Icarus 318 (2019) 230–240 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Ac-H-11 Sintana and Ac-H-12 Toharu quadrangles: Assessing the large and small scale heterogeneities of Ceres’ surface ∗ M.C. De Sanctis a, , A. Frigeri a, E. Ammannito b, F.G. Carrozzo a, M. Ciarniello a, F. Zambon a, F. Tosi a, A. Raponi a, A. Longobardo a, J.P. Combe c, E. Palomba a, F. Schulzeck d, C.A. Raymond e, C.T. Russell f a Istituto di Astrofisica e Planetologia Spaziali, INAF, via del fosso del Cavaliere, 100, 00133, Rome, Italy b Agenzia Spaziale Italiana, Rome, via del Politecnico, 00133 Rome, Italy c Bearfight Institute, 22 Fiddler’s Road, P.O. Box 667, Winthrop, WA 98862, USA d DLR, Institute of Planetary Research, Berlin, Germany e Jet Propulsion Laboratory, Pasadena, CA 91109, USA f Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA a r t i c l e i n f o a b s t r a c t Article history: Mineralogical maps of the Ac-H-11 Sintana and the Ac-H-12 Toharu quadrangles of the dwarf planet Ceres Received 27 April 2017 were produced in order to access the composition of this planetary body. We used data from NASA’s Revised 25 July 2017 Dawn spacecraft, in particular the spectra returned by VIR, the imaging spectrometer on board. Different Accepted 9 August 2017 spectral parameters in the infrared range have been computed to study the composition of this portion Available online 12 August 2017 of Ceres’ surface and its large and small scale variability. We studied the variation and distribution of the Keywords: phyllosilicate bands at 2.73 μm and 3.07 μm, the reflectance at 1.2 μm, and the overall spectrum in specific Dwarf planet Ceres locations. We did not observe variations of the center of the bands at 2.73 μm and 3.07 μm, with the Surface composition exception of a few pixels in the Kupalo crater. We found that this southern region, extending from 0 ° to H2 O ice 180 ° and from 21 °S to 66 °S, show an overall increase of phyllosilicate band intensity from the equatorial Carbonates areas to the southern areas. Superimposed to the large-scale trend, we observe many smaller localized Phyllosilicates variations of band intensity. The observed variations can indicate large and small-scale heterogeneities in Reflectance spectroscopy the abundance of the different species in the Ceres subsurface. However, the small-scale variation that is mostly associated with young craters, can be also due to processes related with impacts, such as de- hydration or delivery of exogenous material that, mixed with the original surface, could change the band intensity. Several craters, such as Kupalo and Juling, show a different composition with respect to the background, displaying water ice and sodium carbonates. © 2017 Published by Elsevier Inc. 1. Introduction Here we will use the data returned by the VIR and FC in- struments in order to study the mineralogical composition of this NASA’s Dawn spacecraft arrived at Ceres on March 6, 2015. It southern region of Ceres. The thermally-corrected reflectance spec- was launched in September 2007 to study the two most massive trum of Ceres ( Fig. 2 ) shows that the 2.6–4.2 μm wavelength region bodies in the asteroid belt: the asteroid Vesta and the dwarf planet is characterized by a broad asymmetric feature, due to H2 O/OH Ceres ( Russell and Raymond, 2011 ). A combined approach of gravi- bearing materials. Within this broad absorption feature are several tational investigation, visible and near-infrared spectroscopic mea- distinct absorptions bands at 2.73, 3.05–3.1, 3.3–3.5, and 3.95 μm. surements (VIR, De Sanctis et al., 2011 ), gamma ray and neutron VIR observations show a strong and narrow absorption centered spectroscopy (GRaND, Prettyman et al., 2011 ) and imaging with at 2.72–2.73 μm. This characteristic absorption feature is distinc- Dawn’s framing camera (FC, Sierks et al., 2011) is used to study tive for OH-bearing minerals because H2 O-bearing phases show the innermost dwarf planet Ceres. a much broader absorption band that is a poor match for Ceres’ spectrum. The Sintana and Toharu quadrangles are located in Ceres’ south- ern hemisphere between 21–66 °S and 0–180 °E ( Fig. 1 ). The Sintana ∗ quadrangle truncates several large craters of the heavily cratered Corresponding author. E-mail address: [email protected] (M.C. De Sanctis). Zadeni South Pole quadrangle. The Kerwan quadrangle and the http://dx.doi.org/10.1016/j.icarus.2017.08.014 0019-1035/© 2017 Published by Elsevier Inc. M.C. De Sanctis et al. / Icarus 318 (2019) 230–240 231 Fig. 1. Scheme of 15 quadrangles used for High Altitude Mapping Orbit (HAMO)- and Low Altitude Mapping Orbit (LAMO)-based regional geologic mapping ( Roatsch et al., 2016 ). The Sintana quadrangle and the Toharu quadrangle are highlighted by a red border. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Haulani quadrangle are on its northern border ( Tosi et al., this is- ters can be mapped on the surface of Ceres with a resolution never sue ; Palomba et al., this issue ). The southern hemisphere of Ceres achieved before. is less heavily cratered than its northern hemisphere ( Hiesinger et VIR acquired data of Ceres during all of the mission phases, al., 2016 ; Marchi et al., 2016 ). A low-crater density (LCD)_ terrain starting from the Approach phase, for a total of two years of oper- was identified by Hiesinger et al. (2016) , centered at 54.2 °E/23.3 °S, ations at the moment of writing ( Russell et al., 2016 ). Ceres was close to the boundary of the Haulani quadrangle ( Krohn et al., observed with increasing spatial resolution from Survey orbit to 2016 ) in the north. Geological maps of these two quadrangles are LAMO orbit. Here we used mainly the HAMO dataset that has a described in Williams et al. (2017) and Schulzeck et al. (2017) . nominal spatial resolution of about 450 m/pix. Data of the Framing Camera (FC), which offer the geolog- 2. Data analysis description ical context of these regions, reach spatial resolutions up to ∼140 m/pixel (HAMO) and ∼35 m/pixel (LAMO). A HAMO mosaic 2.1. Dataset with a mean resolution of 140 m per pixel ( Roatsch et al., 2016 ) has been used ( Fig. 3 ) in this study. The comparison of local spec- The dataset used was acquired by VIR, the mapping spectrom- tral characteristics and surface morphology was done using indi- eter of the Dawn mission ( Russell et al., 2011; De Sanctis et al., vidual FC images. The FC data has been used to construct the 2011 ). Images taken by the Dawn Framing Camera ( Sierks et al., Digital Terrain Models ( DTM ) that give the topography of the re- 2011 ) are also used for context, geological and morphological anal- gions. In this paper we use such maps for comparison with the ysis. VIR is an imaging spectrometer derived from VIRTIS-M aboard derived spectral parameter maps. Rosetta and Venus Express ( Coradini et al., 1998 ). It measures spec- tra between 0.25 and 5.1 μm using two channels: the VIS channel, between 0.25 and 1.05 μm, and the IR, between 1.0 and 5.1 μm. The 2.2. Data analysis high spatial (IFOV = 250 μrad/pixel, FOV = 64 × 64 mrad) and spec- tral ( λVIS = 1.8 nm/band; λIR = 9.8 nm/band) performances al- The VIR data were calibrated and processed in order to ana- low for the identification of spectral features at small spatial scale. lyze them in terms of mineralogy of the different terrains. The The instrument uses a scanning mirror, thus the scene is scanned calibration used here has been improved with respect to the data one line at a time through the slit of the spectrometer. Each line is published in De Sanctis et al. (2012) , in which calibration artifacts made up of 256 pixels, each having a spectrum in the 0.25–5.1 μm were present in the region beyond 2.5 μm. The new calibration range. The set of adjacent images is then stacked to form a 3-D ar- procedure is described in Carrozzo et al. (2016) . Photometric ray called a hyperspectral cube. VIR’s imaging capability, combined effects are corrected for both topographic variations and physical with an extensive spectral range, offers the geological context for characteristics of the regolith using the Hapke approach ( Ciarniello mineralogical investigations: spectra and derived spectral parame- et al., 2017 ). 232 M.C. De Sanctis et al. / Icarus 318 (2019) 230–240 large-scale variation in band centers. For this reason, maps of band centers are not shown. The band at 3.06–3.07 μm has been assigned to different species like brucite ( Milliken and Rivkin, 2009 ) and ammonia-bearing species ( King et al., 1992 ). Brucite, in particular, clearly shows this narrow characteristic absorption. However, ammoniated phyllosili- cates are the best fit of the overall Ceres spectrum ( De Sanctis et al, 2015 ). For this reason, we assign this band to ammoniated species. 3. Maps and parameters 3.1. Reflectance maps The first global image maps in the visible and infrared wave- lengths were derived from the FC data, and VIR data ( Roatsch et al., 2016 ; Ciarniello et al., 2017 ) was made from Survey orbit im- ages. Photometrically corrected reflectance maps in the infrared, at 1.2 μm, are obtained from VIR data taken at different orbital phases and, consequently, at different spatial resolutions ( Fig.