CHRONOLOGY OF FRESH RAYED CRATERS IN ELYSIUM …Elysium Planitia has the largest concentration of...
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CHRONOLOGY OF FRESH RAYED CRATERS IN ELYSIUM PLANITIA, MARS. C. B. Hundal 1 , M. P. Golombek 2 , I. J. Daubar 2 . 1 Dept. 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 recognized by their high-albedo rays on the Moon and Mercury, fresh rayed craters on Mars have only been identified recently in thermal infrared images [1]. The- se rays are believed to be among the youngest geo- morphic features on Mars; closer inspection of them reveals high densities of secondary impact craters [2]. Elysium Planitia has the largest concentration of iden- tified rayed craters on Mars, and their thermal rays overlap one other in a number of regions, including the landing site for the InSight mission [3]. Similar to [4], we use superpositions of secondary craters as seen in High Resolution Imaging Science Experiment (HiRISE) images to determine relative ages among seven fresh rayed craters between 1.5 and 13.9 km in diameter (Fig 1). We further constrain these ages with crater-counting age estimates of their contin- uous ejecta blankets, calculated recurrence intervals, and previous estimates of the ages of these craters from the literature. Secondary Superpositions: Populations of sec- ondary craters from a particular primary tend to exhibit a common set of morphological attributes, particularly at the same distance from the primary. Using methods described in [5], attributes such as typical diameter and distinctive ejecta were noted for each secondary popu- lation. Where both populations were present, we used these attributes to determine source craters and thus superposition relationships. Recurrence intervals: Recurrence intervals are an estimate of the average amount of time between suc- cessive impacts of a given size, over a given area [6]. The intervals in this study are based on the Neukum et al. 2001 production function [7]. Recurrence intervals (Fig. 2) are given for 30%, 50%, and 100% of Mars’ surface for three reasons. First, most of the rayed cra- ters are found in the equatorial region in global moder- ate to high thermal inertia and high albedo Unit C [6]; this unit covers ~30% of Mars. Second, ~50% of Mars is within 30° of the equator where fresh rayed craters have been found [2]. Finally, recurrence intervals are often calculated over 100% of Mars’ area, so those are also included for comparison with the literature, alt- hough they are not as relevant in this case. Crater counts: Crater counts were performed on 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].) 1726.pdf Lunar and Planetary Science XLVIII (2017)
Transcript of CHRONOLOGY OF FRESH RAYED CRATERS IN ELYSIUM …Elysium Planitia has the largest concentration of...
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
recognized by their high-albedo rays on the Moon and Mercury, fresh rayed craters on Mars have only been identified recently in thermal infrared images [1]. The-se rays are believed to be among the youngest geo-morphic features on Mars; closer inspection of them reveals high densities of secondary impact craters [2]. Elysium Planitia has the largest concentration of iden-tified rayed craters on Mars, and their thermal rays overlap one other in a number of regions, including the landing site for the InSight mission [3].
Similar to [4], we use superpositions of secondary craters as seen in High Resolution Imaging Science Experiment (HiRISE) images to determine relative ages among seven fresh rayed craters between 1.5 and 13.9 km in diameter (Fig 1). We further constrain these ages with crater-counting age estimates of their contin-uous ejecta blankets, calculated recurrence intervals, and previous estimates of the ages of these craters from the literature.
Secondary Superpositions: Populations of sec-ondary craters from a particular primary tend to exhibit a common set of morphological attributes, particularly
at the same distance from the primary. Using methods described in [5], attributes such as typical diameter and distinctive ejecta were noted for each secondary popu-lation. Where both populations were present, we used these attributes to determine source craters and thus superposition relationships.
Recurrence intervals: Recurrence intervals are an estimate of the average amount of time between suc-cessive impacts of a given size, over a given area [6]. The intervals in this study are based on the Neukum et al. 2001 production function [7]. Recurrence intervals (Fig. 2) are given for 30%, 50%, and 100% of Mars’ surface for three reasons. First, most of the rayed cra-ters are found in the equatorial region in global moder-ate to high thermal inertia and high albedo Unit C [6]; this unit covers ~30% of Mars. Second, ~50% of Mars is within 30° of the equator where fresh rayed craters have been found [2]. Finally, recurrence intervals are often calculated over 100% of Mars’ area, so those are also included for comparison with the literature, alt-hough they are not as relevant in this case.
Crater counts: Crater counts were performed on 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].)
1726.pdfLunar and Planetary Science XLVIII (2017)
ondaries from only Zunil and were thus not as well constrained (Dilly, Wiltz, and Thila.) Separate crater counts were made on crater floors and ejecta blankets, carefully excluding obvious secondaries. We used the Craterstats 2.0 program [11] to plot differential crater size frequency distributions and calculate best-fit ages based on diameter bins which both had statistically significant numbers of craters and were also larger than the image resolution. Our results are reported in Fig 2. Due to a lack of craters inside Dilly and on its ejecta blanket, crater counts there were inconclusive.
Results: We find that the seven fresh rayed craters range in age from 0.1 to 30 Ma. Zunil is the youngest crater (0.1-1 Ma) by all measures. Corinto is the next youngest, forming before Zunil, but after the crater-dated ACy volcanic unit in Athabasca basin [12], meaning it formed between 0.1-1 and 2.5±0.2 Ma. These results are consistent with recurrence intervals for Zunil and Corinto, which suggest they are the youngest craters in their size ranges on Mars. Tomini is older than Corinto but younger than Naryn based on secondary superpositions, which agrees with the crater age (2-20 Ma [10]) of its ejecta. Tomini secondaries are superposed on Naryn, which is consistent with crater ages of 4-30 Ma [10] for its ejecta. Wiltz and Thila are 3.5±0.2 and 23±6 Ma respectively based on crater counts of their ejecta blankets using [13] isochrons. Dilly’s lack of superposed craters and the apparent sharp contrast of its rays in THEMIS
nighttime thermal images as compared to those of Thi-la and Wiltz suggest it is the youngest of the three. Zunil secondaries are superposed on all three, so they are all older than around 1 Myr. Recurrence intervals of Dilly, Wiltz, Thila and Naryn are all shorter than their likely age implying that (in contrast to Zunil, Corinto and Tomini) they are not the youngest craters in their size range on Mars.
Conclusions: Seven fresh rayed craters in Elysium Planitia range in age from 0.1 to 30 Ma. Available data indicate Zunil is the youngest (0.1-1 Ma), followed by Corinto (1-2.5 Ma), Tomini (2-20 Ma), and Naryn (4-30 Ma). Dilly and Wiltz (~3.5 Ma) are likely younger than Naryn, and Dilly is likely younger than Wiltz. Thila is probably the oldest (~23 Ma).
References: [1] McEwen A. et al. (2005) Icarus 76, 351-381. [2] Tornabene L. et al. (2006) JGR 111, E10006. [3] Golombek M. et al. (2013) LPS XLIV Ab-stract #1691. [4] Golombek M. et al. (2014) LPS XLV Abstract #1470. [5] Hundal C. B. et al. (2017) LPS XLVIII, this volume. [6] Golombek M. et al. (2010) JGR 115, E00F08. [7] Neukum G. (2001) Space Sci. Rev. 96, 55-86. [8] Christensen P. R. et al. (2004) Space Sci. Rev. 110, 85-130. [9] Bloom C. et al. (2014) LPS XLV Abstract #1289. [10] Hartmann W. K. et al. (2010) Icarus 208, 621-635. [11] Michael G. G. and Neukum G. (2010) EPSL 294, 223-229. [12] Vaucher J. et al. (2009) Icarus 204, 418-442. [13] Hartmann W. (2005), Icarus 174, 294-320. [14] Malin M. C. et al. (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].
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