Geologic Mapping of Bakhuysen Crater, Mars: Insights Into Large Basin Impact Cratering Processes

47th Lunar and Planetary Science Conference (2016) 2360.pdf

GEOLOGIC MAPPING OF BAKHUYSEN CRATER, MARS: INSIGHTS INTO LARGE BASIN IMPACT CRATERING PROCESSES. C. M. Caudill1, G. R. Osinski1,2, L. L. Tornabene1, and A. S. McEwen3, 1Centre for Planetary Science and Exploration / Dept. Earth Sciences, University of Western Ontario, London, ON, Canada, 2Dept. Physics and Astronomy, University of Western Ontario, London, ON, Canada, 3Lunar and Planetary Lab, University of Arizona, Tucson, AZ, USA. isolated outcrops of crudely layered light-toned materi- Introduction: Large basin-forming impact events al may represent the stratigraphy of the smooth unit, (>100 km diameter) have played an important role in observed in the northern and southern crater floor. the geological evolution of the terrestrial planets and perhaps even the origin of life [1]. However, there remain few comprehensive studies of large basin impact structures (LBIS) as very few are preserved on Earth. Although impact basin structural models exist [2, 3], models of ejecta emplacement [4, 5, 6] and impact melt emplacement [7] are based on impact craters generally <<100 km diameter; as such, it is not currently under- stood if such models scale to LBIS. Particularly, studies of the morphology, structure, and mineralogy of LBIS proximal ejecta is lacking as it is generally poor- ly preserved on Earth. Mars possesses several well- preserved analogous basin impact structures, having Fig. 1. Bakhuysen Crater floor, showing fractures in a evidence of aqueous interaction [4, 8], and therefore pitted unit. Smooth unit, which embays the pitted unit, also presents an opportunity to understand these structures shown. Subset of CTX image B18_016551_1566_ XN_23S344W (NASA/ASU Space Flight Facility). at the meter to sub-meter scale using current remote sensing datasets. This work focuses on the detailed Crater Exterior / Ejecta. The short-lobed ejecta mapping of the well-preserved ~150-km Bakhuysen blanket consists largely of hummocky terrain, terminat- Crater (15E, -23), which has a modeled age consistent ing non-continuously at various distances from the with the Early Hesperian - Late Noachian period [9], crater rim. Although the ejecta clearly displays a radial utilizing visible images (6-m/pixel CTX [10] and 25 to secondary pattern [4, 5], it is presently unclear whether 50-cm/pixel HiRISE [11]), thermophysical data (100- the ejecta is single or multi-lobed (SLE, MLE; 4, 5). m/pixel day and nighttime thermal infrared images The lobes are best expressed in the northern and north- from THEMIS [12]), and hyper- and multi-spectral eastern ejecta. Possible ramparts remain intact locally data (CRISM [13]). Due to the preservation state, at numerous lobes in the northeastern proximal ejecta Bakhuysen small basin impact structure is mapped here (at ~0.4 crater radii). to extrapolate to larger peak ring basin structures. Thermophysical data show stark boundaries be- Observations: tween units in the ejecta (Fig. 2A). The areas which are Crater Interior. A pitted unit covers most of the bright white in THEMIS Night IR in Fig. 2A have crater floor, with fractures observed in the northern morphologic similarities consistent with pitted material portions of the pitted unit (Fig. 1). Interestingly, unusu- as observed in HiRISE and CTX visible imagery.

ally large pits are located concentrically where a peak ring feature usually exists in LBIS (~0.3 – 0.6 crater rim diameter) [14, 15]. Arcuate mountainous terrain near the crater center possibly represents a central up- lift, partially overprinted by a younger 9 km-diameter crater. The ejecta of the small interior crater has lobes and ramparts consistent with single layer ejecta (SLE) morphology. Rim-incising channels are found all around the crater rim, but are concentrated and largest on the northern rim. Overlying the pitted unit is a smooth, consistent-toned, continuous unit which is observed where the channels meet the break in slope from the Fig. 2. A) Bakhuysen NW – SW ejecta and crater rim, shown terraces to the crater floor and outward. Small (<1 km) on THEMIS NightIR global mosaic (NASA/ASU Space 47th Lunar and Planetary Science Conference (2016) 2360.pdf

Flight Facility). Arrows indicate thermophysical boundaries Although a peak ring structure indicative of larger of ejecta units, with a zoom image one area. Subset zoom impact basin structures [19] is not apparent in image is ~ 22 km across. B) Figure taken from Carter et al. Bakhuysen, the concentrically-emplaced and unusually [16]; Radar observations of lunar impact melt flows. large pits are interpreted here as being formed due to a Discussion: Pitted materials on the crater floor are subsurface ring structure and potentially associated consistent with and interpreted as volatile-rich primary with degassing of volatiles; Tornabene et al. [9] have impactite deposits, as originally proposed by Torna- previously described structural controls of pitted mate- bene et al. [9]. Fractures present in crater floor pitted rial. This interpretation would be consistent with im- units also likely indicate melt-bearing deposits, similar pact basin structural models [2, 3] coupled with distri- to lunar crater fractures described as contraction fea- bution of hydrothermal activity associated with impact- tures of impact melt deposits [9, 17]. related fault and fracture systems [20]. The central fea- In addition to the crater interior pitted materials, the ture of Bakhuysen, although partially obscured by the presence of ejecta lobes and possible ramparts suggest overprinting crater, appears to be an incomplete ringed the interaction of the impact process with subsurface peak-cluster structure as described by Baker et al. [19]. volatiles (liquid or ice; [4, 8]). Robbins and Hynek [5] Preliminary mapping and data processing of classified Bakhuysen ejecta as radial (Rd), with no Bakhuysen indicates features consistent with a modi- indication of a layered ejecta. (No LBIS in that data- fied but well-preserved LBIS, including 1) extensive base have classified layered ejecta.) This work suggests impact melt-bearing deposits inside the crater and on that the unusually extensive and locally continuous the ejecta, 2) a lobed and multi-unit ejecta, 3) post- material in the ejecta, seen as bright areas in THEMIS impact alluvial and fluvial fan deposits, and 4) a dis- Night IR in Fig. 2A, have ponded ejecta morphology tinct central feature structure. These preliminary find- similar to lunar impact melt flows (e.g., Fig. 2B). Fur- ings may suggest that mechanisms involved in craters thermore, the texture of this material is consistent with <100 km in diameter may indeed scale to LBIS, with pitted texture observed elsewhere in the crater and the initial conditions (e.g., target lithology and availa- ejecta as seen in HiRISE and CTX visible imagery. bly of volatiles [21]), being important factors to con- Pooled, pitted materials are generally seen on terraces sider when determining the formation and post-impact or within ejecta topographic lows just beyond crater history of a LBIS. rims [9]. This unit occupies the lowest-lying regions of References: [1] Cockell, C.S. and Lee, P. (2002) the ejecta and appears to thin out at its margins, gradu- Biological Reviews, 77, 279–310. [2] Grieve, R.A.F. et ally yielding to and exposing the underlying ejecta. al. (2008) Meteoritics & Planetary Science, 43, 855– These units are therefore interpreted as ponded, pitted 882. [3] Melosh, H.J. (2015) Bridging the Gap III, units in the proximal ejecta, emplaced on top of the 1003. [4] Barlow, N.G. et al. (2000) JGR, 105, 26733– hummocky, continuous ejecta unit consistent with 26738. [5] Robbins, S.J. and Hynek, B.M. (2012) JGR, models of multi-stage ejecta emplacement [7, 9]. These 177, E05004. [6] Weiss, D.K. and Head, J.W. (2014) features of the ejecta may be significant to extend the Icarus, 233, 131-146. [7] Osinski, G.R. et al. (2011) models of MLE emplacement [4, 5, 6] to LBIS, as well EPSC, 301, 167-181. [8] Carr, M.H. et al. (1977) JGR, as suggest the availability of volatiles which may have 82, 4055–4065. [9] Tornabene, L.L. et al. (2012) Ica- influenced the post-impact crater environment. rus ,220, 348–368. [10] Malin, M.C. et al. (2007) JGR, Biota are thought to flourish in post-impact hydrat- 112, E05S04. [11] McEwen, A.S. et al. (2007) JGR, ed environments [1]. On Mars, one mechanism for the 112, E5. [12] Christensen, P.R. et al. (2004) Space formation of post-impact hydrated substrates are fluvial Science Reviews, 110, 85-130. [13] Murchie, S.M. et fans and deltas. Crater rim breaches are not observed at al. (2007) JGR, 112, E05S03. [14] Baker, D.M.H, et Bakhuysen and hence the channels mapped here are al. (2011) Planetary and Space Science, 59, 1932– interpreted to be internally sourced, carrying sediments 1948. [15] Wood, C.A. (1980) LPSC 11, 2221 – 2241. to the crater floor and developing the smooth units. [16] Carter, L.M. et al. (2011) NASA Goddard Tech- Moore and Howard [18] described deposits in nical Reports, GSFC.JA. 5788. [17] Heather, D.J. and Bakhuysen as being a combination of alluvial fan and Dunkin, S.K. (2003) Icarus, 163, 307–329. [18] Moore, fluvial deltaic, also suggesting that such fans formed J.M. and Howard, A.D. (2005) JGR, 110, E04005, [19] around the Hesperian-Noachian boundary at a period Baker, D.M.H. et al. (2011) Icarus, 214, 377-393. [20] of enhanced precipitation. The light-toned outcrops Osinski, G.R. et al. (2005) Meteoritics & Planetary observed in the smooth units may therefore have ex- Science, 40, 1859–1877. [21] Kenkmann, T. and Scho- posed hydrated mineralogy, which we hope to better nian, F. (2006) Meteoritics & Planetary Science, 41, constrain with requested HiRISE color and stereo im- 1587–1603. ages as well as CRISM spectral data.