GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L22201, doi:10.1029/2012GL053225, 2012

Infiltration of Martian outflow channel floodwaters into lowland cavernous systems J. Alexis P. Rodriguez,1 Mary Bourke,1,2 Kenneth L. Tanaka,3 Hideaki ,4 Jeffrey Kargel,5 Victor Baker,5 Alberto G. Fairén,6 Richard J. ,7 Lynne Bridget,8 Rogelio Linares Santiago,9 Mario Zarroca Hernández,9 and Daniel C. Berman1 Received 24 July 2012; revised 9 October 2012; accepted 10 October 2012; published 30 November 2012. [1] The hydrosphere of has remained mostly con- 1. Introduction cealed within the subsurface for the past 3.5 Gyr. Localized [2] The Martian were excavated by the rupturing of the permafrost-capped crust led to voluminous largest estimated flood volumes in our solar system [Carr, groundwater discharges that carved some of the largest 1979; Clifford and Parker, 2001; Tanaka et al., 2005]. These known channels in the solar system. However, our knowl- edge of the nature of the flows and their ultimate fate remains waters were released from aquifers [Carr, 1979; Clifford and Parker, 2001] and/or surface lakes [Irwin et al., 2004] and incomplete, partly because diagnostic landforms at outflow may have resulted in the formation of transient oceans [Parker channel termini have been largely destroyed or buried. The et al., 1993; Fairén,2003],lakes[Fairén, 2003], continental- Hebrus Valles outflow channels were excavated by fluid dis- scale ice sheets [Kargel et al., 1995], and regionally extensive charges that emanated from two point sources, and they debris flow deposits [Tanaka et al., 2001]. The study region mostly terminate in systems of fractures and depressions (Figure 1a and 1b), centered at 19N., 125E., includes the within the northern plains. Our investigation indicates that Hebrus Valles (HV) and Hephaestus Fossae (HF). It occurs outflow channel floodwaters were captured and reabsorbed along the SE margin of the 2000-km diameter Utopia impact into the subsurface in zones where caverns developed within basin on the lower flank of the Elysium volcanic rise, and it is the northern plains. These findings imply that the study region comprises the only known location in the Martian also positioned along the global topographic dichotomy sepa- rating ancient southern cratered highland terrains and the northern lowlands where the fate of outflow channel dis- Formation (VBF), which consists of rela- charges can be assessed with confidence. We propose that evacuation of subsurface materials via mud volcanism was tively younger sedimentary deposits forming the floors of the northern lowland basins [Tanaka et al., 2005]. an important process in cavern formation. Our conceptual model provides a hypothesis to account for the fate of sedi- ments and fluids from some of the Martian outflow channels. 2. Data Sources and Methods It also reveals a mechanism for lowland cavern formation and [3] This geologic investigation has been performed using upper crustal volatile enrichment after the development of the Environmental Systems Research Institute’s (ESRI) ArcGIS Martian global cryosphere. Citation: Rodriguez, J. A. P., et al. software. Geologic feature mapping (Figure 1b and 1c) and (2012), Infiltration of Martian outflow channel floodwaters into characterization (Figure 2a–2h) were carried out using Mars lowland cavernous systems, Geophys. Res. Lett., 39, L22201, Reconnaissance Orbiter (MRO) Context Camera (CTX, 5.15 to doi:10.1029/2012GL053225. 5.91 m/pixel), and Mars Odyssey Thermal Emission Imaging System (THEMIS) visible wavelength multiband (VIS, 17 to 40 m/pixel/day IR, 100 m/pixel) images. Topographic infor- mation and volumetric estimations have been derived utilizing Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter  1Planetary Science Institute, Tucson, Arizona, USA. (MOLA) digital elevation models (DEMs) at 460 m/pixel 2Department of Geography, Trinity College, Dublin, Ireland. horizontal and 1 m vertical resolutions. 3Astrogeology Science Center, U.S. Geological Survey, Flagstaff, Arizona, USA. 4University Museum, University of Tokyo, Japan. 3. Results 5 Department of Hydrology and Water Resources, University of [4] Cone features interpreted to be mud volcanoes form Arizona, Tucson, Arizona, USA. 6 extensive fields within the northern plains of Mars [Oehler Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, California, USA. and Allen, 2010, 2012], which include the southern Utopia 7Centre for Research into Earth Energy Systems, Department of Earth boundary plains, where the study region is located [Skinner Sciences, University of Durham, Durham, UK. and Tanaka, 2007] (auxiliary material, Text S1).1 At some 8 Institute of Earth Science and Engineering, University of Auckland, locations within the study region, features interpreted as mud Auckland, New Zealand. 9Unitat de Geodinàmica Externa i d’Hidrogeologia, Departament de volcanoes cluster into linear ridges (black arrow in Geologia, Universitat Autònoma de Barcelona, Barcelona, Spain. Figure 2f ). These ridge patterns align with networks of individual pits, pit chains and troughs, all of which show Corresponding author: J. A. P. Rodriguez, Planetary Science Institute, 1700 E. Fort Rd., Ste. 106, Tucson, AZ 85719-2395, USA. variable degrees of integration (Figure 1c, white arrow in ([email protected]) 1Auxiliary materials are available in the HTML. doi:10.1029/ ©2012. American Geophysical Union. All Rights Reserved. 0094-8276/12/2012GL053225 2012GL053225.

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Figure 1. View of the Hebrus Valles and the Hephaestus Fossae regions. (a) MOLA DEM (128 pixels per degree) on Viking MDIM 2.1 mosaic (256 pixels per degree). (b) Geologic feature map. Mapping of the troughs, pits, impact craters, and channel floors was performed using a CTX mosaic on a background THEMIS VIS mosaic. The margins of the fractured mounds and the contact between the boundary plains and the Vastitas Borealis Formation (VBF) were traced on a THEMIS infrared (IR) mosaic at 230 m/pixel. (c) Close-up view on our mapping of the Hebrus Valles terminal zones.

Figure 2f ). At other locations, poorly integrated networks of pendant bars that extend into troughs (e.g., black arrow in linear depressions extend and interconnect these features Figure 2e) are indicative of rapid and unobstructed flow (white arrows in Figure 2g). We suggest that within the study [Baker, 1982, 2009] into the subsurface. Stream disappearance region, collapsed sections of cavern systems are expressed at into sinkholes is common on Earth [Lorenz, 2006] and con- the surface by these linear depressions. The apparent system duits fed by surface streams through sinkholes can transmit connectivity of mud volcanoes and the pit and trough net- large quantities of bed load and suspended load [White,1988]. works suggests a genetic between mud volcanism and [7] An end-member case scenario would indicate that the the development of cavern networks. HV floods and transported sediments were mostly captured [5] The distal reaches of HV form two distributaries, each into the subsurface. The estimated cumulative volume of the of which terminates within isolated pit and trough networks Hebrus outflow channels (206 km3) exceeds that of the trough (sites 1 and 2 in Figure 1c), suggesting drainage of flood- networks in their terminal regions by two orders of magnitude waters into the subsurface [Christiansen and Hopler, 1987; (site 1, 6.5 km3;site2,8.8km3). However, we note that these Carr and Malin, 2000; Tanaka et al., 2005]. The subsurface trough networks likely represent only the portions of the cav- pathway of distal floodwaters is here demonstrated by the ern networks that underwent collapse. In addition, the neigh- identification of local channel incision by knickpoint retreat boring HF pit and trough network has a cumulative volume (Figure 2a–2e), coupled with a decrease in channel capacity of 637.8 km3, suggesting that caverns extensive enough to downstream from some pits (white arrows in Figure 2a and 2d), accommodate the estimated discharge volumes could have and the particularly abrupt transition that the fluvial morphol- developed regionally. On the other hand, although our map- ogy exhibits at site 1 into the branching, rectilinear pit and ping shows that only a few small channels do not terminate in trough networks (white dashes in Figure 2a). the trough networks (Figure 1c), which indicates that a fraction [6] The absence of evidence of ponding upstream of pits of the floods must have escaped infiltration at these sites, we and troughs (e.g., Figure 2b and 2e) and presence of channel do not rule out that extensive fluvial sedimentation occurred

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Figure 2. Close-up views of study region; context and locations shown in Figure 1. All scale bars correspond to 5 km. (a) Site 1. THEMIS VIS images V11629004 and V12852005 centered at 12647′ E., 2035′ N. (b) Zone of knickpoint (black arrow) retreat shown in panel a. CTX image P13_006196_1998 centered at 12643′ E., 2033′ N. (c) Elevation profiles located in panel b. (d) Site 2. Composite of THEMIS VIS images V02617006, V12877003 and V01868003 centered at 125 36′ E., 2134′ N. (e) Zone of knickpoint retreat (black arrow) shown in panel d. CTX image P17_007765_2015 cen- tered at 12540′ E., 2144′ N. (f ) Part of Hephaestus Fossae. CTX image B18_016560_2033 centered at 12270′ E., 2200′ N. (g) Zone north of site 2 where systems of linear depressions interconnect clusters of features interpreted as mud volcanoes. Part of CTX image B18_016705_2048 centered at 12518′ E., 2500′ N. (h) Zone where Hebrus Valles dissects a plain modified by mud volcanism. Part of CTX image P02_001805_1996 centered at 12647′E., 1928′ N. but the deposits are no longer recognizable because they were 2010, 2012]. The high occurrence of mud volcanoes along weathered and removed. boundary plains (Figure 3a) is consistent with the hydraulic head being driven by the relief aquifers extending across the highland-lowland boundary [Rodríguez et al., 2010]. 4. Discussion [10] Possible sites of Late /Early groundwater discharge, perhaps emanating from a groundwater 4.1. Cavern Formation Associated With Northern zone that extended into more elevated terrains in the southern Plains Mud Volcanism cratered highlands [Clifford and Parker, 2001], have been [8] Carr and Malin [2000] suggested that the HF system identified on the flanks of the Elysium rise [Tanaka et al.,2005; may indicate a subsurface karst landscape formed by the Skinner and Tanaka, 2007]. These sites are located at eleva- dissolution of buried carbonate deposits. However, the only tions ranging between approximately 3500 and 5000 m above carbonate deposits that have so far been identified within the the study region. Thus, provided that a regional groundwater northern plains were exhumed from depths of 6 km during zone existed at the time (1 in Figure 3b), this relief may have an impact crater forming event [e.g., Michalski and Niles, provided a hydraulic head (h (relief in meters) x g (for Mars 2010]. At site 1 a channel extends to the floor of a 300 m 3.711 m/s2) Â r (water density, 1 g/cm3)) between 13– deep trough from a zone of knickpoint retreat occurring at 18.5 MPa, values which are comparable to those postulated depths of just 10s of meters of outflow channel dissection to have produced superlithostatic pressures and driven large- (respectively, elevation profiles A-A′ and B-B′ in Figure 2c). scale hydrologic resurfacing in southern circum-Chryse These observations are indicative of caverns having existed at [Andrews-Hanna and Phillips, 2007]. An alternative geo- shallow depths within the study region. In the absence of the logic scenario is that instead of a regional groundwater zone identification of shallowly buried carbonate deposits, we leading to superlithostatic pressures, pressurized groundwa- offer an alternative mechanism for the formation of the ter systems resulted from intrusive magmatism into the pseudo-karst landscape. Our model invokes the role of mud cryosphere, including dikes (5 in Figure 3b), which during volcanism in the formation of subsurface caverns. Early Amazonian Elysium volcanism [Tanaka et al., 2005] [9] High hydraulic pressures are thought to have led to extended 350 km [Rice et al., 2002; Scott et al., 2002; mud volcanism along the SE margins of the Utopia basin and Head, 2003] from the Elysium rise to the [Tanaka et al., 2003; Skinner and Tanaka, 2007], as well as southeast. Craft and Lowell [2012] show that pressurized within other regions of the northern plains [Oehler and Allen,

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Figure 3. (a) MOLA shaded-relief view of the northern plains of Mars showing the highland/lowland boundary margin (orange hachured line; dotted or queried where dissected or obscured). The Vastitas Borealis and units are and the Utopia lahars are pink, based on geologic mapping of the northern plains [Tanaka et al., 2005]. Point symbols designate the locations of landforms mainly identified in THEMIS VIS images and MOLA altimetry data that possibly resulted from ground-volatile activity, in the following interpretive types: Mass flows (red circles), mud volcanoes (yellow triangles), and various troughs and depressions formed by collapse and/or subsidence (white squares). Abbreviated geo- graphic names include (Ar), (UP), Isidis Planitia (IP), (DM), Mensae (CM), Acidalia (AcM), (AcP), (CP), (TT), (AM), (AF), Scandia region (Sc), and (PB). (b) Schematic diagrams showing upper crustal cross- sectional views depicting the hydrogeologic conditions that led to the inferred history of resurfacing. Panels 1–4 and 5– 8 show alternative regional geologic scenarios, respectively involving a three-zone (ice-rich cryosphere [Cr], hydrosphere [Hr] and basement [Bs]) and a two-zone (ice-rich cryosphere and basement) upper crustal structure. The cross-sections extend from Elysium rise to the floor of the Utopia basin. EC stands for Elysium , HV for Hebrus Valles, and VBF for Vastitas Borealis Formation. Surface topography slightly modified from MOLA for a better artistic reconstruction. 1. Pressurization of a continuous groundwater zone produces fractures within the overlying cryosphere, which led to mud volcanism along the southern margin of the Utopia basin. 2. The hydrosphere progressively drains (white dotted arrow) and the conduit feeder systems drain. 3. An igneous intrusion into the cryosphere produces large volumes of melt that are then released at the surface to carve HV. 4. The floods are captured by the evacuated caverns. 5. Dikes intrude an ice-rich cryosphere, which does not confine an underlying groundwater zone. The dikes produce hydrothermal systems, some of which extend to the surface and expel fluids to produce mud volcanism. 6. As the hydrothermal systems cool and water evaporates, the water table drops and evacuates the conduits. Groundwater within some isolated conduits that do not open to the surface freezes (white arrow). 7. A large intrusive magmatic body melts the cryosphere and produces catastrophic discharges that carve HV. 8. The groundwater discharges infiltrate into the evacuated conduits. (c) Schematic diagrams showing stages in proposed scenario for cavern development associated with Hebrus Valles and Hephaestus Fossae. 1. Hydrofractures within the regional cryosphere are produced by overpressure within an underlying hydrosphere. 2. Feeder conduits form along fractures and converge at the eruption sites. 3. Caverns develop underneath the eruption sites and where feeder conduits form dense networks. 4. Fluid drains from the caverns. aquifers could have resulted from the melting and vaporiza- 2005] may have resulted in thermo-karstic and pseudo-karstic tion of regional permafrost deposits by intrusive dikes. They (by suffusion) erosion along feeder conduits, which led to propose that these aquifers could have produced some cata- cavern development and enlargement [Deville et al.,2003; strophic outflow channel discharges. Davies et al., 2007; Davies, 2008] (3 in Figure 3c). [11] Our model proposes that excess pressure within an [12] We note, however, that other than the mud volcanic aquifer(s) produced hydrofractures that penetrate an overlying cones, no large volumes of surficial deposits have been cryosphere (1 in Figure 3c). Fluid circulation along the frac- identified, which could be indicative of the volume of evac- tures led to the development of feeder conduits through which uated subsurface geologic materials significantly exceeding fluid-sediment mixtures erupted to construct mud volcanoes. the volumes of the overlying mud volcanoes, perhaps by as This mechanism is supported by observations on Earth [Brown, much as a few orders of magnitude [Valentine et al., 2010]. 1990; Davies and Stewart, 2005; Stewart and Davies,2006](2 For example, it is possible that the VBF consisted of pri- in Figure 3c). In their models, subsurface flow through exten- marily ice-rich permafrost, and thus relatively low volumes sive and complex networks of feeder conduits converging of lithics were ejected during mud volcanism [Mouginot underneath eruption sites [Morley, 2003; Davies and Stewart, et al., 2012]. Alternatively, a significant portion of the

4of6 L22201 RODRIGUEZ ET AL.: OUTFLOW CHANNEL FLOODWATERS INFILTRATION L22201 deposits could have been subject to weathering and reworking Antarctic Ice Sheet [Eyles, 2006], indicate that, on Earth, by small impact craters (Text S1), and aeolian mobilization fluid-filled glacial caverns can exist at depths significantly during the 3.5 Gyr [Werner et al., 2011] since emplacement and greater than 2 km. In addition, the maximum stable width of are currently degraded beyond possible recognition in the used a cavern increases with the inverse square root of gravita- datasets. tional acceleration [Haruyama, 2009]. Consequently, on [13] Fluid evacuation from the caverns could have resulted Mars caverns within geologic materials that have similar from (1) lowering of the regional water table as aquifers mechanical strength could have 60% wider roofs than on drained and/or hydrothermal systems cooled down (2 and 6 in Earth. If maximum cavern dimensions all scale similarly, Figure 3b), (2) eruptive phases of water expelling mud fol- Martian caverns could be more voluminous than Earth’s, lowed by evaporation, (3) eruptive phases of gases expelling perhaps four times (1.63) greater. mud, or (4) a combination of these processes. We note that alternating eruptive phases of mud, water and gases separated 4.3. Geologic Conditions Leading to Outflow Channel by periods of inactivity are known to occur in terrestrial mud Discharges Following the Cessation of Mud Volcanism volcanism [Deville et al., 2010]. In addition, unerupted [17] The absence of mud volcanoes along the floor of HV water-saturated sediment infilling portions of the feeder ’ (Figure 2h) indicates that regional mud volcanism ceased prior conduits network would have eventually desiccated [e.g., to outflow channel activity. Mud volcanism could have ceased Grimm and Painter, 2009], thereby producing further sub- as aquifers depleted (2 in Figure 3b), and/or hydrothermal surface void space. systems cooled (6 in Figure 3b), which would have led to a 4.2. Geologic Conditions Promoting Structural reduction of hydraulic pressure within the regional aquifer(s) Stability of Evacuated Caverns and a lowering of the water table. The formation of HV during the Early Amazonian [Tanaka et al., 2005] is indicative of a [14] Mud volcanism originating from under-compacted second stage of aquifer development and pressurization, which mud-rich strata within terrestrial clastic basin settings is known could have been produced by the localized subsurface melting to lead to the formation of transient cavern systems, which of an ice-rich cryosphere during intrusive magmatism [Rice upon structural failure lead to zones of surface collapse and et al., 2002] (3 and 7 in Figure 3b). The absence of evidence subsidence [e.g., Brown,1990;Davies and Stewart,2005; indicative of a second stage of mud volcanism following out- Davies et al.,2007;Stewart and Davies, 2006; Roberts et al., flow channel formation suggests that aquifer development did 2011]. In contrast, the inferred magnitude of floodwater not result in regional groundwater dispersion, but instead the infiltration within the study region (4 and 8 in Figure 3b) aquifers were drained during catastrophic discharge. points to the existence of structurally stable caverns that were largely evacuated of fluids and sediments prior to HV outflow [18] Acknowledgments. The Editor thanks the two anonymous channel activity (2, 3 and 6, 7 in Figure 3b; 4 in Figure 3c). reviewers for their assistance in evaluating this paper. [15] During the Early Amazonian, the Martian northern plains are thought to have consisted of extensive permafrost deposits [e.g., Clifford and Parker, 2001; Mouginot et al., References 2012]. 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