Extensive Secondary Impact Cratering in the ExoMars Rosalind Franklin Landing Site at Oxia Planum

IntroductionThe ESA ExoMars Rosalind Franklin (EMRF) rover will launch in 2028, and land in the Oxia Planum region of Mars. The main target is Noachian phyllosilicate-rich deposits [1,2]. This region probably represents the oldest aqueous environments to be explored in situ on Mars. In this study we have investigated extensive secondary craters within the EMRF landing ellipse, to place absolute age markers in the stratigraphic framework of Oxia Planum, and identify the likely primary source crater(s). We pay particular attention to possible secondary craters from Mojave crater, due to its likely recent formation age [3] and importance as a potential source crater for martian meteorites [4]. MethodWe first used the recent Mars catalogue of small impact craters [5, 6] in a small (1 x 105 km2) ‘Oxia Planum study region’. We refined the crater identification through manual crater addition, deletion, movement, and scaling. We produced a final catalogue with an extra 22,209 craters, totalling 381,584 impact craters. We applied the ‘Algorithm for the Secondary Crater Identification’(ASCI) [3] to identify possible primary and secondary impact craters. We then produced a crater size density map of possible secondary craters for the Oxia Planum study region, from which we identified acute triangle-shaped clusters (or ‘cones’) of craters. We carried out crater size-frequency distribution (CSFD) studies of previously-identified units in Oxia Planum [1] to determine model surface ages, both with and without secondary craters removed. We then merged the original catalogue of small impact craters [5, 6], with craters with diameters >1 km [7] in a larger (1.2 x106 km2) ‘context study region’. We used the same methods to identify possible primary and secondary craters, and produced a crater size density map of possible secondary craters. ResultsWe separate our results into (1) the identification and analysis of secondary craters, and (2) the implications for model ages in Oxia Planum.Secondary Craters. Of the impact craters in the smaller Oxia Planum study region, we classified 176,927 (46.4%) as primary craters, with 204,657 (53.6%) classified as secondary craters. The number of possible secondary craters has been revised down from our previous results [8]. We identified at least 13 separate clusters of craters that occur in cone shapes, with each cluster typically up to 40 km long and 20 km wide at the distal ends. These cones are oriented radially away from Mojave crater, with the median direction being 225°, similar to the median direction (222°) to Mojave crater. The cones show a distinctive size distribution of craters, with larger craters limited to the proximal (apex) region, with a gradual transition to smaller craters in the distal zone. The cones in our Oxia Planum study region are located at distances of ~700 to 930 km from Mojave. The larger, context study region contains 2,877,811 impact craters, ranging in size from 29 m to 54.4 km [5-7]. We classified 1,356,995 (47.2%) as primary craters, with 1,520,816 (52.8%) classified as secondary craters. We identify a further 13 cone-shaped clusters of secondary craters in the context study region, with ranges ~275 – 500 km from Mojave.Figure 1. (A) Possible primary (red) and secondary (green) impact craters in our Oxia Planum study region. (B) Secondary crater size density map of same region. EMRF 1s (grey) and 3s (black) ellipse ranges are shown. Model Surface Ages. The removal of secondary craters from CSFD studies does not affect the model surface age of the phyllosilicate units in Oxia Planum. We derive a model surface age of 3.9 Ga for the Noachian layered clay-bearing unit (lNc) of [1] using all our craters, and an identical age when using just our primary craters. This similarity is due to the lack of secondary craters at larger diameters. ImplicationsOur results suggest that there are extensive secondary impact craters in Oxia Planum, with ~4000 secondaries within the EMRF 3s ellipse pattern. It is therefore likely that EMRF will encounter secondary craters during surface operations. The orientation of cone-shaped clusters of small craters indicates that the majority of secondaries are sourced from the Mojave impact crater, although larger, older secondary craters from other sources are also present. Given that the Mojave impact is estimated to have occurred 10.1 Ma [3], these secondaries can be used as absolute stratigraphic markers throughout Oxia Planum, particularly in quantifying the rate of recent and active surface processes. These secondary craters will also be important for target prioritization during in situ studies. References: [1] Quantin-Nataf C. et al. (2021) Astrobiol. 21, 345-366. [2] Mandon L. et al. (2021) Astrobiol. 21, 464-480. [3] Lagain A. et al. (2021) Earth Space Sci. 8, e2020EA001598. [4] Werner S.C. (2014) Science, 343, 1343-1346. [5] Lagain A. et al. (2021) in GSA Spec. Paper 550, 629-644. [6] Lagain A. et al. (2021) Nature Comms. 12, 6352. [7] Robbins S.J. & B.M. Hynek (2012) JGR 117, E05004. [8] Grindrod, P.M. et al. (2023) LPSC 54, #1113.

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