Lunar and Planetary Science XLVIII (2017) 1726.pdf

CHRONOLOGY OF FRESH RAYED CRATERS IN ELYSIUM PLANITIA, MARS. C. B. Hundal1, M. P. Golombek2, I. J. Daubar2. 1Dept. Astronomy, Whitin Observatory, Wellesley College, Wellesley, MA 02481 ([email protected]). 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.

Introduction: While fresh impact craters are easily at the same distance from the primary. Using methods recognized by their high-albedo rays on the Moon and described in [5], attributes such as typical diameter and Mercury, fresh rayed craters on Mars have only been distinctive ejecta were noted for each secondary popu- identified recently in thermal infrared images [1]. The- lation. Where both populations were present, we used se rays are believed to be among the youngest geo- these attributes to determine source craters and thus morphic features on Mars; closer inspection of them superposition relationships. reveals high densities of secondary impact craters [2]. Recurrence intervals: Recurrence intervals are an Elysium Planitia has the largest concentration of iden- estimate of the average amount of time between suc- tified rayed craters on Mars, and their thermal rays cessive impacts of a given size, over a given area [6]. overlap one other in a number of regions, including the The intervals in this study are based on the Neukum et landing site for the InSight mission [3]. al. 2001 production function [7]. Recurrence intervals Similar to [4], we use superpositions of secondary (Fig. 2) are given for 30%, 50%, and 100% of Mars’ craters as seen in High Resolution Imaging Science surface for three reasons. First, most of the rayed cra- Experiment (HiRISE) images to determine relative ters are found in the equatorial region in global moder- ages among seven fresh rayed craters between 1.5 and ate to high thermal inertia and high albedo Unit C [6]; 13.9 km in diameter (Fig 1). We further constrain these this unit covers ~30% of Mars. Second, ~50% of Mars ages with crater-counting age estimates of their contin- is within 30° of the equator where fresh rayed craters uous ejecta blankets, calculated recurrence intervals, have been found [2]. Finally, recurrence intervals are and previous estimates of the ages of these craters from often calculated over 100% of Mars’ area, so those are the literature. also included for comparison with the literature, alt- Secondary Superpositions: Populations of sec- hough they are not as relevant in this case. ondary craters from a particular primary tend to exhibit Crater counts: Crater counts were performed on a common set of morphological attributes, particularly craters that had superposition relationships with sec-

Figure 1. A context image showing the locations of the seven young rayed craters in Elysium Planitia and the future landing site for the InSight mission [3] on a Thermal Emission Imaging System (THEMIS) nighttime image mosaic [8]. Naryn (called Tomini B in [2]), Tomini, Thila, Dilly, Wiltz, and Zunil Craters have thermal rays mapped in orange, light blue, dark blue, pink, lilac, and green respectively. Corinto’s rays, mapped by [9], are divided into two facies, represented by yellow and red respectively (Corinto is called unnamed crater in [10].) Lunar and Planetary Science XLVIII (2017) 1726.pdf

ondaries from only Zunil and were thus not as well nighttime thermal images as compared to those of Thi- constrained (Dilly, Wiltz, and Thila.) Separate crater la and Wiltz suggest it is the youngest of the three. counts were made on crater floors and ejecta blankets, Zunil secondaries are superposed on all three, so they carefully excluding obvious secondaries. We used the are all older than around 1 Myr. Recurrence intervals Craterstats 2.0 program [11] to plot differential crater of Dilly, Wiltz, Thila and Naryn are all shorter than size frequency distributions and calculate best-fit ages their likely age implying that (in contrast to Zunil, based on diameter bins which both had statistically Corinto and Tomini) they are not the youngest craters significant numbers of craters and were also larger in their size range on Mars. than the image resolution. Our results are reported in Conclusions: Seven fresh rayed craters in Elysium Fig 2. Due to a lack of craters inside Dilly and on its Planitia range in age from 0.1 to 30 Ma. Available data ejecta blanket, crater counts there were inconclusive. indicate Zunil is the youngest (0.1-1 Ma), followed by Results: We find that the seven fresh rayed craters Corinto (1-2.5 Ma), Tomini (2-20 Ma), and Naryn (4- range in age from 0.1 to 30 Ma. Zunil is the youngest 30 Ma). Dilly and Wiltz (~3.5 Ma) are likely younger crater (0.1-1 Ma) by all measures. Corinto is the next than Naryn, and Dilly is likely younger than Wiltz. youngest, forming before Zunil, but after the crater- Thila is probably the oldest (~23 Ma). dated ACy volcanic unit in Athabasca basin [12], References: [1] McEwen A. et al. (2005) Icarus meaning it formed between 0.1-1 and 2.5±0.2 Ma. 76, 351-381. [2] Tornabene L. et al. (2006) JGR 111, These results are consistent with recurrence intervals E10006. [3] Golombek M. et al. (2013) LPS XLIV Ab- for Zunil and Corinto, which suggest they are the stract #1691. [4] Golombek M. et al. (2014) LPS XLV youngest craters in their size ranges on Mars. Tomini Abstract #1470. [5] Hundal C. B. et al. (2017) LPS is older than Corinto but younger than Naryn based on XLVIII, this volume. [6] Golombek M. et al. (2010) secondary superpositions, which agrees with the crater JGR 115, E00F08. [7] Neukum G. (2001) Space Sci. age (2-20 Ma [10]) of its ejecta. Tomini secondaries Rev. 96, 55-86. [8] Christensen P. R. et al. (2004) are superposed on Naryn, which is consistent with Space Sci. Rev. 110, 85-130. [9] Bloom C. et al. (2014) LPS XLV Abstract #1289. [10] Hartmann W. K. et al. crater ages of 4-30 Ma [10] for its ejecta. Wiltz and (2010) Icarus 208, 621-635. [11] Michael G. G. and Thila are 3.5±0.2 and 23±6 Ma respectively based on Neukum G. (2010) EPSL 294, 223-229. [12] Vaucher crater counts of their ejecta blankets using [13] J. et al. (2009) Icarus 204, 418-442. [13] Hartmann W. isochrons. Dilly’s lack of superposed craters and the (2005), Icarus 174, 294-320. [14] Malin M. C. et al. apparent sharp contrast of its rays in THEMIS (2006) Science 314(5805), 1573-1577.

Figure 2. Our estimated ages (blue, pink, and light grey bars) compared to past estimates (dark grey [14], dark green, and light green bars [10]). Circles indicate our calculated recurrence intervals based on the Neukum et al. 2001 production function [7].