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Keywords:

  • craters;
  • Mars;
  • volatiles

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Morphology of the Ejecta of DLE Craters
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[1] The Thermal Emission Imaging System (THEMIS) visible (VIS) images provide new insight into the nature and development of the unique ejecta deposits of Martian craters. This study focuses on double-layered ejecta (DLE) craters. To date, over 100 DLE craters have been examined using mainly THEMIS VIS data. Our observations suggest that emplacement of DLE crater ejecta occurred in two stages, with the inner ejecta layer emplaced similar to single-layered ejecta (SLE) crater ejecta. This may have involved both ballistic and flow processes. In contrast, the outer ejecta layer of DLE craters appears to have been emplaced through the high-velocity outflow of materials from tornadic winds generated by the advancing ejecta curtain or base surge. Remarkably, DLE craters lack secondary craters, which suggests that the large ejecta blocks that normally produce such craters may have either been entrained and/or crushed by these winds or fragmented as a result of the presence of water in the target materials. These observations suggest that volatiles (either trapped in the subsurface or in the atmosphere) have played a key role in the emplacement of the ejecta of DLE craters and leaves open the question as to what role volatiles play in the emplacement of ejecta of other types of fluidized ejecta craters (i.e., SLE and MLE craters). Because DLE craters are found in many different regions of Mars, often in close proximity to other types of craters, conditions (e.g., atmospheric density) that produce DLE craters must fluctuate or the Martian crust must be unexpectedly heterogeneous (laterally and vertically). While the degree of heterogeneity has yet to be recognized, recent suggestions about possible Martian climate change raises the possibility of impact into target materials that are periodically wet or that a significantly higher atmospheric pressure may be periodically present.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Morphology of the Ejecta of DLE Craters
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[2] From the late 1970s through the late 1990s the Viking image data set was the main basis for the analysis of ejecta deposits around Martian craters [Mouginis-Mark, 1979a; Barlow and Bradley, 1990]. However, because of its limitations in image spatial resolution only in rare instances where high-resolution data exist [Mouginis-Mark, 1987] could these data support morphologic analysis of impact craters. With the advent of extensive image coverage from the Thermal Emission Imaging System (THEMIS) and the Mars Orbiter Camera (MOC), and the Mars Orbiter Laser Altimeter (MOLA) topographic data, a much more detailed description of the morphology of these craters is now possible. The details of crater morphology revealed in the new data provide a significant leap in information about parameters affected by ejecta flow and can be used as a basis for the understanding of the mechanism(s) of such flow.

[3] The ejecta deposits surrounding many impact craters on Mars have been enigmatic since the first Viking Orbiter images illustrated their nonlunar character [e.g., Carr et al., 1977]. Unlike on the Moon and Mercury, Martian impact craters typically possess lobate deposits that appear to have been fluidized at the time of emplacement [Carr et al., 1977; Gault and Greeley, 1978; Mouginis-Mark, 1981, 1987]. The strong radial pattern, distal ridge (rampart) around the perimeter of the ejecta, and the large radial distance to which continuous ejecta can occur was offered as evidence for flow. In addition, Viking data suggested that fluidized ejecta were emplaced as ground hugging flows of particulate material that may have initially been ejected in ballistic trajectories [Carr et al., 1977; Gault and Greeley, 1978; Wohletz and Sheridan, 1983]. While, fluidized ejecta emplacement models generally assume that the ejecta were a mixture of solid particles (fragmented target rock) and gas/water, the origin of the fluidizing medium necessary to cause flow is controversial. Most authors have attributed fluidization of the ejecta to the presence of volatiles (water or ice) within the target material at the time of crater formation, and that spatial and temporal variations in ejecta range can best be ascribed to target variations, the latitude and altitude of the parent crater, and possible temporal changes in the climate [Mouginis-Mark, 1979a; Horner and Greeley, 1987; Barlow and Bradley, 1990; Barlow and Perez, 2003; Barlow, 2005a]. However, Schultz and Gault [1979], Barnouin-Jha and Schultz [1998], Schultz [1992, 2005], and Barnouin-Jha et al. [1999a, 1999b] have offered evidence that the morphology of these ejecta deposits could mainly be due to the interaction of the ejecta with the atmosphere. They suggest that vortices in the atmosphere caused by an advancing ejecta curtain can produce fluidized ejecta and result in the observed morphology. Also challenging the assumption of fluidization caused by volatiles in the target materials, Schultz [1992] has pointed out that ejecta composed mainly of fine grain materials can flow without an included gas or liquid phase. Such flow generally requires the target material to be initially composed of loose, fine grain materials or that the impact itself produces an enormous amount of fine grain material as part of the excavation process.

[4] The Mars Crater Consortium proposed a nomenclature for most of the fresh impact craters based on Mars using Viking Orbiter images [Barlow et al., 2000] and THEMIS visible (VIS) images [Barlow, 2005b] that includes three primary types of fresh fluidized ejecta impact crater: single-layered ejecta (SLE) craters that possess a single continuous ejecta layer surrounding the primary crater, double-layered ejecta (DLE) craters that include two concentric ejecta layers, and multiple-layered ejecta (MLE) with more than two layers around the parent crater. The SLE and MLE ejecta layers have many similarities and are most likely a continuum of the same crater type (MLE craters are typically larger than SLE craters). However, the morphology of the ejecta of DLE craters is markedly different than either that of SLE or MLE craters (Figure 1).

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Figure 1. (left) Typical example of a SLE crater, 5.2 km diameter, located to the SW of Arsia Mons volcano at 29.9°S, 218.8°E (THEMIS image V06372004). (center) DLE crater, 18.2 km diameter, located in Utopia Planitia at 38.5°N, 99.3°E (part of THEMIS image I04865005). (right) MLE crater, 19.5 km diameter, located in SE Elysium Planitia to the SE of the volcano Albor Tholus at 14.6°N, 155.6°E (part of THEMIS image I08533017).

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[5] The new THEMIS images have shown that DLE craters possess a number of previously undetected morphologic attributes that are strikingly different from those observed on other fluidized ejecta craters (i.e., SLE and MLE craters) or craters with ejecta that appears to have been emplaced purely ballistically [Barlow et al., 2000]. These unique attributes also suggest that DLE craters were created though emplacement mechanisms that may have been somewhat different than those of SLE and MLE crater ejecta and provide information that help constrain models of their emplacement mechanisms. In this investigation, we have focused on DLE craters in an effort to understand conditions required for the formation of their ejecta and as a step in the overall understanding of the formation of Martian fluidized ejecta craters. Thus far, we have identified over 100 fresh appearing DLE craters in the diameter range 5.5 to 29.6 km. Future studies will focus on the ejecta of other crater types.

[6] While most of DLE craters we have found occur mainly in two latitudinal bands, from ∼30° to 50°S and 25° to 60°N, it is clear that DLE craters can be found in both hemispheres, at a variety of elevations and on a variety of terrain types and ages (Figure 2) generally consistent with the findings of Barlow and Perez [2003], Barlow and Bradley [1990] and Barlow [2005a, 2005b]. In addition, in some locations, DLE craters are located near SLE and MLE craters of approximately the same size, erosion state (i.e., age), and appear to be on the same geologic unit (Figure 3). This has important implications to what factors control the development of DLE craters and whether these factors have varied with time.

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Figure 2. Four examples of DLE craters at different locations on Mars outside of Utopia Planitia that show very similar DLE exterior morphologies and a variety of interior structures: (a) 14.9 km diameter crater at 32.6°S, 120.0°E (THEMIS image V08148006), (b) 12.2 km diameter crater at 42.3°S, 218.5°E (THEMIS image V07795008), (c) 18.7 km diameter crater at 37.4°N, 159.0°E (THEMIS image V05924026), and (d) 23.3 km diameter crater at 42.1°N, 23.3°E (THEMIS image V09898024).

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Figure 3. Two craters on the SW flank of the volcano Alba Patera illustrating strikingly different ejecta morphologies despite the inference that the target rock in each case was a set of volcanic flows. (left) A 17.7 km diameter crater (located at 32.7°N, 236.9°E) 1138 m deep and displaying the two ejecta layers that are characteristic of DLE craters. (right) A second crater, 18.2 km in diameter and 1179 m deep, located 130 km to the north. This second crater (located at 29.7°N, 236.5°E) is a typical SLE crater and has abundant secondary craters. Both main images are subscenes of THEMIS image V14221014, which has a resolution of 36 m/pixel. Inserts are MOLA shaded relief images showing the location of each crater.

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[7] As of this writing, the THEMIS instrument and Mars Odyssey spacecraft continue to operate well, thereby creating the opportunity for additional imaging of the Martian surface. However, this present investigation only includes data collected for DLE craters imaged through October 2005 for analysis. Impact craters remain one of the high-priority imaging targets, particularly from the perspective of producing complete coverage of several of the freshest impact craters in the 10–40 km diameter size range. Complete coverage of a crater in that size range requires between 10–20 THEMIS high-resolution VIS frames (at 18 m/pixel resolution). For example, Bacolor, named after a small town on Luzon Island, the Philippines, a 20.5 km diameter crater found in Utopia Planitia (Figure 4) requires 18 individual THEMIS images in order to analyze the crater and ejecta blanket. Additional scenes would be required to search for secondary craters beyond the distal boundary of the continuous ejecta layers.

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Figure 4. Crater, called Bacolor, 20.5 km in diameter and centered at 33.0°N, 118.6°E. Bacolor is one of the freshest double-layered ejecta craters on Mars and illustrates some of the best coverage of a single crater obtained to date by the THEMIS visible (VIS) instrument. The locations of Figures 6, 7, 10, and 12 are shown, as is the MOLA profile presented in Figure 5d. This is a mosaic of THEMIS images V05451015, V09670012, V10294008, V10319007, V11829005, V11854007, V12141005, V12453007, V12765007, V13052008, V13077006, V13364007, V13676010, V13963010, V14300012, and V17058008.

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[8] All craters in this study are morphologically fresh enough to exhibit details of the small-scale morphologic features on both layers of ejecta. In this sample, several stages of degradation and burial of the crater interior are presumed to be included, even though all of the diagnostic features on the ejecta blankets of DLE craters are consistent for all craters. For this reason, many of these craters show postimpact interior deposits, but in spite of this, fine-scale texture on their ejecta blankets has been preserved. Some of these craters, in particular those in the northern plains, appear to have been buried and may be in the process of being exhumed [Kreslavsky and Head, 2002; Tanaka et al., 2003; Boyce et al., 2005].

[9] The improved image resolution provided by THEMIS VIS compared to Viking Orbiter images, and the greater spatial coverage compared with MOC images, have made it possible to more clearly identify which craters should be classified as DLE craters [Barlow, 2005a]. For instance, it is evident that there are additional craters that are DLE craters despite the fact that their far-range ejecta have been buried. Using Viking data, such craters would have been classified as SLE or pedestal craters, but using THEMIS VIS images, the distinctive and diagnostic radial texture on the inner ejecta layer of DLE craters can be easily identified. In addition, despite the higher spatial resolution, such a reclassification of craters as DLE craters was not generally possible using MOC data because of the limited spatial coverage of the images.

2. Morphology of the Ejecta of DLE Craters

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Morphology of the Ejecta of DLE Craters
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[10] All DLE craters in this study have, at least, some of the attributes of fresh craters, but none of the DLE craters investigated appears to be completely pristine. Even the fresh appearing crater Bacolor (Figure 4), which will be used here extensively to illustrate the morphologic features of DLE craters, has a few superposed small impact craters, and fine-scale morphologic features such as dunes that indicate some postimpact modification.

[11] In spite of the effects of modification by the Martian surface processes, the ejecta deposits of DLE craters share a number of morphologic features that distinguish them from other Martian craters [Barlow et al., 2000], including (1) two ejecta layers, (2) strong radial texture composed of grooves and ridges that extends continuously from their rims to the outer edge of their continuous ejecta blankets, and (3) the apparent lack of secondary impact craters. We find the inner layer is typically ∼1–2 R (here R is defined as crater radius measured from the crater rim) wide compared with the outer layer and extends out to ∼3–6 R. This is in reasonable agreement with the ejecta mobility (EM) of DLE craters measured by Barlow [2005b] for the Viking-based database of 1.49 R average for the inner layer and 3.24 R average for the outer layer. Bridges and Barlow [1989] and Barlow [2005b] also find that the outer edge of the inner ejecta layer (Γ = ∼1.04) of DLE craters is similar to that of SLE craters (Γ = ∼1.10), but less lobate than that of their outer ejecta layer (Γ = ∼1.14) or MLE ejecta (Γ = ∼1.18). Neither the inner or outer ejecta layers terminate in a relatively narrow distal rampart in a manner like that of SLE or MLE craters, but instead thicken more gradually toward their outer perimeters. High-resolution images of DLE crater ejecta deposits reveal few features that appear to be blocks on their surface of the ejecta, unlike the examples found on fresh MLE and SLE craters [Mouginis-Mark et al., 2003; Barnouin-Jha et al., 2005; Baratoux et al., 2005]. This lack of blocks may partly account for the remarkable absences of secondary craters around DLE craters (at least out to at least 11 R). In the rest of this section, we will describe the morphology of the ejecta deposits, and the (lack of) secondary craters of DLE craters in more detail.

2.1. Rim

[12] From the rim crest outward to ∼0.05–0.1 R, the slopes are steep, commonly near the angle of repose (Figure 5). Beyond this is a sharp break in slope of the surface to more gentle slopes.

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Figure 5. Profiles from raw MOLA data showing that the ejecta layers of DLE craters are quite diverse. Each profile starts at the rim crest of the crater. For all craters a depression or “moat” (denoted by “M” on each profile) surrounds the rim of the crater, with the relative depth of this moat generally related to the size of the crater. In addition, the inner ejecta layer typically terminates in a broad distal rampart. The size of this rampart is also generally related to the size of the crater. The outer ejecta layer is thinner than the inner layer and is generally of relatively uniform thickness. Like the inner ejecta layer the outer layer terminates in a broad low rampart. Profiles are taken from the following craters and MOLA profiles: (a) 13.4 km crater at 34.9°N, 102.6°E, MOLA orbit 19296; (b) 16.2 km crater at 45.4°S, 25.7°E, MOLA orbit 16620; (c) 8.3 km crater at 42.41°N, 344.98°E, MOLA orbit 20051; (d) Bacolor crater at 33.0°N, 118.6°E, MOLA orbit 20322 (see Figure 4 for location); (e) the crater Arandas, 24.8 km in diameter, located at 42.4°N, 345.0°E, MOLA orbit 12307 (see also Figures 9a and 9b); and (f) 17.4 km crater at 38.0°S, 16.4°E, MOLA orbit 13058.

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[13] The rims of the DLE craters measured in this study are nearly the same as those predicted from empirically derived rim height to diameter relationship by Pike [1977] for lunar craters, and recent work by Steward and Valiant [2006] that take into account regional strength difference in Mars terrains. For example, the rim of the freshest appearing DLE crater, Bacolor, is ∼680 m high (with a measurement error of ∼10%), compared with ∼788 m predicted by Pike [1977], and ∼710 m predicted by Steward and Valiant [2006]. Considering the predicted uplift of the bedrock at the rim of ∼430 m [Melosh, 1989; Roddy, 1978], the initial thickness of ejecta at the rim of Bacolor might have been as much as ∼250 m. In addition, it should also be noted that rim uplift may be accentuated in water-rich targets [Greeley et al., 1980; Steward et al., 2000] and as a result may indicate that little, if any, ejecta is missing from the rim, a possibility that is consistent with the morphologic freshness of Bacolor.

[14] In addition, on the basis of the predicted uplift and topographic relief, the thickness of ejecta at the break in slope at the base of the rim may be <∼100 m around Bacolor, much less than predicted [McGetchin et al., 1973]. This apparent thickness of ejecta may indicate that (1) ejecta have been preferentially eroded from near the rim; (2) rim uplift was not as great as predicted, or (3) the ejecta emplacement process resulted either in less material being deposited on the rim than predicted by present models or collapse because of water in the target materials causing the rim to flow outward [Barnouin-Jha et al., 2005]. The present data do not provide information to resolve this question.

2.2. Inner Ejecta Layer

[15] The morphology of the inner ejecta layer of DLE craters is different than MLE and ballistic ejecta impact craters on Mars. The ejecta in the inner layer of DLE craters is generally relatively thin close to the rim, but thickens outward to form a broad, low rampart-like ridge on the outer edge (Figure 5). This is generally similar to SLE crater ejecta, but the rampart around SLE craters is narrower. This contrasts with ballistic ejecta craters whose ejecta thins rapidly and continuously outward. For MLE craters, the ejecta rapidly thins near the rim but decreases slightly outward in each layer to their terminal rampart [Garvin and Frawley, 1998; Garvin et al., 2000].

[16] In cross section, the inner ejecta layer of all DLE craters is typically thinnest from the rim out to ∼0.5 R (e.g., Figures 5a and 5d) forming a depression that resembles a “moat” around the crater. However, around some DLE craters the topography of the moat can be quite subdued (Figure 5c) in places. On the basis of MOLA measurement (assuming the preexisting surface under the ejecta was a flat plain), ejecta in this moat can be only a few tens of meters or less thick [Garvin and Frawley, 1998; Garvin et al., 2000].

[17] An estimate of ejecta thickness in this zone can be made based on the degree of partial burial of small preexisting impact craters by using their predicted initially morphology compared with the present shape. In the case of the small craters found in the inner ejecta layer of Bacolor shown in Figure 6 the ejecta thickness (at their locations of ∼0.3 R) are estimated to be ∼45 m thick based on the degree of their burial. In Figure 6 crater “a” is ∼1.2 km in diameter while crater “b” is ∼1.75 km diameter. A MOLA profile (orbit 11007) crosses the center of the crater “a” indicates its rim relief is ∼20 m. Assuming that it was fresh before burial, a rim height of ∼65 m (using the rim height to diameter relationship from Roddy [1978] for Meteor Crater) and an ejecta thickness of ∼45 m are estimated. Estimating an ejecta thickness at crater “b” requires scaling its rim height from its diameter [Pike, 1977] (no MOLA trace cross it) and suggests an initial rim height of ∼96 m. Considering the steep appearance of the outer slopes of the rim (assuming them between ∼20°–25°) then the present relief of the rim is ∼42–55 m. As a result the ejecta deposit thickness is estimated to be ∼48 ± 6 m, consistent with that of the crater “a”. This value is also consistent with ejecta thicknesses reported by Garvin and Frawley [1998] for Martian fluidized ejecta craters the size of Bacolor.

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Figure 6. Inner ejecta layer of Bacolor crater (see Figure 4 for location). Crater “a” is ∼1.2 km in diameter, while crater “b” is ∼1.75 km diameter. Using the predicted shape of these craters compared with their present shape suggests that they have been partially buried by ∼45 m of ejecta. This is part of THEMIS image V09670012.

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[18] Outward from the moat, starting at 0.3–0.5 R, the inner ejecta layer of DLE craters typically thickens into a broad annular ridge that resembles a broad rampart (see Figure 5). For Bacolor, ejecta near the outer edge of this annular ridge is ∼200 m thick. On large DLE craters (i.e., >∼15 km diameter) this ridge abruptly terminates in a steep scarp of jumbled and elongate blocks. However, on smaller DLE craters the scarp is lower relief and such blocks are uncommon.

[19] This distribution of ejecta in the inner layer is illustrated by the ejecta thickness function (ETF) [McGetchin et al., 1973; Garvin and Frawley, 1998] for DLE, compared with other types of craters. The ETF describes how ejecta thin away from the parent crater. Garvin and Frawley [1998] found that the exponent of the ETF function (that describes the rate of change in ejecta thickness outward) was commonly larger for Martian craters (average ∼−4, but has considerable variability) than predicted by the equations of McGetchin et al. [1973] for ballistic ejecta (i.e., −3 ± 0.5). Garvin and Frawley [1998] suggested that this was the result of the more rapid thinning of ejecta away from fluidized ejecta craters than away from ballistic ejecta craters. The measurements of Garvin and Frawley [1998] are consistent with the ETF exponent of ∼−4.5 we found for Bacolor, out to ∼0.5 R of the rim (i.e., the outer edge of the moat of DLE craters). However, outward from the outer edge of the moat of Bacolor to the outer edge of its inner ejecta layer (at ∼1R) the ETF exponent becomes positive as ejecta thickness increases. In addition, as much as 25% of the total ejecta may be in this outer ring of material in the inner ejecta layer of large DLE craters.

[20] One of the most prominent and diagnostic features of the inner ejecta layer is its radial texture. This texture is unique to DLE craters and is composed of closely spaced, straight, flat-floored, steep-sided grooves cut into the surface of the inner layer (Figure 7). These features commonly start on the ejecta flap deposits (sometimes even on the steep slopes below the crater rim crest) and extend across the entire inner ejecta layer and many times connect uninterrupted with morphologically different radial grooves and ridges on the outer ejecta layer (see Figure 1). This suggests that the same process produced the radial features on both layers and operated continuously from the rim (or very near it) out to the outer edge of the outer ejecta layer.

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Figure 7. Striations (grooves and ridges) that have developed near the rim of Bacolor. Such features are common on the rims of most DLE craters and suggest that the process that formed them operated from the rim outward. In addition to the radial groove and ridges, sets of troughs develop roughly concentric to the center of the crater on DLE craters. Individual trough segments are generally several hundred meters across and 1–3 km long. In places the grooves cut across the troughs indicating that the troughs formed before the grooves formed. This is part of THEMIS image V12141005. See Figure 4 for location. Inset shows higher-resolution view of the same striations on the ejecta blanket. Mars Orbiter Camera (MOC) image E1003462 is 6.4 m/pixel resolution.

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[21] Individual grooves can be as wide as 300 m, and on the basis of estimates from MOLA profile data, a few tens (generally < 20 m) of meters deep, or as narrow as a few tens of meters and a few meters deep (“Grooves” in Figure 7). The depth of these features appears to be directly related to their width. In addition, the slopes along the sides of the groove are typically relatively steep. Locally, the tops of the ridges in between the grooves are typically flat and appear to be at the same general elevation level on either side of a particular groove, no matter the size of the groove or the width of the ridge. These relationships suggest that the relief on the radial features is a result of erosion of the grooves into the ejecta deposit of the inner layer instead of deposition of materials to form the ridges.

[22] In addition, within the inner ejecta layer, the grooves and ridges cross small preexisting features such as small impact craters (see Figure 6) or hills without changing direction. For large obstacles, such as the hill ∼0.4 R northwest of the rim of the 16 km diameter crater shown in Figure 8a (from its morphology and distance from the crater rim this hill appears to be a mesa like other mesas found in that region and not a block of displaced rim), most of the radial grooves and ridges terminate partly up its craterward side and others continue again a short distance on the other side, but maintain the radial direction (Figure 8b.). The grooves and ridges close to the mesa, on the craterward side, are <50 m wide and <100 m apart. There is no evidence for ejecta deposits on top of this obstacle, although, smooth material is found on the lee side of this mesa. This smooth material is most likely a deposit formed by deceleration of ejecta in the shadow of the obstacle. Considering the momentum of ejecta flowing across the surface (modeled as the velocity required for momentum to roll a rounded meter diameter basalt boulder to the summit of the hill), in order for these obstacles to be topped by the ejecta, it must have exceeded 50 m/s, indicating that ejecta flow must have been at least that velocity. Such an interpretation is consistent with the observation of Baloga et al. [2005].

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Figure 8a. A 16 km diameter crater in W. Utopia Planitia (located at 35.3°N, 88.1°E) illustrating two examples of ejecta flow diverted by preexisting obstacles. The locations of Figures 8b and 11a, which provide higher-resolution views, are shown. This is a mosaic of THEMIS images I09721009 and I10657011.

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Figure 8b. An ∼5 km long and ∼50–75 m high hill ∼0.4 R from the rim of the crater (see Figure 8a for location). Striations run up to this hill and become subdued, or die out, over it but resume on the downrange side (labeled “a”). Ejecta also appear to have been deposited as an apron in the lee of the hill (labeled “b”). This is THEMIS image V10657012.

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[23] In addition to these radial structures, there are features that form in roughly concentric patterns with the crater center. Some of these features are graben-like, flat-floored, steep-sided troughs ∼300–400 m wide and ∼20–30 m deep that develop most commonly in the moat area. They are ∼1–3 km long (“Troughs” in Figure 7), but typically interconnect to form chevron or scallop patterns. In places, they grade into outward facing scarps a few to tens of meters high. The radial grooves and ridges cut across the floors and walls of these troughs (“Grooves” in Figure 7) indicating that the troughs formed before the radial striations, but how much before cannot be determined. The generally concentric nature and graben-like morphology of the troughs suggests that they may be extensional deceleration features produced as the ejecta halted. Alternatively, these features could be deflation features produced as the ejecta out gassed, although this requires outgassing to be extraordinarily rapid in order to occur before groove formation. These features are not observed in the ejecta of other types of fluidized ejecta craters and may indicate differences in rheological behavior between the ejecta of the inner layer of DLE craters and other types of craters.

[24] In addition to these troughs, on some craters like Arandas (43°N, 344°E), the outer edge of the inner ejecta layer has developed sets of ridges that follow the outline of the edge of the ejecta layer and wrap around preexisting obstacles [Carr et al., 1977] (Figures 9a and 9b). The geometry of these ridges suggests that they are produced as the ejecta flowed around and piled up against the obstacles shortly before it stopped. Consequently, these features appear to be compression features similar to pressure ridges.

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Figure 9a. Ridges on some DLE craters such as Arandas (24.8 km, located at 42.4°N, 345.0°E). The box shows the location of Figure 9b. This is a mosaic of THEMIS images V03346003, V09812010, V11035005, V11347008, V12882007, V13194006, and V13793015.

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Figure 9b. Detailed view of the ridges (arrowed) upslope of preexisting craters. These ridges are concentric with the edge of the inner ejecta layer and wrap around preexisting features such as the crater at bottom left. These are most likely to be pressure ridges that developed as the leading edge of the ejecta began to slow and the materials accumulated upslope. This is THEMIS image V09812010.

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[25] The inner ejecta layer of larger DLE craters (i.e., >∼15 km diameter) commonly terminates in a scarp of hummocks, jumbled and elongate blocks of ejecta (“a” in Figure 10), while for smaller DLE craters (i.e., <∼15 km diameter) this zone may have not developed (see Figure 1). Typically, the width of this zone of jumbled blocks is proportional to crater size. On large DLE craters such as Bacolor, this zone can be as much as 6 km wide, with >100 m of relief above the lower outer ejecta layer. Most of the grooves on the inner ejecta layer terminate at this zone indicating that this zone formed after the grooves, although how long afterward cannot be determined form the data. On small DLE craters where this zone is absent or not well developed, the radial grooves frequently cross it uninterrupted to the outer layer (Figure 1).

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Figure 10. Transition zone at Bacolor between the inner ejecta layer and the outer ejecta layer abruptly terminating outward in scarps over 100 m high. This zone typically includes elongate blocks of ejecta concentric with the edge of the zone, suggesting that they may be slump blocks that indicate failure of the slopes along the scarp (labeled “a”). In a few places, small patches of ejecta with radial grooves have been preserved in this zone in spite of the slumping that has occurred along the scarp (labeled “b”). While most radial grooves terminate in this zone, suggesting slumping occurred after formation of the grooves, a few of the largest grooves have been preserved and can be traced across the zone and out into the outer layer. This is part of THEMIS image V12141005. See Figure 4 for location.

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[26] Some of the largest of the blocks along this scarp form ridges several hundred meters wide and several kilometers long that parallel the outline of the scarp. The nature of the large blocks suggests slope failure along the scarp, causing sliding and slumping. This implies that the mechanical properties of the ejecta were first sufficient to bring to rest a thick layer of ejecta, but later at some unspecified time the strength of this layer was exceeded by the shear stress exerted by its own weight and it slumped. Such slumps and slides have been identified from Viking images for the MLE crater Bamburg [Mouginis-Mark, 1979b], but THEMIS data reveal that this slumping is much more common at many other craters.

[27] Occasionally, preexisting topographic features such as small mesas or craters are found in this disrupted zone. The way the ejecta flowed around or over such features provide information about the flow properties of the ejecta at that distance from the crater [Baloga et al., 2005]. For example, the double-layered mesa located ∼11.8 km (1.36 R) from the rim crest of a 17.3 km diameter crater in western Utopia Planitia (at 36.1°N, 87.9°E) (Figure 11) is in the outer edge of the disrupted zone between ejecta layers. MOLA data (orbit 15435) show that this mesa has a maximum height of ∼373 m above the surrounding terrain, with the lower bench of the mesa at ∼150 above the surrounding terrain. The mesa has created a barrier to the radial flow of the ejecta with the inner ejecta layer butting up against it on the side facing the crater up to the level of the lower bench. Thin deposits of ejecta can be traced across the bench and diverted around the upper part of the mesa, but little, if any, is found on top or on the lee side of either level of the mesa (Figure 10 MOC image). This suggests that the outward flow of the inner ejecta layer halted at the mesa, while ejecta of the outer ejecta layer flowed around, but not over or fell from above on to the summit of the mesa. This implies that the velocity of the ejecta at the boundary between ejecta layers was too low for momentum to carry it over the summit of the upper mesa, or a velocity of the <∼75 and 100 m/s (modeled as the velocity required for momentum to roll a rounded meter diameter basalt boulder to the summit of the hill). This is consistent with that found by Baloga et al. [2005] for SLE and MLE craters and is a factor of 2–3 times slower than if they transported along ballistic arc to this location. It should also be kept in mind that while ejecta deposits are not apparent on the summit of the mesa, it is also possible that ejecta may have been deposited there but drained off or are too thin (or scours too small) to be observed with MOC images of ∼7 m/pixel.

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Figure 11. (a) A 17.3 km diameter crater within W. Utopia Planitia (at 36.1°N, 87.9°E) clearly demonstrating the ability of preexisting obstacles to divert the ejecta flow from DLE craters in the zone between ejecta layers. This is THEMIS image V10345016. (b) High-resolution MOC image showing that while the outer layer of ejecta can be traced over the top of the bench of this obstacle and diverted around the upper part of the mesa, no ejecta or scours are found on the summit of the obstacle. This is part of MOC image E1200669 with a resolution of 6.4 m/pixel.

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2.3. Outer Ejecta Layer

[28] From Viking images, the outer ejecta layer of DLE craters was interpreted to be morphologically similar to the MLE craters ejecta [e.g., Carr et al., 1977], but this no longer appears to be the case, based on our analysis of the THEMIS, MOLA and MOC data. The new data have revealed that, like the inner ejecta layer, the outer ejecta layer has unique attributes that distinguish it from ejecta layers of other fluidized ejecta craters as well as from the inner ejecta layer.

[29] Like the inner ejecta layer, one of the most distinctive characteristics of the outer ejecta layer is its radial texture. This texture is composed of numerous radial grooves, troughs, and ridges (Figure 12). Frequently, the troughs are bounded with levee-like ridges that give them a leveed channel appearance that may have formed as a result of the low-velocity flow of ejecta. These landforms can be as much as 150 m across and a few meters to tens of meters deep, nearly that of the thickness of the ejecta deposit.

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Figure 12. Flow paths (labeled “a”) within the outer ejecta layer of Bacolor showing that, in some instances, the flow is not exactly radial to the parent crater. Local obstacles or topographic depressions divert the flow. In the right center part of the image a small tongue of ejecta (labeled “b”) extends past a major lobate ejecta scarp, indicating a complicated flow history during emplacement of the ejecta. This is part of THEMIS image V05451015. See Figure 4 for location.

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[30] While the direction of the grooves on the inner ejecta layer is relatively unaffected by the presence of preexisting obstacles, the radial texture of the outer ejecta layer appears to curve around preexisting topographic features (“a” in Figure 12), although frequently the radial texture also can be traced over low-relief obstacles. In order for the ejecta to develop flow lines with these characteristics, it must be traveling as a near surface flow at relatively low velocity. This is consistent with the channel-like morphology and the relief of these features relative to the layer thickness, and suggests that the radial features are produced by deposition and not erosion like the grooves on the inner ejecta layer.

[31] MOLA-derived topographic data indicate that the outer ejecta layer is commonly only a few meters to tens of meters thick and varies little in thickness (see Figure 4) much like the fluidized ejecta deposits around SLE and MLE craters [Garvin and Frawley 1998; Garvin et al., 2003; Barnouin-Jha et al., 2005; Baloga et al., 2005]. In addition, the outer ejecta layer commonly thickens to a low rampart-like ridge near the outer edge where it terminates in low, lobate scarps. Some of these lobes overlap indicating a complex emplacement history within this layer. In contrast to rampart craters (i.e., SLE and MLE craters) that have pronounced distal ramparts that may be 80–170 m high around large craters [Garvin and Frawley, 1998; Mouginis-Mark and Baloga, 2006], MOLA profiles (Figures 13a and 13b) reveal that, even on large DLE craters such as Bacolor, the height of the DLE rampart is considerably lower, <35 m, and may be as low as 20 m.

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Figure 13a. Distal southern margin of the Bacolor ejecta blanket showing the location of the four MOLA profiles (labeled “a”–“d”) illustrated in Figure 13b. These MOLA data confirm that DLE craters, while they still have distal ramparts, have much less pronounced relief than either SLE or MLE craters. This is a mosaic of THEMIS images V11829005 and V12141005.

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Figure 13b. Profiles of the distal edge of the ejecta blanket of Bacolor shown in Figure 13a. Maximum height of the scarp at the edge of the ejecta is ∼35 m, and craterward (to the left) from the rampart the outer ejecta layer is of the order of 10 m thick. Topographic data derived from the following MOLA orbits (a) 15195, (b) 19972, (c) 13560, and (d) 12818.

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[32] In a few locations, thin subtle, ridge-like features extend off and outward radial from the lobate edge of the outer layer (see “b” in Figure 12). These features are typically a few tens of meters wide with a few meters of relief (judging from their shadows), and extend for a few kilometers or less from the edge of the outer ejecta layer. On Bacolor, these ridge-like features appear to become more subdued and grade into faint radial texture that may extend outward for nearly a crater radius from the outer edge of the outer ejecta layer. However, because these features are so small, their exact size and extent away from the outer edge of the outer ejecta layer cannot be determined. This outer ejecta layer may be caused by flow separation, as predicted by the model of Schultz [1992] or observed at Mount St. Helens [Kieffer, 1981] where the winds generated by the advancing flow carried fine grain materials up and over the near surface atmosphere as the ground hugging debris halted.

[33] These subtle ridge-like features may be the erosional remnants of a thin, easily eroded, outer ejecta deposit such as that observed on some of the freshest DLE craters found in high-latitude regions of Mars (Figure 14). In THEMIS images taken at 36 m/pixel resolution, IR images taken at ∼100 m/pixel resolution and MOLA profiles, such fresh craters have the attributes common to all other DLE craters (e.g., inner ejecta layer with a pronounced inner moat and broad rampart whose surface is cut by straight grooves cut into the inner layer, lack of secondary craters outward form the continuous ejecta), but with the addition of a thin (∼10–25 m thick based on MOLA profile data), relatively smooth ejecta deposit that extends outward from the outer edge of the second ejecta layer (Figure 14). The outer edge of this highly irregularly shaped ejecta layer extends from the outer edge of the second ejecta layer to an average of ∼6–8 R from the crater rim. In addition, there are narrow strands of ejecta that extend to ∼10–12 R from the crater rim. This ejecta layer terminates in a low, steep scarp, but does not appear to have distal rampart ridges.

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Figure 14. Fresh, high-latitude impact crater having the same general morphologic characteristics as all DLE craters but including a thin (uniformly thick at <∼10–25 m), highly irregularly shaped, outer ejecta layer located beyond the second ejecta layer. The crater is 13.0 km in diameter and 1201 m deep and is located at 72°N, 38°E. Boxes show the locations of enlargements in Figure 15. This is a mosaic of THEMIS VIS images V10709002, V11358008, V09798018, and V10085009 and IR image I03669002. The resolution of the VIS images is ∼36 m/pixel, and the IR image is ∼100 m/pixel.

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[34] Remarkably, in places these outer ejecta deposits are dissected by chains of shallow elongate pits oriented approximately radial to the crater (arrows Figure 15). No such pits are found beyond this ejecta layer on the surrounding terrain. The depths of the pits appear to be no deeper than the thickness of the ejecta layer and consequently would be completely erased if the deposits were removed by erosion. While these pits have the appearance of secondary craters, their development on the outer ejecta deposit is inconsistent with current models for the timing of fluidized ejecta emplacement and secondary crater formation [e.g., Melosh, 1989; Mouginis-Mark et al., 2003; Mouginis-Mark and Baloga, 2006] and would be expected to have formed first and be overridden by the continuous ejecta deposits. However, if these pits are secondary craters the ejecta blocks that formed them appear to have been capable of excavating this thin ejecta layer but not the material beneath and/or beyond. This suggests that these ejecta blocks were composed of weak, easily disaggregated materials. Moreover, we suggest that the apparent ease at which this outer ejecta layer erodes compared with the other two ejecta layers (that remain around older craters after its disappearance) is also most consistent with this outer ejecta layer being composed of fine-grain, loosely consolidated, easily eroded materials. Alternatively the pits could be sublimation pits (i.e., alases) or some other type of erosional feature or these craters could be a new type of multilayered crater found only in high-latitude regions. Higher-resolution images (i.e., 18 m/pixel THEMIS, HiRISE and MOC images) of these craters and their ejecta deposits are required to confirm their exact nature.

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Figure 15. Chains of pits (arrows) found in the outer ejecta deposit but not beyond of the crater. These pits lead away from the center of the crater and may be secondary craters from this crater. THEMIS VIS images (a) V11358008, (b) V10709002, and (c) V10709002. The resolution of these VIS images is ∼36 m/pixel. See Figure 14 for the location of these images.

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2.4. Secondary Craters

[35] One of the most remarkable features of DLE craters is that they do not appear to have secondary craters, at least out to ∼11 R from the rim. Even though it had been suggested from Viking-based observations, that at least one DLE crater (i.e., Arandas crater) may have distant secondary craters [Schultz and Singer, 1980], new higher-resolution THEMIS VIS data do not support this contention. In contrast, most fresh SLE and MLE craters (see Figures 3 and 12) show secondary craters similar to those around primary lunar craters [Mouginis-Mark et al., 2003]. Furthermore, we can find no circular depressions within the ejecta layers that suggest that secondary craters were initially formed but were then buried during the outward radial flow of the ejecta.

[36] It is possible that DLE craters formed secondary craters, but they occur beyond 11 crater radii away from the primary crater. While such a relationship between the primary and its secondary is not observed on other planets where high-resolution images of fresh craters are available, it would not be a unique situation on Mars for secondary craters to be absent close to the primary. For example, in the case of the crater Zunil, which has abundant secondary craters, few are found within less than a ∼10 R [McEwen et al., 2005]. This may also be the case for DLE craters, but until additional THEMIS data are collected around fresh DLE craters we will be unable to determine with certainty if any DLE craters produce secondary craters.

2.5. Morphometry of DLE Crater Ejecta

[37] The unique geometry of DLE crater ejecta deposits, as illustrated by the profiles in Figure 3, shows important differences and similarities compared to the ejecta of other types of craters. These provide insight into ejecta emplacement mechanisms. For example, the general shape and volume of the inner ejecta layer of DLE craters are similar to ejecta around SLE craters, while the general shape of the outer ejecta layer of DLE craters and MLE craters ejecta and total ejecta volumes are also similar. Such similarities in distribution of mass may point to similarities in the mechanism of transport of mass around these craters.

[38] In particular, similarities in the overall shape of the inner ejecta layers of DLE craters and SLE craters ejecta (i.e., rapidly thinning ejecta outward to a terminal rampart) suggest similarities in their emplacement. In addition, the relatively short runout distance (i.e., 1–2 R) of the inner ejecta layers of DLE craters and the continuous ejecta of SLE craters (compared with ballistic ejecta from craters of the same size) are counter to the expectation that fluidized ejecta should be relatively more fluid and run out farther than ballistic ejecta. This is most likely a result of different emplacement regimes of these different types of ejecta. Ballistic ejecta are generally emplaced through ballistic sedimentation processes [Oberbeck, 1975], while the fluidized ejecta of SLE and DLE craters may be emplaced through a process involving flow [e.g., Wohletz and Sheridan, 1983; Schultz, 1992; Barnouin-Jha et al., 2005]. Several mechanisms have been proposed for producing relatively short runout distance for fluidized ejecta. These mechanisms generally involve unique target material or atmosphere characteristic.

[39] There is evidence that the concentration of volatiles in the target, and the atmospheric pressure during crater formation may play a key role in controlling the processes of emplacement of fluidized ejecta, while target material characteristics (lithology i.e., initial grain size and coherence) may also be of importance. Wohletz and Sheridan [1983] examined data from dry and wet vapor explosion experiments and suggested that stubby ejecta flows are to be expected for craters where targets contain relatively little water. This is because less water in the ejecta generally meant more friction between particles during flow and as a result shorter transport distances. While this mechanism may predict shorter runout distance for fluidized ejecta, it does not explain how those distances can be shorter than dry ejecta emplaced ballistically. However, McSaveney and Davies [2005] have suggested another possible mechanism that could account for the short runout distance for some fluidized ejecta even though the ejecta may have relatively low viscosity. They proposed that the runout of ejecta over a thick eroded substrate, such as the layered deposits found in the midlatitudes of Mars, would quickly remove energy from the flow because of the erosive nature of flowing particulate materials. While areas like the northern lowland plains [Head et al., 1998, 1999; Boyce et al., 2005] and southern highlands [Soderblom et al., 1973; Malin and Edgett, 2001] may contain easily erodible, layered deposits, DLE craters are also found in areas that show no evidence of such deposits, such as on Tharsis (e.g., Alba Patera). In addition, Schultz [1992] has suggested that such parameters as grain size (lithology), variations in near-surface volatile content of the target and atmospheric pressure can have important effects on the runout distance of ejecta on Mars. While varying such parameters may result in relatively shorter runout distance of the inner ejecta layer, some require surface conditions on Mars to be different from craters in the same area at different times or a Martian crust with considerable lithologic heterogeneous.

[40] While runout distances are similar between SLE ejecta and the inner ejecta layer of DLE crater, the moats around the inner ejecta layer of DLE craters are narrower and the rampart broader and higher than around SLE craters. Schultz [1992, 2005] suggests that moats could be the result of intense erosion of fine-grain ejecta deposits by tornadic-strength vortices of atmospheric gas generated by an advancing ejecta curtain, although such erosion could also occur through the high-velocity outflow of particles and gas associated with base surge. Base surges, sometimes called column surges, are dense, ring-shaped cloud of ejected particles and gas generated by the collapse of a vertical explosion column of ejected fine-grain particles and gas (on Earth most of the gas is derived from the atmosphere) produced as a result of impact or explosion crater formation, and during some highly energetic explosive volcanic eruptions [Young, 1965; Gladstone and Dolan, 1977; Pohl et al., 1977; Kenkman and Schönian, 2005; Wohletz, 1998]. These turbulent, high-velocity clouds of ejecta sweep outward from the crater as a near-surface density flow and are also capable of substantial erosion. Both mechanisms are strongest and scour the surface more near the rim (i.e., in the area of the moats), probably entraining more materials as the flow outward. Outward, as the velocity in the flows drops and deposition is permitted. However, even taking into account materials eroded from the moat, for the outer ejecta layer to be mainly deposited from material carried aloft in a base surge would require nearly 30–40% of the total ejecta volume to be lofted in the explosion column. In any case, these mechanisms require that the ejecta deposits include large amounts of fine grain materials in order for the high-velocity winds to remove significant amounts of materials to produce the moats but only modestly eroding the small preexisting craters found in the moat zone (see Figure 5).

[41] Alternatively, the differences in the geometry of the moats may be a direct consequence of the nature of the materials and their mechanism of emplacement in the inner ejecta layer. For example, differences between DLE craters and SLE craters may only be that the ejecta were emplaced as a granular flow (probably after initial ballistic ejection from the crater) and that the ejecta of DLE craters contained materials with a different particle size distribution, gas concentration or grain size distributions compared with SLE crater ejecta [Wohletz and Sheridan, 1983].

[42] The outer ejecta layer of DLE craters and that of MLE craters also show similarities in overall planimetric shape. A comparison of their shapes with those of the inner ejecta layer of DLE craters and SLE craters suggests more fluid behavior because of their long runout distance. However, smaller-scale flow features, such as the radial texture, on MLE crater ejecta and ejecta of the outer ejecta layer of DLE craters are significantly different and may suggest different emplacement mechanisms. Additionally, these small-scale features (e.g., flow lines) suggest that the two ejecta layers of DLE craters were each emplaced by different mechanisms, while all ejecta layers of MLE craters appear to have been emplaced by the same process.

3. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Morphology of the Ejecta of DLE Craters
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[43] The new THEMIS VIS images have allowed the recognition of unique morphologic characteristics of DLE craters that provide insight into the modes of emplacement of their ejecta. We suggest that the most important of these are the nature and extent of the radial texture, and the lack of secondary craters. All these provide intriguing information about the ejecta of these craters that will be discussed in more detailed below.

3.1. Radial Texture

[44] While the ejecta deposits surrounding SLE and MLE craters show some radial flow lines the distinctive radial texture of DLE crater ejecta is unique to this type of crater. All evidence indicates that these radial features were produced during ejecta emplacement and are a result of their unique morphologic characteristics. These attributes provide insight about the dynamic environment during ejecta emplacement and how it may differ from that of SLE and MLE crater ejecta emplacement. As a starting point in understanding the development of these features, we have focused on terrestrial analogs, and in particular morphologic features around explosion craters and explosive volcanoes that are remarkably similar to the radial texture found on DLE crater ejecta. These radial features are produced by surges, the high-velocity outward flow of solid particles and gas from some explosive volcanoes, explosions or impact events. Most observed surges are base, but in a few instances where explosive volcanic eruptions involved large amounts of water (in many cases are steam explosions) blast surges are produced. These surges typically are produced by the explosive radial expansion of ejected particles and high-pressure gases and may be followed by base surge of materials they have ejected. Both types of surges can produce distinctive radial features.

[45] One of the most studied and best examples of the development of radial features that are similar to those on DLE crater ejecta is the blast deposit that was produced during the May 18, 1980 eruption of Mount St. Helens. This eruption produced a blast surge from a lateral explosion that, according to Kieffer [1981], was a steam-driven multiphase expansion wave that accelerated a mixture of vapor and solid particles from the vent down slope at high velocity. This produced an inner blast zone close to the vent, in which flow was approximately radial to the mountain and whose streamlines (i.e., grooves and ridges) were undeflected by large topographic features. This geometry is similar to characteristics of the grooves in the inner ejecta layer of DLE craters. Remarkably, the streamlines at Mount St. Helens abruptly changed in character in an outer zone where they were deflected by local topographic features [Kieffer, 1981; Hoblitt et al., 1981]. This behavior is similar to the radial features in the outer ejecta layer of DLE craters. Kieffer [1981] proposed that this change in characteristic is the result of a change on the internal speed within the flow from supersonic to subsonic (note that the speed of sound in such multiphase flow is not the same as the speed of sound in the atmosphere around it or the velocity of the flow over the surface). She points out that streamlines produced in subsonic flow are diverted smoothly around obstacles because signals of the obstacle's presence are propagated upstream into the flow, while supersonic flow is diverted around obstacles through shock waves that are oriented downstream from the flow, because the obstacle cannot propagate shock waves upstream. Because of the distance and time needed for spatial and temporal disturbances to propagate into supersonic flow, it tends to be less affected by topographic features than does subsonic flow.

[46] Kieffer and Sturtevant [1988] also studied the erosional development of the furrows found radial to Mount St. Helens in the blast inner zone. They found that while the surge in the inner zone surmounted all topographic features as predicted for such supersonic flow by Kieffer [1981], it left few traces on top of these obstacles producing only small-scale scours up range and thin leeward deceleration deposits, similar to the features shown in Figures 8a and 8b. Kieffer and Sturtevant [1988] suggested that the radial furrows on the inner blast zone were produced by longitudinal vortices resulting from flow instabilities in the boundary layer induced by complex topography and by the cross-flow component of flow subparallel to ridge crests. The diameter and transverse spacing of these vortices was inferred from the distance between furrows to be on the order of the boundary layer thickness (only a few tens of meters). These furrows have approximately the same dimensions as the grooves on most DLE craters. In addition, because the vortices modeled by Kieffer and Sturtevant [1988] are produced by the near surface outward high-velocity flow of particles and gas, presumably they behave in a similar manner to those predicted by Schultz [1992] and Barnouin-Jha et al. [1999a, 1999b] to be produced in the Martian atmosphere by an advancing ejecta curtain. Assuming the similarity in morphology of the radial texture with the streamlines around Mount St. Helens is the result of the same process, then the vortices that produced the radial texture of DLE craters may have been nearly the same size or a little larger.

[47] Furthermore, Wohletz [1998] suggests that in the supersonic parts of surges (and multiphase flows, in general), like in the inner blast zone of Mount St. Helens, erosion would dominate because particle relaxation is too slow to adjust to topographic variability. As a result, particles do not stick to the substrate but rather tend to erode it, whereas for subsonic surges topographic variability influences particle deposition, and deposition is often favored. This is consistent with the deposits left by the blast surge around Mount St. Helens as well as other surge deposits where the surge velocity was particularly high [Hoblitt et al., 1981; Wohletz, 1998]. Using this as a guide, we suggest that the radial texture on the inner ejecta layer is most likely the result of supersonic flow that eroded the grooves into the inner ejecta layer, and as the flow dropped internally to subsonic velocities it deposited materials beyond that layer.

[48] It should also be noted that even though the velocity within a multiphase flow may, like that in the inner blast zone at Mount St Helens, exceed the bulk speed of sound, the velocities of such flows relative to the surface they flow across typically are much less than the speed of sound in air. This is because the sound speed in homogenous two-phase mixtures is, in part, dependent on particle population density [Pai, 1977; Kieffer, 1981; Wohletz, 1998]. As a result, the supersonic velocity obtained in a surge generally is far less that the ballistic velocity of ejected materials. Consequently, for flows with high concentrations of particles such as most volcanic surges and impact crater ejecta, the speed of sound can be as slow as only a few tens of meters/second. This is most likely the cause of the apparent relatively low velocity (<100 m/s) of ejecta at the distance of the boundary between the inner and outer ejecta layers of the crater shown in Figure 11 and could explain the finding of Baloga et al. [2005] that the ejecta of SLE and MLE craters were also emplaced at relatively low velocity (i.e., ∼27–116 m/s). This also strengthens the argument that the production of the radial texture was by a multiphase flow of ejecta particles and gas that was initially internally supersonic but dropped to subsonic velocities at some distance from the source.

[49] Added to this argument is the observation that the radial texture on the outer ejecta layer curves around topographic features, which is consistent with emplacement of materials from a subsonic flow (i.e., such as that in the outer zone of Mount St. Helens). In addition, this is the zone where deposition dominates in terrestrial surges and where longitudinal dunes commonly form by vortices that spin perpendicular to the flow as the surge moves over and around a topographic barrier [Gutmann and Sheridan, 1978; Wohletz, 1998]. We suggest that the deflection of the radial texture around topographic features, the leveed nature of many of the troughs and their large size relative to the thickness of the outer ejecta layer suggests deposition of materials from a subsonic flow simultaneous with development of the radial features. The atmospheric/ejecta interaction model of Schultz [1992] and Barnouin-Jha et al. [1999a, 1999b] could also explain these observations. In their model, the outer ejecta layer is the zone where flow separation occurs behind the ejecta curtain causing the ejecta entrained in the flow to run over and beyond the inner ejecta layer. The velocity of the entrained ejecta most likely rapidly drops when the bulk of the ejecta curtain that generates the vortices has been deposited, and may explain the abrupt morphologic changes of the radial texture at the outer edge of the inner ejecta layer of DLE craters. Consequently, either emplacement mechanism could result in radial features similar to those found on both ejecta layers of DLE craters, but the atmospheric/ejecta interaction model may explain what controls the location of the supersonic/subsonic flow boundary.

[50] The timing of the development of the radial features and the degree of erosion associated with their development also provides insight into the emplacement of the ejecta of DLE craters. For example, the observations that the radial grooves cut the concentric troughs on the inner ejecta layer, and ejecta flap near the rim suggest that the inner ejecta layer had been deposited and its surface stabilized when the grooves formed. While this idea may be consistent with both the base surge and atmosphere/ejecta interaction, it is inconsistent with a blast surge. We expect that for a blast surge to produce such erosion requires generation of large amounts of gas from the target past the end of crater excavation when impact energies, and hence the production of gas from subsurface volatiles (if present) was rapidly declining [Melosh, 1989]. Consequently, blast surge seems an unlikely candidate as a mechanism to produce the high-velocity outflow that scoured the radial grooves in the inner ejecta layer.

[51] While there are other ways of producing radial texture in ejecta around impact craters, (low-velocity flow or ballistic processes [Suzuki et al., 2005a, 2005b]) none fit the characteristics of the radial features around DLE craters better than high-velocity multiphase flow. For example, for ejecta emplacement processes that also produce radial features [Piekutowski, 1977] the resultant radial features are mainly depositional features ridges instead of erosional scours like those found on the inner ejecta layer of DLE craters. In other relatively lower velocity flow processes such as those in landslides where striations commonly form in the direction of flow [e.g., Shreve, 1966; Erismann and Abele, 2001; McSaveney, 2002], these features lack the abrupt change in morphologic character of the radial features on DLE crater ejecta. This suggests a significant difference in flow velocity, if not in process for the DLE craters.

[52] Consequently, we suggest that the nature of the radial texture indicates that the ejecta emplacement process around DLE craters included a high-velocity radial outward flow of solid particles and gas component. This outflow was internally supersonic resulting in scouring out of the grooves in the inner ejecta layer and possibly also producing the moat, while outward its velocity dropped permitting deposition of entrained materials in the outer ejecta layer. The most likely cause for this outflow is high-velocity winds produced by base surge, or by the advancing ejecta curtain.

3.2. Why No Secondary Craters?

[53] As a general rule on planets, impact crater excavation produces large blocks of material ejected from the near-surface zone by stress wave/spall interaction [Melosh, 1989] in ballistic arcs. These large blocks impact the surface beyond the continuous ejecta blanket to form secondary impact craters. Martian DLE craters appear to violate this general rule, and have no observed secondary craters. Other Martian impact craters, even other types of fluidized ejecta craters produce secondary craters [Mouginis-Mark 1979b; Schultz and Singer, 1980; Barlow and Perez, 2003; Mouginis-Mark et al., 2003; McEwen et al., 2005] although Zunil, a SLE crater, produced secondary craters that begin outward of ∼10 radii [McEwen et al., 2005]. Consequently, it is possible that DLE craters produce secondary craters but that they have not been identified yet.

[54] The lack of observed secondary craters around DLE craters does not rule out the ejection of large ejecta blocks. A possible reason for the lack of secondary craters could be the nature of the blocks ejected by DLE craters that prevent them from producing secondary craters. For example, highly fragmented blocks would be ineffective at producing secondary craters, such as is suggested in Figure 15. Such weak and fragmented blocks could be the produced if (1) the target materials were composed of already fragmented materials, such as clastic sedimentary rocks [Mouginis-Mark, 1981], and (2) water was contained in the target materials at the time of impact [Wohletz and Sheridan, 1983; Kieffer and Simonds, 1980]. Alternatively, the blocks could have been crushed by the dynamic pressure in the high-velocity outflow of gas and particles during ejection [Vickery, 1986].

[55] In first considering the affects of the nature of the target material, we note that there are areas of thick sequences of lava flows (e.g., Alba Patera) that contain fresh DLE craters with no secondaries located adjacent to fresh SLE and MLE craters with abundant secondaries (Figure 3). Even if there are local heterogeneities in these thick sequences of volcanic rock, such as interbedded patches of highly friable sedimentary or pyroclastic rock, the rock from the lava flows that surface these areas should produce secondary craters. This relationship suggests that rock type alone most likely is not an important factor in the suppression of secondaries because 1) all these craters have formed in the same target rock type, and 2) the target material is expected to be composed mainly of coherent rock (e.g., basaltic lava flows).

[56] Second, water in target materials has been shown to enhance fragmentation during impact events [Kieffer and Simonds, 1980; Wohletz and Sheridan, 1983]. As a result, if water or ice were in the target rock during crater formation, weak or disaggregated blocks would be produced that had little effect when they reimpacted the surface. This mechanism has important implications to Martian history, and if it is the cause of the lack of secondary craters implies that DLE craters formed in wet targets while SLE and MLE craters formed in dry (or nearly dry) targets. Moreover, because DLE, SLE and MLE craters are located adjacent to one another in the same type of terrain, for this mechanism to work either subsurface water/ice was distributed highly heterogeneously throughout the Martian regolith and/or was present sporadically.

[57] Third, it is also possible that the absence of secondary craters around DLE craters is a result of the same high-velocity outflow of particles and gas that produced their radial texture. If this outflow overtook and entrained the blocks in flight, then the dynamic pressure in the flow would cause their fragmentation [Vickery, 1986]. This outflow could be the result of winds generated by an advance ejecta curtain [Schultz, 1992; Barnouin-Jha et al., 1999a, 1999b], or a base surge and does not require additional volatiles from the target materials. The former explanation is slightly favored over the later because the ejecta-generated winds would have a better chance of catching up with the ejected blocks in flight because they are generated earlier in the ejecta emplacement event. However, this mechanism implies that the winds are much stronger than those generated during SLE and MLE craters ejecta emplacement, and as a result most likely requires higher atmospheric pressure and possibly more entrained fine-grain materials generated at the time of impact. This additional gas could be from a higher-pressure atmosphere at the time of impact or vaporization of volatiles in the target materials that produce a local transient gas cloud. No matter which, this mechanism has important implications to the volatile history of Mars, suggesting dramatic fluctuation in volatiles, either in the atmosphere and/or in the subsurface with time on Mars.

[58] Consequently, the absence of secondary craters around DLE craters suggests that volatiles (i.e., subsurface water and/or ice, and/or atmospheric gas) have played an important role in suppression of these features. The close proximity of fresh SLE, DLE and MLE craters on the same terrain suggests that the amount of volatiles (whether atmospheric or subsurface) available to participate in the cratering process must have fluctuated repeatedly over Martian history.

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Morphology of the Ejecta of DLE Craters
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[59] THEMIS images permit morphologic observations to be made that help constrain current [e.g., Schultz, 1992; Ivanov et al., 1994, 1997; Ivanov and Deutsch, 1999; Ivanov, 1996; Ivanov et al., 1997; Garvin and Baloga 1999; Steward et al., 2001; Baratoux et al., 2002, 2005; Ivanov, 2003, 2005; Fagents et al., 2005; Barnouin-Jha et al., 2005; Baloga et al., 2005; Mouginis-Mark and Baloga, 2006], and future numerical and rheologic models for ejecta emplacement. These new observations place stringent constraints on where the ejecta first hit the ground, how they moved across the preexisting landscape, and how their movement changed with distance from the crater.

[60] On the basis of observations made using these data sets, we find evidence that DLE crater ejecta are emplaced in two distinct stages, and through two different processes, at least one of which requires volatiles to operate. With the addition of new details this two-stage model is similar to the Viking-image-based model proposed by Mouginis-Mark [1981] where the inner ejecta layer was deposited before the outer ejecta layer was emplaced as thin deposits that flowed over and beyond the edges of the inner layer.

[61] We suggest that the shape and stratigraphic relationships of the distinctive radial features of DLE crater ejecta record the high-velocity outflow of materials that affected terrain from the rim of the crater to the outer edge of its continuous ejecta blanket. This outflow eroded the inner ejecta layer, possibly producing the inner moat, and was responsible for emplacement of the ejecta in the outer ejecta layer. The radial features cut across concentric, graben-like structures in the inner ejecta layer (interpreted to be deceleration features) suggesting that the ejecta of the inner layer had already been emplaced and its surface stabilized before the high-velocity outflow cut the radial features into it. Similarities in overall shape of the inner ejecta layer and SLE crater ejecta suggest similarities in ejecta emplacement process.

[62] The high-velocity outflow of materials that produced the radial texture most likely was the result of winds generated by the advancing ejecta curtain that deposited the inner ejecta layer as proposed by Schultz [1992] and Barnouin-Jha et al. [1999a, 1999b], or by a base surge developed by the collapse of an explosion column. If these winds were the product of an advancing ejecta curtain (that most likely also produced the inner ejecta layer), then the outer ejecta deposits may be the result of flow separation of the debris-laden winds that allowed the entrained ejecta to be carried outward even after emplacement of ejecta in the curtain [Schultz, 1992]. However, this detachment mechanism is not required to produce the outer ejecta layer if the winds are generated by a base surge caused by the collapse of an explosion column of debris and gas [Young, 1965; Gladstone and Dolan, 1977]. The high-velocity outflow may have also suppressed the production of secondary craters. It may have entrained and/or crushed large blocks that normally produce secondary craters, or, alternatively, water in the target materials may have caused these large blocks to fragment, hence suppressing secondary crater formation.

[63] The broad geographic distribution of DLE craters (i.e., found over a great range in elevations, terrain types and terrain ages) suggests that the formation of these craters is controlled by factors that occur globally. DLE craters are also found in close proximity with other types of fluidized ejecta craters that do not show the same type of radial texture and the lack of secondary craters that are diagnostic of DLE craters. Presumably the processes that produced these unique features of DLE craters did not operate on these other types of fluidized ejecta craters. This suggests that conditions that control which type of crater forms on Mars (i.e., DLE verse SLE or MLE craters) may fluctuate, and that the observed distribution of DLE craters could be produced by periodically higher atmospheric pressure and/or abundant subsurface water, or alternatively that the Martian crust is much more heterogeneities than expected. This heterogeneity could be the result of the nonuniform distribution (laterally and/or vertically) of subsurface water with time [Barlow and Perez, 2003].

[64] While the degree of heterogeneity has yet to be recognized on Mars [Scott and Tanaka, 1986; Greeley and Guest, 1987], recent suggestions about possible climate change [e.g., Head et al., 2000; Mustard et al., 2001; Neukum et al., 2004] on Mars raise the possibility of impact into target materials that are periodically wet or that a significantly higher pressure atmosphere may be periodically present [Barnouin-Jha and Schultz, 1998]. Such transient features on Mars might have affected the ejecta emplacement process and thus the resultant morphology of an impact crater.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Morphology of the Ejecta of DLE Craters
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[65] We thank Phil Christensen and the THEMIS Science Team for their support of this work, especially Laurel Cherednik, Kelly Bender, and Andreas Dombovari for the targeting of THEMIS and Noel Gorelick, Jim Torson, and the THEMIS software development Team at Arizona State University and the U.S. Geological Survey in Flagstaff for processing and mosaicing of THEMIS images. Harold Garbeil created the software that enabled us to colocate the MOLA profiles with the THEMIS images and wrote the software used to derive Figures 5 and 13. We would like to especially thank Olivier Barnouin-Jha for his careful and insightful review. This work was supported by NASA's Mars Data Analysis Program under grants NAG5-11362 (JMB) and NAG5-13420 (PMM). This is HIGP 1459 and SDEST Publication 6879.

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