Characterization of Ejecta Facies of a Small Lunar Crater in Balmer Basin Using Lroc Data

41st Lunar and Planetary Science Conference (2010) 1932.pdf

CHARACTERIZATION OF EJECTA FACIES OF A SMALL LUNAR CRATER IN BALMER BASIN USING LROC DATA. K. S. Wells,1 J. F. Bell III,1 1Cornell University, 514 Space Sciences Building, Ithaca, NY 14853; [email protected]; [email protected].

Introduction: For the heavily cratered bodies in the Solar System, an accurate description of the impact process, especially the amount and distribution of the ejecta generated, is essential to interpretation of local geology. Impact ejecta modifies and influences the evolution of heavily cratered bodies in dramatic ways. For example, in rays and other distal facies, secondary craters can contaminate crater-counts used for age- dating [e.g., 1, 2]. Indeed, preferential formation and retention of small primary craters on ejecta facies with different material properties can complicate even rela- tive age comparisons of regions where counts of craters with diameters less than 1 km are important [3]. Unfortunately, complete characterizations of ejecta facies of individual impacts are challenging. Large impacts can affect hundreds of thousands of km2 (e.g., at least 560,000 km2 of the lunar surface is covered by Tycho crater rays [3]), and resolution limits constrain the study of smaller impacts. Terrestrial impact crater studies [e.g., 4-5] have been essential to understanding continuous ejecta. Our knowledge of the secondary crater forming ejecta produced in an individual impact event comes mainly from scaled-up laboratory experi- ments or extrapolations from partial examinations of extensive ray systems [e.g., 3, 6-8]. High-resolution m < D < 2 km secondary craters and their ejecta blan- (~1 to 0.5 m/pix) imaging of lunar craters such as kets have been imaged with the LROC NAC, an ana- those obtained by the Narrow Angle Cameras (NAC) logous snapshot of primary ejecta will provide an inva- of the Lunar Reconnaissance Orbiter Camera (LROC) luable reference. [9] can significantly improve on these methods. High- Discussion: We surveyed the unnamed crater and its ejecta to determine the distribution of several fa- resolution imaging is particularly important because cies: secondary craters, debris surges, impact melt, and the ejecta blankets of small craters (D< few km) are (a) ejected boulders. Our preliminary investigation has likely more recent and better preserved than larger, focused on the secondary craters and boulders within older craters and (b) of smaller spatial extent, allowing the first few crater radii (R ), out to distance < 9 R . for a complete snapshot of proximal and distal ejecta c c Boulders and impact melt dominate in the proximal in a similar geologic context. portion of the ejecta blanket. Beyond about 4 R , Using a 0.93 m/pix NAC image (M104061987RE), c boulders give way to extensive secondary crater popu- we investigate the distribution of impact ejecta of a lations. These secondary craters are easily identified small (D~1 km) unnamed lunar primary crater located by their unique morphology: they are shallower and in Balmer Basin (-18.6°, 69.1°), southeast of Mare less circular than the small primary craters, with che- Fecunditatis (Figure 1). We interpret this unnamed vron-shaped rims often pointing toward the parent crater as a primary impact due to its circular planform primary (Figure 2). They are often tightly clustered and roughly symmetric continuous ejecta blanket (Fig- with other secondary craters, sometimes at near- ure 1). However, this type of work also provides valu- saturation levels reminiscent of debris surge formation able input to future investigations of the ejecta from [11], other times occurring in radial chains or in rela- similarly sized secondary craters. 13-cm radar circular tive isolation (Figure 2). polarization ratio (CPR) maps of distal Tycho second- We count 875 boulders and 10,239 primary and ary craters show that their ejecta can be distributed secondary craters ranging in diameter from 4 < D < 20 highly asymmetrically, even when the crater planform 2 m in a ~32 km region around the crater. The size- is relatively circular [10]. Currently, radar CPR maps frequency distributions (SFDs) of the combined popu- resolve these features at 100 m/pixel. Once these 350 41st Lunar and Planetary Science Conference (2010) 1932.pdf

lations were azimuthally averaged as a function of distance from the primary crater (Figure 3). The SFD within 4 Rc shows a much reduced crater density; relatively few secondary craters are visible in this region, where the continuous ejecta blanket is dominated in- stead by boulders, impact melt, and hummocky regions possibly related to debris surges [3, 11]. Further from the primary event, between 4 to 9 Rc, average crater populations roughly follow a -3.5 expo- nential power-law distribution. A departure from the power-law shape occurs at ~8 to 10 m. The boulder SFD appears to follow a steeper power-law, with an exponent near -5.5 (Figure 3). Summary: We have completed a preliminary analysis of the proximal ejecta (d < 9 Rc) of a small (D ~1 km) primary impact crater in the Balmer Basin on the Moon. Examination of the secondary craters in the ~32 km2 surrounding the primary impact shows mod- erately steep power-law size-frequency distributions with turn-over at around 10 m. This may be an obser- vational artifact from counting near the limit of resolution. Debris surges have also been suggested as possi- ble mechanisms for obliterating small secondary craters [3, 12]. Our investigation also reveals 875 boulders with 4 m < D < 20 m, mainly within the first three to four crater radii from the primary event. Beyond this range, the dominant features are secondary craters, found primarily in dense rays and inter-ray clusters. Additional high-resolution coverage of ejecta facies of this and other simple lunar craters will help to determine if such inter-ray clustering is common and how far from the primary event these features occur. Stu- dies like this at other lunar craters can provide an ex- cellent counter-part to laboratory measurements and computer simulations of the production and distribution of impact ejecta, bring the terrestrial field studies into context of pristine lunar examples, and generate additional insight into the kinds of widespread surface modification and regolith generation processes in- itiated by individual impacts. References: [1] Bierhaus, E. B. et al. (2005) Na- ture, 437, 1125-1127. [2] McEwen, A. S. et al. (2005) Icarus, 176, 351-381. [3] Dundas, C. M. and McEwen, A. S. (2007) Icarus, 186, 31-40. [4] Grieve, R. A. F. and Head, J. W. (1983) J. Geophys. Res., 88, A807- A818. [5] Hörz F., et al (1983) Rev. Geophys. and Space Phys., 21, 1667-1725. [6] Artemieva, N. A. and Pierazzo, E. (2003) LPI Contribution 1155, 10-11. [7] Campbell, B. A. et al. (1992) Proc. Lunar Planet. Sci. XXII, 259-274. [8] Hawke, B. R. et al. (2004) Icarus, 170, 1-16. [9] Robinson, M. S. et al. (2005), LPSC, Abstract #1576. [10] Wells, K. S. et al. (2009), LPSC, Abstract # 1778. [11] Oberbeck, V.R. (1975), Rev. Geophys. and Space Phys., 13, 337-362.