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
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9 Concentric Crater Fill in the Northern Mid-Latitudes of Mars:
10 Formation Processes and Relationships to Similar Landforms of Glacial Origin
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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]
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19 Submitted to Icarus: November 6, 2009
20 Revised: Feb 28, 2010
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23 29 Pages
24 16 Figures
25 1 Table ACCEPTED MANUSCRIPT 2
26 Running Head: Concentric crater fill and Amazonian glacial systems
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28 Corresponding Author:
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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
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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].
121 Mapping of crater-filling units at Martian mid-latitudes can be accomplished using both
122 HiRISE and CTX image data in concert (Figure 3). Full-resolution CTX data are sufficient to
123 resolve broad-scale ridges and troughs associated with CCF (Figure 2), while detailed analysis of
124 relationships between “brain terrain” surface textures is facilitated by observation at HiRISE
125 resolution. Full-resolution CTX images spanning Mars Reconnaissance Orbiter orbits 1331-7927
126 from the study region (-30 – 90˚E, 20-80˚N) were inspected for the presence of crater-filling
127 units. Crater-fill units from the 1000 images surveyed were mapped on a sinusoidal equal-area
128 global map (Figure 3).
129 For mapping morphological subtypes, we define “classic” CCF as a crater-interior unit
130 showing lineations concentric with at least some portion of the crater rim and displaying “brain
131 terrain” surface texture (Figure 1a). It is the orientation of the long-axis of “brain terrain” cells
132 that provides the fine structure of CCF lineations, (Figure 2). Topographic ridges and troughs
133 ~100 m wide define the broader concentric lineations associated with CCF and can be clearly
134 resolved in CTX image data (Figure 2) [Levy et al., 2009b]. The distribution of classic CCF along
135 the martian dichotomy boundary is shown in Figure 3. In our survey region, classic CCF is
136 widely distributed between ~27˚ N and ~50˚ N, with scattered examples at higher and lower
137 latitudes (Figure 3).
138 In some craters, CCF lacks clearly defined concentric lineations produced by topography
139 or lacks clearly-defined “brain terrain” cells (Figure 1b). We refer to this CCF as “low-definition” ACCEPTED MANUSCRIPT 8
140 CCF. Low-definition CCF may show widened fractures or linear depressions oriented both
141 concentric-with and radial-to the crater rim (Figure 1b) or may have a muted surface texture,
142 showing only subtle stippling or shallow undulations in broad-scale topography. The latter
143 surface texture is characteristic of higher-latitude low-definition CCF, where continuous latitude-
144 dependent mantle surfaces are thicker and more widespread [Head et al., 2003; Levy et al.,
145 2009b]. The distribution of low-definition CCF along the dichotomy boundary is mapped in
146 Figure 3.
147
148 3. CCF Morphological Relationships
149
150 Having briefly outlined the morphological characteristics of individual CCF deposits, the
151 next step in assessing the origin, development, and modification history of CCF is to document
152 the range of stratigraphic and morphological relationships observed between CCF and other
153 martian landforms [e.g., Squyres, 1979; Squyres and Carr, 1986; Levy et al., 2009b]. Here, we
154 consider relationships observed between both ice-related landforms (e.g., LVF, LDA) [Squyres,
155 1979; Lucchitta, 1984; Squyres and Carr, 1986; Head et al., 2006a; Head et al., 2006b; Head et
156 al., 2008; Holt et al., 2008; Head et al., 2009; Levy et al., 2009b; Plaut et al., 2009] and non-ice-
157 related landforms (such as talus, craters, and dunes).
158
159 3.1 CCF and Host Craters
160 Because CCF is a crater-interior unit, relationships between CCF and its host craters can
161 be used to evaluate CCF-forming processes. Analysis of image and topography data indicates that
162 CCF does not merely line or coat the interior of craters; rather it consists of a flat and high (mesa-
163 like) or convex-up surface that fills host craters (Figure 4). Fill thicknesses can be estimated by
164 comparing predicted crater depths from [Garvin et al., 2002] with observed depths from MOLA ACCEPTED MANUSCRIPT 9
165 or HRSC topographic profiles [e.g., Levy et al., 2009b]. Six measured fill depths vary from ~600
166 m to ~1700 m, and commonly constitute ~75% of expected crater depth (Figure 4).
167 Contacts between CCF material and crater walls have variable morphology. In some
168 instances, the upper surface of CCF material sits well above the crater wall and is topographically
169 separated from the crater wall by a bounding scarp up to ~100 m high (Figure 4). In other cases,
170 CCF surfaces grade from sloping crater walls to the CCF surface—this latter relationship is most
171 typical of craters in which abundant latitude-dependent mantle material rings the crater interior
172 (Figure 2) [Head et al., 2003; Levy et al., 2009b].
173 In some cases, crater-fill material appears to have cross-cut or transgressed over crater
174 rims (Figure 5). For example, one crater in Deuteronilus Mensae (Figure 5) shows low-definition
175 CCF within a crater that is connected to degraded “brain-terrain” surfaces outside the crater by a
176 series of linear, positive-relief ridges. Similar ridges are present to the east and to the west of the
177 crater. Ridges to the east of the crater extend from a nearby valley floor, upslope and onto the
178 outer walls of the crater rim. HRSC topographic profiles of the deposit suggest that the low-
179 definition CCF material is ~300 m lower within the crater and outside the crater than it is on the
180 rim of the crater (Figure 5). The ridges are morphologically similar to drop moraines typical of
181 terrestrial glaciers [Marchant et al., 1993; Benn et al., 2003], as well as martian glaciers [Head
182 and Marchant, 2003; Shean et al., 2005] that form as debris is shed at the glacier margins. If this
183 interpretation is correct, it indicates that previously crater-filling material was at least 300 m
184 higher in the past—sufficient to permit flow over the crater rim and onto the valley floor outside
185 of the crater.
186
187 3.2 CCF and Dunes
188 Zimbelman et al. [1989] hypothesized that CCF is an aeolian feature, primarily on the
189 basis of a layered surface texture visible in Viking images of “classic” CCF terrain; what do high-
190 resolution images suggest about crater-filling aeolian processes (e.g., sediment and dune ACCEPTED MANUSCRIPT 10
191 deposition)? At both high and low latitudes, dunes partially fill some craters (Figure 6). Dunes are
192 seldom present along the entire interior of a crater, and typically drape, rather than fully fill, the
193 crater interior (Figure 6). Dune morphology can be readily distinguished from “brain terrain”
194 textures in HiRISE and CTX images, suggesting that CCF is not primarily composed of mobile
195 aeolian material. Rather, higher-resolution image data reveal that the layered surface texture
196 observed in Viking image data [Zimbelman et al., 1989] is a visual effect produced by closely-
197 spaced lineations composed of “brain terrain” surface texture on topographically undulatory CCF
198 ridges (Figure 2) [Levy et al., 2009b].
199
200 3.3 CCF and Talus
201 In CTX and HiRISE images, contacts between CCF and talus slopes are clearly
202 discernable (Figure 7). Some examples of “low-definition” CCF (e.g., in Deuteronilus Mensae or
203 Arabia Terra, Figure 7) show that “brain terrain” patterned crater-fill material abuts talus slopes
204 associated with crater rims. The contact between such talus slopes and the CCF is abrupt, with
205 CCF material present downslope along a steep scarp. Such relationships are most clearly visible
206 at low latitudes, where annuli of mantling material are not present at the crater/crater-fill contact
207 (e.g., Figure 2). Similar contacts between LVF and talus scarps in Nilosyrtis Mensae were
208 interpreted by Levy et al. [2007] as evidence of the recession of LVF surfaces from a previous
209 high-stand. This model suggests that the talus slope was emplaced down to the level of the
210 (previously higher) LVF margin, and remains as a steep talus slope now that the LVF/CCF
211 surface has vertically ablated. In other locations, CCF is topographically separated from the
212 interior slopes of crater walls by tens to hundreds of meters of relief, suggesting that CCF
213 material is physically distinct from talus shed by crater walls (Figure 4) [Russell et al., 2004;
214 Russell and Head, 2005]. Lastly, CCF is present in craters with a range of rim morphologies,
215 ranging from crisp and relatively well-defined (Figures 1-2, 8) to complexly eroded (Figure 9),
216 suggesting that CCF is not simply composed of mobilized talus shed from crater rims. ACCEPTED MANUSCRIPT 11
217
218 3.4 CCF and LVF
219 CCF and lineated valley fill (LVF) commonly are present in the same geographical areas
220 on Mars [Squyres, 1979; Squyres and Carr, 1986; Head et al., 2006b; Head et al., 2009]. What
221 do contacts between LVF and CCF show? In many locations (Figures 10-11), contacts between
222 CCF and LVF show surficial merging of CCF and LVF lineations. LVF-containing valleys
223 commonly breach craters hosting CCF, both from upslope (Figure 10) and downslope (Figures
224 10-11). Merging of surface lineations occurs in both cases [Head et al., 2006a]. HiRISE analysis
225 of “brain terrain” surface textures on both CCF and LVF show similar groupings of open and
226 closed cells forming characteristic lineations (Figure 12). Topographic profiles across LVF/CCF
227 contacts are gradational and smooth in HiRISE image data, and at both MOLA resolution (~460
228 m/pixel) and HRSC-DTM resolution (~100 m/pixel) (Figures 10-11).
229
230 3.5 CCF and LDA
231 Along with lineated valley fill (LVF), CCF commonly occurs in the same geographical
232 areas as lobate debris aprons (LDA) [Squyres, 1979; Squyres and Carr, 1986; Head et al., 2006b;
233 Head et al., 2009]. In some craters, “brain terrain” surfaced lobes extend from crater walls,
234 towards the crater interior, but do not entirely fill the crater (Figure 13). Lineations on these lobes
235 are commonly concentric with crater rims. In other craters (Figure 14), crater-filling material
236 spills from within the crater down to proximal, LVF-surfaced valley floors. Such fill is commonly
237 lineated along strike, rather than concentrically with the crater, similar to lineations in lobate
238 debris aprons (LDA) [Squyres, 1979; Squyres and Carr, 1986; Head et al., 2006a; Levy et al.,
239 2007; Head et al., 2009].
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241 3.6 CCF and the LDM ACCEPTED MANUSCRIPT 12
242 What relationships are observed between CCF and the martian latitude dependent mantle
243 (LDM) [Kreslavsky and Head, 1999; Kreslavsky and Head, 2000; Mustard et al., 2001; Head et
244 al., 2003; Milliken et al., 2003; Levrard et al., 2004]? Relatively low-albedo mantling material
245 typically overlies some portion of CCF surfaces along the dichotomy boundary (e.g., Figure 2).
246 This mantling material is commonly polygonally patterned by thermal contraction cracks and
247 ranges in thickness from <1 m to several tens of meters [Levy et al., 2009b]. This mantling
248 material overlies CCF “brain terrain” and shows little evidence of deformation associated with
249 CCF flow, consistent with a very youthful age of emplacement (~1-2 Ma), as compared to the
250 more ancient CCF (see below) [Levy et al., 2009b].
251
252 4. Age Estimates
253
254 Crater counting on CCF surfaces poses many of the same challenges as crater counting
255 on LDA and LVF surfaces: small craters are obscured by “brain terrain” cells [Levy et al., 2009b]
256 while larger craters may have undergone a range of modification processes [Mangold, 2003;
257 Kress and Head, 2008]. Two principle morphologies of craters are observed on CCF surfaces:
258 fresh, bowl-shaped craters (Figures 8a, 8b) and “ring-mold” craters (RMCs) (Figures 8a, 8c) that
259 display the same inner ring or inner plateau morphologies described for RMCs on LVF and LDA
260 by Mangold [2003] and Kress and Head [2008]. Both fresh and RMC craters are observed on
261 CCF surfaces, suggesting a complex geological history for CCF.
262 In order to provide an age estimate for the formation of CCF, we avoid counting directly
263 on CCF surfaces. Rather, we identify CTX images in which both filled (CCF) and unfilled craters
264 are present (Figure 15, Table 1). By counting on the ejecta of these craters, it is possible to
265 bracket the formational period of CCF: the CCF must have formed after the filled-crater ejecta
266 was emplaced, but before the unfilled-crater ejecta was emplaced. (Figure 15). Crater counts were
267 made on ten impact crater ejecta deposits surrounding comparably-sized craters with and without ACCEPTED MANUSCRIPT 13
268 CCF deposits. The combined crater count results are summarized in Figure 16. This binned-count
269 approach assumes that CCF deposits formed at approximately the same time throughout the study
270 region. Both counts show significant roll-off at small crater size—in part a resolution effect
271 associated with CTX data and in part due to small-crater resurfacing at high latitudes due to
272 mantle emplacement processes, [e.g., Head et al., 2003; Levy et al., 2009a; Kreslavsky, 2009].
273 Still, counts on unfilled crater ejecta show a quantitatively younger age. Best-fit ages calculated
274 using Hartmann [2005] isochrons and only craters larger than 100 m are ~70 Ma for unfilled
275 craters and ~320 Ma for CCF filled craters; similar to age estimates for LDA and LVF that span
276 ~0.1-1 Ga [Mangold, 2003; Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Kress and
277 Head, 2008; Head et al., 2009]. Visual inspection of isochron plots suggest an age for unfilled
278 craters of ~60-80 Ma, with filled craters forming a population of surfaces that may be up to ~1 Ga
279 in age. The kink in the filled crater curve at the ~0.5 km diameter bin may be indicative of an
280 older population of craters that has undergone a range of resurfacing processes [Neukum and
281 Hiller, 1981; Werner, 2009], or the Amazonian filling of considerably older craters. Both curves
282 indicate, however, that CCF formation is a process typical of the relatively recent Amazonian.
283
284 5. Discussion
285
286 On the basis of the observations presented above, we can evaluate hypotheses regarding
287 the origin and significance of concentric crater fill (CCF). Given the strong morphological
288 dissimilarity between CCF and landforms identified as dunes, we suggest that CCF does not
289 represent a purely aeolian crater-filling process. However, atmospheric processes such as
290 deposition of windblown sediment [e.g., Zimbelman et al., 1989] and snow/ice [Head et al.,
291 2006a; Head et al., 2006b] may be important in CCF formation (see below). Likewise, CCF may
292 be modified by aeolian processes, such as dust-devil redistribution of surface sediment [e.g., van
293 Gasselt et al., 2010, their figure 7]. Nonetheless, clear morphological differences exist between ACCEPTED MANUSCRIPT 14
294 CCF surface textures and dunes typical of martian low and high latitudes (Figure 6). Although
295 CCF deposits bear a superficial resemblance to terrestrial seif dunes, dune-fields typically lack
296 the concentric pattern typical of CCF, and do not display open- and closed-cell “brain terrain”
297 morphology [Levy et al., 2009b].
298 The observation that CCF is commonly elevated above the interior slopes of craters, and
299 in places is markedly convex-up, suggests that CCF is not simply crater rim talus material that
300 has been transported down-slope by ice-assisted creep [e.g., Squyres, 1979; Squyres and Carr,
301 1986; van Gasselt et al., 2010]. Indeed, CCF is found on craters with well-preserved rims, as well
302 as craters that have eroded rims (Figures 1, 4, 8, and 9-10 respectively). Two key morphological
303 observations are difficult to reconcile with such a rock-glacier-like mechanism [Haeberli et al.,
304 2006]: 1) recession of CCF from high stands (implying the removal of a large volume of excess
305 ice) (Figure 7) and 2) the presence of convex-up and high-standing CCF in crater interiors
306 (implying preferential removal of CCF material around crater wall interiors) [e.g., Russell et al.,
307 2004; Russell and Head, 2005] (Figure 4). In both instances, internal rock-rock contact in the
308 rock-glacier model for CCF would provide a uniform surface topography, even as interstitial ice
309 was removed from the deposit. Rather, these morphological observations suggest that
310 significantly excess ice (ice volume exceeding pore space volume) is required to form CCF.
311 What does the presence of landforms that have been interpreted as indicators of debris-
312 covered glacial ice in or on CCF deposits suggest about the origin of concentric crater fill? Ring-
313 mold craters (RMCs) have been interpreted as interactions between impactors and near-surface
314 ice through spallation and differential sublimation [Mangold, 2003; Kress and Head, 2008].
315 “Brain terrain” has been interpreted as the surface expression of a multi-stage sublimation process
316 in the upper portions of CCF, LVF, and LDA surfaces, in which fracture and sediment infill drive
317 differential sublimation of buried ice [Levy et al., 2009b]. Finally, tracing of surface lineations on
318 LVF, LDA, and CCF shows continuity of flow and folding over multiple-km length-scales,
319 strongly suggesting the internal deformation and flow of an ice-rich substrate forming these ACCEPTED MANUSCRIPT 15
320 landforms [Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Dickson et al., 2008; Head
321 et al., 2009; Morgan et al., 2009]. Indeed, the presence topographic ridges and troughs, as well as
322 fractures (both thermal-contraction-driven and resulting from structural failure of deforming ice)
323 are typical for terrestrial debris covered glaciers, and may form parallel or orthogonal to the flow
324 direction [Marchant et al., 2002; Head et al., 2006a,b; Levy et al., 2007]. The combined
325 observation of ring-mold craters, “brain terrain,” and flow-related lineations on CCF all suggest
326 that CCF is composed largely of debris-covered ice that underwent flow and has been
327 modification by sublimation. We interpret CCF to represent crater-interior debris-covered
328 glaciers [e.g., Garvin et al., 2006; Kreslavsky and Head, 2006; Levy et al., 2009b] that initially
329 flowed down-slope from protected crater-rim accumulation zones (e.g., Figure 13) before
330 merging and filling crater interiors (e.g., Figure 1).
331 What is the relationship between CCF and other martian glacial landforms, such as LVF
332 and LDA? The morphological similarity between “brain terrain” surfaces associated with LDA,
333 CCF, and LVF suggest that these units have undergone similar developmental histories. The
334 confluence and merging of CCF, LVF, and LDA lineations suggests that they have similar
335 rheological properties and were active during the same period. The comparable crater retention
336 ages recorded by CCF, LVF, and LDA suggest that similar processes were involved in the
337 formation and modification of these units, over the course of the recent Amazonian. A large body
338 of evidence suggests that LVF and LDA formed as debris-covered glaciers as ice accumulated at
339 high obliquity in protected alcoves along dichotomy boundary slopes, flowed as cold-based
340 glaciers, and eventually sublimated as climate conditions grew colder and/or drier [Forget et al.,
341 2006; Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Dickson et al., 2008; Head et al.,
342 2009; Madeleine et al., 2009]. Morphological similarities between CCF and LVF/LDA suggest
343 that similar mechanisms may have controlled CCF formation and development. Given 1) the
344 detection of buried ice beneath martian LDA and LVF [Holt et al., 2008; Plaut et al., 2009], 2) the
345 thinness of overlying sublimation residue deposits suggested by the presence of ring-mold craters ACCEPTED MANUSCRIPT 16
346 [Mangold, 2003; Kress and Head, 2008], and 3) the hundreds to thousands of meters of fill
347 material still present in CCF deposits, we suggest that CCF may represent a significant reservoir
348 of non-polar martian ice, extending over ~38,000 km2 along the Martian dichotomy boundary.
349 What does CCF morphology indicate about changes in CCF volume over time? The
350 presence of steep talus slopes above some CCF deposits (e.g., Figure 7) suggests that in places at
351 least tens of meters of CCF material has been removed since the cessation of flow. At a larger
352 scale, the presence of “uphill flowing” low-definition CCF deposits (Figure 5), similar to perched
353 glacier deposits observed in Coloe Fossae crater interiors [e.g., Dickson et al., 2008, 2009],
354 suggests than in places at least 300 m of CCF material has been removed. The vertical drop-down
355 of CCF surfaces is consistent with loss of relatively clean glacier ice [Marchant et al., 2002;
356 Kowalewski et al., 2006; Marchant et al., 2007; Marchant and Head, 2007], and is inconsistent
357 with loss of pore ice in grain-supported regolith (e.g., rock-glaciers would not result in a change
358 in surface elevation). This suggests that CCF deposits were once considerably more voluminous
359 than is indicated by their current extent. For many CCF deposits, the presence of an additional
360 300 m of material would over-fill the craters that the CCF currently occupy, suggesting along
361 with the presence of breached CCF craters (e.g., Figures 5, 10-12), the intriguing possibility that
362 CCF deposits were once part of a larger, inter-crater glacial landsystem [Marchant and Head,
363 2008; 2009], remnants of which are preserved as CCF, LVF, and LDA deposits, along with ice
364 buried beneath mid-latitude pedestal craters [Kadish et al., 2008]. If this were the case, then the
365 current extent of CCF represents only the fraction of the maximum extent of ice that has been
366 preserved beneath a debris cover as the glaciation waned.
367 Finally, what is the relationship between CCF and the martian latitude dependent mantle
368 (LDM) [Mustard et al., 2001; Head et al., 2003; Milliken et al., 2003; Levrard et al., 2004]? The
369 difference in age between LVF/LDA/CCF and the LDM by one to two orders of magnitude
370 suggests that they represent distinct periods of ice deposition on Mars that have left a record of
371 unique characteristic landforms. Young, meters-thick LDM deposits drape CCF, LVF, and LDA ACCEPTED MANUSCRIPT 17
372 [Levy et al., 2009b]—in places showing limited evidence of recent deformation as viscous flow
373 features [Milliken et al., 2003]. In contrast, ~100 Ma old LVF, LDA, and CCF show evidence of
374 integrated glacier-like flow over tens of kilometers, and to depths of hundreds to thousands of
375 meters [Forget et al., 2006; Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Dickson et
376 al., 2008; Head et al., 2009; Madeleine et al., 2009]. Accordingly, the transition over the most
377 recent Amazonian from widespread deposition of CCF, LVF, and LDA to latitude-dependent
378 LDM emplacement may represent a transition from more active glacial periods to a more
379 quiescent “ice age” period of martian climate evolution [e.g., Head et al., 2003; Levy et al.,
380 2009b].
381
382 6. Conclusions
383
384 On the basis of morphological analyses of concentric crater fill (CCF) along the martian
385 dichotomy boundary, we conclude that a debris-covered glacier-like formation mechanism is the
386 most consistent explanation for the current distribution and characteristics of CCF, rather than
387 ice-mobilized talus creep or purely aeolian processes. Given strong morphological similarities
388 between surface textures associated with CCF, LVF, and LDA, we suggest that similar debris-
389 covered glacier processes account for the formation of all three landforms. Filled crater depth-
390 diameter analysis indicates that in many locations, hundreds of meters of ice may still be present
391 under desiccated surficial debris. Analysis of breached CCF-containing craters suggests that in at
392 least several locations, CCF-related ice was once several hundred meters higher than its current
393 level, and has sublimated significantly during the most recent Amazonian (between ~60-300 Ma).
394 The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF
395 and LDA glacial landsystems suggests the possibility that CCF represents one component of a
396 significant, Amazonian-age glaciation of mid-latitude Mars.
397 ACCEPTED MANUSCRIPT 18
398 7. Acknowledgements
399
400 We gratefully acknowledge support from the Mars Exploration Program Mars Express
401 HRSC grant JPL 1237163, and the Mars Data Analysis Program grants NNG05GQ46G and
402 NNX07AN95G to JWH. Special thanks to Caleb Fassett for assistance in processing and
403 interpreting crater count information and for image processing; to James Dickson for coordination
404 of image processing and distribution. The thoughtful comments of two anonymous reviewers
405 greatly improved this manuscript.
406
407 8. References
408
409 Benn, D. I., M. P. Kirkbride, L. A. Owen, and V. Brazier, 2003. Glaciated valley landsystems, in
410 Glacial Landsystems, edited by D. J. A. Evans, pp. 372-406, Edward Arnold, London.
411 Dickson, J. L., J. W. Head, and D. R. Marchant, 2008. Late Amazonian glaciation at the
412 dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple
413 glacial phases, Geology, 36, 411-414.
414 Dickson, J. L., J. W. Head, and D. R. Marchant, 2009. Kilometer-thick ice accumulation in the
415 northern mid-latitudes of Mars: evidence for crater-filling events in the late Amazonian at
416 the Phlegra Montes. Earth and Planetary Science Letters, doi:10.1016/j.epsl.2009.08.031.
417 Dobrea, E. Z. N., E. Asphaug, J. A. Grant, M. A. Kessler, and M. T. Mellon, 2007. Patterned
418 ground as an alternative explanation for the formation of brain coral textures in the mid
419 latitudes of Mars: HiRISE observations of lineated valley fill textures, in 7th International
420 Conference on Mars, Abstract #3358, Pasadena, CA.
421 Edgett, K. S., M. C. Malin, and t. C. S. O. Teams (2008. MRO context camera (CTX)
422 investigation primary mission results, in American Geophysical Union Fall Meet,
423 Abstract #P31D-04, San Francisco, CA. ACCEPTED MANUSCRIPT 19
424 Forget, F., R. M. Haberle, F. Montmessin, B. Levrard, and J. W. Head, 2006. Formation of
425 glaciers on Mars by atmospheric precipitation at high obliquity, Science, 311(5759), 368-
426 371.
427 Garvin, J. B., S. E. H. Sakimoto, J. J. Frawley, and C. Schnetzler, 2002. Global geometric
428 properties of martian impact craters, in 33rd Lunar and Planetary Science Conference,
429 Abstract #1255, League City, TX.
430 Garvin, J. B., J. W. Head, D. R. Marchant, and M. A. Kreslavsky, 2006. High-latitude cold-based
431 glacial deposits on Mars: multiple superposed drop moraines in a crater interior at 70N
432 latitude, Meteoritics and Planetary Science, 1659-1674.
433 Haeberli, W., B. Hallet, L. Arenson, R. Elconin, O. Humlum, A. Kaab, V. Kaufman, N. Ladanyi,
434 N. Matsuoka, S. Springman, and D. Vonder Muhll, 2006. Permafrost creep and rock
435 glacier dynamics, Permafrost and Periglacial Processes, 17, 189-214.
436 Hartmann, W. K., 2005. Martian cratering 8: Isochron refinement and the chronology of Mars,
437 Icarus, 174, 294-320.
438 Head, J.W., and D. R. Marchant, 2003. Cold-based mountain glaciers on Mars: Western
439 Arsia Mons, Geology, 31, 641-644.
440 Head, J. W., J. F. Mustard, M. A. Kreslavsky, R. E. Milliken, and D. R. Marchant, 2003. Recent
441 ice ages on Mars, Nature, 426, 797-802.
442 Head, J. W., D. R. Marchant, M. C. Agnew, C. I. Fassett, and M. A. Kreslavsky, 2006a.
443 Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for late
444 Amazonian obliquity-driven climate change, Earth and Planetary Science Letters, 241,
445 663-671.
446 Head, J. W., A. L. Nahm, D. R. Marchant, and G. Neukum, 2006b. Modification of the
447 dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation,
448 Geophysical Research Letters, 33, doi:10.1029/2005GL024360. ACCEPTED MANUSCRIPT 20
449 Head, J. W., D. R. Marchant, and M. A. Kreslavsky, 2008. Formation of Gullies on Mars: Link to
450 Recent Climate History and Insolation Microenvironments Implicate Surface Water Flow
451 Origin, Proceedings of the National Academy of Sciences, 105, 13258-13263.
452 Head, J. W., D. R. Marchant, J. L. Dickson, A. M. Kress, and D. M. Baker, 2009. Northern mid-
453 latitude glaciation in the late Amazonian period of Mars: criteria for the recognition of
454 debris-covered glacier and valley glacier landsystem deposits, Earth and Planetary
455 Science Letters, In Press.
456 Holt, J. W., A. Safaeinili, J. J. Plaut, J. W. Head, R. J. Phillips, R. Seu, S. D. Kempf, P.
457 Chouhardy, D. A. Young, N. E. Putzig, D. Biccari, and Y. Gim, 2008. Radar sounding
458 evidence for buried glaciers in the southern mid-latitudes of Mars, Science, 322, doi:
459 10.1126/science.1164246.
460 Jaumann, R., G. Neukum, T. Behnke, T. C. Duxbury, K. Eichentopf, J. Flohrer, S. van Gasselt, B.
461 Glese, K. Gwinner, E. Hauber, H. Hoffman, A. Hoffmeister, U. Kohler, K.-D. Matz, T.
462 B. McCord, V. Mertens, J. Oberst, R. Pischel, D. Reiss, T. Roatsch, P. Saiger, F.
463 Scholten, G. Schwarz, K. Stephan, M. Whlisch, and the HRSC Camera Instrument Team,
464 2007. The high-resolution stereo camera (HRSC) experiment on Mars Express:
465 instrument aspects and experiment conduct from interplanetary cruise through the nomial
466 mission, Planetary and Space Science, 55, 928-952.
467 Kadish, S. J., J. W. Head, N. G. Barlow, and D. R. Marchant, 2008. Martian pedestal craters:
468 marginal sublimation pits implicate a climate-related formation mechanism, Geophysical
469 Research Letters, 35, doi:10.1029/2008GL034990.
470 Kowalewski, D. E., Marchant, D.R., Levy, J.S., and Head, J.W., 2006. Quantifying low rates of
471 summer-time sublimation for buried glacier ice in Beacon Valley, Antarctica. Antarctic
472 Science 18, 421-428. ACCEPTED MANUSCRIPT 21
473 Kreslavsky, M. A. 2009. Dynamic Landscapes at High Latitudes on Mars: Constraints from
474 Populations of Small Craters. 40th Lunar and Planetary Science Conference,
475 Abstract #2311, The Woodlands, TX, March 23-27.
476 Kreslavsky, M., and J. Head, 1999. Kilometer-scale slopes on Mars and their correlation with
477 geologic units: initial results from Mars Orbiter Laser Altimeter (MOLA) data, Journal of
478 Geophysical Research, 104, 21,911-921,924.
479 Kreslavsky, M. A., and J. W. Head, 2000. Kilometer-scale roughness on Mars: results from
480 MOLA data analysis, Journal of Geophysical Research, 105, 26695-26712.
481 Kreslavsky, M. A., and J. W. Head, 2006. Modification of impact craters in the northern plains of
482 Mars: Implications for the Amazonian climate history, Meteoritics and Planetary Science,
483 41, 1633-1646.
484 Kress, A. M., and J. W. Head, 2008. Ring-mold craters in lineated valley fill and lobate debris
485 aprons on Mars: evidence for subsurface glacial ice, Geophysical Research Letters, 35,
486 doi:10.1029/2008GL035501.
487 Levrard, B., F. Forget, F. Montmessin, and J. Laskar, 2004. Recent ice-rich deposits formed at
488 high latitudes on Mars by sublimation of unstable equatorial ice during low obliquity,
489 Nature, 431, 1072-1075.
490 Levy, J. S., J. W. Head, and D. R. Marchant, 2007. Lineated valley fill and lobate debris apron
491 stratigraphy in Nilosyrtis Mensae, Mars: Evidence for phases of glacial modification of
492 the dichotomy boundary, Journal of Geophysical Research, 112, doi:
493 1029/2006JE002852.
494 Levy, J. S., J. W. Head, and D. R. Marchant, 2009a. Thermal Contraction Crack Polygons on
495 Mars: Classification, Distribution, and Climate Implications from HiRISE Observations,
496 Journal of Geophysical Research, 114, doi:10.1029/2008JE003273 ACCEPTED MANUSCRIPT 22
497 Levy, J. S., J. W. Head, and D. R. Marchant, 2009b. Concentric crater fill in Utopia Planitia:
498 timing and transitions between glacial “brain terrain” and periglacial processes, Icarus,
499 202, doi:10.1016/j.icarus.2009.1002.1018
500 Lucchitta, B., 1984. Ice and debris in the fretted terrain, Mars, Journal of Geophysical Research,
501 89, B409-B418.
502 Madeleine, J.-B., F. Forget, J. W. Head, B. Levrard, F. Montmessin, and E. Millour, 2009.
503 Amazonian norhtern mid-latitude glaciation on Mars: a proposed climate scenario, Icarus,
504 doi:10.1016/j.icarus.2009.1004.1037.
505 Malin, M. C. and Edgett, K. S., 2001. Mars Global Surveyor Mars Orbiter Camera: Interplanetary
506 Cruise Through Primary Mission. Journal of Geophysical Research, 106, 23429-23570.
507 Mangold, N., 2003. Geomorphic analysis of lobate debris aprons on Mars at Mars Orbiter Camera
508 scale: Evidence for ice sublimation initiated by fractures, Journal of Geophysical
509 Research, 108, 8021-8033.
510 Marchant, D.R. and Head, J.W. 2007. Antarctic Dry Valleys: Microclimate zonation, variable
511 geomorphic processes, and implications for assessing climate change on Mars. Icarus
512 192(1), 187-222, doi:10.1016/j.icarus.2007.06.018
513 Marchant, D. R., G. H. Denton, and C. C. I. Swisher, 1993. Miocene-Pliocene-Pleistocene glacial
514 history of Arena Valley, Quartermain Mountains, Antarctica, Geographiska Annaler,
515 75A, 269-302.
516 Marchant, D. R., A. R. Lewis, W. M. Phillips, E. J. Moore, R. A. Souchez, G. H. Denton, D. E.
517 Sugden, N. J. Potter, and G. P. Landis, 2002. Formation of patterned ground and
518 sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land,
519 Antarctica, Geological Society of America Bulletin, 114, 718-730.
520 Marchant, D. R., W. M. Phillips, J. M. Schaefer, G. Winckler, J. L. Fastook, D. E. Shean, D. E.
521 Kowalewski, J. W. Head, and A. R. Lewis, 2007. Establishing a chronology for the ACCEPTED MANUSCRIPT 23
522 world's oldest glacier ice, paper presented at 10th International Symposium on Antarctic
523 Earth Sciences, Santa Barbara, CA.
524 Marchant, D. R., and J. W. Head, 2008. Kilometer-thick ice-sheet in the northern mid-latitudes in
525 the Amazonian: analogs from the East Antarctic Ice Sheet and the dry valleys, in 39th
526 Lunar and Planetary Science Conference, Abstract #2097, League City, TX.
527 Marchant, D. R., and J. W. Head, 2009. The glacial deposits of the northern mid-latitudes:
528 remnants of large-scale plateau glaciation, in 40th Lunar and Planetary Science
529 Conference, Abstract #2355, The Woodlands, TX.
530 McEwen, A. S., M. E. Banks, N. Baugh, K. Becker, A. Boyd, J. W. Bergstrom, R. A. Beyer, E.
531 Bortolini, N. T. Bridges, S. Byrne, B. Castalla, F. C. Chuang, L. S. Crumpler, I. Daubar,
532 A. K. Davatzes, D. G. Deardorff, E. DeJong, W. A. Delamere, E. Z. N. Dobrea, C. M.
533 Dundas, E. M. Eliason, Y. Espinoza, A. Fennema, K. E. Fishbaugh, T. Forrester, P. E.
534 Geissler, J. A. Grant, J. L. Griffes, J. P. Grotzinger, V. C. Gulick, C. J. Hansen, K. E.
535 Herkenhoff, R. Heyd, W. L. Jaeger, D. Jones, B. Kanefsky, L. Keszthelyi, R. King, R. L.
536 Kirk, K. J. Kolb, J. Lasco, A. Lefort, R. Leis, K. W. Lewis, S. E. Martinez-Alonso, S.
537 Mattson, G. McArthur, M. T. Mellon, J. M. Metz, M. P. Milazzo, R. E. Milliken, T.
538 Motazedian, C. H. Okubo, A. Ortiz, A. J. Phillippoff, J. Plassmann, A. Polit, P. S.
539 Russell, C. Schaller, M. L. Searls, T. Spriggs, S. W. Squyres, S. Tarr, N. Thomas, B. J.
540 Thomson, L. L. Tornabene, C. V. Houten, C. Verba, C. M. Weitz, and J. J. Wray, 2009.
541 The High Resolution Imaging Science Experiment (HiRISE) during MRO's primary
542 science phase, Icarus, doi:10.1016/j.icarus.2009.1004.1023
543 Milliken, R. E., J. F. Mustard, and D. L. Goldsby, 2003. Viscous flow features on the surface of
544 Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images, Journal
545 of Geophysical Research, 108, doi:10.1029/2002JE002005.
546 Morgan, G. A., J. W. Head, and D. R. Marchant, 2009. Lineated valley fill (LVF) and lobate
547 debris aprons (LDA) in the Deuteronilus Mensae northern dichotomy boundary region, ACCEPTED MANUSCRIPT 24
548 Mars: constraints on the extent, age, and episodicity of Amazonian glacial events, Icarus,
549 202, 22-38.
550 Mustard, J. F., C. D. Cooper, and M. K. Rifkin, 2001. Evidence for recent climate change on
551 Mars from the identification of youthful near-surface ground ice, Nature, 412, 411-414.
552 Neukum, G., and K. Hiller, 1981. Martian ages, Journal of Geophysical Research, 86, 3097-3121.
553 Oberst, J., F. Scholten, K. Gwinner, B. Giese, M. Waehlisch, T. Roatsch, and G. Neukum, 2005.
554 The topographic mapping performance of the HRSC (High Resolution Stereo Camera) on
555 Mars Express, in American Geophysical Union Fall Meeting, Abstract #G53C-03, San
556 Francisco, CA.
557 Plaut, J. J., A. Safaeinili, J. W. Holt, R. J. Phillips, J. W. I. Head, R. Seu, N. E. Putzig, and A.
558 Frigeri, 2009. Radar evidence for ice in lobate debris aprons in the mid-northern latitudes
559 of Mars, Geophysical Research Letters, 36, doi:10.1029/2008GL036379.
560 Russell, P. S., J. W. Head, and M. H. Hecht, 2004. Evolution of ice deposits in the local
561 environment of Martian circum-polar craters and implications for polar cap history. 35th
562 Lunar and Planetary Science Conference, Abstract 2007, League City, TX.
563 Russell, P. S. and J. W. Head, 2005. Circumpolar craters with interior deposits on Mars: Polar
564 region geologic volatile and climate history with implications for ground ice signature in
565 Arabia Terra. 36th Lunar and Planetary Science Conference, Abstract #1541, League
566 City, TX.
567 Sharp, R. P., 1973. Mars: Fretted and chaotic terrains, Journal of Geophysical Research, 78,
568 4073-4083.
569 Shean, D. E., J. W. Head, and D. R. Marchant, 2005. Origin and evolution of a cold-
570 based tropical mountain glacier on Mars: The Pavonis Mons fan-shaped deposit,
571 Journal of Geophysical Research, 110, doi:10.1029/2004JE002360.
572 Squyres, S. W., 1978. Martian fretted terrain: Flow of erosional debris, Icarus, 34, 600-613. ACCEPTED MANUSCRIPT 25
573 Squyres, S. W., 1979. The distribution of lobate debris aprons and similar flows on Mars, Journal
574 of Geophysical Research, 84, 8087-8096.
575 Squyres, S. W., and M. H. Carr, 1986. Geomorphic evidence for the distribution of ground ice on
576 Mars, Science, 231, 249-252.
577 van Gasselt, S., E. Hauber, and G. Neukum, 2010. Lineated valley fill at the Martian dichotomy
578 boundary: nature and history of degradation. Journal of Geophysical Research, In Press.
579 Werner, S. C., 2009. The global martian volcanic evolutionary history, Icarus, 201, 44-68.
580 Williams, R. M. E., 2006. Latitude dependence of meter-scale surface textures in Deuteronilus
581 Mensae, Mars, in 37th Lunar and Planetary Science Conference, Abstract #1445, League
582 City, TX.
583 Zimbelman, J. R., S. M. Clifford, and S. H. Williams, 1989. Concentric crater fill on Mars: An
584 aeolian alternative to ice-rich mass wasting, paper presented at 19th Lunar and Planetary
585 Science Conference, Houston, TX.
586
587 9. Figure Captions
588
589 Figure 1. “Classic” and “low-definition” concentric crater fill (CCF) on the martian dichotomy
590 boundary. a) Classic CCF. Portion of P15_007028_2164. b) Low-definition CCF. Portion of
591 P16_007437_2090. Illumination from the left and north to image top in both images.
592
593 Figure 2. Comparisons between HiRISE and CTX image data detailing CCF structure at fine and
594 coarse scales. a) and c) show the same patch of CCF surface, showing typical “brain terrain”
595 morphology. HiRISE reveals details of surface structure not visible in CTX images. OC-BT
596 indicates open-cell “brain terrain” (arcuate and cuspate cells that are delimited by a convex-up
597 boundary ridge surrounding a flat-floored depression) and CC-BT indicates closed cell “brain
598 terrain” (arcuate or domed mound features that are commonly ~10-20 m wide, and that may have ACCEPTED MANUSCRIPT 26
599 surface grooves or furrows located near the centerline of the cell’s long axis). LDM Annulus
600 indicates exposures of latitude-dependent mantle material ringing the interior of the crater. b) and
601 d) show the same CCF containing crater (areas shown in a) and c) indicated by white box). CTX
602 clearly resolves large-scale lineations in CCF caused by topographic undulation and by
603 orientation of “brain terrain” cells; HiRISE viewing of the same features show similar structure,
604 but it noisier due to the resolution of finer-scale features. All images excerpted from CTX image
605 P03_002175_2211 and HiRISE image PSP_002175_2210. Illumination from the left. North to
606 image top.
607
608 Figure 3. Above) Map showing the distribution of CCF landforms along the martian dichotomy
609 boundary. CCF deposits larger than ~3 km in diameter are mapped. 614 classic CCF deposits are
610 mapped, as well as 262 low-definition CCF deposits. White line indicates region of most
611 concentrated “classic” CCF detection. Surveyed image data include all CTX images from orbit
612 groups P01 to P17 located in the mapping region spanning -30˚ – 90˚ E and 20˚ - 80˚ N. Base
613 map is MOLA topography. Below) Histogram comparing the distribution of CCF occurrences
614 (dark bars) and surveyed CTX images (light bars) by 5˚ latitude bins. CCF distribution appears to
615 be strongly latitude-dependent, while CTX image distribution is not. CCF occurrences can exceed
616 the number of CTX images in a given latitude bin if multiple examples of CCF are present in a
617 given CTX image.
618
619 Figure 4. a) Classic concentric crater fill that is clearly separated from the interior crater wall
620 talus slopes. White line indicates track of topographic profile. Portion of P14_006570_2241.
621 Illumination from left. North to image top. b) HRSC high-resolution DTM topographic profile
622 across CCF containing crater. Inset shows calculated depth expected given crater diameter from
623 Garvin et al. [2002] and an idealized bowl-shaped profile.
624 ACCEPTED MANUSCRIPT 27
625 Figure 5. a) Low-definition CCF present in a Deuteronilus Mensae crater. “Brain terrain” surface
626 textures extend out of the crater between several arcuate ridges interpreted to be moraines. Box
627 shows location of part c. Portion of P16_007385_2149. Illumination from left. North to image
628 top. b) HRSC high-resolution DTM profile across crater and nearby plains. c) “Brain terrain”
629 knobs present within the distal moraine-like features.
630
631 Figure 6. Dunes in craters are morphologically distinct from CCF. a) Typical low-latitude dunes.
632 Illumination from lower left. Portion of P08_004197_2007. Illumination from left. North to
633 image top. b) Typical high-latitude dunes. Illumination from lower left. Portion of
634 P16_007187_2581. Illumination from lower left. North to image top.
635
636 Figure 7. a) “Brain terrain” patterned “low definition” CCF surface in contact with a crater central
637 peak talus slope. Crater fill material is significantly lower than the steep talus slope. Inset shows
638 higher-resolution view of talus-CCF contact. Portion of P17_007743_2169. Illumination from
639 left. North to image top. A larger view of this crater is found in Figure 14. b) “Brain terrain”
640 patterned “low definition” CCF surface in contact with a crater rim talus slope. Crater fill material
641 is significantly lower than the steep talus slope. Box shows location of part c. c) Higher-
642 resolution view of talus-CCF contact. For b) and c), illumination is from the left, and north is
643 towards image top. Portion of HiRISE image PSP_005752_2130 overlaid on CTX image
644 P17_005752_2130.
645
646 Figure 8. CCF with a range of impact crater morphologies. Portion of P17_007585_2149.
647 Illumination from upper left. North to image top. b) Fresh, bowl-shaped impact crater. c) Three
648 “ring mold” craters [Kress and Head, 2008].
649 ACCEPTED MANUSCRIPT 28
650 Figure 9. CCF in a crater with an extensively eroded rim. Contrast with CCF in craters with intact
651 rims (e.g., Figures 1, 5). Illumination from left. North to image top. Portion of
652 P16_007385_2149.
653
654 Figure 10. a) CCF integrated into lineated valley fill (LVF). Surface lineations characteristic of
655 LVF grade into CCF from image right to center, and from CCF back to LVF from image center to
656 left. Illumination from lower right. North to image left. Portion of P12_005870_2181. b) MOLA
657 gridded topography data extracted from profile from LVF to CCF to LVF. A nearly monotonic
658 down-slope profile suggests integration of CCF into LVF. Regional flow patterns in this region
659 were mapped by Head et al. [2006a].
660
661 Figure 11. a) CCF integrated into lineated valley fill (LVF). Surface lineations characteristic of
662 CCF grade into LVF in a confined valley and then into a broader LVF trunk valley. Illumination
663 from lower right. North to image left. Portion of P04_002653_2216. b) High-resolution HRSC
664 DTM profile from CCF to LVF. A nearly monotonic down-slope profile suggests integration of
665 CCF into LVF.
666
667 Figure 12. Comparison of similar “brain terrain” surface textures on CCF (b) and LVF (c). a)
668 Context image showing CCF integrated with LVF (see Figure 11). Illumination from the left.
669 North to image top. b) Typical CCF “brain terrain.” Illumination from image bottom. Portion of
670 PSP_008296_2175. c) Typical LVF “brain terrain.” Illumination from image bottom. Portion of
671 PSP_009588_2175.
672
673 Figure 13. Partial fill of a crater by lobe-like, rim-concentric, “brain terrain” surfaced deposits.
674 The partial fill material is similar in position and morphology to lobate debris aprons observed
675 elsewhere on Mars. Illumination from left. Portion of P16_007242_2159. ACCEPTED MANUSCRIPT 29
676
677 Figure 14. Lobate debris apron-like flow emerging from a crater and integrating into LVF. This
678 feature suggests a similar origin for CCF, LVF, and LDA. Illumination from left. North to image
679 top. Portion of P04_002442_2221.
680
681 Figure 15. Filled and unfilled craters provide a means of estimating the age of CCF emplacement.
682 The most recent CCF emplacement period must be older than the age determined by counts on
683 unfilled crater ejecta and younger than the age determined by counts on filled crater ejecta.
684 Illumination from upper left. North to image upper right. Portion of P06_003272_2079.
685
686 Figure 16. Crater counts for ejecta of CCF filled craters (white circles) and unfilled craters (black
687 circles). Best-fit ages are ~60 Ma for unfilled craters and ~300 Ma for filled craters, suggesting
688 that CCF formed during the recent Amazonian.
689
690 Table 1. Index of craters used for crater retention age dating. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT