Lunar and Planetary Science XLVIII (2017) 1683.pdf

THE GEOLOGIC HISTORY AND FORMATION OF THE FACULAE IN OCCATOR CRATER ON CERES. J. E. C. Scully1, D. L. Buczkowski2, P. M. Schenk3, A. Neesemann4, C. A. Raymond1, C. T. Russell5, M. T. Bland6, J. C. Castillo-Rogez1, A. Ermakov1, L. C. Quick7, O. Ruesch8, B. E. Schmidt9, 1Jet Propulsion Laborato- ry, California Institute of Technology, Pasadena, California, USA (jennifer.e.scully@jpl.nasa.gov), 2Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA, 3Lunar and Planetary Institute, Houston, Texas, USA, 4Free University of Berlin, Berlin, Germany, 5University of California, Los Angeles, California, USA, 6USGS, Astrogeology Science Center, Flagstaff, Arizona, USA, 7Planetary Science Institute, Tucson, Arizona, USA, 8NASA Goddard Space Flight Center, Greenbelt, MD, USA, 9Georgia Institute of Technology, Atlanta, GA, USA.

Introduction: When the Dawn spacecraft arrived cratered terrain [8,9]. In Ezinu quadrangle, which in- at Ceres in March 2015, bright regions were observed cludes the northern portion of Occator crater, the cra- on the mostly dark surface. The brightest of these re- tered terrain is cross-cut by the medium-sized craters gions are found in the 92-km-diameter Occator crater Occator, Datan, Messor and Ninsar (diameters of 40- [1,2]. The single scattering albedo of the Occator crater 92 km). These craters are associated with geologic bright regions is 0.67-0.80, while that of average Ceres units of intermediate age: they are surrounded by ejec- is 0.09-0.11 [3]. The central bright region in Occator ta deposits (mapped as crater material) and contain crater is Cerealia Facula, and the ancillary bright re- crater terrace material and hummocky crater floor ma- gions in the eastern floor are the Vinalia Faculae. terial in their interiors. The stepped morphology of the Dawn’s Framing Camera has observed the Cerealia crater terrace material is likely the result of slumping and Vinalia Faculae at a high spatial resolution (35 of material from the crater walls. The hummocky m/pixel), which revealed that Cerealia Facula is locat- crater floor material is likely a mass wasting deposit, ed in a ~9 km wide and ~700 m deep pit [1,4,5]. Occa- which formed as material cascaded to the crater floors. tor crater was mapped as a part of the geologic map- In Occator, Datan and Messor craters, these intermedi- ping campaign of Ceres [6,7], and is included in the ate age geologic units are cut by a young geologic unit: geologic maps of two of Ceres’ quadrangles: Occator lobate material, which is interpreted as a mass wasting [8] and Ezinu [9]. In addition, data from the Visible- deposit and has a distinct lobate scarp defining its mar- Infrared Mapping Spectrometer indicate that the Cere- gins. One of the youngest geologic units in all the cra- alia and Vinalia Faculae contain sodium carbonate ters is the talus material, which is interpreted to form [10]. This is different to Ceres’ average surface com- by the late-stage mass wasting of material from the top position, which contains Mg/Ca carbonates. of the crater walls [9]. In order for the Cerealia and Vinalia Faculae to Geologic Map of Occator crater. While Occator have an albedo and composition that is distinct from crater has some geologic units in common with adja- Ceres’ average surface, they must have not mixed with cent impact craters, it also contains many unique geo- the average surface material, and thus must be geologi- logic units: for example, the smooth and knobby lobate cally young. The presence of such geologically young materials. These units are young, are located in the material on the surface of Ceres is intriguing, and here southern and eastern crater floor and share a gradation- we present a detailed geologic map of the interior of al boundary. We interpret that the units were emplaced Occator crater, which is one of the necessary inputs to as flows, based on their lobate margins that cross-cut decipher the formation mechanism of the Cerealia and the underlying crater terrace material and hummocky Vinalia Faculae. crater floor material. In Occator’s northeastern floor, Methods: Our basemap is a 35 m/pixel clear filter the knobby lobate material is cross-cut by the hum- mosaic, obtained by Dawn’s Framing Camera during mocky lobate material, whose distinct lobate margins the Low Altitude Mapping Orbit (LAMO) [7]. Our and surface ridges/troughs strongly suggest that the mapping is also informed by a variety of 140 m/pixel unit was emplaced as a flow. Along with the hum- Framing Camera color filter composite mosaics, ob- mocky lobate material, the Cerealia and Vinalia Facu- tained during the High Altitude Mapping Orbit lae are the youngest features within Occator crater. We (HAMO), and a shape model that has a lateral spacing map the Cerealia and Vinalia Faculae as the Occator of ~135 m/pixel and a vertical resolution of <100 m pit/fracture material bright unit, because of the associa- [12]. We use ESRI ArcMap 10.3 software to create the tion of the Cerealia Facula with the Occator central pit, geologic map, which allows for the mapping to be and the association of both faculae with fractures in georeferenced to the aforementioned datasets. Occator’s floor, investigated in detail by [13]. Results: Discussion and Ongoing Investigation: On the Geologic Context of Occator crater. The oldest basis of our geologic mapping, we propose the follow- unit in the region surrounding Occator crater is the ing geologic history for Occator crater: Lunar and Planetary Science XLVIII (2017) 1683.pdf

a) The Occator-forming impactor hits the cratered e) Further evolution, freezing and upwelling of the terrain and produces ejecta. Soon after, slumping reservoir material produces a localized upwelling and mass wasting form the crater terrace material region, or dome, within the Cerealia Facula in Oc- and hummocky crater floor material. cator’s central pit. Fractures on top of this dome b) Unlike the other medium-sized craters in the re- expose the underlying upwelled material, which gion, the Occator-forming impact excavated to a appears ‘orange’ in color composite mosaics [4]. sub-surface reservoir of partially melted salts and We are currently investigating the geophysical and water ice, similar to the material proposed to form geochemical plausibility of this proposed geologic Ahuna Mons [14]. There are indications of this res- history, possible transport mechanisms of the reservoir ervoir in the gravity data, which suggests that there material from depth, and the absolute timing of the is sub-surface upwelled material in the region sur- emplacement of the various units derived from crater rounding Occator crater [15]. counts. c) The energy from the Occator-forming impact fur- References: [1] Nathues, A., et al. (2015) Nature, ther melts the reservoir material [16], which flows 528, 237-240. [2] Russell, C. T., et al. (2016) Science, out onto the crater floor, initially to form the 353, 1008-1010. [3] Li, J.-Y., et al. (2016) The Astro- smooth and knobby lobate materials, and later to physical Journal Letters, 817:L22, 1-7. [4] Schenk, P., form the hummocky lobate material. Evolution of et al. (2016) AGU Fall Meeting, Abstract #P41C-03. the melted reservoir material, and/or the mode of [5] Jaumann, R., et al. (2017) 48th LPSC, this meeting. emplacement, may be illustrated by the change in [6] Williams, D. A., et al. (2017) 48th LPSC, this meet- texture from smooth-knobby lobate materials to ing. [7] Roatsch, Th., et al. (2016) Planetary and hummocky lobate materials. Space Science, 129, 103-107. [8] Buczkowski, D. L., et d) As commonly occurs in impact craters, the center al. (2016) 47th LPSC, Abstract #1255. [9] Scully, J. E. of Occator crater remains hot for longer than the C., et al. (2016) 47th LPSC, Abstract #1627. [10] De other parts of the crater. In this hot center, at depth, Sanctis, M. C., et al. (2016) Nature, 536, 54-57. [11] the composition of the reservoir material gradually McCord, T. B., et al. (2011) Space Science Reviews, evolves, similar to the incremental compositional 163, 63-76. [12] Preusker, F., et al. (2016) 47th LPSC, evolution of a terrestrial magma chamber [17]. The Abstract #1954. [13] Buczkowski, D. L., et al. (2016) Cerealia and Vinalia Faculae are formed when this 48th LPSC, this meeting. [14] Ruesch, O., et al. (2016) evolved material is emplaced onto the crater floor. Science, 353, 1005. [15] Ermakov, A., et al. (2016) Thus, the dark smooth, knobby and hummocky lo- 47th LPSC, Abstract #1708. [16] Bowling, T. J., et al. bate materials correspond to an early composition (2016) 47th LPSC, Abstract #2268. [17] Marsh, B. D. of the reservoir material, and the bright Cerealia (2000) “Magma Chambers” from the Encyclopedia of and Vinalia Faculae correspond to a later-stage Volcanoes, pp. 191-206. composition of the reservoir material.

Figure 1. Geologic map of Occator crater (left) and the basemap (right). The Cerealia Facula are located in the center of Occator crater, and the Vinalia Faculae are located in the eastern crater floor.