47th Lunar and Planetary Science Conference (2016) 1618.pdf

IMPLICATIONS FOR THE GEOLOGIC EVOLUTION OF CERES, DERIVED FROM GLOBAL GEOLOGIC MAPPING OF LINEAR FEATURES. J. E. C. Scully1, C. A. Raymond1, D. L. Buczkowski2, D. P. O’Brien3, G. Mitri4, S. D. King5, C. T. Russell6, T. Platz7, 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA ([email protected]), 2JHU-APL, Laurel, MD, USA, 3PSI, Tucson, AZ, USA, 4University of Nantes, Nantes, France, 5Virginia Tech, Blacksburg, VA, USA, 6UCLA, Los Angeles, CA, USA, 7MPI for Solar System Research, Göttingen, Germany.

Introduction and previous work: NASA’s Dawn Results: spacecraft is currently orbiting Ceres, a dwarf planet Overview and global distribution. In Figure 1, and the largest object in the asteroid belt (mean grooves are mapped as red lines and chains of diameter of ~940 km). On March 6th 2015, Dawn pits/craters are mapped as yellow lines. Generally, the became the first spacecraft to visit Ceres, which was linear features, in the form of grooves and chains of formerly studied by telescopic observations and pits/craters, are evenly distributed across Ceres’ geochemical modeling [e.g. 1, 2, 3, 4, 5]. surface. However, fewer linear features are mapped in The Dawn Science Team has studied Ceres’ some regions: (1) from ~70-90 °N and ~65-90 °S physical properties, geology and composition. Initial because of inappropriate illumination conditions, and scientific results include the identification of bright (2) in selected equatorial regions, some of which are spots on Ceres’ surface, the brightest of which are associated with the widespread smooth material around located within the floor of Occator crater and may be Kerwan crater [e.g. 10]. sublimating [6]. The average surface composition of We observe grooves to transition into chains of Ceres is consistent with a mixture of ammoniated pits/craters, and that these linear features are phyllosilicates, antigorite, carbonate and a dark commonly organized into sub-parallel sets. Thus, we component [7]. In addition, there are prominent linear interpret that the grooves and chains of pits/craters are features on Ceres’ surface, in the form of grooves and end-members that lie at either end of a continuum. chains of pits/craters. One particular set of linear Intra-crater grooves (green lines in Figure 1) is the features, provisionally called the Regional Linear provisional name we give to distinctive sets of linear Structures, has been initially interpreted as a fracture features that we map, using HAMO data, inside and in system, possibly formed by internally driven tectonics association with some of the most geologically fresh [8]. Many of the remaining linear features are initially impact craters: Azacca, Dantu, Ezinu, Occator, Urvara, interpreted to result from the formation of impact and Yalode craters. However, in initial LAMO data, craters [8]. some intra-crater grooves appear to be chains of pits Here we present a global geologic map of linear rather than grooves, and we also observe intra-crater features on Ceres. We use this map as the basis of our grooves within additional craters, such as Haulani. We detailed classification of all sets of linear features are currently investigating the formation mechanism of observed to date, and the interpretation of their intra-crater grooves. formation mechanisms. We identify cross-cutting Grooves and crater chains interpreted as ejecta ray relationships between the sets of linear features and systems. Morphologically fresh craters such as Dantu other geologic features, and use this to develop a and Occator are the centers of radial sets of grooves global geologic history focusing on the contribution of and chains of pits/craters. Thus, we propose these the linear features to the geologic evolution of Ceres. linear features are ejecta ray systems, which commonly Methods: Ceres science data are primarily form when material is ejected at low angles during acquired during three orbital phases of progressively impact crater formation, and bounces and scours across lower altitude: Survey, High Altitude Mapping Orbit the surface. Thus, in this case, the chains are chains of (HAMO) and Low Altitude Mapping Orbit (LAMO). secondary craters, and not chains of pits. Furthermore, The acquisition of Survey and HAMO data was unlike pits, some of these secondary craters have completed by the submission of this abstract, along visible rims. In future work, we plan to identify further with the collection of initial LAMO data. Therefore, sets of radial grooves and crater chains that form ejecta our global geologic map of linear features is based on a ray systems. ~140 m/pixel HAMO global mosaic of clear filter Grooves and pit chains interpreted as fractures. Dawn Framing Camera images [produced by the The most prominent set of grooves and chains of methods outlined in 9]. Additional mapping will be pits/craters is centered near Erntedank Planum. In this undertaken in regions of interest using the ~35 m/pixel work, we map this set of linear features as a band that LAMO Framing Camera images. We also use Framing extends from ~140 °E to ~320 °E, and ~50 °N to ~25 Camera color filter images and topography data, °S. Future LAMO-based mapping will further refine derived from the Framing Camera images, to inform the boundaries of this set of linear features. In the geologic mapping. agreement with [8], we interpret these linear features 47th Lunar and Planetary Science Conference (2016) 1618.pdf

as the surface expression of sub-surface fractures, form Ahuna Mons. which form as surficial materials drain into the sub- 4. Urvara and Yalode. In agreement with [8], we surface [e.g. 11], because of characteristics such as en observe that the RLS are cross cut by grooves and echelon patterns. Thus, in this case, the chains are secondary crater chains radial to Urvara and Yalode chains of pits, and not chains of secondary craters. We craters. In addition, in this work we observe that the also informally call this set of linear features the floor of Yalode crater may contain partially Regional Linear Structures (RLS). buried/re-activated RLS, which we will further Cross-cutting relationships. investigate with LAMO-based mapping. Thus, 1. Occator. The RLS are partially buried/cross-cut by these cross-cutting relationships indicate that the ejecta, grooves and secondary crater chains from RLS formed prior to Urvara and Yalode craters. Occator crater. Thus, Occator crater likely formed Discussion and future work: after the RLS. We are currently investigating the The aforementioned cross-cutting relationships possibility that subsurface fractures (part of the indicate that the RLS are older than Occator, Dantu, RLS) may be conduits that could allow for the Urvara and Yalode craters, as well as Ahuna Mons, upward flow of the material that forms the Occator making them one of the oldest features on Ceres. In bright spots. future work we will investigate possible mechanisms 2. Dantu. The ejecta from Dantu crater also appears to that could explain the formation of this widespread, partially bury/cross-cut the RLS. relatively old set of fractures on Ceres. 3. Ahuna Mons. Ahuna Mons is a ~4 km high solitary References: mountain that is interpreted as a viscous extrusive [1] McCord T. B. and Gaffey M. J. (1974) Science, 186, 352- dome [12]. The southeasterly part of the RLS 355. [2] Lebofsky L. et al. (1981) Icarus, 48, 453-459. [3] Küppers M. et al. (2014) Nature, 505, 525-527. [4] Castillo-Rogez J. C. and borders the western rim of Ahuna Mons. Using McCord T. B. (2010) Icarus, 205, 443-459. [5] McCord T. B. and LAMO Framing Camera images, we observe the Sotin C. (2005) J. Geophys. Res., 110, EO5009 1-14. [6] Nathues A. northwestern rim of Ahuna Mons to cross cut one et al. (2015) Nature, 528, 237-240. [7] De Sanctis M. C. et al. (2015) of the RLS grooves, indicating that the Ahuna Nature, 528, 241-243. [8] Buczkowski D. L. et al. (2015) AGU, Abstract #P44B-05. [9] Roatsch T. et al. (2015) Planetary and Space Mons construct is younger than this portion of the Science, in press. [10] Williams D. A. et al. (2016) LPSC XLVII, RLS. We are currently investigating the possibility this meeting. [11] Wyrick D. et al. (2004) Journal of Geophysical that a subsurface fracture (part of the RLS) may Research, 109, E06005. [12] Ruesch O. et al. (2015) AGU, Abstract have provided a conduit for ascending material to #P31H-06.

Figure 1. Global geologic map of linear features on Ceres, overlain onto the HAMO-resolution (~140 m/pixel) Framing Camera basemap (mosaics made by DLR and T. Platz, MPS). Red lines are grooves, yellow lines are chains or pits/craters, and green lines are intra-crater grooves.