Journal of Volcanology and Geothermal Research 337 (2017) 62–80

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Journal of Volcanology and Geothermal Research

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Mangala Valles, Mars: A reassessment of formation processes based on a new geomorphological and stratigraphic analysis of the geological units

Giovanni Leone

ETH Zurich, Institute of Geophysics, Geophysical Fluid Dynamics, Room H28, Sonneggstrasse 5, CH-8092 Zurich, Switzerland article info abstract

Article history: Mangala Valles has always been viewed as the typical outflow channel formed by catastrophic floods of water. A Received 9 October 2016 new analysis has shown that the geomorphological traces of fluvial or lacustrine processes within Mangala Valles Received in revised form 1 March 2017 can be better explained by fluid lava flooding the channels and filling pre-existing impact craters. As for the Accepted 10 March 2017 circum-Chryse outflow channels, where no clear source of water or mechanism able to replenish water at its hy- Available online 11 March 2017 draulic head is observed, there is no geologic trace of a sudden removal of a volume of water (ice) necessary to carve Mangala Valles. Neither maars nor rootless cones, typical volcanic features indicative of interaction be- tween lava and ground ice, were found. Past works suggested that the formation of Mangala Valles occurred in late Amazonian age when the climate of Mars was similar to that seen today, that is absolutely not liquid water friendly. The present work shows how the origin of Mangala Valles may go back to Noachian or even Pre-Noachian when other studies have concluded that the climate was not liquid water friendly. Even assuming limited periods of obliquity favourable to liquid water in the history of Mars, which is at odds with the wide- spread presence of unaltered olivine and jarosite, it is very difficult to find plausible mechanisms of aquifer re- charge or signs of catastrophic water release at the Notch of Mangala Valles that could feed the multiple episodes, or even a single episode, of fluvial flooding suggested in the literature. This evidence and other analysis will show that the presence of water and, eventually, ground ice is not incontrovertible in the equatorial regions and should not be given for granted as commonly done so far in the literature. The geomorphological analysis of the Mars Reconnaissance Orbiter (MRO) images provided in this paper, combined with THEMIS and MOLA data, show how Mangala Fossa, from which Mangala Valles originated as a breakout, is an erosional channel formed by the flow of lava in a original tube coming from Daedalia Planum rather than a tectonic graben or the sign of a dike rupturing to the surface. © 2017 Elsevier B.V. All rights reserved.

1. Introduction and eolian erosion (Cutts and Blasius, 1981). Volcanic (Leverington, 2004)andmixedvolcanic-fluvial processes (Keske et al., 2015) Mangala Valles is a ~900–1000 km-long, 165 km-wide, outflow regained consideration. A description of all these mechanisms of forma- channel always thought as formed by catastrophic flooding of water tion will be given in the discussion section. Several other papers have (Baker and Milton, 1974; Ghatan et al., 2005) initiated through mobili- suggested the volcanic origin of the outflow channels (Jaeger et al., zation of shallow aquifers by volcanic intrusions (Tanaka and Chapman, 2007; Leverington, 2007; Leverington, 2011; Leone, 2014) and one of 1990; Dohm et al., 2001; Hanna and Phillips, 2006; Leask et al., 2007a, b; them mentions the problems related to the poor stability of liquid Basilevsky et al., 2009; Burr et al., 2009; Bargery and Wilson, 2011). The water in the low-pressure atmosphere of Mars (Leverington, 2011). head of Mangala Valles is located along the ~220 km-long, ~6 km-wide, New evidence supports a volcanic-only erosional origin for Mangala and ~1 km deep, Mangala Fossa on a volcanic flood plain sloping to- Valles without any water and tectonism involved. The most important wards Amazonis Planitia (Fig. 1). A volcanic-only origin was initially evidence is observed along Mangala Fossa and along the course of proposed for several other outflow channels located in the largest volca- Mangala Valles. Mangala Fossa was interpreted in the past as a graben nic provinces of Mars, but not for Mangala Valles (Carr, 1974). Other opened by the intrusion of a dike from which water trapped in the works suggested alternative hypotheses of formation, including glacial cryosphere spilled out to carve Mangala Valles (Tanaka and Chapman, erosion (Lucchitta, 1982), debris flow (Nummedal and Prior, 1981), 1990; Ghatan et al., 2005; Leask et al., 2007a). However a dike or a sill rupturing to the surface should leave trace of lateral flows (e.g. perpen- dicular to the fissure) all or partially along its length. Magma coming out E-mail address: [email protected]. from tectonic fractures showing vertical and/or horizontal displacement

http://dx.doi.org/10.1016/j.jvolgeores.2017.03.011 0377-0273/© 2017 Elsevier B.V. All rights reserved. G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 63

Fig. 1. MOLA context map of the Sirenum Terra region, the black rectangles indicate the location of the panels shown in Fig. 3. The black arrows indicate the path of the lava flooding that formed the volcanic inter-crater plain indicated with the geologic unit Asp2 (Nsp2). was observed at Prometheus Patera (Leone et al., 2009) and Zal Patera explanation to water or to a dike rupturing onto the surface for the for- (Bunte et al., 2008) on Io, but does not seem to be the case at Mangala mation of both Mangala Fossa and Mangala Valles. The lack of any kind Fossa. Also, the hypothesis of a dike approaching but not reaching the of displacement shown by MOLA profiles along the sides of Mangala surface to potentially warm up the putative cryosphere to release Fossa (Fig. 2) raises doubts about a putative tectonic origin. This does groundwater (Wilson and Head, 2004) was analysed. These hypotheses not necessarily mean that other fossae on Mars might not be of tectonic raised several questions. How much water is required to carve Mangala origin, every situation must be evaluated case by case based on the Valles? Was this water potentially available underground? How can available evidence. water be replenished or a cryosphere be present in sufficient amounts The direct observation of lava flows through high resolution CTX and at the hydraulic head of Mangala Valles to feed multiple episodes of HiRISE imagery, combined with a global geomorphological and mineral- flooding suggested by Tanaka and Chapman (1990), Zimbelman et al. ogical analysis of the surface of Mars, was fundamental to understand (1992), and Basilevsky et al. (2009)? How can liquid water survive in the volcanic origin of Valles Marineris, of several other circum-Chryse the low atmospheric pressure of Mars after phreatomagmatic (explo- outflow channels, and of all the valley networks once thought to be flu- sive) activity? Why does magma coming from depth not raise at differ- vial networks (Leverington, 2004, 2007, 2009, 2011; Leone, 2014, ent heights the sides of Mangala Fossa? Why is no significant tectonic 2016). The same approach will be used here to better understand the displacement observed along Mangala Fossa (i.e. a large scale strike- origin of Mangala Valles. The available geological maps of Mangala slip fault)? Which type of tectonic stress might selectively produce frac- Valles made by Chapman and Tanaka (1993), and particularly by tures inside a pre-existing impact crater and none on its rim crossed by Keske et al. (2015), provided a basis for the discussion and the new in- Mangala Fossa? This paper will answer these questions and will show terpretation of the geological units. An alternative interpretation of sev- how the erosional power of lava flowing in a tube (Greeley et al., eral geologic units will be given in this paper and will be compared to 1998) or in channels (Hurwitz et al., 2010) can provide an alternative that made by Keske et al. (2015), every description and age of the 64 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80

Fig. 2. A) MOLA profiles across the lower course of Mangala Fossa; the white arrows indicate Mangala Fossa in the corresponding profiles numbered from 1 to 5; all the profiles show no significant vertical displacement between the sides of the fossa. B) MOLA profiles across the median course of Mangala Fossa near the Notch of Mangala Valles and the ridge of separation with Daedalia Planum; the white arrows indicate Mangala Fossa in the corresponding profiles numbered from 6 to 10; the profiles show no significant displacement between the sides of the fossa, the difference in height is only related to the local topographic roughness of the ridge. C) MOLA profiles across the source region of Mangala Fossa; the white arrows indicate Mangala Fossa in the corresponding profiles numbered from 11 to 15; also here the profiles do not show any significant vertical displacement except for that related to the topographic roughness of the crater rim crossed by Mangala Fossa. units that will be discussed in this paper refers to their geologic map. section of Mangala Fossa just west of the Notch (Fig. 3a), a process pre- The description of the geomorphology will start from the source of viously seen along Valles Marineris (Leone, 2014). The geomorphology Mangala Fossa through the head of Mangala Valles and then will follow still shows a narrow section of Mangala Fossa immediately west of the its whole course, including all the distributaries, to the mouth at Notch and a wide section to the east (Fig. 3a). The terraces located on Amazonis Planitia. The bed-floors of Mangala Valles and its distributar- both sides of Mangala Valles after the Notch indicate the gradually de- ies are dusty but there is enough visual and already mapped evidence of creasing flow of lava from Mangala Fossa once the eruption faded out. lava flows (Keske et al., 2015) and of unaltered olivine (Ehlmann et al., However, the stream of lava to Mangala Valles was sustained long 2010)toconfirm the flow of fluid lava of likely basaltic composition. The enough to carve its whole course, including several other breakouts exact sources of the lava flows and of the geological units within along the course of Mangala Valles and several tear shaped structures Mangala Fossa were unspecified in the work of Leverington (2007) (henceforth TESS) that will be described in the next sections. The geo- and partially specified in that of Keske et al. (2015) so this work will logical map made by Keske et al. (2015) provided a useful coverage of clarify the possible sources of the geological units, including eventual Mangala Fossa in the section between the two ridges that border correlations outside Mangala Valles. Mangala Valles. The lava flows inside Mangala Fossa were included in Lastly, this work will discuss the formation of Mangala Valles in the their volcanic unit Asp3 that formed the main channel of Mangala context of the overall volcanic history of Mars, which is now enriched of Valles. No overflows were reported along the whole section of Mangala new important information (Leone et al., 2014; Leone, 2016). This work Fossa, as would be expected in a fissure eruption from a dike reaching will provide a better understanding of the formation of single features at the surface, but lava was primarily channelized through the Notch. regional scale, focusing particularly on the nature of Mangala Fossa (e.g. The source of Mangala Fossa shows two nested channels of different erosional vs. tectonic), which is crucial to determine the source(s) of the section and a smaller one separated by the others (black arrows in lava flows that draped Mangala Valles. Fig. 3f). The smallest and northernmost of these channels (indicated by the black arrow on top of Fig. 3f) is separated from the others, but 2. Mangala Fossa and the source of Mangala Valles still retains the typical morphology of the pit chain because has not re- ceived enough lava flows to be well eroded and enlarged. In fact, it is lo- THEMIS and CTX images, supported by MOLA data, show in detail cated in a lateral and more sheltered position from the Daedalia Planum the sources of both Mangala Valles and Mangala Fossa (Fig. 3). The lava flows. The smaller of the two nested channels is at an intermediate source of Mangala Valles is directly connected to Mangala Fossa stage, in which the nested tip still retains the rounded shape of the pit (Fig. 3a), through a bottleneck informally called “the Notch” (Ghatan chain whereas its flooded part looks more like a fossa (black central et al., 2005), and it is geomorphologically very similar to the breakouts arrow in Fig. 3f). The larger channel is at a more advanced stage of observed in volcanic channels along Valles Marineris (Leone, 2014) fossa and crosses the breached rim of an unnamed crater eroded by and Jovis Fossae (Plescia, 2013). The youngest lava flows coming from lava flows coming from Daedalia Planum. The lava flows are still visible Daedalia Planum have embayed a ridge that divides Mangala Valles and clearly channelized in several points to flow into Mangala Fossa from Daedalia Planum (see Fig. 4i for a close-up). These flows appear (southernmost black arrow in Fig. 3f). Thus, the stage of pit chain is geomorphologically very similar to other fluid lava flows erupted from still visible across the ridge that separates Mangala Valles from Daedalia Arsia Mons (Crown and Ramsey, 2016). The lava flows coming from Planum (Fig. 3b) as well as at the source of Mangala Fossa (Fig. 3f). Daedalia Planum, channelized into Mangala Fossa, were able to slightly MOLA data along the whole course have shown that there is neither enlarge a wedge into the ridge and this wedge continues towards a pit vertical nor horizontal displacement between the sides of Mangala chain across the ridge (Fig. 3b). Immediately beyond the western side Fossa (Fig. 2a, b, and c). The rims of the craters and the ridges crossed of the ridge, the floor of Mangala Fossa is enlarged and contains a thin by Mangala Fossa retain their original shapes and show no displace- central chain of mounds and isolated mesas morphologically similar to ment. This is particularly evident on a crater crossed along the lower those visible in central Coprates Chasma. Mangala Fossa shows a break- course and centred at coordinates 19.19 S–152.43 W (Fig. 3c and e); out about 50 km from the ridge, becoming Mangala Valles (Fig. 3a), and the northern and southern rim appear perfectly intact. All that can be continues its course to the west (Fig. 3c). The local gradient of topogra- observed in the area is a tributary crossing the nearby ridge on the east- phy, declining both to the west and to the north as inferred by the altim- ern rim (Fig. 3e), and a distributary on the western rim with sub-parallel etry in Fig.1 of this paper and by the profile in Fig. 2 of Leverington overflows according to the gradient of topography, which declines from (2007), favoured the opening of the lateral breakout and diverted the east to west (Fig. 3c). main stream of lava towards Mangala Valles. The opening of the break- MOLA data also gave important information on the depth and the out was further aided by the presence of a bottleneck in the narrowest section's shape of Mangala Fossa along its length. Mangala Fossa is G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 65

Fig. 3. A) The mosaic of the CTX image P04_002646_1616_XI_18S149W, centred at 18.47 S–149.16 W, and the CTX image P05_002857_1629_XN_17S149W, centred at 17.14 S–149.74, shows the source area of Mangala Valles; here lava flowing into Mangala Fossa enlarged the bed-floor opening lateral breakouts, the most important of which is Mangala Valles, and eroded the walls forming mesas and a central discontinuous ridge. Both sides of Mangala Valles show terraces that sign the decreasing level of the lava stream. B) The mosaic of the CTX image B04_011322_1621_XI_17S148W, centred at 18.01 S–148.23, and the CTX image P21_009410_1619_XN_18S147W, centred at 18.14 S–147.77 W, shows a wedge carved in the ridge that separates Daedalia Planum from Mangala Valles; the wedge is mantled of lava flows coming from Daedalia Planum and continues into a pit chain along Mangala Fossa. C) Context THEMIS mosaic image for the Mangala Fossa – Mangala Valles source system, the labelled white rectangles refer to the corresponding panels. D) Context THEMIS image for the source of Mangala Fossa, the white polygon refers to the corresponding labelled panel f. E) The CTX image D05_029282_1604_XN_19S152W, centred at 19.76 S–152.03 W, shows the track of Mangala Fossa across a ridge; pits are visible only at both ends of the track suggesting that the lava tube feeding the crater to the west might be deep enough, or that the material of the ridge might be consolidated enough, to avoid total collapse of the roof. F) The mosaic of the CTX image G03_019273_1643_XN_15S140W, centred at 15.8 S–140.28 W, the CTX image F02_036402_1645_XN_15S140W, centred at 15.61 S–140.62 W, and the CTX image B16_016082_1643_XN_15S141W, centred at 15.8 S–140.92 W, shows the source area of Mangala Fossa; the white arrows indicate the channels where the lava flows. similar to Coprates Chasma due to the same mechanisms of erosion in be no more than ~5–6 km. The slope might be inferred by the difference original lava tubes (Leone, 2014): of height between the two sides of the ridge (around 500 m). This infor- mation will be useful to roughly estimate the flow of lava that can pass fl a) The attest in its upper course where there is more direct access to through it per unit of time. lava filling coming from Daedalia Planum. b) The deepest in its median and lower course where there is less di- 3. The upper course of Mangala Valles rect access to lava filling from Daedalia Planum; profiles reveal an – – average depth of 600 400 m along the lower course and 1000 The upper course of Mangala Valles is characterized by TESS, 500 m along the median course while the average depth at the mapped as Asp2 unit and likely shaped as teardrops by the stream of – fi source and along the upper course is only 400 200 m; pro les lava corresponding to the unit Asp3 along the main channel. The walls along the median and the lower course still show a V-shaped of the main channel exhibit terraces indicative of several episodes of fl bed- oor. flooding (Fig. 4c), or decreasing level of lava during a single episode, and several parts of chaotic terrain (Fig. 4b). The chaotic terrain was It is worth noticing that Mangala Fossa does not receive significant mapped in two distinct geological units, (polygonal blocks with dissect- contributions from tributaries. Some lava flows try to outflank the ing canyons) Apb and (small knobs) Ask units (Keske et al., 2015). Al- ridge from the south, but they do not reach Mangala Fossa (black arrows though surrounded and evidently embayed by the Asp3 volcanic unit in Fig. 3c). The only possible feeder could be the lava tube located in the these TESS were interpreted as of fluvial origin (Keske et al., 2015), as- shallow underground below the ridge along Mangala Fossa as indicated suming the term “fluvial origin” as formed by a flow of liquid water. by the pit chain shown in Fig. 3b. The width of such a tube can be esti- The Apb and Ask units are still part of the same island in which the mated by the width of the pit chain observed on the ridge and should Asp2 unit is also present and appear embayed by the Asp3 unit 66 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80

Fig. 4. The geological units indicated in the panels of this figure are Keske's units. A) Context image for the upper-median course of Mangala Valles; several episodes of lava flows formed the main channel on the stratifications of older lava flows; flowing lava breached craters rims located along the main channel walls and flooded them up; the lava stream in the middle of the channel formed also tear-eye shaped structures; the labelled white rectangles refer to the panels of this figure. B) The mosaic of the CTX images P05_002857_1629_XN_17S149W and P03_002079_1639_XN_16S149W, centred at 16.52 S–149.36 W, shows in detail the erosional mechanism of the fluid lava forming Mangala Valles; the main stream of lava (see black arrows) carved the main channel leaving only small or no mounds (Unit Asp3) of the older Asp2 unit on its floor, formed a large central island and several embayments (Apb units) on the walls of the channel; rivulets of lava carved a network of smaller channels in the central island. C) The CTX image P05_002857_1629_XN_17S149W, centred at 17.19 S– 149.31 W, shows a tear-eye shaped structure in the main channel of Mangala Valles; the arcuate structure of the impact crater shields the terrain behind it from the erosional power of the lava stream forming the classic tear-eye shape; terraces are still visible in the terrain protected by the impact crater suggesting several episodes of lava flooding; the obstacle of a mound formed other two bifurcated sub-channels where the stripes of lava left by the stream are still visible (black arrows). D) The mosaic of the CTX images G18_025339_1648_XN_15S150W, P01_001459_1656_XN_14S150W, P05_002857_1629_XN_17S149W, and P16_007182_1626_XN_17S150W, centred at 14.57 S–150.03 W, provides a lateral perspective on a breached crater located along the western wall of Mangala Valles; the lava stream was strong enough to breach the rim of the crater and flood it completely; the black arrows indicate the point of entrance of the lava into the crater and the point of exit located further downslope; the crater's floor appears completely flooded. E) The mosaic of the CTX images P19_008619_1680_XI_12S150W and P19_008474_1688_XN_11S150W, centred at 12.53 S–150.10 W shows an impact crater with its eastern rim completely eroded away by lava flooding along the median course of Mangala Valles, the dashed black line shows a possible contour of the original rim inferred from the shape of the crater; it is still visible the stream of lava on the channel's bed-floor and the point of contact with the terraces on the right side of the channel (black arrows). F) The CTX image P02_001934_1677_XN_12S151W, centred at 12.38 S–151.19 W, shows the perspective of another crater (see rectangle labelled f in panel a to find its location) with the rim breached by the lava flooding further ahead along the left side of Mangala Valles; here the lava flow fronts coming from the channel encounter the flow fronts coming out from the flooded crater (the floor is indicated in the image) and interfere like the waves of the sea encountering the backwash from the shore; the black arrows indicate the main stream of lava, the lava flowing out from the crater, and their points of interference in the main channel. G) This THEMIS mosaic, centred at 13.38 S–146.36 W, shows the northern portion of the ridge that separates Mangala Valles from Daedalia Planum; the white line indicates the Keske et al. (2015) border between the Asp1 and Asp2 geological units; the black arrow indicates the breach in the northwestern rim of the flooded crater, from this point onwards the lava flows coming from Daedalia Planum, marked with AHv, feed the Asp1 unit. H) The mosaic of the CTX images P15_006760_1658_XN_14S148W, B01_010188_1655_XI_14S148W, P13_006259_1680_XN_12S149W, and P22_009832_1677_XN_12S149W, centred at 13.36 S– 148.35 W, provides a perspective on the geological contacts between the Asp1, Asp2, and Ags units; the white solid line is the geological contact between the units Asp1 and Asp2, the white dashed line is the geological contact between the units Asp2 and Ags; the Asp2 unit appears superposed on both the units Asp1 and Ags. I) The CTX image B20_017625_1641_XI_15S147W, centred at 16.29 S–147.13 W, shows a perspective of the portion of the eastern side of the ridge dividing Mangala Valles from Daedalia Planum; the ridge here appears obliterated by an impact crater with the eastern rim eroded away by the lava flows flooding in from the east; the black arrows indicate the various embayments of lava in the ridge and in the remaining crater rim. G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 67

(Fig. 4b). The related presence of the Asp2 unit on this and other islands the possible water sources. The geomorphological analysis of the located within the channel and the superposition relationships among Fig. 4b shows how the Asp2 unit is a volcanic unit located on top of the various units involved suggest that the Apb and Ask units were the stratigraphic column on both sides of the channel, including the just chunks of Asp2 volcanic terrain eroded by the Asp3 flow unit. Al- islands located in the middle of the channel. Then, the Asp3 volcanic ready for this reason the differentiation between Apb, Ask, and Asp2 unit carved its way across this sequence of layers forming Mangala should not have been made. The ages obtained from the crater counts Valles as we see it today. Lava of the Asp2 unit flooded the corridor be- by Keske et al. (2015) show some visible inconsistencies. The Ags unit tween two ridges, starting from the inter-crater plains in Terra Sirenum was considered younger than the Asp2 unit although the Asp2 is super- (Fig. 1). A contributing source of the Asp2 lava flooding, tracked back as posed on it (Fig. 5). The contact between these units was even far as Terra Sirenum, seems also the couple of Noachian volcanoes interpreted as fluvial cataracts (Keske et al., 2015), without details on Sirenum Mons and Sirenum Tholus.

Fig. 5. The geological units indicated in the panels of this figure are Keske's units. A) THEMIS context image for the upper-median course of Mangala Valles focused on the cataracts located along its eastern side; the labelled white rectangles refers to the panels of this figure. B) The mosaic of the CTX images P03_002079_1639_XN_16S149W, D15_033053_1645_XN_15S149W, and P14_006615_1645_XN_15S149W, centred at 15.44 S–149.11 W, shows a lateral perspective on the putative cataracts located at the contact between the Asp2 and Ags units and indicated by the black arrows; it is clearly evident from the cataracts that the lava flooding forming the Asp2 unit is superposed on the Ags unit. C) The mosaic of the CTX images P14_006615_1645_XN_15S149W, D15_033264_1645_XN_15S149W, and B01_010188_1655_XI_14S148W, centred at 15.08 S–149.05 W, shows a lateral perspective on several streamlined (TESS) features within the Ags unit just downhill of the cataracts; the cataract originated from the Asp2 unit (black arrow), which thus cannot be carved by the Ags unit; the Asp2 unit does not show any fluvial incision suggesting that the cataract was formed by the Asp2 volcanic flooding rather than by a subsequent fluvial activity. D) The mosaic of the CTX images B01_010188_1655_XI_14S148W, P09_004347_1636_XI_16S148W, centred at 14.36 S–148.53 W, shows a lateral perspective of the contact between the Ags and the Asp2 further downhill with respect to the previous panel; the situation is not changed, the Asp2 unit is above the Ags unit and formed cataracts of lava flooding; terraces formed by the lava flooding are visible in the image and are indicated by the black arrows. E) A zoom into the CTX image P09_004347_1636_XI_16S148W, centred at 14.17 S–148.59 W, concludes the survey of the cataracts on the eastern side of Mangala Valles; also here is clearly visible in a lateral perspective that the Asp2 unit is superposed on the Ags unit and it is not interested by any incision typical of fluvial activity. F) The mosaic of the CTX images P22_009832_1677_XN_12S149W, P13_006259_1680_XN_12S149W, B01_010188_1655_XI_14S148W, and B09_013234_1704_XN_09S149W, centred at 12.45–149.34 W, shows the perspective of another cataract on the contact between the Ags and Asp2 units; there are streamlined features characterizing the Ags unit and not the Asp2 unit; the Ast unit is also superposed on the Ags unit because cuts it in the middle of the channel. 68 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80

Another geological inconsistency is the differentiation between Hrt islands in the southern section of the main channel, were mapped both and Nh units made by Keske et al. (2015). These units were formed by as Hrt and Asp2 units with Ast terraces (Fig. 4c). In the sector of the the same flow front (the ridge) dividing Daedalia Planum from Mangala channel imaged in Fig. 4d these units were mapped as a whole new Valles. A similar argument applies to the other ridge on the western side Alk unit, which also includes the terraces of the Ast unit. of Mangala Valles. The eHv was formed by the same flooding episode Moving further northward along the left side of the channel, it is pos- that formed the Asp2 unit. The Asp3 unit, erupted from Daedalia sible to see an impact crater with its eastern rim completely eroded Planum and channelized through Mangala Fossa, is the result of the ep- away and its floor flooded by the stream of lava (Fig. 4e). The breach isode that produced a subsequent erosional stage controlled by the pre- in the rim of this crater was considered the result of Asp3 volcanic existing topography (Fig. 4b) and thus formed Mangala Valles. The Asp3 flooding but its floor not mapped as an Asp3 unit by Keske et al. flow started to erode the Asp2 unit and then continued downwards (2015). Another breached and flooded crater further to the north, also through the whole stratigraphic sequence eroding older units, being located on the same left side of the channel, was instead mapped as the Ags unit below the Asp2. The mounds of the Ask unit were previous- Asp3 (Fig. 4a and f). There is even evidence of backwash lava flows ly polygonal blocks similar to those of the Apb unit before being further out of the crater interfering with the lava stream in the channel (Fig. 4f). eroded and rounded by the stream of the lava flow that formed the Asp3 Moving eastward, away from the main channel, it is still possible to unit. The space among the mounds, which appears filled by the lavas of follow the stratigraphic sequence of volcanic layers seen in the upper the Asp3 unit, was once the network of dissected channels still visible in course of Mangala Valles. The Asp2 unit is still on top of the stratigraphic the Apb unit. The network of dissected channels of the Apb unit ends up column but a new and older Asp1 unit suddenly appears to the north into two larger channels that leave the island to the north, showing how just below the Asp2 unit in the map of Keske et al. (2015). Their geologic the Asp3 lavas were able to cross the whole island. The same network of map indicates as the source of this unit a breach 1.5 km wide into the dissected channels is also observed in the Apb units located on the right northwest rim of a filled crater obliterating the northern side of the side of the main channel, to the east of the island, along the direction of ridge that divides Daedalia Planum from Mangala Valles (Fig. 4g). the stream of (Asp3) lava (Fig. 4b). The eastern part of the island, la- First, the breach in the crater rim is not a geologic boundary, lava in belled as Asp2 unit, is not eroded because located on a topographic the unit AHv cannot suddenly become another unit Asp1 after passing high and thus out from the reach of the Asp3 lava flows. Although the the breach; second, its age cannot become older than that of the unit stream of (Asp3) lava eroded the older (Asp2) flow fields, forming a Asp2 because it is superposed on it. So an older Asp1(=AHv) than TESS around an impact crater located in the middle of the main channel Asp2 in the median course of Mangala Valles is a geological inconsisten- (Fig. 4c), the channel at the left of the TESS was mapped as being of flu- cy. Another comparison of ages comes from the superposition relation- vial origin (Ast) while the channel at the right of the TESS was mapped ships between the Asp2 and Ags units, the latter still being considered a as volcanic (Asp3) by Keske et al. (2015). The analysis of the position of younger unit of fluvial origin by Keske et al. (2015). The geomorpholog- the units in Fig. 4c shows that the Asp3 unit appears younger than the ical analysis has shown that the Ags unit is an older volcanic layer than Ast unit just before the TESS, but suddenly Keske et al. (2015) the Asp2 unit instead, being the latter superposed on the former (Fig. interpreted it as older (Ast superposed on Asp3) just after the TESS 4h). The same superposition relationship was observed to the south (top left corner of the Fig. 4c). The observations in this area revealed (Fig. 4b) and in the cataracts (Fig. 5). The geomorphological analysis other geological inconsistencies among the units mapped by Keske et of these units was facilitated by the observation of the available CTX mo- al. (2015). A few mounds not completely eroded away by the Asp3 saics from different perspective to improve the view and thus the inter- flows are what remain of the older Asp2 flows in the middle of the chan- pretation. The Ags unit is found only on the eastern side of the main nel (Fig. 4d). These mounds, already seen at the erosional stage of larger channel along the upper course of Mangala Valles (Fig. 5a), where the

Fig. 6. The geological units indicated in the panels of this figure are Keske's units. A) THEMIS context image for the lower-median course of Mangala Valles, here the main channel forms several breakouts into the paleoflows and small areas of chaos terrains on its eastern side; the main channel breaches a crater rim on its western side to feed Labou Vallis as far as Medusae Sulci; the labelled white rectangles refers to the panels of this figure. B) The CTX image D15_033040_1731_XN_06S155W, centred at 6.98 S–155.85 W and taken at the mouth of Labou Vallis, shows the classic feature of interference between flows coming from opposite directions (black arrow at the centre of the image). C) The mosaic of the CTX images P13_006180_1717_XN_08S153W, P22_009753_1700_XN_10S153W, B02_010531_1704_XI_09S153W, and B05_011665_1707_XI_09S154W, centred at 9.11 S–153.30 W, shows a close-up of the bed-floor of Labou Vallis after the double breach of two impact craters' rims flooded with lava from Mangala Valles; the double breach in the two craters' rims is completely visible in panel a; lava of the Asp3 eruptive episode carved the bed-floor of Labou Vallis, labelled as Ast unit by Keske et al. (2015) here, and outflanked a dam of rough terrain Hrt creating TESS labelled as Alk unit; on the top centre of the panel is clearly visible another impact crater double breached (the black arrows indicate entrance and exit) by the stream of the lava. D) The mosaic of the CTX images P01_001591_1708_XN_09S151W and P01_001525_1700_XN_10S151W, centred at 9.34 S–151.27 W, shows the sector of Mangala Valles where is located the breakout that forms Labou Vallis; the white arrows indicate the subdivision of the Asp3 lava flow in one stream going to Labou Vallis (black arrow on the left) and one continuing along Mangala Valles (black arrow on the right); the Asp3 flow is also circumventing a piece of rough terrain (Hrt) visible at the centre of the image; the geologic units in the image are mapped according to Keske et al. (2015) but the striking continuity between the Asp3 and the Ast units is particularly visible in the left main channel (black arrow) and it is commented in the text. E) The mosaic of the CTX images P01_001591_1708_XN_09S151W and P01_001525_1700_XN_10S151W, centred at 8.39 S– 151.28 W, shows a sector of Mangala Valles characterized by chaotic terrains (Apb units on the right side of the image); lava of the volcanic unit Asp3 flowed and formed the network of canyons located among the polygonal blocks and the knobbed terrains mapped as Apb and Ask units, respectively; the Ast unit characterizing the main channel visible on the left side of the image is the continuation of the Asp3 unit seen in the previous panel d; the black arrow indicates the remnants of an impact crater breached and filled by flowing lava; the dashed semi-circular line indicates the original rim, extrapolated from the shape of the crater, removed by the Mangala Valles main channel. F) The mosaic of the CTX images P01_001591_1708_XN_09S151W, P01_001525_1700_XN_10S151W, and P12_005758_1699_XI_10S151W, centred at 7.52 S–151.35 W, shows the bifurcation of Mangala Valles into two channels; the channel on the left is further subdivided into two sub-channels heading to Sabis Vallis and Minio Vallis, respectively; a thin stripe of the Asp2 volcanic unit is what remains of the dam that deviated the stream of lava towards the narrow passage (black arrow) leading to the Minio Vallis sub-channel; this narrow passage also allowed the course of Mangala Valles to continue towards the northeast in the image; although the channels were mapped as Ast fluvial unit by Keske et al. (2015), the morphology of the channel's bed- floor is the same of the Asp3 volcanic unit seen in panel d. G) The mosaic of the CTX images D19_034820_1727_XN_07S152W, P17_007538_1699_XN_10S152W, P15_007037_1751_XN_04S152W, and P01_001591_1708_XN_09S151W, centred at 7.13 S–152.08 W, shows a circular feature informally dubbed “The Little Roundabout” in this work; this feature was likely formed by lava flowing along Sabis Vallis during the Asp3 episode finding its way around a crater previously filled by the Asp2 lava flooding episode; the main point of entrance of lava is indicated by the black arrow; the lack of depth of Sabis Vallis in this particular sector indicates that lava was likely slowed down by the topography so that thermal erosion predominated over mechanical erosion. H) The mosaic of the CTX images P15_006826_1738_XN_06S151W, P02_001934_1750_XN_05S152W, P01_001591_1708_XN_09S151W, D19_034820_1727_XN_07S152W, P21_009331_1742_XN_05S152W, and P01_001591_1708_XN_09S151W, centred at 6.27 S–152.07 W, shows an area located between the Sabis Vallis and Minio Vallis sub-channels; Minio Vallis forms another roundabout feature dubbed “The Big Roundabout”; the black arrow on the right side of the image indicates a point of bifurcation where Minio Vallis forms a sinuous secondary channel that merges Sabis Vallis to the west, the black arrow on the left side of the image indicates the point where the secondary channel gets to Sabis Vallis. I) The mosaic of the CTX images G05_020025_1735_XN_06S150W, B21_017902_1741_XN_05S150W, P12_005547_1741_XI_05S150W, and P13_006114_1748_XN_05S151W, centred at 6.01 S–150.39 W, shows islands of eroded terrain along the main channel of Mangala Valles; the ffc units with an exclamation mark are based on the interpretation made by Keske et al. (2015), an alternative interpretation is lava of the Asp3 unit filling the crater as also explained in the text; the dashed semi-circular line indicates the original rim, extrapolated from the shape of the crater, removed by the Mangala Valles main channel. G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 69 cataracts are located, and disappears in the lower course (Keske et al., discharge into the Medusae Sulci, the stream of lava is still visible on 2015). The Asp2 unit appears superposed on the Ags unit all over the the bed-floor (Fig. 6b). The formation of Labou Vallis is quite interesting. contact between the two units (Fig. 5b to f). Even the Ast unit, The stream of lava opened a breach by both thermal and mechanical interpreted as fluvial and older than the Ags unit by Keske et al. erosion into the rim of an impact crater, according to a mechanism (2015), appears superposed on the Ags unit (Fig. 5f). It is evident in all well explained by Hurwitz et al. (2010),andflooded in. Another hy- the images how the putative fluvial erosion of the Ags unit is not affect- pothesis in which lava may have flooded through pre-existing inlets ing the Asp2 unit. Morphologically, any fluvial activity that formed the and outlets of the craters formed by water (Goudge et al., 2012) should Ags unit must have eroded the Asp2 unit all the way along the channel show typical features formed by the interaction of lava with previous but this is not the case (Fig. 5f). The Ags unit surrounding the stream- water (ice) left in the interstitial pores of the ground, but these are not lined features is located stratigraphically below the Asp1 and Asp2 present here as well. Nevertheless, aided by the local topography, the units. This would indicate that these areas were probably produced by lava flows did not get out from the crater back into the Mangala Valles the lava flows of a flooding episode preceding the emplacement of the channel but opened another breach on the opposite side of the crater Asp2 unit. No volcanic features typical of interaction between lava and rim and flooded into a confining larger crater continuing through all ice are observed in the images thus suggesting that no ground ice was the impact craters encountered along the way (Fig. 6a and c). While present before the emplacement of the Asp2 unit. opening its way across the craters and across a dam of rough Hrt terrain, the lava stream carved the Labou Vallis' main channel towards the 4. The median course of Mangala Valles and its distributary Medusae Sulci through a small network of interconnected channels Labou Vallis (Fig. 6c). According to the geologic map of Keske et al. (2015), lava of the Asp3 unit breached the impact crater and formed Labou Vallis. The The median course of Mangala Valles is characterized by an in- continuity of the Asp3 stream of lava from Mangala Valles is very easy creased number of breakouts, the main of which is Labou Vallis, which to see but, strangely, was suddenly interpreted as Ast unit beyond the formed a network of interconnected channels and some chaotic terrains largest impact crater crossed by Labou Vallis in the Keske et al. (2015) (Fig. 6a). Lava of the Asp3 unit flowed westward to Labou Vallis to map (see also Fig. 6c). Other inconsistencies are the TESS labelled as 70 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80

Alk, although these were clearly carved into the Hrt unit, and the floors 6. Discussion of some impact craters labelled as “flat craters floors” when it is even clearer that they were flooded by the same lava of the Asp3 unit The formation of Mangala Valles is discussed here within a necessar- (Fig. 6c). Similar inconsistencies can be seen going back to the main ily wide and global context of all the various factors that influenced the channel of Mangala Valles where the Asp3 lava flows were mapped environment all over the history of Mars. The geomorphology of water again as scoured terrain Ast (Fig. 6d and e), TESS were mapped as Alk and lava are quite similar (i.e. Dietterich and Cashman, 2014)andonly (Fig. 6c), a new unit Asp4 appeared as continuation of Asp3/Ast the analysis of several factors characterizing the environment of Mars (Fig. 6d), a terracing was mapped as Ats just before the entrance of may allow the exclusion of water in favour of lava as the likely fluid Labou Vallis (Fig. 6d), and a knobbed terrain was mapped as scoured that carved the channels. Among these factors, the low atmospheric terrain Ast (Fig. 6d). Near the entrance to Labou Vallis, Mangala Valles pressure assumes a key role for the instability of water. The same envi- formed a first bifurcation: an eastern channel and a western channel di- ronmental factors do not affect the eruption of lava flows and the forma- vided by an island of Hrt and Asp2 units (Fig. 6d). Keske et al. (2015) tion of rivers of lava on the surface of Mars. mapped the western channel as Ast unit and the eastern channel as Asp3 unit although it is clear that both units were formed by the same 6.1. Analysis of the mechanisms of formation eruptive episode Asp3 (Fig. 6e). The Asp2 unit is here reduced by lava erosion in a thin dam that closes the central bed-floor (Fig. 6f). From As already mentioned in the introduction, several other mechanisms this point onwards, Mangala Valles bifurcated again in two distinct of formation other than volcanic processes were suggested for Mangala channels: the right branch heading to north-north-east and the left Valles. These will all be discussed here to understand which one might branch heading to north. The left branch is subdivided in two other be more plausible for the formation of the whole system Mangala channels: Sabis Vallis and Minio Vallis. These channels formed interest- Fossa – Mangala Valles. ing circular features similar to roundabouts in the Asp2 unit that filled pre-existing impact craters. The first of these features is found along 6.1.1. Glacial erosion Sabis Vallis and is here informally named “The Little Roundabout” The sculpture by glacial erosion was suggested as an obvious mech- (Fig. 6g). Sabis Vallis surrounded the Little Roundabout (Fig. 6g) and anism after the observation of the brines deposited on the surface of the opened a breach (Fig. 6h) into a couple of nested impact craters (see planet (Lucchitta, 1982). However, this mechanism remains highly hy- also Fig. 6a for a larger context view). These craters, logically crossed pothetical and not supported by any direct observation of glaciers in by Asp3 lava as seen in Fig. 6d, were simply mapped as flat floored cra- the equatorial regions of Mars. These glaciers should already be visible ters by Keske et al. (2015). Minio Vallis formed “The Big Roundabout” at the resolution of the MRO imagery but nothing was ever seen so around a larger crater (Fig. 6h), this informal name is also given here far. Observations have instead shown that water and dry ice sublimate for the first time. From these roundabouts onwards, sinuous channels between night and day in the cold weather of Mars at equatorial lati- carved their own paths across the Asp2 and Hrt units connecting tudes (Thomas and Schimel, 1991; Thomson and Schultz, 2007). The Minio Vallis to Sabis Vallis (Figs. 6h, 7b and c). Along the main channel observations at the Phoenix landing site have shown how ice sublimat- of Mangala Valles the stream of lava of the Asp3 unit eroded a crater and ed in about four sols even at higher latitudes (around 66° North) than left a crescent shaped sediment mapped as flat-floored crater (ffc) unit Mangala Valles when brought to the surface from the shallow under- by Keske et al. (2015), see exclamation mark in Fig. 6i. A semi-circular ground (Smith et al., 2009). There is no surviving glacier, underground dashed black line inferred from the shape of the crater indicates the ice, or any other under formation observed at equatorial latitudes be- missing part (Fig. 6i). tween ±30° (Cutts and Blasius, 1981; Dundas et al., 2014; Brough et al., 2016). Some well-formed glaciers should be observable at the equa- tor in the current cold climate if a source of atmospheric water was 5. The lower course and the mouth of Mangala Valles available, but there is no reported case. Lucchitta (1982) also proposed that wet based ice would have moved readily by slipping over the Minio Vallis is characterized by several breakouts that formed a ground. Bargery and Wilson (2011) postulated that this particular pro- complex network of sub-channels (Fig. 7d; see Fig. 7a for enlarged cess has negligible erosional capability on Mars and further detail will be view). This network is as complex as the one coming out from the given in the next subsection. Even assuming a temporary increase of the roundabouts. A breakout from Minio Vallis discharged directly into temperature by punctuated volcanism (Halevy and Head, 2014), the Amazonis Planitia whereas other breakouts formed sub-channels bring- main problem for the stability of ice or liquid water on the surface of ing lava to Minio Vallis (Fig. 7d). A few chaotic terrains are found along Mars is the low atmospheric pressure. Minio Vallis (Fig. 7e). A flat-topped deposit of remnant lava flows can be found along the course of Sabis Vallis as far as its mouth towards Eu- 6.1.2. Dike intrusion menides Dorsum; these are likely lava flows of the Asp3 unit (Fig. 7f). The mechanism suggested by Bargery and Wilson (2011) is based on A close-up of the lava flows observed at the mouth of Mangala Valles a short-lived (17.9 h) turbulent flow of water-ice slurry located beneath (Fig. 7d and h) shows consistency with those observed at the mouth ice rafts, which becomes of an admitted (by the same authors) negligi- of Sabis Vallis (Fig. 7g), Minio Vallis (Fig. 7h), and Labou Vallis ble erosional capability as soon as the flow becomes laminar. Even as- (Fig. 6b). This observation indicates that the lava flows of the Asp3 suming that sufficient water might be available at the source of unit discharged directly to Amazonis Planitia along the various distrib- Mangala Valles and stay liquid above the volcanic heat, which is already utaries. The flow fronts and their superposition relationships observed unrealistic because of the low pressure environment of Mars, the mech- in Amazonis Planitia suggest that the Asp3 flows coming from Mangala anism proposed by Bargery and Wilson (2011) is also afflicted by a lack Valles are older than the lHt flows coming from the channel located at of information on grain size distribution, including unrealistic sediment the south of Amazonis Mensa (Fig. 7h), exactly the opposite of what es- load of the turbulent flow phase, unrealistic porosity of the putative timated by the crater counts in Keske et al. (2015). The geomorpholog- source aquifer, and unrealistic volume of water stored underground ical analysis of the Mangala Valles' mouth revealed that the origin of when compared to the eventual trace of subsidence/collapse that a sud- Padus and Asopus Vallis is not fluvial, as stated by Keske et al. (2015), den and catastrophic removal of such a large volume of water ice should but rather volcanic. A breakout of Mangala Valles diverted Asp3 leave on the surface. The estimates of water volumes made by Bargery lava into the lateral Hrt unit to the east and fed the flows that and Wilson (2011) to carve Mangala Valles were based on the unrealis- carved Padus Vallis and then Asopus Vallis as a breakout of Padus Vallis tic sediment load of 40% suggested by Komar (1980), and even 100% at (Fig. 7i). its distal end, whereas water volumes up to two or three orders of G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 71

Fig. 7. A) Context image for the lower course of Mangala Valles showing its complex network of breakouts and sub-channels, including Sabis and Minio Vallis. B)ThemosaicoftheCTX images P02_001934_1750_XN_05S152W and D09_030851_1756_XI_04S152W, centred at 5.15 S–152.12 W, provides detail on the network of sub-channels that fed Sabis and Minio Vallis; the geologic units taken from Keske et al. (2015) are indicated as well as in the other panels; on the left side of the image is possible to see the S-shaped curve formed by Sabis Vallis; the black arrows indicate the points of entrance and exit of sub-channels connecting Minio Vallis to Sabis Vallis. C) The mosaic of the CTX images P02_001934_1750_XN_05S152W and D09_030851_1756_XI_04S152W, centred at 5.45 S–152.17 W, shows the area immediately south of the previous panel B located between the Big Roundabout and the flooded nested craters visible in panel A; the area shows a particularly complex network of sub-channels connecting Minio Vallis to Sabis Vallis; again, the black arrows indicate the points of entrance and exit of the sub-channels; the exclamation mark indicates that a Asp2 unit has been interpreted as Hrt in Keske et al. (2015). D) The mosaic of the CTX images B17_016412_1749_XN_05S151W, P13_006114_1748_XN_05S151W, P12_005547_1741_XI_05S150W, and B08_012667_1747_XI_05S150W, centred at 5.00 S–150.55 W, covers a large area located between Mangala Valles and Minio Vallis; Minio Vallis has several breakouts that form sub-channels returning back to Minio Vallis and another reaching directly Amazonis Mensa instead; the floor of the impact crater crossed by Mangala Valles was mapped as ffc unit by Keske et al. (2015), the exclamation mark indicates that it should rather be mapped as Asp3 unit; the same applies to the Ahc unit located along the bed-floor of the breakout heading directly to Amazonis Mensa. E) The CTX image P02_001934_1750_XN_05S152W, centred at 4.41 S–152.06 W, provides a close-up of the polygonal blocks inside Minio Vallis; the Ask! unit indicates the bed-floor of a sub- channel connecting Minio Vallis to Sabis Vallis; the Asp2 unit on top of the polygonal blocks was mapped as Apb by Keske et al. (2015), as well as the Asp3 unit on the bed-floor of the sub-channel was mapped as Ask, and thus are indicated with an exclamation mark. F) The mosaic of the CTX images D09_030772_1756_XI_04S152W, P20_008975_1789_XI_01S153W, P16_007393_1788_XN_01S153W, P18_007894_1778_XN_02S154W, and P21_009397_1777_XI_02S154W, centred at 2.56 S–153.32 W, shows the mouth of Sabis Vallis; although this area was not mapped geologically by Keske et al. (2015), some of their units were indicated here on the basis of their geomorphological similarity with the observed terrain; the question mark on the Asp2 unit indicates some degree of uncertainty in the recognition of the unit. G) The CTX image P11_005402_1775_XN_02S152W, centred at 3.30 S–152.10 W, provides a close-up of the Asp3 lava flows that filled the bed-floor of Sabis Vallis. H) The mosaic of the CTX images B17_016412_1749_XN_05S151W, P13_006114_1748_XN_05S151W, P12_005547_1741_XI_05S150W, P22_009476_1756_XN_04S150W, P04_002435_1752_XI_04S150W, and B18_016768_1769_XN_03S150W, centred at 3.30 S–151.03 W, shows the confluence of the Asp3 flow discharged onto Amazonis Planitia from the mouth of Minio Vallis and from Mangala Valles; the bed-floor of Minio Vallis is filled with the Asp3 lava flows and the geomorphology of these flows is similar to that seen at the mouth of Sabis Vallis and Mangala Valles; the lava flows of the lHt unit are evidently superposed on the lavas of the Asp3 unit, the black arrow indicates the point of contact, so they should be younger instead. I) The mosaic of CTX images P19_008263_1749_XI_05S150W, P12_005692_1772_XI_02S150W, and P19_008553_1750_XI_05S149W, centred at 4.50 S–149.30 W, shows the network of sub-channels feeding Padus and Asopus Vallis; a breakout of Mangala Valles (starting from the low left corner of the image) is directly feeding Padus Vallis; a lateral breakout of Padus Vallis debouches into the impact crater to the east, floods its floor with lava, and then comes out of the crater to the north as the distributary Asopus Vallis (the white arrows indicate the direction of the lava); the floor of the crater is mapped as ffc by Keske et al. (2015), although it is clear that the lava of the Asp3 unit (mapped as Ast) coming from Mangala Valles floods its floor, so it is indicated with an exclamation mark. 72 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 magnitude were suggested as more plausible (Andrews-Hanna and (Bridges et al., 2012). By the way, no desert or pyroclastic deposit area Phillips, 2007; Leverington, 2011), and on the unrealistic permeability on Earth shows outflow channels similar to those observed on Mars. of 10−7 m2, ~300 times larger than the most permeable aquifer on Earth (Ghatan et al., 2005). Even assuming eventual fractures increasing 6.1.4. Debris flow permeability underground, such hyperconcentrated flows are not possi- The debris flow hypothesis by Nummedal and Prior (1981) for the ble at all through erosion alone, wall collapse is needed all along the formation of the outflow channels and associated chaos terrains was course of Mangala Valles (Kleinhans, 2005; Bargery and Wilson, geomorphologically based on the observation of poor resolution Viking 2011). The complete coverage of images of Mangala Valles showed images and was thought to be triggered by underground fluid escape. In that such a wall collapse may have occurred only in a limited sector of the images published in their paper, the mounds of the chaos terrains the upper course (Fig. 4b) and thus not enough to saturate all the vol- may indeed appear as blocks of landslide material formed by retrogres- ume of water required to carve the channel. The already low surface sive slumping of the head of the channel. They dismissed wind (Cutts pressure of 6.1 mbar assumed by Bargery and Wilson (2011) to keep and Blasius, 1981)andlava(Carr, 1974) as possible erosional agents be- the slurry stable might have been even overestimated given that mea- cause wind failed to explain the sinuosity of the channels and because surements within Gale crater showed variations from 2 to 6 mbar “no volcanic deposits were positively identified in the region”.Although (Guzewich et al., 2016). The source of Mangala Valles is located at a the former statement about the wind is plausible, the second statement height well above Gale crater (around 0 km of height, Fig. 1) and thus about the lack of volcanic deposits is more difficult to accept given that likely at a lower average pressure than Gale. Such a lower pressure the outflow channels are located within large volcanic provinces and on would favour the phreatomagmatic activity proposed by Wilson and the slopes of large volcanic centres. Furthermore, Nummedal and Prior Head (2004) because would enhance the explosivity of the putative vol- (1981) admitted the difficulty in assessing the triggering mechanism cano ice interactions. However, there is no geomorphological trace of of the retrogressive slumping from geomorphology alone. They specu- explosive activity (i.e. maars, tuff rings, tuff cones) in the whole area lated on a variety of triggering mechanisms, including seismic events, of Mangala Fossa – Mangala Valles but just evidence of fluid and quiet meteorite impacts, and gradual reduction of internal strength, but ig- lava flows. nored how lava carved channels on the slopes of well visible volcanoes. Observations along Valles Marineris, one of the larger and spectacular 6.1.3. Eolian erosion outflow channels that Carr (1974) excluded from being formed by Cutts and Blasius (1981) disagreed with the sudden release of lava erosion, have shown that indeed landslides enlarged the channel groundwater suggested by Carr (1979) and with the debris flow by but these were triggered by the erosional flow of lava into the bed- Nummedal and Prior (1981) dismissing these hypotheses as “catastro- floor according to Leverington (2009, 2011) and Leone (2014). Last phist views” based on “events that transformed the surface of Mars in but not least, Mangala Valles is not even mentioned in the Nummedal no more than a few days”. Cutts and Blasius (1981) noticed the absence and Prior (1981) work as one of the possible examples for the debris of glaciers on the surface of the planet outside the polar areas and point- flow mechanism of formation. ed out that the eolian hypothesis is based on processes “that acted over an extended period of time” still at work on Mars and thus implicitly 6.1.5. Tectonic origin preferable to “catastrophic events of which there is little evidence”.Al- Mangala Fossa was also interpreted as a graben of tectonic origin though the views of Cutts and Blasius (1981) may be partially shared (Tanaka and Chapman, 1990; Basilevsky et al., 2009; Leverington, here, the main problem with eolian erosion is that their model is both 2007). This view is not supported by the evident lack of both vertical statistically unlikely and unrealistic (Carr, 1979). The process is statisti- and horizontal displacement all along both its sides and along some cra- cally unlikely because the wind should selectively remove the materials ters that the fossa crosses. There are morphological similarities between from the ground only at the places where the outflow channels are lo- the crater described in Section 2 and other floor-fractured craters (ffc) cated. Winds blow all over the planet but the outflow channels are seen on the Moon (Schultz, 1976) and on Mars (Leone, 2016)but only in the equatorial regions. Cutts and Blasius (1981) suggested that these have nothing to do with large scale tectonic stresses. These craters the material removed from the equatorial regions at a rate of can be better explained with thermal and mechanical erosion by lava 1600 × 106 tons/year was transported to the north polar region of the (Hurwitz et al., 2010). The putative extensional pull that would have planet. They also stated that continuous eolian transport is highly unre- opened the putative fractures is neither accommodated by any com- alistic suggesting periodic events of transport of the order of 106 years pressional feature of proportional extent nor by any subduction zone occurring only 1/10% of the time. I can share the view that wind could elsewhere. Even invoking stresses formed by the rise of magma in the be periodical (i.e. Ullán et al., 2017), even assuming preferential zones crust does not help, faults on Io formed by this mechanism have over pyroclastic deposits in which excavation may initiate, but I do shown either vertical or horizontal displacement with lava spilling out not see why these same zones cannot be refilled with material all along the length of the fracture (Bunte et al., 2008; Leone et al., transported by the wind from elsewhere. Global observation of dunes 2009). Tectonic processes on Earth have also shown a minimum vertical on Mars has indeed shown that 75% of the dunes are located between displacement rate of 0.5 mm/a (Fuhrmann et al., 2014); at such a rate a 70 and 85°N of latitude, but common sediment transport is towards vertical displacement of 5 km on both sides of the walls should have the east and the west thus suggesting sediment source in local layered formed in just 10 Ma and be visible along Mangala Fossa. The floor of deposits (Hayward et al., 2013). The eolian hypothesis was previously Mangala Fossa should have been lowered also across the ridge as tec- debated by Greeley et al. (1976), who noticed a mismatch between tonic fractures ignore the topography. However, this does not seem to the direction of the channels and the direction of the winds, but Cutts be the case even assuming the 800–700 Ma of formation time estimated and Blasius (1981) invoked different atmospheric patterns in the past by Keske et al. (2015). The pit chain crossing the ridge along Mangala to justify the mismatch. However, invoking winds that may carve sinu- Fossa has an alternative explanation for its formation and this is not re- ous outflow channels directly proportional to the size of the volcanic lated to tectonism. It was already shown at Noctis Labyrinthus and provinces is more difficult to envision. Leone (2014) pointed out the Valles Marineris how pit chains can also be related to the presence of proportionality between the size of the outflow channels and the size lava tubes or underground feeders (Wyrick et al., 2004; Leone, 2014). of the related volcanic provinces, something that might be more com- Thus, the pit chain that is visible on the ridge separating Mangala Valles fortably explained by eruption rates directly proportional to the extent from Daedalia Planum might be the sign of a shallow lava tube. Other of the volcanic provinces (Wilson and Head, 1994). The observation of authors have suggested the formation of pit chains with collapse pro- the dunes on Mars has also shown sand fluxes similar to those of Victo- cesses related to lava tubes on Mars (Cushing et al., 2007; Léveillé and ria Valley, Antarctica, implying similar rates of landscape modification Datta, 2010) and to generic collapse processes associated to chasmata G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 73 on Venus (Wyrick et al., 2004). Other processes related to surface man- of the initial formation time of the lava tube that then evolved into ifestation of dykes and faulting were suggested (i.e. Wilson and Head, Mangala Fossa by lava erosion during the late stages of activity of 2002; Wyrick et al., 2004) but these are not applicable to Mangala Arsia Mons. MOLA charts in Fig. 8 show how common are these thick Fossa for the lack of lateral flows coming out from the fossa along its flow fronts in the largest volcanic provinces of Mars. whole length as already seen on Io (i.e. Bunte et al., 2008) and on Valles Marineris (Leone, 2014). The possibility that the pit chains may have a 6.1.6. An erosional origin by alternate events of flowing lava and water on tectonic origin is excluded for the whole Mangala Fossa as well as was the Mars surface excluded for Valles Marineris but this does not exclude tectonic hypoth- Fluvial events alternate to lava flows were also proposed by Keske et eses of formation for other pit chains on Mars. Also, from the observa- al. (2015) but several inconsistencies in their geological mapping were tion of Fig. 8, it should be quite straightforward to understand that the found. Keske et al. (2015) interpreted the origin of the Asp3 unit as spill- ridge might be the front of a previous large lava flow coming from ing from the ridge bordering the eastern side of Mangala Valles, but the Arsia Mons. Upon cooling, this ridge protected Mangala Valles from fur- lava flows in the Asp3 unit are morphologically very similar to the lava ther lava flooding. This interpretation should not be surprising consider- flows found at the source and in the section of Mangala Fossa located ing the huge size and the huge amounts of lava erupted by the Tharsis within Daedalia Planum. The possibility that the origin of the Asp3 volcanoes, Elysium Mons, and even Tyrrhenum Mons in their stages of unit interpreted by Keske et al. (2015) could be only the debouchment formation. The same reasoning can also be applied to the wrinkle ridges, of the original lava tube along Mangala Fossa, indicated by the pit chain previously interpreted as of tectonic origin (Plescia, 1991), which can be crossing the ridge, is thus more than intriguing. Aside that they never smaller lava flow fronts instead (Leone, 2016). It is worth noting here suggested a plausible source for the liquid water, the Ags – Apb – Ask that Mars never had plate tectonics forming mountains (O'Rourke and units interpreted by Keske et al. (2015) as the result of fluvial activity Korenaga, 2012) so the preferred interpretation in this work, supported eroding the Asp2 volcanic unit are not in direct continuity with the by the geomorphological analysis, leans towards the volcanic hypothe- Notch of Mangala Valles but appear rather patchy in the upper and me- sis. Given that water cannot stay liquid on Mars today or in the past (see dian course and absent in the lower course. Where present, these “fluvi- also Section 6.3), and cannot produce ridges this large, the ridge border- al” units are limited to the contact among the Asp2 unit and the Ast and ing Mangala Valles might be a previous lava flow front probably coeval Asp3 units in a few sectors along the main channel. The Ast unit in

Fig. 8. A) Global MOLA context image for the Western Tharsis volcanic province; the white arrows indicate the arcuate ridges at the border between the westernmost edge of the Tharsis lava fields and Mangala Valles; these ridges are the remnants of previous lava flow fronts, with respect to the latest flows mantling Daedalia Planum; Mangala Fossa and the Sirenum Fossae might be the result of the erosional evolution of the lava tubes formed during the eruptions that emplaced the ridges. B) Global MOLA context image for the Northern Tharsis volcanic province; the white arrows indicate the arcuate ridges emplaced as the fronts of lava flows erupted from Olympus Mons and from Alba Mons. C) Global MOLA context image for the Elysium volcanic province; the white arrows indicate the arcuate ridges emplaced as lava flow fronts erupted from Elysium Mons. D) Regional MOLA context image for the Tyrrhena volcanic province where Tyrrhenum Mons is located; the white arrows indicate the arcuate flow fronts forming ridges. 74 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 particular showed the same geomorphological characteristics of the vol- of the source area of Mangala Valles (Fig. 3c), it is reasonable to infer canic Asp3 unit and was observed in evident continuity to it along both that the lava flows have not been erupted by any fissure but travelled Mangala Valles and Labou Vallis. So the mapping as Ast from here on- from the lava fields of Daedalia Planum to cross the ridge through the wards is not shared in this paper, the continuity as Asp3 lava flow unit tube along Mangala Fossa. A bottleneck or a dam into Mangala Fossa lo- is recommended instead. This new interpretation is also supported by cated to the west of the ridge favoured the opening of a breakout that the similarity of morphology of the Asp3 lava flows seen near the source formed the Notch and thus Mangala Valles. The gradient of topography of Labou Vallis (Fig. 6d) with those seen along the upper course of declining from south to north from the Notch, see again Fig. 1 of this Mangala Valles (Fig. 4c). Lava of the Asp3 unit eroded the Asp2 unit paper and Fig. 2 of Leverington (2007), and the strength of the stream forming channels and polygonal blocks or knobby terrains that were favoured the channelling of lava through Mangala Valles in a network then mapped as Apb and Ask, respectively, by Keske et al. (2015).An- of dissected channels according to the mechanism of the preferential other inconsistency is the mapping of flooded craters as ffc units. The pathways suggested by Bleacher et al. (2015). The channelized stream recommended interpretation all along Mangala Valles would be that of lava formed the TESS and then further deepened the Mangala Valles' fluid lava forming the Asp3 unit carved the bed-floor of Mangala Valles channels mainly by mechanical erosion. Topography is also important through the Hrt and Asp2 units, the Ast and ffc units should be replaced for the mechanism by which lava opens its way through craters forming by the Asp3 unit, and the Ats unit should be replaced by Asp2 and/or inlets and outlets, thermal erosion is more effective on gradual slopes even by Hrt unit (Table 1). The different degree of erosion of the same whereas mechanical erosion is more effective on steeper slopes unit does not and should not make another unit. As seen in the upper (Hurwitz et al., 2010). Many craters crossed by lava through inlets and and median course of Mangala Valles, and as seen all along Valles outlets are visible both along Mangala Valles and Labou Vallis. This ob- Marineris (Leone, 2014), the chaotic terrains are also here largely the re- servation combined with the arguments against the stability of water sult of lava erosion of the volcanic Asp2 unit. The terraces and the im- on Mars given in Section 6.3 ahead and with the acknowledgement of pact craters located between the Asp2 and the Asp3 units were the Asp3 as volcanic unit contributes to strengthen the role of lava for flooded by lava (Figs. 4d, g, 6c, e, i), thus suggesting that the Asp2 unit the formation of Mangala Valles. It is thus likely that the formation of was eroded by the lava flows that formed the Asp3 unit in all the Mangala Valles might not be linked to the latest episodes of volcanism Mangala Valles' channels. of Arsia Mons (i.e. lHt), but more related to a previous event (Asp3) to which Mangala Fossa had direct access through the shallow lava tube 6.1.7. Erosional origin by lava flows only indicated by the pit chain. So it is important to explore the timeline of The geomorphological observation of the source of Mangala Fossa the volcanic events to see if there is any consistency with this view. showed a direct continuity in the system of the channels carved by the lava flows of the Asp3 unit as far as the mouth of Mangala Valles. 6.2. Formation ages of Mangala Valles Lava tubes are frequently associated with eruptions of fluid lava flows (Cushing et al., 2007; Léveillé and Datta, 2010) and have shown erosion- The oldest ages estimated from crater counts for the initial activity of al evolution in fossae first and outflow channels afterwards at Valles Arsia Mons range from 3.54 Ga (Werner, 2009) to 3.2 Ga (Neukum and Marineris on the opposite side of Tharsis with respect to Mangala Valles Hiller, 1981). The ages inferred for the terrains on which the Mangala (Leone, 2014). So it would be perfectly plausible that the same volcanic Valles are carved, including the ridge mapped as rough terrain (Hrt), processes at Valles Marineris occurred at Mangala Valles as well. A lava range from 3.59 to 3.77 Ga (Keske et al., 2015). The geo-chrono-strati- tube may have thus formed naturally during the eruptions that graphic map of Mars indicates these terrains as Hesperian (Tanaka et emplaced the flow fronts bordering Mangala Valles, at the edge of al., 2014) so there is a substantial agreement among these ages regard- Daedalia Planum, and may have then evolved into a fossa. The location less of the uncertainties related to the crater counts (Robbins et al., of the original lava tube can be easily tracked because it was along the 2013). There are several issues that must be taken into account when direction of Mangala Fossa heading into the wedge of the ridge (Fig. estimating absolute ages through crater counts (Leone, 2016): a) the re- 1b). This wedge was already interpreted as flooded by volcanic lavas duced flux of large impactors after the Late Heavy Bombardment (LHB) (Leverington, 2007) and as the source from which lava came out and (i.e. Bottke et al., 2007), if LHB ever happened (i.e. Morbidelli et al., moved to the east (Basilevsky et al., 2009). This interpretation was 2012), may have dramatically altered the clock and thus explain the ap- based on the observation at low resolution of some of the latest flow parently young lava flows of Arsia Mons from which Mangala Fossa fronts that seem to depart from the base of the ridge and move to the originated; b) the ages of the geo-chrono-stratigraphic map of Mars east (Fig. 4a). The observation of the high resolution CTX within the con- (Tanaka et al., 2014), for example, rely on a crater database that has text of the THEMIS imagery suggests that these lava flows came from high uncertainties with the quasi-circular depressions (Robbins and Daedalia Planum instead and: a) flooded and almost buried an already Hynek, 2012), there are many craters that are half-buried under the well formed Mangala Fossa (Fig. 3f); b) embayed the ridge dividing lava flows just beyond the ridge bordering Mangala Valles and Daedalia Daedalia Planum from Mangala Valles (Fig. 4i); and c) were even Planum (Leone, 2016), many more are surely buried under the flanks of bounced back by the ridge like the waves of the sea when reach a Arsia Mons making this volcano older than it appears; c) an evident ex- shore (Fig. 4i).Thisisconfirmed by a CTX mosaic of a section of the ample of such a discrepancy is also given in another region of Mars, the ridge that shows how the lava flows embay it and do not erupt from it Cumberland sample was dated 4.21 ± 0.35 Ga through K-Ar measure- (Fig. 4i). A global view of the main volcanic provinces of Mars shows ments with respect to a younger (3.7–3.5 Ga) formation age of Gale cra- how common are the ridges similar to those bordering Mangala Valles ter estimated through crater counts (Farley et al., 2014). Leone (2016) and how their more or less arcuate patterns follow the borders or are showed how the lava flows coming from Tyrrhenum Mons reached, even part of huge lava fields (Fig. 8). Thus, from the overall observation breached, and filled Gale crater via Farah Vallis finding an exit via Peace Vallis through the erosional mechanism explained by Hurwitz et al. (2010). So the age of the Gale lava filling might be strongly indic- Table 1 ative of the age of Tyrrhenum Mons' activity. These issues raise the in- List of the recommended changes in the units proposed by Keske et al. (2015) as imaged in fl Fig. 4c. triguing hypothesis that even the youngest lava ows of Daedalia Planum might not be as young as shown in the Tanaka et al. (2014) Figure of reference Current unit Recommended unit geo-chrono-stratigraphic map of Mars thus bringing the clock back in Fig. 4c Ast Asp3 time along the whole stratigraphic column. The Asp3/Asp4 volcanic Fig. 4c Ats Asp2 and/or Hrt unit of Mangala Valles was even dated between 0.4 and 0.3 Ga (Keske Fig. 4c ffc Asp2 or Asp3 et al., 2015). Such a relatively recent formation age for the volcanism G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 75 was also suggested for some late episodes on the flanks of Arsia Mons mineralogical evidence that will be discussed in the next section will dated with the crater counts (Werner, 2009). However, such young further show how the climate of Mars was evidently not liquid water ages are quite difficult to reconcile with both the thermal history and friendly since the Noachian. The low pressure of the Martian atmo- the thick crust of the planet. The observational evidence shows that sphere does not even allow the stability of dry ice or water ice that sub- the Asp3 lava flows were channelized into Mangala Fossa through limate at middle-low latitudes despite the low temperatures and the Daedalia Planum, so it is difficult that the volcanism of Arsia Mons presence of salts observed at the Phoenix landing site (Smith et al., might have been prolonged until the Amazonian (Leone, 2016). The 2009). Only the very low temperatures at the polar caps allow stability self-consistent hypothesis of formation for the Martian dichotomy and of ice on the surface. HiRISE observations of craters exposing ice have the consequent onset of volcanism (Leone et al., 2014), validated by shown total instability at equatorial latitudes where the Mangala Valles the discovery of twelve volcanic alignments (Leone, 2016), suggests are located and time limited stability above 40–50° of latitude (Dundas that the volcanic activity of Arsia Mons and Sirenum Mons, from et al., 2014). which the units Asp2 and Asp3 within Mangala Valles derived, peaked in the Pre-Noachian (4.5–4.1), declined in the Noachian (4.1–3.9 Ga), 6.3. Analysis of the mineralogy and implications for water and completely ended in the Hesperian (3.5 Ga ago at the latest). Keske et al. (2015) estimated a volcanism of Amazonian ages through The analysis of isotopic data from carbon, nitrogen, and argon sug- crater counts but they did not make any correlation to the thermal his- gested a big loss of volatiles during the first 500 Ma (Gillmann et al., tory of Mars to see if there is any consistency. The decline of Martian vol- 2011; Lammer et al., 2013; Kurokawa et al., 2014; Krasnopolsky, 2015; canism and the consequent shutdown of its magnetic field (~4.1–4.0 Ga Villanueva et al., 2015). Such a loss is consistent with an acute phase ago; Acuña et al., 1999; Lillis et al., 2008; Williams et al., 2008)revealthe of volcanic degassing coeval to the strongest phase of volcanism that transient nature of the internal heat of the planet. This view is supported Mars ever had as consequence of the SPGI (Leone, 2016). The water by other thermal models that suggested Hesperian as maximum limit degassed is the sum of the water originally contained in magma coming for volcanism on Mars (Hauck and Phillips, 2002; Grott et al., 2005) from the mantle plus the water encountered by magma along the rise in with an exception extending until Amazonian (Baratoux et al., 2011). the crust. This loss is supported by authors who suggested that early However, Baratoux et al. (2011) suggested that a thick crust would pre- Mars (4.5 Ga ago) had a global equivalent layer of 137 m available to vent volcanic activity even in case of a still internally hot Mars. A thick fill the lowlands with an ocean (Villanueva et al., 2015). However, the crust is a plausible hypothesis for West Tharsis where Mangala Valles argument that all this degassed water formed an ocean in the lowlands is located (Williams et al., 2008). Based on the southern polar giant im- is not supported by the widespread presence of unaltered olivine depos- pact (SPGI) model ages of Sirenum Mons, Sirenum Tholus, and Arsia ited by that same coeval early volcanism, a global map shows that both Mons (Leone, 2016), volcanic centres which contributed to the forma- the lowlands and the highlands have strong predominance of unaltered tion of the Asp2 flows (Fig. 1), and on the crater count ages of Xiao et olivine (Ehlmann et al., 2010). The time necessary for the alteration of al. (2012), I would place the Asp3 unit that formed the Mangala Fossa olivine into serpentine due to contact with water is restricted to a nar- – Mangala Valles system between 4.05 and 3.99 Ga (see Table 2). A row window between 100 and 10 k years (Oze and Sharma, 2007; later formation is excluded because lava tubes do not cross solid obsta- Stopar et al., 2006). The widespread presence of unaltered olivine in No- cles. Given that the Asp3 unit of Mangala Fossa and Mangala Valles achian volcanic terrains (McSween et al., 2006; Rogers and Bandfield, crosses both Asp2 and Hrt units, and that Asp2 unit crosses the corridor 2009; Ehlmann et al., 2010), including Mangala Valles, rules out the op- made by Hrt unit, the conclusion is that Asp3 follows both Asp2 and Hrt. tion of recharge through atmospheric water (i.e. rain), even for limited So a lava tube formed during the Hrt eruptive event was later exploited warm periods of 10 ka obtained through favourable obliquity (Jakosky by the Asp3 eruptive event to form and enlarge Mangala Valles. Calcula- et al., 1995; Laskar et al., 2004). Also problematic for the hypothesis of tions in Section 6.4 will show how the timescale of the eruptive events is periodic fluvial events alternate with volcanic events at Mangala Valles much shorter than absolute geologic age intervals. So the events that are: a) the visual lack of those volcanic features (i.e. maars, tuff rings, formed Hrt, Asp2, and then Asp3 units are essentially coeval on a geo- and rootless cones) typical of the interactions of lava with ice that a flu- logic timescale, although the Asp3 event followed the other two events. vial event should leave in the interstitial pores of the ground; b) the lack Being all this volcanism older than Hesperian and Amazonian ages, re- of a mechanism of water replenishment (Leverington, 2011) and tec- spectively, the letters H and A will be both changed into the N of Noachi- tonic faulting at the head of Mangala Valles to support multiple flooding an in Table 2. Noachian is conservatively chosen as trade-off between episodes suggested by Tanaka and Chapman (1990);c)andtheabsence Pre-Noachian and Hesperian because also supported by the crater of evidence for a sudden removal of a volume of terrain more or less counts ages of Xiao et al. (2012). A prolonged volcanic activity from equivalent to the volume of Mangala Valles to support even a single ep- Pre-Noachian to Amazonian appears unsustainable for the above-men- isode of flooding suggested by Bargery and Wilson (2011). tioned thermal models and obviously unlikely. Keske et al. (2015) even The few serpentine observed at Nili Fossae and other few locations placed the putative fluvial activity (Ags unit older than Asp3) of on Mars, but not Mangala Valles, must have been formed in the shallow Mangala Valles in Amazonian times when the climate of Mars already subsurface where water was still at pressures high enough to circulate reached its cold and dry current state. A wet climate found only at in 400–600 °C hydrothermal fluids (i.e. Evans, 2004, 2010), and then Mangala Valles is unlikely and in stark contrast with the available cli- erupted with lava onto the surface. Furthermore, the close association mate models that lean towards globally dry and cold (Forget et al., of unaltered olivine with jarosite found at the Opportunity landing site 2013; Wordsworth et al., 2013; Wordsworth et al., 2015). The in Meridiani Planum (Madden et al., 2004) and at the Curiosity landing site in Gale crater (Vaniman et al., 2013) suggests a plausible scenario Table 2 that jarosite may have formed directly from the small amount of Renamed units with reassessed ages based on the reference ages of Sirenum Mons and water contained in fluid lava and then preserved by a dry environment Sirenum Tholus (Xiao et al., 2012; Leone, 2016). Nrt to Nsp3 might be coeval units from immediately afterwards. Glauconite, often associated to carbonates and 4.05 to 3.99 Ga ago. gypsum and believed to form at surface temperature on Earth (Hillier, Revised ages Old name New name 1995), was not found on Mars and only prehnite provided some evi-

4.05–3.99 Ga Hrt Nrt dence of hydrothermal alteration in the subsurface (Ehlmann et al., 4.05–3.99 Ga Asp2 Nsp1 2011). So it is reasonable to infer that most of the water was degassed 4.05–3.99 Ga AHv Nsp2 and was lost to space (Kurokawa et al., 2014; Villanueva et al., 2015) 4.05–3.99 Ga Asp3 Nsp3 with scarce or none available at liquid state to flow on the surface of 3.99–3.97 Ga lHt Nsp4 Mars. The unaltered jarosite excludes that a putative ocean lasted for 76 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 less than 100 years or 10 ka because it rapidly forms iron oxyhydroxides 20,000 km3 of eruption volume suggested by Leverington (2007).This in humid climates (Madden et al., 2004). Even the study of the valley result does not also keep into account the volume that has been filled networks watersheds on the highlands suggested an arid to hyper- by the latest lava flows that draped the bed-floor of the channels, arid Noachian paleoclimate (Irwin et al., 2011). which would increase the total volume that must have been carved by the putative previous water. Now, considering the two-three orders of 6.4. Volumetric calculations of Mangala Valles and implications for water magnitude of water required for the erosion (Andrews-Hanna and Phillips, 2007; Leverington, 2011), it is clear that even the highest esti- Keske et al. (2015) suggested a single fluvial event caused by dike in- mate of ~40,000 km3 of water volume made by Leask et al. (2007a) trusion between 800 and 700 Ma ago, with a recurrence of alternate vol- and ~50,000 km3 by Ghatan et al. (2005),regardlesswhether canic and fluvial episodes. The hypothesis of the single fluvial event is discharged in 46 or 90 days, or 17.9 h, is impossible to store in the lim- not new in the literature (i.e. Ghatan et al., 2005; Bargery and Wilson, ited volume of Mangala Fossa and keep liquid for such a long time in the 2011) but the sources of water are always hypothetical, still given for low pressure environment of Mars. Thus, the only way to accommodate granted, and not supported by the above-mentioned mineralogical the permeability of the aquifer is a mechanism of replacement for water. and environmental arguments. Leone (2014) showed why chaos ter- However, it is very difficult to imagine a mechanism of replacement for rains are not an unequivocal morphological evidence of collapse for the additional water required at the hydraulic head of Mangala Valles in melting of ground ice because they can be related to erosion of flowing the liquid water unfriendly Martian environment. The hydraulic head of lava. Except for polar areas and middle-high latitudes, it is very hard to Mangala Valles is topographically too high for the model of recharge of find a clear and direct evidence of glaciers or underground water ice at the global groundwater system by basal melting of ice-rich polar de- equatorial latitudes (Cutts and Blasius, 1981; Dundas et al., 2014; posits (Carr, 2002; Leverington, 2011) and ice is totally unstable at Brough et al., 2016). Other authors (i.e. Souness et al., 2012; Levy et equatorial latitudes (Dundas et al., 2014). Last but not least, the hypoth- al., 2014) have proposed debris aprons as glacier-like forms (GLF). De- esis of Mangala Fossa as potential graben that could accommodate the bris aprons are not direct evidence of glaciers but may also be the result subsidence following the removal of underground ice is not viable. of landslides formed by dry or lava-induced mass wasting processes Due to the topographic gradient decreasing from east to west along (McEwen, 1989; Soukhovitskaya and Manga, 2006; Bigot-Cormier and the fossa, the effective length of the Mangala Fossa that could ideally Montgomery, 2007; Leone, 2014; Leone, 2016). Another recent attempt contribute with water (ice) to form Mangala Valles is the ~50 km seg- to provide some evidence of underground water ice using the sounding ment between the Notch and the ridge where the pit chain ends. radar instrument (SHARAD) of the MRO (Karlsson et al., 2015)pro- Based on topographic measurements with available MOLA data, such a duced ambiguous results because the dielectric constant for water segment is 1 km deep and 6 km wide at maximum for a total volume used in the experiments is very similar to that of dacitic lava or porous of ~300 km3. This volume is very far away even from the minimum vol- basalt (Russell and Stasiuk, 1997); porous basalt could have been plau- umes estimated for Mangala Valles. Even assuming the total length of sibly formed by the above-mentioned volcanic degassing. Although the 220 km considered by Ghatan et al. (2005), which is not at all actively interpretation of the lava flows is shared and well supported in Keske et contributing of course, the total volume would be 1320 km3 and still al. (2015) paper, the occurrence of fluvial events mentioned above in very far away from the volumes estimated for Mangala Valles. The anal- the Keske et al. (2015) and in other work (Tanaka and Chapman, ysis of the mechanical erosion by lava, based on similar calculations 1990; Wilson and Head, 2004; Ghatan et al., 2005; Leask et al., 2007b; done for Athabasca Valles (Keszthelyi et al., 2014), provided numbers Basilevsky et al., 2009; Bargery and Wilson, 2011) still remains prob- more consistent with the volumetric measurements of Mangala Valles lematic in the environment of Mars which is unfriendly to liquid instead. Given that no more of 10% of the erupted volume is water both today and in the past. entrained/eroded rock (Keszthelyi et al., 2014), a minimum volume of Leask et al. (2007a) based their volumetric estimates of the channels 180,000 km3 of basaltic lava must have been erupted to carve the upon those made by Tanaka and Chapman (1990), who indicated a total 18,000 km3 of Mangala Valles. Lava was mostly discharged to Amazonis eroded volume of 3 × 1012 m3. This result was obtained assuming an av- Planitia with a minor amount draping the channels (i.e. Figs. 3c, e, 5d, erage depth of 100 m by 30 km of width over a total length of 1000 km 6g, h, 7g, and h). Assuming a laminar flow of an average thickness of for the Mangala Valles-Fossa system. Available MOLA profiles, where 100 m inferred from the thickness of the lava deposits still draping the crossing the channels, revealed a range of depths spanning an average channel, a density of 3000 kg/m3 for basaltic lava (i.e. Leone and of 200 m from the upper course of Mangala Valles to Labou Vallis and Wilson, 2001), a viscosity of 10,000 Pa s, an average half-filled 5 km of an average of 800 m from Labou Vallis to the mouth of Mangala Valles diameter lava tube/feeder (inferred from the width of the pit chain) with a upper limit of 1000 m at the source and at the mouth. A survey crossing the ridge along Mangala Fossa can provide a rough flow rate based on available CTX images has returned some reliable results on of ~0.272 km3/s, based on a flow velocity of 13.875 m/s calculated the average width of the Mangala Valles channel system. Starting from with the combined use of the Jeffreys and Manning equations the conservative estimate of the 5 km wide pit chain on the ridge (Keszthelyi and Self, 1998; Harris et al., 2007). Assuming a single epi- along Mangala Fossa (Fig. 3b) and the 5.5–6 km wide Notch (Fig. 3a), sode, this means that the 180,000 km3 of lava would have been erupted the main channel of Mangala Valles enlarges progressively reaching a in about 661,764 s, roughly corresponding to 7 Earth days. Assuming a maximum of 165 km in the TESS sector described in Fig. 4. Then the turbulent regime the flow velocity would be 8.6 m/s on the basis of a main channel progressively narrows to 25 km measured at the source single slope value of 0.01, although slope is variable along Mangala of Labou Vallis (Fig. 5d). At this point, slightly more than 500 km have Fossa and Mangala Valles, and on an average friction coefficient of been run with an average channel width of ~100 km. From here to the 0.05 estimated from various eruptions (Baloga et al., 1995), the flow mouth of Mangala Valles the main channel splits in a network of sub- rate would have been 0.157 km3/s with an episode lasting 1,146,496 s, channels of variable width from the 4 km of Sabis Vallis (Fig. 6g) to roughly corresponding to 13 Earth days. In any case, it would have the 6 km of Minio Vallis (Fig. 6h). For practical reasons, from this been a longer episode compared to the 17.9 h of time estimated by count are even excluded many other sub-channels less than 3 km of Bargery and Wilson (2011) for their catastrophic release of water. Al- width that, of course, also provide their volumetric contribution. A sim- though these are still rough estimates based on parameters that have ple sum of all these estimates gives a total conservative volume of some degree of uncertainty, the order of magnitude of time required roughly 18,000 km3, 6 times the 3000 km3 estimate made by Tanaka for the eruptive episode forming Mangala Valles is not terribly different and Chapman (1990) and nearly twice the 10,000 km3 trade-off from the 30 Earth days obtained for a smaller eruptive episode forming estimate by Leask et al. (2007a), but consistent with the 13,000– a 150 m deep mechanically eroded channel within an Elysium Planitia 20,000 km3 range suggested by Ghatan et al. (2005) and the impact crater (Hurwitz et al., 2010) or the 580 Earth days estimated G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 77 for Mangala Valles by Leverington (2007). This might thus explain why d) The volume of water required to carve Mangala Valles is now com- the ~900 km long Mangala Valles are still small if compared to the main pared to the volume of the source and no match was found; replenish- circum-Chryse outflow channels (Kasei Valles and Valles Marineris), ment for multiple episodes would require rain, which is at odds with otherwise it would have been much larger if the influx of lava was predominant worldwide unaltered olivine and jarosite, or basal re- higher or lasted longer. Mars had an unusually strong volcanism with charge of aquifers, which is impossible at the height of Mangala Valles very high eruption rates because of the giant impact that formed the di- head; there is neither presence of collapse features at the Notch nor chotomy (Leone et al., 2014) and twelve volcanic alignments (Leone, maars or tuff rings in the whole section of Mangala Fossa between 2016), which also explains why the volcanic features are much larger the ridge and the Notch able to justify the volume of Mangala Valles than those seen on Earth (Wilson and Head, 1994). There is no other and volcano ice explosive activity; I conclude that no study has con- comparable example anywhere in the solar system of such high erup- vincingly demonstrated the presence or the sudden removal of tions rates and of so large and aligned volcanic edifices like those seen ground ice at the source of Mangala Valles. on Tharsis or elsewhere on Mars (Leone, 2016). e) Both heat flux and thick crust of the planet were unable to support prolonged volcanism until the Amazonian; the many inconsistencies 7. Conclusions in the superposition relationships and the high degree of uncertainty related to the crater counts cast doubts on the ages of the geological A volcanic origin for all the geological units was interpreted from the units proposed by Keske et al. (2015). geomorphological analysis of both Mangala Fossa and Mangala Valles through the use of MRO images, combined with volumetric calculations The geological unit Asp3 mantling the floor of Mangala Fossa was al- obtained from MOLA data, and from the careful observation of all the ready interpreted as lava flowing into a graben following the gradient of possible sources and types of the geological units. It is unlikely that topography (Keske et al., 2015) and such interpretation is partially water has ever been involved as main erosive agent for the formation shared in this paper, Mangala Fossa is a channel formed by lava erosion of Mangala Valles. No rootless cones or maars are visible all along and not a graben. The unit Asp3 refers to the last of the youngest flows Mangala Valles and Labou Vallis thus confirming no previous presence that essentially formed Mangala Valles, although not absolutely the of groundwater (ice) left by previous putative fluvial activity. Some youngest in time. The Asp3 unit eroded the pre-existing Asp2 unit, rootless cones were seen at Athabasca Valles (Keszthelyi et al., 2014) emplaced in the corridor formed by two lava flow fronts (unit Hrt) com- and rarely at Valles Marineris (Leone, 2014) but none at Mangala Valles. ing from Arsia Mons. Sirenum Mons and Sirenum Tholus likely contrib- There is objectively no way to demonstrate previous presence of any uted together with Arsia Mons to the origin of the lava flooding that water or underground ice at Mangala Valles because there is nothing formed the unit Asp2 (see black arrows in Fig. 1). So the ages of forma- that may remotely recall any lava-water interaction out there. Lava is tion of these volcanic centres were taken as a point of reference for the the only visible and documented presence all along Mangala Valles ages of the units in the new reassessment. The Noachian ages of from the source to the mouth. The calculations of the volumes and the Sirenum Mons and Sirenum Tholus suggested in this work come from erosion rates of lava gave realistic and better results than water. Lava Xiao et al. (2012) whereas the age of Arsia Mons from the ages of the is stable in the environment of Mars whereas water not. The widespread alignment to which all these volcanic centres belong as consequence mineralogical evidence of unaltered olivine and jarosite in Noachian ter- of the SPGI (Leone, 2016). The ages in Table 2 arejustindicativeinter- rains of the equatorial regions, where the outflow channels (and vals of time in which the eruptive events related to the correspondent Mangala Valles) are located, suggests that water did not flow on the sur- units may have occurred. From the superposition relationships of the face since the past. Assuming that water was flowing or ponding in the volcanic units described so far, a possible sequence of events (from past from the presence of phyllosilicates is unjustified, both under- the oldest to the youngest) might be as follows: ground and surface volcanic activity can explain their formation. This evidence strengthens the hypothesis for a volcanic-only origin of • Lava flows from Arsia Mons emplaced the Hrt unit, which formed the Mangala Valles and the conclusions listed here: original lava tube (that after became Mangala Fossa) within the ridge that divides Daedalia Planum from Mangala Valles; lava flows from a) Mangala Valles originated from Mangala Fossa through the only Arsia Mons emplaced the Asp2 unit also with the contribution of source observed in the whole area: volcanic activity of Arsia Mons Sirenum Mons and Sirenum Tholus; these are the oldest units and formed a lava tube and the ridge that divides Mangala Valles from may have occurred during the Noachian (from 4.05 to 3.99 Ga ago) ac- Daedalia Planum; the presence of the lava tube is inferred from a cording to the ages estimated by Xiao et al. (2012) and Leone (2016). pit chain still visible on the ridge; the ridge is plausibly a lava flow • Lava flows from Arsia Mons emplaced the AHv unit, which is super- front thick enough to prevent total collapse of the roof along its sec- posed on the Asp2 unit near Amazonis Mensa, and embayed the Hrt tion, similar ridges have been observed around the main volcanic unit (from 4.05 to 3.99 Ga). centres on Mars (Fig. 8). • Lava flows from Arsia Mons emplaced the Asp3 unit (superposed on b) The original lava tube then evolved in Mangala Fossa upon erosion by AHv), enlarged the original tube, and formed Mangala Fossa, including lava; lava flowing on the surface, aided by both decreasing topography the breakout that flooded and carved Mangala Valles (from 4.05 to from south to north and from east to west carved Mangala Fossa, 3.99 Ga). formed Mangala Valles as a breakout of Mangala Fossa and Labou • New flows, corresponding to the lHt unit in the map of Keske et al. Vallis as a breakout of Mangala Valles, opened its way through pre- (2015) and superposed on the AHv unit, flooded and partially covered existing impact craters, and at last discharged into Amazonis Planitia; the Asp3 unit at Amazonis Mensa (probably from 3.99 to 3.97 Ga). it is demonstrated that fluid lava can travel very far from its sources even on cold planetary surfaces characterized by low to very low at- The sequence of emplacement that comes from the new interpreta- mospheric pressure, the same cannot be demonstrated for water. tion should be as follows (from the oldest to the youngest): Hrt, Asp2, c) The geomorphological analysis of Mangala Fossa, supported by the AHv, Asp3, and then lHt. So, I preferred to rename these units according analysis of MOLA profiles all along its length, has shown neither to a timely order, again from the oldest to the youngest with the correct vertical or horizontal movements nor lateral lava flows that may sug- age letter in Table 2: Hrt becomes Nrt; Asp2 becomes Nsp1; AHv be- gest a tectonic origin or a dike rupturing to the surface; topography comes Nsp2; Asp3 becomes Nsp3; lHt becomes Nsp4, so that the origi- determined the straightness of Mangala Fossa as well as determined nal AHv unit of Keske et al. (2015) on Daedalia Planum is now split into straight sections of Valles Marineris and Kasei Valles on the opposite Nsp2 and Nsp4. These renamed units were mapped in Fig. 9.The flank of Tharsis. bedfloor of Mangala Fossa, Mangala Valles, Labou Vallis, Sabis Vallis, 78 G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80

Fig. 9. Geologic map of the Mangala Valles – Mangala Fossa – Daedalia Planum system; the map focuses mainly on the location of the geologic units and gives only an indicative time interval because the ages based on crater counts have high uncertainty. and Minio Vallis is now included in the Nsp3 unit. No one of all the flu- showing the volcanic origin of the outflow channels (Valles Marineris, vial units interpreted by Keske et al. (2015) is confirmed in the new in- Kasei Valles, Athabasca Valles, and Mangala Valles), the striking evi- terpretation because all the units have volcanic origin. The geologic map dence of large volcanic provinces associated with outflow channels on in Fig. 9 does not carry any structural information because there are no Mars, all the claims of fluvial activity on Mars must be seriously tectonic features to report, the previously interpreted wrinkle ridges reconsidered. might essentially be lava flow fronts as it was also concluded by Leone (2016). The indicative geologic ages reported here are based on the cra- Acknowledgements ter counts and the SPGI modelling found in the literature. Noachian age for the geologic units is a conservative estimate, based on SPGI model, The author acknowledges the support of the ETH Research Commis- but even Pre-Noachian age cannot be excluded. Reliable ages can only sion grant ETH-03 10-1 for the completion of this work. Gaetano Di be estimated with accurate geochronologic measurements in situ as Achille has contributed with helpful discussions to clarify the current done at Gale crater by the Curiosity lander (Farley et al., 2014). Hence- views about water on Mars. Two anonymous reviewers and the editor forth, Mangala Valles and Mangala Fossa should be considered as lava Malcolm Rutherford have contributed with their helpful comments to channels formed by mechanical and thermal erosion by lava and thus the improvement of the original version of this manuscript. of volcanic-only origin with no water involved. Lastly, SHARAD experi- ments done in several locations of Mars have shown how radar cannot References distinguish porous lava from water because they have similar dielectric constant. In no case presence of glaciers and/or glacier-like landforms Acuña, M.H., Connerney, J.E.P., Ness, F.N., Lin, R.P., Mitchell, D., Carlson, C.W., McFadden, J., Anderson, K.A., Rème, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. was observed at the locations of the radar experiments. After the origi- Global distribution of crustal magnetization discovered by the Mars Global Surveyor nal works following the Viking missions, the growing number of works MAG/ER experiment. Science 284 (5415), 790–793. G. Leone / Journal of Volcanology and Geothermal Research 337 (2017) 62–80 79

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