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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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Concentric Crater Fill in the Northern Mid-Latitudes of Mars: 9
Formation Processes and Relationships to Similar Landforms of Glacial Origin 10
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Joseph Levy1*, James W. Head1, and David R. Marchant2 12
1 Department of Geological Sciences, Brown University, Providence, RI, 02912 13
Ph. 401-863-3485 Fax. 401-863-3978 Email. [email protected] 14
2 Department of Earth Science, Boston University, Boston, MA, 02215 15
* Now at Department of Geology, Portland State University, Portland, OR, 97201 16
Ph. 503-725-3355 Email. [email protected] 17
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Submitted to Icarus: November 6, 2009 19
Revised: Feb 28, 2010 20
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29 Pages 23
16 Figures 24
1 Table25
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Running Head: Concentric crater fill and Amazonian glacial systems 26
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Corresponding Author: 28
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Joseph Levy 30
Department of Geology 31
Portland State University 32
P.O. Box 751 33
Portland OR 97207-0751 34
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Abstract: 36
Hypotheses for the formation of concentric crater fill (CCF) on Mars range from ice-free 37
processes (e.g., aeolian fill), to ice-assisted talus creep, to debris-covered glaciers. Based on 38
analysis of new CTX and HiRISE data, we find that concentric crater fill (CCF) is a significant 39
component of Amazonian-aged glacial landsystems on Mars. We present mapping results 40
documenting the nature and extent of CCF along the martian dichotomy boundary over -30˚ – 90˚ 41
E latitude and 20˚ - 80˚ N longitude. On the basis of morphological analysis we classify CCF 42
landforms into “classic” CCF and “low-definition” CCF. Classic CCF is most typical in the 43
middle latitudes of the analysis area (~30-50˚ N), while a range of degradation processes results 44
in the presence of low-definition CCF landforms at higher and lower latitudes. We evaluate 45
formation mechanisms for CCF on the basis of morphological and topographic analyses, and 46
interpret the landforms to be relict debris-covered glaciers, rather than ice-mobilized talus or 47
aeolian units. We examine filled crater depth-diameter ratios and conclude that in many locations, 48
hundreds of meters of ice may still be present under desiccated surficial debris. This conclusion is 49
consistent with the abundance of “ring-mold craters” on CCF surfaces that suggest the presence 50
of near-surface ice. Analysis of breached craters and distal glacial deposits suggests that in some 51
locations, CCF-related ice was once several hundred meters higher than its current level, and has 52
sublimated significantly during the most recent Amazonian. Crater counts on ejecta blankets of 53
filled and unfilled craters suggests that CCF formed most recently between ~60-300 Ma, 54
consistent with the formation ages of other martian debris-covered glacial landforms such as 55
lineated valley fill (LVF) and lobate debris aprons (LDA). Morphological analysis of CCF in the 56
vicinity of LVF and LDA suggests that CCF is a part of an integrated LVF/LDA/CCF glacial 57
landsystem. Instances of morphological continuity between CCF, LVF, and LDA are abundant. 58
The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF 59
and LDA, suggests the possibility that CCF represents one component of the significant 60
Amazonian mid-latitude glaciation(s) on Mars. 61
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Keywords: Mars, Mars Climate, Mars Surface62
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1. Introduction 63
64
Concentrically-lineated, crater-filling deposits (concentric crater fill, or CCF, Figure 1) 65
have been described at mid-high latitudes on Mars since the Viking era, and have long been 66
associated with a suite of features typical of fretted terrain (e.g., lobate debris aprons, LDA; and 67
lineated valley fill, LVF) [Sharp, 1973; Squyres, 1978; 1979; Malin and Edgett, 2001]. 68
Hypotheses explaining the formation of these unusual landforms range from entirely water-free 69
processes to those requiring the deposition and maintenance of hundreds of meters of nearly pure 70
ice. For example, Zimbelman et al. [1989] accounts for CCF by the emplacement and removal of 71
aeolian layers. Invoking limited quantities of water, Sharp [1973] explains fretted terrain as 72
debris derived from bedrock recession along scarps due to sublimation of ground ice or the 73
emergence of groundwater. Squyres [1978] suggests that lineated valley fill results from the 74
mobilization and flow of talus by diffusionally-emplaced pore ice; while Squyres [1979] and 75
Squyres and Carr [1986] extend this rock-glacier-like mechanism [e.g., Haeberli et al., 2006] to 76
lobate debris aprons and concentric crater fill. A greater role for internally-deforming ice in LVF 77
processes was proposed by Lucchitta [1984], drawing explicit analogs between lineated valley fill 78
and polythermal Antarctic glaciers. In a study of large craters in the Vastitas Borealis Formation, 79
Kreslavsky and Head [2006] suggest a range of crater-filling mechanisms to account for the 80
filling of high-latitude craters on Mars, including debris-covered glacial processes [see also, 81
Garvin et al., 2006]; similar to the debris-covered glacier mechanism invoked by Levy et al. 82
[2009b] to account for CCF morphology in Utopia Planitia. The potential for the current presence 83
of significant quantities of ice in CCF and LVF was suggested by the documentation of abundant 84
examples of integrated, glacier-like flow features in LVF and LDA across the Martian dichotomy 85
boundary [e.g., Head et al., 2006a; Head et al., 2006b, 2009; Levy et al., 2007; Morgan et al., 86
2009], the presence of ring-mold craters in LVF/LDA deposits [Kress and Head, 2008], and the 87
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discovery of significant volumes of nearly pure ice within lobate debris aprons and lineated valley 88
fill described by Holt et al. [2008] and Plaut et al. [2009] using SHARAD radar data. 89
Advances in understanding of CCF and related landforms have followed from the 90
improving spatial resolution and coverage of spacecraft data, including image and topographic 91
datasets. Here, we use Context Camera (CTX) [Edgett et al., 2008] and High Resolution Imaging 92
Science Experiment (HiRISE) [McEwen et al., 2009] image data, coupled with HRSC high-93
resolution stereo DEMs [Oberst et al., 2005; Jaumann et al., 2007] and supplemented by MOLA 94
topographic data, to address the following outstanding questions related to CCF: 1) What are the 95
morphological characteristics of CCF at high resolution, and what does the morphology suggest 96
about CCF composition? 2) Are there morphological subtypes of CCF, and if so, what is their 97
geographical distribution? 3) If ice is involved in CCF processes, is any still present? 4) What is 98
the relationship between CCF and other crater-filling materials (e.g., dunes) on Mars? 5) What is 99
the relationship between CCF and LVF/LDA? 5) What do relationships between LVF/LDA/CCF 100
processes suggest about the origin and development of martian glacial landforms? We focus on 101
CCF found along the Martian dichotomy boundary (particularly within -30 - 90° E and 20 - 80° 102
N), in the vicinity of a range of landforms that have been interpreted as features of glacial origin. 103
104
2. Mapping Mid-Latitude Crater-Filling Units 105
106
Concentric crater fill (CCF) has been classically described in regions containing abundant 107
lineated valley fill (LVF) and/or lobate debris aprons (LDA) as a crater-interior unit characterized 108
by multiple rings or lineations concentric with the crater rim [Squyres, 1979] (Figure 1a, 2). At 109
Mars Orbiter Camera (MOC) resolution (1.5-20 m/pixel), the surface texture associated with 110
LVF, LDA, and CCF lineations appears mounded and furrowed, a texture described as “brain or 111
corn-like pit and mound textures” [Malin and Edgett, 2001], “pit-and-butte” texture [Mangold, 112
2003] or “knobs—brain coral” [Williams, 2006]. Detailed analysis of HiRISE images has led to a 113
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description of CCF surface texture as “brain coral terrain” [Dobrea et al., 2007] or “brain terrain” 114
[Levy et al., 2009b]. For succinctness and consistency, we will refer to this surface texture 115
throughout as “brain terrain.” HiRISE images reveal textural differences between “brain terrain” 116
mounds or “cells”—particularly “closed cells” (arcuate or domed mound features that are 117
commonly ~10-20 m wide, and that may have surface grooves or furrows located near the 118
centerline of the cell’s long axis) and “open cells” (arcuate and cuspate cells that are delimited by 119
a convex-up boundary ridge surrounding a flat-floored depression) (Figure 2) [Levy et al., 2009b]. 120
Mapping of crater-filling units at Martian mid-latitudes can be accomplished using both 121
HiRISE and CTX image data in concert (Figure 3). Full-resolution CTX data are sufficient to 122
resolve broad-scale ridges and troughs associated with CCF (Figure 2), while detailed analysis of 123
relationships between “brain terrain” surface textures is facilitated by observation at HiRISE 124
resolution. Full-resolution CTX images spanning Mars Reconnaissance Orbiter orbits 1331-7927 125
from the study region (-30 – 90˚E, 20-80˚N) were inspected for the presence of crater-filling 126
units. Crater-fill units from the 1000 images surveyed were mapped on a sinusoidal equal-area 127
global map (Figure 3). 128
For mapping morphological subtypes, we define “classic” CCF as a crater-interior unit 129
showing lineations concentric with at least some portion of the crater rim and displaying “brain 130
terrain” surface texture (Figure 1a). It is the orientation of the long-axis of “brain terrain” cells 131
that provides the fine structure of CCF lineations, (Figure 2). Topographic ridges and troughs 132
~100 m wide define the broader concentric lineations associated with CCF and can be clearly 133
resolved in CTX image data (Figure 2) [Levy et al., 2009b]. The distribution of classic CCF along 134
the martian dichotomy boundary is shown in Figure 3. In our survey region, classic CCF is 135
widely distributed between ~27˚ N and ~50˚ N, with scattered examples at higher and lower 136
latitudes (Figure 3). 137
In some craters, CCF lacks clearly defined concentric lineations produced by topography 138
or lacks clearly-defined “brain terrain” cells (Figure 1b). We refer to this CCF as “low-definition” 139
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CCF. Low-definition CCF may show widened fractures or linear depressions oriented both 140
concentric-with and radial-to the crater rim (Figure 1b) or may have a muted surface texture, 141
showing only subtle stippling or shallow undulations in broad-scale topography. The latter 142
surface texture is characteristic of higher-latitude low-definition CCF, where continuous latitude-143
dependent mantle surfaces are thicker and more widespread [Head et al., 2003; Levy et al., 144
2009b]. The distribution of low-definition CCF along the dichotomy boundary is mapped in 145
Figure 3. 146
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3. CCF Morphological Relationships 148
149
Having briefly outlined the morphological characteristics of individual CCF deposits, the 150
next step in assessing the origin, development, and modification history of CCF is to document 151
the range of stratigraphic and morphological relationships observed between CCF and other 152
martian landforms [e.g., Squyres, 1979; Squyres and Carr, 1986; Levy et al., 2009b]. Here, we 153
consider relationships observed between both ice-related landforms (e.g., LVF, LDA) [Squyres, 154
1979; Lucchitta, 1984; Squyres and Carr, 1986; Head et al., 2006a; Head et al., 2006b; Head et 155
al., 2008; Holt et al., 2008; Head et al., 2009; Levy et al., 2009b; Plaut et al., 2009] and non-ice-156
related landforms (such as talus, craters, and dunes). 157
158
3.1 CCF and Host Craters 159
Because CCF is a crater-interior unit, relationships between CCF and its host craters can 160
be used to evaluate CCF-forming processes. Analysis of image and topography data indicates that 161
CCF does not merely line or coat the interior of craters; rather it consists of a flat and high (mesa-162
like) or convex-up surface that fills host craters (Figure 4). Fill thicknesses can be estimated by 163
comparing predicted crater depths from [Garvin et al., 2002] with observed depths from MOLA 164
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or HRSC topographic profiles [e.g., Levy et al., 2009b]. Six measured fill depths vary from ~600 165
m to ~1700 m, and commonly constitute ~75% of expected crater depth (Figure 4). 166
Contacts between CCF material and crater walls have variable morphology. In some 167
instances, the upper surface of CCF material sits well above the crater wall and is topographically 168
separated from the crater wall by a bounding scarp up to ~100 m high (Figure 4). In other cases, 169
CCF surfaces grade from sloping crater walls to the CCF surface—this latter relationship is most 170
typical of craters in which abundant latitude-dependent mantle material rings the crater interior 171
(Figure 2) [Head et al., 2003; Levy et al., 2009b]. 172
In some cases, crater-fill material appears to have cross-cut or transgressed over crater 173
rims (Figure 5). For example, one crater in Deuteronilus Mensae (Figure 5) shows low-definition 174
CCF within a crater that is connected to degraded “brain-terrain” surfaces outside the crater by a 175
series of linear, positive-relief ridges. Similar ridges are present to the east and to the west of the 176
crater. Ridges to the east of the crater extend from a nearby valley floor, upslope and onto the 177
outer walls of the crater rim. HRSC topographic profiles of the deposit suggest that the low-178
definition CCF material is ~300 m lower within the crater and outside the crater than it is on the 179
rim of the crater (Figure 5). The ridges are morphologically similar to drop moraines typical of 180
terrestrial glaciers [Marchant et al., 1993; Benn et al., 2003], as well as martian glaciers [Head 181
and Marchant, 2003; Shean et al., 2005] that form as debris is shed at the glacier margins. If this 182
interpretation is correct, it indicates that previously crater-filling material was at least 300 m 183
higher in the past—sufficient to permit flow over the crater rim and onto the valley floor outside 184
of the crater. 185
186
3.2 CCF and Dunes 187
Zimbelman et al. [1989] hypothesized that CCF is an aeolian feature, primarily on the 188
basis of a layered surface texture visible in Viking images of “classic” CCF terrain; what do high-189
resolution images suggest about crater-filling aeolian processes (e.g., sediment and dune 190
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deposition)? At both high and low latitudes, dunes partially fill some craters (Figure 6). Dunes are 191
seldom present along the entire interior of a crater, and typically drape, rather than fully fill, the 192
crater interior (Figure 6). Dune morphology can be readily distinguished from “brain terrain” 193
textures in HiRISE and CTX images, suggesting that CCF is not primarily composed of mobile 194
aeolian material. Rather, higher-resolution image data reveal that the layered surface texture 195
observed in Viking image data [Zimbelman et al., 1989] is a visual effect produced by closely-196
spaced lineations composed of “brain terrain” surface texture on topographically undulatory CCF 197
ridges (Figure 2) [Levy et al., 2009b]. 198
199
3.3 CCF and Talus 200
In CTX and HiRISE images, contacts between CCF and talus slopes are clearly 201
discernable (Figure 7). Some examples of “low-definition” CCF (e.g., in Deuteronilus Mensae or 202
Arabia Terra, Figure 7) show that “brain terrain” patterned crater-fill material abuts talus slopes 203
associated with crater rims. The contact between such talus slopes and the CCF is abrupt, with 204
CCF material present downslope along a steep scarp. Such relationships are most clearly visible 205
at low latitudes, where annuli of mantling material are not present at the crater/crater-fill contact 206
(e.g., Figure 2). Similar contacts between LVF and talus scarps in Nilosyrtis Mensae were 207
interpreted by Levy et al. [2007] as evidence of the recession of LVF surfaces from a previous 208
high-stand. This model suggests that the talus slope was emplaced down to the level of the 209
(previously higher) LVF margin, and remains as a steep talus slope now that the LVF/CCF 210
surface has vertically ablated. In other locations, CCF is topographically separated from the 211
interior slopes of crater walls by tens to hundreds of meters of relief, suggesting that CCF 212
material is physically distinct from talus shed by crater walls (Figure 4) [Russell et al., 2004; 213
Russell and Head, 2005]. Lastly, CCF is present in craters with a range of rim morphologies, 214
ranging from crisp and relatively well-defined (Figures 1-2, 8) to complexly eroded (Figure 9), 215
suggesting that CCF is not simply composed of mobilized talus shed from crater rims. 216
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217
3.4 CCF and LVF 218
CCF and lineated valley fill (LVF) commonly are present in the same geographical areas 219
on Mars [Squyres, 1979; Squyres and Carr, 1986; Head et al., 2006b; Head et al., 2009]. What 220
do contacts between LVF and CCF show? In many locations (Figures 10-11), contacts between 221
CCF and LVF show surficial merging of CCF and LVF lineations. LVF-containing valleys 222
commonly breach craters hosting CCF, both from upslope (Figure 10) and downslope (Figures 223
10-11). Merging of surface lineations occurs in both cases [Head et al., 2006a]. HiRISE analysis 224
of “brain terrain” surface textures on both CCF and LVF show similar groupings of open and 225
closed cells forming characteristic lineations (Figure 12). Topographic profiles across LVF/CCF 226
contacts are gradational and smooth in HiRISE image data, and at both MOLA resolution (~460 227
m/pixel) and HRSC-DTM resolution (~100 m/pixel) (Figures 10-11). 228
229
3.5 CCF and LDA 230
Along with lineated valley fill (LVF), CCF commonly occurs in the same geographical 231
areas as lobate debris aprons (LDA) [Squyres, 1979; Squyres and Carr, 1986; Head et al., 2006b; 232
Head et al., 2009]. In some craters, “brain terrain” surfaced lobes extend from crater walls, 233
towards the crater interior, but do not entirely fill the crater (Figure 13). Lineations on these lobes 234
are commonly concentric with crater rims. In other craters (Figure 14), crater-filling material 235
spills from within the crater down to proximal, LVF-surfaced valley floors. Such fill is commonly 236
lineated along strike, rather than concentrically with the crater, similar to lineations in lobate 237
debris aprons (LDA) [Squyres, 1979; Squyres and Carr, 1986; Head et al., 2006a; Levy et al., 238
2007; Head et al., 2009]. 239
240
3.6 CCF and the LDM 241
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What relationships are observed between CCF and the martian latitude dependent mantle 242
(LDM) [Kreslavsky and Head, 1999; Kreslavsky and Head, 2000; Mustard et al., 2001; Head et 243
al., 2003; Milliken et al., 2003; Levrard et al., 2004]? Relatively low-albedo mantling material 244
typically overlies some portion of CCF surfaces along the dichotomy boundary (e.g., Figure 2). 245
This mantling material is commonly polygonally patterned by thermal contraction cracks and 246
ranges in thickness from <1 m to several tens of meters [Levy et al., 2009b]. This mantling 247
material overlies CCF “brain terrain” and shows little evidence of deformation associated with 248
CCF flow, consistent with a very youthful age of emplacement (~1-2 Ma), as compared to the 249
more ancient CCF (see below) [Levy et al., 2009b]. 250
251
4. Age Estimates 252
253
Crater counting on CCF surfaces poses many of the same challenges as crater counting 254
on LDA and LVF surfaces: small craters are obscured by “brain terrain” cells [Levy et al., 2009b] 255
while larger craters may have undergone a range of modification processes [Mangold, 2003; 256
Kress and Head, 2008]. Two principle morphologies of craters are observed on CCF surfaces: 257
fresh, bowl-shaped craters (Figures 8a, 8b) and “ring-mold” craters (RMCs) (Figures 8a, 8c) that 258
display the same inner ring or inner plateau morphologies described for RMCs on LVF and LDA 259
by Mangold [2003] and Kress and Head [2008]. Both fresh and RMC craters are observed on 260
CCF surfaces, suggesting a complex geological history for CCF. 261
In order to provide an age estimate for the formation of CCF, we avoid counting directly 262
on CCF surfaces. Rather, we identify CTX images in which both filled (CCF) and unfilled craters 263
are present (Figure 15, Table 1). By counting on the ejecta of these craters, it is possible to 264
bracket the formational period of CCF: the CCF must have formed after the filled-crater ejecta 265
was emplaced, but before the unfilled-crater ejecta was emplaced. (Figure 15). Crater counts were 266
made on ten impact crater ejecta deposits surrounding comparably-sized craters with and without 267
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CCF deposits. The combined crater count results are summarized in Figure 16. This binned-count 268
approach assumes that CCF deposits formed at approximately the same time throughout the study 269
region. Both counts show significant roll-off at small crater size—in part a resolution effect 270
associated with CTX data and in part due to small-crater resurfacing at high latitudes due to 271
mantle emplacement processes, [e.g., Head et al., 2003; Levy et al., 2009a; Kreslavsky, 2009]. 272
Still, counts on unfilled crater ejecta show a quantitatively younger age. Best-fit ages calculated 273
using Hartmann [2005] isochrons and only craters larger than 100 m are ~70 Ma for unfilled 274
craters and ~320 Ma for CCF filled craters; similar to age estimates for LDA and LVF that span 275
~0.1-1 Ga [Mangold, 2003; Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Kress and 276
Head, 2008; Head et al., 2009]. Visual inspection of isochron plots suggest an age for unfilled 277
craters of ~60-80 Ma, with filled craters forming a population of surfaces that may be up to ~1 Ga 278
in age. The kink in the filled crater curve at the ~0.5 km diameter bin may be indicative of an 279
older population of craters that has undergone a range of resurfacing processes [Neukum and 280
Hiller, 1981; Werner, 2009], or the Amazonian filling of considerably older craters. Both curves 281
indicate, however, that CCF formation is a process typical of the relatively recent Amazonian. 282
283
5. Discussion 284
285
On the basis of the observations presented above, we can evaluate hypotheses regarding 286
the origin and significance of concentric crater fill (CCF). Given the strong morphological 287
dissimilarity between CCF and landforms identified as dunes, we suggest that CCF does not 288
represent a purely aeolian crater-filling process. However, atmospheric processes such as 289
deposition of windblown sediment [e.g., Zimbelman et al., 1989] and snow/ice [Head et al., 290
2006a; Head et al., 2006b] may be important in CCF formation (see below). Likewise, CCF may 291
be modified by aeolian processes, such as dust-devil redistribution of surface sediment [e.g., van 292
Gasselt et al., 2010, their figure 7]. Nonetheless, clear morphological differences exist between 293
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CCF surface textures and dunes typical of martian low and high latitudes (Figure 6). Although 294
CCF deposits bear a superficial resemblance to terrestrial seif dunes, dune-fields typically lack 295
the concentric pattern typical of CCF, and do not display open- and closed-cell “brain terrain” 296
morphology [Levy et al., 2009b]. 297
The observation that CCF is commonly elevated above the interior slopes of craters, and 298
in places is markedly convex-up, suggests that CCF is not simply crater rim talus material that 299
has been transported down-slope by ice-assisted creep [e.g., Squyres, 1979; Squyres and Carr, 300
1986; van Gasselt et al., 2010]. Indeed, CCF is found on craters with well-preserved rims, as well 301
as craters that have eroded rims (Figures 1, 4, 8, and 9-10 respectively). Two key morphological 302
observations are difficult to reconcile with such a rock-glacier-like mechanism [Haeberli et al., 303
2006]: 1) recession of CCF from high stands (implying the removal of a large volume of excess 304
ice) (Figure 7) and 2) the presence of convex-up and high-standing CCF in crater interiors 305
(implying preferential removal of CCF material around crater wall interiors) [e.g., Russell et al., 306
2004; Russell and Head, 2005] (Figure 4). In both instances, internal rock-rock contact in the 307
rock-glacier model for CCF would provide a uniform surface topography, even as interstitial ice 308
was removed from the deposit. Rather, these morphological observations suggest that 309
significantly excess ice (ice volume exceeding pore space volume) is required to form CCF. 310
What does the presence of landforms that have been interpreted as indicators of debris-311
covered glacial ice in or on CCF deposits suggest about the origin of concentric crater fill? Ring-312
mold craters (RMCs) have been interpreted as interactions between impactors and near-surface 313
ice through spallation and differential sublimation [Mangold, 2003; Kress and Head, 2008]. 314
“Brain terrain” has been interpreted as the surface expression of a multi-stage sublimation process 315
in the upper portions of CCF, LVF, and LDA surfaces, in which fracture and sediment infill drive 316
differential sublimation of buried ice [Levy et al., 2009b]. Finally, tracing of surface lineations on 317
LVF, LDA, and CCF shows continuity of flow and folding over multiple-km length-scales, 318
strongly suggesting the internal deformation and flow of an ice-rich substrate forming these 319
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landforms [Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Dickson et al., 2008; Head 320
et al., 2009; Morgan et al., 2009]. Indeed, the presence topographic ridges and troughs, as well as 321
fractures (both thermal-contraction-driven and resulting from structural failure of deforming ice) 322
are typical for terrestrial debris covered glaciers, and may form parallel or orthogonal to the flow 323
direction [Marchant et al., 2002; Head et al., 2006a,b; Levy et al., 2007]. The combined 324
observation of ring-mold craters, “brain terrain,” and flow-related lineations on CCF all suggest 325
that CCF is composed largely of debris-covered ice that underwent flow and has been 326
modification by sublimation. We interpret CCF to represent crater-interior debris-covered 327
glaciers [e.g., Garvin et al., 2006; Kreslavsky and Head, 2006; Levy et al., 2009b] that initially 328
flowed down-slope from protected crater-rim accumulation zones (e.g., Figure 13) before 329
merging and filling crater interiors (e.g., Figure 1). 330
What is the relationship between CCF and other martian glacial landforms, such as LVF 331
and LDA? The morphological similarity between “brain terrain” surfaces associated with LDA, 332
CCF, and LVF suggest that these units have undergone similar developmental histories. The 333
confluence and merging of CCF, LVF, and LDA lineations suggests that they have similar 334
rheological properties and were active during the same period. The comparable crater retention 335
ages recorded by CCF, LVF, and LDA suggest that similar processes were involved in the 336
formation and modification of these units, over the course of the recent Amazonian. A large body 337
of evidence suggests that LVF and LDA formed as debris-covered glaciers as ice accumulated at 338
high obliquity in protected alcoves along dichotomy boundary slopes, flowed as cold-based 339
glaciers, and eventually sublimated as climate conditions grew colder and/or drier [Forget et al., 340
2006; Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Dickson et al., 2008; Head et al., 341
2009; Madeleine et al., 2009]. Morphological similarities between CCF and LVF/LDA suggest 342
that similar mechanisms may have controlled CCF formation and development. Given 1) the 343
detection of buried ice beneath martian LDA and LVF [Holt et al., 2008; Plaut et al., 2009], 2) the 344
thinness of overlying sublimation residue deposits suggested by the presence of ring-mold craters 345
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[Mangold, 2003; Kress and Head, 2008], and 3) the hundreds to thousands of meters of fill 346
material still present in CCF deposits, we suggest that CCF may represent a significant reservoir 347
of non-polar martian ice, extending over ~38,000 km2 along the Martian dichotomy boundary. 348
What does CCF morphology indicate about changes in CCF volume over time? The 349
presence of steep talus slopes above some CCF deposits (e.g., Figure 7) suggests that in places at 350
least tens of meters of CCF material has been removed since the cessation of flow. At a larger 351
scale, the presence of “uphill flowing” low-definition CCF deposits (Figure 5), similar to perched 352
glacier deposits observed in Coloe Fossae crater interiors [e.g., Dickson et al., 2008, 2009], 353
suggests than in places at least 300 m of CCF material has been removed. The vertical drop-down 354
of CCF surfaces is consistent with loss of relatively clean glacier ice [Marchant et al., 2002; 355
Kowalewski et al., 2006; Marchant et al., 2007; Marchant and Head, 2007], and is inconsistent 356
with loss of pore ice in grain-supported regolith (e.g., rock-glaciers would not result in a change 357
in surface elevation). This suggests that CCF deposits were once considerably more voluminous 358
than is indicated by their current extent. For many CCF deposits, the presence of an additional 359
300 m of material would over-fill the craters that the CCF currently occupy, suggesting along 360
with the presence of breached CCF craters (e.g., Figures 5, 10-12), the intriguing possibility that 361
CCF deposits were once part of a larger, inter-crater glacial landsystem [Marchant and Head, 362
2008; 2009], remnants of which are preserved as CCF, LVF, and LDA deposits, along with ice 363
buried beneath mid-latitude pedestal craters [Kadish et al., 2008]. If this were the case, then the 364
current extent of CCF represents only the fraction of the maximum extent of ice that has been 365
preserved beneath a debris cover as the glaciation waned. 366
Finally, what is the relationship between CCF and the martian latitude dependent mantle 367
(LDM) [Mustard et al., 2001; Head et al., 2003; Milliken et al., 2003; Levrard et al., 2004]? The 368
difference in age between LVF/LDA/CCF and the LDM by one to two orders of magnitude 369
suggests that they represent distinct periods of ice deposition on Mars that have left a record of 370
unique characteristic landforms. Young, meters-thick LDM deposits drape CCF, LVF, and LDA 371
ACCEPTED MANUSCRIPT 17
[Levy et al., 2009b]—in places showing limited evidence of recent deformation as viscous flow 372
features [Milliken et al., 2003]. In contrast, ~100 Ma old LVF, LDA, and CCF show evidence of 373
integrated glacier-like flow over tens of kilometers, and to depths of hundreds to thousands of 374
meters [Forget et al., 2006; Head et al., 2006a; Head et al., 2006b; Levy et al., 2007; Dickson et 375
al., 2008; Head et al., 2009; Madeleine et al., 2009]. Accordingly, the transition over the most 376
recent Amazonian from widespread deposition of CCF, LVF, and LDA to latitude-dependent 377
LDM emplacement may represent a transition from more active glacial periods to a more 378
quiescent “ice age” period of martian climate evolution [e.g., Head et al., 2003; Levy et al., 379
2009b]. 380
381
6. Conclusions 382
383
On the basis of morphological analyses of concentric crater fill (CCF) along the martian 384
dichotomy boundary, we conclude that a debris-covered glacier-like formation mechanism is the 385
most consistent explanation for the current distribution and characteristics of CCF, rather than 386
ice-mobilized talus creep or purely aeolian processes. Given strong morphological similarities 387
between surface textures associated with CCF, LVF, and LDA, we suggest that similar debris-388
covered glacier processes account for the formation of all three landforms. Filled crater depth-389
diameter analysis indicates that in many locations, hundreds of meters of ice may still be present 390
under desiccated surficial debris. Analysis of breached CCF-containing craters suggests that in at 391
least several locations, CCF-related ice was once several hundred meters higher than its current 392
level, and has sublimated significantly during the most recent Amazonian (between ~60-300 Ma). 393
The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF 394
and LDA glacial landsystems suggests the possibility that CCF represents one component of a 395
significant, Amazonian-age glaciation of mid-latitude Mars. 396
397
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7. Acknowledgements 398
399
We gratefully acknowledge support from the Mars Exploration Program Mars Express 400
HRSC grant JPL 1237163, and the Mars Data Analysis Program grants NNG05GQ46G and 401
NNX07AN95G to JWH. Special thanks to Caleb Fassett for assistance in processing and 402
interpreting crater count information and for image processing; to James Dickson for coordination 403
of image processing and distribution. The thoughtful comments of two anonymous reviewers 404
greatly improved this manuscript. 405
406
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586
9. Figure Captions 587
588
Figure 1. “Classic” and “low-definition” concentric crater fill (CCF) on the martian dichotomy 589
boundary. a) Classic CCF. Portion of P15_007028_2164. b) Low-definition CCF. Portion of 590
P16_007437_2090. Illumination from the left and north to image top in both images. 591
592
Figure 2. Comparisons between HiRISE and CTX image data detailing CCF structure at fine and 593
coarse scales. a) and c) show the same patch of CCF surface, showing typical “brain terrain” 594
morphology. HiRISE reveals details of surface structure not visible in CTX images. OC-BT 595
indicates open-cell “brain terrain” (arcuate and cuspate cells that are delimited by a convex-up 596
boundary ridge surrounding a flat-floored depression) and CC-BT indicates closed cell “brain 597
terrain” (arcuate or domed mound features that are commonly ~10-20 m wide, and that may have 598
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surface grooves or furrows located near the centerline of the cell’s long axis). LDM Annulus 599
indicates exposures of latitude-dependent mantle material ringing the interior of the crater. b) and 600
d) show the same CCF containing crater (areas shown in a) and c) indicated by white box). CTX 601
clearly resolves large-scale lineations in CCF caused by topographic undulation and by 602
orientation of “brain terrain” cells; HiRISE viewing of the same features show similar structure, 603
but it noisier due to the resolution of finer-scale features. All images excerpted from CTX image 604
P03_002175_2211 and HiRISE image PSP_002175_2210. Illumination from the left. North to 605
image top. 606
607
Figure 3. Above) Map showing the distribution of CCF landforms along the martian dichotomy 608
boundary. CCF deposits larger than ~3 km in diameter are mapped. 614 classic CCF deposits are 609
mapped, as well as 262 low-definition CCF deposits. White line indicates region of most 610
concentrated “classic” CCF detection. Surveyed image data include all CTX images from orbit 611
groups P01 to P17 located in the mapping region spanning -30˚ – 90˚ E and 20˚ - 80˚ N. Base 612
map is MOLA topography. Below) Histogram comparing the distribution of CCF occurrences 613
(dark bars) and surveyed CTX images (light bars) by 5˚ latitude bins. CCF distribution appears to 614
be strongly latitude-dependent, while CTX image distribution is not. CCF occurrences can exceed 615
the number of CTX images in a given latitude bin if multiple examples of CCF are present in a 616
given CTX image. 617
618
Figure 4. a) Classic concentric crater fill that is clearly separated from the interior crater wall 619
talus slopes. White line indicates track of topographic profile. Portion of P14_006570_2241. 620
Illumination from left. North to image top. b) HRSC high-resolution DTM topographic profile 621
across CCF containing crater. Inset shows calculated depth expected given crater diameter from 622
Garvin et al. [2002] and an idealized bowl-shaped profile. 623
624
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Figure 5. a) Low-definition CCF present in a Deuteronilus Mensae crater. “Brain terrain” surface 625
textures extend out of the crater between several arcuate ridges interpreted to be moraines. Box 626
shows location of part c. Portion of P16_007385_2149. Illumination from left. North to image 627
top. b) HRSC high-resolution DTM profile across crater and nearby plains. c) “Brain terrain” 628
knobs present within the distal moraine-like features. 629
630
Figure 6. Dunes in craters are morphologically distinct from CCF. a) Typical low-latitude dunes. 631
Illumination from lower left. Portion of P08_004197_2007. Illumination from left. North to 632
image top. b) Typical high-latitude dunes. Illumination from lower left. Portion of 633
P16_007187_2581. Illumination from lower left. North to image top. 634
635
Figure 7. a) “Brain terrain” patterned “low definition” CCF surface in contact with a crater central 636
peak talus slope. Crater fill material is significantly lower than the steep talus slope. Inset shows 637
higher-resolution view of talus-CCF contact. Portion of P17_007743_2169. Illumination from 638
left. North to image top. A larger view of this crater is found in Figure 14. b) “Brain terrain” 639
patterned “low definition” CCF surface in contact with a crater rim talus slope. Crater fill material 640
is significantly lower than the steep talus slope. Box shows location of part c. c) Higher-641
resolution view of talus-CCF contact. For b) and c), illumination is from the left, and north is 642
towards image top. Portion of HiRISE image PSP_005752_2130 overlaid on CTX image 643
P17_005752_2130. 644
645
Figure 8. CCF with a range of impact crater morphologies. Portion of P17_007585_2149. 646
Illumination from upper left. North to image top. b) Fresh, bowl-shaped impact crater. c) Three 647
“ring mold” craters [Kress and Head, 2008]. 648
649
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Figure 9. CCF in a crater with an extensively eroded rim. Contrast with CCF in craters with intact 650
rims (e.g., Figures 1, 5). Illumination from left. North to image top. Portion of 651
P16_007385_2149. 652
653
Figure 10. a) CCF integrated into lineated valley fill (LVF). Surface lineations characteristic of 654
LVF grade into CCF from image right to center, and from CCF back to LVF from image center to 655
left. Illumination from lower right. North to image left. Portion of P12_005870_2181. b) MOLA 656
gridded topography data extracted from profile from LVF to CCF to LVF. A nearly monotonic 657
down-slope profile suggests integration of CCF into LVF. Regional flow patterns in this region 658
were mapped by Head et al. [2006a]. 659
660
Figure 11. a) CCF integrated into lineated valley fill (LVF). Surface lineations characteristic of 661
CCF grade into LVF in a confined valley and then into a broader LVF trunk valley. Illumination 662
from lower right. North to image left. Portion of P04_002653_2216. b) High-resolution HRSC 663
DTM profile from CCF to LVF. A nearly monotonic down-slope profile suggests integration of 664
CCF into LVF. 665
666
Figure 12. Comparison of similar “brain terrain” surface textures on CCF (b) and LVF (c). a) 667
Context image showing CCF integrated with LVF (see Figure 11). Illumination from the left. 668
North to image top. b) Typical CCF “brain terrain.” Illumination from image bottom. Portion of 669
PSP_008296_2175. c) Typical LVF “brain terrain.” Illumination from image bottom. Portion of 670
PSP_009588_2175. 671
672
Figure 13. Partial fill of a crater by lobe-like, rim-concentric, “brain terrain” surfaced deposits. 673
The partial fill material is similar in position and morphology to lobate debris aprons observed 674
elsewhere on Mars. Illumination from left. Portion of P16_007242_2159. 675
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676
Figure 14. Lobate debris apron-like flow emerging from a crater and integrating into LVF. This 677
feature suggests a similar origin for CCF, LVF, and LDA. Illumination from left. North to image 678
top. Portion of P04_002442_2221. 679
680
Figure 15. Filled and unfilled craters provide a means of estimating the age of CCF emplacement. 681
The most recent CCF emplacement period must be older than the age determined by counts on 682
unfilled crater ejecta and younger than the age determined by counts on filled crater ejecta. 683
Illumination from upper left. North to image upper right. Portion of P06_003272_2079. 684
685
Figure 16. Crater counts for ejecta of CCF filled craters (white circles) and unfilled craters (black 686
circles). Best-fit ages are ~60 Ma for unfilled craters and ~300 Ma for filled craters, suggesting 687
that CCF formed during the recent Amazonian. 688
689
Table 1. Index of craters used for crater retention age dating. 690
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