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The Surface Composition of Ceres• Ezinu Quadrangle Analyzed By Icarus 318 (2019) 124–146 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus The surface composition of Ceres’ Ezinu quadrangle analyzed by the Dawn mission ∗ Jean-Philippe Combe a, , Sandeep Singh a, Katherine E. Johnson a, Thomas B. McCord a, Maria Cristina De Sanctis b, Eleonora Ammannito c, Filippo Giacomo Carrozzo b, Mauro Ciarniello b, Alessandro Frigeri b, Andrea Raponi b, Federico Tosi b, Francesca Zambon b, Jennifer E.C. Scully d, Carol A. Raymond d, Christopher T. Russell e a Bear Fight Institute, 22 Fiddler’s Road, P.O. Box 667, Winthrop, WA 98862, USA b Istituto di Astrofisica e Planetologia Spaziali-Istituto Nazionale di Astrofisica, Rome, Italy c Agenzia Spaziale Italiana, Rome, Italy d Jet Propulsion Laboratory, Pasadena, CA, USA e Institute of Geophysics and Planetary Physics, University of California Los Angeles, CA, USA a r t i c l e i n f o a b s t r a c t Article history: We studied the surface composition of Ceres within the limits of the Ezinu quadrangle in the ranges Received 5 December 2017 180–270 °E and 21–66 °N by analyzing data from Dawn’s visible and near-infrared data from the Visi- Accepted 22 December 2017 ble and InfraRed mapping spectrometer and from multispectral images from the Framing Camera. Our Available online 5 January 2018 analysis includes the distribution of hydroxylated minerals, ammoniated phyllosilicates, carbonates, the Keywords: search for organic materials and the characterization of physical properties of the regolith. The surface of Dwarf planet Ceres this quadrangle is largely homogenous, except for small, high-albedo carbonate-rich areas, and one zone Surface composition on dark, lobate materials on the floor of Occator, which constitute the main topics of investigation. (1) Mineralogy Carbonate-rich surface compositions are associated with H 2 O ice rich crust. Weaker absorption bands of Spectroscopy hydroxylated and ammoniated minerals over the carbonate-rich areas can be explained by higher abun- dances of carbonates at the topmost surface. (2) Dark, smooth lobate materials at the foot of Occator’s northeastern wall possibly reveal fresh slumping of phyllosilicate-rich materials with fine grain size, or local enrichment in carbon-rich materials such as tholins. (3) The deeper absorption band depth of OH and NH 4 , on the rim of several impact craters, is one observation that is consistent with a stratification of the phyllosilicate abundance that has been inferred previously from global investigations. ©2018 Elsevier Inc. All rights reserved. 1. Introduction fication of molecular groups and minerals by the Visible and In- fraRed mapping spectrometer (VIR) ( De Sanctis et al., 2011 ), mul- (1) Ceres is the largest object in the main asteroid belt, the tispectral data from the Framing Camera (FC) ( Sierks et al., 2011 ), only one with global hydrostatic compensation ( Dermot, 1979; Mil- and the elemental composition by the Gamma-Ray and Neutron lis et al., 1987; Park et al, 2016 ), classified as a dwarf planet 1 and, Detector (GRaND) ( Prettyman et al., 2011 ). because its unique characteristics are linked to the origin and evo- The surface of Ceres is essentially made of aqueously altered lution of the solar system, it is the second main objective of the silicates, similar to the composition of carbonaceous chondrites Dawn mission ( Russell et al., 2011 ). Ceres’ thermodynamics, chem- ( Johnson et al., 1973; 1975; Larson et al., 1979 ; Gaffey and Mc- istry and internal structure are investigated remotely by Dawn’s in- Cord, 1979; Lebofsky et al., 1981; Fanale and Salvail, 1989; Rivkin struments, which are dedicated to observe evidence of differenti- et al., 2006; Milliken and Rivkin, 2009 ). The ubiquitous presence ation processes, material redistribution by meteoroid impacts, and of absorption band at 2.7 μm is characteristic of the widespread effects of space weathering. One main objective of the mission is distribution of hydroxyl-rich (OH) minerals. An absorption band at mapping the surface composition of Ceres, particularly the identi- 3.1 μm, also detected everywhere on Ceres, is attributed to N –H + bonds in ammoniated minerals (structural NH4 ) (De Sanctis et al., 2015 ). Both absorption bands have been linked to the presence of ∗ Corresponding author. phyllosilicates on Ceres ( De Sanctis et al., 2015 ), where Mg-rich E-mail address: jean-philippe_combe@bearfightinstitute.com (J.-P. Combe). phyllosilicates exhibit only the OH absorption at 2.7 μm and NH - 4 1 http://astro.cas.cz/nuncius/ . https://doi.org/10.1016/j.icarus.2017.12.039 0019-1035/© 2018 Elsevier Inc. All rights reserved. J.-P. Combe et al. / Icarus 318 (2019) 124–146 125 rich phyllosilicates have both (the ammonium substitutes into a a similar principle to the analysis of Vesta ( Combe et al., 2015 )): large cation site). The observed lateral heterogeneities in the dis- we typically paired the absorption band depths at 2.72 and 3.1 μm tribution of the two absorption band depths are interpreted as rel- (both sensitive to phyllosilicates), and the absorption band depths ative abundance variations of these two phyllosilicate species (and at 3.4 and 4.0 μm (both sensitive to carbonates). We also used only these two). To date, no shifting in position of these two ab- three-color composites (which can be applied with any combina- sorption bands could be measured, which leads to the conclusion tion of three spectral parameters). that fluctuations in the geochemistry of the exposed upper layer Within the boundaries of the Ezinu quadrangle, we defined sev- are too small to be reported, and that no other type of phyllosili- eral topics of investigation, and here we summarize the results: cates is observed ( Ammannito et al., 2016 ). The origin of those phyllosilicates is likely endogenous. Ammo- (1) Areas around flow features have weaker absorption bands of nia could have been delivered on Ceres from other bodies origi- phyllosilicates and stronger absorption bands of carbonates, nating from the outer solar system ( De Sanctis et al., 2015 ), al- suggesting a relationship between the abundance of these min- though at places such as Dantu crater–where the impact reaches erals and H2 O ice in the subsurface, possibly hydrothermal pro- very deep into Ceres’ crust–the 3.1 μm band is very strong–rather cesses. pointing to a higher abundance of ammonium in the subsurface (2) Rims of a few large impact craters (Ezinu, Occator) exhibit ( Stephan et al., 2017b,c ). Bright carbonates, also of endogenous ori- deeper absorption band depths at 2.72 and 3.1 μm than the gin, have been discovered in the areas of highest albedo (faculae) floor or the surrounding terrains, which may reveal either more with an absorption band at 4.0 μm ( De Sanctis et al., 2016 ) that intense aqueous alteration or grain size sorting, or composi- are the strongest on the floor of the Occator crater, and at 3.4 μm. tional variations in the stratigraphy. Aliphatic organics, also likely endogenous, are found in a region (3) On the northern walls, rims and ejecta of the Occator crater, a that includes the Ernutet crater, where the surface exhibits a strong strong positive spectral slope in the visible and up to 1.25 μm, absorption band at 3.4 μm ( De Sanctis et al., 2017 ) and a positive without the absorption bands distinctive of organic materials spectral slope in the visible ( Schröder et al., 2017 ). Finally, exposed is observed. This is a rare finding on Ceres that may indicate either different physical properties (grain size) from the rest H2 O ice was discovered in nine small areas at latitudes above 30° by the presence of absorption bands at 1.28, 1.65 and 2.00 μm of Ceres’ surface, or a distinct composition of the topmost mi- ( Combe et al., 2016, 2017 ), which is likely the result of recent ex- crometers of the surface. posure of buried water ice, consistent with a higher H2 O content in the subsurface as a function of latitude ( Prettyman et al., 2017 ). 2. Dawn’s spectral dataset and methods To date, the distributions of each of the surface components de- tected by the Dawn mission have only been studied separately. Dawn began orbiting Ceres on 6 March 2015 at a 14,0 0 0 km At present, a comparative distribution can be performed in de- radius (23 April–9 May), followed by a 4900 km (6–30 June), a tail at high resolution (140 m) from maps of VIR spectral param- 1950 km radius orbit (17 August–23 October), and an 850 km or- eters ( Frigeri et al., 2017 ), which may lead to refined interpreta- bit (16 December 2015–October 2016). Reflectance spectroscopy tions on a case-by-case basis. The analysis of Ezinu quadrangle is the main technique available on the Dawn spacecraft to study is also the opportunity to open new questions about the detec- the surface composition remotely; it is sensitive to the molecular tion and distribution of carbonates and phyllosilicates on Ceres. bonds, minerals and physical properties of the first micrometers of Indeed, the presence of these two mineralogical species has been the surface. Multispectral, high-resolution images from FC and data inferred only from the identification of absorption bands beyond from the imaging spectrometer VIR are used in this study. 2.7 μm, although they are both expected to have absorption bands between 1.4 and 2.5 μm. In this paper, we discuss the reasons for 2.1. FC multispectral images and albedo maps their non-detection, and we open the exploration of alternative in- terpretations such as the possible presence of carboxylic acids (e.g. The Framing Camera has a two-dimension detector Applin et al., 2016 ) instead of ammoniated phyllosilicates and car- (1024 × 1024 pixels) and seven bandpass filters, plus a clear bonates.
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