Accepted Manuscript Concentric Crater Fill in the Northern Mid-Latitudes of Mars: Formation Pro‐ cesses and Relationships to Similar Landforms of Glacial Origin Joseph Levy, James W. Head, David R. Marchant PII: S0019-1035(10)00146-6 DOI: 10.1016/j.icarus.2010.03.036 Reference: YICAR 9395 To appear in: Icarus Received Date: 6 November 2009 Revised Date: 24 February 2010 Accepted Date: 31 March 2010 Please cite this article as: Levy, J., Head, J.W., Marchant, D.R., Concentric Crater Fill in the Northern Mid-Latitudes of Mars: Formation Processes and Relationships to Similar Landforms of Glacial Origin, Icarus (2010), doi: 10.1016/ j.icarus.2010.03.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT 1 1 2 3 4 5 6 7 8 9 Concentric Crater Fill in the Northern Mid-Latitudes of Mars: 10 Formation Processes and Relationships to Similar Landforms of Glacial Origin 11 12 Joseph Levy1*, James W. Head1, and David R. Marchant2 13 1 Department of Geological Sciences, Brown University, Providence, RI, 02912 14 Ph. 401-863-3485 Fax. 401-863-3978 Email. [email protected] 15 2 Department of Earth Science, Boston University, Boston, MA, 02215 16 * Now at Department of Geology, Portland State University, Portland, OR, 97201 17 Ph. 503-725-3355 Email. [email protected] 18 19 Submitted to Icarus: November 6, 2009 20 Revised: Feb 28, 2010 21 22 23 29 Pages 24 16 Figures 25 1 Table ACCEPTED MANUSCRIPT 2 26 Running Head: Concentric crater fill and Amazonian glacial systems 27 28 Corresponding Author: 29 30 Joseph Levy 31 Department of Geology 32 Portland State University 33 P.O. Box 751 34 Portland OR 97207-0751 35 [email protected] ACCEPTED MANUSCRIPT 3 36 Abstract: 37 Hypotheses for the formation of concentric crater fill (CCF) on Mars range from ice-free 38 processes (e.g., aeolian fill), to ice-assisted talus creep, to debris-covered glaciers. Based on 39 analysis of new CTX and HiRISE data, we find that concentric crater fill (CCF) is a significant 40 component of Amazonian-aged glacial landsystems on Mars. We present mapping results 41 documenting the nature and extent of CCF along the martian dichotomy boundary over -30˚ – 90˚ 42 E latitude and 20˚ - 80˚ N longitude. On the basis of morphological analysis we classify CCF 43 landforms into “classic” CCF and “low-definition” CCF. Classic CCF is most typical in the 44 middle latitudes of the analysis area (~30-50˚ N), while a range of degradation processes results 45 in the presence of low-definition CCF landforms at higher and lower latitudes. We evaluate 46 formation mechanisms for CCF on the basis of morphological and topographic analyses, and 47 interpret the landforms to be relict debris-covered glaciers, rather than ice-mobilized talus or 48 aeolian units. We examine filled crater depth-diameter ratios and conclude that in many locations, 49 hundreds of meters of ice may still be present under desiccated surficial debris. This conclusion is 50 consistent with the abundance of “ring-mold craters” on CCF surfaces that suggest the presence 51 of near-surface ice. Analysis of breached craters and distal glacial deposits suggests that in some 52 locations, CCF-related ice was once several hundred meters higher than its current level, and has 53 sublimated significantly during the most recent Amazonian. Crater counts on ejecta blankets of 54 filled and unfilled craters suggests that CCF formed most recently between ~60-300 Ma, 55 consistent with the formation ages of other martian debris-covered glacial landforms such as 56 lineated valley fill (LVF) and lobate debris aprons (LDA). Morphological analysis of CCF in the 57 vicinity of LVF and LDA suggests that CCF is a part of an integrated LVF/LDA/CCF glacial 58 landsystem. Instances of morphological continuity between CCF, LVF, and LDA are abundant. 59 The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF 60 and LDA, suggests the possibility that CCF represents one component of the significant 61 Amazonian mid-latitude glaciation(s) on Mars. ACCEPTED MANUSCRIPT 4 62 Keywords: Mars, Mars Climate, Mars Surface ACCEPTED MANUSCRIPT 5 63 1. Introduction 64 65 Concentrically-lineated, crater-filling deposits (concentric crater fill, or CCF, Figure 1) 66 have been described at mid-high latitudes on Mars since the Viking era, and have long been 67 associated with a suite of features typical of fretted terrain (e.g., lobate debris aprons, LDA; and 68 lineated valley fill, LVF) [Sharp, 1973; Squyres, 1978; 1979; Malin and Edgett, 2001]. 69 Hypotheses explaining the formation of these unusual landforms range from entirely water-free 70 processes to those requiring the deposition and maintenance of hundreds of meters of nearly pure 71 ice. For example, Zimbelman et al. [1989] accounts for CCF by the emplacement and removal of 72 aeolian layers. Invoking limited quantities of water, Sharp [1973] explains fretted terrain as 73 debris derived from bedrock recession along scarps due to sublimation of ground ice or the 74 emergence of groundwater. Squyres [1978] suggests that lineated valley fill results from the 75 mobilization and flow of talus by diffusionally-emplaced pore ice; while Squyres [1979] and 76 Squyres and Carr [1986] extend this rock-glacier-like mechanism [e.g., Haeberli et al., 2006] to 77 lobate debris aprons and concentric crater fill. A greater role for internally-deforming ice in LVF 78 processes was proposed by Lucchitta [1984], drawing explicit analogs between lineated valley fill 79 and polythermal Antarctic glaciers. In a study of large craters in the Vastitas Borealis Formation, 80 Kreslavsky and Head [2006] suggest a range of crater-filling mechanisms to account for the 81 filling of high-latitude craters on Mars, including debris-covered glacial processes [see also, 82 Garvin et al., 2006]; similar to the debris-covered glacier mechanism invoked by Levy et al. 83 [2009b] to account for CCF morphology in Utopia Planitia. The potential for the current presence 84 of significant quantities of ice in CCF and LVF was suggested by the documentation of abundant 85 examples of integrated, glacier-like flow features in LVF and LDA across the Martian dichotomy 86 boundary [e.g., Head et al., 2006a; Head et al., 2006b, 2009; Levy et al., 2007; Morgan et al., 87 2009], the presence of ring-mold craters in LVF/LDA deposits [Kress and Head, 2008], and the ACCEPTED MANUSCRIPT 6 88 discovery of significant volumes of nearly pure ice within lobate debris aprons and lineated valley 89 fill described by Holt et al. [2008] and Plaut et al. [2009] using SHARAD radar data. 90 Advances in understanding of CCF and related landforms have followed from the 91 improving spatial resolution and coverage of spacecraft data, including image and topographic 92 datasets. Here, we use Context Camera (CTX) [Edgett et al., 2008] and High Resolution Imaging 93 Science Experiment (HiRISE) [McEwen et al., 2009] image data, coupled with HRSC high- 94 resolution stereo DEMs [Oberst et al., 2005; Jaumann et al., 2007] and supplemented by MOLA 95 topographic data, to address the following outstanding questions related to CCF: 1) What are the 96 morphological characteristics of CCF at high resolution, and what does the morphology suggest 97 about CCF composition? 2) Are there morphological subtypes of CCF, and if so, what is their 98 geographical distribution? 3) If ice is involved in CCF processes, is any still present? 4) What is 99 the relationship between CCF and other crater-filling materials (e.g., dunes) on Mars? 5) What is 100 the relationship between CCF and LVF/LDA? 5) What do relationships between LVF/LDA/CCF 101 processes suggest about the origin and development of martian glacial landforms? We focus on 102 CCF found along the Martian dichotomy boundary (particularly within -30 - 90° E and 20 - 80° 103 N), in the vicinity of a range of landforms that have been interpreted as features of glacial origin. 104 105 2. Mapping Mid-Latitude Crater-Filling Units 106 107 Concentric crater fill (CCF) has been classically described in regions containing abundant 108 lineated valley fill (LVF) and/or lobate debris aprons (LDA) as a crater-interior unit characterized 109 by multiple rings or lineations concentric with the crater rim [Squyres, 1979] (Figure 1a, 2). At 110 Mars Orbiter Camera (MOC) resolution (1.5-20 m/pixel), the surface texture associated with 111 LVF, LDA, and CCF lineations appears mounded and furrowed, a texture described as “brain or 112 corn-like pit and mound textures” [Malin and Edgett, 2001], “pit-and-butte” texture [Mangold, 113 2003] or “knobs—brain coral” [Williams, 2006]. Detailed analysis of HiRISE images has led to a ACCEPTED MANUSCRIPT 7 114 description of CCF surface texture as “brain coral terrain” [Dobrea et al., 2007] or “brain terrain” 115 [Levy et al., 2009b]. For succinctness and consistency, we will refer to this surface texture 116 throughout as “brain terrain.” HiRISE images reveal textural differences between “brain terrain” 117 mounds or “cells”—particularly “closed cells” (arcuate or domed mound features that are 118 commonly ~10-20 m wide, and that may have surface grooves or furrows located near the 119 centerline of the cell’s long axis) and “open cells” (arcuate and cuspate cells that are delimited by 120 a convex-up boundary ridge surrounding a flat-floored depression) (Figure 2) [Levy et al., 2009b].