Icarus 265 (2016) 70–78 Contents lists available at ScienceDirect Icarus journal homepage: www.journals.elsevier.com/icarus Origin of circular collapsed landforms in the Chryse region of Mars ⇑ Manuel Roda a,b, , Maarten G. Kleinhans b, Tanja E. Zegers b, Rob Govers b a Universitá degli Studi di Milano, Dipartimento di Scienze della Terra, Via Mangiagalli, 34, 20133 Milano, Italy b Universiteit Utrecht, Faculty of Geosciences, Heidelberglaan 2, 3584 CS Utrecht, The Netherlands article info abstract Article history: The quasi-circular collapsed landforms occurring in the Chryse region of Mars share similar morpholog- Received 29 June 2015 ical characteristics, such as depth of collapse and polygonally fractured floors. Here, we present a statis- Revised 20 October 2015 tical analysis of diameter, maximum and minimum depth, and amount of collapse of these features. Accepted 21 October 2015 Based on their morphometric characteristics, we find that these landforms have a common origin. In par- Available online 27 October 2015 ticular, the investigated landforms show diameter-depth correlations similar to those that impact craters of equivalent diameters exhibit. We also find that the observed amount of collapse of the collected fea- Keywords: tures is strongly correlated to their diameter. Furthermore, the linear relation between minimum filling Geological processes and pristine depth of craters, the constant ratio between collapse and the amount of filling and the frac- Ices Impact processes tured and chaotic aspect of the filling agree with melting and subsequent collapse of an ice layer below a Mars, surface sediment layer. This interpretation is consistent with a buried sub-ice lake scenario, which is a non-climatic mechanism for producing and storing abundant liquid water under martian conditions. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction the crater. The high amount of collapse can be explained by the withdrawal of magma sills or by the subsidence produced after Numerous quasi-circular collapsed features are present in the the water discharge from the melted cryosphere (Sharp, 1973; Chryse region of Mars and they share similar morphological char- Cabrol et al., 1997; Chapman and Tanaka, 2002; Ogawa et al., acteristics (Fig. 1): deeply collapsed quasi-circular areas with 2003; Leask et al., 2006; Meresse et al., 2008). More commonly intensively fractured floor characterized by polygonal tilted blocks proposed scenarios are based on the increase in pressure of global very different in size (Chapman and Tanaka, 2002; Glotch and aquifer with consequent disruption of the cryosphere and fractur- Christensen, 2005; Rodriguez et al., 2005b; Meresse et al., 2008). ing, followed by intensive groundwater discharge and removal of It has been suggested that these circular landforms originated from sediment by water, that would explain the collapse (Carr, 1979; the collapse of buried impact craters and can occasionally merge Clifford, 1993; Andrews-Hanna and Phillips, 2007; Marra et al., together forming larger chaotic terrains (Sato and Kurita, 2005; 2014a,b). Sato et al. (2010) propose a model of earth fissuring of Rodriguez et al., 2005a). Despite that, different classifications of sediments within the crater as result of differential compaction these objects occur in the literature. Some of them are classified consequent to the increase of groundwater level. The groundwater as (Fig. 1a) chaotic terrains (Carr et al., 1973; Sharp, 1973; Carr, piping active below the crater floor toward the rims would be 1980; Rotto and Tanaka, 1995) and other as (Fig. 1b) floor- responsible for the collapse. Some authors have highlighted the fractured craters (FFC, Schultz and Orphal, 1978; Korteniemi, role of gas hydrated dissolution and release in the collapsing pro- 2003; Korteniemi et al., 2006). The third types includes circular cess (Milton, 1974; Lambert and Chamberlain, 1978; Hoffman, collapsed features with fractured and broken floor within larger 2000; Komatsu et al., 2000; Tanaka et al., 2001; Max and Clifford, chaotic terrains (Fig. 1c and d). This is important, because different 2001; Montgomery and Gillespie, 2005; Kargel et al., 2007). classifications are the basis for different explanations for their ori- Warner et al. (2011) suggest a mechanism of subsurface volume gin (see Bamberg et al. (2014) for an extensive review). loss consequent to the effusion of groundwater to the surface along Based on their similarity with lunar FFC, Schultz and Orphal linear zones of preexisting regional fractures pattern where (1978) suggest uplift and fracturing of crater floor and filling sed- localized deep basins can develop. Collapse over subterranean iment consequent to the rising of magma intrusion underneath groundwater bodies appears to have played a significant role in the generation of chaotic terrains within the study region. Rodriguez et al. (2003, 2005a,b) invoked the drainage of extensive ⇑ Corresponding author at: Universitá degli Studi di Milano, Dipartimento di water-filled caverns in southern circum-Chryse as a major causative Scienze della Terra, Via Mangiagalli, 34, 20133 Milano, Italy. http://dx.doi.org/10.1016/j.icarus.2015.10.020 0019-1035/Ó 2015 Elsevier Inc. All rights reserved. M. Roda et al. / Icarus 265 (2016) 70–78 71 -36 W -34 W -32 W -30 W -28 W -26 W -24 W -22 W 38 6 39 5 3 4 N 2 1 47 2 N 0 4 32 12 0 37 13 14 0 50 100 150 200 250 0255075100125 km a km -2 S 7 7 b 17 5 11 10 -4 S 15 0 42 43 48 22 23 16 -6 S 49 8 -2 S 44 -8 S 24 18 19 0 40 80 120 160 200 0306090120150 20 km c km d -30 W -28 W -26 W -24 W -20 W -18 W -16 W -14 W -6440 m 7127 Fig. 1. Some examples of quasi-circular collapsed landforms in the Chryse region of Mars. They all represent deeply collapsed quasi-circular areas with intensively fractured floor characterized by polygonal tilted blocks very different in size. (a) Chaotic terrains (1,2-Hydaspis Chaos; 32-Hydraotes Chaos): they show a quite irregular shape but some circular features are clearly detectable from the distribution of fractures. (b) Floor-fractured craters. (c) Large chaotic terrains (43-Aureum Chaos; 44-Arsinoes Chaos) in which several small circular collapsed features can be detectable. (d) Low collapsed area (49-Iani Chaos) with some small circular collapsed features inside. DTM MOLA mosaic (Zuber et al., 1992) over shaded relief HRSC images (Jaumann et al., 2007). condition leading to regional highland collapse. In subsequent statistical relationships to evaluate whether they show common investigations, Zegers et al. (2010) and Roda et al. (2014) identified distributions and therefore a possible common origin. Next we will geologic evidence indicative of chaos generation resulting from the focus on the morphological imprint of the evolution of the crater collapse of sedimentary strata into underlying subsurface lakes. infill. On the basis of their morphometric characteristics, we will Although all proposed scenarios predict fracturing and collapse, distinguish between the formation scenarios proposed to explain they are expected to result in very different morphometric charac- the evolution of these landforms. teristics. For example, the maximum subsidence resulting by an unrealistic complete discharge of a 20-km deep aquifer is limited 2. Method to 1.2 km (using a depth-porosity relation suggested by Clifford et al. (2010)). Intrusions of magma may be followed by higher col- For about fifty quasi-circular collapsed landforms around lapse during the withdrawal of magma (Meresse et al., 2008). The Chryse region (Fig. 2) we measure the main morphometric charac- sub-ice lake scenario requires the existence of an original crater teristics such as the diameter and the maximum and minimum filled with ice which drives the resulting amount and morphology depth with respect to the surrounding, non-collapsed area of the collapse (Zegers et al., 2010; Roda et al., 2014). (Fig. 3). We also measure the collapse as difference between the In order to discern the origin and the mechanism of the evolu- maximum and minimum depth and it is considered as the tion of the quasi-circular collapsed features we investigate observed collapse depth. This represents the collapse achievable. whether the chaotic terrains are really different from In fact the amount of collapse can be higher if the measured floor-fractured craters and then which mechanism can explain minimum depth is generated by collapse with respect to the the peculiar morphology of these landforms. We will first focus surrounding areas. In this case the collapse would coincide with on their origin. We analyze the diameter, maximum depth and the maximum depth. However, the minimum depth can be only observed collapse of the collapsed landforms and we study their the result of pre-existing difference in topography. Since it is not 72 M. Roda et al. / Icarus 265 (2016) 70–78 -45° W -40° W -35° W -30° W -25° W -20° W -15° W 40 29 41 10° N 38 39 5° N 2 1 5 6 47 32 3 35 12 0° 28 13 4 36 37 22 14 23 52 49 7 24 11 17 15 25 10 42 -5° S 48 51 33 16 30 50 43 26 8 31 27 45 19 44 18 46 20 -10° S 21 34 0 200 400 600 800 1,000 9 km Fig. 2. Location of collected landforms around Chryse region. See Supplementary material for the reference. Elevation map based on MOLA mosaic. min depth min max collapse depth pristine depth min filling Fig. 3. Representation of measured and calculated parameters on a hypothetic cross section of a collapsed feature. The observed collapse depth is the difference between maximum and minimum depth, the pristine depth is obtained from four different empirical relations and the minimum filling is the difference between pristine and maximum depth.