An investigation of Martian northern high-latitude and polar impact crater interiors: Atypical interior topographic features and cavity wall slopes

Authors


Corresponding author. E-mail: abacasto@nd.edu

Abstract

Abstract– We examine Martian northern high-latitude and polar impact craters (NPICs) to better understand the north polar materials and polar processes. We examine topographic characteristics for 346 NPICs and compare them to global fit data (e.g., Garvin et al. 2003; Boyce and Garbeil 2007) as well as to a small set (N = 92) of southern high-latitude and polar impact craters (SPICs). We find that the NPIC population above 57° N is significantly shallower than the global crater population. This suggests that the NPICs (1) were initially shallow due to target properties of polar geologic units; (2) were once deeper, but have been infilled due to polar processes; or (3) a combination of both. Indeed, many of the NPICs exhibit considerable noncentral peak interior topographic features (IFTs), which may be indicative of infilling processes. The NPIC IFTs also appear to display trends in their preferential orientation within the crater cavity; some SPICs display similar interior features, but do not show a clear preference in their orientation within the crater cavity. In addition, the NPIC population displays cavity wall slope trends that seem to indicate steepening of slopes with increasing crater diameter in comparison to the global slope trend (Garvin et al. 2003). These trends suggest that the NPICs are unique in their geometry when compared to the global data set as well as with the SPICs further indicating that the north polar region may exhibit target properties and polar processes not seen in the south polar region or elsewhere on Mars.

Introduction

Impact craters on Mars have served a variety of purposes in the past such as determining the amount of volatiles in the surface at the time of impact (e.g., Barlow 2006), determining wind direction from intercrater dunes (e.g., Tsoar et al. 1979), surface age dating (e.g., Neukum et al. 1975), and serving as landing sites (e.g., Gusev crater; Cabrol et al. 1996). Several recent studies have examined morphologic characteristics of Martian craters both as a global data set and as polar features. Fourteen ice-associated craters in the north polar region were the focus of research by Garvin et al. (2000) who proposed separate depth–diameter relationships for polar craters and nonpolar craters. Garvin et al. (2003) proposed empirically derived equations to examine global crater morphology trends based on gridded digital elevation model measurements such as crater depth, rim height, central peak height and diameter, cavity shape, and inner cavity wall slope. While most studies examine fresh impact craters, Boyce and Garbeil (2007) concentrated their attention on pristine impact craters within various locations on Mars to determine a depth–diameter relationship, although they also examined fresh impact craters. Malinksi et al. (2012), whose work is ongoing, examine impact craters 21–30 km in diameter in an effort to identify temporal changes in morphology between craters. Although Garvin et al. (2000) focused their work on the north polar region, their data set was relatively sparse at 109 impact craters. On the opposite end, the work by Boyce and Garbeil (2007) examined >6000 impact craters throughout many regions on Mars, but excluded craters in some of the high-latitude and polar regions.

We expand the data set begun by Garvin et al. (2000) for north polar impact craters, which also incorporates more than 100 craters not included in the expansive set by Boyce and Garbeil (2007). Our principal objective is to investigate the morphologic properties and prevalence of craters with massive, noncentral peak, interior topographic features (ITFs) in the northern high-latitude and polar region of Mars. We compare these measured properties with global population trends as defined by Garvin et al. (2000, 2003) and Boyce and Garbeil (2007). We employ the following impact crater topographic characteristics and trends (1) crater geometry such as depth, diameter, and cavity wall slope; (2) attributes of raised topography within the crater cavity such as location within the cavity and its relative dimensions; and (3) regional variations of crater morphologies.

Data Collection

We examined 346 northern high-latitude and circumpolar impact craters (Fig. 1a, Table 1) in the range of 57–82° N with high-resolution (256 pixels/degree) gridded digital elevation models (DEMs) from the Mars Orbiter Laser Altimeter (MOLA) on the Mars Global Surveyor. This latitude range was selected to encompass a broad range of geologic materials and ages (Tanaka et al. 2005) within the high latitudes while also incorporating craters with unusual IFTs such as the well-known Korolev crater. We selected 57° N as the terminus of the study area in keeping with the work of Garvin et al. (2000), who determined 57° N as the southern edge of the polar terrains based on the winter frost findings of Thomas et al. (1992).

Figure 1.

 Location map of impact craters in this study over MOLA shaded relief topography a) 346 north polar impact craters in the latitude range 57–82° N and b) 92 south polar impact craters in the latitude range 58–85° S.

Table 1.   Location and average morphologic data for North Polar Impact Craters (NPICs) in this study.
Id no.LatitudeE longitudeAvg. diam. (km)Avg. depth (m)N-slope (°)aS-slope (°)bAvg. slope (°)c
  1. aNorth (equator-facing) cavity wall slope.

  2. bSouth (pole-facing) cavity wall slope.

  3. cAverage cavity wall slope of all cardinal and ordinal directions (N, W, SW, etc.).

  4. Missing ID nos. (154, 284, 290, 327) were found to be duplicates of other craters.

178.179.146.81150.7511.7710.728.60
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7169.45162.6314.85510.0015.5710.8911.74
7271.43170.2913.17510.5015.189.5714.31
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7664.93155.5213.96613.502.085.803.21
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7865.13178.0046.612098.2512.9610.1110.46
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8060.03153.0411.71307.006.517.627.05
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8463.19165.1415.26570.756.105.146.28
8563.55170.2315.71728.252.456.265.94
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8761.75171.0117.35685.008.4510.6610.24
8860.20173.568.77249.754.086.067.51
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9176.98195.6021.621165.507.333.918.06
9273.91186.3329.441575.0018.0616.9915.74
9372.84188.446.02271.7512.0211.0410.81
9470.90193.5937.642287.753.704.156.60
9569.44194.067.91212.2513.168.2313.10
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9768.43189.3212.111005.505.893.234.19
9868.13199.3517.78992.754.285.025.20
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10067.13202.1217.12776.502.626.215.51
10166.95200.777.81143.0017.2811.8415.58
10266.61199.126.52267.009.4614.6812.72
10367.29192.9114.49702.252.695.394.51
10464.83209.4222.731377.007.867.919.21
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31162.3481.7010.15295.006.127.598.82
31261.6970.7212.27432.502.987.405.56
31357.9158.6653.132257.256.365.905.46
31458.0862.1723.531340.003.463.054.33
31558.2767.7014.92502.005.534.015.35
31658.2274.7418.28893.253.618.229.57
31760.4576.267.08223.253.697.268.12
31861.3778.468.67333.7515.4213.8815.08
31961.6079.598.19247.756.692.496.33
32060.4183.8111.73341.2516.719.9616.40
32160.6687.7432.251563.2524.7522.4016.97
32260.5989.6918.87678.0011.8913.3012.76
32358.2489.6111.45451.0011.523.148.34
32458.7582.4011.36394.002.494.683.77
32577.46101.429.80141.7511.8010.168.44
32677.1289.1031.09742.508.6310.1914.86
32874.96101.7513.50254.252.983.714.47
32971.2199.199.28188.5017.8917.6618.18
33070.16103.2236.161590.7511.968.017.92
33167.9092.8427.172032.504.584.294.90
33266.4199.7315.81359.256.613.284.37
33364.5496.2414.38510.754.948.436.66
33463.68110.215.58206.258.056.988.61
33563.86116.039.04331.009.375.897.84
33667.34120.008.87354.753.814.424.49
33767.09113.5720.24862.007.606.087.42
33867.80109.387.06537.502.813.193.19
33962.43103.3912.94434.5018.0713.6916.39
34061.8491.356.10314.0010.528.699.92
34161.6692.665.80148.7514.2212.4414.12
34260.4397.568.75261.5013.8411.5310.96
34358.8592.265.34180.258.549.987.93
34457.8093.625.81204.505.9518.4410.72
34559.0795.479.28420.5011.2016.8315.60
34659.76103.158.74263.504.785.074.07
34758.68112.439.31242.008.129.9111.18
34858.53116.8316.95587.0013.3210.3112.49
34960.39115.8615.85471.752.925.594.01
35060.96106.905.10136.7519.786.3311.96

A small set of 92 (Fig. 1b, Table 2) southern high-latitude and circumpolar impact craters within the latitudinal range of 57–85° S was also examined for comparison with their northern and global counterparts. This latitude range was selected to correspond directly with the north polar crater range. Craters in the southern hemisphere are generally regarded as older and more degraded than northern craters creating difficulties in measuring morphologic characteristics, thus we chose to focus the majority of our measurements on the fresher, northern craters while incorporating a subset of southern high-latitude and polar impact craters (SPICs). To avoid bias in selecting the SPICs, a list of randomly generated coordinates was compiled between 57 and 85° S; the impact crater closest in proximity to each coordinate was selected. Although the craters in this study are found in both high-latitude and polar locations, for the purposes of being concise we will refer to all craters in later text as “polar” for shorthand.

Table 2.   Location and average morphologic data for South Polar Impact Craters (SPICs) in this study.
Id no.LatitudeE longitudeAvg. diam (km)Avg. depth (m)N-slope (°)aS-slope (°)bAvg. slope (°)c
  1. aNorth (equator-facing) cavity wall slope.

  2. bSouth (pole-facing) cavity wall slope.

  3. cAverage cavity wall slope of all cardinal and ordinal directions (N, W, SW, etc.)

1−83.36.416.5795.110.410.112.9
2−80.26.314.7654.19.313.614.4
3−80.565.28.2335.98.39.39.5
4−80.296.840.7910.418.24.98.1
5−79.0127.248.81277.910.310.29.0
6−80.4130.013.4641.08.710.29.2
7−85.3252.89.4302.63.52.95.6
8−78.9228.848.31128.59.98.810.0
9−79.8251.412.1363.32.68.38.2
10−81.6303.912.0589.98.210.48.1
11−80.6280.515.5473.69.16.79.2
12−81.2340.623.4975.88.314.611.1
13−76.922.0104.61982.55.919.112.8
14−72.519.214.0461.38.37.37.9
15−71.219.210.2292.36.08.18.0
16−65.521.134.71070.99.28.49.3
17−63.314.415.3675.18.811.710.4
18−58.412.512.7534.311.614.213.2
19−80.539.014.6805.412.114.013.0
20−70.748.313.6714.611.08.39.9
21−64.341.734.81037.812.28.49.9
22−62.442.163.81560.616.712.315.6
23−60.343.014.0385.46.37.77.0
24−75.087.727.4905.912.517.412.9
25−72.578.931.0989.010.110.211.3
26−66.572.48.5217.56.54.25.9
27−62.874.117.1287.54.510.46.6
28−59.176.326.81568.912.517.214.3
29−61.161.7121.42563.419.010.310.8
30−58.186.571.21925.113.012.411.7
31−75.698.644.51760.616.013.313.8
32−69.8104.549.41570.514.014.613.5
33−65.6111.836.71968.811.816.115.4
34−63.7103.841.51186.910.016.812.6
35−58.1104.252.52290.810.613.312.8
36−72.2117.0106.82233.84.813.511.2
37−75.3128.322.2946.112.111.012.9
38−69.0134.235.71907.111.612.815.4
39−65.6140.947.11980.417.414.014.2
40−61.4136.015.7775.19.912.312.6
41−59.0137.523.81309.411.613.614.0
42−68.2151.437.21099.410.413.510.8
43−74.9163.323.8641.44.49.56.6
44−70.5159.046.11642.18.016.011.2
45−65.1167.627.11453.012.612.613.2
46−61.5165.910.9417.48.910.77.3
47−59.9160.731.0955.613.012.112.5
48−64.5158.637.01964.115.813.413.3
49−66.1161.532.3931.47.313.910.3
50−68.6162.754.7819.44.57.48.0
51−76.9183.840.01179.97.88.48.4
52−70.5193.515.1664.08.111.79.0
53−66.7186.535.3710.18.511.39.0
54−63.9192.321.8629.06.610.08.3
55−59.0193.824.21305.419.117.214.7
56−68.6191.6108.31658.311.07.910.1
57−72.8204.753.81990.312.111.413.4
58−74.6194.358.11548.513.17.28.8
59−74.8187.067.01438.511.17.68.8
60−73.2225.066.71710.012.411.312.1
61−71.6225.718.9777.49.612.511.4
62−67.8227.224.31180.413.413.715.0
63−64.3227.611.6533.812.011.110.7
64−60.3225.925.11605.318.119.516.9
65−70.9229.375.31967.417.09.113.3
66−73.9229.758.31731.916.011.813.1
67−73.9234.945.51114.89.38.312.0
68−78.6216.472.91264.84.85.38.7
69−77.9234.363.31639.99.311.011.8
70−79.6235.557.01259.112.43.610.3
71−73.1249.629.91035.612.713.511.9
72−71.3254.132.3942.310.79.69.9
73−65.6251.039.5944.67.67.610.2
74−64.4255.313.5489.58.211.18.9
75−59.3257.08.9415.110.116.812.4
76−66.3242.640.71960.616.114.315.1
77−75.1277.015.2891.86.810.710.3
78−69.8288.112.9640.512.612.29.0
79−66.9280.713.8535.47.97.89.1
80−62.8282.340.2923.811.515.812.1
81−58.5278.58.2340.610.09.48.9
82−69.9271.3110.61870.69.47.311.0
83−75.7317.77.3405.59.211.912.0
84−69.4317.211.1404.38.310.28.9
85−65.3307.121.91269.810.713.012.6
86−61.2318.918.1801.011.410.812.9
87−61.0316.419.71248.921.114.115.4
88−74.8344.031.11186.820.417.617.1
89−68.5346.058.41216.813.519.411.6
90−66.8342.611.5539.38.96.28.8
91−62.1349.132.2642.810.08.57.7
92−59.1348.015.4564.110.412.210.8

Constrained by the resolution of the data (Zuber et al. 1992), only circular features larger than 5 km in diameter (rim to rim) were considered for analysis. From a potential 559 circular crater-like features, only 346 were selected in the north polar region. Some features were eliminated because (1) they were determined to be too modified to be able to give precise morphologic data, as their rims were not easily discernable in visual inspection or (2) the elevation of the crater cavity floor was above that of the surrounding plains indicating that the crater had undergone substantial modification, was a pedestal crater, or was a volcanic caldera rather than an impact crater. Both northern high-latitude and polar impact craters (NPICs) and SPICs were systematically selected according to these criteria.

Impact crater locations and their morphologic properties were determined using the IDL-based program, Gridview (Roark et al. 2004). Depth and diameter measurements were averaged from four topographic profiles encompassing each of the cardinal and ordinal directions extracted from gridded MOLA data for each crater (Fig. 2, Tables 1–3). For this study, the diameter was defined as the rim-to-rim distance, where the rim was identified as the local topographic high. The crater depth is given as the vertical distance between the crater rim crest and the crater floor, where the crater floor was identified as the local topographic minimum in profile. Previous studies have suggested a possible latitudinal trend for impact crater wall slope symmetry in the Martian northern hemisphere (Kreslavsky and Head 2006; Parsons and Nimmo 2009). To thoroughly consider this suggestion, the interior crater cavity wall slopes were measured directly by Gridview from each of the four topographic profiles (Tables 1–3); only the top one third of each cavity wall was measured (Fig. 2b) to avoid potential effects of cavity modification such as dune fields.

Figure 2.

 “Textbook” complex impact crater (D ∼ 43.6 km) centered at 63.72° N, 11° E. a) MOLA shaded relief image of crater with lines representing the orientations of profiles taken for data measurements. b) Cross-section of west–east transect shown in 2a with gray lines approximating where the slope of cavity wall was measured.

Table 3.   Range, average, and standard deviation of morphologic characteristics from NPICs and SPICs in this study as well as the global fit trends as defined by Garvin et al. (2000, 2003) and Boyce and Garbeil (2007).
Crater groupLatitude rangeNo. of cratersDiameter (km)Depth (m)N, equator-facing slope (°)S, pole-facing slope (°)
RangeAverageSDRangeAverageSDRangeAverageSDRangeAverageSD
NPIC57–82° N3465.1–130.113.710.4126.5–2560.0585.0474.10.98–27.89.3550.9–27.89.54.9
SPIC57–85° S927.2–121.434.925.5217.5–2563.41081.0558.92.6–21.110.73.82.9–19.511.33.5
Boyce and Garbeil (2007) 80° S–80° N60477–49   d = 0.381D0.52Pristine complex (km)      
      d = 0.315D0.52Fresh complex (km)      
Garvin et al. (2003) Global6000    d = 0.21D0.81Simple (km)   s = 28.40D0.18   Fresh simple
      d = 0.36D0.49Complex (km)   s = 23.82D0.28   Fresh complex
Garvin et al. (2000) 57–90° N109    d = 0.03D1.04Polar complex (km)      
      d = 0.19D0.55Nonpolar complex (km)      

Many of the NPICs that were examined contain significant ITFs, uncommon in nonpolar craters and unlike typical central peaks, which have been noted by several authors (Garvin et al. 2000; Armstrong et al. 2005; Sakimoto 2005; Brown et al. 2008; Hovius et al. 2009) and typically cover at least one third of the diameter or depth of the crater cavity. A small subset of ITFs within NPICs has topography exceeding that of a predicted central peak height (Garvin et al. 2003) and sometimes that of the present surrounding plains. Similar-looking features have also been identified in some impact craters near the south pole (e.g., Russell and Head 2005; Westbrook et al. 2009). The approximate compass direction of the long axis and general morphology of each feature were noted. These features are typically asymmetrical deposits that lie in a variety of orientations either directly adjacent to the crater cavity wall (Figs. 3a and 3b) or in a more central location (Fig. 3c) on the crater floor. Levy et al. (2010) examined features similar to that in Fig. 3b in great detail and concluded that they were formed by a glacier-like mechanism, although the formation mechanisms for features in Figs. 3a and 3c are still under debate.

Figure 3.

 Satellite imagery and topographic profiles of an example from each crater group with IFTs indicated by arrows: a) CTX image B02_010407_2587_XN_78N028W and MOLA profile of group 1, 20.3 km diameter crater (Id no 262) centered at 78.60° N, 331.77° E. b) HiRISE images ESP_016603_2415_RED & ESP_017737_2415_RED and MOLA profile of group 2, 12.2 km diameter crater (Id no 33) centered at 61.0° N, 24.14° E c) CTX image P18_008222_2424_XN_62N121W and MOLA profile of group 3, 31.4 km diameter crater (Id no 173) centered at 62.10° N, 238.19° E.

MOLA DEMs interpolate between track lines, producing an inherent level of uncertainty with the data. Smith et al. (2001) calculated the vertical precision of the MOLA instrument based on work by Gardner (1992) and Abshire et al. (2000). On smooth slopes (<∼2°), the vertical precision is approximately 37.5 cm and increases to 10 m for a 30° slope; interior cavity wall slopes of the craters in this study are all less than 30°, so the associated uncertainty should be less than 10 m. Horizontal precision is latitude-dependent with the number of shots per DEM grid cell increasing with latitude (Som et al. 2008), thus the precision of the MOLA DEM will be highest in the high-latitude and polar regions. Each footprint is approximately 168 m in diameter and spaced approximately 300 m apart, but analyses indicate that random errors in position are less than 100 m (Neumann et al. 2001). Because the point data density is variable and may introduce a latitude dependence, we use high-resolution grids in preference over the raw MOLA data tracks, which may not singularly be representative of asymmetrical crater interiors. High-resolution polar grids are invaluable for measuring these features that are not precisely characterized by single orbital tracks.

Geologic Context

The NPICs lie within seven main geologic units varying in age from Early Hesperian to Late Amazonian as mapped by Tanaka et al. (2005) and inferred to be composed of volcanic, ice, and sand-derived materials; these units are discussed in order of relative age with the approximate percentage of craters from this study contained in each unit. Because geologic units are mapped from images, one assumes that the mapped units are surface units, obviously with varying depths. It is well known that the northern region is mantled as shown by the work of Frey et al. (2002) on buried basins as well as others on polygonal ground (e.g., Kostama et al. 2006; Levy et al. 2010). We sort the craters in this work in accordance with the units defined by Viking, Mars Global Surveyor, and Mars Odyssey missions (Tanaka et al. 2005) as it is a well known and accepted surface area classification that may well affect either target properties, surface modification processes, or both. What lies underneath could perhaps be a factor, but is beyond the scope of this work.

The Late Hesperian Alba Patera unit, characterized by lobate shield flows (Tanaka et al. 2005), contains approximately 2.0% of the craters from this study while approximately 6.9% of the craters superpose Late Amazonian–Early Hesperian aged crater ejecta and melt unit (AHc). Approximately 68.5% of our study’s craters lie within the Early Amazonian Vastitas Borealis Complex (ABvi & ABvm), which is characterized by plains-forming material containing polygonal trough systems and thumbprint terrain that are likely ice or volcanic features (Tanaka et al. 2005). The Early Amazonian Scandia unit (ABs), which overlays Abvi, contains approximately 18.5% of the craters in this study and is composed of knobs and mesas that are likely due to volatile-rich, eruptive material (Tanaka et al. 2003) or degraded, ice-cored glacial moraines (Fishbaugh and Head 2005). The Late Amazonian dune fields (Olympia Undae, Abo) contain approximately 2.9% of the NPICs while approximately 1.2% of the craters lie in the similarly aged Planum Boreum 2 (ABb2), which is characterized as a plateau capping unit mantled by residual water-rich polar ice and containing swirling troughs (Tanaka et al. 2005).

The SPICs lie in a broader age range of geologic units. Approximately 63.0% of the SPICs in this study lie within the Noachian-aged portion of the south polar plateau sequence (Npl1, Npl2, Nple, Nplr, and Nplh) that is characterized by rough, heavily cratered terrain likely composed of volcanic and impact-related units (Tanaka and Scott 1987). The Early Hesperian ridged plains (Hr), distinguished by broad, planar, moderately cratered surfaces with long ridges likely from local lava flows, contain approximately 10.9% of the SPICs, while the Early to Mid-Hesperian Dorsa Argentea formation (Hdu and Hdl) contain approximately 13.0% in its volcanic polar plains (Tanaka and Scott 1987). The Amazonian polar-layered deposits (Apl) contain approximately 9.8% of the craters in its ice and dust deposits while the remaining 3.3% lie within the impact breccia-derived (Greeley and Guest 1987) Noachian Hellas Assemblage (Nhl) and Hesperian-Noachian undivided material (HNu) characterized by plateau sequence and basement rocks (Tanaka and Scott 1987).

Discussion of Morphology Trends

Depth–Diameter Relationship

Our depth (d) and diameter (D) relationships show that the NPIC population poleward of 57° N is notably shallower than the global fresh crater population as defined by Garvin et al. (2003) for both simple (D < 7 km) and complex (D > 7 km) craters (Fig. 4a). In comparison with global fit lines determined by approximately 6000 craters (Garvin et al. 2003; Table 3), few craters lie on or above the complex crater fit line (d = 0.36*D0.49), while none of the craters lie on or above the simple crater fit line (d = 0.21*D0.81). Craters lying above these lines are deeper than the average global fresh crater population, which are defined by clear ejecta blankets and cavity rims that have been subjected to relatively little degradation. Some of the largest NPICs seem to approach the average global fresh crater depth line, but most have dramatically shallower depths than predicted by the global data as defined by these best-fit lines. This indicates that either (1) the NPICs were initially shallower than the global average due to polar target properties or (2) that they were once much deeper than at present, but have subsequently been filled by polar processes over time.

Figure 4.

 Depth–diameter relationships of polar impact craters in this study with global best-fit lines determined by Garvin et al. (2000, 2003) and Boyce and Garbeil (2007) show that a) NPICs are predominantly shallower than global trends for fresh and pristine craters and some are deeper than predicted based on polar trends. SPICs b) show similar depth–diameter trends to the NPICs until D ∼ 20 km, where SPICs become systematically shallower than NPICs with similar diameters.

The latter proposition is supported by the work of Garvin et al. (2000, Table 3) who suggested that fresh north polar region craters are best fit by the power equation d = 0.03D1.04 (Fig. 4a), while nonpolar craters in the northern hemisphere are best fit by d = 0.19D0.55, where d and D are measured in kilometers. The difference in exponents between the two regions indicates that in the northern hemisphere, fresh complex polar craters with D > 43.2 km are predicted to be deeper than similarly sized nonpolar craters while the opposite is true for craters with < 43.2 km (Garvin et al. 2000). Most of the craters in this study with > 20 km are deeper than the polar power law trend described by Garvin et al. (2000), yet shallower than the global complex trend described later by Garvin et al. (2003). Garvin et al. (2000) also note that ice-associated craters poleward of 70° N have an even steeper power law as reflected by their greater depths than predicted by the global data set.

In addition to the work by Garvin et al. (2000, 2003), this study is similar in scope to the work of Boyce and Garbeil (2007), but has several distinctions. Most importantly, while many of our selected craters overlap, our study contains 127 craters that were not included in the more than 6000 crater data set by Boyce and Garbeil (2007). These craters are predominantly located within the north polar-layered deposits, the lower member of Alba Patera, and various units within the Vastitas Borealis Formation as mapped by Tanaka et al. (2005). Within the Vastitas Borealis, the craters in our study are located in the Ridged and Arcadia (approximately 180–210° E), Knobby (approximately 0–60° E), and Mottled (approximately 60–120° E; 150–180° E) members. Secondly, the craters in this study are relatively “fresh,” north polar features while Boyce and Garbeil (2007) focused their study on “fresh” as well as “pristine” craters over the entire surface of Mars. They calculated best-fit global population lines (Table 3) for pristine (= 0.381D0.52) and fresh (d = 0.315D0.52) complex craters 12–49 km; these lines lie above and just below the Garvin et al. (2003) complex crater line, respectively (Figs. 4a and 4b).

In examining depth–diameter relationships for craters residing in different geologic units, there is no apparent trend for craters within the most-polar units (ABs, ABo, ABb2) in contrast to craters in slightly lower latitude units. There does, however, seem to be an unexpected unit-dependent diameter relationship for the NPICs. Most of the geologic units (ABs, ABo, ABvm, ABb2, HTa) only contain craters less than 24 km in diameter, with one outlier for ABs. In contrast, craters more than 24 km in diameter are predominantly found in the interior Vastitas Borealis unit (ABvi), although this unit also has some smaller craters as well; the impact crater melt and ejecta unit (AHc) consistently contains craters on either side of the 24 km diameter marker—likely because its material is largely composed of the preimpact target material and the unit is widespread over the entire region. The smaller craters (D < 24 km) are preferentially found in younger units (ABo, ABb2, ABs); the lack of many smaller craters in the ABvi seems to indicate an erosional or mantling resurfacing event for ABvi that would affect the crater population by eliminating smaller craters (Michael and Neukum 2010). If a single resurfacing event or events did occur, it must have taken place before the formation of the youngest units in the Early Amazonian. One problem with this supposition is that the craters in the Late Hesperian Alba Patera unit all fall below the 24 km diameter boundary, although this may not be a problem if the resurfacing event was local to the polar region and did not stretch down to 60–63° N and 240–270° E.

A Noachian-aged ocean for the northern hemisphere of Mars has been proposed in the past (e.g., Parker et al. 1989, 1993) as a manner in which to explain the topographic dichotomy boundary between the hemispheres. More recently, some (Kreslavsky and Head 2002; Carr and Head 2003) have suggested that the Vastitas Borealis Formation may represent sediments from a much younger ocean or large-scale flooding; these studies were conducted with the use of an older map of the northern plains of Mars that suggested the Vastitas Borealis was Hesperian in age (Tanaka and Scott 1987) rather than the map used in this study (Tanaka et al. 2005), which suggests that the Vastitas Borealis formation is Amazonian in age. Nevertheless, there appears to be some evidence of a resurfacing event occurring more recently than the Noachian. A local resurfacing event could have left behind sediments that would obscure smaller craters from direct observation, thus creating the subsequent dominance of larger craters in the region. Alternatively, the influx and removal of an ice sheet during a period of low obliquity in the Amazonian (Fagan et al. 2010) may have similarly erased evidence of smaller craters at least locally within the Borealis Volcanic Field and potentially beyond.

SPICs, like NPICs, also appear to be shallower than the global crater populations, both pristine and fresh, as defined by Garvin et al. (2003) and Boyce and Garbeil (2007) (Fig. 4b). In particular, large (≥∼25 km diameter) south polar craters are approximately 30–40% shallower than northern craters of similar diameter. This dramatic depth difference is not observed when comparing smaller SPICs and NPICs. Depth–diameter relationships for SPICs separated by their respective geologic units are inconclusive, although craters in the polar-layered deposits (Apl) tend to be shallower and those in Dorsa Argentea (Hdu and Hdl) tend to be smaller in diameter than the rest of the SPIC population. This may indicate that properties of the Dorsa Argentea volcanic plains prevent the formation of larger craters indicating a stronger target material than similar volcanic plains in the region such as the Noachian plateau sequence (Npl1, Npl2, Nple, Nplr, Nplh) or the Hesperian ridged plains (Hr). Similarly, the ice and dust that dominate the south polar-layered deposits could be responsible for the shallower depths of craters either by an initial target property causing more collapse during modification or by allowing massive infilling from ice and eolian-led processes.

Noncentral Peak Interior Topographic Features

Simple craters are characterized by a parabolic profile without any substantial interior topography unlike a complex crater, which is most often distinguished by a relatively flat cavity floor and a large-scale uplift, called a central peak (Melosh 1989). Approximately 138 of the NPICs in this study have unusual ITFs, which set them apart from a typical simple or complex crater profile. These ITFs display positive relief that differs from central peaks in their overall shape and are at least twice the diameter of a central peak predicted for a given crater diameter (Garvin et al. 2003). In addition, while central peaks typically terminate in a peak at their maximum height, the ITFs are predominantly rounded or flat on top like a pingo or mesa and are circular to elliptical in plan form. Unlike the layered deposits in the mid-latitude regions described by Malin and Edgett (2001) many of these features do not have visible layering in high-resolution photographs, although recent work has examined the layers within the Korolev crater ITF using radar sounding (Moore et al. 2012). ITFs are found within both the simple and the complex crater groups, with approximately five times as many complex craters as simple craters. The distinctive raised ITF within these craters can be separated into three groups depending on where the feature lies within the crater cavity (1) directly adjacent to the cavity wall at a preferred location within the crater (Fig. 3a); (2) directly adjacent to the cavity wall in a circumferential orientation (Fig. 3b); or (3) offset from the crater cavity wall and in a more central location within the crater cavity (Fig. 3c). Nearly 69% of the NPICs with an ITF fall into group 1 while the rest fall into groups 2 (approximately 19%) and 3 (approximately 12%).

Group 1 NPICs display a definite trend in preferred location within the crater cavity. Nineteen NPICs have ITFs that lie directly adjacent to the western crater cavity wall with five more lying at altitudes within 15° of due west. In addition, 21 NPICs have ITFs lying adjacent to the eastern crater cavity wall with three more lying at altitudes within 15° of due east. Together, these show that 48 of the 95 craters in group 1 have features lying directly adjacent to crater cavity walls within 15° of due east or due west (Fig. 5a). Interestingly, there also appears to be a slight trend in the latitude–longitude location of these 48 NPICs (Fig. 5b), although they do not appear to lie exclusively in any particular geologic unit. In addition, 23 impact craters have ITFs preferentially located in the NE or SW regions of the cavity and these appear to have a longitudinal trend (Fig. 6a) with the exception of one outlier located at approximately 62° N, 270° E. A similar clustering trend is seen in craters with the ITFs preferentially found in the northern or southern regions of the cavity (Fig. 6b), but no apparent longitudinal trend can be discerned for those preferentially found in the NW and SE regions of the cavity (Fig. 6c).

Figure 5.

 Distribution of preferred location of raised interior topography of the 92 group 1 NPICs displays a) an overarching E–W preference and b) possible latitude/longitude trends of affected craters with IFTs lying within 15° of due E or W. Impact craters shadowed in gray have deposits within 15° of due west whereas impact craters not in shadow have deposits within 15° of due east. Arrows indicate the compass direction placement of raised topography within the crater cavities.

Figure 6.

 Locations of NPICs with group 1 interior topographic features showing a preferential location adjacent to the a) NE or SW cavity wall, b) N or S cavity wall, and c) NW or SE cavity wall. Arrows indicate the compass direction placement of raised topography within the crater cavities.

Some NPICs’ ITFs exceed the predicted height of a central peak and come near to or may even surpass the level of the present surrounding plains. These features seem to be found predominantly in the NPIC group closest to the pole (>70° N). These craters do not appear to lie in any one particular geologic formation nor do they and other craters within the same region necessarily display identical characteristics. Interestingly, the unusual topography within 75% of these most poleward craters lies in the western half of the crater cavity (Table 4), which may be due to insolation effects (e.g., Russell et al. 2004).

Table 4.   Location and diameter of northern craters with ITFs exceeding the level of the surrounding plains.
Id no.Latitude (°)E longitude (°)Avg. diam. (km)Raised relief cavity location
379.160.924.4SW
481.6190.118.9W
681.3254.818.2W
978.6347.012.8W
1081.188.88.7W
1181.4117.311.1SW
6572.8164.682.0NE
11977.2214.352.4NW
21674.3319.217.0SSE
21771.2309.018.9SSE
26278.6331.820.3W
32677.189.131.1NE

While the NPICs show distinctive interior topography trends, the selected SPICs do not, although approximately one quarter of the SPICs display topographic features are similar in appearance to the NPIC’s ITFs (e.g., Fig. 7). The NPICs show a clear E–W preference in wall placement of ITFs within the cavity, which is not seen with the SPICs (Fig. 8a). There may be some suggestion of a slight trend for such features in SPICs lying adjacent to the NW and SW crater walls, but the evidence is inconclusive with such a small data set (Fig. 8b). In addition, there do not appear to be any SPICs in this data set with features rising above the surrounding plains. This does not, however, rule out the possibility of the existence of such a crater outside this dataset, as there are a vast number of impact craters near the south pole that were not selected. Of the SPICs selected that have raised interior topography, half are located within the Noachian-aged south polar plateau sequence and one third are located within the Amazonian polar-layered deposits (Tanaka et al. 2005), which likely account for the shallow nature of the SPICs.

Figure 7.

 Mosaic of THEMIS images (V06174001, V25618009, V17631012) of a SPIC (∼ 46 km) centered at 70.5° S, 159° E showing a raised interior topographic feature offset from the crater cavity wall.

Figure 8.

 a) Rose diagram of distribution of wall preference placement of interior topographic features within NPICs (open black petals) and SPICs (gray petals) displaying group 1 interior topography. b) SPIC distribution of group 1 topography wall preference.

Several authors have suggested formation mecha-nisms of these positive relief features within crater cavities such as a glacial origin (Russell and Head 2005), frost mounds or pingos (e.g., Hovius et al. 2009), build-up of seasonal frost (Westbrook et al. 2009), atmospheric deposition of water vapor (Brown et al. 2008; Conway et al. 2011), or asymmetries in local energy balance due to solar insolation and radiative effects of crater walls (Russell et al. 2004). However, others have disputed some of these mechanisms as being unlikely; for example, Hovius et al. (2009) rejected the atmospheric condensation hypothesis due to dissimilarity in the layering distribution, patterns between craters with mounds, and the polar cap deposits, as well as the lack of craters located near cold traps. Burr et al. (2009) determined that the proposed mechanisms of formation for the intercrater mounds was unlike processes that form terrestrial pingos and thus do not support the hypothesis that the mounds are pingos.

Some evidence may exist that the group 1 ITFs are reliant on prevailing wind directions, where the position of the deposit in the crater may indicate the wind direction during the period of dune field migration across the floor (Hayward et al. 2009). The most-polar craters, however, do not have any obvious correlation with wind directions derived from crescentic dunes (Thomas and Gierasch 1995), although wind-streak data may be a viable alternative for exploring prevailing wind directions. Although it is possible that wind may play a role in the creation or modification of the cavity features in group 1 craters, insufficient data for wind indicators prove inconclusive and there is little evidence to support the influence of wind on group 1 craters north of 73° N. Eolian processes also do not appear to play a dominant role in the creation or modification of group 2 or 3 features.

Crater Cavity Wall Slope

Previous studies (Kreslavsky and Head 2006; Parsons and Nimmo 2009) have suggested a possible latitudinal trend for impact crater wall slope symmetry in the Martian northern hemisphere, but we find no statistically significant trend for cavity symmetry normalized by crater diameter. Similarly for the SPICs, we find the normalization of crater slope differences to be statistically unrelated to latitude (Fig. 9b). Although it may appear that NPICs increase in symmetry with latitude (Fig. 9a), this is a visual deception due to the small sampling of craters at the highest latitudes. If craters do not become increasingly more symmetrical or asymmetrical with latitude, then this indicates that insolation properties associated with latitude bear little effect on the overall slope. However, some NPIC cavity wall slopes do display a latitude trend. North (equator-facing) wall slopes appear to become shallower as latitude increases, but south (pole-facing) wall slopes exhibit greater fluctuations throughout the latitude range. Other cavity wall slopes (W, E, NE, NW, SE, SW) do not display any definitive trends. This is likely due to the uneven distribution of MOLA orbit paths, leaving gaps, which makes the western and eastern wall slope data less reliable in gridded data than the north and south walls that are sampled in every orbit track (Kreslavsky and Head 2006).

Figure 9.

 Crater cavity N and S wall slope differences normalized by diameter versus latitude for NPICs (a) and SPICs (b).

Garvin et al. (2003) empirically determined global fit relationships between crater cavity wall slope (s) and diameter (D) using MOLA topographic grids for a global set of approximately 6000 Martian impact craters. Simple craters (D < 7 km) were found to follow s = 28.40*D−0.18, while complex craters (7 ≤ < 100 km) were best described by s = 23.82*D−0.28. In an effort to understand the crater cavity slopes within the polar regions in comparison to the global dataset, we normalize the average measured cavity slope to the predicted slope calculated from the Garvin et al. (2003) equation for complex craters. We find that approximately 30% of the NPICs’ and approximately 66% of the SPICs’ average cavity wall slopes are steeper than the predicted value based on the parameters determined by Garvin et al. (2003). This disparity appears to have a diameter dependence where all NPICs with diameters greater than 25 km have steeper slopes than the global fit. The SPICs have a similar deviation at 25 km, although approximately 10% of the craters larger than 25 km are less steep than predicted (Fig. 10). This finding may indicate the influence of target properties in the polar regions that allow impact craters to favor steeper slopes at larger diameters. On the basis of the work of Garvin et al. (2003), a 25 km diameter crater would have a predicted cavity wall slope of approximately 9.67°, whereas the measured average wall slope of a similarly sized NPIC and SPIC is 17.27° and 16.91°, respectively. These steeper slopes may indicate stronger target materials that would be able to support the weight of the slope rather than collapsing into the crater cavity (Boyce et al. 2006). Because these steeper slopes are only found in the larger (D > 25 km) craters, we may be seeing the influence of a stronger target material beneath the surface that only the largest craters are able to excavate. Smaller craters have a shallower depth of excavation, which would prevent them from reaching such a theoretical stronger material.

Figure 10.

 Average measured crater cavity slope normalized by predicted cavity slope (Garvin et al. 2003) displaying dependence on diameter (D). Normalized values >1 indicate steeper slopes than predicted, values <1 indicate shallower slopes than predicted. All NPICs (gray dots) with > 25 km have steeper cavity slopes than predicted; 90% of the SPICs (black dots) with > 25 km have steeper slopes than predicted.

The steepest N and S cavity wall slopes for NPICs are both approximately 27.8° while the steepest slopes for SPICs are approximately 21.1° and 19.5°, respectively. Kreslavsky and Head (2003) confirmed that crater slopes in north and south polar regions are rarely steep, but that in the equatorial region, slopes steeper than 30° are ubiquitous. Because of the similar behavior of north and south polar crater slopes, the lack of steep (i.e., >30°) slopes is likely due to polar climate or specific target properties (Kreslavsky and Head 2006). Melting of ice during summer periods of high obliquity may promote down-slope movement of material, causing steep slopes to be reduced north of 50° N (Kreslavsky and Head 2003). Kreslavsky and Head (2006) also note a sharp drop in the frequency distribution of slope steepness at approximately 20° based on 130 craters, 10–25 km in diameter and north of 52° N. We confirm this drop in frequency continues with a larger NPIC data set (346 craters) for both north (equator-facing) and south (pole-facing) wall slopes (Fig. 11a), but does not appear to be a phenomenon which affects the SPICs (Fig. 11b). Kreslavsky and Head (2006) suggested that the step at 20° for north polar craters could be caused by (1) an active (continuously or episodically) erosion process indicating strong climate change in the Late Amazonian or (2) active slope degradation throughout the entire Amazonian with a sharp onset at 20° slope. The lack of a similar trend with SPICs would indicate (1) more craters need to be sampled to be able to obtain a similar trend, (2) the process causing such a step in the north polar region does not occur in the south polar region— suggesting a possible climatic difference, or (3) the influence of a south polar-specific process, which preferentially affects slopes less than 7.5° as well as those greater than 15° such as a combination of resurfacing and erosion to create a normal distribution in slope (Fig. 11b).

Figure 11.

 Frequency distribution of crater cavity wall slopes for a) NPIC north (equator-facing, dark gray) and south (pole-facing, light gray) walls showing a steep drop at approximately 20° slope in agreement with Kreslavsky and Head (2006). A similar plot is shown for b) SPIC north (dark gray) south (light gray) wall slopes, which show a drop in frequency with slopes <7.5°  and >15° S.

Conclusions

The purpose of this study was to investigate the morphologic properties of NPICs in comparison to global datasets for fresh and pristine craters. After thoroughly examining the NPICs and a small set of SPICs, it appears that the two polar sets differ from each other in their geometry and when compared to a global dataset. To start, NPICs and SPICs are substantially shallower than the global fresh crater populations as determined by Garvin et al. (2000, 2003) and Boyce and Garbeil (2007). Although Garvin et al. (2000) may suggest that large polar craters were deeper than nonpolar craters in the northern hemisphere, current observations of shallower depths indicate probable massive infilling. In addition, large (>25 km diameter) SPICS tend to be shallower than both the measured NPIC population as well as the global fresh crater populations. Moreover, a lack of NPICs with D < 24 km in certain geologic units may be indicative of a recent resurfacing event such as an ocean, massive flooding, or advance and retreat of an ice sheet during the Amazonian in the vicinity of Vastitas Borealis.

NPICs exhibit a clear trend in the location of non-central peak ITFs with a preferential altitude adjacent to the east or west crater cavity wall. SPICs with similar features do not display such a definite trend for preferred cavity wall location as with the NPICs. There may be a slight directional trend of preference for the NW or SW cavity wall, but the data are inconclusive without further data collection and analysis of SPICs. In addition, while some NPICs have cavity features that rise above the level of the surrounding plains, the SPICs of this study do not appear to have such characteristics. Because the SPIC data set is small compared with the overwhelming total number of impact craters in the south polar region, it is possible that there are some impact craters with such cavity characteristics that were not sampled here.

Large (> ∼25 km) NPICs have steeper slopes than predicted by the work of Garvin et al. (2003), which may suggest the tapping of a stronger unit below the surface; a similar deviation can be seen for SPICs suggesting that the occurrence may be a general polar phenomenon related to both polar climate and target properties. Finally, NPICs have a sharp drop in wall slope frequency at approximately 20° slope while SPICs experience a drop in frequency at <7.5° and >15°. This difference in slope frequencies suggests that the north polar and south polar regions, although similar, may exhibit different target properties resulting in different slope and general morphologic characteristics.

In summary, the NPICs and SPICs both differ from the global data set suggesting the potential for polar-specific modification in contrast to the mid-latitude and equatorial regions. However, the depth–diameter ratio, location of ITFs, and slope frequencies imply that the NPICs are inherently different from the SPICs and we conclude that the north polar region exhibits such characteristics due to a distinctive combination of modification processes and possible subsurface target properties.

Acknowledgments

Acknowledgments— We graciously thank the University of Notre Dame Lilly Fellowship for funding this research and Dr. Clive Neal for providing constructive feedback. We also wish to express our deep gratitude to the constructive reviews of Dr. Joseph Boyce and Dr. Joseph Levy, whose criticisms greatly improved this manuscript.

Editorial Handling— Dr. Gordon Osinski

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