Formation of double-layered ejecta craters on Mars: A glacialsubstrate model

David K. Weiss1 and James W. Head1

Received 27 June 2013; revised 15 July 2013; accepted 19 July 2013; published 7 August 2013.

[1] A class ofMartian impact craters with particularly unusualejecta characteristics (double-layered ejecta (DLE) craters) arepreferentially located in the midlatitudes in both hemispheresof Mars. Unlike today, decameters thick deposits of snow andice occupied these same latitudes for significant periodsduring the Amazonian period. We assess the hypothesis thatthe unusual double-layer morphology could be related toimpact into a snow and ice glacial substrate followed bylandsliding of ejecta off of the structurally uplifted rim. Wefind that many characteristics of DLE craters (e.g., latitudinaldistribution, lack of secondaries, landslide-like textures,evidence for overthrusting, relation to other crater types, etc.)are consistent with such an origin. Citation: Weiss, D. K., andJ. W. Head (2013), Formation of double-layered ejecta craters onMars: A glacial substrate model, Geophys. Res. Lett., 40,3819–3824, doi:10.1002/grl.50778.

1. Introduction

[2] Martian impact craters often possess unusual ejectacharacteristics relative to ballistically dominated lunar andMercurian impact crater ejecta: lobate ejecta deposits appearto have been fluidized during their emplacement, althoughthe mode of fluidization is debated [Carr et al., 1977; Gaultand Greeley, 1978; Mouginis-Mark, 1979, 1981; Woronow,1981; Boyce and Mouginis-Mark, 2006].[3] Particularly enigmatic amongst the fluidized ejecta

craters are double-layered ejecta (DLE) craters (Figures 1aand 1b), located in the mid-high latitudes of Mars in bothhemispheres (Figure 1e) [Boyce and Mouginis-Mark, 2006]and characterized by two ejecta facies (Figure 1a): the innerfacies possesses a distinctive radial texture of parallel ridgesand grooves. The major unusual features of DLE cratersinclude the following (Figure 1): (1) a regional, midlatitude-dependent distribution [Boyce and Mouginis-Mark, 2006];(2) a paucity of secondary craters [Boyce and Mouginis-Mark, 2006]; (3) a characteristic radial grooved texture[Boyce and Mouginis-Mark, 2006] and megablock distribution(see supporting information) [Wulf et al., 2013]; (4) apparentsuperposition of the inner ejecta facies over the outer [Carret al., 1977; Barlow and Perez, 2003; Osinski et al., 2011;

Wulf et al., 2012]; (5) an unusual annular depression at the baseof the rim structural uplift and enhanced positive annular topog-raphy at the distal edge of the inner facies [Boyce andMouginis-Mark, 2006]; (6) an association with other distinctivecrater types (pedestal (Pd) craters, excess ejecta (EE) craters[Kadish and Head, 2011]; Figure 1f), and (7) a sharp rim crest.[4] Many of these unusual characteristics have been

interpreted to be related to several factors: (1) the presenceof the Martian atmosphere [Schultz and Gault, 1979;Schultz, 1992], (2) the presence of volatiles within theregolith target materials [Mouginis-Mark, 1981; Barlow andBradley, 1990; Costard, 1994; Barlow and Perez, 2003;Barlow, 2005; Osinski, 2006; Boyce and Mouginis-Mark,2006], (3) some combination of these factors [Barlowet al., 2005; Boyce and Mouginis-Mark, 2006, Komatsuet al., 2007], (4) a base surge [Boyce and Mouginis-Mark,2006], (5) impact melt overtopping the crater rim [Osinski,2006; Osinski et al., 2011], or (6) impact into a subsurfaceice layer [Senft and Stewart, 2008]. Prior models, however,have been unsuccessful in reconciling their predictions withobservations (see supporting information).[5] Noting the latitudinal distribution (Figure 1e), we

explore a new hypothesis, the glacial substrate model, whichattributes the unusual characteristics of DLE craters(Figure 1b) to the presence of decameters thick surface snowand ice (glacial deposits) that existed periodically in mid-high latitude regions due to spin-axis/orbital variations[Laskar et al., 2004]. Recent work (synthesized in Headand Marchant [2009]) shows that nonpolar regions haveoften been covered with decameters of snow and ice [Headet al., 2005; Meresse et al., 2006; Head et al., 2006;Kadish et al., 2009; Pedersen et al., 2010; Dickson et al.,2008; Kress and Head, 2008]. Could the presence of a glacialsubstrate, such as these regional ice layers, in earlier Martianhistory have been a factor in producing the unusual morphol-ogy of DLE craters?

2. Testing the Hypothesis

[6] We examine each of the seven unusual characteristicsof DLE craters enumerated above (Figure 1b) and test theseagainst the glacial substrate model to assess its plausibilityand usefulness for further analysis.

2.1. Latitude Dependence

[7] DLE craters are primarily located in the mid-highlatitudes of Mars (Figure 1e) [Boyce and Mouginis-Mark,2006], and new mapping [Barlow and Boyce, 2013] hasshown that DLE craters are present in the latitudinal bands25°N–80° N and 30°S–70°S. A wide variety of studies ofdifferent landforms, summarized in Head and Marchant[2009], have established the presence of nonpolar surfacesnow and ice deposits during extended periods of the

Additional supporting information may be found in the online version ofthis article.

1Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

Corresponding author: D. K. Weiss, Department of Geological Sciences,Brown University, 324 Brook Street, Box 1846, Providence, RI 02912,U.S.A. (David_Weiss@brown.edu)

©2013. American Geophysical Union. All Rights Reserved.0094-8276/13/10.1002/grl.50778

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GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 3819–3824, doi:10.1002/grl.50778, 2013

Amazonian in these latitudinal bands (Figure 1e). Theselandforms include latitude-dependent mantles [Mustardet al., 2001; Head et al., 2003], Pd craters [Kadish et al.,2009], EE craters [Black and Stewart, 2008], lobate debrisaprons, lineated valley fill, and plateau ice deposits [Headet al., 2010]. We hypothesize that the icy substrate layerbeneath Pd and EE craters, often at least 50 m thick, may alsobe related to DLE craters.

2.2. Paucity of Secondary Craters

[8] DLE craters exhibit a scarcity of evident secondarycraters [Boyce and Mouginis-Mark, 2006]. This has beenexplained by the fragmentation of ejecta blocks due to (1)atmospheric turbulence [Schultz, 1992], (2) volatiles withinthe regolith ejecta blocks and target material [Boyce andMouginis-Mark, 2006], (3) crushing of ejecta blocks by

dynamic pressure in high-velocity outflow of gas duringejection [Boyce and Mouginis-Mark, 2006], in addition to(4) burial of secondary craters by ejecta flow [Mouginis-Mark, 1979;Osinski, 2006], and (5) primarily near-rim ejectadeposition [Senft and Stewart, 2008]. Alternatively, in the gla-cial substrate model, if secondary craters were emplaced whendecameters of ice were present, the small size and relativelyshallow depths of the secondary craters would be likely to pre-vent their preservation subsequent to the sublimation loss ofthe glacial substrate during a later climate regime.

2.3. Radial Textures

[9] DLE craters exhibit radial striations that extend fromthe crater rim to the outer edge of the inner ejecta deposit.These striations have been suggested to result from radialscouring of wind vortices by the advancing ejecta curtain

Figure 1. Characteristics and distribution of double-layer ejecta (DLE) craters on Mars. (a) Morphology and topography ofa typical DLE crater (34.7°N, 120.5°E; CTX image (P20_008833_2149, overlain on MOLA gridded topography (~463 m/pixel)). (b) MOLA altimetric profile (NW to SE) of the DLE crater (see red line in Figure 1a) derived from MOLA griddedtopography. Illustrated are the main distinctive features of DLE craters. (c) Comparison between the Sherman landslide(photo by Austin Post, U.S.G.S, 1965) and (d) the inner ejecta scarp of a DLE crater (blue box in Figure 1a). Troughs (whitearrows) crosscut the radial striations in both cases. (e) Global distribution of DLE craters [Barlow, 1988] (http://webgis.wr.usgs.gov/pigwad/down/mars_crater_consortium.htm) and Pd craters equatorward of ~60°N and 65°S [Kadish et al., 2009];Pd craters are less common at higher latitudes [Kadish et al., 2009]. Red diamond corresponds to the location of Figure 1a.(f) Crater diameter and icy substrate thickness relationship for a variety of impact crater types. Approximate average valuesfor each crater type populations are plotted, while the horizontal and vertical lines correspond to the range of values.

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[Schultz and Gault, 1979; Mouginis-Mark, 1981, 1987;Schultz, 1992], or a base surge [Boyce and Mouginis-Mark,2006]. Radial striations, however, are also common in terres-trial and Martian landslide material (Figure 1c) [Shreve,1966; Lucchitta, 1978]. Orthogonal troughs, similar to thosecommonly found in terrestrial landslides [Shreve, 1966],crosscut the radial grooves in the inner ejecta facies of manyDLE craters (Figure 1d). In landslides, lateral stresses fromadjacent “drivetrains” cause pervasive splitting as the land-slide spreads out, forming the grooves [Shreve, 1966; DeBlasio, 2011b]. Lucchitta [1978] and De Blasio [2011b]suggested that the presence of grooves is enhanced by lowcoefficients of friction. Further, lack of confinement of alandslide seems to correlate with the presence of longitudinalgrooves [Lucchitta, 1978; 1979]. Thus, the radial nature (lackof confinement) and icy substrate of a DLE crater inner layerlandslide is consistent with the presence of longitudinalgrooves on the landslide surface. The larger scale of theMartian grooves (~100 m across; Figure 1c) is consistentwith the larger sizes of the landforms themselves (Shermanlandslide runout ~5 km [Shreve, 1966], Martian landsliderunout ~7–20 km). The large Martian landslides in VallesMarinaris, Capris Chasma, and Ganges Chasma [Lucchitta,1979; De Blasio, 2011a], which have anomalously largerunout distances compared with terrestrial landslides, alsodisplay grooves of identical dimensions to those found onthe inner layer of DLE craters. We note that lower frictionat the base of an ice-rock landslide enhances slippage/splitting of landslide material [De Blasio, 2011b]. In ourscenario, we interpret the radial striations to be due tolandsliding of the outer rim material, enhanced by craterrim structural uplift and an underlying snow and ice layer.The larger dimensions of the landslide, radial nature of thelandslide surface (crater rim), and the lubricating basal ice/snow layer enhance splitting to generate wider grooves thantheir terrestrial counterparts.

2.4. Overthrust

[10] The inner ejecta facies has been shown to overlie theouter ejecta facies [Carr et al., 1977; Barlow and Perez,2003; Osinski et al., 2011; Wulf et al., 2012], and radialoutward flow is interpreted to have led to an overthrust ofthe outer facies by the inner facies [Wulf et al., 2012]. Wesuggest that the overthrust event was a landslide of outerrim material promoted by structural uplift and the ice-lubricated glacial substrate.

2.5. Annular Depression

[11] Annular depressions are observed at the base of theuplifted rim crests of DLE craters, contrary to the usual ten-dency of ejecta to thicken toward the rim crest. The floor ofDLE annuli depressions lie at an elevation very close to thatof the outer ejecta layer (Figure 1b). The annuli around DLEcraters have been explained through scouring [Schultz, 1992;Boyce andMouginis-Mark, 2006] or impact into a subsurfaceice sheet [Senft and Stewart, 2008]. Alternatively, if uplift-induced landsliding occurred due to the lubrication of aglacial substrate, one might predict a topographic low at thebase of the structural uplift in the proximal part of the land-slide area corresponding to the edge of the area interpretedto be the origin of the landslide. The location of this zonecan be explained by the termination of the structural uplift,

extending out as far as ~1.5 R from the rim [Settle andHead, 1979].

2.6. Relation to Other Crater Types

[12] DLE craters are linked by their areal distribution andtheir glacial substrate [Kadish and Head, 2011] to othercrater types, Pd craters [Kadish et al., 2009], EE craters[Black and Stewart, 2008], and perched (Pr) craters [Meresseet al., 2006; Kadish and Head, 2011] which have latitudinaldistributions similar to that of DLE craters. Kadish and Head[2011] proposed that Pd/EE/Pr craters (Figure 1f) share thecommon theme of formation on a glacial snow and ice layer.The classification of EE craters shows that the majority of EEcraters have DLE morphology [Kadish and Head, 2011].[13] Excess ejecta (EE) craters as identified by Black and

Stewart [2008] are craters with volumes of material abovethe pre-impact surface (Vabove) larger than the crater cavityvolumes (Vcavity) by factors of 2.5 to 28.5 [Kadish andHead, 2011]. In the proposed EE crater formation, an icy sur-face layer is present at the time of impact [Black and Stewart2008]. The impact excavates material below the icy surfacelayer, and the resultant ejecta deposit covers the icy layerproximal to the crater cavity. Black and Stewart [2008]propose that the ejecta deposit will insulate the underlyingicy layer proximal to the crater cavity and slow sublimation.Climate change causes the regional icy layer to sublime, andthe terrain outside the crater ejecta deposit will disappear,leaving an anomalously large volume: that of the persistentunderlying icy layer protected by the ejecta deposit.[14] Kadish and Head [2011] observe that there may exist

EE craters too large to detect: the volume of “excess” ejectabecomes proportionately smaller as the crater diameter (andthus ejecta volume) increases, making the positive identifica-tion of an EE crater more difficult at larger diameters (>25km). Since most EE craters are also DLE craters and sincelarge EE craters would be difficult to recognize as EE craters,larger DLE craters may also be EE craters that formedthrough impact into an icy surface layer.[15] On the basis of the substrate thickness/crater diameter

relationships for these craters (Figure 1f), it is clear that DLEcraters exhibit a larger average crater diameter and larger up-per limit in crater diameter than EE craters. This is consistentwith formation in a glacial substrate: the larger DLE cratersare not classified as EE craters because their Vabove/Vcavity

signal is drowned out by their larger ejecta volumes.

2.7. Rim Crest

[16] The outer rim crests of DLE craters appear to be steep[Boyce and Mouginis-Mark, 2006]. In concert with the otherlines of evidence, this is consistent with the sliding of ejectaoff of the structurally uplifted inner rim facilitated by a glacialsubstrate, resulting in the production of the inner facies.

3. Toward a Quantitative Glacial Substrate Model

[17] If the morphological characteristics of DLE craterscan be explained through a landslide mode of ejecta off anunderlying glacial substrate, does the angle of the structuraluplift play a role in governing ejecta stability? An analysisof the force balance of ejecta sliding on a glacial substrateat the proximal edge of the structurally uplifted rim crestconstrains the structural uplift angles of DLE and EE craterswhich would facilitate a landslide of the near-rim ejecta.

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[18] If the ejecta is considered to be a distinct unit overly-ing the pre-impact substrate, the balance of forces acting onthe ejecta can be divided into (1) normal force upwards intothe ejecta, (2) gravitational force acting downwards parallelto the sliding plane, and (3) frictional force (governed bythe static coefficient of friction, μ) between the ejecta andthe substrate. Thus, the outer rim crest angle (θ) at whichejecta overlying a glacial substrate becomes unstable can berepresented by θ = tan�1(μ). Terrestrial landslides on glaciersgenerally have dynamic μ values ranging from 0.03–0.1[Sosio et al., 2008]. While terrestrial landslides occur muchcloser to the melting point of ice than surface temperaturesof Mars in the Amazonian, the high temperature environmentof the area proximal to the crater following the impact [e.g.,Wrobel et al., 2006] will serve to decrease the static μ byproducing melt which may trigger the landslide, a processwhich would be enhanced by deposition of even a moder-ately heated ejecta facies. Alternatively, if the landslideoccurred in the latter stages of crater formation, the initialhorizontal velocity of ejecta at the time of deposition maybe enough to overcome the friction, facilitating the landslide,and so we apply dynamic terrestrial coefficients of friction topost-impact landslides on Mars.[19] The force balance has been calculated for a 1 kg mass

of ejecta overlying a glacial substrate to determine what rimcrest angles produce an ejecta instability (i.e., the angle atwhich net force on the ejecta is> 0; Figure 2b). We find thatejecta overlying a glacial substrate (μ = 0.1) is unstable at cra-ter rim structural uplift angles that exceed ~5.7° (Figure 2b).[20] Comparing our calculation results with an approxima-

tion for structural uplift [Stewart and Valiant 2006] as a func-tion of crater diameter and crater radii (R) from the rim crest(Figure 2a), we find that the ejecta instability angle (~5.7°) isreached on the structural uplift of craters 0.1 R to 0.5 R fromthe rim crest up to a crater diameter of ~25 km. This suggeststhat beyond ~25 km, the structural uplift angle values forcraters are too low for ejecta overlying a glacial substrate tobe unstable, predicting that craters >25 km in diametershould not exhibit DLE morphology. The crater diameter-frequency distribution for DLE craters (Figure 2c) shows that

DLE craters do indeed fall off drastically> ~25 km,suggesting that EE craters beyond ~25 km do not haveDLE morphology because their low structural uplift anglesprevent landsliding of the ejecta proximal to the rim crest.DLE craters are generally not present in the ~1 km diameterrange (Figure 2c) because their excavation depths are tooshallow to excavate beneath the glacial substrate. Becausethe value of μ is uncertain, we acknowledge the possibilitythat the ejecta instability angle may be even higher, thuspredicting DLE morphology to fall off at smaller craterdiameters: Figure 2b shows that the diameter-frequency ofDLE craters may begin falling off after ~15 km.

4. Conclusions

[21] The proposed glacial substrate model (Figure 3) forDLE crater formation is interpreted to have involved thefollowing steps: (1) Deposition of a snow and ice layer,producing a glacial substrate (Figure 3a). (2) Impact of aprojectile, penetrating down into the regolith below the glacialice, causing excavation, rim structural uplift, and ejectaemplacement (Figure 3b); secondary craters form primarilyin the glacial substrate. (3) In the latter stages of formationof the ejecta facies, the inner facies is emplaced in a landslidemode, enhanced by the glacial substrate-lubricated, structur-ally uplifted rim (Figure 3c). This produces the sharp craterrim crest and the distinctive annular topographic low. (4)Following this emplacement, global climate change leads toremoval of the surrounding icy substrate, destroying evidenceof superposed secondary craters (Figure 3d).[22] We conclude that numerous characteristics of Martian

DLE craters can be plausibly explained by the presence of aglacial substrate (snow and ice layer) present during theimpact events. Further analysis and testing of this modelcan be undertaken by (1) dating of individual DLE cratersand comparison of these ages with the chronology of icedeposits [e.g., Head and Marchant, 2009] and (2) compari-son of the diameter distribution of EE craters with SLEmorphology to the predicted size range.

Figure 2. (a) Structural uplift relationship approximated using the rim height and structural uplift functions adapted fromStewart and Valiant [2006] and Craddock et al. [1997]. (b) Force balance calculation for a 1 kg mass of ejecta under twoconditions: overlying a glacial substrate (blue line) and overlying a regolith substrate (black line). For μ =0.1, a structural upliftangle of ~5.7° will cause ejecta overlying a glacial substrate to be unstable (net force> 0). (c) Crater diameter-frequencydistribution: DLE craters fall off beyond ~25 km.

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[23] Acknowledgments. We gratefully acknowledge support from theNASA Mars Data Analysis Program NNX09A146G to JWH.[24] The Editor thanks an anonymous reviewer for his/her assistance in

evaluating this paper.

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WEISS AND HEAD: A GLACIAL SUBSTRATE MODEL

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