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Mangala Valles Outflow Channel, Mars doi: 10.1111/j.1365-3121.2005.00642.x Evidence for remnants of ancient ice-rich deposits: Mangala Valles outflow channel, Mars Joseph S. Levy and James W. Head Department of Geological Sciences, Brown University, Providence, RI 02912, USA ABSTRACT High-resolution spacecraft data reveal the presence of a the remnant of floodwaters trapped in channel lows. This distinctive unit on the upper reaches of the floor of Mangala interpretation further supports the likelihood that climate Valles, an ancient outflow channel on Mars. In contrast to conditions at the time of emplacement were similar to the abundant evidence for scour, intense erosion, and hydro- hyper-arid cold desert conditions of today. Frozen floodwaters dynamic shaping typical of the floors and margins of Mangala from the ancient martian subsurface could still be preserved in and other outflow channels, this unit is smooth-surfaced, has these deposits, representing attractive and accessible exobio- cuspate margins, is superposed on the scoured valley floor, and logical exploration objectives. is extensively pitted. We interpret this unit to be the sublimat- ion lag deposit derived from an ice thermal boundary layer on Terra Nova, 17, 503–509, 2005 the aqueous flood that formed the outflow channel system, and Greeley, 1994; Ghatan et al., 2005), undulatory and rounded in some Introduction this unit (Fig. 3) is smooth-surfaced, locations, and scalloped in others, Outflow channels represent one of the has arcuate and cuspate margins, and forming arcuate serrations (Figs 3– most dramatic manifestations of the has a host of unusual surface features 5). Where the unit meets the adjacent presence, during the Late Hesperian- including round pits and ring struc- walls of the channel, or where there is Amazonian periods, of huge quantities tures often containing huge angular a contact between the smooth unit of liquid water on the surface of Mars. blocks (Fig. 7). We assess several poss- and the scoured unit on which it is Features associated with outflow ible origins for this unit and the asso- superimposed, the level and uniform channels show evidence for catastro- ciated features, and conclude that the surface of the smooth unit is bevelled. phic water release from the subsurface, most plausible explanation is an ice- From these relationships we can deter- extensive overland flow and erosion, rich remnant created by a combination mine the minimum thickness of the ponding in low-lying regions and ulti- of ponding and proximal ice-cover unit: c. 10–15 m. At the proximal end mate return to other water reservoirs deflation during the waning stages of of the channel, the unit is composed of (e.g. Baker et al., 1991; Carr, 1996; the outflow channel flood event. two layered subunits, displaying sim- Kreslavsky and Head, 2002). The fate ilar morphology but different spatial of the water released and emplaced on extent (Fig. 5). Description and setting of the the surface in these events has always smooth unit been a matter of controversy (e.g. Description of associated surface Carr, 1996) and the nature of such Mangala Valles is an c. 800 km long, features groundwater and its relation to poten- outflow channel located south-west of tial exobiological environments has Tharsis (Fig. 1). It extends northward Impact craters been of extreme interest (e.g. McKay from a break in one of the Memnonia and Stoker, 1989; McKay and Davis, Fossae graben, across the southern Both the smooth unit and the sur- 1991). highlands, and terminates in the nor- rounding scoured terrain display im- High-resolution images from MGS thern lowlands at the dichotomy pact craters. Primary impact craters and Odyssey reveal an unusual unit on boundary. The geologic units mapped are distinguished by raised rims and a the floor of the Mangala Valles out- on the valley floor include Amch generally circular shape (Figs 3–7). flow channel (Figs 1–7). In contrast to (younger, early Amazonian channel Craters range in diameter from 20 to abundant terrain showing scour and material), AHmch (older, late-Hespe- 560 m. Impact crater counts between hydrodynamic shaping typical of the rian/early-Amazonian channel mater- the scoured surface and the smooth floors and margins of Mangala Valles ial) and Hmp (late Hesperian plains surface are generally similar, with the and other outflow channels (e.g. Tan- material) (Zimbelman et al., 1994). scoured terrain appearing slightly old- aka and Chapman, 1990; Zimbelman The smooth unit, Amch, stretches er than the superimposed smooth unit, et al., 1992, 1994; Craddock and from the head of Mangala Valles consistent with the observed strati- ()18.1 N, 210.6 E) to points at least graphic relationships. 400 km north ()11.7 N, 208.8 E), Correspondence: Joseph S. Levy, Depart- varying in width from 10 km to Pits ment of Geological Sciences, Brown Uni- <400 m. MOLA altimetry demon- versity, 324 Brook Street, Providence, RI strates that the unit occurs predomi- The most striking features observed on 02912, USA. Tel.: +1 401 863 3485; nantly in the lowest regions of the the smooth unit are abundant, shal- fax: +1 401 863 2058; e-mail: joseph_ low, round pits (Figs 3–7). Angular [email protected] channel. The margins of the unit are Ó 2005 Blackwell Publishing Ltd 503 Evidence for remnants of ancient ice-rich deposits • J. S. Levy and J. W. Head Terra Nova, Vol 17, No. 6, 503–509 ............................................................................................................................................................. Fig. 2 THEMIS context images for THEMIS images (a) V01454001, (b) V06647001, (c) V04762003, and (d) V04400003. Landmarks in each of the context images can be used to locate high-resolution image locations using Fig. 1 (Above) MOLA topographic map of Mars showing major surface regions and the maps provided in Fig. 1. Black lines the Mangala Valles region, boxed, south-west of Tharsis. (Lower left) Shaded relief are synthetic features associated with map of the Mangala Valles region, derived from MOLA data. (Lower right) Sketch longitude and latitude lines. map of the Mangala Valles/Memnonia Fossae region showing the location and lowest portions of the Mangala Valles distribution of the features and deposits associated with the Mangala Valles outflow outflow channel, and are similar in age channel. Mv is Mangala Valles regions not associated with notable features. Mf is one to the adjacent and underlying inten- of the Memnonia Fossae graben. Cr marks cratered upland terrain. Sm denotes the smooth unit, and Sc the scoured unit. Significant craters are marked with filled circles. sely scoured valley floor. They show a, b, c, and d mark the locations of THEMIS images V01454001, V06647001, evidence for pitting, marginal scallop- V04762003, and V04400003, respectively. After Zimbelman et al. (1994). ing, and a distal braided ridge. boulders are found at the centres of features are often of a comparable size many pits near the proximal end of to the pits observed in the centre of c the channel (Fig. 7). Most of the pits the smooth unit. Other portions of the Fig. 3 A portion of THEMIS image with blocks have a wide raised rim; margin are more broadly scalloped V4400003. A scalloped margin delin- however, none of the empty pits have (Fig. 3). eates the contact between the smooth raised rims. Pits with blocks tend to be unit (right) and the scoured unit (left). larger than empty pits; however, the The rounded and cuspate morphology Distal sinuous ridge largest pits are empty (Figs 4 and 5). of the margin suggests that embayments into the smooth unit may be the result of Clusters of pits show signs of coales- In several locations, the width of the sublimation pits forming near the edge cence, forming elongated rectangular unit decreases towards the distal end of the unit. Longer embayments may be depressions with long axes sub-paral- of the channel, developing into a the result of several pits coalescing near lel to the channel (Fig. 4). There is no sinuous feature several hundred the edge of the smooth unit. Arrows apparent spatial preference for pitting metres wide (Fig. 6). The sinuous indicate features interpreted to be scal- density. portion of the smooth unit remains loping associated with pit formation in the lowest reaches of the channels. (bottom and top) and more rounded Scalloped margins and serrations scalloping (centre). Similar margins have Summary of characteristics been observed in the Tiu/Ares Valles The margins of the smooth unit are regions of Chryse Planitia and have been often serrated with scalloped or cus- The deposits are flat and smooth, at interpreted as thermokarst features pate features (Fig. 3). These margin least 10–15 m thick, occur in the (Tanaka, 1997, 1999). 504 Ó 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 6, 503–509 J. S. Levy and J. W. Head • Evidence for remnants of ancient ice-rich deposits ............................................................................................................................................................. the Mangala Valles outflow channel tions of the channel system (Ghatan event. Following extensive flooding et al., 2005). Furthermore, if global and scouring of the valley, flow veloci- climate conditions were similar to pre- ties would have decreased and flood- sent conditions (as suggested by the waters would have receded until the presence of glacier-like deposits in the last stages ponded in low-lying por- Mangala Valles source region, Head a b Fig. 4 (a) A portion of THEMIS image V01454001 showing the intensely pitted surface of the smooth unit and the contact between the smooth unit and the scoured unit (right). Pits show some signs of coalescence. We measured the diameters of all Discussion and interpretation 404 pits and 2137 craters in THEMIS images V01454001, V0440003, and V04762003. The pits range in diameter from 100 to 600 m, with a mean of 230 m and standard The basic characteristics and strati- deviation of 60 m.
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