Possible Impact Melt and Debris Flows at Tooting Crater, Mars

Home , Dilly (crater), Stokes (Martian crater), Tooting (crater)

Icarus 209 (2010) 369–389

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Possible impact melt and debris ﬂows at Tooting Crater, Mars

Aisha R. Morris a,*, Peter J. Mouginis-Mark b, Harold Garbeil b a Department of Earth Sciences, Syracuse University, 204 Heroy Geology Lab, Syracuse, NY 13244, United States b Hawaii Institute of Geophysics and Planetology, University of Hawaii, 1680 East-West Road, Honolulu, HI 96822, United States article info abstract

Article history: Candidate examples of impact melt flows and debris flows have been identified at Tooting crater, an Received 15 July 2009 extremely young (<2 Myr), 29 km diameter impact crater in Amazonis Planitia, Mars. Using HiRISE and Revised 27 May 2010 CTX images, and stereo-derived digital elevation models derived from these images, we have studied Accepted 28 May 2010 the rim and interior wall of Tooting crater to document the morphology and topography of several flow Available online 9 June 2010 features in order to constrain the potential flow formation mechanisms. Four flow types have been identified; including possible impact melt sheets and three types of debris flows. The flow features are all Keywords: located within 2 km of the rim crest on the southern rim or lie on the southern interior wall of the crater Cratering 1500 m below the rim crest. Extensive structural failure has modified the northern half of the crater Geological processes Mars, Surface inner wall and we interpret this to have resulted in the destruction of any impact melt emplaced, as well Volcanism as volatile-rich wall rock. The impact melt flows are fractured on the meter to decameter scale, have ridged, leveed lobes and flow fronts, and cover an area >6 km Â 5 km on the southern rim. The debris flows are found on both the inner wall and rim of the crater, are 1–2 km in length, and vary from a few tens of meters to >300 m in width. These flows exhibit varying morphologies, from a channelized, leveed flow with arcuate ridges in the channel, to a rubbly flow with a central channel but no obvious levees. The flows indicate that water existed within the target rocks at the time of crater formation, and that both melt and fluidized sediment was generated during this event. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction have the potential to provide information regarding the distribution of landforms that could support or refute the idea that For years impact craters on Mars have been studied in order to ejecta are fluidized by target volatiles during the cratering event examine the spatial and temporal variations in surface properties (Mouginis-Mark and Garbeil, 2007). of the target materials (Barlow and Perez, 2003; Carr et al., 1977; Recent high resolution imaging data from the Context Imager Mouginis-Mark, 1981, 1979; Osinski, 2006). Several different lo- (CTX; 6 m/pixel) (Malin et al., 2007) and the High Resolution Imag- bate ejecta morphologies are observed on Mars (Barlow et al., ing Science Experiment (HiRISE; 0.25–0.30 m/pixel) (McEwen 2000) and an ongoing debate concerns the mode of ejecta emplace- et al., 2007a) have provided an unprecedented view of the surface ment for these craters. The two proposed models are formation by geology of several pristine, geologically recent craters. This spatial fluidization of the solid ejecta through their interaction with vola- resolution has enabled detailed geomorphic and topographic anal- tiles derived from the target (Carr et al., 1977; Mouginis-Mark, ysis of features associated with the craters, and has reinforced the 1979), or by interaction of ejecta with the atmosphere (Barnouin-Jha role of volatiles in the impact cratering process (McEwen et al., and Schultz, 1998; Schultz and Gault, 1979). The former mecha- 2007b; Mouginis-Mark et al., 2007; Tornabene et al., 2007). Here nism is the most widely accepted and has been used as evidence we use these CTX and HiRISE data to identify the distribution for volatiles in the martian crust at the time of crater formation and geomorphology of several different types of lobate flow fea- (Barlow et al., 2000). In almost all cases, the age (i.e., degradation tures on the southern rim and interior wall of Tooting crater state) of the crater precludes the confident identification of land- (Fig. 1) that may relate to the distribution of volatiles. In addition forms considered to be most diagnostic of volatiles being in the tar- to the image data, we have produced several CTX and HiRISE ste- get at the time of crater formation. However, the study of the reo-derived digital elevation models (DEMs) of the interior and impact ejecta of very young, large craters on Mars is believed to southwest rims of Tooting crater where particularly interesting flows can be found. These elevation data (see Appendix A for a description of their derivation) enable the slope of the rim units * Corresponding author. Fax: +1 315 443 3363. on which the flows formed to be determined, as well as the eleva- E-mail address: [email protected] (A.R. Morris). tion of the flows relative to the rim crest.

Fig. 1. Mosaic of THEMIS VIS images detailing Tooting crater and surrounding ejecta blanket. Inset image shows black arrow pointing to the location of Tooting crater on Amazonis lava ﬂows to the west of the Olympus Mons aureole. Tooting crater is located at 23.4°N and 207.5°E. The Interior crater cavity is 29 km across. The white arrow points in the inferred downrange direction (Mouginis-Mark et al., 2007). The white box outlines a depression adjacent to braided channels, which may have served as a source for volatiles prior to the impact cratering event.

In this analysis, we first describe the distribution, morphology valid to the meter scale, despite the presence of a dusty mantle and topography of four types of flows we have identified on the in this region of Mars. rim of Tooting crater, and then focus on the topographic analysis of a specific flow to evaluate its mode of formation and relationship to the cratering event itself. We propose that the results of our 2. Possible age and geometry of Tooting crater analysis of the four flow types will contribute to understanding the flow formation sequence, provide information on the apparent Tooting crater is 29 km in diameter, is located at 23.4°N, preferential distribution of the flows, and elucidate the role of vol- 207.5°E, and is classified as a multi-layered ejecta crater (Barlow atiles in the crater formation and ejecta emplacement. There are et al., 2000). Inspection of THEMIS VIS images of Tooting crater two mechanisms proposed for the origin of the flows described reveals that there are 13 smaller impact craters P54 m in diameter in the following sections. The first mechanism is a formation due superposed on the ejecta blanket, which has an area of 8120 km2 to the flow of impact melt formed from the target rock due to (Mouginis-Mark and Garbeil, 2007). Using the 2004 iteration of the shock wave compression and release during the impact event martian crater-count isochron (Hartmann, 2005; his Table 2) gives (Melosh, 1989). The second proposed formation mechanism is the an age for Tooting crater that is <2 Myr. mobilization of impact ejecta either by the incorporation of vola- Tooting crater formed on virtually flat, young, layered basalt tiles during the impact event, or by the release of volatiles into lava flows within Amazonis Planitia (Tanaka et al., 2005), where the newly formed crater cavity that subsequently mix with and there appear to have been no major topographic features prior to remobilize the already emplaced ejecta. We note that the small- the impact event (Mouginis-Mark and Garbeil, 2007). The depth est-scale morphology of Tooting crater may be muted by the pres- of the crater and the thickness of the ejecta blanket have been ence of cm-thick dust, but we do note that our observations are determined by subtracting the elevation of the surrounding A.R. Morris et al. / Icarus 209 (2010) 369–389 371

Table 1 The measured depth of the crater floor below the northern rim Summary of morphometric data for Tooting crater. Data are a combination of crest was determined by Mouginis-Mark and Garbeil (2007) to measurements of the crater geometry and ejecta by Mouginis-Mark and Garbeil be 1567 m and the measured depth of the crater floor below the (2007), and updated values using the DEM derived from CTX images. southern rim crest is 1851 m. These rim height and crater depth Location 23.4°N 207.5°E numbers can now be updated using the CTX DEM and are Diameter 29 km 1871 m and 2257 m respectively (Table 1). The width of the final Area of ejecta blanket 8120 km2 Rim height 367 m (min) 981 m (max) crater cavity appears to have been increased by structural failure Rim height/diameter ratio 0.012 (min) 0.034 (max) (large-scale slumping to produce 2 km wide terrace blocks Crater depth 1871 m (north) 2257 m (south) 900–1500 m below the rim crest) on the north/northeastern half Depth/diameter ratio (d/D) 0.064 (north) 0.078 (south) of the crater that is essentially absent on the south/southwestern Age 62 Myra half of the crater (Fig. 2). The proposed pre-slump diameter is a Based upon the number of superposed impact craters larger than 54 m (4 26.4 km (Fig. 2), about 2.5 km less than the existing crater diam- pixels) in diameter and the cratering chronology of Hartmann et al. (2005). eter (29 km). Garvin et al. (2003) established the relationship d = 0.36D0.49, where d is the depth and D the diameter, both in landscape (À3872 m) from the individual MOLA elevations of the km. Using this relationship and the proposed pre-slump diameter, crater rim and ejecta blanket (Mouginis-Mark and Garbeil, 2007). Tooting crater is predicted to have a depth of 1874 m, consistent

Fig. 2. Portion of CTX image P03_002158 detailing Tooting crater cavity and the proximal rim materials. Tooting crater is 29 km in diameter and the southern floor is nearly 200 m deeper than the northern floor. The dashed black line identifies proposed pre-terracing crater diameter of 26.4 km and the dashed white lines indicate terrace blocks. The white circle identifies the location of two small north rim flows (not studied here in detail). White boxes outline areas covered by subsequent figures. F1, F2, F3 and F4 indicate figures associated with flow Types 1–4. 372 A.R. Morris et al. / Icarus 209 (2010) 369–389

Table 2 Summary of ﬂow types.

Flow Location Source region Attributes Interpretation type Type 1 Southern crater Disseminated smooth Numerous flows up to 700 m long, 50–200 m wide Impact melt flows rim crest area at crest of crater Smooth at meter-scale rim Polygonally fractured Direction of flow both Few boulders towards and away Few superposed craters from crater cavity Lobate flow fronts Festoon ridges on several flows Type 2 Southern crater Largest flow emanates Largest flow Debris flows ejecta blanket from collapsed terrain 1260 m long, 225 m wide Largest flow Traverses slope of 4.5° Sediment-rich flow Hummocky surface at meter Lineated channel carved by secondary water-rich flow Smaller flows Sediment-rich flows Smaller flows have Scale indistinct source Rubbly levees regions Lineated channel Smaller flows Smooth surface at meter scale Sharp levees Coalescing flows Type 3 SE crater interior Region of highly 1570 m long, 250–500 m wide Debris flows terraced region dissected/gullied Traverses slope of 8.2° Low energy terrace blocks Proximal portion of flow Sediment-rich Hummocky surface Discrete pulses of flow Boulders 2–5 m across emplacement Discontinuous levees Distal portion of flow 6 discrete lobes 10–15 m across Smooth with steep margins Narrow levees, <3 m wide Few boulders Type 4 SW crater rim Ridged region adjacent 1900 m long, 75–310 m wide Debris flow to rim crest Traverses general topographic slope of 2.3° Sediment mobilized Narrow, channeled proximal region (9.5° slope) by impact-released volatiles Convex down-flow transverse ridges within channel Sediment-rich, degree Tri-lobed distal region of coherency (boulders on surface) Boulders from 90 cm up to several meters in diameter on surface of flow

with the depth measured from the CTX data on the northern floor. tures exhibit a range of flow morphologies. In addition, there are However, we note that the southern floor is 1.2 times deeper than two small flows observed on the northern rim. We discuss all of this prediction. This unusually large depth/diameter ratio is taken these flows in following sections. To our knowledge, prior to this to be another indicator of the morphometric ‘‘freshness” of Tooting study, none of these flow types have ever been documented for a crater. martian impact crater. This is most likely a consequence of the The structural failure of the southern crater wall appears to be combination of the relative morphometric freshness of the crater less extensive than that observed on the north/northeastern wall and the high resolution imagery acquired by HiRISE, and suggests of the crater where two major terrace blocks are identified that Tooting crater presents a rare opportunity to study the crater- (Fig. 2). A study of the distribution of the ejecta volume reveals ing process at the tens of kilometer-scale on Mars. that, for a given radial distance from the cavity rim, the ejecta blanket is 100 m thicker on the northeastern portion of the ejecta 3.1. Flow Type 1: smooth, polygonally fractured material blanket than on the southwestern portion (Mouginis-Mark and Garbeil, 2007). Additionally, Mouginis-Mark and Garbeil (2007) Several flows can be identified on the southern rim of Tooting note that the maximum radial distance of the ejecta is greatest at crater (Table 2, Fig. 3). Extensive smooth, polygonally fractured an azimuth of 30° (NE) and smallest at 210° (SW). Based upon material with lobate flow fronts is the largest unit mapped in this the asymmetry in the distribution of the ejecta blanket (both the study (Fig. 3b). As revealed in the HiRISE-derived DEM, the frac- maximum range and the ejecta volume), the northeastern direc- tured materials are present at elevations between 200 m above tion is the inferred downrange direction (Fig. 1) resulting from a and 400 m below the break in slope that defines the edge of presumed oblique impact (Mouginis-Mark and Garbeil, 2007). the southern rim (Fig. 3). HiRISE images show that the materials extend radially away from the edge of the cavity rim to a distance 3. Types of flows observed in HiRISE and CTX data of at least 6 km. Where the material is present on the edge of the rim, it exhibits some degree of coherence, as there are obvious Four types of flow features are observed in CTX and HiRISE overhangs and a high degree of erosion beneath the overhang images, either on the outer rim or the inner wall of the southern (Fig. 4a). The horizontal extent of the rim overhang is unknown, portion of Tooting crater (Fig. 2, boxed areas), and these flow fea- although there could be at least several meters of material A.R. Morris et al. / Icarus 209 (2010) 369–389 373

Fig. 3. (a) Portion of HiRISE image PSP_001538_2035 detailing extensive sheet-like fractured flows and ridged lobate flows (Type 1 flows). The center of the crater is towards the top of the image. Contours derived from HiRISE stereo pair, and have a contour interval of 25 m. Elevations are given relative to the MOLA datum (Zuber et al., 1992). See Fig. 2 for location. Locations of Figs. 4a and b and 5 are identified. (b) Geomorphic interpretation of Fig. 3a detailing surface units described in text. Black lines in smooth, fractured material unit are flow fronts that do not form distinct, discrete flow lobes. Dashed line indicates crest of local topographic high. suspended above the fluted rim and eroded talus slopes. The mate- We note that there are very few boulders on the surface of the rials in this region exhibit decameter- to meter-scale polygonal fractured materials resolved in the HiRISE images (minimum size fractures (Fig. 4b). These fractured materials appear to be laterally of features resolved is 3 pixels, or 90 cm in a 30 cm/pixel image, continuous, although near the tops of steep slopes they have McEwen et al., 2007a), although some flow fronts do exhibit a con- formed lobes (Fig. 3). centration of boulders (Fig. 4b). On the surface of the southern rim, 374 A.R. Morris et al. / Icarus 209 (2010) 369–389

These lobes exhibit a diversity of features, including transverse ridges, raised levees and boulders on their surfaces and margins. Two of the most prominent lobate features appear to originate at scarps near the crest of a local topographic high and extend north toward the crater center within topographic lows created along the boundaries between incipient slump blocks (Fig. 5).

3.2. Flow Type 2: hummocky and smooth lobes

In addition to the smooth, fractured materials and the ridged, lobate materials on the southern rim of Tooting crater, there are several hummocky and smooth flows that appear in one 2.4 km by 2.7 km area between the polygonally fractured materials to the east and the smooth and pitted terrain to the west (Table 2, Fig. 6). These flows are grouped together based on their spatial proximity and absence of these flow morphologies in any other areas of Tooting crater imaged by HiRISE. The surfaces of these flows vary from hummocky on the meter to tens of meter scale on the largest flows to nearly smooth with sharp levees on the smaller flows. These surface morphologies appear to be mutually exclusive. Each flow will now be discussed. The largest of these flows, Flow 2, is 1260 m long and 225 m wide, hummocky and appears to emanate directly from collapsed terrain (Fig. 6a and b). Our geologic mapping of the rim indicates that this collapsed terrain is most likely parts of the original crater ejecta blanket that subsequently structurally-failed after emplacement. The DEM produced from CTX data indicates that the source area for Flow 2 is at an elevation of about À3650 m, and the distal end of the flow is at an elevation of À3750 m, indicating the flow traverses a slope of 4.5° (Fig. 6c). This flow has levees of rubbly material, each 60–70 m across, that enclose a lineated channel region that is 86 m across at its widest. The channel is 490 m long and is characterized by linear features parallel to the direction of flow. The channel also contains several isolated, elongate features (longitudinal bars) that are similar in morphology to the levees. The largest of the elongate features is 5–6 m across and 18 m long. The central channel terminates in a low, smooth mound 36 m long (parallel to the flow direction) and 15 m across, 618 m from the distal end of the flow. The outer margin of Flow 2 is characterized by individual lobes on the scale of tens of meters or less in horizontal extent (parallel to the flow direction). The flow overlies material that is interpreted to be undivided sediments as well as regions of fractured material and blocky material. The distal end of Flow 2 is slightly digitate and overlies hummocky pitted ter-

Fig. 4. Portion of HiRISE image PSP_001538_2035 detailing extensive sheet-like rain similar to that described in Mouginis-Mark and Garbeil (2007) fractured flows and ridged, lobate flows. (a) Close-up of rim overhang of within the crater cavity (Fig. 6d). polygonally fractured flows with HiRISE-derived contours overlain on image. The smooth, smaller flows (not studied in detail here) typically Illumination is from the lower left. Black arrows indicate location of rim overhang. exhibit coalescing morphologies, with enclosed inliers of the pre- Note long shadows on slope below rim. See Fig. 3 for location. (b) Close-up of existing surface (‘‘kipukas”) visible within some flows (Fig. 6e). fractured sheet materials (at right) and ridged flow with a boulder-strewn surface at lower left. Illumination from the lower left. Boulders on flow surface are 2–5 m The flows appear to widen and narrow along their length. Some in diameter. of the shortest of the small flows are on the order of tens of meters long and only a few meters across. The source regions for the smaller, smooth flows are not as obvious as for Flow 2 and the other the presence of meter-scale boulders and fractured material ap- rubbly, large flows in the region. pear to be mutually exclusive. There are very few impact craters on the surface of the fractured materials, and those craters that 3.3. Flow Type 3: hybrid lobe (Flow 3) are observed in the HiRISE images are typically <40 m in diameter. In addition to the polygonally fractured materials, there are also A third type of flow is a large flow on the terraces in the interior ridged, lobate, flow-like features present on the rim of the cavity southwestern region of Tooting crater (Table 2, Fig. 7). It is the only (Fig. 5). The lobate features generally grade into the fractured flow that we have found so far on the interior of Tooting crater. The material without any obvious stratigraphic contact (Fig. 3). The source region for this flow lies between two discrete wall terrace lobes north of a local topographic high (marked by a dashed black blocks and it is characterized by a centripetal network of channels line in Fig. 3b) flowed toward the crater cavity, whereas the lobes that flow toward the crater floor. The flow is 1570 m long and its to the south of the high flowed south/southwest, away from the width varies between 250 and 500 m. The CTX DEM indicates crater cavity (Fig. 3). Precise measurement of individual flow that the probable source for Flow 3 occurs at an elevation of widths and lengths is difficult because the features often coalesce. À4600 m and the distal end of the flow is at an elevation of A.R. Morris et al. / Icarus 209 (2010) 369–389 375

Fig. 5. (a) Portion of HiRISE image PSP_001538_2035 detailing lobate ﬂows within and adjacent to fractured, sheet-like materials. Flow direction is toward crater cavity, to the north. Location of this sub-scene is shown in Fig. 3. HiRISE-derived contours at 25 m intervals. North is to the top. See Fig. 3 for location. b) Geomorphic sketch map identifying units described in text. Contours are at 25 m intervals and are referenced to the MOLA datum.

À4850 m, indicating the flow traverses a slope of 8.2°. The source main case study due to the availability of HiRISE stereo coverage area is 1500 m below the rim crest and 400 m above the pres- of the flow and the ability to study its entire length from the in- ent-day crater floor (Fig. 9). The upper reaches of the flow are char- ferred source region to the distal lobes. Flow 4 is located within acterized by large boulders (2–5 m across) and discontinuous the unit of rim material containing blocks that may lack an over- levees (Fig. 8a) whereas the distal 600 m of the flow is composed lying ejecta layer. A sketch map of the area (Fig. 10b) identifies of 6 discrete lobes that are each 10–15 m in width (Fig. 8b). the units in this region. The flow is primarily emplaced on blocky The distal lobes appear to be smooth with steep margins, promi- ejecta with linear ridges that trend radial away from the crater nent narrow levees (<3 m wide), and appear to have very few boul- cavity (RS, Fig. 10b). Within this unit, there are also several ders on their surfaces. The distal lobes do not appear to coalesce; smooth sediments or flow materials occupying the areas between rather, they appear to have progressively overridden or have been linear ridges. Uphill from, and adjacent to, the ridged, blocky unit deflected by previously emplaced distal lobes. The transition from is a mountainous, massive unit with smooth sediments or per- hummocky and boulder-rich to smooth with steep levees is spa- haps other flow materials (MS, Fig. 10b). In addition to the mas- tially correlated with the termini of several large channel features sive and ridged units there is a unit composed of pitted terrain on the slump block (Fig. 8b). (PT, Fig. 10b), which appears to be similar to the pitted terrain mapped in the region of Flow 2 and on the main Tooting crater 3.4. Flow Type 4: leveed, ridged, channelized flow (Flow 4) floor (Mouginis-Mark and Garbeil, 2007), as well as has been observed in several other morphometrically fresh impact craters on The fourth flow type is located on the west–southwest rim of Mars (Tornabene et al., 2007). We hypothesize that the pitted ter- Tooting crater (Table 2, Fig. 10). This flow was selected as our rain is caused by the degassing of impact-generated sediments 376 A.R. Morris et al. / Icarus 209 (2010) 369–389

Fig. 6. (a) Portion of HiRISE image PSP_007406_2035 detailing region of smooth and hummocky flows. See Fig. 2 for location. White box outlines location of (b). Black boxes outline location of Fig. 15b and d. (b) Distribution of flows. Area shown is the same as (a). (c) CTX-derived contour map of the area shown in (a). Flow 2. Note channel near collapsed source region. Box outlines location of (c). (e) Small smooth flow. Illumination is from the west. Arrows point to enclosed portions of the pre-existing surface (kipukas). Direction of flow is towards the lower left. and incorporated volatiles that slumped off of the inner walls 4. Flow 4 geomorphology and topography during the crater modification stage. Melosh (1989) describes complex crater collapse as a rapid process for lunar craters 4.1. Dimensions that was most likely coincident with the end of cavity formation. In the case of Tooting crater it is likely that impact melt rock A series of 19 cross-flow topographic profiles were acquired would also have been incorporated into this mix of volatiles from the HiRISE DEM at a down-flow spacing of 40–75 m and impact-derived sediments, and would result in a period of (Fig. 11), and measurements of flow thickness, flow width, channel thermal equilibration for a time after the impact. The pitted depth and channel width were recorded at each profile location materials are typically observed in the interiors of impact crater (Fig. 11). For each profile, flow thickness was measured from the cavities, as well as in topographic lows on the terrace blocks elevation of the substrate just west of the flow to the top of the and near-rim crest depressions on the outer walls of fresh craters. flow (maximum elevation of the associated profile). The height of Within the crater interiors, the pitted materials are undisturbed the western levee was used due to the emplacement of post-flow by terrace formation and superposed by sediment fans, suggest- sediments on the eastern side of the flow. Flow width was mea- ing the pitted material formed during an intermediate phase of sured across the entire flow, including the levees. The channel crater evolution. depth was measured from the top of the levees to the surface of A.R. Morris et al. / Icarus 209 (2010) 369–389 377

Fig. 6 (continued)

the channel, and channel width was measured across the channel (between profiles 4 and 8; Fig. 11). To accentuate small-scale vari- (between the interior margins of the levees). In addition to the ations in topography, the longitudinal profile down the center of topographic measurements, flow length was measured from the the flow was detrended by removing the long-wavelength topo- HiRISE images. Based on slope measurements of the adjacent ter- graphic slope using a least squares polynomial fitting routine in rain, the general topographic slope is approximately 2.3°. The por- IDL. tion of the flow where the channel developed has a slope of 9.5° Flow 4 is 1900 m in length and 1050 m from the top edge of (between profiles 4 and 12; Fig. 11) and the steepest portion of the the cliff to the lobate, distal toe (Fig. 10). Three distinct sub-lobes flow (the cliff identified in Fig. 10b) has a slope of nearly 18° characterize the distal portion of the flow (Fig. 10). The flow 378 A.R. Morris et al. / Icarus 209 (2010) 369–389

Fig. 7. (a) Local topography near Flow 3 derived from CTX images P01_001538_2035 and P03_002158_2034, See Fig. 2 for location. Illumination is from the west. (b) Portion of HiRISE image PSP_005771_2035 detailing the morphology of the southwestern inner wall of Tooting crater overlain by sketch map of region shown in (a). Direction of flow is towards the top of the image. Lines on yellow unit indicate individual channels. Locations of Figs. 8a and b are identified. width varies from a minimum of 75 m where the flow was chan- over lobes. The flow eventually overtopped the small ridge and was neled between ridges and subsequently travelled over the cliff channeled over the cliff between two of the linear ridges, forming a (Fig. 10b) to a maximum of 310 m at the widest part of the three lobate flow with levees, a channel and discrete distal lobes. Just sub-lobes in the distal reaches of the flow (Fig. 12a). The thickness upslope from the cliff, a series of braided channels intersects the of the flow varies from a minimum of 0.5 m at the apex of the cliff upper margin of the lobate flow. to a maximum of 6 m just up-flow from the division into the three sub-lobes (Fig. 12a). 4.3. Channel, levees and flow margins

4.2. Source area The channel begins at the edge of the cliff and ends in a slight rise just up-flow from the start of the distal sub-lobes (Fig. 11). The source area for Flow 4 is just south of the crater rim crest The characteristics of the northwest and southeast levees differ. and it is confined by one of the linear ridges in the ‘‘NE–SW trend- The NW levee is typically rough, with a mottled surface that ing linear ridges with sediments or flow materials” unit (Fig. 10b). appears morphologically similar to the surface of the distal lobes. Upslope from the cliff, the material that eventually comprises Flow Along much of the channel, the SE levee is smooth but irregularly 4 appears to have ponded behind the ridge and formed small spill- fractured, although the smooth levee disappears and a levee A.R. Morris et al. / Icarus 209 (2010) 369–389 379

Fig. 8. Portions of HiRISE image PSP_005771_2035 detailing Flow 3 morphology. North is up. See Fig. 7 for location. (a) Close-up of upper region of Flow 3. Note boulder-rich, hummocky surface. Example boulders are identified by arrows and boulder widths above the arrows. (b) Close-up of distal lobes of Flow 3. Note smooth flows with distinct levees towards the bottom of the image. Direction of flow is towards the top of the image.

similar in texture to that on the NW appears 350 m from the cliff Channel width is weakly correlated with flow width (Table 3). As (570 m from Profile 1; Fig. 11). The width of the channel is channel width increases, the width of the flow typically increases 61 ± 20 m. The narrowest portion of the channel is located at (Fig. 13b). Channel depth is anti-correlated with flow width (Table the apex of the cliff, where the channel is 30 m wide. At the base 3). As channel depth increases, flow width decreases (Fig. 13b). The of the cliff, the channel widens out to 74 m. Within the channel exception to this is Profile 5, where the flow is 96 m wide, but the are convex down-flow transverse ridges. The average spacing be- channel is 4 m deep and 37 m wide. Because the channel is re- tween ridge crests is 8 m. The individual ridges are not resolved stricted to the region of the slope between NE–SW trending ridges, in the DEM, and as a result we interpret that they are <1 m high. it is likely the channel formed in this region as a result of flow con- Using the HiRISE images and DEM, the width and depth of the finement (Fig. 10). channel were measured at each profile location and plotted against The margins of the flow exhibit a lobate nature, and the lobes the distance from Profile 1 (Fig. 12). Channel width and depth are are spatially correlated with slight topographic highs that cross inversely correlated; the deepest parts of the channel are also the the interior of the flow (Fig. 11). The lighter toned small lobes on narrowest parts of the channel. The channel is deepest (4m) the edges of the flow can be traced across the interior region, form- and narrowest (30–40 m) between profiles 4 and 6, which is also ing topographically higher ‘‘fronts” that are obliquely angled to the the steepest section of the channel. Between profiles 7 and 12, flow direction. The lobes on the margins of the flow are 10–20 m the channel is notably shallower (1–2 m) and wider (65–82 m), in length, and a few meters in width. These bright topographically and the topographic gradient is also generally less steep than the high features are called pulse fronts (PF, orange lines drawn on the gradient between profiles 4 and 6. Channel width appears to be flow in Fig. 10b). correlated with flow thickness (Table 3). As channel width in- At least three large-scale flow lobes characterize the region of creases, the thickness of the flow increases (Fig. 13a). Channel the distal portion of the flow (Fig. 10). The two outer lobes appear depth is anti-correlated with flow thickness (Table 3, Fig. 13a). to be bifurcated components of the same flow and the central, 380 A.R. Morris et al. / Icarus 209 (2010) 369–389

Fig. 9. Oblique view of southwestern inner crater wall and Flow 3 derived from topography and CTX images from Fig. 7c. The rim crest is 1720 m above the crater floor. Black arrow indicates location of toe of Flow 3. Vertical exaggeration is 2.25. farthest-reaching lobe was emplaced separately. The lobe farthest (2) gravity-driven, fluid-assisted debris flows. We base these inter- to the west is 27 m wide and 4 m thick, the central lobe is pretations on the published flow morphology characteristics and 146 m wide and 4 m thicker than the western lobe, and the east- typical locations of each type of flow in terrestrial and extraterres- ern lobe is 91 m wide and 2 m thick (Profile 17, Fig. 11). The dis- trial environments (Hawke and Head, 1977; Iverson, 1997; Iverson tal toe of the central lobe superposes pitted terrain similar to that and Vallance, 2001; Major, 1997; Mangold and Costard, 2003; observed on the northern crater floor and at the distal end of Flow Melosh, 1989). Assuming a relatively homogenous target material, 2(Fig. 10b). were both the discrete flows and the Type 1 fractured flows all There are 1 m-high linear features that are generally parallel flows of impact melt origin, we would expect to see differences to the flow direction, identified in the sketch map by purple lines, in rheology that might be related to radial distance from the rim and they are located primarily along the channel (Fig. 10b). The crest, with a greater proportion of melt (and, hence, less viscous most prominent of the linear features is located between the cen- flows) closer to the crater cavity. The ratio of ‘‘cold” clasts to liquid tral channel and the smooth SE levee. In addition to the channel- melt in impact melt-dominated flows would be expected to in- bounding features, there are also flow-parallel linear features near crease with radial distance away from the crater rim. The effect the region where the distal portion of the flow is divided into three on the impact melt flow rheology of increasing the clast content sub-lobes (Fig. 10b). might be reflected in the subsequent flow morphology expressed The surface of the flow is covered by boulders that range from at the meter scale. Similar changes in the rheology have been de- the minimum resolvable size (90 cm) up to several meters across scribed for the impact melt sheet at Manicouagan crater due to (Fig. 10b). Above the cliff, the boulders are concentrated in the cen- the inclusion of cold lithics within melt rock (Simonds et al., ter of the flow, often occurring in ‘‘boulder chains.” Below the edge 1976). Alternatively, the morphological differences in the discrete of the cliff the boulders are mostly located along the outer margins flows may be the result of heterogeneities in the target, differences of the flow lobes. A few boulders are located in the upper reaches in the volatile content of the emplaced ejecta deposits, or the var- of the channel, just below the base of the cliff, but otherwise the iable influence of underlying topography. Based on our observa- channel is devoid of large boulders. Toward the distal end of the tions, we now suggest that the Type 1 flows (fractured flows flow, near the dispersed, lobate margin, boulders are concentrated with lobate flow fronts) on the southern rim of the crater are along the margins of the western, central, and eastern sub-lobes. sheets of impact melt and Flows 2–4 are debris flows.

5.1. Impact melts 5. Origin of flows The location of the Type 1 fractured flows on the southern rim The diversity of flow morphologies identified here, as well as of the crater and the morphology of the flows are both consistent their respective locations, argues against a common origin for all with observations of materials interpreted to be cooled sheets flows identified on the southern rim and interior wall of Tooting and flows of impact melt on lunar craters such as Copernicus crater. We propose two separate mechanisms for the origin of (Howard, 1975; Howard and Wilshire, 1975), Necho (Hawke and the flows: (1) flows of impact melt/melt-bearing material and/or Head, 1977) and King (Heather and Dunkin, 2003). The ridged A.R. Morris et al. / Icarus 209 (2010) 369–389 381

Fig. 10. (a) Portion of HiRISE image PSP_002646_2035 detailing Flow 4 (case study flow). See Fig. 2 for location. Flow direction is from the upper right to the lower left of the image. Black boxes outline location of Fig. 15a and c. (b) Sketch map of region shown in (a). lobes within the flow suggest that the material behaved as a cohe- debris flows, we would expect that debris flows would be (a) sive, laminar material, as opposed to a turbulent flow. It is also pos- sourced in highly dissected material, (b) have narrow levees, and sible that the melt sheet may have been relatively clast-free at the (c) if they are sediment flows, we would not expect to see large time that it was emplaced on the crater rim. The fractured material boulders on the surface. All of the discrete flows in this study are exhibits a high degree of coherence, as demonstrated by the over- closely associated with channels, dissected terrain and apparent hang of material over the rim edge (Fig. 4a), and this would be con- sedimentary flow features. The source for Flow 2 appears to be a sistent with an impact melt origin. In addition, were these flows region of collapse adjacent to a local topographic high (Fig. 6), sug- composed of sedimentary material, it is unlikely that a desiccated gesting a late-stage outflow of material from the subsurface. Flow 3 sedimentary deposit would maintain sufficient mechanical appears to be sourced down-slope of a highly dissected wall block strength to form the steep slopes observed on the crater rim. The on the inner wall of the cavity (Fig. 7). An oblique view of the inner surfaces of the fractured flows appear to have very few craters, wall of the crater (Fig. 9) reveals that Flow 3 is down-slope from suggesting that they were emplaced after the termination of the dissected terrain and sedimentary fans; these sedimentary features primary phase of ejecta fallout. Studies of lunar craters indicate can first be seen at an elevation of À4450 m, and they converge be- that during an impact event, one of the last materials to be em- tween fluted rim material and dissected wall material at an eleva- placed is any impact melt that may be produced (Melosh, 1989; tion of À4550 m (Fig. 7a). It is interesting to note that the distal end Wilhelms, 1987). Additionally, impact melt deposits are typically of Flow 3 exhibits a morphology similar to the pyroclastic-flow emplaced onto the proximal region of the ejecta blanket deposits emplaced during the 1980 eruption of Mount St. Helens (Wilhelms, 1987). The ridged, lobate flow features in the fractured (Fig. 14). These volcanic flow deposits were fluidized by a gas sheet flows are morphologically similar to lobate, channelized phase, likely primary magmatic gases and entrained atmosphere flows of impact melt on King crater, on the far side of the Moon (Wilson and Head, 1981). The upper reaches of Flow 4 are also spa- (Heather and Dunkin, 2003). Differences in the surface morphology tially associated with dissected terrain adjacent to the mountain of the flows inferred here to be impact melt, such as the short flow block to the northwest of the flow. Flow 4 is located just down- with numerous boulders on the surface (Fig. 4b), may be attributed slope from a region of ponded material with extensive channels to a varying clast content of the melt. that emanate from the smooth pond and intersect with Flow 4 just above the cliff (Fig. 10). The spatial correlation between the 5.2. Debris flows discrete flows and channels likely to have formed by the action of liquid water suggests that the discrete flows are the product of Here we define debris flows as gravity-driven flows of granular the interaction of impact ejecta and liquid water. material mixed with less-dense inter-granular liquid or gas (Iverson Flows 2–4 exhibit morphologies distinct from the fractured, and Vallance, 2001). Based on previous work examining terrestrial lobate flow-front group, as well as morphologies distinct from each 382 A.R. Morris et al. / Icarus 209 (2010) 369–389

Fig. 11. Portion of stereo-derived DEM of Flow 4, generated from HiRISE images PSP_002646_2035 and PSP_002580_2035. Black vertical bar near right edge of image is artifact of DEM creation. Image is color-coded topography with 5 m contours over shaded relief. Outline of Flow 4 is shown in black. Illumination is from the northwest. Nineteen cross-flow topographic profiles are identified by white lines and associated numbers. The red line identifies the down-flow topographic profile. See Fig. 11 for contours. other. The discrete flows appear to be types of sediment/debris water in the sediment and determined that in the experiments, flows and we interpret the morphologic differences to be due to arcuate surface ridges were observed only on deposits from variations in the amount of incorporated volatiles. All three flows unsaturated flows, suggesting that ridges may be used as indica- have channels with levees and Flows 3 and 4 both exhibit narrow, tors of relative water content of the flow source (Major, 1997). flow-parallel linear features on the outer levees. The existence of Additionally, Major (1997) notes that unsaturated flows accrete the narrow linear ridges is reminiscent of the levees observed on toward the source region from the distal end of the deposit and water-laden debris flows from mud volcanoes in Azerbaijan on saturated flows primarily accrete in vertical increments by Earth (Hovland et al., 1997). Individual flows identified by Hovland progressive overriding of previously deposited material. The et al. (1997) (their Fig. 6) have the same narrow ridges and similar morphologies of the unsaturated experimental deposits appear pressure ridges (albeit at a smaller scale) as the ones identified to be similar to Flow 4. here at Tooting crater. This similarity indicates that the morphol- A comparison of Flow 2 and Flow 4 suggests that there may be ogy of these specific Tooting flows is consistent with the morphol- local variations in the volatile content of the target material and ogy of known debris flows, and may not be flows of impact melt. the resultant ejecta. Flow 2 is located on the southern rim of the Flow 2 lacks narrow, linear ridges, and the levee thickness and crater and Flow 4 is located on the southwestern rim of the crater. morphology are the same as the thickness and morphology in Flow 2 emanates from a collapse feature, and the surface of the the post-channel portion of the flow, suggesting that plug flow flow is rubbly and covered with boulders, consistent with the did not form the channel. Rather, a secondary event, such as the re- down-slope movement of collapse debris (Fig. 6a). The source for lease of liquid water, likely carved the lineated channel. The smal- Flow 4 is likely to be sediments collected on the southwestern ler, smoother flows near Flow 2 appear to lack the rubbly surface rim of Tooting crater, with the volatiles derived from the deposited present on the larger, longer Flow 2 (Fig. 6). The difference in flow material on the mountain block to the northwest. The surface of morphology could reflect the relative abundances of liquid and Flow 4 lacks extensive boulder populations (Fig. 10a) and has a incorporated ejecta, as Flow 2 originates at an obvious collapse channel with well-formed levees, whereas the channel on Flow 2 structure, whereas the smaller flows do not. The smaller flows appears to be a late-stage feature that eroded the pre-existing flow. may represent slower, less catastrophic seepage of volatiles (per- In addition, Flow 4 exhibits pulse fronts down-flow (Fig. 10b), haps even subsurface seepage) from the topographic high adjacent while Flow 2 does not (Fig. 6). The channel in Flow 4 is occupied to the region of Flow 2 and the associated flows. by material exhibiting concave-down arcuate ridges and the chan- Terrestrial studies of the depositional processes of experimen- nel in Flow 2 appears essentially emptied (or highly dissected at a tal debris flows from flume experiments find that the resulting late stage), with flow-parallel lineations and longitudinal bars deposits are similar to Flow 4 in that they are lobate, with steep within the channel (Fig. 15a and c). The distal toes of the flows blunt margins and marginal levees (Major, 1997). The experi- differ, as Flow 4 widens out into a tri-lobed flow with blunt, steep ments were run down a 95 m long, 2 m wide chute with a slope margins (Fig. 15b) and Flow 2 narrows down-slope with somewhat of 31° (Major, 1997). Individual experiments varied the amount of digitate, individual lobes (Fig. 15d). A.R. Morris et al. / Icarus 209 (2010) 369–389 383

Fig. 12. Graphs of detrended elevation, flow width, flow depth, channel width and channel depth for Flow 4. Measurements were obtained from stereo-derived DEM of Flow 4 and HiRISE image PSP_002646_2035. Numbers correlate to locations of cross-flow profiles identified in Fig. 11 and the location of the detrended elevation profile is identified by a black line in Fig. 11. (a) Graph of detrended elevation, flow width and flow thickness. (b) Graph of detrended elevation, channel width and channel depth.

Comparison of Flow 4 flow and channel dimensions with the Table 3 Mauna Loa, Hawaii, 1984 Flow 1A ‘a’a flow (Glaze and Baloga, Correlation coefficients for Flow 4 and Mauna Loa 1984 Flow 1A lava flow. Mauna Loa 2006) provides insight into the flow morphometry compared to a data are from Table 2 in Glaze and Baloga (2006). Flow 4 measurements were known lava flow (Table 3). Surprisingly, there are few terrestrial obtained from the HiRISE images and DEM. Correlation coefficients determined by lava flows where the geometry of the flow has been tabulated, plotting measurements against each other (similar to plots in Figs. 12 and 13). but one such flow is the Mauna Loa 1984 Flow 1A. This ‘a’a flow Measurements Correlation coefficients (R) is 13 km in length, and thickens from 5 m near the source to Flow 4 Mauna Loa 1984 Flow 1Aa 15 m at the distal toe. Generally, the measurements of Flow 4 mor- Flow width vs. distance 0.67 0.68 phometry are more correlated than those for the measured ‘a’a Channel width vs. distance 0.85 0.57 flow, suggesting Flow 4 behaves in a manner that is inconsistent Flow thickness vs. distance 0.65 0.37 with observed behavior of a known basaltic lava flow. In addition, Channel width vs. flow width 0.62 0.40 we do not believe that Flow 4 is a pahoehoe flow, as pahoehoe Flow width vs. flow thickness 0.44 0.21 Channel width vs. flow thickness 0.84 0.05 flows typically do not form large-scale channels and levees (Kil- Channel depth vs. distance À0.75 n/d burn, 2000). Channel depth vs. flow width À0.48 n/d Topography may play a role in the channel measurements dis- Channel depth vs. channel width À0.86 n/d cussed above. Examination of the topographic gradient of Flow 4 Channel depth vs. flow thickness À0.93 n/d between individual profiles indicates that generally, as topographic n/d = no data. gradient increases, flow width and flow thickness decrease a Data from Table 2 in Glaze and Baloga (2006). (Fig. 16a). In contrast, the local gradient appears to have a limited 384 A.R. Morris et al. / Icarus 209 (2010) 369–389

Fig. 13. Graphs of channel width and depth plotted against flow thickness and flow width. Measurements were obtained from stereo-derived DEM of Flow 4 and HiRISE image PSP_002646_2035. Numbers correlate to locations of cross-flow profiles identified in Fig. 11. (a) Channel width and channel depth plotted against flow thickness. (b) Channel width and channel depth plotted against flow width.

effect on channel width and depth (Fig. 16b). Generally, as local sources for the volatiles are (1) the ‘‘bleeding” of water from the gradient increases, channel depth increases and channel width de- freshly exposed aquifer within the newly formed crater cavity or creases. Unfortunately, detailed topographic data for the measure- (2) in situ dewatering of the impact ejecta and wall blocks. Precipita- ments made on the Mauna Loa flow are not available, but tion from the atmosphere is also possible, but probably unlikely, as it topography may have played a key role in the flow measurements would have to be highly localized. A previous study of Mojave crater, obtained from Glaze and Baloga (2006). Mars, has suggested that impact-induced precipitation provided The discrete flows observed on Tooting crater exhibit a range of water that formed channels and fan features observed on much of morphologies at HiRISE scale, from very smooth, with few boulders the crater interior (Williams et al., 2004). Presumably, the effect of (e.g. distal region of Flow 3, Fig. 8b), to rubbly and boulder-rich precipitation would be evident on all portions of the crater, and as (e.g. Flow 2, Fig. 6b). The differences in the flow morphologies flow features are nearly absent on the northern rim, it is unlikely that may indicate differences in formation mechanism for the discrete, rain contributed to the volatiles in the ponded sediments. A recent channelized flows. The lobate margins with prominent levees in high-resolution study of Mojave Crater has provided evidence for the channel region of Flow 4 suggest that surges occurred during depositional fan formation due to impact-induced overland fluid flow formation, similar to the observed formation of large-scale flow, although the amount and source of the fluid have not been con- experimental debris flows (Iverson, 1997; Major, 1997). The strained (Williams and Malin, 2008). Dewatering of the exposed cav- many-lobed margins and lack of pulse fronts of Flow 2 suggest that ity wall, or of the emplaced ejecta seem to be most plausible, and each individual lobe was emplaced individually, rather than as a both have been suggested by several previous studies (McEwen coherent body with static margins. Assuming that the source of et al., 2007b; Mouginis-Mark et al., 2007; Tornabene et al., 2007). the material in the flows was similar crater ejecta, the main con- The target material in which Tooting crater was formed is inter- trols on the resulting flow morphology may be the relative preted to be young, layered basalt flows (Tanaka et al., 2005). In amounts of volatiles and sediment, the size distribution of the sed- HiRISE and CTX images of Tooting crater, we observe layers in iment, and the underlying topography. the walls, slumped wall blocks, and on the outer portion of the crater rim not covered by the proposed impact melt, and agree that 5.3. Distribution of volatiles these are layered basalt flows. Additionally, we observe a high degree of dissection on the southwestern wall block in the crater The discrete, channelized debris flows (Flows 2–4) examined in interior (identified as ‘‘dissected wall material” in Fig. 7), as well this study appear to be related to the incorporation of volatiles and as just upslope from the location where Flow 4 crosses the apex the resulting mobilization of the ejecta sediments. The two potential of the cliff face on the crater rim (Fig. 10b). The presence of A.R. Morris et al. / Icarus 209 (2010) 369–389 385

of a pre-existing aquifer in the young, layered basalt ﬂows is observed to the southeast of the crater ejecta blanket, where braided channels intersect the north side of a depression (white box in Fig. 1). The depression may have been a source for liquid water that ﬂowed on the surface and formed the braided channels. The channels suggest liquid was present on or near the surface in this region. It appears that the volatiles were present prior to the impact, as secondary craters from the cratering event that formed Tooting crater have been found on the channels. Additionally, the dissected near-rim crater ejecta (Fig. 10) suggests that a low-viscosity material, possibly liquid water, was also present on the surface adjacent to the crater cavity.

5.4. Flow distribution

The flows on the rim and interior wall of Tooting crater are dominantly observed on the southern and southwestern side of the crater. A survey of the entire rim and interior wall of the crater reveals that only two small flows are present on the northern rim of the crater (the general location is identified in Fig. 2). The location of the flows is coincident with the inferred up-range portion of the crater cavity. Additionally, large-scale terrace formation is more extensive on the northern, western, and northeastern portions of the crater (Fig. 1), coincident with the preferential deposition of the material ejected from the cavity (Mouginis-Mark and Garbeil, 2007). Based on known crater formation mechanics, we infer that the proposed impact melt flows formed at the terminal end of the crater excavation stage (prior to terrace formation), and were likely equally distributed within the transient crater cavity. The lack of flow features on the northern, eastern and western portions of the rim may be the result of one of two related scenarios. In the first scenario, the flows formed equally at all azimuths, but were subsequently reworked and incorporated into the crater deposits during terrace formation on most parts of the crater floor. In partic- ular, we envision the mixing of fragmented, hot, impact melt and volatile-rich sediments which interact to disrupt and destroy the original melt. In the second scenario, terrace formation occurred after impact melt emplacement and prior to debris flow formation, resulting in the inability of the northern half of the crater to form debris flows due to the emplacement of the volatile-rich wall rock and near-rim ejecta onto the crater floor during terrace formation. In the first scenario, we assume that impact melt flows and debris flows formed equally at all azimuths. We propose that terrace formation post-dated flow emplacement, physically destroying (by mechanical disruption and mixing with other wall material) any impact melt flows and/or debris flows present on the outer rim of the northern/northeastern portion of the crater, and subsequently bury- ing the flows on the interior wall. Dewatering of the target material and/or impact ejecta preceded terrace formation, resulting in debris flows on the outer rim and inner cavity walls. In the second scenario, we assume terrace formation occurred prior to debris flow emplacement, but after impact melt deposition Fig. 14. Pyroclastic flow deposits from Mount St. Helens Volcano, Washington, and flow. In this scenario, we propose that volatile-rich materials showing the morphology of a granular flow fluidized by magmatic gases and were emplaced on the crater floor during terrace formation, which entrained atmosphere. Compare to Fig. 8b Flow 3 morphology. Photos courtesy of primarily occurred on the northern half of the crater. On the USGS-Cascades Volcano Observatory. (a) Vertical aerial view of Mount St. Helens flows showing July 22 and August 7, 1980 pyroclastic flows. Flow direction is from southern half of the crater, the volatile-rich wall materials top left. Photo by N. Banks, Skamania County, Washington, July 31, 1980. Portion of remained intact (i.e., there was little slumping of the inner wall) Fig. 298, USGS Professional Paper 1250. (b) Oblique aerial view of lobes of and subsequently dewatered in situ, forming the debris flows pyroclastic-flow deposits of July 22 and August 7, 1980 eruptions of Mount St. observed on the south/southwestern region of the crater. Helens, showing generally constant width and thickness of deposits. Same area as Based on our observations and a few key assumptions, the fol- (a). Star indicates same feature. Direction of flow is towards lower left. Photo by J.W. Head, Skamania County, Washington, 1980, Fig. 299, USGS Professional Paper 1250. lowing is our preferred interpretation. Although we cannot explic- itly determine the relative timing of flow emplacement and dissected material on the wall blocks on the southern interior of terrace formation, the proposed large-scale failure of a volatile-rich the crater suggests that volatiles were present in the target mate- wall has implications for other attributes of the crater interior. The rial at the time of crater formation. In addition, potential evidence crater cavity is nearly 200 m shallower on the northern floor than 386 A.R. Morris et al. / Icarus 209 (2010) 369–389

Fig. 15. Comparison of the surface morphology of Flows 4 and 2. In all cases, the illumination is from the left. Locations of boxes are shown in Fig. 10 (for (a) and (c)) and Fig. 6 (for (b) and (d)). Scale is the same for all four images and north is up. (a) Portion of HiRISE image PSP_002646_2035 detailing a region of the central channel of Flow 4. (b) Portions of HiRISE image PSP_007406_2035 detailing a region of the central channel of Flow 2. (c) Portion of HiRISE image PSP_002646_2035 detailing the distal lobe of Flow 4. Illumination is from the southwest. (d) Portions of HiRISE image PSP_007406_2035 detailing the distal lobes of Flow 2. the southern floor (Mouginis-Mark and Garbeil, 2007). Dewatering ered lava flows (Tanaka et al., 2005). Our observation of ter- channels, absent on the crater interior and rim near the terrace race blocks with extensive dissection at higher elevations blocks on the northern wall, may have been preferentially mechan- and the absence of flow features on the stratigraphically ically destroyed, while the pitted floor appears to be located primar- lower cavity wall suggests a pre-impact layered target, ily where the terrace blocks formed. The presence of the pitted where the upper layers were volatile-rich in comparison to terrain on the northern half of the crater floor may suggest that the lower layers. any volatile-rich material present in the crater cavity wall and on (2) An impactor approached from the southwest, moving north- the rim was slumped into the cavity and subsequently dewatered. east, and struck the layered basaltic lava flows, forming the The subsequent loss of volatiles from the floor material resulted in transient crater cavity and impact melt, and resulting in the formation of pits due to the mass deficits within the crater floor. ejection of volatile-rich impact ejecta. We have observed that the volatile source may have been strat- (3) Emplacement of impact melt on the transient crater cavity ified within the target material, as some terrace blocks appear to and rim crest. have dewatering channels, while others at a lower elevation in (4) The transient crater cavity began collapsing, destroying (by the crater cavity lack channels and obvious dissection (Figs. 7 mechanical disruption and incorporation with impact-gen- and 9). The only two flow features observed on the northern crater erated sediments, as well as volatiles from the upper layers rim are located between the two major terrace blocks, consistent within the cavity) the impact melt on the cavity walls and with the first scenario, suggesting the debris flows formed at all crater rim except in the southern portion of the crater. Dur- azimuths around the crater but were preferentially obscured by ing cavity collapse, the near-rim impact melt flows (frac- structural failure on the northern half of the crater. tured flows) began flowing both toward and away from the cavity, forming ridged, lobate flow features (Fig. 5). An 6. Proposed sequence of flow formation incipient terrace block began forming on the southern wall of the crater, causing the impact melt flows (Fig. 5) to flow Based on the above observations and a few key assumptions, we toward the excavated cavity, but cavity collapse ended suggest the following sequence of events for the formation of the before a major terrace block could form on the southern rim. flows observed on the southern/southwestern rim and interior (5) The impact melt flows on the southern crater rim cooled, wall of Tooting crater: forming contraction fractures on the surface of the material. Concurrently, the ejecta below the polygonally fractured (1) Prior to the impact event, the surface of Amazonis Planitia material began dewatering, resulting in the formation of was essentially featureless and composed of a series of lay- smooth sediment flows observed on the southern half of A.R. Morris et al. / Icarus 209 (2010) 369–389 387

Fig. 16. Graphs of ﬂow width and thickness and channel width and depth plotted against local gradient. Measurements were obtained from stereo-derived DEM of Flow 4 and HiRISE image PSP_002646_2035. (a) Flow width and ﬂow thickness plotted against local gradient. (b) Channel width and channel depth plotted against local gradient.

the crater, including the sediment fan observed upslope tiles (either water or melted subsurface ice) and sediment on from the collapse forming Flow 2 (Fig. 6a). an over steepened slope. The rubbly surface morphology of (6) The debris flows formed on the outer rim and interior wall Flow 2 is distinct from the morphology of Flows 3 and 4. These terraces, as volatiles were released from the ejecta on the flows (3 and 4) appear to be the result of less fluidized material outer rim and target material within the crater cavity. moving down-slope. The ability of Flow 4 and the upper (7) Cavity collapse ends, but the interior rim materials on the region of Flow 3 to support boulders on the surface are consis- southern wall continue to bleed water to produce the fans of tent with Flows 3 and 4 comprising material with less fluidiza- material that extend over the pitted terrain on the crater floor. tion than of Flow 2. Flow 3 originated near the top of the crater wall in a highly dissected region of the interior wall. There is An alternative sequence of events is also possible if the flows no evidence for down-flow pulses of material in Flow 3, and formed after terrace formation: the channels within the levees are smooth, appear to be fairly elevated with respect to the surrounding terrain, and lack any (4) During transient cavity collapse, the near-rim impact melt obvious plastic surface deformation. The multiple lobes (fractured flows) began flowing both toward and away from observed in the distal reaches of Flow 3 suggest a protracted the cavity (Fig. 5). period of flow formation. Flow 4 originates on the outer sur- (5) Terrace formation ceases, emplacing volatile-rich material face of the edge of the crater rim and exhibits evidence of both on the northern crater floor. Similar to Step 7 in Scenario plug flow and multiple down-flow pulses of material. 2, volatile-rich material is emplaced on the crater floor. (6) The last flows to form were the more discrete flows, namely 7. Conclusions Flows 2–4, but even these had diverse origins that may indicate a difference in timing. Flow 2 appears to have formed The ongoing debate concerning the emplacement of multi-lay- from collapse of the emplaced non-melt-bearing component ered ejecta craters seeks to determine whether the ejecta were flu- of the ejecta blanket, and was most likely an outburst of vola- idized by volatiles in the target (Carr et al., 1977; Mouginis-Mark, 388 A.R. Morris et al. / Icarus 209 (2010) 369–389

1979) or by entrainment of atmosphere (Barnouin-Jha and Schultz, diameter) indicate that these craters are among the youngest cra- 1998; Schultz and Gault, 1979). Thus the identification of volatile- ters in their size class, occur preferentially on young volcanic plains, rich sediment flows at Tooting crater is important because it indi- and may have formed preferentially in volatile-rich targets (Torna- cates that water existed within the target rock at the time of bene et al., 2006). Thus rayed craters such as Zunil, Zumba, Gratteri, impact and that at least some of the ejecta that now lies beyond Tomini, Corinto and Dilly may be potential areas to target for iden- the rim crest contained water. tification of additional flows comparable to the ones described here We have observed four different types of flows on the rim and for Tooting crater. We hope that additional fresh, large craters in the interior wall of Tooting crater. Coherent, low-viscosity impact melt diameter range 10–30 km will be imaged by HiRISE and CTX, and was emplaced on the rim of the crater and cooled fairly rapidly advocate that stereo coverage will be particularly helpful in the while collecting and flowing short distances on slopes both toward evaluating the geomorphic characteristics of flow features should and away from the cavity. Sediment flows with variations in the they be seen to exist. Additionally, Tooting crater may provide a initial emplacement mechanism (collapse versus flow of water-la- good start for comparison to the fresh impact melt deposits just den sediment on slopes) and in the amount of volatiles are ob- being revealed by the Lunar Reconnaissance Orbiter for the Moon. served. Our investigation reveals that the flows (impact melt and debris flows) appear to be preferentially located on the southern Acknowledgments rim and interior wall. The absence of similar flows in the northern part of the crater may be due to the destruction of impact melt and This research formed part of the dissertation research of A.M. volatile-rich crater ejecta by terrace formation. These features pro- and benefited from numerous comments and encouragement from vide evidence for volatiles in the subsurface that were liberated by F. Scott Anderson, Sarah Fagents, Scott Rowland and Ross Suther- the impact event. land. In addition, Alfred McEwen, Chris Okubo and Livio Tornabene The relative timing of flow formation and crater relaxation im- are thanked for their efforts to acquire the excellent HiRISE cover- plies spatial differences in subsurface volatile distribution. If flows age of Tooting crater that is now available. Livio Tornabene is also formed equally around the crater, the volatile source is likely to thanked for his lively and enthusiastic formal review of an earlier have been laterally extensive. The observation that terrace blocks version of this manuscript. This research was supported by NASA near Flow 3 exhibit vertical variations in gullying suggests that Grant NNX06AC61G from the Planetary Geology and Geophysics the volatile source is likely not vertically extensive. If the flows Program and by NASA Grant NNG0-5GL98G from the Mars Funda- formed after the cessation of terrace formation, the volatile source mental Research Program. is likely less laterally extensive. Formation of debris flows in an area restricted to the southern outer rim of the crater suggests that Appendix A. Derivation of digital elevation models the volatile source was laterally heterogeneous and localized to the region of the flows. Our method for the generation of the digital elevation models for We propose that the preferential preservation of the impact Tooting crater is based on the stereo-mapping techniques described melt and the formation of debris flows on the southern portion by Stevens et al. (2004). We use the PDS EDR versions of the HiRISE of Tooting crater is likely related to the lack of large-scale terrace images available at http://hirise-pds.lpl.arizona.edu/PDS/EDR, and collapse on the southern half of the crater. In order to test this use ISIS 3 routines for mosaicking and removal of camera distor- hypothesis, we suggest a survey of the rim deposits and interior tions similar to that described by Kirk et al. (2008). (The images cavities of the freshest and best-preserved fluidized ejecta craters are not mapped into a map projection at this stage.) For the CTX im- (either the SLE or MLE craters defined by Barlow et al., 2000) age preprocessing, no mosaicking is required but calibration again >15 km in diameter, jointly using CTX and HiRISE images together. is performed using the ISIS 3 programs, mroctx2isis and ctxcal. The presence and/or absence of materials as identified above may The processing of the stereo pair to parallax determination and sub- provide clues to the precise process(es) occurring in the formation sequent height determination data uses software written in-house. of the impact-related flow features. For example, if impact melt is We rely on a series of frequency domain cross correlation kernel identified at distances >1 crater radius away from the crater rim operations to refine the auto-matching with the finest kernel at (the maximum distance on the Moon as observed by Hawke and 8 Â 8 pixels. This means that the spatial resolution of the derived Head (1977)), then our interpretations of Tooting crater may need DEMs is of the order of 2 m for HiRISE and 50 m for CTX. Retrieval to be revisited. Furthermore, the identification of the combination of emission angles using the ISIS program CAMPT for the input pair of large terrace blocks, a notably shallower floor, and extensive of images allows for the conversion from parallax to relative height flow features on a crater rim will indicate that the preferential and absolute elevations are provided by a polynomial surface fitting preservation of flow features on Tooting crater is not the result routine utilizing all the available individual elevation points de- of a lack of slumping, and that these flows may have a unique ori- rived from the Mars Orbiter Laser Altimeter (MOLA) instrument, gin because of unusual target properties. which typically amounted to 3000 data points for the CTX stereo The preservation of the flows of sediment, likely lubricated by pair and 500 data points for each of the HiRISE stereo pairs. An water released during or closely after (perhaps only a few days?) analysis of the predicted heights from the DEM and the MOLA shot an impact event, could be unique to Tooting crater; our inspection values has a correlation of 95–99%. We used the following image of craters imaged by HiRISE reveals that evidence of sediment flows pairs to produce the DEMs used here: at older craters on Mars is lacking and implies that if such flows ever CTX images: existed they most likely have been destroyed by subsequent crater P01_001538_2035 and P03_002158_2034 modification. The combination of attributes for Tooting crater (a HiRISE images: young, fairly large crater that formed in young, layered volcanic tar- PSP_002580_2035 and PSP_002646_2035 get materials) may be the key to identifying preserved impact melt PSP_001538_2035 and PSP_002158_2035 and impact-induced debris flows. Nevertheless, a search of high- resolution data may reveal additional craters with similar morphologic features in targets with high mechanical strength. Potential References targets include fluidized ejecta craters that formed on volcanoes Barlow, N.G., Perez, C.B., 2003. Martian impact crater ejecta morphologies as such as Ascraeus Mons, Arsia Mons, Olympus Mons, Alba Patera indicators of the distribution of subsurface volatiles. J. Geophys. Res. 108 (E8). and Elysium Mons. Analyses of rayed craters on Mars (2–10 km doi:10.1029/2002JE002036. A.R. Morris et al. / Icarus 209 (2010) 369–389 389

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