Icarus 239 (2014) 186–200

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Martian Low-Aspect-Ratio Layered Ejecta (LARLE) craters: Distribution,characteristics, and relationship to pedestal craters

http://dx.doi.org/10.1016/j.icarus.2014.05.0370019-1035/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Fax: +1 928 523 1371.E-mail addresses: Nadine.Barlow@nau.edu (N.G. Barlow), jboyce@higp.hawaii.

edu (J.M. Boyce), carinc@uw.edu (C. Cornwall).

Nadine G. Barlow a,⇑, Joseph M. Boyce b, Carin Cornwall c

a Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011-6010, USAb Hawai’i Institute for Geophysics and Planetology, University of Hawai’i, Honolulu, HI 96822, USAc Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310, USA

a r t i c l e i n f o

Article history:Received 4 October 2013Revised 19 May 2014Accepted 25 May 2014Available online 14 June 2014

Keywords:Mars, surfaceGeological processesCrateringImpact processes

a b s t r a c t

Low-Aspect-Ratio Layered Ejecta (LARLE) craters are a unique landform found on Mars. LARLE craters arecharacterized by a crater and normal layered ejecta pattern surrounded by an extensive but thin outerdeposit which terminates in a sinuous, almost flame-like morphology. We have conducted a survey toidentify all LARLE craters P1-km-diameter within the ±75� latitude zone and to determine their morpho-logic and morphometric characteristics. The survey reveals 140 LARLE craters, with the majority (91%)located poleward of 40�S and 35�N and all occurring within thick mantles of fine-grained deposits whichare likely ice-rich. LARLE craters range in diameter from the cut-off limit of 1 km up to 12.2 km, with 83%being smaller than 5 km. The radius of the outer LARLE deposit displays a linear trend with the craterradius and is greatest at higher polar latitudes. The LARLE deposit ranges in length between 2.56 and14.81 crater radii in average extent, with maximum length extending up to 21.4 crater radii. The LARLElayer is very sinuous, with lobateness values ranging between 1.45 and 4.35. LARLE craters display anumber of characteristics in common with pedestal craters and we propose that pedestal craters areeroded versions of LARLE craters. The distribution and characteristics of the LARLE craters lead us to pro-pose that impact excavation into ice-rich fine-grained deposits produces a dusty base surge cloud (likethose produced by explosion craters) that deposits dust and ice particles to create the LARLE layers. Saltsemplaced by upward migration of water through the LARLE deposit produce a surficial duricrust layerwhich protects the deposit from immediate removal by eolian processes.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

Fresh martian impact craters are typically surrounded by singlelayer ejecta (SLE), double layer ejecta (DLE), or multiple layer ejecta(MLE) deposits (Barlow et al., 2000). Here we report on a uniqueejecta morphology where martian craters with these morphologiesalso include a thin (typically <10-m-thick) outer extensive deposit(Fig. 1). This outer deposit extends from six to over 21 crater radiifrom the crater, compared to normal layered ejecta blankets whichusually are less than three crater radii in extent. We find thattypical thicknesses of these outer deposits are <10 m.Consequently, the aspect ratio (the ratio of the thickness to thelength of a deposit) of these deposits is very low, with values onthe order of �10�5. These aspect ratio values are comparable to

those of some volcanic ignimbrite surge deposits, which have beentermed Low-Aspect-Ratio Ignimbrite (LARI) deposits. Following thenomenclature of these volcanic surges, we have named thisunusual crater class the Low-Aspect-Ratio Layered Ejecta (LARLE)craters. The extensive outer deposit is henceforth called the LARLElayer to distinguish it from the normal ejecta deposit immediatelysurrounding the crater.

We have conducted a survey to identify all LARLE craters P1-km-diameter in the ±75� latitude zone on Mars. In order to beclassified as a LARLE crater, the crater must meet the followingmorphologic criteria:

� The outer LARLE layer is very thin, with a thickness less thanapproximately 10 m.� The LARLE layer terminates in a sinuous ‘‘flame-like’’ edge.� The LARLE layer has a maximum extent of at least 6.0 crater

radii.

Fig. 1. Example LARLE craters. (a) Daytime IR THEMIS mosaic of 10.8-km-diameter Lonar crater, centered at 72.99�N 38.30�E. (b) Daytime IR THEMIS mosaic of unnamed 5.5-km-diameter LARLE crater centered at 68.27�N 266.36�E.

N.G. Barlow et al. / Icarus 239 (2014) 186–200 187

� The LARLE layer cannot be classified as a normal layered, pedes-tal, or radial morphology based on its morphology (Barlow et al.,2000). The thickness, and thus aspect ratio, of the depositsallows us to distinguish between a LARLE layer and a normalejecta deposit.

The resulting LARLE crater database (Table 1) includes the fol-lowing information for each crater:

� Latitude and (east) longitude of the crater center.� Diameter of the crater.� Geologic unit on which crater occurs, obtained from the USGS

1:15,000,000-scale geologic maps of the planet (Scott andTanaka, 1986; Tanaka and Scott, 1986; Greeley and Guest,1987) and the northern plains (Tanaka et al., 2005).� Perimeter of outer edge of the LARLE layer.� Area covered by the LARLE layer.� Average ejecta mobility ratio (EM), which is the ratio of the

deposit length measured from crater rim to crater radius, ofthe LARLE layer (Section 2.5).� Maximum EM of the LARLE layer, measured from the crater rim

to the furthest extent of the LARLE deposit (Section 2.5).� Lobateness (a measure of sinuosity) of the LARLE deposit

(Section 2.5).� Type of normal ejecta deposit (SLE, DLE, MLE, etc.) associated

with the crater exclusive of the LARLE layer.

Up to five example visible and/or daytime infrared THEMISimages covering the LARLE crater and comments about the crateralso are included in the database, although these are not includedin Table 1.

In this paper, we report on the results of our survey and thecharacteristics of the LARLE craters. We compare the characteris-tics of LARLE craters and pedestal craters and suggest that thetwo crater types are linked through an evolutionary process. Theresults reported here are utilized in a companion paper describingour proposed base surge formation mechanism for emplacement ofthe LARLE deposit (Boyce et al., submitted for publication).

2. Distribution and characteristics of martian LARLE craters

2.1. Distribution

We utilized Mars Odyssey Thermal Emission Imaging System(THEMIS) visible (VIS; 18 m/pixel resolution) and daytime infrared(IR; 100 m/pixel resolution) imagery (Christensen et al., 2004) toidentify all LARLE craters P1-km-diameter in the ±75� latitudezone. Higher latitudes were not included in the survey due tointerference from the martian polar caps and generally poorerimage resolution associated with atmospheric conditions. MarsReconnaissance Orbiter Context Camera (CTX; 6 m/pixel resolution(Malin et al., 2007)) and High Resolution Imaging Science Experi-ment (HiRISE; 0.3 m/pixel resolution (McEwen et al., 2007))images were used to investigate selected LARLE craters in moredetail. We have identified 140 LARLE craters within the studyregion.

Fig. 2 shows the distribution of the LARLE craters identified inthis study. Three regions of LARLE concentrations are apparent inthis map: Group 1 craters are concentrated in the high northernlatitudes between 35�N and 75�N, Group 2 craters are the southernhemisphere counterpart in the 40–75�S zone, and Group 3 cratersare equatorial LARLE craters distributed between 3�N and 11�S.Group 1 contains 89 craters, which constitutes 64% of all LARLEcraters in this study. There are 39 LARLE craters (28%) in Group 2and 12 (8%) in Group 3. The three groups of LARLE craters showa strong correlation with the distribution of regions of current highH2O concentrations as revealed by the Mars Odyssey NeutronSpectrometer data analysis (Feldman et al., 2004).

Table 2 shows the frequency of LARLE craters both in rawcounts and per unit area as a function of geologic unit (Scott andTanaka, 1986; Tanaka and Scott, 1986; Greeley and Guest, 1987;Tanaka et al., 2005). LARLE craters occur on geologic units rangingin age from Amazonian to Noachian, although most (62%) arefound on Amazonian-aged units. Group 1 LARLE craters show astrong correlation with Early Amazonian Vastitas Borealis unitson the northern plains map of Tanaka et al. (2005), particularlythe Scandia (ABs), interior (ABvi), and marginal (ABvm) units

Table 1LARLE database.

Lat Long Dia (km) Geoa T et al. NPb Perimeter (km) Rd (km) Average EM Max EM Ejecta typec

�75.75 20.35 3.6 s 214.718 13.13 7.3 9.5 SLERS�72.84 70.60 1.5 Npl1 83.637 6.18 8.3 10.2 Un�71.65 229.54 1.8 s 127.086 8.99 10.0 12.3 SLERS�71.07 96.96 4.9 Hr 244.138 14.09 5.8 9.7 SLERS�70.51 244.30 7.5 Npl2 498.22 35.58 9.5 11.3 SLERS�70.10 112.89 10.3 Hr 330.763 25.28 4.9 8.1 MLERS�66.95 65.58 2.0 Hr 208.488 13.40 13.4 19.6 SLERS�66.61 89.48 1.2 Npl2 77.733 3.13 5.2 10.2 SLERS�65.78 26.22 4.7 Hr 227.163 16.82 7.2 9.5 DLERS�65.57 222.52 2.1 Npld 191.965 10.25 9.8 16.0 SLERS�64.87 91.89 4.7 cs 318.948 21.04 9.0 11.8 DLERS�63.74 283.66 3.1 Npld 95.546 7.09 4.6 6.1 SLERS�63.41 71.67 1.5 Hr 98.511 5.39 7.2 10.3 SLERS�59.00 50.43 6.9 Hr 602.654 27.93 8.1 12.4 DLERS�57.58 226.77 2.6 Npl1 106.169 6.33 4.9 10.2 SLERS�56.90 123.18 2.3 Npld 229.509 12.20 10.6 14.2 SLERS�56.05 36.65 1.5 Hr 121.967 8.27 11.0 12.7 SLERS�55.96 137.62 2.8 v 184.16 8.86 6.3 8.5 SLERS�55.62 46.34 2.6 Hr 191.053 11.13 8.6 14.0 SLERS�54.74 341.45 2.9 cb 141.385 9.84 6.8 7.1 SLERS�52.79 228.26 3.1 Npl1 180.934 13.76 8.9 9.3 SLERS�52.75 222.62 3.3 Nplr 109.856 7.24 4.4 7.8 SLERS�50.69 158.93 6.0 Nplr 196.359 17.56 5.9 6.4 DLERS�50.23 210.49 2.1 Hr 69.975 4.27 4.1 7.0 SLERS�49.73 219.33 2.0 Hr 93.453 6.58 6.6 7.7 SLERS�49.50 172.05 1.7 Npl2 122.076 8.95 10.5 15.4 SLERS�48.24 155.24 2.5 Nplr 84.152 6.55 5.2 7.4 SLERS�48.10 242.41 3.3 Hpl3 193.608 11.48 7.0 8.2 SLERS�47.34 140.35 1.8 Hpl3 82.884 6.64 7.4 8.0 SLERS�47.15 94.86 1.6 Nm 65.187 3.80 4.8 8.5 Un�46.29 353.46 1.7 Npl2 74.263 4.67 5.5 6.8 SLERS�46.25 158.56 3.9 Nplr 156.205 12.58 6.5 7.4 SLEPC�45.92 152.69 3.4 Nplr 135.893 8.74 5.1 7.0 SLERS�45.92 9.53 3.3 Nplr 133.733 9.49 5.8 9.0 SLERS�45.42 25.70 10.3 Npl1 370.592 22.76 4.4 6.0 MLERS�44.31 139.27 4.2 cb 272.82 20.09 9.6 11.0 SLERS�44.03 176.23 2.2 Npl1 69.921 5.10 4.6 8.1 SLERS�43.20 202.47 1.8 cb 145.91 5.78 6.4 12.1 SLERS�40.04 286.45 6.0 Hpl3 258.682 15.70 5.2 8.2 DLERS�10.81 176.25 2.6 Aml 78.222 6.45 5.0 7.7 SLERS�3.02 13.70 3.8 Hpl3 191.801 12.63 6.7 12.9 SLERS�2.92 224.08 2.2 AHt3 87.879 5.83 5.3 8.0 SLERS�1.19 193.02 5.8 Npl1 AHAals 152.505 7.42 2.6 7.4 SLERS�0.84 149.42 4.4 Aml AAm 145.445 9.59 4.4 9.4 SLEPC�0.37 150.00 4.4 Aml AAm 257.846 8.77 4.0 6.0 SLERS

0.44 148.35 4.8 Aml AAm 173.218 8.89 3.7 7.0 SLEPC1.01 194.08 2.4 HNu AHAa1s 104.78 5.84 4.9 6.5 Un1.16 207.16 2.4 Amm AAm 139.21 6.18 5.2 10.5 Un1.39 218.55 1.4 Amu 77.281 5.68 8.2 12.6 Un2.32 154.22 4.1 Aml HBu2 150.353 10.12 4.9 10.6 SLERS2.43 230.68 3.4 Amu 181.385 10.73 6.3 9.2 SLERS

35.17 223.36 4.5 Hal Nn 130.026 8.28 3.7 6.1 MLERS36.24 221.65 2.4 Npl2 Nn 83.427 5.87 4.9 6.8 SLERS36.60 90.83 2.0 Hvk ABvi 85.143 6.84 6.8 7.8 SLERS38.23 15.79 2.0 Hr ABd2 154.39 7.36 7.4 12.6 SLERS38.49 159.93 2.0 Hr HNn 84.284 6.89 6.9 9.2 SLERS38.57 161.25 2.9 Hr HNn 138.2 8.95 6.2 7.7 SLERS39.40 75.19 1.7 AHpe HBu2 102.55 5.02 5.9 8.0 SLERS40.22 187.88 1.7 Aa4 AAa1n 79.738 5.10 6.0 12.0 SLERS40.52 175.69 1.4 Hvg ABvm 125.35 9.60 13.7 19.0 DLERS42.45 222.96 3.4 Aa1 HTa 179.86 14.32 8.4 12.0 SLERS43.06 184.56 3.7 Aa4 AAa1n 255.188 15.98 8.6 10.3 DLERS43.95 19.99 5.4 Hch Nn 295.453 15.35 5.7 8.3 SLEPC45.38 165.69 2.0 Aps HNn 85.17 7.74 7.7 11.2 SLERS45.66 135.93 2.4 Hvg ABvi 204.584 10.93 9.1 10.8 SLERS46.58 150.12 5.1 Hvk ABvi 226.178 21.57 8.5 9.6 SLEPC48.26 42.59 4.1 Apk HNu 182.421 13.85 6.8 8.0 SLEPC48.44 162.29 2.9 HNu ABvm 207.302 14.94 10.3 12.9 SLEPC49.26 214.56 2.4 Aa1 ABs 93.808 8.39 7.0 8.0 SLERS49.58 195.96 6.0 Aa1 ABvi 189.415 16.19 5.4 7.0 DLERS49.75 82.83 2.4 Hvm ABvi 217.213 14.13 11.8 14.8 SLEPC51.72 172.01 2.6 Aps ABvm 224.781 18.92 14.6 14.9 SLERS52.09 167.08 7.0 Aps ABvm 663.045 27.64 7.9 10.4 DLERS53.65 252.00 1.9 Hal 138.398 10.00 10.5 11.5 SLERS

188 N.G. Barlow et al. / Icarus 239 (2014) 186–200

Table 1 (continued)

Lat Long Dia (km) Geoa T et al. NPb Perimeter (km) Rd (km) Average EM Max EM Ejecta typec

53.92 256.17 3.7 Hal 433.979 22.60 12.2 13.4 DLERS54.28 136.73 3.1 Hvk ABvi 257.125 18.71 12.1 19.2 SLEPC54.93 77.77 3.7 Hvm ABvi 335.508 27.39 14.8 21.1 SLERS55.67 102.41 3.1 Hvm ABvi 255.157 14.40 9.3 13.0 SLERS56.20 163.36 2.4 Hvm ABvi 150.93 10.54 8.8 8.8 SLERS56.40 327.24 4.4 Hvm ABvi 223.271 15.20 6.9 9.3 SLERC57.27 249.53 2.7 Hal HTa 133.284 8.77 6.5 15.6 SLERC57.70 270.55 2.7 Hal ABs 133.408 12.36 9.2 9.9 SLERS58.34 95.87 3.7 Hvm ABvi 282.443 14.62 7.9 11.4 DLERS58.47 105.84 3.4 Hvm ABvi 160.435 15.57 9.2 9.6 SLERS58.62 68.32 2.0 Hvm ABvi 98.98 8.66 8.7 9.2 SLERS58.63 149.98 2.4 Hvm ABvi 153.094 10.78 9.0 13.0 SLEPC58.66 217.23 1.7 Hvk ABs 108.987 8.81 10.4 13.2 SLERS58.87 209.64 1.0 Hvk ABvm 73.435 2.91 5.8 13.6 SLERS59.17 171.34 1.9 Hvm ABvi 115.3 9.20 9.7 11.1 SLERS59.37 244.98 4.1 Aa1 ABs 254.324 21.38 10.4 14.0 SLERS59.55 85.68 1.4 Hvm ABvi 104.977 6.41 9.2 19.4 DLERS59.71 12.95 2.4 Hvk ABvi 143.221 9.59 8.0 15.6 SLERS60.07 92.42 1.4 Hvm ABvi 78.585 5.58 8.0 9.7 SLEPC60.88 96.72 1.7 Hvm ABvi 137.052 7.39 8.7 11.2 SLERS61.05 119.32 2.4 Hvk ABvi 116.46 8.51 7.1 9.3 SLERS61.31 224.80 2.0 Aa1 ABs 103.788 7.19 7.2 9.5 DLERS62.14 27.13 2.7 Hvk ABvi 121.137 8.33 6.2 9.0 SLERS63.71 49.63 2.8 Hvk ABvi 139.225 9.58 6.9 9.6 SLERS64.23 245.23 3.4 Aa1 ABs 169.522 14.01 8.2 10.1 SLERS64.88 149.32 2.0 Hvk ABvi 106.836 8.16 8.2 9.9 SLERC65.00 113.81 1.4 Hvk ABvi 86.72 5.81 8.3 11.7 SLERd65.33 259.03 2.2 Aa1 ABvm 268.734 13.11 11.9 17.0 Un65.50 181.80 3.7 Hvk ABvi 129.353 9.80 5.3 7.9 SLERC65.59 3.02 6.3 Hvk ABvi 356.605 25.55 8.1 13.5 SLERS65.80 334.80 2.7 Hvk ABvi 177.182 13.65 10.1 17.1 DLERS65.82 150.26 1.9 Hvk ABvi 131.594 8.62 9.1 11.8 DLERS65.86 153.75 1.7 Hvk ABvi 106.615 7.54 8.9 13.2 DLERS67.03 18.41 2.4 Hvk ABvi 156.664 12.87 10.7 13.3 SLERS67.40 172.58 1.5 Hvk ABvi 124.621 5.52 7.4 10.9 DLERS67.67 124.35 8.2 Hvk ABvi 369.345 22.40 5.5 8.5 DLERS67.69 152.88 9.4 Hvk ABvi 439.382 27.90 5.9 11.9 MLERS67.70 27.97 4.1 Hvk ABvi 312.017 19.70 9.6 13.1 SLERS67.79 356.21 2.7 Hvk ABvi 143.706 9.17 6.8 10.8 SLERS67.80 109.38 7.3 Hvk ABvi 487.122 31.58 8.7 11.0 DLERS67.81 348.69 2.9 c ABvi 125.039 7.12 4.9 10.1 SLERS67.85 151.81 1.4 Hvk ABvi 89.925 6.22 8.9 10.1 SLERC67.97 34.54 1.7 Hvk ABvi 119.783 8.60 10.1 14.8 SLERS68.27 266.36 5.5 Am ABs 724.963 26.11 9.5 16.4 DLERS68.30 235.40 10.9 Am ABvi 339.002 31.72 5.8 10.0 SLERS68.40 189.30 12.2 Am ABs 781.92 45.76 7.5 10.5 DLERS68.78 94.13 2.4 Hvk ABvi 133.998 9.51 7.9 10.2 DLERS68.93 150.55 2.8 Hvk ABvi 200.807 11.97 8.6 9.7 DLERS69.00 199.30 7.5 Am ABs 293.517 18.75 5.0 7.3 DLERS69.05 138.03 2.0 Hvk ABvi 147.588 8.67 8.7 14.6 DLERS69.22 356.35 5.5 Hvk ABvi 251.736 22.85 8.3 11.1 SLERS69.28 156.40 1.7 Hvk ABvi 121.064 10.01 11.8 13.6 SLERS69.68 69.17 5.9 Hvk ABvi 342.313 16.17 5.5 8.9 SLERS70.00 308.40 3.1 Hvr Abo 120.515 8.29 5.4 9.7 SLERS70.53 122.62 2.9 Am ABvi 206.199 12.68 8.7 12.9 DLERS70.67 230.32 3.6 Am ABs 236.373 18.53 10.3 10.8 SLERS71.28 89.22 5.5 Hvk ABvi 357.92 18.69 6.8 10.1 DLERS71.60 292.80 4.4 Am ABvi 208.053 12.82 5.8 9.0 SLERS72.08 354.75 5.6 Hvk ABvi 356.317 22.05 7.9 10.3 SLERS72.45 126.45 2.8 Am ABvi 318.92 10.27 7.3 21.4 SLERS72.60 182.00 1.7 Am ABvi 87.456 5.83 6.9 8.0 SLERS72.70 197.60 1.4 Am ABs 61.805 2.66 3.8 6.9 Un72.96 1.38 1.7 Hvk ABvi 91.192 7.11 8.4 10.8 SLERC72.99 38.30 10.8 Hvk ABvi 1481.384 59.71 11.1 12.3 DLERS73.58 62.07 6.9 Hvk ABvi 400.526 31.15 9.0 18.2 SLERS73.86 152.24 4.7 Am AHc 222.487 19.85 8.5 11.7 SLERS

a Geo column consists of the geologic units from Scott and Tanaka (1986), Tanaka and Scott (1986), and Greeley and Guest (1987).b T et al. NP column consists of the geologic units from the Tanaka et al. (2005) northern plains geologic map.c Ejecta types follow the nomenclature in Barlow et al. (2000), where SLE is a single layer ejecta blanket, DLE is double layer ejecta, and MLE is multiple layer ejecta. R refers

to a distal rampart, P is a pancake structure, S indicates sinuous outer edge, and C indicates circular planform to the ejecta deposit. Un stands for unclassifiable ejectamorphology at present image resolutions.

N.G. Barlow et al. / Icarus 239 (2014) 186–200 189

which are interpreted as fine-grained volatile-rich sediments(Tanaka et al., 2008). Group 2 LARLE craters are concentrated onsmooth materials (s), Noachian-aged dissected cratered units

(Npld), Noachian- and Hesperian-aged ridged plains (Nplr, Hr),and partially buried crater units (cb), which are interpreted as vol-canic units intermixed with sedimentary layers from eolian and

Fig. 2. Distribution map of LARLE craters P1-km-diameter over MOLA shaded relief map. The distribution of the 140 LARLE craters in this study can be divided into threeregions: 35–75�N (Group 1), 40–75�S (Group 2), and 3�N–11�S (Group 3).

Table 2Frequency of LARLE craters as a function of geologic unit.

Geologic unit Area (106 km2) Number Number/area(10�6 km�2)

Group 1 ABs 1.814 12 6.62ABvi 14.180 55 3.88ABvm 2.154 7 3.25HTa 0.947 2 2.11AAa1n 1.381 2 1.45HNn 2.767 4 1.45ABo 0.693 1 1.44Hal 1.831 2 1.09HBu2 1.127 1 0.89Nn 3.812 3 0.79

Group 2 s 0.437 2 5.48Npld 0.618 3 4.85Hr 2.272 10 4.40Nm 0.236 1 4.24cb 0.778 3 3.86Nplr 1.725 6 3.48cs 0.552 1 1.81Hpl3 1.758 3 1.71Npl2 3.143 4 1.27Npl1 13.990 5 0.36v 0.041 1 0.24

Group 3 Aml 0.613 5 8.16HNu 0.191 1 5.24AHt3 0.365 1 2.74Amu 0.937 2 2.13Npl1 0.532 1 1.88Hpl3 0.580 1 1.72Amm 0.884 1 1.13

190 N.G. Barlow et al. / Icarus 239 (2014) 186–200

fluvial processes (Scott and Tanaka, 1986; Tanaka and Scott, 1986;Tanaka, 1986; Greeley and Guest, 1987). Group 3 LARLE craters arelargely found in or adjacent to the lower member of the MedusaeFossae Formation deposits (Aml), composed of fine-grained high-porosity materials which could be either ice-rich or an anoma-lously low-density ice-poor material (Watters et al., 2007; Carteret al., 2009). The Medusae Fossae Formation deposits have been

proposed to be tephra erupted from Apollinaris Mons (Kerberet al., 2011) and correlate with high chlorine content (Boyntonet al., 2007). Sinuous ridges are common in the Aml unit and areinterpreted as inverted fluvial channels, suggesting that the unitformed under water-rich conditions and that the unit may stillretain a high volatile content (Zimbelman and Griffin, 2010).

2.2. Sizes

LARLE craters range in diameter from the cut-off diameter of1 km up to 12.2 km with a median diameter of 2.8 km. Most LARLEcraters are small, with 82% being less than 5 km in diameter and71% < 4 km, although this result may be skewed towards largerdiameters since our analysis did not include LARLE craters smallerthan 1-km-diameter due to resolution effects. Fig. 3 shows thediameter-latitude distribution of the 140 LARLE craters in thisstudy. All LARLE craters P6-km-diameter are found poleward of40� latitude in both the northern and southern hemispheres andthe largest LARLE craters are found at the high northern latitudes(i.e., Group 1). However, the largest LARLE craters are not foundat the highest latitudes.

The area (A) covered by the outer LARLE deposits ranges from35.5 km2 to over 13,300 km2. This area includes the sum of theaverage radius of the deposit (Rd) and the radius of the crater (Rc):

A ¼ pðRd þ RcÞ2 ð1Þ

Rearranging Eq. (1) allows us to calculate the average radius ofthe deposit through measurement of the LARLE deposit area andthe crater radius:

Rd ¼ffiffiffiffiAp

r� Rc ð2Þ

We then compare Rd to Rc (Fig. 4) and find an approximately lin-ear relationship between the extent of the LARLE deposit and thecrater radius. A linear regression analysis shows that the averageradius of the LARLE deposit is related to the radius of the crater by

Rd ¼ 1:5075þ 6:4989Rc ð3Þ

N.G. Barlow et al. / Icarus 239 (2014) 186–200 191

The linear regression gives a better fit (larger coefficient ofdetermination (R2)) than either a power law or polynomialfunction. As with crater diameter (Fig. 3), the average radius ofthe LARLE deposit is largest at higher latitudes in both hemispheres(LARLE Groups 1 and 2) and the largest Rd values are found inGroup 1 craters (Fig. 5). However since LARLE deposits with smal-ler extent exist at both high and low latitudes and the largest Rd arenot exactly correlated with the highest latitude, there is no uniquefunction relating Rd to a specific latitude.

Fig. 4. Mean radius of deposit versus crater radius. An approximately linearrelationship (Eq. (2)) exists between the mean radius of the LARLE layer and thecrater radius.

Fig. 5. Mean radius of LARLE deposit versus latitude. The LARLE craters with thegreatest radius tend to occur at the higher latitudes.

2.3. Surface morphology

The surface morphology of LARLE layers is like no other layeredejecta deposit on Mars. The LARLE layer begins at the outer bound-ary of the inner ejecta deposits, suggesting that the processes thatformed this outer layer only operated outward of this boundary.There are no obvious stratigraphic relationships indicating thatthe material comprising the LARLE deposit was emplaced eitherbefore or after the formation of the inner ejecta layers. Most cratersdisplaying a LARLE deposit also display a normal layered ejectablanket adjacent to the crater rim. LARLE craters with a SLE ejectamorphology comprise 101 (72%) of the 140 craters in this study.Twenty-eight LARLE craters are associated with a DLE morphology(20%), the MLE morphology is seen around four (3%) LARLE craters,and the remaining seven craters have insufficient image resolu-tions to classify the inner ejecta morphology. These results areconsistent with the general observation that most craters <13-km-diameter at high latitudes are either SLE or DLE craters(Mouginis-Mark, 1979; Costard, 1989; Barlow and Bradley, 1990;Barlow and Perez, 2003) and indicate that LARLE craters are notunusual in terms of their associated normal layered ejectamorphologies.

The LARLE craters appear morphologically fresh with typicallyonly minor modifications from surficial depositional processes(i.e., infilling eolian materials and/or ice-rich deposits). Normal lay-ered ejecta blankets also appear relatively fresh except for somesuperimposed depositional materials. The LARLE deposit is theregion which displays the largest range in preservation. Some LAR-LE deposits are very fresh, whereas others show evidence of ero-sion along the outer edge of the deposit. Crater-related pittedmaterials, such as those seen on the crater floor and ejecta depositsof fresh craters elsewhere on the planet (Tornabene et al., 2012;Boyce et al., 2012a), have not been observed with the LARLEcraters.

The surfaces of the freshest LARLE layers commonly exhibitradial, curvilinear ridges and dune-like landforms, similar to thelobate morphologies of terrestrial seif-type linear dunes (Fig. 6a).These features become more separated by inter-ridge troughs as

Fig. 3. Latitudinal distribution of LARLE craters as a function of diameter. Thelargest LARLE craters tend to occur at higher latitudes.

distance from the rim increases (Fig. 6b). The troughs appear tobe nearly as deep as the thickness of the deposits, which suggestsdeposition in a turbulent radial fluid flow (e.g., Greeley andIverson, 1987; Tsoar, 2001). These ridges and troughs extend tothe outer edge of the LARLE deposit, resulting in the outer edgeof the layer commonly terminating in ragged, feathery orflame-like shapes (Fig. 6c). This is another distinctive feature offresh LARLE layers, in contrast to the lobate distal edges of ejectadeposits associated with normal layered ejecta craters. Unlikemany SLE, DLE, and MLE morphologies, the LARLE layers lack distaland marginal rampart ridges. Since ejecta ramparts often containlarger blocks of material (Baratoux et al., 2005), this suggests thatthe LARLE layer material lacks these blocks and is composed offine-grained particles.

A few fresh LARLE craters exhibit long, narrow, ray-like depositsof ejecta that extend outward for over 20Rc (Fig. 7). Unlike mostejecta rays, these features do not include abundant secondary cra-ters and appear to be continuous deposits of approximately thesame thickness as the rest of the LARLE layer. The largest and bestpreserved of these features exhibit edges that have a ragged flame-like appearance similar to the outer edge of the LARLE layer(Fig. 6c). Similar ray-like deposits are seen around small craterselsewhere on the planet (McEwen et al., 2005; Tornabene et al.,2006), but these craters have no associated LARLE deposit. Thissuggests that the mechanism producing these ray-like deposits iscommon across the planet but the formation and/or retention ofthe LARLE deposit requires special circumstances.

Although not typically associated with the ray-like deposits,secondary craters are often found in association with fresh LARLEcraters (Fig. 8a). These secondary craters appear to be an early phe-nomenon in the excavation of these craters, as indicated by the

Fig. 6. Detailed morphology of LARLE layer. (a) Example of the morphology of seif linear dunes in Israel. This region is centered at 31.11�N 34.28�E. (b) Detail of the LARLElayer north of 4.2-km-diameter Farim crater showing lobate features morphologically similar to the seif linear dunes in (a) (CTX image B19_016929_1364). (c) Lonar, one ofthe freshest and largest LARLE craters, displays radial, curvilinear ridges, and dune-like landforms on the LARLE deposit. At increasing distance from the crater rim, themorphologic features become more separated by inter-ridge swales that appear to be nearly as deep as the thickness of the deposits. The outer edge of the LARLE layercommonly terminates in ragged, feathery or flame-like shapes (THEMIS VIS image V20518001).

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ejecta of the thick inner ejecta layers overriding or partially bury-ing these craters. LARLE deposits can be traced across some ofthese secondary craters, indicating that the LARLE layers are quitethin and are deposited after the secondary craters form (Fig. 8b).Unlike secondary craters found around other fresh layered ejectacraters, relatively few ejecta blocks are associated with the LARLEcrater secondaries. We suggest that this paucity of blocks may bea result of (1) their burial by the thin LARLE deposit, (2) blockswere not produced because the secondary craters excavatedmainly the fine-grained mantle materials instead of competentbedrock, (3) the process of transport and deposition of LARLE ejectadestroyed such blocks, or (4) a combination of two or more of thesepossibilities. The lack of blocks also is consistent with the lack ofobserved ramparts for the LARLE deposits.

2.4. Thickness and aspect ratio

Even the most extensive LARLE layers are usually so thin thataccurate measurement of their thickness is difficult using MOLAdata (Boyce et al., 2012b). Accurate measurement of their thicknessis further hampered by the fact that the LARLE layer is typicallyperched on thin, regional mantle deposits that often have erodedback to the edge of the layer. However, there are a few craterswhere reasonable estimates of LARLE layer thickness are possible.For example, Fig. 9 shows Vaduz, a 1.85-km-diameter LARLE cratermapped by Schaefer et al. (2011). Boyce et al. (submitted forpublication) suggest that Schaefer et al.’s (2011) estimated thick-

ness (�25 m) for the LARLE deposit actually included two units,an upper high-albedo LARLE layer that forms cliffs and the under-lying slope-forming, low-albedo regional climate-driven mantle(see Fig. 4 of Boyce et al., submitted for publication). Based on sha-dow lengths, Boyce et al. (submitted for publication) estimate thatthe heights of the cliffs formed by the upper layer of material (i.e.,the LARLE layer) is �7 ± 2 m. Using the Schaefer et al. (2011) esti-mate of �25 m as the total thickness, Boyce et al. (submitted forpublication) suggest that the mantle beneath the Vaduz LARLElayer is �14 ± 4 m.

The LARLE layers appear to vary little in thickness with distancefrom the crater rim. McGetchin et al. (1973) found that the ballis-tically-emplaced ejecta thickness (d) of explosion craters decays asa power law function of the distance (r) from the center of thecrater:

d ¼ 0:14R0:74t

rRt

� ��3:0

ð4Þ

where Rt is the radius of the transient crater cavity. The EjectaThickness Function (ETF) of martian low-latitude layered ejectadeposits displays a decay exponent of ��4.0 ± 0.5 (Garvin andFrawley, 1998; Boyce and Mouginis-Mark, 2006), suggesting thatlayered ejecta blankets experience a more rapid thinning with dis-tance from the rim. In contrast, the approximately constant thick-ness of the LARLE layer suggests that the decay exponent is nearzero. This is similar to the decay exponent for ejecta deposits ofpolar craters, which Garvin et al. (2000) found to be near �0.5,

Fig. 7. Lonar crater ray material southeast of the crater. This ray extends about 11Rc

away from the rim of Lonar. A few small craters have formed along the featuresuggesting they are secondary craters (THEMIS IR image I46050003).

N.G. Barlow et al. / Icarus 239 (2014) 186–200 193

although other characteristics of these polar craters are more simi-lar to normal layered ejecta blankets. The ETF of the LARLE layerssuggests that they are emplaced by processes unlike the transport

Fig. 8. LARLE secondaries. (a) Secondary craters are abundant around fresh LARLE cratersV27756015). (b) Secondaries, such as these found on the south side of Lonar crater, areindicating the LARLE layer is very thin (THEMIS VIS image V21260004).

and emplacement process of ballistic ejecta and other types of lay-ered ejecta.

The width and estimated thickness of the LARLE layers result inaspect ratios that are so low that they rival those of Low-Aspect-Ratio Ignimbrite (LARI) deposits on Earth (Dade, 2003). We havecalculated the aspect-ratios of three of the freshest LARLE craters(Lonar, Farim, and Vaduz) and find values of approximately8.3 � 10�5, 6.6 � 10�5, and 3.5 � 10�4, respectively (Fig. 10). Thesedimensions are substantially different from those of layered ejectadeposits or terrestrial flows such as landslides, debris flows, andmost pyroclastic flows.

LARLE layers show evidence of flow over low-relief topographicfeatures (Fig. 11), unlike normal layered ejecta deposits which tendto be deflected around pre-impact topographic features (Carr et al.,1977). This ability to flow over low-relief topography also has beenobserved in LARI deposits and has been used as evidence that LARIsare dilute, suspension-driven surges (Dade and Huppert, 1996).

2.5. Ejecta mobility and lobateness

The radial extent of layered ejecta blankets surroundingmartian impact craters is quantified through the ejecta mobility(EM) ratio, which is the ratio of the radial extent of the ejectadeposit (measured outward from the crater rim) to the craterradius (Mouginis-Mark, 1979; Barlow, 2005). We therefore candefine the ejecta mobility ratio for the LARLE deposit as

EM ¼ Rd

Rcð5Þ

We have calculated EM in two ways for the LARLE deposit: anaverage EM and the maximum EM. The average EM (Ave EM) isobtained from using the average radius of the deposit calculatedusing Eq. (2). The maximum EM (Max EM) is computed using thegreatest radial extent of the LARLE deposit. Max EM is used asone of the criteria for classifying a crater as a LARLE crater (Max

. These secondary craters formed on the east side of Lonar crater (THEMIS VIS imagecovered by the layered ejecta blanket (top) but are visible through the LARLE layer,

Fig. 9. (a) THEMIS daytime IR mosaic image of the 1.85-km-diameter LARLE crater named Vaduz. Crater is centered at 38.23�N 15.79�E. Insert shows the location of the highresolution image in (b). (b) Detail of the transition zone from the normal layered ejecta blanket (bottom) surrounding Vaduz to the LARLE layer (top) (CTX imageG13_023302_2186). Vaduz displays more erosion than Lonar, providing insight into how the LARLE deposits degrade over time.

Fig. 10. Aspect-ratio of various types of geophysical flow deposits, includingnormal layered ejecta blankets for martian craters, martian landslides, terrestrialpyroclastic flows, terrestrial mass wasting deposits, base surge deposits fromexplosion craters, and the LARLE layers for three craters (Lonar, Farim, and Vaduz).Note that LARLE deposits have similar aspect ratios to the various surge and LARIdeposits, but their AR’s are much smaller than deposits produced by otherprocesses. Also note that the aspect-ratios of base surge deposits generated byexplosion craters are substantially lower than any other flow type. This is becauseeven though their EM values are similar to LARLE layers, they are relatively muchthinner. The LARLE layer and explosion crater data are from Boyce et al. (2014),martian layered ejecta data are from Boyce et al. (2010), and the remaining data arefrom Dade (2003).

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EM > 6.0), whereas we utilize Ave EM in our comparison studieswith the EM of the pedestal and layered ejecta craters (Sections3 and 4). There is generally a linear trend between Ave EM andMax EM, although some scatter is observed.

Ave EM ranges between 2.56 and 14.81 with a median of 7.44.Max EM ranges between our minimum defining value of 6.0 up to21.4, with a median of 10.2. Fig. 12 shows that the highest values ofboth Ave EM and Max EM are associated with LARLE craters athigher latitudes and that the highest values are associated withGroup 1 LARLE craters. However, there is a large range in bothAve EM and Max EM values at a specific latitude and once again

the highest EM values are not necessarily correlated with the high-est latitudes.

Boyce et al. (2012b) conducted a simple test to determine if therun-out distance of LARLE layers could be explained by the samemechanism that emplaces impact-generated base surge. Theytested this by comparing the aspect ratios and run-out distancesof LARLE layers predicted for Lonar and Farim craters under mar-tian conditions using the physics-based model of Dade andHuppert (1996) and Dade (2003) and found remarkable agreementwith observed values. This model was developed to predict thebehavior of dusty, turbulent, dilute, suspension-driven gravity cur-rents like those thought to be produced by explosion crater basesurges, low aspect ratio ignimbrites (LARI), and dust storms.

We can quantify the sinuosity of the LARLE layer terminus byagain using a parameter originating from layered ejecta morphol-ogy studies (Barlow, 1994). This parameter is called the lobateness(C) and is related to the area (A) and outer perimeter (P) of theLARLE deposit:

C ¼ P

ð4pAÞ1=2 ð6Þ

A circular deposit has a lobateness value of 1.0 and C increasesas the outer edge of the deposit becomes more sinuous. The lobate-ness of the LARLE deposits range from 1.45 to 4.35 with a medianvalue of 2.05. No correlations are seen between C and parameterssuch as latitude, crater diameter, Ave EM, or Max EM.

2.6. Thermal inertia

Thermal inertia estimates of eight LARLE crater deposits wereobtained using the KRC thermal model (Kieffer, 2013). The modelis one-dimensional and, for this study, assumed vertical homoge-neity. All observations in this study are at low latitudes wherethere is little or no evidence of frost or subsurface ice and seasonalinsolation variations are not pronounced enough for subsurfacelayering to substantially affect the surface temperatures(Bandfield and Feldman, 2008; Kieffer, 2013). The latitude con-straint severely limited the number of LARLE craters for whichwe could obtain good thermal inertia data. THEMIS band nine,pre-dawn observation brightness temperatures were used as

Fig. 11. LARLE material from 4.2-km-diameter Farim crater (bottom) has flowedover the rim of an older impact crater (top) and shows little deflection around therim. Farim is centered at 44.31�S 139.27�E (CTX image B19-016929_1364).

Fig. 12. Ejecta mobility (EM) ratio of the LARLE layer as a function of latitude.Average EM (a) and maximum EM (b) both show a latitudinal dependence, with thehighest EM values occurring for LARLE craters at higher latitudes.

Fig. 13. Graph of temperature versus thermal inertia, showing the effect of north(azimuth 0) and south-facing (azimuth 180) slopes at a latitude of 40�N on apparentthermal inertia estimates.

N.G. Barlow et al. / Icarus 239 (2014) 186–200 195

model input along with location, season, elevation (derived fromMOLA), and surface albedo (derived from TES). Observations wererestricted to late spring/early summer observations with dust andice visible opacities less than 0.4. Dust and ice opacities werederived from late afternoon THEMIS observations of the same Marsyear and within ±10� Ls of the early morning observation. Slope andazimuth values of the crater ejecta have been ignored due to lack of

high-resolution slope data of the ejecta debris. Separate runs of theKRC thermal model with various inputs of slope and azimuth showthat north and south-facing slopes have variable pre-dawn surfacetemperatures that would affect thermal inertia estimates. How-ever, this difference is �50 thermal inertia units (TIU) for a surfacewith a thermal inertia of �350 TIU at latitude 40� between northand south facing slopes of 45� (Fig. 13). This difference is wellwithin acceptable error for thermal inertia estimates (Fergasonet al., 2006). Effects of east and west-facing slopes on derived ther-mal inertia values were less pronounced. In this case the differentslopes showed a modeled thermal inertia difference of less than15 TIU at a slope of 45� and a thermal inertia of �350 TIU. Regard-less, no systematic trends in thermal inertia are apparent withrespect to slope azimuth, and the potential variations in derivedthermal inertia due to slopes are much smaller than the variationsof surface units.

Fig. 14a shows the thermal inertia map for the 1.7-km-diameterLARLE crater located at 39.40�N 75.19�E. The image shows that thenormal layered ejecta blanket has a higher thermal inertia thaneither the LARLE layer or the surrounding regolith. This is typicalof ejecta blankets that are not located in dusty areas. The high ther-mal inertia of the ejecta blanket suggests that it either is composedof larger particle sizes or is partially indurated. The highest thermalinertia values coincide with the inner ramparts of the layeredejecta blanket, consistent with prior observations that ejecta ram-parts typically contain high concentrations of boulders (Baratouxet al., 2005). A HiRISE image from within the normal layered ejectablanket (Fig. 14b) shows that this region is composed of a poorlysorted mixture of large boulders and finer sediments comparedto the surrounding mantle outside the ejecta blanket (Fig. 14c).This is consistent with the thermal inertia estimates, where thepresence of boulders in the layered ejecta blanket results in an ele-vated apparent thermal inertia. The thermal inertia of the LARLElayer (Fig. 14a) is similar to or just slightly higher than the sur-rounding mantle and is lower than the values associated withthe layered ejecta ramparts. This suggests that the LARLE layer isprimarily composed of finer-grained particles than the bouldersseen on the inner layered ejecta blanket (Fig. 14b) but it could beslightly more indurated than the surrounding mantle.

3. Comparison of LARLE and pedestal craters

Many of the characteristics discussed in the previous section forLARLE craters are similar to those associated with pedestal (Pd)

Fig. 14. (a) Thermal inertia map (derived from I03861002) superimposed onunnamed 1.7-km-diameter crater located at 39.40�N 75.19�E, on a THEMIS daytimemosaic with CTX image D02_028113_2196_XN_39N284W used for shading. (b)HiRISE view of the transition in texture between the normal layered ejecta blanket(below arrows) and the LARLE deposit (upper part of image) (location 1 on (a)). (c)HiRISE view of the texture of the mantling deposit outside of the LARLE layer(location 2 on (a)). (Figures (b and c) from HiRISE image ESP_028113_2195).

Fig. 15. Pedestal craters, such as this 3.4-km-diameter example centered at 62.4�N99.4�E, are found on plateaus elevated above the surrounding terrain. Marginal pits,seen surrounding the base of this crater’s pedestal, have been interpreted asresulting from the sublimation of ice in the underlying mantle material (THEMISVIS image V13602003).

196 N.G. Barlow et al. / Icarus 239 (2014) 186–200

craters. Pd craters are small (generally <5-km-diameter) craterssurrounded by a plateau elevated above the surrounding terrain(Fig. 15) (Barlow et al., 2000; Barlow, 2006). Most are found pole-ward of 33�N and 40�S, with a small equatorial concentrationfound in the Medusae Fossae Formation (Barlow, 2006; Kadishet al., 2009). A normal layered (SLE or DLE) ejecta blanket is oftenseen surrounding the crater but the plateau extends further, typi-cally between 3 and 6 crater radii from the crater rim, althoughvalues can reach 13Rc. Kadish et al. (2010) used MOLA topographyto measure the heights of 2300 pedestals and found that most are<60 m. Some Pd craters display pits around the edge of the pedestal(Fig. 15) (Kadish et al., 2008) and SHARAD radar studies of 97 Pdcraters indicate the material comprising the pedestal has a lowdielectric permittivity, consistent with either low-porosity silicatesor a mixture of ice and silicate (Nunes et al., 2011). The observedcharacteristics of Pd craters have led to a model that these craters

form in ice-rich fine-grained materials with the region immedi-ately surrounding the crater undergoing armoring by some processassociated with crater formation. Subsequent sublimation of theice during periods of low obliquity results in a lowering of thesurrounding terrain, leaving the crater and its armored pedestalelevated (Kadish et al., 2009).

Table 3 compares the distribution and characteristics of Pd(from Kadish et al., 2009, 2010) and LARLE craters (this study).As the table shows, there are a number of similarities betweenthese two crater types, including:

� Most Pd and LARLE craters are <5 km in diameter.� Pd and LARLE craters occur in the same geographic regions on

Mars: at high northern latitudes (poleward of 33–35�N), at highsouthern latitudes (poleward of 40�S), and in the equatorialregion mainly within the Medusae Fossae Formation (Fig. 16).� Pd and LARLE craters usually display a normal layered ejecta

blanket in addition to their pedestal/LARLE layer.� The LARLE layer and pedestal of Pd craters extend much further

than the normal layered ejecta blanket seen adjacent to thecrater rim.

The major differences between Pd and LARLE craters are in themeasured EM and C values, which are both smaller for the Pdpedestal than they are for the LARLE layer. In addition, only afew LARLE craters show evidence of being elevated above the sur-rounding terrain, which is the distinguishing characteristic of Pdcraters. Vaduz is one LARLE crater which does show evidence ofbeing elevated: the estimated thickness of the LARLE layer plusthe underlying mantle for Vaduz (�25 m; Schaefer et al., 2011) isclose to the lower range of pedestal heights measured by Kadishet al. (2010). These observations strongly suggest that LARLE andPd craters form by a similar mechanism, but that Pd craters haveundergone more modification than LARLE craters. We will developthis evolutionary connection in more detail in Section 4.

Table 3Pd and LARLE comparison.

Pedestala LARLE

Number of craters 2696 140Diameter range

(km)0.7–8.0 1.0–12.2

Distribution Mostly poleward of 33�N and 40�S; equatorial examples concentrated inMedusae Fossae Formation

Mostly poleward of 35�N and 40�S; equatorial examples concentrated inMedusae Fossae Formation

EM range 1.2–13.0 (Max) 6.0–21.4 (Max)2.6–14.8 (Ave)

C range 1.0–2.5 1.45–3.45Ejecta ‘‘Normal’’ layered ejecta superposed on extensive pedestal ‘‘Normal’’ layered ejecta associated with LARLE layerLayer thickness Generally <60 m Generally <10 m

a From Kadish et al. (2009, 2010).

Fig. 16. Distribution of LARLE (circles) and pedestal (triangles) craters on MOLA shaded relief map. The LARLE crater analysis covers the ±75� latitude zone (this study) andthe pedestal crater analysis is for the +65� to �72� latitude zone (Kadish et al., 2008). As seen in this map, LARLE and pedestal craters occur in the same regions. Most arelocated at high polar latitudes, with a few equatorial examples largely found in the thick fine-grained deposits of the Medusae Fossae Formation (MFF).

N.G. Barlow et al. / Icarus 239 (2014) 186–200 197

4. Discussion

The question arises as to whether or not LARLE craters are trulydistinct from the normal layered ejecta craters found across Mars.To address this question, we have investigated the morphologicand morphometric characteristics of the crater cavities and normallayered ejecta deposits of the LARLE craters and compared them tolayered ejecta craters elsewhere on Mars (i.e., non-LARLE craters).Fig. 17 shows the EM ratio of layered ejecta morphologies of non-LARLE craters, the inner (‘‘normal’’) layered ejecta deposits of LAR-LE craters, and the outer LARLE deposits. EM of the non-LARLE andinner LARLE layered ejecta deposits overlap, indicating that theLARLE layered ejecta deposits are not unusually extensive in com-parison the layered ejecta deposits elsewhere on the planet. Onlythe outer LARLE layers are distinctly different in terms of EM. Inaddition, we find that the depth-to-diameter relationships of LAR-LE craters are not unusual compared to non-LARLE polar craters onMars (Fig. 18). The similarity in crater sizes, ejecta morphology,EM, and crater depth–diameter values indicate that LARLE cratersdo not display unusual layered ejecta crater characteristics exceptfor the LARLE layer itself. Thus LARLE craters are normal layeredejecta craters except for the presence of the LARLE layer. The LARLE

crater depth–diameter relationships are similar to those of freshcraters in the polar regions of Mars, supporting the morphologicevidence that LARLE craters are very fresh.

Table 4 compares the distribution, EM, and C of the LARLE lay-ers found in this study to the same characteristics of non-LARLESLE, DLE, and MLE craters (Barlow and Bradley, 1990; Barlow andPerez, 2003; Barlow, 2005). The table shows that there are a num-ber of differences between the LARLE layer and normal layeredejecta blankets, including larger values of median EM (Fig. 17)and median C. These results suggest that the LARLE deposit isemplaced by a different mechanism than the layered ejecta blan-kets. Formation models of layered ejecta blankets propose thatthe ejected material is entrained either in a gas cloud producedby vaporization of subsurface volatiles (Carr et al., 1977; Wohletzand Sheridan, 1983; Mouginis-Mark, 1987; Stewart et al., 2004)or within the martian atmospheric gases (Schultz and Gault,1979; Schultz, 1992; Barnouin-Jha et al., 1999a, 1999b; Suzukiet al., 2007), although a dry granular flow mechanism also has beenproposed (Shinbrot et al., 2004; Wada and Barnouin-Jha, 2006).Observations of layered ejecta craters on Mars (Barlow, 2005)and Ganymede (Boyce et al., 2010) argue that subsurface volatilesplay the dominant role in the emplacement of these morphologies.

Fig. 18. Depth to diameter relationships of LARLE craters (large open circles)compared with martian polar crater from the global MOLA-based data of Boyce andGarbeil (2007). Dark circles are craters on Noachian-aged terrains and gray trianglesare craters on younger geologic units. Note that the depth to diameter ratio of allLARLE craters is near the top of the plot (i.e., deepest craters), indicating that all arein the fresh impact crater range.

Fig. 17. Comparison of ejecta mobility (EM) ratio for different martian ejectadeposits. The plot show that the inner (normal layered) ejecta layers of LARLEcraters (filled diamonds for inner layers and filled triangles for outer layers) havesimilar EM to non-LARLE layered ejecta craters on Mars (open diamonds andcrosses, respectively). The LARLE layers (filled circles) display much higher EMvalues than any of the layered ejecta deposits. This suggests that LARLE craters aremorphologically just layered ejecta craters with an anomalously long run-out outerlayer (i.e., the LARLE layer). The outer layer of normal double layer ejecta cratersalways display larger EM than the inner layer—this transition is indicated by thedashed horizontal line.

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These materials are ejected along ballistic trajectories during theexcavation stage of crater formation (Melosh, 1989), are entrainedwithin the vapor cloud, and subsequently are emplaced along thesurface as a ground-hugging flow (Carr et al., 1977; Baloga et al.,2005; Baratoux et al., 2005; Boyce et al., 2010; Osinski et al., 2011).

The two- to five-times greater radial extent of the LARLE depositcompared to layered ejecta morphologies, combined with theapproximately 2� greater lobateness, suggest that the materialcomprising the LARLE deposit was much more fluid. In our com-panion paper (Boyce et al., submitted for publication), we notethe similar low aspect ratios of the LARLE deposit to base surgedeposits generated during nuclear and volcanic explosions

Table 4Comparison of LARLE, SLE, DLE, and MLE characteristics.

LARLE SLE

Distribution Poleward of 35�N and 40�S; equatorial in MFF GlobalAve EM range 2.56–14.81 0.20–6.Median Ave EM 7.44 1.53C range 1.45–4.35 1.00–3.

SLE, DLE, and MLE data from Barlow (2005).

(Fig. 10). Base surges are common features with any highly explo-sive event and consist of dust clouds produced mainly by primaryejecta erosion and mobilization of dislodged surface material. Thecomparatively low concentrations of suspended particles in thesedust clouds mean that their dynamics are controlled by the flowproperties of interstitial atmospheric gas. Boyce et al. (submittedfor publication) suggest that these base surges are low-viscosity,Newtonian flows with flow properties approximating those ofthe atmospheric gas and, consequently, they can travel great dis-tances driven only by their weight.

The key to why the LARLE deposits are retained only around aselect group of craters when base surges should occur in all cra-ter-forming events (Oberbeck, 1975; Osinski et al., 2011) is thematerial in which the LARLE craters are found. A common themeamong all of the LARLE craters is that they are found in regions cov-ered by thick deposits of fine-grained materials (Section 2.1). Ter-restrial observations show that base surge deposits are thickestwhere easily eroded clastic mantling materials can be added tothe surge. The ice-rich, fine-grained nature of the materials inwhich LARLE craters form provides abundant vapor from thevaporization of the target ice and abundant fines to be incorpo-rated into the surge. Thus the conditions are ideal for the creation,deposition, and preservation of a base surge deposit. The ice-richmantle is thicker and more prevalent in the high-latitude martiannorthern plains, which could explain the observed latitude depen-dence of the LARLE craters with the largest diameters and EM. Therestricted size range of LARLE craters also can be explained interms of the thickness of the ice-rich mantle. Craters which aretoo small will not have the necessary ejecta velocities to producethe base surge deposit. Alternately, craters which are above a cer-tain size range will excavate through the mantle layer and theejecta volume will overwhelm the volume of the eroded fine-grained material needed for the LARLE deposit.

LARLE and Pd craters display enough similarities to suggest theyform by a similar mechanism. We propose that fresh craters form-ing within the fine-grained mantling deposits will initially displaythe LARLE characteristics. The LARLE deposit can be armoredagainst the environmental erosional processes by formation of aduricrust as water vapor diffuses into and out of the fine-grainedmaterials, transporting salt-rich materials (such as sulfates(Massé et al., 2010) or chlorides (Osterloo et al., 2008)) to the sur-face where these salts harden into a surficial intergranular cement(Jakosky and Christensen, 1986). Subsequent erosional processessuch as sublimation from the ice-rich mantles causes the sur-roundings to lower, leaving the LARLE deposit elevated above thesurroundings and transforming the LARLE crater into a pedestalcrater. The ice-rich mantle retained below the surficial duricrustand thin LARLE deposit undergoes sublimation along the exposededges, and thus the original LARLE deposit migrates inward towardthe crater as it is eroded from below. This explains the lower EMand C of the pedestal craters, which are simply eroded LARLE cra-ters. The marginal pits observed along the base of some pedestalcraters (Fig. 15) show that this process of erosion from backwa-sting still continues today. A few LARLE craters do show morpho-logic evidence that they are beginning to transition into a

DLE (outer) MLE (outer)

Poleward of �40�N and 40�S Primarily between 30�N and 40�S60 1.20–10.60 0.30–4.70

3.24 2.1757 1.01–2.27 1.02–1.74

N.G. Barlow et al. / Icarus 239 (2014) 186–200 199

pedestal crater, such as indications of cliff formation at the edge ofthe LARLE layer combined with transitions to EM and C valuesmore typical of Pd craters. These observations further supportthe proposed evolutionary sequence between the two crater types.

5. Conclusions

Low-Aspect-Ratio Layered Ejecta (LARLE) craters are distinctfrom the normal layered ejecta craters found on Mars. Our surveyhas revealed the presence of 140 LARLE craters P1-km-diameter inthe ±75� latitude zone, with the majority (92%) located poleward of35�N and 40�S. All LARLE deposits display larger EM and C valuesthan normal layered ejecta morphologies as well as morphologiccharacteristics suggesting a different emplacement mechanism.High-resolution images and thermal inertia data suggest that theLARLE deposits are thin layers composed of fine-grained particlesemplaced as a dusty base surge cloud resulting from impact intoice-rich, fine-grained mantling deposits. The similarity in distribu-tion, size, and characteristics of LARLE and pedestal craters leads usto propose that pedestal craters are eroded versions of LARLEcraters.

The base surge mechanism is analogous to the turbulent grav-ity-driven current seen in association with large chemical andnuclear explosions and is described in more detail in our compan-ion paper (Boyce et al., submitted for publication). A base surge islikely to occur during formation of most martian impact craters,but normally this deposit is quite thin and removed quickly byeolian activity. We propose that the LARLE deposit remains inthe specific regions where these craters are found due to the for-mation of a salt-rich duricrust produced as liquid water in theice-rich base surge deposit diffuses upward carrying dissolved ionsto the surface. This cemented duricrust layer is sufficient to protectthe LARLE deposit from immediate erosion and explains the‘‘armoring’’ of the plateaus associated with pedestal craters.Sublimation of ice in the mantle underlying the duricrust eventu-ally removes the LARLE/pedestal layer through backwasting. Thusboth LARLE and pedestal craters can be explained by the normalinteraction of the cratering process with the specific environmen-tal conditions in which these craters are found.

Acknowledgments

JMB was supported under the NASA/ASU THEMIS Team Con-tract A2910. The authors thank Dan Berman and an anonymousreviewer for their comments which have improved the manuscript.We also thank the MO THEMIS, MGS MOLA, and MRO CTX and HiR-ISE teams for their data products which have been utilized in thisstudy.

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