keszthelyi et al., 2014b; Cataldo et al., 2015 ). An erupted lava total magma effused to form Kasei Valles (e.g., Keszthelyi and Self, volume of 5 × 10 6 km 3 satisfies this criterion, and is considered 1998; Keszthelyi et al., 2014b; Cataldo et al., 2015 ) may be the best to be a reasonable estimate of the magma volume that may have available means for determining the minimum volume of magma been involved in development of Kasei Valles. necessary for incision of a given system, and would suggest that the estimate of ∼5 × 10 6 km 3 is a minimum requirement for 6. Discussion development of Kasei Valles. If the mare-style volcanic units of the northern lowlands are The basic geological and morphological properties of the Kasei assumed to have an average thickness of 2 km (e.g., Frey et al.,

Valles system are aligned with those of ancient volcanic channels 2002; Salvatore et al., 2010 ), then a magma volume of 5 × 10 6 km 3 on Venus, the Moon, Mercury, and Earth. As with many of these would be sufficient to entirely constitute these mare units across channels, Kasei Valles heads at a topographic depression that most of Chryse Planitia as well as parts of southern Acidalia Plani- marks sites of effusion of voluminous low-viscosity magmas, tia. A volcanic origin for Kasei Valles and the other circum-Chryse terminates at topographic lows that are thickly mantled by flood outflow channels would thus imply that these systems contributed lavas, and shows evidence for having acted as a conduit for volu- substantially to the emplacement of ridged plains in Chryse minous low-viscosity flows. The Echus Chasma region is hypothe- Planitia and the lowlands to the north. Though the Shallow Radar sized here to be the surface expression of a major ancient igneous instrument (SHARAD) onboard the Mars Reconnaissance Orbiter plumbing system, with other possible sites of volcanic effusion has not yet detected subsurface reflectors of note within Chryse including regions characterized by chaotic or upwardly-deformed Planitia (e.g., Putzig et al., 2014 ), negative gravity anomalies here terrain. On the basis of the existence of large volcanic channel suggest mass deficits in the subsurface that have previously been systems on other solar system bodies, and thermal considerations interpreted as the products of buried channel sediments emplaced related to the insulation of flows by fixed or mobile surface crusts, by floods that formed the outflow channels ( Zuber et al., 20 0 0 ). the most voluminous lava flows erupted within the Kasei Valles More broadly, the Vastitas Borealis Formation, which forms an ex- system are expected to have had the capacity to flow for distances tensive mantle across much of the northern lowlands, is believed in excess of 20 0 0 km. Kasei Valles is the largest outflow system by some workers to have been emplaced as a result of aqueous on Mars (e.g., Carr, 1996 ), and is larger than the volcanic systems discharges from the outflow channels (e.g., Kreslavsky and Head, of other rocky bodies ( Leverington, 2014 ). An igneous origin for 20 02; Kleinhans, 20 05; Salvatore and Christensen, 2014, 2015 ). Kasei Valles would thus make this system the largest, though not A volcanic origin for Kasei Valles and the other circum-Chryse the longest, volcanic channel system in the solar system. outflow channels would necessitate reconsideration of the origins Though the physical processes involved in lava erosion are not of these gravity anomalies (e.g., Dombard et al., 2004 ) and the yet well understood, and results generated using available nu- Vastitas Borealis Formation (e.g., Catling et al., 2012 ). merical models should therefore be treated with caution ( Dundas A total lava volume of millions of cubic kilometers is compa- and Keszthelyi, 2014; Cataldo et al., 2015 ), the rates of mechan- rable to the original volumes of individual Large Igneous Provinces ical and thermal erosion estimated in this study are consistent on Earth, which range from ∼1 × 10 6 km 3 to 50 × 10 6 km 3 with volcanic origins for Kasei Valles. Total incision rates of at (e.g., Coffin and Eldholm, 1994 ). The Large Igneous Provinces are 52 D.W. Leverington / Icarus 301 (2018) 37–57 generally believed to have formed as a result of eruptive events for volcanic systems ( Leverington, 2011; Hopper and Levering- related to mantle plumes (e.g., Ernst and Buchan, 2004; Ernst ton, 2014 ). Close similarities between the basic properties of et al., 2005; Howell et al., 2014; Hole, 2015 ). Formation of the all outflow channels on Mars suggest development by common Valles Marineris canyon system itself, of which Echus Vallis is a processes. A volcanic origin for all outflow channels on Mars peripheral component, is hypothesized by some workers to have would dramatically reduce predictions of near-surface water vol- involved voluminous magmatic intrusion, withdrawal, and effusion umes, and correspondingly would resolve previous contradictions (e.g., Schonfeld, 1979; Leone, 2014 ). Voluminous effusion of lava at between such predicted volumes and those estimated on the the surfaces of large rocky bodies is very likely to have required basis of independent geochemical and mineralogical considera- the accumulation of magmas in deep sub-crustal reservoirs in tions ( Leverington, 2011 ). A volcanic origin would also the order to have involved sufficient pressure to drive magmas to the formation mechanisms of the Martian outflow channels to those surface without stranding them in neutral buoyancy zones (e.g., that produced volcanic channels elsewhere in the solar system, Head and Wilson, 1991, 1992, 2017; Wilson and Head, 1981, 2017; broadening the relevance of the Martian systems to the study of Leverington, 2011 ). Ultimately, if the conditions that drove eruptive magma generation, intrusion, effusion, and flow on other rocky processes at the Kasei Valles system are to be properly understood, bodies of the solar system (and vice versa), and increasing the the overall structure and geometry of associated igneous plumbing likelihood that early development of large volcanic channels is systems will need to be more precisely determined or inferred. typical of all large rocky bodies ( Leverington, 2014 ). On Earth, large Exactly how large and how deep were the magma reservoirs volcanic channels of the Archean and Proterozoic are in numerous within which partial melts accumulated? To what extent did cases closely associated with Ni–Cu ± (PGE) sulfide mineralization regional tectonics affect the migration of magmas toward the that developed as a direct result of thermomechanical erosion of surface here? What were the interior conditions that promoted substrates by turbulent ultramafic flows (e.g., Groves et al., 1986; long-term magmatism in this region? Barnes et al., 1987, 2011; Lesher and Campbell, 1993; Lesher and Much remains to be learned regarding even the most basic Arndt, 1995; Barnes, 2006; Houlé et al., 2012; Staude et al., 2016 ). aspects of magma flow and incision (e.g., Dundas and Keszthelyi, Such processes involved the scavenging of metals from silicate 2014; Cataldo et al., 2015 ). The lack of availability of large and magmas by sulfide liquids, as well as the mass concentration of active volcanic channel systems for field-based study on Earth has sulfides by flow processes operating within large volcanic conduits hindered direct observation and measurement of relevant flow (e.g., Burns and Fisher, 1990; Lesher and Campbell, 1993; Arndt parameters for all but the smallest channel systems. Available et al., 2005; Baumgartner et al., 2015, 2017 ). Might these or related numerical methods have necessarily, in part, been extended from igneous ore-generating processes have operated during volcanic fluvial methods, despite uncertainties regarding the manner in development of Kasei Valles or other Martian outflow channels which processes of magma flow might differ from those of aque- ( Baumgartner et al., 2015 , 2017 )? ous flow. For example, it is not yet clear how the erosive effects At present, a lack of detailed information regarding the surface of solid particles moved via traction and suspension might differ and subsurface character of the Kasei Valles system makes it diffi- between magmatic and aqueous systems, or how erosive effects cult to entirely rule out past contributions to channel development related to bubble formation and collapse may differ between such by aqueous processes. Nevertheless, though many uncertainties systems ( Dundas and Keszthelyi, 2014 ). In the future, numerical remain regarding the nature of involved mechanisms and envi- models for quantification of magma flow conditions must be fur- ronmental conditions, volcanic processes can credibly account for ther developed and validated. Relatively large volcanic eruptions the existence and basic properties of Kasei Valles without the involving silicate magmas continue to occur on Jupiter’s moon Io need to invoke the past action of aqueous processes for which (e.g., Blaney et al., 1995; et al., 2010; Williams et al., 2011b ), there is no clear evidence and no realistic solar system analog. and detailed future study of such events may play an important The very existence of enormous volcanic channel systems at the role in the refinement of methodologies for the modeling of large surfaces of multiple rocky bodies confirms that ancient igneous mafic and ultramafic effusions. plumbing systems had a capacity for development of large outflow Future study of Kasei Valles will also ideally involve the de- channels. Aqueous processes arguably complicate hypotheses of tailed modeling of spatial variations in the nature and effects of Martian channel formation without offering additional explanatory magma flow. For example, magma temperatures must decrease power beyond that provided by volcanic processes, rendering with distance from eruption centers, and lava viscosities corre- even hybrid aqueous-volcanic models unsatisfactory and unnec- spondingly will increase with distance as crystal contents increase essary. Assumptions of aqueous origins for the Martian outflow and as interstitial melt compositions change (e.g. Williams et al., channels currently predominate, but support for such viewpoints 1998, 20 0 0b, 20 01, 2011a; Keszthelyi et al., 2006; Hurwitz et al., is expected to decline as the dramatic differences between the 2012; Chevrel et al., 2013; Robert et al., 2014; Sehlke and Whit- magnitudes of ancient planetary volcanic processes and those of tington, 2015; Baumgartner et al., 2017 ). The effects of mechanical their modern terrestrial counterparts become better quantified and thermal incision therefore must vary with distance from and more broadly recognized. eruption centers, with the capacity for incision generally greatest near eruption centers and least along the most distant channel reaches. Though uncertainties regarding the forms of ancient 7. Conclusions landscapes and the detailed character of past volcanic events will always present challenges, future models of systems such as The properties of the Kasei Valles outflow system are aligned Kasei Valles are likely to produce advances in understanding by with those of ancient volcanic channels on Venus, the Moon, considering aspects of the spatial and temporal distribution of the Mercury, and Earth. As with many of these systems, Kasei Valles combined effects of mechanical and thermal erosion as a function heads at a topographic depression that marks sites of effusion of distance, topographic slope, substrate assimilation, and magma of voluminous low-viscosity magmas, terminates at topographic evolution (e.g., Williams et al., 1998, 20 0 0b, 20 01, 2011a; Chevrel lows that are thickly mantled by flood lavas, and shows evidence et al., 2013; Cataldo et al., 2015; Baumgartner et al., 2017 ). for having acted as a conduit for voluminous low-viscosity flows. Most outflow channels on Mars have sizes and morphological Echus Chasma, which forms the head of the Kasei Valles system, is complexities intermediate between those of Kasei Valles and hypothesized here to be the surface expression of a major ancient Hrad Vallis, both of which are considered here to be candidates igneous plumbing system. D.W. Leverington / Icarus 301 (2018) 37–57 53

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