Intra-crater glacial processes in central Utopia Planitia, Mars

Icarus 212 (2011) 86–95

Contents lists available at ScienceDirect

Icarus journal homepage: www.elsevier.com/locate/icarus

Intra-crater glacial processes in central Utopia Planitia, Mars Geoffrey Pearce a,⇑, Gordon R. Osinski a,b, Richard J. Soare c a

Dept. of Earth Sciences, University of Western Ontario, London, ON, Canada N6A 5B7 Dept. of Physics and Astronomy, University of Western Ontario, London, ON, Canada N6A 5B7 c Dept. of Geography, Dawson College, Montreal, QC, Canada H3Z 1A4 b

a r t i c l e

i n f o

Article history: Received 19 June 2009 Revised 26 November 2010 Accepted 1 December 2010 Available online 17 December 2010 Keyword: Mars, Surface

a b s t r a c t We describe and interpret a series of previously unidentified glacial-like lobes (34–43°N; 107–125°E) that were discovered as part of a survey of large (D > 5 km) impact craters in Utopia Planitia, one of the great northern plains of Mars. The lobes have characteristics that are consistent with a glacial origin. Evidence includes curvilinearity of form, lineations and ridges, and surface textures that are thought to form by the sublimation of near-surface volatiles. The lobes display morphologies that range from wedge-shaped to near-circular to elongate. The flow directions are towards the northern walls in the case of craters with large single lobes, and in all directions in the case of the largest (D > 30 km) craters. Concentric crater fill is also interspersed within craters of our study region, with such craters having much higher filling rates than those with flow lobes. We suggest that the impact crater population in southwest Utopia Planitia demonstrates a spectrum of glacial modifications, from low levels of filling in the case of craters with elongate lobes at one extreme, to concentric crater fill in highly-filled craters at the other. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction A variety of landforms possibly related to late-Amazonian glacial activity (<300 Ma) have been identified on Mars. They include: lineated fill and lobate debris aprons at Deuteronilus, Protonilus, and Nilosyrtis Mensae (located between 25–40°N, 15–75°E) (e.g., Colaprete and Jakosky, 1998; Dickson et al., 2008; Head et al., 2006a; Levy et al., 2007; Lucchitta, 1984; Squyres, 1978, 1979); lobate debris aprons on the flanks of Tharsis Montes and within the Hellas impact basin (Fastook et al., 2005, 2008; Head et al., 2005; Kargel and Strom, 1992); morainal and esker-like features in valleys and mid-latitude impact craters (Arfstrom and Hartmann, 2005; Berman et al., 2005, 2009); and, concentric fill craters (located between 30–50°N and S) (Colaprete and Jakosky, 1998; Levy et al., 2009a; Squyres and Carr, 1986; Squyres, 1989). Based on data returned by the Shallow Subsurface Radar (SHARAD) aboard the Mars Reconnaissance Orbiter, dielectrical properties matching those of massive (nearly pure) glacial-like ice were identified at several sites in the northern and southern hemispheres where lobate debris aprons occur (Holt et al., 2008; Plaut et al., 2009; Safaeinili et al., 2009). This lends indirect or circumstantial support to glacial hypotheses of morphologically similar landforms in other regions. Moreover, widespread terrain smoothening in both hemispheres between 30° and 60° latitude ⇑ Corresponding author. Address: 110 Seigniory ave., Pointe Claire, QC, Canada, H9R 1J7. E-mail addresses: [email protected], [email protected] (G. Pearce). 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.12.001

has been attributed to the presence of a relatively thin (up to several tens of meters thick) and sublimating dusty-ice mantle (Head et al., 2003; Mustard et al., 2001). Impact craters are useful targets for identifying glacial features because their interiors function as cold traps, shielding volatiles from the ablative effects of insolation or wind and preserving icy bodies that would otherwise be removed in an open plain (Dickson et al., 2007, 2008; Greeley et al., 2004; Head et al., 2003, 2006b; Hecht, 2002; Levy et al., 2007, 2009a,b; Russell et al., 2004). In addition, the initial geometry of impact craters is relatively predictable from their diameter (e.g., Garvin et al., 2003; Stewart and Valiant, 2006) – with variation the result of different subsurface properties and the gravitational collapse of shock-weakened rock. This facilitates the identification of post-impact modifications and allows for reasonably accurate estimates of the depth and volume of fill material. Highly-filled craters with concentric rings (i.e., concentric crater fill) were the first martian intra-crater landform thought to be glacial in origin (Squyres, 1979). The hypothesis proposed by Squyres (1979) and Squyres and Carr (1986) suggested that the concentric rings were formed by the mass wasting of regolith rich in interstitial ice on crater walls (Squyres, 1979; Squyres and Carr, 1986). While a non-icy composition for the filling material has also been suggested (Zimbelman et al., 1989), a recent reassessment of concentric crater fill using High Resolution Imaging Science Experiment (HiRISE) imagery interprets the feature as being debris covered glaciers (Levy et al., 2009a). This is inferred from the presence of coral-patterned terrain that has been found exclusively on

G. Pearce et al. / Icarus 212 (2011) 86–95

martian landforms, such as lineated valley fill, that share characteristics common to glacial features on Earth (Levy et al., 2009a, 2010). It has been suggested that this coral-patterned terrain forms by the sublimation of glacial ice overlain by a veneer of debris (Dobrea et al., 2007; Levy et al., 2009a, 2010; Williams et al., 2008). A number of other interesting and putatively glacial intra-crater landforms also have been observed on Mars. One such feature that has received considerable discussion are arcuate ridges near the inner walls of mid-latitude craters; numerous traits of form show similarities to glacial depositional features on Earth (e.g., Arfstrom and Hartmann, 2005; Berman et al., 2005, 2009; Hartmann et al., 2003; Howard, 2003; Marchant and Head, 2003). For example, Arfstrom and Hartmann (2005) propose that many ridges strongly resemble terrestrial moraines in appearance. They show a curvilinearity that is common to push moraines, double crests that may represent separate stages of advance, and conformity in scale with moraines on Earth. Berman et al. (2005) performed a statistical study of arcuate, moraine-like ridges in the Newton basin and noted that their orientation (pole-ward or equator-ward) correlates strongly with latitude, with ridges in craters below 44° occurring preferentially near pole-ward facing walls and those above 44° near equator facing walls. It is thought that this latitude-dependent orientation is caused by fluctuating insolation parameters that occur during shifts towards periods of higher planetary obliquity (Berman et al., 2005). The correlation between latitude and flow orientation has also been demonstrated for putative ice-rich flow features within large (D > 20 km) craters in Arabia Terra and terrain east of the Hellas basin (Berman et al., 2009). In Utopia Planitia, one of the great northern plains, there has been some discussion of potential glacial features within impact craters (i.e. Levy et al., 2009a) and of ice-rich mantling in the plains (e.g. Mustard et al., 2001; Milliken et al., 2003; Tanaka et al., 2005; Soare and Osinski, 2009). However, neither the distribution nor many of the fine details of these features have been studied in any detail. Here we present the results of a study, based on data drawn from impact craters in Utopia Planitia, whose aim is to address these two shortcomings. We do this by presenting a map of the intra-crater features, identifying key latitudinal and longitudinal constraints, and by discussing key aspects of these features, heretofore unaddressed in the literature.

2. Intra-crater survey, landform morphology and distribution

87

Boyce et al. (2005) and Stewart and Valiant (2006). According to this method, crater depth is calculated as the lowest value of all MOLA points across a given crater subtracted from the average height of points that cross its rim (i.e., Boyce et al., 2005). Stewart and Valiant (2006) estimate that crater depth and rim height calculations within the diameter range of our study are accurate to within a few percent on smooth terrain, which is common throughout our study area. Crater diameter calculations are accurate to 5% or less using THEMIS imagery (19 m/pixel) on a 5 km crater, with most of this error being the result of the natural variability in crater radius (Stewart and Valiant, 2006). Imagery at THEMIS resolution or higher was available for all but 4 of the 191 craters. Given the potential error of these measurements, our calculations of crater depth, diameter, and depth/diameter should be highly accurate. 2.2. Lobate flow features Our survey shows the presence of a variety of lobate features between 34–43°N; 107–125°E located on crater floors and walls (Fig. 1, Table 1). In general, the lobes share characteristics that include curvilinear fronts, lineations and troughs, and a pitted, sometimes high-albedo surface texture (Figs. 2–4). The shape of the lobate features varies from elongate (e.g., Fig. 2d) to wedge-shaped (e.g., Fig. 3). While smaller craters have single lobes, the largest craters (D > 30 km) have multiple lobes whose fronts are clearly identifiable but whose lateral boundaries are often faint or indistinguishable (e.g., Fig. 4b). Estimates of the thickness of the lobes made from MOLA point transects and photoclinometry suggest that the lobes range from as few as 10–15 m (Fig. 4) to potentially over 100 m in thickness (Fig. 2a–c). The northern front of the lobe shown in Fig. 2a, for instance, is 100 m below the lobe surface between the terrace at the southern wall and the crater’s central peak (Fig. 2c). In another crater with multiple-tongue-shaped lobes on its floor (Fig. 3), the lobate floor material is 25–30 m thick where it is incised by the debris fan at the bottom of the south-eastern crater wall (Fig. 3c). A minimum height of 400 m of filling material is estimated at the center of this crater’s floor using a power law from Garvin et al. (2003) to predict initial crater depth. We cannot know how much of the filling the lobate material is responsible for, however, as antecedent processes such as dust infilling may have contributed significantly to the filling of the crater as well. This is discussed below.

2.1. Landform survey area and methods 2.3. Concentric crater fill and arcuate ridges To evaluate the possible extent of glacial processes in Utopia Planitia we have conducted a search of large impact craters (D P 5 km, n = 191) across a broad latitudinal and longitudinal area (85–125°E, 30–60°N). Five kilometers was chosen as a minimum diameter so as to be able and generate accurate elevation profiles of the craters using MOLA point data, and because it is small enough to yield a large sample. We performed our survey by identifying the position of all craters using a MOLA hill-shade background image and THEMIS day and night-time infrared imagery. We then searched for all imagery of the craters taken by the Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) visible wavelength camera onboard Mars Odyssey, and the High Resolution Imaging Science Experiment (HiRISE) and Context Imager (CTX) onboard the Mars Reconnaissance Orbiter mission. CTX images provide complete and high resolution coverage of many of the craters in our survey and were georeferenced in ArcGIS 9.3 using standard processing techniques for mapping purposes. Crater depths were calculated using Mars Orbiter Laser Altimeter (MOLA) point tracks according to the method described in

Previously discussed landforms pointing to landscape modification by glacial processes also have been identified within the craters of our survey region, with the most widespread being concentric crater fill (CCF) (Fig. 1). CCF is found in shallow craters (i.e., those with very deep fill) (e.g., Fig. 5). We have included in our results those craters where ridges in CCF form at least a half-circle, thereby omitting some of the more subdued, and potentially degraded, examples. Concentric crater fill displays a latitudinal distribution similar to that of the lobate features, but with a broader longitudinal occurrence (Fig. 1). Near-wall arcuate ridges (i.e., Arfstrom and Hartmann, 2005; Berman et al., 2005; Hartmann et al., 2003; Howard, 2003; Marchant and Head, 2003) are also interspersed within craters in the region (Fig. 6a). The arcuate ridges (n = 7) occur in craters between 90–125°E and 36–43°N. 2.4. Age of lobate features Crater size–frequency distribution (CSFD) analyses relying on sub-kilometer impact craters are commonly used to obtain age

88

G. Pearce et al. / Icarus 212 (2011) 86–95

Fig. 1. MOLA hill-shade map overlain with geological units from Tanaka et al. (2005) showing location of concentric crater fill and intra-crater glacial-like lobes. Craters with glacial-like lobes overlie early-Amazonian (<100 Ma) units including the Vastitas Borealis and Elysium sub-units, respectfully (ABvi) and (AHEe and AEtb).The numbers indicate craters with corresponding figures. Red ‘‘x’’ indicate craters with near-wall arcuate ridges. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 This table showing the location and dimensions of craters with possible glacial flow features in Utopia Planitia. Figures that correspond with these craters are indicated in the right-most column. Crater location and dimensions

Lobe characteristics

Figure

Latitude

Longitude

Diameter (km)

Depth (km)

d/D

Single or multiple lobes

Flow direction

Average Width (km)

Length (km)

Surface area (km2)

34.5 34.6 34.85 34.9 34.9 36 36.2 36.4 36.6 39.5 40.4 41.2 43

123 123.5 117.5 102.6 111.55 101.4 125.1 107.75 121.7 105.4 120.2 124.1 115.7

28.3 10.5 14.2 14.1 9.3 15.8 22.5 10.5 8.5 11.5 38.5 35.0 31.5

2.1 0.8 1.5 1.3 1.0 1.5 1.6 1.5 0.9 1.22 2.2 2.4 2.1

0.07 0.08 0.11 0.09 0.11 0.09 0.07 0.14 0.10 0.11 0.06 0.07 0.07

Single Single Single Single Single Multiple Single Single Single Multiple Multiple Single Single

N N N N N N N NW N S n, s, e, w n, s, e, w n, s, e, w

7 8 8 5 5 3 15 2 5 0.3 6 8 8

9 8 7 7 3 18 14 3.5 4 3 47 5 8

36 64 56 35 15 54 210 7 20 0.9 282 40 64

estimates for features with surface areas and impact records that are comparable to the features discussed here. This having been noted, there is substantial evidence that the great majority of sub-kilometer impact craters are formed by secondary impacts and are unfit for use in surface-age estimates (e.g., Bierhaus et al., 2005; McEwen et al., 2005; McEwen and Bierhaus, 2006; Preblich et al., 2005). Therefore, we suggest that it is not possible to obtain a reliable and narrow estimate for the absolute age of the lobate features using CSFD statistics due to the small surface areas of the features in question and the complete absence of large (>1 km) impact craters overlying them. This last observation is still consistent with a late-Amazonian (<100 Ma) age for the features using the isochrons from Hartmann and Neukum (2001), even if it cannot be used to arrive at an accurate prediction. By contrast, the relative age of the lobate features can be derived from the cross-cutting relationships associated with them. Three relationships are important in this regard. First, some of the features

3 2a–c 7

2d–f 6 4

are within craters that overlie early to mid Amazonian (<1 Ga) geologic units as identified in recent mapping of the region (Tanaka et al., 2005). These units include the expansive Vastitas Borealis Formation (VBf) and Tinjar Valles units (AEta, and AEtb) (Fig. 1). The lobate features, therefore, formed no earlier than their host craters, which are early-Amazonian in age. Second, debris aprons cross cut the lobate features in several craters (e.g., Fig. 3d). This shows that these lobes formed prior to that of the aprons (10 Ma) (Malin and Edgett, 2000). Third, glacial-like deposits in some of the larger craters overlie debris aprons, implying that the former postdates the aprons (Fig. 4f). Based on these relationships, and the complete absence of large (D > 1 km) impact craters overlying these features, we suggest that these glacial lobes formed no later than the very late-Amazonian period (<100 Ma); with regard to the lobes that cross cut the debris aprons (e.g., Fig. 4f) they must have formed even more recently than the gully-associated debris flows (potentially as recently as within the last 10 Ma).

G. Pearce et al. / Icarus 212 (2011) 86–95

89

Fig. 2. (a) Large glacial-like lobe within a 14 km impact crater at 118°E, 35°N. Lobe thickness is  50 m, with the deepest sections possibly exceeding 100 m. Lobe flow has occurred around the terrace at the southern wall and the highly covered central peak. The inset at the bottom right shows a typical ‘‘ring-mold’’ crater (D  200 m). The latter could be due to a small asteroid impacting into ice-rich substrate (Kress and Head, 2008). Section of CTX image P04_0024252152. (b) Lineated and pitted surface of lobe from HiRISE image PSP_008899_2150. (c) Profile across above crater showing the estimated thickness of a flow lobe and the location of the crater wall terrace and central uplift. The solid black line is a portion of MOLA profile 18150. (d) An elongate flow lobe in a 10.5 km impact crater at 108°E, 36°N. The feature is overlain by debris aprons on the south-eastern wall. Section of CTX image P15_006724_2167. (e) Section of HiRISE image PSP_006724_2165 showing the distal reaches of a debris apron (bright, channelized material) overlying a flow lobe. (f) Section of HiRISE image PSP_006724_2165 showing the irregular and fractured surface of the flow lobe at HiRISE resolution.

3. Landform origin 3.1. Glacial origin? Could the flow lobes be the result of volatile-rich material having been deposited within the craters by atmospheric processes and then mobilized by glacial processes or mass-wasting? This hypothesis is supported by numerous features and characteristics of the flows, as well as the crater wall and floor materials, that are suggestive of glacial landscapes on Earth. These include troughs and lineations that mirror the local flow direction (e.g., Fig. 2b), steep-walled and curvilinear fronts (e.g., Figs. 2a and d, 4, and 6b), and ridges that are comparable in scale and morphology to terminal and lateral moraines (e.g., Figs. 3b and 4b–d). The size of the flows (Table 1) is also within a size range that is typical of valley and outlet glaciers (e.g., Drewery, 1986; Easterbrook, 1999; Nye, 1959). As an assemblage, the list of physical characteristics we have outlined is uniquely associated with glaciated environments on Earth (e.g., Fig. 6c). Analysis of the finer characteristics of the

flows reveals further analogical detail. For example, one HiRISE image shows sharp-crested, curvilinear ridges at the fronts of a series of flows (Fig. 4b–d). Intriguingly, these flows have secondary ridges that are located at regular intervals (<50 m) in the upslope direction (Fig. 4b). The morphology and regular spacing between the secondary ridges is analogous to observations of recessional moraines that form by pauses, or minor re-advances, during periods of glacial retreat on Earth (Easterbrook, 1999; Price, 1970). In another HiRISE image, the steep walls of two smaller flows are discernible (Fig. 6b) with debris piled in front of them. The fronts may represent well-preserved terminal moraines that are composed of material that was transported as the flows breached the arcuate ridges that sit beside them (highlighted in the sub-frames of Fig. 6b). Glacial accumulation is not possible in the latitudinal band in which these flows occur (35–43°N) under the currently cold and arid climatic paradigm (e.g., Head et al., 2006a,b; Kargel and Strom, 1992). This implies that relict ice from past glaciations will gradually degrade unless it is buffered from the sublimating effects of

90

G. Pearce et al. / Icarus 212 (2011) 86–95

Fig. 3. (a) Lobate floor material in a 23 km impact crater at 125°E, 36°N. The crater is substantially filled and the lobate material has flowed towards the north. (b) Close-up of tongue-shaped features near the center of the crater. (c) Area near the south-eastern wall where debris aprons have cross cut the lobate material, indicating that the former are more youthful than the latter. All images are from CTX P17_007554_2154. (d) Crater profile showing estimated level of filling. The solid line is from MOLA profile 15138. The level of filling was estimated from a power law for estimating initial crater depth from Garvin et al. (2003).

Fig. 4. (a) A variety of glacial-like features in a 39 km impact crater at 120°E, 40°N. Glacial flow appears to have occurred down the crater wall in all directions with the maximum advance occurring just before the rise of the central peak. Note the change of albedo between the flow material and the central peak. Composite of CTX images P03_002214_2207 and P22_009479_2206. (b) Several series of flow features with curvilinear ridges at their fronts. (c) Tongue-shaped lobes near the central peak of the crater. (d) Close-up of front of tongue-shaped lobes showing moraine-like ridges and polygonal patterned ground. All images in Fig. 4b–d are from HiRISE image PSP_010679_2205. (e) Section of CTX P22_009479_2206 showing bright, pitted material overlying gully alcoves. (f) Section of CTX P03_002214_2207 showing the mantle material overlying channel and debris apron material downslope from alcoves. The arrows indicate the contact between the mantling material and the crater wall.

the atmosphere. There is evidence of such degradation on flows in the largest craters (D P 30 km) where a high-albedo pitted surface texture (Fig. 4a, b, and e) occurs. The latter is thought to be a marker of volatile-rich layers undergoing sublimation (e.g., Milliken

et al., 2003; Mustard et al., 2001). Conversely, a patch of low albedo material that appears to be wind-transported sediment is located to the south-west of the central peak in the last example (Fig. 4a). This sediment overlies the fronts of several flows (e.g.,

G. Pearce et al. / Icarus 212 (2011) 86–95

91

a debris-type composition, with relict glacier ice potentially preserved beneath this surface layer. 3.2. Debris flow? Debris flows are flowing and saturated masses of clay-to-boulder-size material that are mixed with water, ice, or both water and ice (Easterbrook, 1999). Could the lobes be associated with, or the result of, debris flows? We suggest that a debris flow origin is unlikely based on the morphology of the lobes and of their host craters. Firstly, there are no features that are indicative of the displacement of debris that comes close to equalling the volume of material mobilized by the flow lobes. Secondly, while there are gullies upslope from the flow lobes in most of the craters, the alcoves of these gullies have volumes comparable to their debris aprons (e.g., Figs. 2d and 3c), and are much too small to be a likely source of material for the flow lobes. Based on these constraints, we suggest it is likely that the flowing material was supplied, at least primarily, by another source. 3.3. Rock glacier?

Fig. 5. (a) Section of CTX image P03_002175_2211 centered at 41°N, 99.5°E showing a crater with concentric fill. The crater has a d/D of 0.03 and is estimated to be filled with 1.15 km of filling material using the power law from Stewart and Valiant (2006) to estimate initial crater depth. (b) Section of MOLA point profile from orbit 12303 showing profile across crater.

transverse dunes at the front of the lobe in Fig. 4d). The aeolian features that are present on this dark patch are absent in areas that are marked by the high-albedo pitted material. This contrast in albedo between the flows and surrounding crater wall and floor material is not present in smaller craters with large single lobes (e.g., Figs. 2, 3 and 6). Instead, the flows in these smaller craters have an irregular surface texture that is crossed by steep-walled ridges and sharp fractures (e.g., Fig. 2f). We suggest that the morphology of these flows is consistent with the surface having

Rock glaciers share physical characteristics with ice glaciers – such as lobate and tongue-shaped forms and curvilinear ridges – even though their composition is primarily angular rock with interstitial ice (e.g., Barsch, 1996; Wahrhaftig and Cox, 1959). This raises questions as to whether some of the flows, particularly those that appear to have debris at their surface (e.g., Fig. 2a and d), are associated with the occurrence of a rock glacier. A high degree of equifinality is recognized in rock glaciers, with similar forms resulting from periglacial (i.e., formation of interstitial ice between angular fragments by freeze–thaw) and glacial (the covering of a glacial body by talus) processes (e.g., Barsch, 1996; Wahrhaftig and Cox, 1959). We suggest that a periglacial rock-glacier origin is unlikely for the same reason as a debris flow origin is unlikely: there are no features indicative of the displacement of rock fragments that come close to equalling the volume of material mobilized by the flow lobes. Concerning a buried-ice rock-glacier origin, there is no morphological evidence for crater wall talus having contributed heavily to the surface of the flows. A much more

Fig. 6. (a) Series of arcuate ridges in an 11.5 km impact crater at 105°E, 39°N (CTX P01_001357_2198). (b) Flow features that have overridden arcuate ridges from HiRISE PSP_001357_2200. (c) Glacier folds that form under constricted flow from an Alaskan glacier that resemble features in (b). Photo courtesy of Don McCully.

92

G. Pearce et al. / Icarus 212 (2011) 86–95

Fig. 7. (a) Section of CTX image P13_006210_2140 showing a 14 km impact crater at 102.6°E; 34.9°N. (b) Close-up of the CTX image shown in (a) highlighting arcuate ridges emplaced at section of eroded lobate floor material.

Fig. 8. Graph showing d/D by latitude and the latitudinal distribution of craters within our study area. The graph shows that the average d/D value is much lower above 43°N.

plausible explanation of flow morphology and origin is that the flows were covered primarily by the gradual accumulation of wind-blown dust and sediment. 3.4. Flow direction The flow direction of the lobes in the smaller craters is typically pole-ward (Table 1, Figs. 2 and 3). The only exception is a pair of relatively small flow lobes that appear to have flowed equatorward across their crater floor, overriding part of a series of arcuate ridges (Fig. 6). Although equatorward flow is unusual in crater bowls at this latitude (39.5°N) (Berman et al., 2005, 2009) this flow direction is supported by the resemblance of the southern portion of these lobes to glacial folds found in terrestrial environments (Fig. 6c) the latter form when glacial flow fronts become restricted. Curvilinear ridges at the front of these lobes are oriented concave to the south, which would be consistent with southwards flow and a glacial fold origin. Finally, frontal and lateral ramparts

around the lobes are thickest to the south of the arcuate ridge, which is consistent with the lobes displacing material southwardly from the arcuate ridges. The generally pole-ward orientation of the flow lobes is consistent with the findings of Berman et al. (2005, 2009), in that intra-crater glacial flow is primarily pole-ward below 44° latitude. All of the flow lobes in our survey occur below 44°N. By contrast, the largest craters exhibit lobes that seem to have flowed down the crater walls from a variety of orientations (Fig. 4e) and that often occur parallel to one another (Fig. 4b). The maximum margin of advance by the flows in these craters is distinguishable not only by the change in topography at the front of the lobes, but also by the transition from a bright, high-albedo surface to a lower albedo surface around crater central peaks (Fig. 4a). At first glance, the multi-directional flow lobes in these larger craters may appear to contradict recent research concluding that pole-ward intra-crater flow predominantly is pole-ward below 44° latitude (i.e. Berman et al., 2005, 2009; Head et al., 2008). We suggest that our findings are compatible with these

G. Pearce et al. / Icarus 212 (2011) 86–95

models, however, because the slopes of the walls of these larger craters are more gentle than those in the smaller craters discussed by Berman et al. (2005) and Head et al. (2008); thus orientation may not have the critical influence on insolation that it does in smaller, more steeply-walled craters (i.e., Fig. 2a and d).

4. Discussion 4.1. Late-Amazonian glacial complex We suggest that the flows we have identified are part of a large assemblage of potentially glaciated landscapes on Mars (including lineated valley fill, lobate debris aprons, and concentric crater fill). Crater size–frequency distribution analyses in past studies have led to the general consensus that these landscapes formed during the last few hundred Ma (e.g., Head et al., 2006a,b; Levy et al., 2009a,b). The driving force behind potential glacial activity is thought to be cyclic fluctuations in the obliquity of Mars’s spin axis and, by consequence, cyclic fluctuations in the planet’s insolation parameters (e.g., Laskar and Robutel, 1993; Laskar et al., 2004). Changes in insolation parameters initiate climate change and a recent Mars General Circulation Model (GCM) predicts that these fluctuations can cause the equatorward redistribution of polar volatiles (e.g., Madeleine et al., 2009). Given sufficient time, such volatile redistribution may result in the growth of icy bodies and the initiation of glacial flow in sub-polar regions. We have presented a glacial landform hypothesis that is based primarily on analogy with glacial landforms on Earth; the occurrence of some of these Earth-like features at different sites on Mars provides support for our interpretation. For example, the lineated surface (e.g., Fig. 2a) and curvilinear form that is present on the intra-crater flows we have identified are common to lineated valley fill and lobate debris aprons (Head et al., 2006a,b) on Mars as well as glaciers on Earth (Easterbrook, 1999). A complementary approach is to analyze features on these glacial-like landforms that do not have Earth-based analogues but that may also be indicative of the presence of glacial ice. Two features in particular have been discussed in the recent literature. First, small (< several hundred meters) impact craters whose shape resembles a truncated torus (i.e., ‘‘ring-mold craters’’) that are present on lineated valley fill, lobate debris aprons, and concentric

93

crater fill, but not in the terrain surrounding them (Kress and Head, 2008; Levy et al., 2010). On the basis of laboratory simulations of impacts formed in ice and the physics of impact cratering in layered material, Kress and Head (2008) hypothesize that these ‘‘ring-mold craters’’ form by impacts into nearly pure ice that is covered by a veneer of debris. Second, networks of coral-shaped terrain that are visible at HiRISE resolution on lineated valley fill, lobate debris aprons, and concentric crater (i.e., Dobrea et al., 2007; Levy et al., 2009a, 2010; Williams et al., 2008), but not on the terrain surrounding them. The morphology of coral-shaped terrain and its spatial association with glacial-like landforms led Levy et al. (2009a) to propose that it forms by the sublimation of glacial ice that is overlain by a veneer of debris (Levy et al., 2009a). Our investigation shows that ‘‘ring-mold craters’’ (e.g., Figs. 2a and 7b) and coral-shaped terrain (e.g., around the central peak of the crater shown in Fig. 2b) are present on some of the flows we have identified. Although hypotheses for the formation of ‘‘ring-mold craters’’ and coral-shaped terrain cannot be evaluated without access to analogous landforms, their spatial association with glaciallike landforms suggests that their origin may be associated with the presence of near-surface ice. 4.2. Glacial activity in Utopia Planitia Past studies have identified features of possible glacial flow origin within crater interiors in Utopia Planitia (e.g., Levy et al., 2009a; Pearce et al., 2010). The distribution of these flows had not previously been clarified, however. We show that glacial-like modifications are observed within large craters at all longitudes in our survey (85–125°E) and within a relatively narrow latitudinal band (35–44°N) (Fig. 1). The extent of potential glacial modifications has been inferred by the distribution of lobate flows (Table 1), concentric crater fill, and by the presence of near-wall arcuate ridges. Our findings suggest that, in Utopia Planitia, the distribution of concentric crater fill is more abundant than that of lobate flows, arcuate ridges, or other potential markers of glacial flow (i.e., clustering of lobate flows in the south-east, as shown in Fig. 1). 4.3. Crater filling The depth/diameter (d/D) ratio of impact craters decreases as they fill over time (Boyce et al., 2005) so that low d/D values are

Fig. 9. Graph showing d/D by longitude. The d/D values do not show a pronounced longitudinal trend.

94

G. Pearce et al. / Icarus 212 (2011) 86–95

a useful indicator of crater filling. We have plotted the d/D values of all craters (D P 5 km) within our survey zone (85–125°E, 30–60°N) by longitude and latitude in order to asses regional crater filling trends. Our results show that d/D values have a significantly lower average between 40–60°N (d/Dav = 0.03, n = 107) than they do between 30–40°N (d/Dav = 0.1, n = 84) (Fig. 8), but that the d/D values do not display a strong longitudinal trend (Fig. 9). Using regional power laws from Stewart and Valiant (2006) to estimate initial crater depth, and comparing these estimates with their present depth, our results are consistent with the great majority of craters north of 40° (103 out of 107) having been filled by at least 650 m of material. The latitudinal crater filling trend observed in Utopia Planitia may explain the absence of glacial-like lobes north of 43°N. The highly-filled craters that are ubiquitous north of 43° have smooth topography and short walls relative to less-filled craters of a similar diameter – two factors that would tend to inhibit glacial flow. It is interesting to note that concentric crater fill occurs in highlyfilled craters, but does not occur north of 43° within our study zone (Fig. 1). Recent work favours an ice-rich glacial flow origin of concentric crater fill (Head et al., 2006a; Levy et al., 2009a, 2010) and the strong latitudinal trend in crater filling begs the question whether the filling material in craters absent of concentric crater fill might be ice-rich as well. North of 52° within our study area, for example, is the high latitude ‘‘smoothed zone’’ described by Kreslavsky and Head (2002, 2006). Craters and plains alike in this zone are draped by material that is marked by concavity at the sub-kilometer scale (Fig. 10), and which could have been emplaced by the gradual accumulation of an ice-rich mantle during the last 450 Ma (Kreslavsky and Head, 2002, 2006). Against the backdrop of past research, our observations are consistent with the suggestion that the martian mid-latitudes in the northern hemisphere experienced a geologically recent ice age (e.g., Head et al., 2003, 2005), and that large craters in this area may have been subjected to widespread glacial flow (e.g., Berman et al., 2005, 2009; Head et al., 2008; Levy et al., 2010). In particular,

Utopia Planitia and the regions immediately to the west of our study area, such as Arabia Terra (e.g., Berman et al., 2009; Levy et al., 2010), and Deuteronilus and Nilo Syrtis Mensae (e.g., Head et al., 2006a,b; Levy et al., 2007) appear to represent a hub of recent northern mid-latitude glacial activity. 5. Summary We have shown that features of potential glacial flow origin are widespread within a latitudinal band (35–44°N, 85–125°E) in Utopia Planitia. These features include lobate flows, concentric crater fill, and arcuate ridges. The lobate flows are of particular interest as they display characteristics that are common to glacial systems on Earth. These characteristics include: a lobate form, curvilinear ridges and troughs on their surface, and sharp-crested ridges at their fronts that resemble terminal and recessional moraines found in valley glacial settings. The flows are also overlain by uniquely martian landforms that are thought to be representative of near-surface ground ice, including ‘‘ring-mold craters’’ (Kress and Head, 2008) and coral-shaped terrain (Levy et al., 2009a). While absolute dating of these features remains elusive, due to their very small surface area, a relative age indicates that these flows formed in the very recent geological past, with some examples (e.g., Fig. 4) post-dating the most recent episode of gully activity within their craters. The presence of concentric crater fill within many of the highlyfilled craters in our study zone, as well as the presence of glaciallike lobes in less-filled craters, raises the possibility that ice is the primary crater filling agent on a regional scale. In this case, the lobate flow features may represent a transitional morphology that is replaced by concentric crater fill and smoothened topography as filling levels increase (i.e., Kreslavsky and Head, 2006; Levy et al., 2010). Our findings suggest that Utopia Planitia experienced substantial late-Amazonian glacial activity and is supportive of recent studies suggesting that glacial accumulation was widespread in the northern mid-latitudes (e.g., Berman et al., 2009; Dickson et al., 2008; Head et al., 2006a,b; Levy et al., 2010). Acknowledgments G.R.O. is supported by an Natural Sciences and Engineering Research Council of Canada (NSERC) Industrial Research Chair sponsored by MDA Space Missions and the Canadian Space Agency (CSA). Funding from the NSERC Discovery Grant is gratefully acknowledged. This study represents a component of the M.Sc. thesis of GP. We thank the MOC, THEMIS, CTX and HiRISE instrument teams for making the outstanding imagery available and without which this research would not have been possible. CTX imagery courtesy of NASA/JPL/MSSS. HiRISE imagery courtesy of NASA/ JPL/University of Arizona. We are very appreciative for the insightful feedback provided by Joseph Levy and an anonymous reviewer. References

Fig. 10. (a) Section of CTX image P19_008399_2390 centered at 58.7°N, 87.3°E showing a crater in the high latitude ‘‘smoothed zone’’. The crater has a d/D value of 0.05 and is estimated to have 950 m of filling material. (b) Section of MOLA point profile from orbit 19213 showing profile across crater.

Arfstrom, J., Hartmann, W.K., 2005. Martian flow features, moraine-like ridges, and gullies: Terrestrial analogs and interrelationships. Icarus 174, 321–335. Barsch, D., 1996. Permafrost creep and rockglaciers. Permafrost Periglacial Process. 3, 175–188. Berman, D.C., Hartmann, W.K., Crown, D.A., Baker, V.R., 2005. The role of arcuate ridges and gullies in the degradation of craters in the Newton Basin region of Mars. Icarus 178 (2), 465–486. Berman, D.C., Crown, D.A., Bleamaster, L.F., 2009. Degradation of mid-latitude craters on Mars. Icarus 200, 77–95. Bierhaus, E.B., Chapman, C.R., Merline, W.J., 2005. Secondary craters on Europa and implications for cratered surfaces. Nature 437, 1125–1127. Boyce, J.M., Mouginis-Mark, P., Garbeil, H., 2005. Ancient oceans in the northern lowlands of Mars: Evidence from impact crater depth/diameter relationships. J. Geophys. Res. 110, E03008. doi:10.1029/2004JE002328.

G. Pearce et al. / Icarus 212 (2011) 86–95 Colaprete, A., Jakosky, B.M., 1998. Ice flow and rock glaciers on Mars. J. Geophys. Res. – Planets 103, 5897–5909. Dickson, J.L., Head, J.W., Kreslavsky, M.A., 2007. Martian gullies in the southern midlatitudes of Mars: Evidence for climate-controlled formation of young fluvial features based upon local and global topography. Icarus 188 (2), 315–323. Dickson, J.L., Head, J.W., Marchant, D.R., 2008. Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases. Geology 36, 411–414. Dobrea, E.Z.N., Asphaug, E., Grant, J.A., Kessler, M.A., Mellon, M.T., 2007. Patterned ground as an alternative explanation for the formation of brain coral textures in the mid latitudes of Mars: HiRISE observations of lineated valley fill textures. In: Proceedings of the 7th International Conference on Mars. Abstract #3358. Drewery, D.J., 1986. Glacial Geologic Processes. Edward Arnold, London. 276pp. Easterbrook, D.J., 1999. Surface Processes and Landforms, second ed. Prentice Hall Publishing House, 546pp. Fastook, J.L., Head, J.W., Marchant, D.R., 2005. Tropical mountain glaciers on Mars: Accumulation conditions, formation times, glacier dynamics, and implications for planetary spin-axis obliquity values. American Geophysical Union, Fall Meeting 2005. Abstract #P34A-03. Fastook, J.L., Head, J.W., Marchant, D.R., Forget, F., 2008. Tropical mountain glaciers on Mars: Altitude-dependence of ice accumulation, accumulation conditions, formation times, glacier dynamics, and implications for planetary spin-axis/ orbital history. Icarus 198, 305–317. Garvin, J.B., Sakamoto, S.E.H., Frawley, J.J., 2003. Craters on Mars: Global geometric properties from gridded MOLA topography. Paper presented at 6th International Conference on Mars, Calif. Inst. of Technol., Pasadena, Calif. Greeley, R. et al., 2004. Wind-related processes detected by the Spirit rover at Gusev crater, Mars. Science 305, 819–821. Hartmann, W.K., Neukum, G., 2001. Cratering chronology and the evolution of Mars. Space Sci. Rev. 96, 165–194. Hartmann, W.K., Thorsteinsson, T., Sigurdsson, F., 2003. Martian hillside gullies and icelandic analogs. Icarus 162, 259–277. Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent ice ages on Mars. Nature 426, 797–802. Head, J.W. et al., and HRSC Co-Investigator Team, 2005. Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars. Nature 434, 346–351. Head, J.W., Marchant, D.R., Agnew, M.C., Fassett, C.I., Kreslavsky, M.A., 2006a. Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for late Amazonian obliquity-driven climate change. Earth Planet. Sci. Lett. 241, 663–671. Head, J.W., Nahm, A.L., Marchant, D.R., Neukum, G., 2006b. Modification of the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation. Geophys. Res. Lett. 33 (8). doi:10.1029/2005GL024360. Head, J.W., Marchant, D.R., Kreslavsky, M.A., 2008. Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin. Proc. Natl. Acad. Sci. USA 105, 13258–13263. Hecht, M.H., 2002. Metastability of water on Mars. Icarus 156, 373–386. Holt, J.W. et al., 2008. Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars. Science 322, 1235–1238. Howard, A.D., 2003. Tongue ridges and rumpled crater floors in midsouthernlatitude martian craters. Lunar Planet. Sci. XXXIV. Abstract #1065. Kargel, J.S., Strom, R.G., 1992. Ancient glaciation on Mars. Geology 20, 3–7. Kreslavsky, M.A., Head, J.W., 2002. Mars: Nature and evolution of the surface of young latitude-dependent water-ice-rich mantle. Geophys. Res. Lett. 29 (15), 1719. doi:10.1029/2002GL015392. Kreslavsky, M.A., Head, J.W., 2006. Modification of impact craters in the northern plains of Mars: Implications for Amazonian climate history. Meteorit. Planet. Sci. 41, 1633–1646. Kress, M.A., Head, J.W., 2008. Ring-mold craters in lineated valley fill and lobate debris aprons on Mars: Evidence for subsurface glacial ice. Geophys. Res. Lett. 35. doi:10.1029/2008GL035501. Laskar, J., Robutel, P., 1993. The chaotic obliquity of the planets. Nature 362, 608– 612. Laskar, J., Gastineau, M., Joutel, F., Robutel, P., Levrard, B., Correia, A., 2004. Longterm evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170, 343–364. Levy, J.S., Head, J.W., Marchant, D.R., 2007. Lineated valley fill and lobate debris apron stratigraphy in Nilosyrtis Mensae, Mars: Evidence for phases of glacial modification of the dichotomy boundary. J. Geophys. Res. 112. doi:10.1029/ 2006JE002852.

95

Levy, J.S., Head, J.W., Marchant, D.R., 2009a. Concentric crater fill in Utopia Planitia: History and interaction between glacial ‘‘brain terrain’’ and periglacial mantle processes. Icarus 4. doi:10.1016/j.icarus.2009.02.018. Levy, J., Head, J.W., Marchant, D.R., 2009b. Thermal contraction crack polygons on Mars: Classification, distribution, and climate implications from HiRISE observations. J. Geophys. Res. – Planets 114, E01007. doi:10.1029/ 2008JE003273. Levy, J., Head, J.W., Marchant, D.R., 2010. Concentric crater fill in the northern midlatitudes of Mars: Formation processes and relationships to similar landforms of glacial origin. Icarus 209, 390–404. Lucchitta, B., 1984. Ice and debris in the fretted terrain, Mars. J. Geophys. Res. 89, B409–B418. Madeleine, J.B., Forget, F., Head, J.W., Levrard, B., Montmessin, F., Millour, F., 2009. Amazonian mid-latitude glaciation on Mars. Icarus 203, 390–405. Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2334. Marchant, D.R., Head, J.W., 2003. Tongue-shaped lobes on Mars: Morphology, nomenclature, and relation to rock glacier deposits. In: 6th International Conference on Mars. Abstract #3091. McEwen, A.S., Bierhaus, E.B., 2006. The important of secondary cratering to age constraints on planetary surfaces. Annu. Rev. Earth Planet. Sci. 34, 535–567. McEwen, A.S., Preblich, B.S., Turtle, E.P., Artemieva, N.A., Golombek, M.P., Hurst, M., Randolph, L.K., Burr, D.M., Christensen, P.R., 2005. The rayed crater Zunil and interpretations of small impact craters on Mars. Icarus 176, 351–381. Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images. J. Geophys. Res. 108 (E6), 5057. doi:10.1029/2002JE002005. Mustard, J.F., Cooper, C.D., Rifkin, M.K., 2001. Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice. Nature 412, 411–414. Nye, J.F., 1959. The motion of ice-sheets and glaciers. J. Glaciol. 3, 493–507. Pearce, G., Osinski, G.R., Soare, R.J., 2010. Intra-crater glacial flow and ice mantling in Adamas Labyrinthus, northern plains of Mars. Lunar Planet. Sci. 41. Abstract #2490. Plaut, J., Safaeinili, A., Holt, J.W., Phillips, R., Head, J.W., Seu, R., Putzig, N., Frigeri, A., 2009. Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars. Geophys. Res. Lett. 36, L02203. Preblich, B., McEwen, A., Studer, D., 2005. Mapping rays and secondary craters from Zunil, Mars. Lunar Planet. Sci. XXXVI. Abstract #2112. Price, R.J., 1970. Moraines at Fjallsjokull, Iceland. Arctic Alpine Res. 2, 27–42. Russell, P.S. et al., 2004. Evolution of ice deposits in the local environment of martian circumpolar craters and implications for polar cap history. Lunar Planet. Sci. 35, League City, TX. Abstract #2007. Safaeinili, A., Holt, J., Plaut, J., Posiolova, L., Phillips, R., Head, J.W., Seu, R., 2009. New radar evidence for glaciers in Mars Phlegra Montes region. Lunar Planet. Sci. XXXX. Abstract #1988. Soare, R.J., Osinski, G.R., 2009. Stratigraphical evidence of late Amazonian periglaciation and glaciation in the Astapus Colles region of Mars. Icarus 202, 17–21. Squyres, S.W., 1978. Martian fretted terrain: Flow of erosional debris. Icarus 34, 600–613. Squyres, S.W., 1979. Distribution of lobate debris aprons and similar flows on Mars. J. Geophys. Res. 84, 8087–8096. Squyres, S.W., 1989. Urey prize lecture – Water on Mars. Icarus 79, 229–288. Squyres, S.W., Carr, M.H., 1986. Geomorphic evidence for the distribution of ground ice on Mars. Science 231, 249–252. Stewart, S.T., Valiant, G.J., 2006. Martian subsurface properties and crater formation processes inferred from fresh impact crater geometries. Meteorit. Planet. Sci. 41, 1509–1537. Tanaka, K.L., Skinner, A.J., Hare, T.M., 2005. Geologic map of the northern plains of Mars. US Geological Survey, Scientific Investigations Map #288. Wahrhaftig, C., Cox, A., 1959. Rock glaciers in the Alaska Range. Geol. Soc. Am. Bull. 70, 383–436. Williams, K.E., Toon, O.B., Heldmann, J.L., McKay, C., Mellon, M.T., 2008. Stability of mid-latitude snowpacks on Mars. Icarus 196, 565–577. Zimbelman, J.R., Clifford, S.M., Williams, S.H., 1989. Concentric crater fill on Mars: An aeolian alternative to ice-rich mass wasting. Proc. Lunar Planet. Sci. Conf. 19, 397–407.