46th Lunar and Planetary Science Conference (2015) 2488.pdf

SECONDARY CRATERS ASSOCIATED WITH THE RAYED CRATER ZUMBA, , . F. C. Chuang1, D. A. Crown1, and L. L. Tornabene2, 1 Planetary Science Institute, 1700 E. Ft. Road, Suite 106, Tucson, AZ 85719 USA; 2 Centre for Planetary Science and Exploration, Dept. of Earth Sciences, Western University, London, ON N6A 5B7, Canada ([email protected]).

Introduction: Zumba is a 2.9 km diameter impact Secondary crater rays, a few of which extend crater located in Daedalia Planum (center: 28.6° N, further than 200 km, were first mapped using THEMIS 226.9° E), an area of elevated plains at the southern data. After detailed examination of the rays and other margin of the region. The Daedalia Planum areas within the study region, secondaries were mapped area is dominated by expansive lava flow fields and on 5-6 m/pixel CTX images (49 total), which covered also includes embayed and isolated massifs of ancient 72% of the mapping area. Secondary craters are most highland materials, degraded rims, and often observed in groups within dark areas of the areas of rugged terrain [1-8]. The Zumba surface, presumably due to removal of bright dust by occurred on the lava flows, which from previous impact processes. In this study, only solitary and dense geologic mapping were identified as part of the groups of secondary craters (hereafter termed -age formation [1]. secondary fields) with associated dark deposits were One of most distinctive features of Zumba is the mapped and stored as polygon features using ArcGIS prominent rayed pattern emanating from the impact 10.2.2 software. site. In Thermal Emission Imaging System (THEMIS) Mapping Results: A total of 12,793 secondary infrared data, the rays are highly distinct, with some fields were mapped, covering a cumulative area of reaching hundreds of kilometers in length [9]. For 2459.2 km2. Statistics for the population of secondary Zumba and several other rayed craters on Mars [9,10], fields are provided in Figure 1. The vast majority of the rays correlate well with dense clusters of secondary fields (n=12,310; 96.2%) are small (0.005-1 secondaries that are best observed in Mars Orbiter km2). We find that small to medium-sized secondary Camera (MOC), Context Camera (CTX), and High fields are widespread throughout the 200 km radial Resolution Imaging Science Experiment (HiRISE) area whereas larger secondary fields (10-50 km2) are images [9-12]. This correlation is in contrast to other associated with and concentrated within the large craters on Mars with ray-like patterns in thermal data secondary crater rays. (e.g., and [13,14]), but that lack dense In order to analyze the distribution of Zumba clusters of secondaries. Instead, these patterns secondaries, we divide the mapping area into eight represent either ejecta rays or air-blasted surfaces. We primary 45° sectors (Figure 2). Within each of these refer to the rays in this study as secondary crater rays. sectors, we also divide the distance into 50 km We have recently completed detailed mapping of increments from the center of Zumba out to 200 km, Zumba secondary craters to investigate their for a total of 32 individual sectors. Sectors with distribution and density as a function of distance from coverage gaps occur within 100-200 km from Zumba. the impact site. Given the young age (potentially 100 For the entire study region, the percentage of area ka) [15], good state of preservation, and relatively covered by secondary fields in any given sector ranges uniform geology of the impact region, this information from ~0.095% to 19.4%. We find that the highest areal provides insight into the cratering process (e.g., coverages occur within 100 km of Zumba (a1-h1 & a2- direction and angle of impactor), a means to assess h2) and that beyond this, sectors c3-4, f2-3, and g2-4 ages derived from small crater populations [15,16] and exhibit the highest areal coverages due to the longest the spallation process on a body with an atmosphere secondary crater rays located in these areas. If we [17], and to gauge the volcanic compositions and consider each 50 km "ring" of sectors (8 sectors in stratigraphy at the impact site and where secondary each ring, A-D in Fig. 2) around Zumba, the peak craters formed (see [3]). Previous studies of Martian cumulative number and greatest areal coverage of rayed craters suggest that the Zumba impact site may secondaries occurs in ring B (Table 1). These values have a provided a source for Martian SNC meteorites decrease in rings C and D. [3,9]. Overall, the data show that from A to D, the Data, Methods, and Study Region: Global 100 average mean size of secondary fields decreases, which m/pixel THEMIS daytime and nighttime IR mosaics is consistent with the expectation of lower numbers and were used as the primary base in studying the regional smaller sizes of secondary craters with greater distance geologic features of Daedalia Planum and crater from the impact site. For rings A and B, the average Zumba. The mapping area consists of the entire region mean sizes are much larger (minimally ~3.6X) than for within a 200 km radius of Zumba. rings C and D. This is consistent with our earlier result 46th Lunar and Planetary Science Conference (2015) 2488.pdf

that most secondaries are located within 100 km of the Table 1. Statistics for areal coverage by Zumba secondary impact site. fields in sector rings A-D*. We have also made preliminary estimates of the Ring count min max med mean±stdev crater density for mean-sized secondary fields (0.193 (n) (km2) (km2) (km2) (km2) 2 km ) in the study area. Of the nine secondary fields A 354 0.0030 6.38 0.133 0.336±0.635 2 selected (within ±500 m of the mean), we count 5-18 B 490 0.0015 11.1 0.065 0.284±0.800 2 craters for a density of ~25.9-93.3/km . Assuming that C 383 0.0037 2.81 0.026 0.077±0.222 this density is solely due to the Zumba impact, D 386 0.0017 2.85 0.023 0.049±0.155 ~63,693-229,443 secondary craters were produced. * see fig. 2 for defined sector rings These values are ~1-3 orders smaller than estimates of secondary craters for the 10 km diameter rayed crater [10,11]. However, Zunil's population may be an overestimate due to the overlap of secondary craters produced by crater Corinto [18]. References: [1] Scott and Tanaka (1986) USGS Misc. Invest. Ser. Map I-1802-A. [2] Dohm et al. (2001) USGS Misc. Invest. Ser. Map I-2650. [3] Lang et al. (2009) JVGR, 185, doi:10.1016/j.jvolgeores.2008.12.014. [4] Scott (1981) USGS Misc. Invest. Ser. Map I-1274. [5] Scott and Tanaka (1981) USGS Misc. Invest. Ser. Map I-1281. [6] Crown et al. (2011) LPSC 42, Abstract #2353. [7] Crown et al. (2013) LPSC 44, Abstract #2499. [8] Bleacher et al. (2007) JGR, 112, doi:10.1029/2006JE002873. [9] Tornabene et al. (2006) JGR, 111, doi:10.1029/2005JE002600. [10] McEwen et al. (2005) Icarus, 176, 351-381. [11] Preblich et al. (2007) JGR, Figure 1. Histogram of areal coverage of Zumba secondary 112, doi:10.0129/2006JE002817. [12] Calef et al. (2009) fields showing high positive skew (~18). Approximately JGR, 114, doi:10.1029/2008JE003283. [13] Williams and 96% of secondary fields are 0.005-1 km2 in size. Malin (2008) Icarus, 198, doi: 10.1016/j.icarus.2008.07.013. [14] et al. (2011) Icarus, 211, doi:10.1016/j.icarus.2010.10.014. [15] Hartmann et al. (2010) Icarus, 208, doi:10.1016/j.icarus.2010.03.030. [16] Hartmann (2007) Icarus, 189, doi:10.1016/j.icarus.2007.02.011. [17] Melosh (2001) Impact Cratering, 245 pp. [18] Golombek et al. (2014) LPSC 45, Abstract #1470.

Figure 2. Azimuthal distribution of mapped Zumba secondary fields (black polygons). Hot- cool colors represent the percent area covered by secondaries in each sector (45° circumferential arc and 50 km wide from the crater center, 32 total). Highest densities are within 100 km of Zumba and in sectors containing the longest rays (c3-4, f2-3, g2-4). Statistics for each 50 km "ring" of eight sectors (A-D) are shown in Table 1. Background composed of THEMIS day IR base mosaic with overlying CTX images.