Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6354.pdf

CONTINUOUS EJECTA DEPOSITS OBSERVED BEYOND LAYERED EJECTA RAMPARTS ON MARS. L. L. Tornabene1, J. L. Piatek2, N. G. Barlow3, J. Boyce4, R. Sopocco1, R. Capitan1, A. S. McEwen5, G. R. Osinski1, S. J. Robbins6, W. Watters7, 1Centre for Planetary Science and Exploration and Dept. of Earth Sci- ences, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B, ([email protected]), 2Dept. of Geological Sciences, Central Connecticut State Univ., New Britain, CT, 3Dept. Physics and Astronomy, Northern Arizona Univ., Flagstaff, AZ, 4 Hawaii Institute of Geophysics and Planetology, University of Hawai'i, Honolulu, HI, 5LPL, University of Arizona, Tucson, AZ, 6Southwest Research Institute, Boulder, CO, 7Dept. Astronomy, Whitin Observatory, Wellesley College, Wellesley, MA.

Introduction: Continuous ejecta deposits on inations of these craters are actively underway, Mars are generally divided into two main morpholog- inlcuding initial measurements of the extent of their ic types: “radial” and “layered”. While radial ejecta “Beyond-Rampart Continuous Ejecta” (BRaCE) faci- represents the most common morphologic type on the es, which is forthcoming. Moon and Mercury [1], layered ejecta is the dominant Beyond-Rampart Continuous Ejecta (BRaCE) morphology for well-preserved craters on Mars facies: HiRISE observations show abundant flows (>90% for craters ≥ 5 km in diameter) [2,3]. Volatile emanating from what are interpreted to be volatile- or ice content within (or on) the target [e.g., 4], and/or rich impact melt-bearing deposits that lie atop well- effects from interactions between the ejection process preserved layered ejecta (LE) blankets [14; see Fig. with the atmosphere [e.g., 5], have all been proposed 16]. These abundant flows appear to coalesce into a to explain layered ejecta. Layered ejecta is also dis- continuous off-rampart ejecta facies that is generally tinct from radial with respect to its morphometric characterized as: relatively thin, smooth, sinuous and profile [e.g., 6] including a distinctive terminal edge gently undulating with various quasi-radial features. that manifests as a ridge or scarp, commonly referred There are three quasi-radial features observed associ- to as a “rampart” [4,7-8]. Importantly, the layered ated with BRaCE: dense clusters of irregular depres- ejecta rampart has often been assumed to be the ter- sions, hummocky “islands” or patches, and linea- minus of the continuous ejecta facies and the start of ments with the best examples of these latter features discontinuous facies. occurring near the terminal edge of the BRaCE facies. Based on observations with Mars Reconnaissance Pit clusters are often sinuous, and are observed to Orbiter (MRO) images, we describe a subtle, relative- divert and completely wrap around pre-existing ob- ly thin, but continuous deposit that flow off of, and stacles – a behavior inconsistent with secondary crater terminates well beyond (up to 5 crater radii) the lay- chains. The pits are relatively shallow with respect to ered ejecta rampart of several of the best-preserved their diameters and have scalloped edges; as such, craters on Mars (Fig. 1.). they are similar to the pits described by [14], and even Background and General Methods: Emphasized exhibit similar associations between the pit size and in this abstract are observations and morphological density to deposit thickness. This interpretation is mapping results based on HiRISE [9] and CTX [10] further supported by the observed continuity with images for two of the best-preserved simple-to- both ponded and pitted, and smooth deposits (i.e., complex transitional Single Layer Ejecta (SLE) cra- impact melts) observed on the LE [14; see Fig. 16]. ters on Mars: Resen and Noord. Resen is approxi- Rougher and higher-standing hummocky “islands” mately 7.6 km in diameter and located in Hesperia or patches are also observed, and are almost identical Planum (108.88°E, 27.94°S). Noord is approximately in apperence to hummocky terrains that are larger and 7.8 km in diameter and located in Noachis Terra more abundant within the LE. They also show distinct (348.74°E, 19.27°S). These craters were selected similarities to the overall morphology of radial ejecta based on the general characteristics outlined in [11- deposits observed around well-preserved simple cra- 13], including similar sizes, different target materials ters. When completely mapped, these hummocky and because they are two of the best-preserved craters patches are notably associated with the crater as they that have experienced minimal post-impact degrada- are radial, occur circumferentially, and are within a tion for their size. confined range to the primary. They are also observed Although we emphasize results from Resen and to embayed, coated and sometimes partially buried by Noord here, similar features have been observed in the smooth and pitted portions of the BRaCE facies; HiRISE and CTX images of additional well-preserved as such, they must predate the emplacement of these craters on Mars (~12 craters thus far; e.g., Gratteri, materials. We interpret them to be portions of an ini- Zunil, Corinto, etc.), including the initial recognition tial emplacement of radial ejecta deposits that crop of these features [14-15], and inspite of the lacking out of the smooth and pitted materials that comprise HiRISE coverage of the more distal portions of ejecta. the bulk of the BRaCE facies. We also note the pres- Improving HiRISE coverage and more detailed exam- ence of rare occurances of pre-impact target-surface Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6354.pdf

features, which are notably more absent crater-ward latitudes. Indeed, some LARLEs are found at lower of the margin of the BRaCE facies. Such pre-existing latitudes, but these are much rarer (11; ~8%) and oc- terrain has a distinctive tone (lighter) and is compara- cur very specifically in deposits interpreted to be al- tively smoother in appearance when compared to the tered ash deposits [18] (e.g., Medusa Fossae Fm.). hummocky patches interpreted to be Resen-related. Thus, we suggest that LARLE and pedestal craters are The most diagnostic feature of BRaCE facies is not unique ejecta classes, but may merely be expres- the marked scarcity of herringbone features and a sions of various degrees of preferential preservation distinct absence of secondary crater chains crater- of the BRaCE facies with the enhanced run-out dis- ward from its distal margins. Indeed, abundant pre- tances and differences in preservation being likely existing target-surface features, and both partially attributed to target properties and local conditions, buried and scoured herringbone features and second- respectively. ary crater chains, are only observed beyond the termi- Conclusions: We describe a continuous, relatively nal edge of the BRaCE facies. With increasing dis- thin, smooth and pitted ground-hugging flow facies of tance from this boundary, the secondaries become ejecta that is observed well beyond the terminal ram- sharper, deeper and better-preserved. This is con- part of some of the best-preserved layered ejecta cra- sistent with crater ejecta becoming progressively dis- ters on Mars. Despite similarities to LARLE, the clear continuous away from the BRaCE margin. We sug- relationship of these beyond-rampart deposits to vola- gest that the emergence of these features marks the tile-rich impact pitted and smooth deposits observed general contact between continuous ejecta and discon- on the layered ejecta suggests an important role not tinuous ejecta for LE craters, and not the LE rampart. only for target volatiles in their formation, but also for impact melt-production and emplacement. The recog- nition of these continuous beyond layered ejecta ram- part deposits calls into question how we define crater ejecta on Mars (continuous vs. discontinuous), and possibly other bodies; as such, these deposits warrant further detailed study to make additional constrains on ejecta classification, our understanding of how ejecta blankets form, and how they degrade over time. References: [1] Barlow N.G. et al. (2000), JGR, 105, 26733-26738. [2] Barlow N.G. (1988), Icarus 75, 285–305. [3] Barlow N.G. et al. (2007), 7th Mars, [4] Carr et al. (1977), JGR 82, 4055-4065. [5] Schultz P.H. and Gault D.E. (1979), JGR, 84, 7669-7687. [6] Komatsu et al. (2007), Icarus, 112. [7] McCauley J.F. (1973), JGR 78, 4123-4137. [8] Mouginis-Mark P.J. (1977), JGR, 84, 8011-8022. [9] McEwen A.S. et al. (2007), 112. [10] Malin M.C. et al. (2007), JGR, 112. [11] Tornabene L. L. et al. (2015). LPSC 46, Abstract #2531. [12] Tornabene L.L. et al (2016) LPSC 47,

Fig. 1. 3D perspective of Resen Crater ejecta facies Abstract #2879. [13] Piatek J. L. et al (2015) LPSC looking North (v.e.= 0.4x). Generated from a CTX 46, Abstract #2654. [14] Tornabene L.L. et al. ortho image and DTM. A slightly darker-toned ejecta (2012), Icarus 220, 348–368. [15] Boyce J.M. et al. facies (corresponding to the BRaCE facies) can be (2014) PCC meeting, #1405. [16] Osinski G.R. et al. observed ~2 to 5 crater radii from the layered ejecta (2011), EPSL, 310, 167-181. [17] Christensen et al. rampart (orange line). Well-preserved Resen second- [18] Barlow N.G. et al. (2014), JGR, 239, 186–200. aries (red arrows) are prominent in the foreground and [19] Boyce J.M. et al. (2015) Icarus, 245, 263-272. away from the edge of the BRaCE facies. Acknowledgements: The authors wish to thank The BRaCE facies is similar to, and is likely relat- NASA’s Mars Data Analysis Program ed to, Low-Aspect Ratio Layered Ejecta (LARLE), (NNX15AM41G) for supporting this work, and which were proposed to form as a ground-hugging acknowledge Tornabene’s personal Canadian-based base surge [18-19]. However, unlike LARLEs, all support from the NSERC Discovery Grant programme (RGPIN/04215-2014) and the Canadian Space Agen- craters with BRaCE deposits have generally shorter run-outs (<6 crater radii), are notably superior in cy (CSA) (14EXPUWO-002). preservation, and occur equator-ward of the mid-