Geologically Recent Gully–Polygon Relationships on Mars: Insights From
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Icarus 201 (2009) 113–126 Contents lists available at ScienceDirect Icarus www.elsevier.com/locate/icarus Geologically recent gully–polygon relationships on Mars: Insights from the Antarctic Dry Valleys on the roles of permafrost, microclimates, and water sources for surface flow ∗ J.S. Levy a, ,J.W.Heada, D.R. Marchant b,J.L.Dicksona, G.A. Morgan a a Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA b Department of Earth Science, Boston University, 675 Commonwealth Ave., Boston, MA 02215, USA article info abstract Article history: We describe the morphology and spatial relationships between composite-wedge polygons and Mars- Received 29 May 2008 like gullies (consisting of alcoves, channels, and fans) in the hyper-arid Antarctic Dry Valleys (ADV), Revised 17 October 2008 as a basis for understanding possible origins for martian gullies that also occur in association with Accepted 22 December 2008 polygonally patterned ground. Gullies in the ADV arise in part from the melting of atmospherically- Available online 21 January 2009 derived, wind-blown snow trapped in polygon troughs. Snowmelt that yields surface flow can occur Keywords: during peak southern hemisphere summer daytime insolation conditions. Ice-cemented permafrost Mars surface provides an impermeable substrate over which meltwater flows, but does not significantly contribute Earth to meltwater generation. Relationships between contraction crack polygons and sedimentary fans at the Geological processes distal ends of gullies show deposition of fan material in polygon troughs, and dissection of fans by Ices expanding polygon troughs. These observations suggest the continuous presence of meters-thick ice- Regoliths cemented permafrost beneath ADV gullies. We document strong morphological similarities between gullies and polygons on Mars and those observed in the ADV Inland Mixed microclimate zone. On the basis of this morphological comparison, we propose an analogous, top–down melting model for the initiation and evolution of martian gullies that occur on polygonally-patterned, mantled surfaces. © 2009 Elsevier Inc. All rights reserved. 1. Introduction gullies can form by dry avalanche processes alone (Treiman, 2003; Pelletier et al., 2008). Gullies on Mars are a class of geologically young features, ini- Concurrent with advances in understanding of gully processes tially interpreted to have formed by surficial flow of released on Mars, modeling and observational studies have documented the groundwater (Malin and Edgett, 2000, 2001; Mellon and Phillips, distribution and origin of various types of martian thermal con- 2001), and which may still be active (Malin et al., 2006). Mar- traction crack polygons (Mellon, 1997; Mangold, 2005; Levy et al., tian gullies are geomorphic features composed of a recessed al- 2008a). Despite the observation of polygonally patterned ground cove, one or more sinuous channels, and a depositional fan or in gullied terrains on Mars and Earth (Malin and Edgett, 2000, apron (Malin and Edgett, 2000). Alternative hypotheses for the 2001; Bridges and Lackner, 2006), and an increasing awareness source of gully-carving fluids include obliquity-driven melting of of the importance of polygonally patterned permafrost in the de- near-surface ground ice (Costard et al., 2002), melting of dust-rich velopment of terrestrial polar fluvial systems (Fortier et al., 2007; snow deposits (Christensen, 2003), and melting of atmospherically Levy et al., 2007a; Levy et al., 2008b), there has been little analy- emplaced frost and/or snow (Hecht, 2002; Dickson et al., 2007a; sis of the interactions between thermal contraction crack polygons Head et al., 2007; Dickson and Head, 2008; Williams et al., and gullies on Mars. 2008). Complementing gully formation models, recent GCM re- In this contribution, we explore interactions between gullies sults (Forget et al., 2007) predict the deposition and potential for and polygons in the Mars-like Antarctic Dry Valleys (Marchant and melt of up to 25 mm/yr of water ice at martian northern midlat- ◦ Head, 2007), and then assess similarities and differences with fea- itudes (∼30–50 N) during obliquity conditions modeled to have tures observed on Mars. We first summarize recent research on occurred within the past 10 My (and potentially within the past the spatial distribution, formation, and modification of gullies and <1My;Laskar et al., 2004). Other workers have proposed that polygons in selected regions of the Antarctic Dry Valleys (ADV). In the next section we show how gully development on polygonally * Corresponding author. Fax: +1 401 863 3978. patterned ADV surfaces affects gully morphology and enhances E-mail address: [email protected] (J.S. Levy). water-flow processes. Further, we show how the morphology of 0019-1035/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2008.12.043 114 J.S. Levy et al. / Icarus 201 (2009) 113–126 Fig. 1. Perspective view of a portion of the South Fork study area in upper Wright Valley, Antarctica. Black arrows indicate channels on the southern wall of the valley and white arrows indicate large alcoves present in the dolerite bedrock, approximately 1000 m above the valley floor. The dark, tongue-shaped lobe of dolerite boulders at center is approximately 300 m wide. Inset. Boxed region showing a small concavity present in the colluvium slope. White arrow indicates the center of the depression. Channels enter into and emanate from the concavity. polygons is altered by proximity to developing gullies. We describe are termed equilibrium landforms (Marchant and Head, 2007). Gul- such reciprocal modification relationships as “gully–polygon sys- lies and polygons are the two dominant equilibrium landforms on tems.” inland-mixed zone valley walls. We then analyze HiRISE images that document the interplay between polygonally patterned ground, ice-cemented permafrost, 2.1. Gully–polygon systems in the ADV and gullies on Mars. If strong morphological similarities exist be- tween gullies and polygons observed on Mars and those docu- In the inland mixed zone of the ADV, gullies are character- mented in the ADV, then this evidence would suggest that, to ized by a recessed alcove, sinuous channels with seasonally moist a first order, some martian gullies formed and were modified hyporheic zones (McKnight et al., 1999; Gooseff et al., 2002; by processes analogous to those occurring in ADV gully–polygon Levy et al., 2008b), and one or more distal fans (Figs. 1 and 2). The systems. Such morphological comparisons can help constrain the hyporheic zone is the area marginal to and beneath a stream that physical and hydrological properties of gully flow. We address exchanges water with the stream channel. Within and adjacent to concerns over equifinality (similar morphologies produced by dif- most gullies, dry, ice-free sediment overlies sediment that is ce- ferent processes) by focusing our analysis on morphological rela- mented by pore ice. The lower depth of this pore ice is unknown, tionships that illustrate specific spatial and stratigraphic relation- but its surface, called the “ice-cement table,” is fairly uniform and ships. occurs on average at about 15–20 cm depth (Bockheim et al., 2007; Levy et al., 2007b). Typically, the ice-cement table deepens with increasing distance from isolated snow banks and gully chan- 2. The Antarctic Dry Valleys (ADV) nels. In ADV areas with extensive pore ice, the ground commonly The Antarctic Dry Valleys are a suitable laboratory for under- shows well-developed thermal contraction crack polygons (Berg standing the geomorphological effects of water moving through and Black, 1966). All gullies save one observed in the Wright Valley temperature-dependent phase transitions (freezing, melting, sub- study site are present on polygonally-patterned slopes (Levy et al., limation, evaporation). On the basis of summertime air tempera- 2008b; Morgan et al., 2008). Across the ADV, active and recently ture, relative humidity, soil temperature, and soil moisture con- active gullies are typically present in association with contraction- ditions, the ADV region is divided into three microclimate zones. crack polygons; relict gullies in the coldest and driest portion of The three zones include a coastal thaw zone, an inland mixed the ADV that have been inactive for up to 10 My (Lewis et al., zone, and a stable upland zone (Marchant and Denton, 1996; 2007) typically lack polygons characteristic of the Wright Valley Marchant and Head, 2007). In the inland mixed zone, melting, site. The most common polygons present in the South Fork area are evaporation, and sublimation occur, whereas in the stable up- composite-wedge polygons (Levy et al., 2008b). Composite-wedge land zone, sublimation is the dominant phase transition (Ragotzkie polygons are those in which alternating layers of sand and ice fill and Likens, 1964; Marchant et al., 2002; Kowalewski et al., 2006; thermal contraction cracks (Berg and Black, 1966). Importantly, ar- Marchant and Head, 2007). The stable upland zone is interpreted eas in the Dry Valleys that lack pore ice within the upper ∼1m to be closely analogous to Mars under current, average climate of soils tend to lack all varieties of thermal contraction crack poly- conditions, whereas the inland mixed zone may be a good analog gons (Marchant and Head, 2007). for more clement martian conditions produced by orbitally-driven climate change (Marchant and Head, 2007) or for short duration 2.1.1. ADV gully water sources peak temperature and insolation conditions. Landforms that are Soil-temperature measurements indicate that melting along the produced in equilibrium with microclimate conditions in each zone ice-cement table in the inland mixed zone is uncommon, and Geologically recent gully–polygon systems on Mars and Earth 115 Fig. 2. Summary of key observations from gully–polygon systems in the South Fork of upper Wright Valley, Antarctica. (a) Polygon troughs accumulate wind-blown snowbanks that contribute meltwater to gully flow. (b) Trough excavated through dry colluvium across a downslope-oriented polygon trough. The ice-cement table is depressed along polygon troughs, channelizing flow over the ice-cement table.