Lunar and Planetary Science XXXV (2004) 1185.pdf

GEOLOGIC EVOLUTION OF DAO VALLIS, . David A. Crown1, Leslie F. Bleamaster III1, and Scott C. Mest2, 1Planetary Science Institute, 1700 E. Ft. Rd., Suite 106, Tucson, AZ 85719, [email protected], 2Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260.

Introduction: Dao, Harmakhis, and Reull Valles targets for evaluation of the astrobiological potential extend through the cratered highlands and of Mars, 2) evaluation of potential Martian lacustrine sedimentary plains of the eastern Hellas region of and glacial activity (which have been proposed to Mars. Dao Vallis, along with its tributary Niger occur in the Hellas region [10-11]), and 3) Vallis, extends for ~1200 km from the eastern margin assessment of atmospheric and endogenic models for of Hadriaca Patera into , where it and ages of gully formation [12-18]. contributed sediments and water/ice to the basin Formation of Dao Vallis: Geomorphic and floor. The Dao Vallis system (~6-50 km wide) is topographic analyses suggest the following characterized by two steep-walled source evolutionary sequence: 1) withdrawal of underlying depressions, regions of subsided plains, and support and potential removal of subsurface volatiles; prominent central canyons whose walls display 2) vertical collapse and surface fracturing; 3) sapping gullies with associated depositional aprons covering and surface runoff facilitated by collapse-induced parts of canyon floors. Analyses using Viking Orbiter topography; 4) wall collapse and erosion; and 5) images have suggested that Dao Vallis formed by a resurfacing of canyon floors. This general sequence combination of collapse of sedimentary and volcanic of events appears to have occurred in various parts of plains and surface and subsurface flow events, as Dao Vallis at different scales and at different times, well as late-stage wall collapse that served to enlarge and currently Dao Vallis preserves evidence of each its canyons and source areas [1-3]. The present of these different stages. study is designed to utilize MOC images, MOLA Collapse of plains: Analyses of Viking images topographic data, TES data, and THEMIS daytime indicated that regions of slumped plains separate the and nighttime images to evaluate and refine the source depressions of the Dao/Niger system from the geologic evolution of Dao Vallis. main central canyons [1]. Collapse and related Geologic Significance: The three circum-Hellas subsurface migration of volatiles were thought to be valles represent a stage in regional geologic history potentially triggered by volcano-ice interactions in intermediate to formation of channels and valleys the Martian subsurface. These slumped zones, also within highland terrains in the Late and seen at [1-4, 19-20], were thought Early Epochs and debris aprons and gullies to represent an initial stage in vallis formation. in the Period [1-5]. Channeled and MOLA topographic data clearly show smooth plains in the eastern Hellas region are subsidence of plains along the path of Dao Vallis. dissected by the and provide MOLA profiles show slumped plains up to 500 m evidence for contemporaneous fluvial activity; the below their surroundings, although the magnitude of emplacement and erosion of a sequence of plains subsidence is typically less and shows a high degree adjacent to may be directly tied to of spatial variability. Collapse continues following flooding associated with this channel system [6-7]. establishment of deep canyon segments. Viking and Dao, Harmakhis, and Reull Valles are classified THEMIS images show collapse of blocks at the as outflow channels, although they do not exhibit margins of Dao Vallis which serve to increase distinctive streamlined islands as are found with the canyon width significantly and have been largely Chryse outflow channels [8] and the hybrid nature of responsible for creation of its observed planform Reull Vallis has also been recognized [3, 9]. Because morphology. Collapsed regions range in size from of its prominent central canyon, its proximity to 10's to 100's of km2; individual remnant blocks are Hadriaca Patera, connection to the Hellas Basin, and commonly a few km2 in size. The significant vertical the recent identification of gullies on its walls, Dao displacement and large areal extent of collapsed Vallis has long been considered a landform of plains is consistent with large amounts of void space significance on Mars. The nature, magnitude, and at depth and removal of subsurface volatiles. history of volatile-driven processes associated with Surface runoff: Small channels and lineations Dao Vallis have important implications for Martian parallel to canyon walls observed in Viking images geologic and climate studies, including: 1) the role of have been cited as evidence for surface runoff in volcanism in melting ground ice and subsequent conjunction with the formation of Dao Vallis [1]. generation of hydrothermal systems that are critical Dao Vallis truncates sinuous channels in the Lunar and Planetary Science XXXV (2004) 1185.pdf

surrounding plains, suggesting both nearby and and possibly current transport of wall material into geologically concurrent fluvial activity. Classification the canyon. TES data for canyon floors show a of Dao Vallis as a confined outflow channel suggests, significant range of thermal inertia values (~250 – in a general way, a contribution of flooding in its 500 J/m2 K s1/2), and the deepest parts of the canyon development, although features diagnostic of surface generally appear to have the highest thermal inertias. flow have not thus far been identified. Current Conclusions: 1) Analyses of Viking, THEMIS, analyses of topography and small-scale surface and MOC images show a consistency of geologic morphology are not consistent with sustained surface processes across a range of spatial scales and suggest runoff over significant lengths, but cannot rule out an evolutionary sequence that applies to various catastrophic flooding in earlier stages of vallis segments of Dao Vallis through time. 2) Collapse is a formation. Viking, THEMIS, and MOC images show dominant process for lateral and vertical growth and small channels that may delineate local scouring from occurs throughout Dao Vallis’ evolution. 3) The surface flow, which may be promoted by the irregular magnitude and areal extent of collapse suggest topography caused by collapse and the presence of significant void space at depth that presumably subsurface water or ice. Local surface flow is also reflects a volatile-rich substrate. 4) Fractures and associated with gullies on Dao Vallis’ walls. topographic irregularities created by collapse Sapping: The zones of subsided plains that promote sapping processes that cause growth of Dao separate the central canyons of Dao and Niger Valles Vallis; localized runoff redistributes materials. 5) from their source depressions have been cited as Longitudinal and cross-sectional profiles are evidence of subsurface flow in Viking-based studies. consistent with a collapse-dominated system and The higher spatial resolution of THEMIS and MOC suggest no sustained surface flow over significant images suggests that sapping is a significant process lengths in the currently preserved canyon system. 6) in the development of Dao Vallis at a range of scales. Canyon walls exhibit a suite of geologically recent, Images illustrate that small linear fractures initially volatile-driven processes that contribute debris to the formed due to collapse of plains can evolve into canyon floor. 7) Morphology of canyon floor network patterns. Individual linear depressions widen deposits suggests downslope movement in viscous and take on an irregular form through scarp retreat flows of debris and water/ice. 8) Variability of and capture of small topographic obstacles. The canyon floors suggest potential to examine deposits resulting morphologies are indicative of sapping of different types and exposure ages and rocks of processes, and mimic the large-scale patterns different formation age within limited area via high- associated with the entire Dao Vallis system. resolution images or rover. Geologic context suggests Canyon Wall and Floor Morphology: Canyon strong potential for subsurface ice. walls are diverse morphologically and include ice- References: [1] Crown D.A. et al. (1992) Icarus, 100, rich mantling deposits, debris flows, slump blocks 1-25. [2] Crown D.A. and Mest S.C. (2001) LPSC XXXII, and associated erosional grooves, and alcove-gully- Abstract #1344. [3] Mest S.C. and Crown D.A. (2001) apron systems that dissect and redistribute mantling Icarus, 153, 89-110. [4] Price K.H. (1998) USGS Misc. and wall materials [2, 12, 18, 21-22]. Canyon floors Invest. Ser. Map I-2557. [5] Leonard G.J. and Tanaka K.L. (2001) USGS Geol. Invest. Ser. Map I-2694. [6] Mest S.C. include coherent blocks and rounded knobs emplaced and Crown D.A. (2002) USGS Geol. Invest. Ser. Map I- via wall failure, surrounded by unconsolidated 2730. [7] Mest S.C. and Crown D.A. (2003) USGS Geol. materials displaying both smooth and lineated Invest. Ser. Map I-2763. [8] Baker V.R. (1982) The surfaces. Unconsolidated floor materials may have Channels of Mars, Univ. Texas Press. [9] Carr M.H. (1981) been derived from canyon walls and/or failed blocks. The Surface of Mars, Yale Univ. Press. [10] Kargel J.S. and Lineation patterns extend away from source slopes Strom R.G. (1992) Geology, 20, 3-7. [11] Moore J.M. and and show deflection around topographic obstacles on Wilhelms D.E. (2001) Icarus, 154, 258-276. [12] Arfstrom the canyon floor. Dune fields are observed but many J.D. (2002) LPSC XXXIII, Abstract #1174. [13] Christensen areas of the canyon floor are dune free. P.R. (2003) Nature, 422, 45-47. [14] Costard F. et al. Comparison of daytime and nighttime THEMIS (2002) Science, 295, 110-113. [15] Hartmann W.K. et al. data allow variations in the thermophysical properties (2002) Icarus, 162, 259-277. [16] Hartmann W.K. (2001) Space Sci. Rev., 96, 165-194. [17] Lee P. et al. (2001) of deposits to be delineated and dusty and rocky LPSC XXXII, Abstract #1809. [18] Malin M.C. and Edgett regions to be identified. Dao Vallis’ floor deposits K.S. (2000) Science, 288, 2330-2335. [19] Bleamaster L.F. shows a high degree of variability and an overall and Crown D.A., this issue. [20] Squyres S.W. et al. (1987) increase in rockiness relative to the surrounding Icarus, 70, 385-408. [21] Milliken R.E. et al. (2003) JGR, geologic units. This suggests that canyon floors are 108, doi:10.1029/2002JE002005. [22] Arfstrom J.D. (2003) not significantly mantled and is consistent with recent LPSC XXXIV, Abstract #1208.