Krupac crater in Sirenum Fossae, Mars: Understanding fresh impact cratering processes
Authors: Vijayan Sivaprahasam , Harish Nandal , Sivaraman Bhalamurugan 
Published: October 2020

Krupac crater in Sirenum Fossae, Mars: Understanding fresh impact cratering processesS. Vijayan, Harish and Bhalamurugan SivaramanPlanetary Science Division, Physical Research Laboratory, Ahmedabad, India.vijayansiva@gmail.com&#160;Introduction: The formation and evolution of impact craters can be studied using the fresh craters with distinguishable rays on Mars. Fresh craters with rays on Mars provide ample evidence for their preservation of impact event, records the post-impact modification, which reveals the recent geological activities. This fresh preserved ejecta is used as a chronological marker to understand the Amazonian geological activities. Apart from this, the fresh craters are also used to explore the target materials, which got exposed and excavated by the impacts. Apart from rays, such craters are potential targets for gullies, fan-like deposits, RSL, boulders, rim alteration, etc. Earlier studies [1, 2] identified rayed craters with few km diameter and inferred that they preserved the recent geologic records and could be possible events for the Martian meteorite source. Also, such fresh craters can be used as a potential analogy for the experimental impact cratering. In this initial study, we report geological records preserved within the Krupac crater (~1 km diameter) with fresh ejecta preserved around it (Figure 1a). This crater is located on the eastern side of the Eridania basin [3] in the Sirenum Fossae region. Krupac crater is located within the Simois Colles region. This part is geologically diverse and appeared to contain isolated paleolakes [3]. The rim of the Simois Colles region contains Al-Phyllosilicates [3], and some of the floor deposits indicates the presence of Olivine. The Krupac crater target material is thus a mixture of mafic and hydrated minerals.Figure 1. a) Krupa crater and its fresh ejecta from HiRISE image (ESP_059256_1415), b) HiRISE colour image (ESP_059256_1415) shows the undissected rim, RSL, and unaltered rim on southern side.Results and Discussion: The fresh ejecta is observed to span up to six crater radii, and it is not as far spread as other bright rayed crater reported by [2]. This ~1 km diameter craters ejecta spread is almost isotropic, but few rays are extended far than this 6 crater radius. Such discontinuous ejecta is challenging to distinguish due to their faded nature. The crater interior hosts gullies, large alcoves on the northern wall, which are possibly formed due to the pole facing activities within the crater. The equator facing wall of the crater completely lacks any such features and retains its pristine rim. These incised gully activities are one of the recent activities within the crater. The incised alcoves on the northern wall altered the rim of this fresh crater significantly. This incising alcove suggests that the rim wall is getting altered or extended due to this. This incision is varying around the crater and this helps to decipher about the differential erosion occurring within the crater. On average this incision is ~20 m from the pristine crater rim wall on the northern side (Figure 1b). This rim incision due to the alcove is altering the crater significantly [4], and some of the undissected rim is also present on the northern side as patches. This is the evidence for the differential weathering over the crater wall. Thus, such fresh crater rim incision reveals how crater rim erodes/alters over time.On the crater rim, Recurring Slope Linea (RSL) are observed in large numbers [5,6,7] (Figure 1b). A small notable variation in the RSL length within HiRISE temporal images is observed. They are observed on top of the rim and over the terraced-like wall deposits within the crater. Mineralogically analysis using the CRISM suggests a noisy spectral signature over the crater region. A first degree polynomial fit for the median ratioed CRISM spectra extracted around the crater suggest presence of Phyllosilicate/clay signature. The indistinct spectra (Figure 2a) also shows a clay mixed carbonate-like signature. The carbonate absorption bands also occur/overlap with phyllosilicate Mg/Fe-OH hydroxyl bands [8]. It is unclear due to indistinguishable absorption; the detected minerals represent the exact surface mineralogy. Further analysis is needed to decipher their exact hydrated nature. There is no notable spectral signature from the RSL locations. The host surface of this crater holds the deposits of Fe/Me-phyllosilicates with light-toned deposits [3]. This Krupac crater is located on the sloped surface, which suggests that the earlier transported sediments were the host for this crater. Thus, the heterogeneous target for this Krupac crater and their younger nature is one good analogy for experimental cratering. &#160;&#160;Figure 2. a) CRISM median ratioed spectra with notable absorption around ~2.29-2.31 &#181;m and ~2.5 &#181;m, b) crater size-frequency distribution estimated age for Krupac crater is ~4 Ma.The Crater size-frequency distribution for the Krupac crater suggests a younger age of ~4 Ma (Figure 2b). This young Late Amazonian age, along with the preserved ejecta ray suggest that this Krupac crater preserved the recent geological activities and exposed the Noachian aged target materials. The presence of RSL, aqueous minerals may indicate that such fresh craters could be potential target for bio-signatures studies [6]. Overall, the Krupac crater is one of the well-preserved craters on Mars to decipher 1) rim-incision on the pole/equator facing walls, 2) RSL activities over the crater wall, 3) exposure of subsurface hydrated target materials and infer their heterogeneous mixtures, and 4) the well-preserved crater with ejecta and boulders can be used for comparing laboratory-based impact cratering experiments.&#160;References: [1] McEwen A. S. et al., Icarus, 2005 [2] Tornabene et al., JGRPlanets, 2006 [3] Solmaz Adeli., et al; JGRPlanets, 2015 [4] Stucky de Quay et al., JGR Planets, 2019, [5] Chojnacki et al, JGRPlanets, 2016, [6] McEwen. A. S. 2018, LPSC2018. [7] Bishop, J. L. et al., LPSC 2019. [8] Brown, A. J. et al., JGRPlanets, 2020. &#160;