Mapping rays and secondary craters from the Martian crater Zunil

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006je002817, 2007 Mapping rays and secondary craters from the Martian crater Zunil Brandon S. Preblich, 1 Alfred S. McEwen, 1 and Daniel M. Studer 1 Received 21 August 2006; revised 8 November 2006; accepted 30 November 2006; published 17 May [1] Zunil, a 10.1 km rayed crater in Elysium Planitia, Mars, produced more than secondary craters 15 m in diameter. We mapped Zunil s rays from thermal IR THEMIS nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters (candidate secondaries) up to 3600 km range. Ray segments were mapped up to 450 km east of Zunil and up to 1700 km to the west. Both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia, which are abundant west of but not east of Zunil. Nevertheless, our interpretation is that Zunil was created by a moderately oblique impact from the east. Zunil secondaries are abundant over all terrain types except the Medusae Fossae Formation (MFF). Given the likely age of Zunil (<100 Ma), parts of the MFF must be eroding at 0.08 m/ma. The size-frequency distribution of the secondaries in Zunil s rays and probable distant secondaries ( km to the west of Zunil) have a cumulative power law exponent near 5, whereas secondaries between the major rays have an exponent near 3.4. We modeled the sizevelocity relationship for Zunil s ejected fragments; it is consistent with the predictions of Melosh (1984) for spallation of a strong surface layer and demonstrates that the inverse size-velocity correlation continues up to at least 2 km/s. Citation: Preblich, B. S., A. S. McEwen, and D. M. Studer (2007), Mapping rays and secondary craters from the Martian crater Zunil, J. Geophys. Res., 112,, doi: /2006je Introduction 1.1. Origins of Small Impact Craters [2] Relative and absolute age estimates for Martian terrains are essential for understanding the geologic history of Mars, including climate changes and potential habitability. There are diverse terrains on Mars that have no or few large (>1 km diameter) superimposed impact craters, so only small craters are available to attempt dating via crater statistics. However, there has been a longstanding controversy concerning the relative abundances of small primary craters ( primaries ) and distant secondary craters ( secondaries ) that do not show obvious clustering or other morphologies marking them as secondaries (reviewed by McEwen and Bierhaus [2006]). [3] How many distant, seemingly random (or not obviously clustered) secondaries can a large primary crater produce, and how are they distributed? At present we can t answer this question for the Moon because the numbers of small craters on the mostly ancient terrains have reached an equilibrium state; the smaller the diameter range, the sooner equilibrium is reached. We know that Tycho produced at least 10 6 secondaries within its bright rays, but cannot estimate the number of secondaries originally produced at sizes below 100 m or at all sizes between and beyond the rays [Dundas and McEwen, 2006]. Europa may be the ideal world for this type of study because there are only a few 1 Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA. Copyright 2007 by the American Geophysical Union /07/2006JE large (>10 km diameter) primary craters over a uniformly young surface, but the high-resolution imaging coverage is quite poor [Bierhaus et al., 2001, 2005]. In contrast, Mars is well imaged and has extensive areas that are not saturated with small craters, although effects of the atmosphere complicate interpretations. [4] Zunil s secondary population is an excellent case to address these issues as it is apparently quite young, the environment of Mars leads to distinctive ejecta blankets on very recent craters over most surrounding terrains, and we have good imaging coverage at visible and thermal IR wavelengths [McEwen et al., 2005; Tornabene et al., 2006]. For example, Zunil enables us to extend the measurements of fragment size versus fragment ejection velocity [e.g., Vickery, 1986, 1987; Nakamura et al., 1992] to velocities greater than 1 km/s, and this size-velocity relation is key to understanding why secondaries have a different sizefrequency distribution (SFD) than small primaries [McEwen and Bierhaus, 2006]. The primary versus secondary SFDs are key to determination of which diameter ranges might be dominated by secondaries and thus useless for dating via crater statistics. The study of secondaries can also provide insight into cratering mechanics, the origin of Martian meteorites, and the nature of surface materials on Mars. [5] One way to describe the SFD of the cratering of a surface area is Nð DÞ ¼ kd b ð1þ where N is the cumulative number of craters per square kilometer greater than or equal to a diameter, D. The 1of18

2 constant k is related to the crater spatial density, and b is the power law exponent or slope. Three alternative ways to describe the SFD (reviewed by Hartmann et al. [1981]) are the differential format (add 1 to b in differential counts to compare it with the slope in cumulative format), the R plot (divide the differential distribution by 3 power law slope), and the logarithmic-differential format (same b as cumulative format). All three differential formats avoid the masking of structure in a cumulative distribution and avoid the statistical pitfalls of fitting slopes to cumulative data [Chapman and Haefner, 1967]. In this work we use the logarithmic-differential format, which uses p 2 intervals between diameter bins. [6] A production function is a model for the number of craters over a specified diameter range produced per unit area, per time period. Model production functions for small craters on Mars have been derived from lunar production functions rather than measurements on Mars [Hartmann and Neukum, 2001; Hartmann, 1999, 2005; Neukum et al., 2001; Ivanov, 2001]. These production functions assume that the primary objects striking Mars are asteroids (with the contribution of comets being insignificant), and have the same SFD as objects striking the Moon. This mathematical transference from the Moon to Mars relies on the assumption that craters of all sizes are dominantly primaries. If secondaries are significant, then the equations are incorrect. [7] Shoemaker [1965] first proposed that distant secondaries could be abundant on the Moon. Secondary craters are the result of a larger primary crater s ejecta fragments impacting the surface. Some fragments are ejected with relatively low velocities and will land near the primary crater. These low-energy impact craters have irregular shapes and are shallower than fresh primaries [Pike and Wilhelms, 1978]. Sometimes a fragment will break up in early or middle flight, and the pieces will separate and create a cluster of craters. The cores of rays on the Moon and Mars often consist of elongated clusters of secondaries. In addition, herringbone patterns are sometimes produced from interactions of simultaneous ejecta [Oberbeck and Morrison, 1973]. These are all good identifiers of secondaries. However, when the fragments are ejected with a relatively high velocity, they will land a great distance away from the primary crater (assuming the ejection angle is not close to 90 ), and they will not have such irregular shapes due to relatively high energy impacts. Since these highvelocity fragments have longer flight times than low-velocity fragments, they are able to separate to a greater degree. This produces much looser clusters of craters, and no herringbone patterns will result from these impacts. (There are many tight clusters of very small (<20 m) craters on Mars, probably due to breakup as the objects impact the atmosphere [Melosh, 1989]; these could be either primaries or secondaries.) As a result of these factors, secondaries produced by high-velocity fragments become difficult to distinguish from small primaries, and this difficulty becomes most pronounced with increasing range from the primary. [8] Some workers have argued that craters of all sizes are dominated by primaries, and that distant secondaries are relatively inconsequential in number [Moore et al., 1980; Neukum and Ivanov, 1994; Ivanov, 2006]. The primary production functions on relatively young volcanic plains on the Moon and Mars between 1 km and 50 km diameter have b 2 [Hartmann et al., 1981; Tanaka, 1986, Barlow, 1988; Strom et al., 2005; Tanaka et al., 2005]. Neukum and Ivanov [1994] argued that the SFD slope steepens for small primary craters throughout the solar system, as shown by the steep SFD slope (b 3.1 to 3.7) of sharp (relatively recent) craters smaller than 1 km diameter on the asteroid Gaspra [Chapman et al., 1996]. However, Bottke and Chapman [2006] presented evidence that Gaspra had a unique recent cratering history, and most asteroid surfaces have been bombarded by a shallower distribution of small primary craters. In addition, recent models of the SFDs of near-earth asteroids predict that there should be only a modest steepening of the SFD slope at small diameters [Bottke et al., 2005; O Brien and Greenberg, 2005]. Ivanov [2006] revisited the arguments for primary origin of small lunar craters, chiefly because (1) the assumption that small craters superimposed over four young well-dated lunar craters are primaries gives consistent results and (2) modeling of lunar crater sizes based on small fireballs and explosions detected in Earth s atmosphere can account for the numbers of small craters. [9] The relative numbers of primaries and secondaries are not necessarily the same on the Moon and Mars [McEwen and Bierhaus, 2006]. For example, the current Martian atmosphere could disrupt and decelerate many high-velocity carbonaceous asteroids and comets that would make small primary craters on an atmosphereless body [Chappelow and Sharpton, 2005]. [10] Hartmann [2005] argued that distant secondaries could be present in significant numbers, but that they are nevertheless useful for dating surfaces or processes. He assumes that distant secondaries are widespread and uniform in their large-scale spatial distribution, and doesn t consider the temporal clustering to be important over long timescales. How accurate are these assumptions? Zunil provides a case study to begin to address these and other questions Zunil Secondaries [11] Large concentrations of small, bright-rayed craters (Figure 1) are present in Elysium Planitia [McEwen et al., 2005]. (Note that Elysium Planitia was recently redefined ( to encompass a smaller area than the previous definition, and now closely corresponds to the region referred to as the Cerberus Plains in previous publications.) These bright ejecta craters appear fresh, as evidenced by sharp crater rims and rays that extend out to a 5 10 crater diameters. These fresh craters also have a distinctive appearance in THEMIS (Thermal Emission Imaging System) infrared (IR) images [Christensen et al., 2004, 2005], which is a result of their thermal inertia (TI) values (Figure 2). They often have the following facies in the visible and thermal IR: (1) relatively low albedo, high- TI interiors and rims, (2) intermediate-albedo, intermediate to high TI inner ejecta, and (3) relatively high albedo, low- TI rays and outer ejecta [see McEwen et al., 2005, Figure 3]. There are also craters where the high-albedo ejecta seems to be missing (Figure 2); we refer to these as dark ejecta craters. The dark material of the two inner facies may be produced when craters are large enough to excavate an underlying dark, rocky substrate. The outer facies must 2of18

3 Figure 1. Mars Orbital Camera (MOC) [Malin et al., 1992] images showing bright ejecta craters. (left) Portion of R at 3.10 m/pixel. The largest craters in this image are 50 m in diameter; the smallest are at the limits of resolution. (right) Portion of R at 4.58 m/pixel. The largest craters in this image are 115 m in diameter, and the bright ejecta has been partially eroded and streaked by the winds. North is 7 to the right of up in these images. consist of very fine-grained (dust-sized) material, which covers the outer ejecta and rays in the Martian environment. There are two possible explanations for the absence of the bright ejecta surrounding dark ejecta craters from Zunil. One possibility is that the bright ejecta was removed by aeolian erosion, but not the more resistant or voluminous inner, darker ejecta layer. Moving west to east across the Zunil field, the bright ejecta is increasingly streaked and eventually disappears (see Figure 3) [see also McEwen et al., 2005, Figure 4]. A second possibility is that the bright ejecta is present, but cannot be distinguished from the bright background; in some cases the outer facies can be seen in nighttime thermal IR images but not visible images (Figure 2). There are areas where both bright and dark ejecta craters exist in a region (Figure 3); only the largest craters still have some bright ejecta. There are also places where most of the small craters have no apparent bright or dark ejecta, yet they occur in chains radial to Zunil and are probable Zunil secondaries (Figure 4). [12] Zunil seems unusual since it has few secondaries and no rays within 16 crater radii (80 km). The closest secondaries to Zunil make up a cluster of craters 12 to 27 m in Figure 2. (left) Zunil s rays in THEMIS nighttime IR image I at 100 m/pixel. (middle and right) MOC image E at 6.17 m/pixel. Notice the correspondence between bright, warm interiors of craters in the THEMIS nighttime IR image and the dark ejecta craters in the MOC image. North is up on the THEMIS nighttime IR image. 3of18

4 Figure 3. Bright and dark ejecta craters that both exist in the same area. The bright ejecta is being eroded and streaked by the wind and only persists around the largest craters and may have initially been present around the smaller dark ejecta craters. (left) A portion of MOC image R (right) Portion of MOC image E Both images are 6 m/pixel. North is 7 to the right of up. diameter about 6 km northwest of the rim of Zunil. Even though some secondaries are located relatively close to Zunil, their density is very low. There are also no large blocks (20 m) near its rim (Figure 5), so we expect that the absence of nearby secondaries is because the target material breaks up into small blocks, perhaps due to jointing, and those blocks ejected at low velocities do not make discernable craters. The largest Zunil secondary crater found to date is only 230 m in diameter, whereas typically the largest secondary is 5% of the primary crater s size [Melosh, 1989], which would be 500 m in this case. Most of Zunil s secondaries were created by smaller (<20 m) blocks or clumps ejected at high velocities, creating equidimensional craters that would be hard to identify as secondaries if not for the IR rays and fresh ejecta, which tie them to Zunil in this region. [13] In this paper we present detailed maps of the locations of Zunil s rays and secondary craters, and present measurements of the number, distance, and size-frequency distribution of these craters. We explore how thermal inertia and geologic units affect observations of rays and bright/ dark ejecta craters, and interpret the full extents of the secondary field. We also estimate the ejecta size-velocity relationship at high velocities (>1 km/s). Finally we discuss the significance of these results to dating young surfaces, which is essential to understanding the recent geologic and climate history of Mars. 2. Mapping Zunil s Rays and Secondary Crater Field [14] In order to map the area around Zunil (7.7 N, 166 E), we first extracted a MOLA (Mars Orbiter Laser Altimeter) [Smith et al., 2001] shaded relief map in Simple Cylindrical projection from tharsis/shademap.html [Neumann et al., 2001] that extended 1700 km (29 of latitude and equatorial longitude) from Zunil in every direction. Next, we map projected (using ISIS: and overlaid (with Photoshop) all available THEMIS daytime IR images in the region over this shaded relief map. We used all THEMIS IR Figure 4. Portion of MOC image E , located at 7.8 N, E, 120 km west of Zunil. All except the largest crater lack bright or dark ejecta yet occur in chains radial to Zunil. North is 7 to the right of up. 4of18

5 Figure 5. Mosaic of Zunil using 5 MOC images at 3 5 m/pixel. No impact craters superimposed on Zunil have been found. The unit covering the flat bottom of the crater is densely pitted, but these pits do not have raised rims or other characteristics of impact craters. The largest blocks in the near-rim ejecta are 10 m diameter. North is up. images available as of summer 2004 for this base map. Projected THEMIS nighttime IR images were next overlaid using the daytime IR mosaic as a base map. In order to overlay the MOLA map the map-projected images needed slight rotations and cross-track scale changes to compensate for deficiencies in the THEMIS camera model available at this time. These IR images were high-pass filtered to create a uniform mosaic, which preserves morphologic but not radiometric information. [15] The nighttime (typically 3 5 AM) IR mosaic was used for identifying and mapping rays because the low-ti rays (containing relatively high-ti spots marking the larger craters) are easiest to identify on the nighttime IR mosaic (e.g., Figure 2). Daytime IR images are affected by a combination of topography, albedo, and TI, whereas predawn temperatures are affected primarily by TI. Also, many of the daytime IR images were acquired in the late afternoon when temperature differences due to variations in TI are small. From the nighttime IR mosaic, rays were found to extend in every direction from Zunil (Figure 6). The rays depicted in Figure 6 were identified either due to their overall TI contrast with the surrounding area or due to strings of large secondary craters (or clusters of craters) that are seen in the nighttime IR. The IR rays are asymmetric, suggesting an oblique impact from the east or somewhat from the northeast. However, this apparent asymmetry may be in part due to a combination of lower TI in the east (discussed below) and increased aeolian erosion to the east [see McEwen et al., 2005, Figure 4]. Zunil s longest IR ray extends 1100 km to the northwest of Zunil. However, there are small segments of IR rays up to 1700 km to the west and southwest of Zunil (beyond the edge of Figure 6). The shortest IR ray extends 450 km to the east. The average extent of the IR rays to the north, south, and west is 750 km. Typically, the IR rays are about 2 6 km in width. The closest IR rays to Zunil are about 150 km away. [16] To further understand the nature of the rays, the secondary crater density was studied in cross sections of rays in two MOC images (Figure 7). The density is highest near the center of the ray, and decreases with distance to the edge of the ray, until the density becomes the same as the local density outside of the ray. These data demonstrate a strong correlation between the location of the low-ti material rays seen in the IR and a high crater density. 5of18

6 Figure 6. MOLA shaded relief map with Zunil s rays as seen in the nighttime thermal IR. Rays are mapped in yellow and outlined in orange to make them more apparent. The map extends from latitude 6 S to18 N and longitude E. There are additional small ray segments to the west and south, beyond the edges of this map. [17] We mapped the locations of bright and dark ejecta craters over a 3300 km wide shaded relief map, using every THEMIS VIS and MOC image available by late Figure 8 displays the bright and dark ejecta craters found in THEMIS VIS images (18 36 m/pixel; craters generally larger than 100 m diameter); most are within 1000 km of Zunil. These craters are not necessarily Zunil secondaries, especially the dark ejecta craters. The dark (rocky) ejecta may be more resistant to aeolian modification than is the bright ejecta, and some of the dark ejecta craters could predate Zunil. We interpret most of the bright ejecta craters on this map as secondaries from Zunil, as evidenced by their close association with Zunil and its rays, and observations that the bright ejecta are being removed by wind, so they must be recent. The dark ejecta craters are confined to the relatively recent lava flows, as discussed below, and Zunil happened to form in the middle of this recent field of flood lavas. Many of the dark ejecta craters are Zunil secondaries, as they are concentrated in rays (e.g., Figure 2). Figure 9 displays the bright and dark ejecta craters found in narrowangle MOC images over this region; in this map, bright ejecta craters are found in abundance much further from Zunil, out to the boundaries of the map (1700 km west and south of Zunil), in contrast to Figure 8. This is consistent with secondary crater diameter decreasing with distance from Zunil (MOC images at m/pixel allow detection of smaller craters than do THEMIS VIS images at 18 m/pixel). Ray segments oriented radial to Zunil also extend 1700 km west and southwest of Zunil, to the edges of the area mapped. Another noteworthy observation is that the bright/dark ejecta craters are common in between the rays, albeit at a lower density. We interpret the bright ejecta craters up to 1700 km to the west as most likely from Zunil because they are in a somewhat continuous band of craters that can be traced back to Zunil and there are ray segments radial to Zunil. The distribution of rays and bright ejecta craters is shown in Figure 10, which provides the best data to interpret the approximate extent of the Zunil secondary field. Bright ejecta craters more than 1700 km west of Zunil could also be Zunil secondaries, as discussed below. 3. Size-Frequency Distribution of the Secondary Craters [18] In order to estimate the sizes and number of secondary craters produced by Zunil, we measured crater populations in 36 areas, which include areas inside, outside, and beyond its IR rays (Figure 11 and Table 1). We counted all craters with bright or dark ejecta that could be seen in the MOC or THEMIS images. Some examples of these counts are plotted in Figure 12. Zunil s rays contain large numbers of secondary craters, with an average density of 95 craters/km 2 15 m in diameter. There are also numerous bright/dark ejecta craters between the rays, with an average density of 12 craters/km 2 15 m in diameter. The counts outside 700 km are not labeled as inside or between the IR rays in Table 1 because IR rays are not usually identifiable beyond that range. These crater counts include only bright and dark ejecta craters (the vast majority of the craters in these counts have bright ejecta rather than dark ejecta), and we do not attempt to interpret craters that lack the bright/dark ejecta to be either secondaries (from Zunil or elsewhere) or primaries. However, there are secondaries for which the bright/dark ejecta is not apparent, as evidenced by some of the obvious chains and clusters of secondaries near Zunil, which do 6of18

7 Figure 7. Crater density (>30 m) plotted against cross-sectional distance from the center of the ray. Below the plots the corresponding MOC images are shown, with sections counted (divided by lines parallel to the rays). (left) MOC image M at 5.87 m/pixel. (right) MOC image M at 5.87 m/pixel. North is 7 to the right of up in these images. not have distinctive ejecta (Figure 4). The bright/dark ejecta in these locations either does not form or is not seen because of erosion or a lack of contrast with the surface. Therefore our results may constitute a lower boundary of the number of secondaries produced by Zunil, but this deficiency is probably less than a factor of two. Only the craters 5 pixels in diameter were used for determining the SFD, due to the difficulty of achieving complete counts with fewer pixels. Five pixel diameter craters seem to mark the completeness limit for binned MOC images, which are the great majority of MOC images. Observations 34, 35, and 36 are all from unbinned (1.5 m/pixel) images but only the larger diameter craters (15 m) in these three counts were used in subsequent SFD fits because the counts appear to roll over below 15 m. We interpret this rollover as most likely due to observational limits even though greater than 5 pixels, because the full-resolution MOC images have a poorer signal-to-noise ratio than the binned images and because the point spread function of the optics is larger than a pixel [Malin et al., 1992]. The rollover may also be influenced by background roughness at small scales or it could mark a true rollover in crater production over these areas. However, we note that large numbers of small (<10 m) bright ejecta craters are present in many MOC images (e.g., Figure 1, left), but we cannot accurately measure their diameters until HiRISE images are available [McEwen et al., 2007]. [19] The average best fit SFD of bright/dark ejecta craters inside the rays is 4.9 log D + 7.9, and that between the rays is 3.4 log D + 5.0, for diameters from 15 m to 150 m. We determined linear fits to the log of the differential data. The SFD slope inside the rays is significantly steeper (b 4.9) than that for bright/dark ejecta craters between the rays (b 3.4). This difference in slope is somewhat puzzling, and may be an intrinsic difference in the fragment distribution, or it may be a result of contamination of counts by non-zunil craters. This potential contamination would not significantly affect the results much inside the rays because of the density of Zunil craters. However, we do not believe that the counts are significantly contaminated by non-zunil craters because the bright/dark ejecta craters are so distinctive and appear identical over a terrain whether inside or outside of a ray, and similar craters are rare away from the secondary fields of 7of18

8 Figure 8. Bright ejecta craters (in red) and dark ejecta craters (in yellow) found in THEMIS VIS images, overlaid on a shaded relief map. Available VIS coverage is illustrated by transparent purple rectangles. Crater density is not to be inferred from this map or those in Figures 9 11 as the coverage and image quality are not uniform. Bright/dark ejecta crater areas appearing in NE SW slanted rectangular shapes are artifacts due to the shape and slant of the VIS images. The map extends from latitude 20 S to 34 N and longitude E. Figure 9. Bright ejecta craters (in red), and dark ejecta craters (in yellow) found in MOC images, overlaid on a shaded relief map. MOC coverage is illustrated by transparent purple rectangles (covered in places by the bright and dark ejecta markings). Bright/dark ejecta crater areas appearing in N S rectangular shapes are artifacts due to the shape of the MOC images. The map extends from latitude 20 S to 34 N and longitude E. 8of18

9 Figure 10. Rays (yellow- orange) and bright ejecta craters (in red, from VIS and MOC images), overlaid on a shaded relief map and color-coded thermal inertia (TI) from Putzig et al. [2005]. We interpret the bright ejecta craters within and between the ray segments as likely Zunil secondaries, but they may not show up on areas of low TI. This map provides a guide to estimating the extent of the secondary field. White arrows point out some of the distant ray segments. The map extends from latitude 20 S to34 N and longitude E. rayed craters. The explanation we favor for this difference in SFD slope within rays versus between rays is an intrinsic difference in the fragment size, velocity, and/or ejection angle distributions. 4. Discussion and Interpretation 4.1. Effects of the Target Materials Geologic Units [20] In order to determine how the target material affects the distribution of rays and bright ejecta craters from Zunil, we overlaid the IR rays on top of a regional geologic map (mostly from Lanagan [2005] with southern portion from Scott and Carr [1978]). IR rays are found on lava that postdates Marte Valles, older Cerberus lavas, ridged plains surfaces that are interpreted to be Hesperian flood lavas [Tanaka, 1986], and ancient highland terrains (Figure 13). To the southwest of Zunil, the IR rays reach the edge of the Medusae Fossae Formation (MFF), but are not seen over this terrain. Four small segments of the IR rays are seen about 1700 km to the southwest of Zunil (Figure 10), which suggests that the rays did indeed extend over and past the MFF. MOC coverage intersects one of these distant rays, and confirms its existence with the presence of dense bright ejecta craters in this location. [21] The bright and dark ejecta crater map was also overlaid on top of the terrain map (Figure 13). This map shows that these craters are also found mostly on pre-and post-marte Valles lavas, ridged plains surfaces, and highland terrains. Dark ejecta craters are almost exclusively found on the lava flow units. Bright and dark ejecta craters are not found over large portions of the MFF, but these craters are present both north and south of this region. MOC images of yardang-covered terrains within the mapped MFF unit do not show any clear bright/dark ejecta craters and, in fact, show almost no small craters. In a few places there are features that could be interpreted as the eroded remnants of small craters. There are some bright and dark ejecta craters seen on the western part of what is mapped as the MFF (Figure 13), but in these cases the terrain map is inaccurate on small scales, and the terrain is not MFF in these exact areas. [22] Since no evidence of either IR rays or bright/dark ejecta craters are seen over the MFF, this must mean that this surface is eroding at a much faster rate than the surrounding terrains. This idea is supported by the widespread yardangs (erosional landforms) covering this region and previous interpretations of the physical properties of this material [Bradley et al., 2002; Hynek et al., 2003]. The 9of18

10 Figure 11. Zunil, its rays (in orange), bright ejecta craters found in MOC images (in red), dark ejecta craters found in MOC images (in yellow), and locations of MOC images used for crater counts (numbers correspond to those in Table 1), overlaid on a shaded relief map. The map extends from latitude 20 S to 34 N and longitude E. minimum depth of erosion can be estimated if one takes the diameter of the largest secondary crater produced at this distance from Zunil (75 m in diameter at 1000 km distance), and applies the typical depth-to-diameter ratio of secondaries (0.11) [Pike and Wilhelms, 1978; McEwen et al., 2005]. From this simple calculation, we estimate that the MFF has eroded 8.25 m. McEwen et al. [2005] argued that Zunil s age is probably less than 10 Ma. Using 100 Ma for an upper limit to the plausible age of Zunil gives an erosion rate of >0.08 m/ma for the MFF, which is many orders of magnitude greater than the erosion rate of 10 5 m/ma reported for the Pathfinder landing site [Golombek and Bridges, 2000] Thermal Inertia and Albedo [23] The bright ejecta craters are often found in intermediate-ti regions around large, rayed primary craters but are not commonly found over low-ti surfaces (<150 J m 2 K 1 s 1/2 ). Figure 10 shows an anticorrelation between low TI and observed IR rays and bright ejecta craters, with the exception of the area within a radius of 750 km around Zunil, where the bright ejecta craters are larger and/or the dust is thinner. For instance, bright ejecta craters are not found in the low-ti region around Elysium Mons (northwest corner of Figure 10) and few are found in the very low TI region more than 450 km east of Zunil. Tornabene et al. [2006] reported that a set of rayed craters best seen in the thermal IR (almost) all occur in thermophysical Unit C of Mellon et al. [2000] and Putzig et al. [2005], which consists of intermediate TI and intermediate albedo. The partial exception is Zunil, the largest of these rayed craters. The correlation with thermophysical properties is further supported in Figure 14, where we found few bright or dark ejecta craters (in MOC subphases M07- M12) in low-ti areas in this 9000 km wide map, with the exception of the area around Zunil. Both fresh craters and rayed primaries are most easily seen over moderate thermal inertia, moderate-albedo terrains. This does not necessarily indicate that bright ejecta craters do not exist in other regions, but simply that the bright ejecta is not apparent when the background terrain has a similar TI and albedo Interpretation of Distant Bright and Dark Ejecta Craters [24] We have mapped bright/dark ejecta craters in all available (as of late 2005) MOC and THEMIS images within a 1700 km radius of Zunil (Figures 8 and 9). We interpret the bright/dark ejecta craters between ray segments (up to 450 km east and 1700 km west) as most likely from Zunil because they are in a somewhat continuous band of craters that can be traced back to Zunil, and there are at least segments of rays that are radial to Zunil. We cannot be sure that the mostly dark ejecta craters in Marte Valles are from Zunil, as the distribution of dark ejecta craters is strongly correlated with the distribution of lava flows (Figure 13). 10 of 18

11 Table 1. Zunil Secondary Crater Counts Number in Figure 11 MOC Image Number Scale, m/pixel Number of Craters Area, km 2 Inside or Between Rays? Best Fit Log N/km 2 Distance (Direction) From Zunil N (15m)/km 2 1 R inside 4.75 log D km (W) M between 3.71 log D km (W) M inside 5.14 log D km (N) M inside 4.64 log D km (NW) E inside 4.94 log D km (NW) M between 4.42 log D km (NW) M bot inside 6.55 log D km (W) M bot inside 4.64 log D km (W) M top inside 5.36 log D km (W) M top between 3.17 log D km (W) E inside 4.35 log D km (W) R inside 4.83 log D km (W) E inside 4.30 log D km (W) M bot between 2.64 log D km (W) M top between 2.91 log D km (W) R between 3.75 log D km (W) M mid inside 6.00 log D km (W) M top inside 5.47 log D km (W) E between 2.44 log D km (W) M bot between 4.34 log D km (W) M log D km (W) E log D km (W) E log D km (W) E log D km (W) E log D km (W) FHA log D km (W) R log D km (W) M log D km (W) R log D km (W) M log D km (W) M log D km (W) E log D km (E) M log D km (S) E between 3.00 log D km (W) E inside 3.03 log D km (W) M log D km (W) 53.5 [25] How much further do Zunil s secondary craters extend to the west and southeast? To answer this question, we looked at every MOC image in the MGS mission subphases M07 M12 (taken September 1999 to February 2000) in the region shown in Figure 14. We chose this group of subphases because it contained a large number of images that were well distributed over the area around Zunil. Since we searched only a few subphases, it was feasible to examine MOC images a very large distance from Zunil. In this survey, we found that the fresh craters (most have bright ejecta) extend up to 3600 km to the west of Zunil in a more or less continuous band, which we interpret as the downrange direction (see section 4.3). Note that the hydrodynamic simulation of Zunil [McEwen et al., 2005] produced secondaries up to slightly more than 3200 km downrange of the oblique impact; more distant craters could not form due to escape from Mars or atmospheric deceleration of the smaller (and higher-velocity) fragments. Thus it is plausible that the Zunil secondary field extends 3600 km to the west. An alternative hypothesis is that much of this terrain is especially suitable for preservation of bright ejecta craters, many of which may be older than Zunil. [26] Figures 10 and 14 also illustrate that there are bright/ dark ejecta craters around Gusev Crater, to the southsoutheast of Zunil. The small craters encountered by the Spirit rover in Gusev crater have been interpreted as secondaries [Golombek et al., 2006], but none of these have distinctive bright or dark ejecta. Despite the large areal gap in bright/dark ejecta craters between Zunil and Gusev, we cannot rule out the possibility that these fresh craters may be associated with the Zunil impact, because the gap is largely covered by MFF which may be rapidly eroding and would have erased any Zunil secondaries. Likewise the bright/dark ejecta craters that extend north of Zunil, beyond the IR rays, may or may not be from Zunil. [27] Do these bright/dark ejecta craters extend further to the southeast? Figure 14 shows the small bright/dark ejecta craters around rayed craters Gratteri and Zumba to the southeast of Zunil as well as Dilly and Tomini to the northwest of Zunil [Tornabene et al., 2006]. These four craters (2 to 7.4 km diameter) were formed by relatively recent impacts, as evidenced by preservation of their rays, best seen in the thermal IR; some of these are better preserved than those of Zunil. Gratteri (6.9 km in diameter) and Zumba (3.3 km in diameter) have ray lengths of 595 km and 207 km respectively, whose sizes and directions are schematically illustrated in Figure 14. The orientation of the Gratteri rays are consistent with the bright/dark ejecta craters found near them, so these and others further east are not likely to be secondaries from Zunil. [28] Tornabene et al. [2006] identified two other craters, Dilly and Tomini, both of which are located near Elysium Mons. While we cannot be certain that there is no contamination of secondaries from these two rayed craters in the 11 of 18

12 Figure 12. Plots of selected crater counts from Table 1. All data points from craters 5 pixels are plotted. Count 11 is located within MOC image E (3.09 m/pixel). Count 18 is located within MOC image M (5.87 m/pixel). Count 22 is located within MOC image E (3.09 m/ pixel). Count 35 is located within MOC image E (1.48 m/pixel). Solid line is a saturation equilibrium model [Hartmann, 1999]; none of the counts in Table 1 appear to reach saturation at measured diameters. Error bars are simply the square root of the number of craters in each diameter bin. Zunil secondary crater counts, there are reasons as to why their effects should be minimal within 1700 km of Zunil. Dilly (2 km in diameter) is located 630 km to the northwest of Zunil, within the extent of Zunil s rays. While it is close to Zunil, it should have comparatively few secondaries 15 m in diameter and they are not likely to extend far enough to be able to contaminate counts hundreds of kilometers away, due to Dilly s small size and due to the short (50 km) extent of its rays. Tomini (7.4 km in diameter) is located 2500 km to the westnorthwest of Zunil. It has rays that extend km, and is large enough to create many secondaries, although probably not as many as Zunil. Tomini was most likely an oblique impact as suggested by a possible forbidden zone to its south-southwest [Tornabene et al., 2006], and so most of its secondaries should be to its north-northeast (downrange), which is not directed toward Zunil. However, we found many bright ejecta craters in its southeastern ray. If the ray extended 1000 km to the southeast of Tomini, it would cross Zunil s western ray of bright/dark ejecta craters (1700 km away from Zunil). It is possible that Tomini s ray of bright ejecta craters extended this far because there are bright ejecta craters found along what would be its southeastern path. One might think that most of the bright ejecta craters on the western edge of the map are from Tomini because this area is closer to Tomini than Zunil. However, Tomini is smaller than Zunil, and so the range of its secondaries should be smaller, if all else were equal. Tomini s secondaries in this area will most likely have the same characteristics as Zunil s and they, too, will be small due to the large distance from Tomini. Pending further mapping, we cannot rule out the possibility that this area may include Tomini secondaries. [29] In summary, we consider it highly likely that bright/ dark ejecta craters up to 450 km east and 1700 km west of Zunil originated predominantly from Zunil, and we consider it plausible but less certain that the small fresh ejecta craters extending west-southwest km are dominated by Zunil secondaries. Some of the bright/dark ejecta craters near Gusev crater or more than 750 km north of Zunil could also be Zunil secondaries, but those further east are probably not Did a Moderately Oblique Impact Produce Zunil? [30] The formation and/or detectability of IR rays and bright ejecta craters is clearly influenced by thermophysical properties of the surface layer. This raises the question of whether the asymmetric distribution of rays and secondaries around Zunil was really due to an oblique impact or due to 12 of 18

13 Figure 13. Bright ejecta craters (in blue) and dark ejecta craters (in yellow) found in MOC and VIS images, over a terrain map. Crater density is not to be inferred from this map. Map extends from latitude 20 S to34 Nand longitude E. Top portion of the terrain map is from Lanagan [2005], and bottom portion is from Scott and Carr [1978]. thermophysical properties. Maybe abundant secondaries exist to the east of Zunil that cannot be identified because of the extremely low TI and high albedo of the surface. However, there are also distinctive strings of large secondaries or clusters of secondaries to the south and west of and radial to Zunil (e.g., Figure 4). We do not see such morphologies more than 450 km east of Zunil, which should be recognizable in spite of the lack of distinctive ejecta. Furthermore there are theoretical and experimental results that indicate that oblique impacts should produce larger numbers of secondary craters [e.g., Artemieva and Ivanov, 2004] (see review by McEwen and Bierhaus [2006]), and Zunil certainly produced a large number. The other rayed craters described by Tornabene et al. [2006] show clear evidence for oblique impacts. The preponderance of the evidence suggests that a moderately oblique (30 to 60 from vertical) impact from the E NE created Zunil Number of Secondary Craters Produced by Zunil [31] To estimate the total number of secondary craters produced by Zunil, the area around Zunil was divided into regions as shown in Figure 15. The resulting numbers of secondaries in each region are estimated in Table 2. The average crater density in each region (Table 1) was multiplied by the area of each region to estimate the total number of craters. Region 1 consists of the IR rays within 750 km radius of Zunil, and region 2 is nonray areas within this circle. Region 3 includes bright/dark ejecta craters that seem to be continuously abundant outward from Zunil, except where interrupted by the Medusae Fossae Formation (discussed above). Region 4 includes mostly dark ejecta craters in Marte Vallis; we are not confident that these originated from Zunil. Region 5 includes bright/dark ejecta craters that are far removed from Zunil but not in the downrange direction, so again we are not confident that they are Zunil secondaries. There are probable Zunil secondaries north of region 2 that were excluded, but the numbers are relatively minor. [32] We estimate that Zunil produced a total of probable secondaries 15 m in diameter within a 1700 km range, from regions 1, 2, and 3. This estimate does not include bright/dark ejecta craters found in regions 4 and 5, since it is possible that they are non-zunil craters. Also excluded from our estimates are bright/dark ejecta craters located more than 1700 km to the west of Zunil, discussed above. If these are secondaries from Zunil, then the total estimate could increase by as much as a factor of two. We 13 of 18

14 Figure 14. Extended MOLA shaded relief map depicting Zunil and MOC image outlines where bright/ dark ejecta craters were found (in white) and where they were not found (in black), from MGS mission subphases M07-M12. Rayed craters and their IR rays (in orange) are depicted with approximate direction and length. There are few bright/dark ejecta craters in the low-ti areas (<150 J m 2 K 1 s 1/2 ) except near Zunil and in Marte Valles. Map extends from latitude 50 S to45 N and longitude E. Figure 15. Regions outlined for the estimation of the number of secondary craters (see Table 2). Rays (in yellow-orange), bright ejecta craters found in MOC images (in red), and dark ejecta craters found in MOC images (in yellow) are overlaid on a shaded relief map. The map extends from latitude 20 S to 34 N and longitude E. 14 of 18

15 Table 2. Number of Secondary Craters From Zunil Region Average N (15 m)/km 2 Total Area, km 2 Total Number of Craters (15 m) 1. IR Rays Between rays (0 km to 700 km from Zunil) Region west/southwest of rays (700 km to 1700 km from Zunil) Region east of rays (700 km to 1700 km from Zunil) Region south of rays (700 km to 1700 km from Zunil) Total from regions 1, 2, and 3 (probable Zunil secondaries) Total from all regions also excluded small craters that lack bright or dark ejecta, some of which are likely to be Zunil secondaries (Figure 4). In spite of these uncertainties, the correct order of magnitude for craters 15 m is probably 10 8 and perhaps as high as 10 9 for craters 10 m. For probable secondaries down to 10 m in diameter within 1700 km range, we can extrapolate to a total of secondaries using b = 5 down to 10 m; secondaries using b = 4; and secondaries using b = 3, depending on how much the actual distribution rolls over below 15 m. In contrast, McEwen et al. [2005] made a rough estimate of 10 7 craters 10 m, which now appears too low by 1 2 orders of magnitude Inverse Size-Velocity Distribution of Ejected Fragments [33] How can secondary craters be more numerous than primary craters at small diameters? Since primaries are clearly more abundant than secondaries when diameters are large, secondaries must have a steeper SFD in order for their number at a given diameter to catch up and surpass primaries below a crossover diameter. Hartmann [1969, 2005] argued that the SFD of the fragments should be the same whether produced by impacts on a terrestrial planet or impacts on asteroids, and assumes that the SFD of craters would also be similar. One possible explanation for a difference in SFD between secondaries and small primaries can be found in the work of Melosh [1984], where he modeled an inverse size-velocity relationship for fragments ejected by spallation (low-shock, high-velocity ejection of near-surface material). This means that, on average, larger spalled fragments travel relatively slowly and smaller fragments travel relatively quickly. Vickery [1986, 1987] observed this relationship when she studied secondaries from three primaries: Copernicus (Moon), Aristillus (Moon), and Crater Dv (Mars), with diameters of 93 km, 55 km, and 26 km, respectively. All three primaries demonstrated an inverse size-velocity relationship. There have been other theoretical, experimental, and secondary crater studies confirming the reality of the size-velocity relation, which may occur in the main crater excavation flow as well as spallation of a thin surface layer (reviewed by McEwen and Bierhaus [2006]). As McEwen and Bierhaus [2006] demonstrated, the size-velocity relationship produces a much steeper SFD slope for secondary craters than the SFD of the fragments. Thus, even though both primaries and secondaries may be produced by populations of rocks that have about the same SFD, the SFDs of the craters must be very different. Asteroid fragments reaching Mars do not have an inverse size-velocity distribution because interplanetary projectiles have velocities that are set by their orbits (at Mars, these projectiles all have velocities of 10 ± 5 km/s). [34] Secondaries should dominate primaries at small sizes if the steep SFD slope continues for more than the 1 or 2 orders of magnitude in size range needed to catch up to the primary distribution [e.g., McEwen et al., 2005, Figure 16] and this is true for distant secondaries if the size-velocity relation applies to ejection velocities greater than 1 km/s. The Melosh [1984] model makes predictions out to an ejection velocity of 10 km/s, but the author expressed doubts about the validity of these predictions at velocities >1 km/s. The data of Vickery [1986, 1987] extend up to 1 km/s, as do laboratory experiments [e.g., Nakamura et al., 1992]. Zunil s distant secondary craters provide an opportunity to understand the inverse size-velocity relationship for fragments with ejection velocities >1 km/s. We plotted the largest Zunil secondaries in various MOC images against distance from Zunil (Figure 16). All of the craters plotted in Figure 16 were from MOC images west of Zunil. The general trend is that the largest secondaries (230 m) were found near Zunil, and the maximum crater diameter generally decreased with distance from Zunil. At a distance of 900 km, the largest secondaries are around 100 m in diameter. Two craters with diameters of 140 m and 175 m and a distance of 1600 km (circled in Figure 16) are outside of the observed trend. These two craters could be primaries. [35] A solid line tracing the upper envelope of the largest secondaries with range is shown in Figure 16, which has a negative slope indicative of an inverse size-velocity distri- Figure 16. Largest secondary in a given MOC image plotted against that MOC image s distance from Zunil. The solid line is an upper envelope of the data. The two circled data points are interpreted to be non-zunil craters. The solid line represents the expected crater size if 25 m diameter fragments were ejected at a range of velocities (at an ejection angle of 45 ) to show the expected function if there was no size-velocity correlation or anticorrelation. This dashed line shows a positive correlation between crater diameter and range, clearly in contrast with the observations. 15 of 18