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10 Floods from fossae: a review of -aged extensional–tectonic megaflood channels on

DEVON M. BURR, LIONEL WILSON and ALISTAIR S. BARGERY

Summary and from unnamed fractures on and the The four youngest megaflood channels on Mars – aureole (Mouginis-Mark and Christensen, , , Grjota´ Valles and Athabasca 2005). These channels show that aqueous flows from fossae Valles – date to the Amazonian Period and originate at fis- have occurred throughout volcanic provinces on Mars. The sures. The channels show common in-channel morpholog- focus of this chapter is megaflood flow; thus, this review ical indications of flood activity (streamlined forms, lon- focuses on the four largest known and best investigated to gitudinal lineations, scour), as well as evidence for vol- date of these channels. These four channels are roughly canic, tectonic, sedimentary and/or glacial/ground ice pro- an order of magnitude smaller in each dimension than the cesses. The fissure sources and channel termini have varied largest flood channels, the -aged circum-Chryse expressions, suggesting various triggering mechanisms and channels (Coleman and Baker, this volume Chapter 9), fates for the floodwaters. Possible triggering mechanisms but still discharged geomorphically significant volumes of include magmatic processes (dyke intrusion), tectonic pro- water. cesses (extensional faulting) and a combination of both Because the youth of these megaflood channels is types of processes. Surface morphology suggests that each noteworthy, this chapter starts with a brief overview of age- of these mechanisms may have operated at different times dating of young features on Mars before then giving the and locations. Upon reaching the surface, the water likely inferred ages of these tectonic megaflood channels. Next, would have fountained at least a few tens of metres above the morphology of each channel is reviewed, including the surface, producing some water and/or ice droplets at the source morphology, in-channel bedforms, any inferred the fountain margins. The likely sources of the floodwater flow characteristics, and possible floodwater sinks. Then a are subsurface of a few kilometres’ thickness and synopsis is provided of the possible mechanisms that may a few tens of degrees Celsius in temperature. have triggered groundwater release from fossae and led to surface flood flow. Finally, some thermal and mechanical 10.1 Introduction aspects of the inferred groundwater movement to and within Megaflooding on Mars has varied in origin and the source fossae are discussed. amount throughout the history of the planet. During the Period, the most ancient period, flooding orig- 10.2 Ages inated from crater basins (Irwin and Grant, this vol- 10.2.1 Issues related to dating of Amazonian-aged ume Chapter 11). During the Early Hesperian Epoch, channels megafloods originated at often set within The most recent period on Mars is the Amazo- chasmata (Coleman and Baker, this vol- nian (Tanaka, 1986), lasting from ∼3.0 Ga to the present ume Chapter 9). During the Amazonian Period, the most (Hartmann and , 2001). The absolute dating of the recent period, megaflooding originated from fossae pro- surface features on Mars is accomplished by comparison duced by extensional tectonism. of actual size–frequency distributions with This chapter provides a review of the four megaflood model size–frequency distributions for fixed ages, referred channels originating at fossae that experienced flow dur- to as ‘isochrons’ (e.g. Hartmann and Neukum, 2001). Crater ing the Amazonian Period. These megaflood channels are counts in support of dating a flood in a channel are per- the largest examples of a suite of aqueous flow channels formed on terrain in the channel interpreted to have been that originate at volcanotectonic fissures. created or modified by the floodwaters, e.g. flood-scoured originate from the Elysium Fossae on the flanks of Ely- channel floors. The actual size–frequency distributions of sium Mons (Mouginis-Mark et al., 1984). Other smaller impact craters on channel floors indicate the age of only the channels originate at the Ceraunius and Olympica Fossae most recent, geomorphically effective flood. This supposi- in the northwest region (Mouginis-Mark, 1990), tion is founded upon the fact that floods are ‘self-censoring’,

194 Megaflooding on and Mars, ed. Devon M. Burr, Paul A. Carling and Victor R. Baker. Published by Cambridge University Press. C Cambridge University Press 2009. Floods from fossae on Mars 195 i.e., more recent, larger floods have been shown to erase the channel and no evidence for exhumation has been deduced sedimentological evidence of previous smaller floods (e.g. for other Amazonian-aged fissure-headed channels. House et al., 2001). Likewise on Mars, previous smaller Bearing in mind these issues with regard to age- floods would not be visible in the channel floor cratering dating of young channels, the ages of each of the four record, although the existence and ages of previous floods largest tectonic outflow channels are now discussed. may be indicated by flood terraces (Berman and Hartmann 2002, Burr et al., 2002b). Ideally, to derive the age of a 10.2.2 Channel ages flood from crater counts, a flood should have erased all Mangala Valles The oldest and largest tectonic outflow craters completely but this may not always be the case (see channel is Mangala Valles (Plate 23). The most upslope e.g. Figure 13 of Burr et al., 2002b). southern end of Mangala Valles coincides with the Mem- A second issue with regard to age-dating of young nonia Fossae, flat-floored features inferred to be a graben flood channels is the uncertain effect of secondary system formed at the coincidence of dyke emplacement and craters. Secondary craters are those formed by impact normal faulting (see Hanna and Phillips, 2006). Flooding of material thrown out from the surface of a planet by a down Mangala is inferred to have coincided with faulting (larger) primary impact. Because secondaries are smaller on the Memnonia Fossae on the basis of crater counts and than primaries, they are erased more quickly; thus, they stratigraphic relationships (Tanaka and Chapman, 1990) as affect the age-dating primarily of the young surfaces mapped at 1:500 000-scale on Viking images (Chapman of Mars, where they are subject to the least amount and Tanaka, 1993; Craddock and , 1994; Zimbel- of erasure and where larger craters are not present for man et al., 1994). Two periods of coeval faulting and flood- use in age-dating. The discovery of efficient secondary ing originally were inferred from Viking data, the first dated production in young flows on Mars (McEwen et al., as late Hesperian age and the second originally dated at the 2005) raises the question of whether Martian isochrons, Hesperian–Amazonian boundary (Tanaka and Chapman, derived from and tied to radiometrically dated lunar 1990). Subsequent analysis on improved images interpreted samples, properly account for secondary crater production the youngest materials in Mangala Valles as Amazonian in (see McEwen and Bierhaus, 2006, for a review). Because age, based on superposition relationships and lack of craters of significant secondary production, dates of the inferred greater than 2 km in diameter (Zimbelman et al., 1992). youngest flood channel have varied by up to two orders of magnitude (compare the ages of in Burr Marte Vallis Marte Vallis (Plate 24) is the largest and et al., 2002b and McEwen et al., 2005). However, even oldest of the three Amazonian-aged outflow channels that with this variance, this channel still retains an age-date surround the plains, where it is located at the of late Amazonian. Older surfaces should have still less eastern end of the plain (Burr et al., 2002b). The channel error and, overall, age-dating of features to within the originally was interpreted as being continuous with one or Amazonian Period appears robust (Hartmann, 2005). Each both of the other two Cerberus plains channels (Tanaka and of the four fissure-headed channels discussed below has Scott, 1986; Berman and Hartmann, 2002, Figure 1; Ples- been inferred to have seen flooding during the Amazonian cia, 2003). However, topography from the Mars Orbiter Period (see next section, ‘Channel ages’). Laser Altimeter (MOLA; et al., 1998) and crater A final point about the ages of these channels is the counts on the channel floor both suggest that the three Cer- possibility of channel exhumation. The documentation of berus channels are spatially and temporally distinct (Burr extensive layered terrain on Mars (Malin and Edgett, 2000) et al., 2002b; Berman and Hartmann, 2002). Marte Vallis and of exhumation of inverted fluvial channels from within has been dated by counting craters on the rock record (Williams and Edgett, 2005; Williams (MOC) images (Malin and Edgett, 2001) of both the chan- et al., 2005) suggest that Martian outflow channels also nel floor, inferred to date to the last flood, and the dark lobate could be exhumed. Extensive burial and partial exhuma- material that embays much of the channel, inferred to date tion of one fissure-headed channel (Athabasca Valles) has since the last flood. These crater counts produced similar been argued on the basis of inferred scour upslope of its ages spanning a few to ∼200 Ma (Berman et al., 2001; Burr source (Edgett and Malin, 2003). In such a case, the derived et al., 2002b; Berman and Hartmann, 2002). model crater-count age for the channel floor would reflect an exposure age, not a formation age. However, in this Grjota´ Valles A previously unnamed channel to the north particular case, the scour has an alternative interpretation, of the Cerberus plains (Plate 25) (Burr et al., 2002b) has namely, as an effect of floodwater gushing upslope from been named Grjota´ Valles (Plescia, 2003). Age-dating of the fissure due to the force of the water eruption (Burr the Grjota´ Valles floor is affected by its weathered char- et al., 2002b; Head et al., 2003; Manga, 2004). Evidence acter, its indistinct flood boundaries, and the presence of for the proposed exhumation is not apparent within this some aeolian (Burr et al., 2002b). Despite these 196 Devon M. Burr et al. factors, crater counts on MOC images of inferred flood- have tectonic sources, either fissures or graben. The chan- formed surfaces – i.e., longitudinally grooved surfaces nels also all show common in-channel morphological indi- adjacent to streamlined forms – fall along the isochrons cators of flooding, including streamlined forms, scour and of Hartmann (1999) and give an age of ∼10–40 Ma (Burr longitudinal lineations. However, the morphologies, both et al., 2002b). at their sources and at their termini, vary. Channel location, source and channel morphology, and possible floodwater Athabasca Valles Athabasca Valles (Plate 26) are at the sinks are discussed below. western end of the Cerberus plains. They have the most pristine geomorphology of all the Amazonian-aged, fissure- 10.3.1 Mangala Valles headed channels, and crater counts on the channel floor Centred near 15◦ S 210◦ E, Mangala Valles (Plate have yielded the youngest model ages. Crater counts on 23) are located in the southern highlands just of MOC images of the longitudinally grooved terrain in the the hemispheric dichotomy boundary of Mars. They are topographic channel give an age of 2–8 Ma (Burr et al., situated to the west of the Tharsis rise and to the south 2002b) based on comparison with the Hartmann (1999) of , and begin at an elevation of about isochron model. Other crater counts within the topographic 0 m. From this origination point at Mangala Fossa, one channel give an age of 3 Ma (Werner et al., 2003) based of the Memnonia Fossae, the channel stretches on the Neukum and Ivanov (1994, described by Hartmann ∼850 km northward to its dual outlets at the south side and Neukum, 2001) model. This 3 Ma unit is not charac- of the eastern (Plate 23). The terised morphologically but is attributed to volcanism; the upper (southern-most) ∼550 km of the channel is a single most recent flood erosion in that study is dated to 1.6 Ga, reach situated between north–south trending blocks with a possible last fluvial event at 30 Ma (Werner et al., (Zimbelman 1989). The most proximal ∼150 km show 2003). These geological units of Werner and colleagues primarily scoured terrain with a few smaller streamlined are defined on the basis of the match of crater counts forms, and the next ∼400 km show less scour with with model isochrons, not with morphological characteri- much larger streamlined forms behind impact craters or sation, nor are the locations of the areas counted described; other obstacles. The upper reach expands rapidly from together this makes it difficult to independently evaluate 5.5 km through a notch in an eroded impact crater north the origin of the units (see also McEwen et al., 2005, of Mangala Fossa to ∼50 km in width within a few p. 376). Widespread pitted mounds in the channel have been kilometres downstream. Channel depth varies from a few interpreted as hydrovolcanic rootless cones, which form by tens of metres to a few hundred metres, and the overall lava overrunning water-rich or ice-rich ground (Greeley slope of the upper reach is ∼0.0005 m m−1 (∼0.03◦). and Fagents, 2001; Lanagan et al., 2001; Lanagan, 2004; Downslope (northward) of this upper reach, the channel Jaeger et al., 2007). Because water is unstable near the sur- diverges into two separate branches. These two divergent face of Mars in the present climate, the presence of rootless branches are as narrow as approximately 10 km and as cones implies that lava emplacement was preceded closely deep as 1000 m in some locations. The northwestward by flooding (Lanagan et al., 2001; Lanagan, 2004). The branch has a number of in-filled impact craters along its age of 3 Ma is derived by Werner et al. (2003) from a MOC path, whereas the north-northeastward branch shows a image that shows features of the type interpreted by Lana- more anastomosing plan-view form. gan et al. (2001) and Lanagan (2004) to be rootless cones. The channel system was inferred to have been On this basis, an age of 3 Ma, if correctly attributed to formed by catastrophic flooding on the basis of stream- lava, would still imply flooding of a similarly young age. lined bars within the channel (see Carling et al.,this Subsequent investigation with Thermal Emission Imaging volume Chapters 2 and 3) and braided plan-view mor- Spectrometer (THEMIS) infrared (IR) images (Christensen phology (Milton, 1973; Sharp and Malin, 1975). The et al., 2003) and crater counts on MOC images showed that oblique orientation of the most proximal streamlined forms ∼80% of the craters on the Athabasca Valles grooved chan- (Tanaka and Chapman 1990) originally suggested that nel floor are secondaries (McEwen et al., 2005). Because water may have been released from more than one loca- this efficient secondary production called into question tion (Zimbelman et al., 1992). The most recent anal- the use of isochrons for young surfaces, the channel was ysis, using THEMIS and MOLA data (Ghatan et al., re-dated using statistical methods, with an age of 1.5– 2005), suggests that the groundwater filled and overflowed 200 Ma (McEwen et al., 2005). The possible exhumation the source trough, generating oblique overland flow, and of Athabasca Valles is discussed in the previous section. that this sheet flow then coalesced into channelised flow through the 5.5 km wide notch to the north side of the 10.3 Morphology source trough. The time scale for the initial trough fill- As shown above, all four known outflow channels ing may have been from as little as ∼2 hours (Leask that experienced megaflooding in the Amazonian Period et al., 2007) up to 107 hours (Ghatan et al., 2005), Floods from fossae on Mars 197 indicating the considerable uncertainty associated with these estimates. The context and morphology of Mangala Valles suggest a complex history providing multiple hypothe- ses. Early interpretations of Mangala Valles as an out- flow channel suggested the origin to have been breach- ing of a large surface reservoir or (Sharp and Malin, 1975). In subsequent analysis, the origination of the chan- nel at a fissure or graben radial to the Tharsis rise was interpreted as indicating that water release resulted from fault-induced cracking into a perched (Tanaka and Chapman, 1990). To other workers, the proximity of the channel to the Tharsis rise suggested floodwater generation by Tharsis magmatic melting of near-surface ground ice, which would have then migrated in the subsurface to the tectonic fissures to be released to flow down the channel (Zimbelman et al., 1992). Under this scenario, water release likely would have been artesian (Zimbelman et al., 1992). The most recent analyses of the Mangala Vallis source region by Ghatan et al. (2005) and Leask et al. (2006) also inferred tectonic tapping of a pressurised aquifer, with Tharsis being the most likely source of the groundwater. This most recent hypothesis builds on the global hydro- sphere model of Clifford (1993) and Clifford and Parker Figure 10.1. Portion of THEMIS visible image V23283003 located ◦ ◦ (2001), in which the crust of the planet is divided into the near 12.2 S 208.7 E showing pitted terrain in Mangala Valles, cryosphere (the uppermost portion that remains below the where irregular pits were interpreted to be the effects of ice blocks et al. water freezing temperature) and the hydrosphere (a sub- deposited by the flood flow (Zimbelman , 1992). North is up, illumination is from the left. cryospheric zone where liquid water accumulates), and on the interpretation of the graben as reflecting subsurface dykes (Wilson and Head, 2002). In this hypothesis, dyke ice blocks stranded during early stage sheet flow of melt- emplacement resulted in graben formation and cracking of water and ice, possibly in association with temporary ice the cryosphere, which released pressurised groundwater to dams (Zimbelman et al., 1992). Lobate ridges deposited the surface; the minimum duration of the flooding is esti- on the Mangala Fossa margins adjacent to the Mangala mated to have been 1–3 months (Ghatan et al., 2005; Leask Valles channel origin (Figure 10.2) have been interpreted et al., 2007). The interpretation of sub-parallel symmetric to be glacial deposits, with the glaciation being the conse- ridges around the eastern end of Mangala Fossa as dunes quence of floodwater release (Head et al., 2004). A candi- emplaced by a phreatomagmatic eruption plume resulting date opportunity for ice formation is the cooling of the thin from dyke emplacement, cryosphere cracking and magma– sheet of water that would have spread across the floor of the groundwater mixing (Wilson and Head, 2004) is consistent Mangala Fossa graben in the first stages of water release with this hypothesis. from graben-bounding fractures (Leask et al., 2007). Addi- As an alternative to the idea of a pre-pressurized tional surface ice formation could have occurred during the aquifer due to cryosphere growth, the high aquifer pore filling of the graben if the temperature of the released water pressures responsible for the flooding at Mangala Valles were less than 4 ◦C; the fact that water has its maximum may have been a direct result of the stress release during density at this temperature suppresses convection as the the tectonic event forming the graben (Hanna and Phillips, water is cooled from above by evaporation at its surface 2006). The presence of chaos within a crater near the Man- (Leask et al., 2007). gala Valles source graben may be a consequence of ele- The terminal pathways and eventual sinks for vated palaeopore pressures and thus provide circumstantial Mangala Valles floodwaters are ambiguous. According evidence consistent with this model (Hanna and Phillips, to the interpretation of Ghatan et al. (2005), the topo- 2006). graphic data suggest that the eastern branch formed first In addition to catastrophic aqueous flow, but was later pirated by the western branch, whereas phreatomagmatism and tectonism, the channel also shows previous crater statistics support the opposite sequence evidence of ice processes. The pitted proximal channel (Chapman and Tanaka, 1993). Ghatan et al. (2005) sug- floor (Figure 10.1) has been interpreted to be the result of gest that, as flooding subsided, residual water froze and 198 Devon M. Burr et al.

fan is similar to a terraced fan in Coprates that is interpreted as representing erosion and redistribution of fan material during lake-level drops (Weitz et al., 2006; see also Di Achille et al., 2006). Thus, this fan-shaped deposit may indicate ponding and episodic recession of Mangala Valles floodwaters within this crater. However, any path between the outlet of the northwestern branch terminus and this impact crater is largely covered by the Amazonian-aged Medusae Fossae Formation. In summary, morphology and modelling of Man- gala Valles suggest a tectonovolcanic release of pressurised groundwater resulting in a catastrophic water flood and possible glaciation during the late Hesperian to early Ama- zonian Epochs.

10.3.2 Marte Vallis In its current topographic expression, Marte Vallis originates near 180◦ E, 10◦ N at the eastern side of the Cer- berus plains at an elevation of approximately −3100 m. Sit- uated between the structure to the northwest and southern highland outliers to the southeast, Marte Vallis stretches first ∼250 km northeastward and then ∼750 km north-northeastward to Amazonis Planitia (Plate 24). The Figure 10.2. Portion of THEMIS visible image V06597003 located ◦ ◦ channel system is consistently broad, spanning several tens near 18 S 211.5 E, showing ridges around the source region of Mangala Valles, interpreted to be glacial deposits (Head et al., of kilometres in width, and a few tens of metres deep, with ∼ ◦ 2004). North is up, illumination is from the left. an overall slope of 0.0002 m/m (0.013 ). It is embayed by a dark lobate material interpreted as lava (Plescia, 1990). This dark embaying material surrounds a number sublimated, leaving a residue in the deepest parts of the of brighter streamlined forms. The low, contrasting albedo channel. of this embaying material caused the channel’s stream- Additional sinks for the floodwater are suggested lined forms and anastomosing morphology to be readily by morphology and may have been different for the visible in Viking images, leading to its early identification northeastern and northwestern branches. The northeast- as an outflow channel (Tanaka and Scott, 1986). Multiple ern branch terminates at the southern side of a broadly levels along the sides of Marte Vallis are interpreted as level plains unit surrounding Medusae Fossae Formation terraces (e.g. Berman and Hartmann, 2002), which may deposits (Tanaka and Chapman, 1990). Approximately reflect multiple flood events (Burr et al., 2002b). Pirating 300 kilometres to the north, a group of streamlined forms of anastomosing channels within Marte Vallis as inferred with impact craters or knobs at their southern ends is appar- from MOC images also suggests at least two episodes of ent in images and MOLA topography (centred near 1◦ N flooding (Fuller and Head, 2002). The (post-embayment) 207◦ E). The kilometre-scale size and large width:length depth of the channel is only on average a few tens ratio of these forms distinguish them from the aeolian of metres. streamlining (yardangs) that overprints this region and sug- Marte Vallis may have had its source in the other gest their formation by floodwater flow. The forms are large (more western) Cerberus flood channels (Tanaka and in width relative to the northeastern branch but also shal- Scott, 1986; Berman and Hartmann, 2002; Plescia, 2003), lower. A possible scenario for their formation is that upon although MOLA topography and crater counts on MOC exiting the deep and narrow northeastern branch, the Man- images indicate that the three Cerberus channels – Marte, gala Valles floodwaters spread out and became more qui- Grjota´ and Athabasca – are distinct (Burr et al., 2002b). escent, depositing their sediment load in the lee of flow To the southwest (i.e., upslope) of the first topographic obstacles. expression of Marte Vallis, MOLA topography of the east- The northwestern branch terminus is largely ern Cerberus plains shows broad, shallow channels lead- embayed by Medusae Fossae Formation deposits. To the ing from a subdued linear depression, which is collinear west of those deposits is a 75 km diameter impact crater from the (Plate 24). The broad, shallow with a terraced fan sourced by a short broad . This channels are not visible on the southern side of the linear Floods from fossae on Mars 199 depression. This topography is the basis for the hypothesis blocks, Marte Vallis sits between Orcus Patera and southern that Marte Vallis originated at a section of the Cerberus highland outliers, and Athabasca Valles are bounded by an Fossae graben that is now buried by subsequent lava flows (see next section). In contrast, (Burr et al., 2002b). The Cerberus plains are interpreted Grjota´ Valles have no such apparent topographic structures as being filled with lava from the Fossae and other vol- to confine surface flow. The distributed nature of the out- canic vents (Plescia, 1990, 2003; Sakimoto et al., 2001). flow at the fissure-source of the channel may be another The broad, shallow channels stretching from the subdued factor in this distributed plan-view morphology (Burr and linear depression (interpreted as a buried fissure segment) Parker, 2006). are hypothesised to have conveyed the floodwater from the The sink for the floodwaters is uncertain. Some of now-buried fissure to the topographic channel that remains the floodwater appears to have flowed southward toward the visible today, and then to have been buried by lava (Burr Cerberus plains (Burr et al., 2002b; Berman and Hartmann, et al., 2002b). Another possible source for the Marte Val- 2002; Plescia, 2003) but the termination of this southward lis floodwaters is breaching of a lake (Scott and Chapman, flow is poorly defined (Burr and Parker, 2006). Inferred 1991) empounded behind a debris dam, ice dam or wrinkle rootless cones in the channel imply that some of the water ridge (Moller et al., 2001). Evidence for this lake is lacking infiltrated (Burr and Parker, 2006). However, infiltration but may be buried beneath subsequent lava. The presence of may have been minimised by freezing (see Clifford and rootless cones in Amazonis Planitia (Lanagan et al., 2001; Parker, 2001; see Burr et al., 2002b for discussion) and Greeley and Fagents, 2001) supports the idea derived from the inferred rootless cones are limited in extent and open the channel’s larger scale morphology (Tanaka and Scott, to other possible interpretations (Burr et al., 2008). Root- 1986; Fuller and Head, 2002) that Amazonis Planitia was less cones in the far western Amazonis Planitia (Lanagan the sink for the Marte Vallis water flows. et al., 2001; Lanagan, 2004; Greeley and Fagents, 2001), which is to the northeast of Grjota´ Valles, raise the possi- 10.3.3 Grjota´ Valles bility that some floodwater may have gone to the northeast, Grjota´ Valles are the least well-defined of the tec- and streamlined forms hint at flow to the northeast (Burr tonic outflow channels. Located in remnants of southern and Parker, 2006). However, the continuation of that flow highland terrain north of the Cerberus plains, they are as far as Amazonis Planitia is not discernible in available oriented east–west and centred near 165◦ E, 15◦ N (Plate data (Burr and Parker, 2006). Previous suggestions for the 25). The channel originates at the most northern major lack of an obvious sink – embayment by lava, blanket- Cerberus fossa as indicated by local scour, orientation of ing by tephra, aeolian erosion/deposition, mass wasting nearby flood-formed features (scattered mesoscale stream- and poor data coverage (Burr et al., 2002b) – now appear lined forms and longitudinal lineations), and MOLA topog- unlikely, given the improved extent and quality of MOLA raphy (Burr et al., 2002b; Plescia, 2003). Recent mapping and THEMIS data used in recent mapping (Burr and Parker, using THEMIS and MOC images shows that the origin 2006). This mapping, in conjunction with recent modelling of the floodwater was distributed discontinuously over a (Bargery and Wilson, 2006), suggests that the floodwaters 250 km stretch of fissure as measured end to end; the of Grjota,´ with their shallow depth and widely distributed summed lengths within this stretch from which water actu- nature, either evaporated or froze and sublimated during ally emerged total ∼150 km (Burr and Parker, 2006). The and/or after flow. On this basis, the sink for the floodwa- elevation of this distributed source ranges from approxi- ters is hypothesised to have been primarily the atmosphere mately −2100 at the eastern end to −2400 m at the western (Burr and Parker, 2006). end. The water flowed both to the north and to the south of the fissure, anastomosing through the remnant highland 10.3.4 Athabasca Valles terrain knobs in which it originated, thus making a con- Athabasca Valles have the most pristine morphol- clusive determination of flow cross-sectional area difficult. ogy of any outflow channel system on Mars. The main The better-defined, northern branch is up to several tens of topographic channel originates at two locations along the kilometres in width and of order ten metres in depth on southernmost major member of the Cerberus Fossae near average, although a small number of short segments are 10.2◦ N 157.2◦ E (Plate 26). The easternmost of these linear up to a few tens of metres deep. The overall slope of the sources is ∼18 km in length along the fossae and the west- channel is ∼0.0006 m/m (0.035◦). ern source is ∼22 long, with elevations of approximately This broadly distributed plan-view morphology and −2400 m. The channels emanating from these two sources small channel depth may be due to the lack of any sig- come together ∼20 km to the south of the fossae to create nificant surface topography controlling channel develop- a single channel. About 40 km below this confluence, the ment (Burr et al., 2002a; Burr and Parker, 2006). Man- Athabasca Valles become distributary, with smaller chan- gala Valles are located between north–south trending fault nels breaching through the wrinkle ridge on the southern 200 Devon M. Burr et al.

formed by deposition of sediment-rich ice blocks (Gaidos and Marion, 2003; see also Burr et al., 2005). Pitted mounds and cones have also been hypothesised to be collapsed pin- gos (Burr et al., 2005; Page and Murray, 2006). Some areas of patterned ground in the channel have been hypothesised to be thermal contraction polygons (Burr et al., 2005) or hyperconcentrated flow deposits (Rice et al., 2002). Recent tectonic activity in the Athabasca Valles region is indicated by small in-channel fissures and fis- sure morphology. The fissures at which the flood channel originates have sharp edges (such as are shown in Fig- ure 10.3), which has been interpreted as evidence of post- lava flow tectonism (Berman and Hartmann, 2002). Alter- natively, this sharp-edge morphology may be a result of melting due to dyke intrusion into ice-rich ground (Head et al., 2003). Smaller, open, sharp-edged fissures also cut across the channel floor. The presence of such fissures in a streamlined form (Burr et al., 2005) indicates that they formed more recently than the flooding that created the streamlined forms. Some of these small fissures are filled, possibly with lava and/or with flood sediments, whereas some are open (Keszthelyi et al., 2004b; Burr et al., 2005). Figure 10.3. Portion of MOC image M02-01973 located near This variation suggests that tectonic activity is interleaved 10.4◦ N 156.2◦ E, showing a segment of the Cerberus Fossae. The with volcanic and/or flood events. fossae are sharp edged, indicating recent tectonism and/or melting of ice-rich ground. The dark lobate features to the north, south and at The pristine morphology of the Athabasca Valles the southeastern tip are interpreted as spatter-fed lava. See Plate 26 has provided some indication of the floodwater flow charac- for location. North is up, illumination is from the left. teristics. A set of transverse linear forms in the channel were analysed and shown to have morphology (Burr et al., 2004). Inferred to be flood-formed dunes, they indi- side of the channel and stretching southward to the Cer- cate that the floodwater flow was subcritical at the location berus plains. Overall, the channel is ∼350 km in length, up and time of their formation (Burr et al., 2004). Temporary to 100 m in depth, with a slope of ∼0.0005 m/m (0.03◦). in-channel floodwater ponding due to hydraulic damming The topographic channel varies from ∼15 km to ∼25 km and associated sedimentation has been hypothesised on in width, although the floodwaters may have locally over- the basis of a cluster of streamlined forms (Figure 10.4) flowed it (Keszthelyi et al., 2004b). (Burr, 2005). According to this hypothesis, a crater and The Athabasca Valles system shows evidence for its ejecta hydraulically dammed the floodwaters and pro- volcanism, tectonism, sedimentation during temporary duced deposition of flood sediments, which were stream- ponding, glacial processes and ground ice. Volcanic pro- lined behind in-channel obstacles during ponded water out- cesses are indicated by dark, lobate features surrounding flow. The layering of these streamlined forms (Figure 10.4) the Cerberus Fossae (Figure 10.3) that have been inter- may thus be made up of layered flood sediments, likely preted as (spatter-fed) lava flows (Burr et al., 2002a; Head interleaved with lava flows, volcanic ash deposits and dust et al., 2003). Platy-ridged terrain on the edge of the channel (Burr, 2005). has been interpreted as analogous to rafted lava plates in Morphology shows that at the distal end of the chan- (Keszthelyi et al., 2000, 2004a). The hummocky or nel, Athabasca Valles floodwaters flowed to both the east- polygonal material in the channel may also be lava (Burr ern and western Cerberus plains. A shallow spillway, Lethe et al., 2002b; Jaeger et al., 2007). Various pitted mounds Vallis, is seen in MOLA topography and more recent visi- in the channel have been interpreted as being composed ble wavelength images, indicating that some water flowed of lava, including hypothesised rootless cones (Lanagan eastward (Plescia, 2003). The past presence of a lake in et al., 2001; Jaeger et al., 2007) and/or basaltic ring struc- the western Cerberus plains is suggested on the basis of tures (Jaeger et al., 2003, 2005). MOLA topography, which shows a shallow basin, and Mars Similar features also have been hypothesised to be Orbiter Camera images showing scarps and benches, which the result of glacial and ground ice processes. A localised are interpreted to be shorelines (Lanagan and McEwen, set of ring features has been hypothesised to be holes 2003; Lanagan, 2004). The interpretation of pitted mounds Floods from fossae on Mars 201

processes because they have received little analysis to date. Marte Vallis is embayed by lava flows, which may have covered post-flood modification. Table 10.1 summarises the processes that are interpreted variously to have occurred in each channel.

10.4 Mechanisms triggering water release Possible water release mechanisms for floods from fissures can be classified as volcanic, tectonic or a combi- nation of both types.

10.4.1 Volcanic The geometries of the sources of tectonic outflow channels suggest that if a volcanic mechanism is involved in causing catastrophic groundwater release, it involves subsurface dyke emplacement (Wilson and Head, 2002). Chaotic terrain has been interpreted elsewhere to be the result of sill intrusion and resultant cryosphere disrup- tion (Leask et al., 2007) but is not evident at the source for any of these channels. (A region of chaotic terrain is located along the Mangala Valles source graben near 19.3◦ N, 207.3◦ E (Hanna and Phillips, 2006) but this chaos ∼ Figure 10.4. Mosaic of MOC narrow-angle images showing cluster is 200 km west of the channel and confined within a of streamlined forms in Athabasca Valles, inferred to have been crater.) The dyke-induced mechanism relies upon the pres- formed by hydraulic damming/ponding, surrounded by longitudinal ence of a sub-cryosphere aquifer system that already has lineations. Scattered small impact craters with bright ejecta are been pressurised, as predicted by Carr (1979) and Clifford secondaries from (McEwen et al., 2005). See Plate 26 for (1993), a consequence of the growth of a global subsur- location of mosaic. North is up, illumination is from the left. (Image face cryosphere over geological time. As detailed in Head credit: Malin Space Science Systems.) et al. (2003) for Athabasca Valles, this proposed mecha- nism entails dyke emplacement resulting in cracking of this as rootless cones both within this basin and at the mouth cryosphere. In addition to causing local surface eruptions of Athabasca Valles also supports the hypothesis of dis- producing spatter-fed flows or phreatomagmatic deposits, tal floodwater ponding and suggests that the infiltration of this cracking provides a pathway for groundwater to rise floodwaters was quickly followed by lava flows (Lanagan from the pressurised aquifer to the surface. Water foun- et al., 2001). The morphology of the material at the channel tains are hypothesised to have resulted at the surface with mouth on which the inferred cones sit has been interpreted the water then draining downslope. Subsequent collapse of as lava (Keszthelyi et al., 2000, 2004a; see also Jaeger et al., the ice-rich soil adjacent to the dyke as a wave of heating 2007), which would have covered any flood deposits and is spreads away from it could have produced the sharp-edged consistent with the hypothesis of rootless cones. Alterna- morphology of the fissures. The interpretation of geomor- tively, the same platy-ridged material in the western Cer- phic features associated with the Athabasca Valles source berus plains has also been interpreted as the remnants of region as spatter-fed flows, collapse pits and exposed dyke an ice-covered lake left by floodwaters from the Cerberus tops supports this hypothesised mechanism (Head et al., Fossae (Murray et al., 2005; Kossacki et al., 2006), and 2003), as does the presence of possible phreatomagmatic pitted mounds have been interpreted as . For either deposits (Wilson and Head, 2004) and exposed dyke tops interpretation, the Athabasca Valles sink would be primar- (Leask et al., 2007) linked to the Mangala Valles source ily the western Cerberus plains, with smaller proportions graben. The use of aquifer permeabilities equal to those of floodwater flowing to the east. of young aquifers on Earth can produce the ampli- In summary, the morphology of the four tudes and durations of the discharges estimated from sur- Amazonian-aged, tectonic outflow channels suggests sev- face topography at Athabasca Valles (Manga, 2004). How- eral similar processes in addition to floodwater flow. To ever, it is not clear that this is the case at Mangala Valles; some extent, the number of additional processes may be the problem is the length of time for which the high dis- partially a function of the amount of analysis a channel has charge rate appears to have been maintained, rather than the received; Grjota´ Valles may show the fewest additional size of the initial water volume flux (Leask et al., 2007). 202 Devon M. Burr et al.

Table 10.1. Morphological processes in fossae-fed channels

Mangala Marte Grjota´ Athabasca

Approximate location 15◦ S 210◦ E (western 10◦ N 180◦ E (east 15◦ N 165◦ E (north 10◦ N 155◦ E(westof Tharsis flank) of Cerberus of Cerberus plains) Cerberus plains) plains) Approximate 0m −3100 m −2100 to −2400 m −2400 m elevation at source Source morphology Widened fossa segment, Buried by Cerberus Distributed along Dual source locations on with overflow through plains lava ∼250 km of fossa fossa notch in eroded impact crater rim Possible sink(s) NW branch: ponding in Ponding, freezing/ Infiltration combined Mostly ponding in crater w/terraced fan sublimation and with some freez- western Cerberus NE branch: broad infiltration in ing/sublimation plains, freezing/ flow northward Amazonis sublimation, infiltration Planitia Process Inferred geologic evidence Volcanism Dyke emplacement,a Embaying lava Embaying lava flowsl Lava flows at source,j,m phreatomagmatic flows,i lava burial dyke emplacement,n dunesb of sourcej rootless cones,o,p basaltic ring structuresq,r Tectonics Rotated fault block,c Sharp-edged fissuress,n graben formationd,e Glaciation, ground Kettle holes,f moraines Kettle holes,t ice wedge ice and other glacial polygons,u pingosu depositsg Flood sedimentation Unit Amchh In Amazonis Dunes,v streamlined Planitia sinkk forms,m,j hyperconcentrated flow depositsw

Sources: a Wilson and Head (2002) i Plescia (1990) q Jaeger et al. (2003) b Wilson and Head (2004) j Burr et al. (2002b) r Jaeger et al. (2005) c Zimbelman (1989) k Fuller and Head (2002) s Berman and Hartmann (2002) d Tanaka and Chapman (1990) l Burr and Parker (2006) t Gaidos and Marion (2003) e Craddock and Greeley (1994) m Burr et al. (2002a) u Burr et al. (2005a) f Zimbelman et al. (1992) n Head et al. (2003) v Burr et al. (2004) g Head et al. (2004) o Lanagan et al. (2001) w Rice et al. (2002) h Ghatan et al. (2005) p Greeley and Fagents (2001)

Repeated short-duration high-discharge floods (e.g. Hanna gested that the elevated water pressures might have been and Phillips, 2006) can explain both the high discharges produced by groundwater circulation induced by Tharsis and the large volumes of water required. magmatism, Tharsis-centred tectonic uplift, or compaction of saturated sediments by the load imposed by lava flow 10.4.2 Tectonic emplacement. Tectonic mechanisms for catastrophic groundwater A more recent hypothesis invokes dyke-induced release involve faulting, but do not require intrusive vol- extensional tectonism as the causal mechanism of all canism to induce the faulting. A correlation between fault- aspects of the process (Hanna and Phillips, 2006). Based ing and floodwater release was first proposed for Mangala on a recent model of the Martian crust (Hanna and Phillips, Valles on the basis of the similar crater-count model ages 2005), this mechanism involves a tectonic pressurisation of the flood channel and the associated faults (Tanaka and of the aquifers as a result of the release of the exten- Chapman, 1990). The mechanism entailed breaching of sional stresses in the crust, with excess pore pressures up pressurised perched aquifers by the faulting. It was sug- to 10 MPa being produced. This extensional pressurisation Floods from fossae on Mars 203 results in a discharge of water to the surface through the ent mechanisms may have operated at different locations tectonically generated fault. Although other factors, such as or times. perched aquifers, –ice interactions, or phreatomag- matic aquifer pressurisation, may also be involved in par- 10.5 Thermal and mechanical aspects of ticular cases, they are not necessary; Hanna and Phillips water release (2006) show that extensional tectonism alone is suffi- If the origin of a fracture through the cryosphere is cient to produce groundwater discharges and flood volumes purely tectonic, the thermodynamics of water rise through equal to those modelled for surficial flow in Athabasca and the fracture is relatively straightforward. If the water Mangala Valles. being released is derived from just below the base of the cryosphere, thus having a temperature only just above its 10.4.3 Combined factors freezing point, and if it rises only slowly, adiabatic cool- The catastrophic release of groundwater may ing may cause partial freezing (Gaidos and Marion, 2003). require both volcanic and tectonic processes. The model of However, given the large discharge rate and water volume Zimbelman et al. (1992) for Mangala Valles proposed that estimates (at least ∼106 to 107 m3 s−1 and up to at least groundwater was generated through melting of ground ice 20 000 km3, respectively; Wilson et al., this volume Chap- as the geotherm was steepened by enhanced regional vol- ter 16) it seems likely that water is tapped from a wide range canic activity during Tharsis formation, and subsequently of aquifer depths. Then even in the absence of volcanic this groundwater was released through fractures associated heat sources the mean temperature of the released water with the Mangala Fossa graben where it intersects a north– may be substantially above the freezing point. As shown in south trending fault. However, the melting of ground ice in Wilson et al., hindered convection in the pore spaces occu- this way may not have been able to provide a great enough pied by aquifer water (Ogawa et al., 2003) probably reduces water volume or discharge to have carved the Mangala the vertical temperature difference across a few-kilometer- Valles (McKenzie and Nimmo, 1999; Ghatan et al., 2005), thick aquifer by a factor of ∼2 below the ∼100 ◦Cimplied so this scenario may not be appropriate for that particular by the geotherm. As a result, the mean water temperature channel system (see Ghatan et al., 2005, for discussion). in an aquifer with a few kilometres’ vertical extent may Nevertheless, the very recent tectonic and volcanic activity be up to at least ∼30 ◦C and possibly as much as ∼50 ◦C that has been documented in Athabasca Valles suggests that (Wilson et al., this volume Chapter 16). a combination of the two processes may well be possible If a dyke intrusion is involved, there are more fac- in other situations. tors to consider. On the basis of a detailed morphological As an example, in a more recent model of the Man- analysis, Leask et al. (2006) proposed that the Mangala gala system, Wilson and Head (2002) suggested that the Fossa graben acting as the source of the Mangala Valles is Mangala Fossa graben was produced by faulting above one the consequence of two dyke intrusions, each dyke having of a series of dykes (the Memnonia Fossae) propagating an implied width of ∼250 m with its top located at a depth laterally from a mantle source beneath . The varying between ∼150 m and ∼550 m below the local sur- locations and radial orientations of these dykes, and oth- face. These width and depth values lie within the range pre- ers inferred to be responsible for the graben of the nearby dicted theoretically by Wilson and Head (2002) for dykes Sirenum, Icaria, Thaumasia and Claritas Fossae, are con- sourced from the crust–mantle boundary on Mars. Leask trolled by the strength and orientation of the regional tec- et al. (2006) ascribed part of the present depth of Mangala tonic stress field. The widths and depths of the graben Fossa to two episodes of of the graben floor are determined by a combination of the width of the dyke as heat released from the dykes melted nearby cryosphere as its upper tip approaches the surface and the depth at ice, with water escape to the surface allowing compaction which upward propagation ceases (Rubin, 1992; Schultz to occur. Through a heat-sharing calculation, Leask et al. et al., 2004). The dyke width is controlled by the interac- (2006) showed that if the cryosphere contained 10% by tion between the excess pressure in the dyke magma and volume ice, a plausible amount based on the crust model of the regional horizontal extensional tectonic stress deviator Hanna and Phillips (2005), and if all of the heat released by prior to dyke injection, as described by Rubin and Pollard the dyke were contained in a hydrothermal system confined (1987). Water migrates through pathways provided partly by the graben faults, the maximum temperature reached by by the propagation of the dyke and partly by the graben the hydrothermal water would have been ∼40 ◦C. The vol- boundary faults. ume of that water, summed over the ∼200 km length along In summary, multiple factors can account for the strike of the graben, would have been ∼155 km3 in each inferred catastrophic discharges of water from fissures. event, a total of ∼310 km3. If the ice melting had extended Given evidence for recent volcanism and tectonism on to a greater lateral distance than the graben faults, then the Mars, a combination of factors is reasonable, and differ- absolute maximum amount of water that could have been 204 Devon M. Burr et al. generated by cooling each dyke to a temperature infinitesi- yields a typical pathway width of 2.3 m and water rise mally above the melting point would have been ∼1650 km3, speed of ∼20 m s−1 along the ∼200 km horizontal extent providing a total of ∼3300 km3 of water just above its of the graben to provide the ∼107 m3 s−1 estimated dis- freezing point, though this efficiency of heat transfer from charge (Leask et al., 2006). All of these water rise speed magma to ice could never be approached in practice. From and pathway width combinations lead to minimal cooling measurements of the volume of rock eroded to form the of the water during its ascent. Mangala Valles channels and an estimate (Komar, 1980) of Water rise speeds of ∼20 to more than 50 m s−1 the maximum sediment-bearing capacity of a water flood imply that water emerging at the surface would have formed (≤40% by volume), calculations of the minimum amount a fountain with a height of at least 50 to as much as ∼400 m. of water required to flow through the system give esti- The mechanics of water spreading away on the surface from mates ranging from 8600 km3 (Hanna and Phillips, 2006) such a fountain has much in common with the formation to between 13 000 and 30 000 km3 (Ghatan et al., 2005). of pyroclastic density currents from fountains of gas and Thus it is extremely unlikely, in this case at least, that particles in some explosive volcanic eruptions (Wilson and melted cryosphere ice contributed more than a small frac- Heslop, 1990). Thus, as it falls onto the surface, the water tion of the water released, and tapping of a pre-existing from the fountain causes a dynamic pressure equal to the sub-cryosphere aquifer seems required. stagnation pressure of the water flow, one-half of the water Along most of its length the Mangala Fossa graben density times the square of its speed. The speed on reaching is ∼2 km wide, and this implies that the graben boundary the ground is essentially the same as the speed with which faults, dipping at ∼60◦ near the surface, extended down- water is projected upward into the fountain, ∼20–50 m s−1, ward for ∼1.5 km into the ∼4 km thick cryosphere. Thus it and so the pressure in the vicinity of the release point will seems inevitable that water was transferred through the be in the range ∼0.2 to ∼1.5 MPa. These pressures will lower part of the cryosphere along one or both of the suppress water vapour formation and will minimise release margins of the dyke. Thermal interaction between chilling of any dissolved carbon dioxide in the core of the fountain magma and melting ice is likely to be inherently unstable, and in the immediate vicinity of the release point. However, and it is not surprising that evidence of phreatomagmatic on the outer edges of the fountain, and on the top surface of explosive activity is seen at the eastern end of Mangala the water flowing away from the fountain, both processes Fossa (Wilson and Head, 2004). The fact that such explo- would occur, and would lead to instabilities in the water– sive activity is not seen at other sites of water release atmosphere interface and formation of water droplets. In hypothesised to have resulted from dyke intrusion sug- the case of fountains formed from water at a temperature gests that the distribution of ice-bearing pore space in very close to the triple point, extraction of latent heat of the cryosphere is heterogeneous. A stable contact between evaporation could lead to freezing of some of the droplets, dyke margin and crustal rock may then exist in some as discussed by Gaidos and Marion (2003). Some of these places, with minimal or no cryosphere ice melting and issues, together with the dynamics of water flow away from upward aquifer water flow, whereas water may rise rela- its source, are considered by Wilson et al. (this volume tively freely in other places through cavities created by an Chapter 16). initial violent but short-lived dyke–cryosphere interaction. Alternatively, an absence of evidence for explosive activ- 10.6 Summary and implications ity at other sites may suggest an amagmatic water-release The four most recent flood channels on Mars all mechanism. head at tectonic features. These channels can be grouped The typical widths of the open pathways required in space and time (Table 10.1). Mangala Valles are the old- for water discharge, together with the flow speeds of est of the four, flowing during the late Hesperian and early water through them, can be estimated from the relation- Amazonian Epochs (Tanaka and Chapman, 1990; Zimbel- ship between pressure in the aquifer system and pathway man et al., 1992). Modelling suggests that the channels wall friction. Using varying assumptions about the origin may have formed through repeated short-duration, high- of excess pressure in the aquifer, Head et al. (2003) and discharge events (Hanna and Phillips, 2006; Manga, 2004), Manga (2004) find average pathway widths in the range whereas geological mapping suggests only one period of 1–2.5 m and water rise speeds up to ∼60 m s−1. Combin- flow (Ghatan et al., 2005). The channels originate from the ing these rise speeds, pathway widths and the ∼30–40 km Memnonia Fossae graben system located off the western length along strike of the pathways, Head et al. (2003) and flanks of the Tharsis rise at an elevation of about 0 m. Manga (2004) show that the water discharges of order 1–2 The other three channels are located to the east, × 106 m3 s−1 estimated for the Athabasca Valles channels north and west of the Cerberus plains and originate from by Burr et al. (2002a) can readily be provided. A simi- the Cerberus Fossae. The elevations at their origination lar analysis for the Mangala Fossa/Mangala Valles system sites range from approximately −3100 m to −2100 m. Floods from fossae on Mars 205

Water flowed in the channels at different times during the We thank Jeffery Andrews-Hanna and Jim Zimbelman for con- late to very late Amazonian Epoch (Burr et al., 2002b; structive and helpful reviews. McEwen et al., 2005). During the Amazonian, the loci of flow progressed from the east (Marte Vallis) to the west References (Athabasca Valles) ends of the fossae. This situation mirrors Bargery, A. S. and Wilson, L. (2006). Modelling water flow with the migration of volcanic activity on the fossae, which has bedload on the surface of Mars. Lunar and Planetary been interpreted on the basis of superposition of lava flows Science Conference XXXVII, Abstract 1218, Lunar and to have moved westward with time (Lanagan and McEwen, Planetary Institute, Houston, Texas (CD ROM). 2003). This correlation is consistent with (but does not Berman, D. C. and Hartmann, W. K. (2002). Recent fluvial, vol- require) a volcanic triggering mechanism and admits the canic, and tectonic activity on the Cerberus Plains of Mars. possibility that the regional stress field in this region is not Icarus, 159, 1–17. static. Berman, D. C., Hartmann, W. K. and Burr, D. M. (2001). Marte The likely sources of the floodwater are subsurface Vallis and the Cerberus Plains: evidence of young water aquifers with thicknesses of at least a few kilometres and flow on Mars. Lunar and Planetary Science Confer- ence XXXII, Abstract 1732, Lunar and Planetary Institute, temperatures of a few tens of degrees Celsius. Mechanisms Houston, Texas (CD ROM). that may have triggered groundwater release from these Burr, D. M. (2005). Clustered streamlined forms in Athabasca aquifers to the surface could have been volcanic, tectonic Valles, Mars: evidence for sediment deposition dur- or a combination of the two. Geomorphological evidence, ing floodwater ponding. Geomorphology, 69, 242– or lack thereof, suggests that each of these mechanisms may 252. have operated at different times and locations. Upon reach- Burr, D. M. and Parker, A. H. (2006). Grjota´ Valles and impli- ing the surface, the water would likely have fountained at cations for flood sediment deposition on Mars. Geo- least a few tens of metres above the surface, suppressing physical Research Letters, 33, L22201, doi:10.1029/ water vapour formation and carbon dioxide release within 2006GL028011. the fountain but promoting water and/or ice droplet forma- Burr, D. M., McEwen, A. S. and Sakimoto, S. E. H. (2002a). tion at the margins. Continued discussion of the resultant Recent aqueous floods from the Cerberus Fossae, Mars. Geophysical Research Letters, 29 (1), doi:10.1029/ surface processes is provided in Wilson et al. (this volume 2001GL013345. Chapter 16). Burr,D.M.,Grier,J.A.,McEwen,A.S.andKeszthelyi,L.P. In large part, Mars exploration is the search for evi- (2002b). Repeated aqueous flooding from the Cerberus dence of life. Because life as known requires liquid water, Fossae: evidence for very recently extant, deep ground- sites of astrobiological interest are those where liquid water . Icarus, 159, 53–73. existed, including catastrophic flood channels (National Burr, D. M., Carling, P. A., Beyer, R. A. and Lancaster, N. Academy of Sciences, 2007). These flood channels must (2004). Flood-formed dunes in Athabasca Valles, Mars: have had sources within subsurface aquifers, the depths of morphology, modeling, and implications. Icarus, 171, 68– which may be consistent with modelled water table depth 83. (see Burr et al., 2002b). At present, the surface of Mars Burr, D. M., Soare, R. J., Wan Bun Tseung, J.-M. and Emery, J. P. is inhospitable to life (e.g. Carr, 1996; National Academy (2005). Young (late Amazonian), near surface, ground ice features near the equator, Athabasca Valles, Mars. Icarus, of Sciences, 2007) but these source aquifers, located in 178, 56–73. volcanic terrain, may provide or have provided both liq- Burr, D. M., Bruno, B. C., Lanagan, P. D. et al. (2008). Mesoscale uid water and energy to sustain life. To the extent that the raised rim depressions (MRRDs) on Earth: a review flood channels are draped in lava (see Jaeger et al., 2007), of the characteristics, processes, and spatial distribu- their utility as an astrobiology target (see Fairen´ et al., tions of analogs for Mars. Planetary and Space Science, 2005) may be reduced. However, sediments from the sub- doi:10.1016/j.pss.2008.11.011. surface deposited in post-magmatic flooding may provide Carr, M. H. (1979). Formation of Martian flood features by release chemical or spectroscopic evidence of a putative subsurface of water from confined aquifers. Journal of Geophysical biosphere, to match the geophysical modelling of the type Research, 84, 2995–3007. provided here. Ongoing exploration of the surface of Mars Carr, M. H. (1996). Water on Mars. Oxford: Oxford University will continue to deepen our understanding of the aqueous Press. Chapman, M. G. and Tanaka, K. L. (1993). Geologic Map of the subsurface as well. MTM -05152 and -10152 Quadrangles, Mangala Valles Region of Mars, scale 1:500,000. U.S. Geological Survey Acknowledgements Investigations Series Map, I-2294. All MOLA topography figures (Plates 23 through 26) Christensen, P. R. and 21 others (2003). Morphology and com- were made in Generic Mapping Tools (Wessel and Smith, 1998) position of the surface of Mars: Mars Odyssey THEMIS with data from the Planetary Data System (Smith et al., 2003). results. Science, 300 (5628), 2056–2061. 206 Devon M. Burr et al.

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