The Surface Composition of Ceres• Ezinu Quadrangle Analyzed By

Home , Ezinu (crater)

Icarus 318 (2019) 124–146

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Icarus

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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 abundances 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 detected 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 interpretations 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. filter. To recreate a multispectral dataset, images acquired with Geological mapping of Ceres ( Williams et al., 2017 ) also pro- a different filter have to be co-registered and projected. The vides the opportunity to study the possible relationship between maps used in this study are a controlled mosaic of clear-filter morphological features (craters, ejecta, flows) and all the surface images acquired during the low-altitude mapping orbit ( Fig. 1 ), components detected by Dawn to date. The objective is to investi- providing context for the surface features, and a photometrically- gate the implications of those relationships in terms of evolution, corrected albedo mosaic of clear-filter images acquired during the internal structure and activity state of Ceres. high-altitude mapping orbit ( Fig. 2 ) ( Schröder et al., 2017 ). In this study, we focus on a region of Ceres in the range 180– The photometric correction relies on the calculation of a shape 270 °E and 21–66 °N, identified as quadrangle Ac-H-4 Ezinu, named model based on stereophotoclinometry ( Park et al., 2016 ). A three- after a large impact crater (110.5 km in diameter) with a center lat- color composite of band ratios (Red: R(965 nm)/R(750 nm); Green: itude of 42.97 °N and a center longitude of 195.83 °E. A geological R(750 nm); Blue: R(440 nm)/R(750 nm), defined as the Clementine study of this area ( Scully et al., 2017 ) is our reference for the inter- color composite ( Pieters et al., 1999 )) enhances the true color of pretation of all morphological features. In addition to the analysis Ceres, where blue colors indicate a negative spectral slope and red of the absorption band depth of hydroxylated minerals at 2.7 μm a positive spectral slope ( Fig. 3 ). and of ammoniated phyllosilicates at 3.1 μm ( Frigeri et al., 2017 ), we calculated the absorption band depth at 4 μm that is sensitive 2.2. VIR processing and spectral parameter maps to carbonates, the absorption band depth at 3.4 μm, and the spectral slope around 1.1–1.25 μm (both sensitive to aliphatic organics). 2.2.1. VIR spectra and calibration Absorption bands at wavelengths longer than 3 μm require the re- VIR is an imaging spectrometer developed by the Agenzia moval of a thermal emission contribution. In order to facilitate the Spaziale Italiana (ASI), Istituto Nazionale di AstroFisica (INAF) and interpretation of multiple spectral parameters, we performed the Galileo Avionica with two focal planes, one dedicated to visible simultaneous analysis of any combination of two spectral param- wavelengths between 0.25 and 1.1 μm, and one to the near infrared eters by using a two-dimensional, multi-color scheme (following between 1.0 and 5.1 μm. The detectors are two-dimensional arrays 126 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 1. Mapping of Ceres in the visible from Dawn’s Framing Camera clear filter images acquired during the Low Altitude Mapping Orbit. This controlled mosaic, not corrected for photometric effects, mostly illustrates the surface morphological features. The white rectangle marks the limits of the Ezinu quadrangle. (A) Global map in equirectangular projection. (B) Map of Ezinu quadrangle in Lambert conformal conic projection with latitude of projection origin at 90 °N, center meridian at 225 °, standard parallels at 21 ° N and 66 °N respectively. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion. Credits: Deutsches Zentrum für Luft- und Raumfahrt (DLR). with 256 spatial samples and 432 spectral wavelengths, with an the surface of Ceres to the sun in AU, and π is a constant that instantaneous field of view (IFOV) of 250 × 250 μrad ( De Sanc- appear when calculating the flux emitted from a sphere and re- tis et al., 2011 ). Depending on Dawn’s orbital altitude, the spatial ceived over the surface of a disk. Additional processing steps (ther- resolution of VIR at Ceres varies from 11 km/pixel to 70 m/pixel. All mal emission correction, photometric correction and calculation of the results presented here are from the near-infrared detector. Cal- spectral parameters) are described in the following subsections. ibration from raw data in digital numbers to reflectance factor (I/F)

(Hapke, 1981), including minimization of artifacts, was done using 2.2.2. Thermal emission correction of VIR spectra the techniques described by Carrozzo et al. (2016): Ceres orbits the sun at distances of 2.56 to 2.98 AU and has

πI d 2 an average albedo lower than 10%; it receives and absorbs enough I /F = , energy to have daytime surface temperatures ranging from 180 to F 0 240 K. As a consequence, thermal emission contributes as an ad- where F0 is the solar flux (irradiance) at 1 AU (Astronomical Unit), ditive signal that may become significant with respect to reflected I is the calibrated radiance measured by VIR, d is the distance from signals at wavelengths beyond 3 μm. The exponential increase of J.-P. Combe et al. / Icarus 318 (2019) 124–146 127

Fig. 2. Photometrically-corrected albedo of Ceres’ surface in the visible from Dawn’s Framing Camera clear filter images, acquired during the High Altitude Mapping Orbit. (A) Global map in equirectangular projection. The white rectangle marks the limits of Ezinu quadrangle. (B) Map of Ezinu quadrangle in Lambert conformal conic projection. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion. Credits: Deutsches Zentrum für Luft- und Raumfahrt (DLR). thermal emission as a function of wavelength creates asymmetric high-albedo faculae), and since the thermal emission dominates and variable distortions in the shape of absorption bands of inter- the signal beyond 3.5 μm, we make the assumption that an opti- est at 3.4 and 4 μm. In addition, absorption bands in reflectance be- mum thermal emission correction of a given spectrum corresponds come thermal emission peaks, which partially or completely cancel to a minimum value of the standard deviation calculated between out the absorptions. 3.5 and 5 μm (159 wavelength channels). The calculation is per- We performed a thermal emission correction of VIR I/F spec- formed iteratively so that the corrected reflectance R and thermal tra by fitting a Planck function, solely based on the pixel-by- emissivity E follow the Kirchoff law of R + E = 1. This first step is pixel analysis of the spectral shapes in VIR data: it is a com- also necessary prior to photometric correction. mon approach for the processing of reflectance spectra (e.g. Clark 1979; Clark et al., 2011; Besse et al. 2013; Tosi et al., 2014 ). In the present study, we assume that I/F is equivalent to bihemi- 2.2.3. Photometric correction spheric reflectance (although no photometric correction has been Photometric effects have two causes that can be corrected sep- performed), and it makes the approximation that the surface cov- arately. (1) Variations of the geometry of illumination and observa- ered by one pixel is characterized by a single temperature (more tion are due to topography at scales larger than a pixel size. A disk accurate correction would require considering surface roughness function correction minimizes these effects and makes the sur- properties ( Bandfield et al., 2008 ). Since the spectra of Ceres have face appear smooth. We use a version of the Akimov disk-function low reflectance and little variability across the surface (except the (Akimov, 1975 and Eq. (29) in Shkuratov et al., 1999) that has no free parameter and has been proven to yield adequate results 128 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 3. Three-color composite of Ceres’ surface in the visible from Dawn’s Framing Camera color ﬁlter images acquired during the High Altitude Mapping Orbit. Red: R(965 nm)/R(750 nm); Green: R(750 nm); Blue: R(440 nm)/R(750 nm). (A) Global map in equirectangular projection. The white rectangle marks the limits of the Ezinu quadrangle. (B) Map of the Ezinu quadrangle in Lambert conformal conic projection. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion. Credits: Deutsches Zentrum für Luft- und Raumfahrt (DLR). on Ceres ( Schroeder et al., 2017 ), as well on Vesta ( Combe et al., (2) Variations as a function of the phase angle are due to mul- 2015 ): The Akimov disk function D function (Eq. (29) in Shkuratov tiple scattering caused by roughness at scales smaller than a pixel et al. (1999) has no free parameter: it depends only on the phase size and physical parameters such as porosity of the surface mate- ( α), incidence ( i ), emergence ( ε) and the azimuth ( ψ) between the rial. For this correction, we use the entire VIR dataset at Ceres; for planes of i and ε, that are transformed into photometric latitude b each wavelength we calculate a histogram of I/F as function of the and photometric longitude l ( Eq. (1 )). phase with 1 ° intervals. Then for each value of the phase angle, we π α interpolate linearly the values of the histogram. − cos π−α l 2 α/ ( π−α) D ( α, b, l ) = ( cos b ) (1) cos ( l ) 2.2.4. VIR spectral parameters Maps of absorption band depth at 2.72 μm ( Fig. 4 ) and 3.1 μm where ( Fig. 5 ), and spectral slopes in the ranges 1.1–1.8 μm and 1.1–1.8 μm cos i = cos b cos ( l − a ) (2) are commonly used in the analysis of 13 quadrangles of the surface of Ceres ( Frigeri et al., 2017 ). The linearly striped aspect of certain parts of the map are due to the spatial sampling of VIR data. VIR cos ε = cos b cos ( l ) (3) scans the surface primarily from the motion of the Dawn spacecraft relative to the ground. Consequently, gaps between each pro- cos a = cos i cos ε + sin i sin ε cos ψ (4) jected line of the data occur when the read-out time of a data line J.-P. Combe et al. / Icarus 318 (2019) 124–146 129

Fig. 4. Absorption band depth of hydroxylated minerals at 2.72 μm of Ceres’ surface calculated from reflectance spectra acquired by VIR-IR. (A) Global map in equirectangular projection. The white rectangle marks the limits of the Ezinu quadrangle. (B) Map of Ezinu quadrangle in Lambert conformal conic projection. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion. exceeds the time it takes for the FOV to shift by more than one Another absorption centered at 4 μm is sensitive to CO3 ( = ( / ) = time the size of a projected pixel. In the maps presented in this and is defined as b1 mean I F[3 . 6687 .. 3 . 7735] μm , b2 ( / ) = ( / ) ) paper, there is usually no gap–and hence no striping–over areas mean I F[3 . 9530 .. 4 . 0100] μm and b3 mean I F[4 . 0760 .. 4 . 1235] μm that have been observed multiple times; however there are regions ( Fig. 9 ). Maps are provided in ( Fig. 10) . that have been observed only once or twice where gaps between individual lines have not been filled to date by additional data. In addition we calculated the spectral slope in the range 1.09– 2.2.5. Two-dimension color scales 1.25 μm, as one parameter to be used in the search for organic In order to facilitate the interpretation of spectral parameters, molecules ( Fig. 6 ), as it can help to identify spectrally peculiar re- it is convenient to display several of them on the same map. Red- gions. Maps are provided in Fig. 7 . green-blue color composites are commonly used to represent three We also calculated band depth parameters using the definition different datasets; however, the drawback is that it is not possible of BD = 1 − 1 / 2 × ( b1 + b3 ) / b1 for two more absorptions. One to represent all the colors of the map in a single color table (a centered at 3.4 μm is sensitive to both CO3 and CH and is defined real one would have three dimensions), which limits the accuracy as (b1 = mean ( I /F . .. . μ ) , b2 = mean ( I /F . .. . μ ) [3 1490 3 2154] m [3 3855 3 4707] m and relevance of the interpretations. It is, however, possible to rep- and b3 = mean ( I /F . .. . μ ) ) , defined in Fig. 6 . Maps are [3 5841 3 6410] m resent two parameters using two-dimensional color tables based provided in ( Fig. 8) . on mixtures of four or more main colors (e.g. Combe et al., 2015 ) ( Fig. 11 ). The interpretation of the colors can be facilitated by dis- 130 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 5. Absorption band depth of ammoniated minerals at 3.1 μm of Ceres’ surface calculated from reﬂectance spectra acquired by VIR-IR. (A) Global map in equirectangular projection. The white rectangle marks the limits of the Ezinu quadrangle. (B) Map of Ezinu quadrangle in Lambert conformal conic projection. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion. playing a two-dimensional scatterplot of the dataset at the same on a map, the four coordinates of each pixel are calculated for scale or as an overlay to the color table. the start, mean and end times of the integration. This calculation This technique can be applied to any pair of spectral parame- is done using the NAIF toolkit and SPICE kernels ( Acton, 1996 ), ters. For Ceres, we use it for simultaneous visualization of the OH similar to what has been done on Vesta (e.g. Frigeri et al., 2015; and NH4 absorption bands at 2.72 and 3.1 μm respectively ( Fig. 12 ), Combe et al., 2015 ). for the CO3 absorption bands at 3.4 and 4 μm (Fig. 13), and for the 1.09–1.25 μm spectral slope and the CH absorption bands at 3.4 μm

( Fig. 14 ). 3. Results: surface composition of the Ezinu quadrangle

The surface of this quadrangle has homogenous composition, 2.2.6. Mapping VIR spectral parameters similar to Ceres’ average, with a few exceptions such as the ﬂoor, VIR acquires data using the relative motion of the spacecraft rims and ejecta of Occator (the high-albedo faculae are investi- (push-broom techniques) or an internal rotating mirror to create gated by Longobardo et al. (this issue) , an unnamed crater cen- images. The readout time between the acquisitions of two con- tered at 222 °E, 61 °N, the ﬂoor and rim of the Ezinu crater, and secutive lines of an image creates a coverage gap. In addition, the a few, local areas scattered about the quadrangle, associated with spacecraft attitude may be adjusted during an observation by VIR, small impact craters: on the quadrangle maps, rectangles identify which creates further gaps and complex shapes of the surface cov- subsets of interest that are detailed in the discussion. In order to ered by a given pixel. In order to precisely represent VIR pixels interpret the surface composition of Ceres, the spectral parameters J.-P. Combe et al. / Icarus 318 (2019) 124–146 131

The surface of the Ezinu quadrangle has a similar phyllosilicate composition as the rest of Ceres. Broad areas with weaker absorption bands at 2.72 and 3.1 μm exist, such as a region with a trian- gular shape that is defined by the southern limit of the quadrangle from 210 to 240 °E, by the southeast corner of the quadrangle, including Ninsar crater, which is located at 50 °N, 230 °E, and by several fresh impact craters and ejecta deposits from 60 to 66 °N and from 200 to 250 °E. Small, fresh craters and ejecta on the floor and southern rim of Ezinu also have weaker absorption bands. Con- versely, strong absorption bands at 2.72 and 3.1 μm are found in the northeast wall, rim and ejecta of the Occator crater. The floor of the Ezinu crater has a relatively weak 3.1 μm absorption band, although it has an average 2.72 μm absorption band depth. The opposite characteristics are observed on a small, high albedo area at 63 °N, 235 °E which exhibits a strong absorption band at 3.1 μm and weak absorption band at 2.72 μm. Laboratory spectra of phyllosilicates exhibits absorption bands between 1.0 and 2.5 μm; however these absorption features are not detected on Ceres. The reason for the absence of ligand–metal (Mg–OH or Fe–OH) is likely the low signal-to-noise ration associated to a very low-albedo surface ( < 10%) that absorbs at all wavelengths. All the absorption bands on Ceres are weak, including the OH absorptions associated with the fundamental vibration mode at 2.7 μm and beyond: it is therefore expected that the intrinsi- cally small absorption bands of phyllosilicates remain undetected on Ceres. In comparison, that same fundamental vibration feature on Mars is about one magnitude stronger.

3.2. Carbonates

Carbonates have two absorption bands at ∼3.4 and ∼4.0 μm, as illustrated in Fig. 9 B. The position of the center of these absorption bands is sensitive to the cation (Ca, Mg, Fe or Mn). The two absorption band depth parameters at 3.4 μm and 4 μm presented in the Fig. 6. Illustration of the search of organic molecules with calculation of the spectral slope between 1.09 and 1.25 μm (grey rectangle on the left), and an absorption method section are adapted to the spectra observed on Ceres. Ar- band depth centered at 3.4 μm (three grey lines labeled b1, b2 and b3 on the right). eas with strong 3.4 μm and 4 μm absorption bands that have aver- (A) Example spectrum from Ceres at Ernutet.(B) Example spectra of aliphatic organ- age or lower than average albedo, and negative or average spectral ics ( Moroz et al., 1998 ). slope may be also related to the presence of carbonates, because the high albedo is mostly a function of light scattering properties of the surface, and not necessarily a function of the chemical com- sometimes have to be combined. We chose to organize the descrip- position. For example, it may only require a small fraction (a few tion of the results based on the various components that have been percent) of a low albedo component such as the average composi- detected to date by Dawn. tion of Ceres to make a carbonate-rich surface appear like the rest of the Ceres surface in the visible. Inside of the Ezinu quadrangle, most areas with a strong ab- 3.1. Hydroxylated and ammoniated minerals sorption at 4.0 μm have also a strong absorption aft 3.4 μm, with a higher albedo than surrounding terrains, and negative or average Absorption bands at 2.72 and 3.1 μm are ubiquitous on Ceres, spectral slope at wavelengths longer than 1 μm; these characteris- and they are related to hydroxyl and ammonia as part of the tics are all consistent with carbonate-rich surface materials. Some composition of phyllosilicates ( De Sanctis et al., 2015; Ammannito areas with strong absorption bands at 3.4 μm and 4.0 μm with aver- et al., 2016 ). The constant positions of the two absorption bands age or lower-than-average albedo may also be related to the pres- across the surface of Ceres indicate a homogenous composition. ence of carbonates. The largest carbonate-rich area occurs in the The depth of the absorption bands is sensitive to abundance and range 60.6 °–62.4 °N and 220.4 °–225.0 °E; this region surrounds the grain size only. However, VIR spectra at 3.1 μm can be further dis- H2 O-rich zone inside of a small, fresh impact crater whose details torted by thermal emission, which can cause spectral parameters are presented in Combe et al, 2017 , and its morphology is charac- to provide erroneous estimates of absorption band depth. Conse- terized by ﬂow features that are consistent with underground wa- quently, when observations from multiple mission phases are mo- ter ice ( Schmidt et al, 2017 ). Other carbonate-rich areas are also in saicked, the edge of individual projected images may exhibit a the interior or in the ejecta of fresh impact craters; however, they sharp contrast due to thermal emission. In this range of wave- are not associated to visible ﬂow features. lengths, however, the thermal emission correction ( Section 2.2.2 ) Laboratory spectra of carbonates exhibit also absorption bands results in seamless 3.1 μm absorption band depth parameter maps, of C –O vibration overtones between 1.5 and 2.7 μm ( Fig. 9 B). On which illustrates its effectiveness. A two-dimensional scatter plot Ceres, these spectral features are generally not detected, except on of the two absorption band depth parameters indicates a diffuse the highest albedo faculae (Occator). The absence of detection of correlation of the two, as also reported in Ammannito et al. (2016) ; overtone bands on Ceres carbonate-rich materials could be par- however, important differences on the maps can be noticed at all tially explained by the small amplitude of these absorptions com- scales. pared to those of fundamental vibration modes at longer wave- 132 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 7. Spectral slope of Ceres’ surface between 1.09 and 1.25 μm calculated from reﬂectance spectra acquired by VIR-IR. (A) Global map in equirectangular projection. The white rectangle marks the limits of the Ezinu quadrangle. (B) Map of the Ezinu quadrangle in Lambert conformal conic projection. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion.

lengths, and the resulting low-signal–to-noise ratio, as we inter- 3.3. Search for organic-rich materials preted for the phyllosilicates ( Section 3.1 ). However, carbonates- rich areas have a relatively higher albedo compared to phyllosili- A strong positive spectral slope in the visible and the near- cates, and for which VIR spectra should have higher signal-to-noise infrared up to 1.25 μm is observed at the foot of the northeastern ratio. The high albedo implies that a large proportion of photons rim of the Occator crater, on an area identiﬁed as dark, smooth undergo surface scattering instead of absorption, which does not lobate materials ( Scully et al., 2017 ). The combination of positive favor the detection of overtone absorption bands. In addition, car- spectral slope with low albedo materials is rare on Ceres; these bonates on Ceres are expected to contain Fe such as siderite, as spectral characteristics may be due to a different molecular com- indicated by chemical analysis of carbonaceous chondrite mete- position, which might be due to certain types of aliphatic organics. orites ( Takir et al., 2015; Schäfer et al., 2016 ) and Gamma-ray spec- No surface combining a strong absorption band at 3.4 μm and a troscopy ( Lawrence et al., 2017 ). Fig. 9 B shows that Fe-bearing car- positive spectral slope in the range 1.10–1.25 μm is found within bonates exhibit much weaker overtone absorption bands between Ezinu quadrangle, which indicates that the type of materials found 1.5 and 2.7 μm than spectra of non-Fe bearing carbonates, and thus in Occator are not the same as those detected in Ernutet ( De Sanc- those bands may not be detectable by VIR. tis et al., 2017 ). J.-P. Combe et al. / Icarus 318 (2019) 124–146 133

Fig. 8. Absorption band depth at 3.43 μm (sensitive to organic molecules and carbonates) of Ceres’ surface calculated from reﬂectance spectra acquired by VIR-IR. (A) Global map in equirectangular projection. (B) Map of Ezinu quadrangle in Lambert conformal conic projection.

4. Interpretation and discussion In the northern portion of the Ezinu quadrangle, the geological context is different from Occator: no doming or radial fractures are

4.1. Indications of the formation of carbonates by hydrothermal observed. The presence of exposed H2 O ice surrounded by high- processes albedo, CO3 -rich materials associated to a fresh crater (Fig. 15) (1 to 10 Myr old, according to Hughson et al. (2017) , both inside the Weaker absorption bands of phyllosilicates and stronger absorp- crater cavity and in the ejecta, is similar to the case of the Oxo tion bands of carbonates occur around areas where H2 O ice are crater (Singh et al., 2017). exposed or around ﬂow features. This observation suggests a rela- H2 O is not stable at the surface of Ceres (Hayne and Aharon- tionship between the abundance of these minerals and H2 O ice in son, 2015; Shorghorfer et al., 2016; Combe et al., 2016; Landis the subsurface. CO3 -rich materials have been discovered initially as et al., 2017; Formisano et al., 2016), and both Oxo and the H2 O-rich the main component of the faculae in the ﬂoor of Occator, which is crater at 222 °E, 61 °N are not located within a persistent shadow characterized by a central doming and a network of radial fractures region ( Platz et al., 2016 ). These characteristics are indicative of a

(Buczkowski et al., 2017; Scully et al., 2017). Since the formation of mechanism that has exposed buried H2 O ice, such as an impact

CO3 requires relatively high temperatures in an H2 O-rich environ- with meteoroid or a landslide, or that has allowed H2 O to migrate ment, it can only take place in the interior of Ceres, or as a result from the subsurface towards the surface ( Combe et al., 2017 ). Since of the heating due to an impact crater that can trigger hydrother- CO3 -rich minerals form by aqueous processes and are seen in the mal processes ( De Sanctis et al., 2016 ). ejecta, the likely scenario is their formation in the interior by hydrothermal processes as described in ( De Sanctis et al., 2016 ), fol- 134 J.-P. Combe et al. / Icarus 318 (2019) 124–146

These areas correspond to small impact craters and their ejecta ( Fig. 17 ), without any other remarkable geological feature or spec-

tral signature of H2 O ice-rich materials. These observations are not contradictory to the ﬁrst two examples: it is indeed not surprising

to not ﬁnd exposed H2 O ice remaining today or to not observe ﬂow features from such small impacts. However, since hydrothermal processes are the most likely mechanism for the formation of car-

bonates, the detection of CO3 -bearing minerals in small craters all about the Ezinu quadrangle is another indicator of the widespread

presence of H2 O on the surface or in the subsurface of Ceres in the past. A global analysis by Palomba et al. (2017b) indicates that small, high-albedo areas are not systematically carbonate-rich, as a result of a different origin and evolution process.

4.2. Complex and diverse surface composition of the northern ﬂoor, wall and rim of Occator crater

In Fig. 18 , a small area on the ﬂoor of Occator, at the foot of the northeastern rim of Occator crater, is distinctive because of its positive spectral slope in the visible, as seen in FC images, and up to 2.4 μm ( Fig. 19 ). It contrasts with surrounding terrains that have a relatively negative spectral slope. The 3.4 μm absorption band is detected, although not particularly strong. Geologically, this area consists of lobate, smooth and dark materials ( Scully et al., 2017 ). It is also adjacent to a portion of the wall of Occator crater with a strong 3.4-μm absorption band an average spectral slope. We explored several hypotheses to explain the nature of these materials.

4.2.1. Characterization of the dark, lobate materials on the ﬂoor of Occator 4.2.1.1. Hypothesis of organic components. The search for organic molecules on Ceres is justiﬁed because of the spectral similarities between its surface and carbonaceous chondrite meteorites samples. The positive spectra slope and the detection of an absorption band at 3.4 μm are common characteristics of organic components. On the smooth, lobate materials of Occator, this absorption band is not as deep as the one observed in Ernutet ( Fig. 6 ), and there-

Fig. 9. Illustration of the search for carbonates with calculation of absorption band fore the detection of organic molecules is only a working hypothe- depths centered at 3.4 μm (three grey lines labeled b1, b2 and b3) and 4 μm (three grey lines labeled b3, b4 and b5). (A) Example spectrum from Ceres at Occator’s sis at this stage of the study. Moroz et al. (1998) revealed that the largest facula – Cerealia Facula –and from high-albedo surface materials at (222 °E, type of organic molecule may affect dramatically the shape and 61 °N), (185 °E, 23 °N) and (266 °E, 25 °N). (B) Example spectra of carbonates [John depth of the CH absorption band at 3.4 μm ( Fig. 20 ). After scal-

Hopkins University spectra library]. ing the spectra of kerite, asphaltite and wurtzilite to make their spectral slope comparable, their respective absorption band depths decrease in that order. Assuming that the dark, lobate materials in lowed by an impact that occurred between 1 and 10 Myr, which Occator contain solid bitumens as in Ernutet, their chemistry must exposed and ejected CO3 -rich materials. Most H2 O ice that may be different so that their reflectance spectra have a positive spectra have been ejected or exposed during the impact was likely subli- slope and only a weak CH absorption band at 3.4 μm. mated by the transient heat. The H2 O ice observed today is either Moroz et al. (1998) noted that increasing H/C and O/C ratios the remainder of the exposed H2 O ice from the impact, or an in- and decreasing aromacity (the fraction of carbon atoms involved dication that subsurface H2 O was able to migrate to the surface in aromatic units) such as noted from asphaltite to wutzilite, re- more recently, possibly through fractures in the floor at the cen- sults in higher reflectance at shorter wavelengths, steeper (more ter of the crater created by the impact. While no fractures are ob- positive) spectral slope in the visible, and broader and stronger ab- served today at 222 °E, 61 °N, some of them are visible in Oxo crater sorptions from 3 μm and beyond, effectively masking the CH ab- ( Combe et al., 2016 ; Hughson et al., 2016). sorption band at 3.4 μm and making it appear weaker. Vilas and Datan crater and several other craters that are superimposed Smith (1985) also noted that the change of spectral slope may on each other, and are centered at about 250 °E, 60 °N, have floors, be caused by the changes of structure and H-content of hydrocar- walls and ejecta made of carbonate-rich material ( Fig. 16 ). Al- bons. However, wutzilite has an overall larger absorption feature though no exposed H2 O ice has been detected so far in this region, between 2.5 and 4 μm and a very strong band at 2.3 μm that dif- the large lobate features, inside and outside the craters, suggest fers from the VIR spectra. Since organics exposed at the surface that H2 O ice was (or still is) present in the subsurface (Scully et al., should gradually lose their volatile components and become pro-

2017). The association of H2 O ice (possibly in the past), high albedo gressively more carbonized (Moroz et al., 1998), the assumption of material and carbonates is similar to the previous example at organic materials in Occator with higher H/C and O/C ratios than 222 °E, 61 °N, or to Oxo crater. in Ernutet could imply more freshly exposed organics in Occator, Several other locations with bright materials inside of the Ez- which is consistent with lobate materials created during the strati- inu quadrangle have absorption bands compatible with carbonates. graphically youngest geological period of Ceres ( Scully et al., 2017 ). J.-P. Combe et al. / Icarus 318 (2019) 124–146 135

Fig. 10. Absorption band depth of carbonates at 4.00 μm of Ceres’ surface calculated from reﬂectance spectra acquired by VIR-IR. (A) Global map in equirectangular projection. (B) Map of Ezinu quadrangle in Lambert conformal conic projection.

Fig. 11. Illustration of the construction of a two-dimensional color scale for the simultaneous representation of two spectral parameters. (A) Choice of the colors and their location in the table. (B) Smoothing by convolution with a Gaussian ﬁlter. 136 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 12. Mapping of OH-bearing and NH 4 -bearing minerals on Ceres with simultaneous representation of absorption band depths at 2.72 and 3.1 μm using a two-dimensional color composite. (A) and (B) Two-dimensional scatterplot of the two spectral parameters representing the entire surface of Ceres. (C) Two-dimensional color scale. (D) Overlay of the color scale with the scatterplot from the Ezinu quadrangle data. (E) Global map in equirectangular projection. The white rectangle marks the limits of the Ezinu quadrangle.(F) Map of the Ezinu quadrangle in Lambert conformal conic projection. White rectangles indicate the limits of seven subsets illustrated as close-up views in the discussion.

In addition, according to Moroz et al. (1998) , an increase in terials falling off the wall of a crater is a plausible scenario, since grain size for an asphaltite (gilsonite) sample results in an over- small grains may more likely stay at higher elevation than large all darkening of the surface ( Fig. 21 A), and a weaker (and slightly grains. broader) CH absorption centered at 3.4 μm ( Fig. 21 B). Although the Besides the hypotheses of aliphatic organic molecules, other spectral slope may decrease slightly as the grain size increases, it organic, not-well-characterized substances such as tholins ( Khare may not vary enough to become similar to the spectral slope of et al., 1994 ) may have also formed on Ceres. Tholins are made of other components of Ceres. Grain size sorting by large-grained ma- H, C, N and were originally synthesized as analogs for aerosols in J.-P. Combe et al. / Icarus 318 (2019) 124–146 137

Fig. 13. Search for CO 3 -rich surface materials on Ceres with simultaneous representation of absorption bands depths at 3.4 and 4 μm using a two-dimension color composite. (A) and (B) Two-dimensional scatterplot of the two spectral parameters representing the entire surface of Ceres. (C) Two-dimensional color scale.(D) Overlay of the color scale with the scatter plot from Ezinu quadrangle data. the atmosphere of Titan. Although the chemical processes that take objects are also relevant to this comparison, as their reddish sur- place in the lab experiments are likely different from those that oc- face has been modeled using tholins (e.g. Dalle Ore et al., 2009 ). In cur naturally on celestial objects of the solar system ( Bernard et al., Fig. 22 , black-brown tholins created as Titan analogs ( Bernard et al., 2006 ), the compositions and spectral characteristics are relevant 2006) present a positive spectral slope in the visible and near- analogs. For example, on the nucleus of comet 67P/Churyumov– infrared up to 2.5 μm, with weak absorption features compared to Gerasimenko, portions of the surface with more positive spectral the total amplitude of the spectrum, and a broad, deep absorption slope than the average may be due to tholins, distinct to other band at 3 μm that can mask or mimic the ubiquitous absorption organic absorbers at 3.4 μm ( Capaccioni et al., 2015 ). Kuiper belt band observed on Ceres. Tholins synthesized as Triton’s analogs 138 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 14. Search for organic-rich surface materials on Ceres with simultaneous representation of the spectral slope between 1.09 and 1.25 μm and the absorption band depth at 3.4 μm using a two-dimensional color composite. (A) and (B) Two-dimensional scatterplot of the two spectral parameters representing the entire surface of Ceres. (C) Two-dimensional color scale. (D) Overlay of the color scale with the scatterplot from Ezinu quadrangle data.

( Owen et al., 2001 ) have similar shape. This spectrum is also sim- 4.2.1.2. Hypothesis of grain size sorting on slumping materials. Ex- ilar to the surface of Iapetus, which has been interpreted as “con- periments on the Murchison carbonaceous chondrite meteorite sistent with the presence of abundant organic material, such as CM2 indicate that smaller grain sizes result in more positive spec- N-rich tholins” ( Brown et al., 2006 ). Since all spectra of Ceres tral slopes ( Izawa et al., 2016 ). As noted by Stephan et al. (a, b, have absorption bands characteristic of phyllosilicates, tholins, if in press) , a more positive spectral slope in the visible associated present, must be mixed with other minerals in smaller proportions to absorption band depths similar to the average surface material than to the low-albedo areas of Iapetus, where a model ﬁts the may correspond to older surfaces and “appear in association with surface spectrum with 36% of tholin ( Buratti et al., 2005 ). recent mass wasting deposits such as slumping” at the foot of the J.-P. Combe et al. / Icarus 318 (2019) 124–146 139

Fig. 15. Fresh impact crater at 222 °E, 61 °N (rectangle subset c on the quadrangle maps) containing a small area of exposed H 2 O ice on the floor, near the shadow, and high albedo, carbonate-rich materials on the wall and ejecta. (A) FC clear filter image. (B) Spectral slope between 1.090 and 1.250 μm. (C)Absorption band depth at 2.72 μm. (D) Absorption band depth at 3.4 μm. (E) Absorption band depth at 3.1 μm. (F) Absorption band depth at 4 μm. (G) Two-dimensional color composite of the absorption band depth at 2.72 μm and at 3.1 μm. (G) Two-dimensional color composite of the absorption band depths at 3.4 μm and at 4 μm. southern wall of the Dantu crater and Haulani crater, suggesting sorption bands at 2.7 and 3.1 μm are strong, as are the rest of the a decrease in grain size as a result. The dark, lobate materials on eastern side of the crater, rim and ejecta. Furthermore, this part the floor of Occator have a strong positive spectral slope and ab- of the Occator floor is distinctively unique at the scale of Ceres sorption band depth at 3.4 and 4.0 μm that are not particularly for its occurrence in the middle of OH-rich and NH4 -rich materi- strong, which is consistent with this hypothesis; however, the ab- als, its large area, and its occurrence at the foot of the tallest wall 140 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 16. Datan crater and two other impact craters superimposed to each other at 250 °E, 60 °N (rectangle subset e on the quadrangle maps) feature lobate features, suggesting

flows containing buried H 2 O ice on the floor and ejecta and high albedo carbonate-rich materials ( Scully et al., 2017 ). (A)FC clear filter image. (B) Spectral slope between 1.090 and 1.250 μm. (C) Absorption band depth at 2.72 μm. (D) Absorption band depth at 3.4 μm. (E) Absorption band depth at 3.1 μm. (F) Absorption band depth at 4 μm. (G) Two-dimensional color composite of the absorption band depth at 2.72 μm and at 3.1 μm. (G) Two-dimensional color composite of the absorption band depth at 3.4 μm and at 4 μm. of Occator. Although other smooth, lobate materials at the foot of dued. This hypothesis, however, is not completely supported by Occator’s northeastern wall are also consistent with slumped ma- measurements of absorption band depth. Reflectance spectra re- terials, their spectral slope is not as positive and it does not af- sult from absorption and scattering processes. For smaller grains, fect the entire range of wavelength from the visible to 2.5 μm. One the ratio volume/area is lower, which increase the scattering (and possible explanation is that a fresh slumping of phyllosilicates may thus the albedo) and decrease the absorptions (e.g., Cooper and pulverize the materials in fine grains, resulting in a strong positive Mustard, 1999 ). Surface roughness also determines the amounts of spectral slope. Then, over time, as the surface is weathered and is scattering, in which the material compaction and particle aggrega- contaminated by ejecta, those spectral characteristics become sub- J.-P. Combe et al. / Icarus 318 (2019) 124–146 141

Fig. 17. Several small impact craters ( < 1 km 2) at various locations inside of the Ezinu quadrangle (from left to right, rectangle subsets a, b, d, g on the quadrangle maps), have distinctive high albedo, carbonate-rich materials. –First column: FC clear ﬁlter controlled mosaic from LAMO. – Second column: FC clear ﬁlter albedo mosaic from HAMO. – Third column: Two-dimensional color composite of the absorption band depth at 2.72 μm and at 3.1 μm. –Fourth column: Two-dimensional color composite of the absorption band depth at 3.4 μm and at 4 μm. – (A)–(D): Crater at 185E, 22 N. (E)–(H): Crater at 230E, 30 N. (I–L): Central, southern part of Ezinu crater. (M–P): Crater at 266E, 26 N. 142 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 18. Northern portion of the Occator crater (rectangle subset f on the quadrangle maps). (A) FC clear ﬁlter controlled mosaic from LAMO. (B) FC clear ﬁlter albedo mosaic from HAMO. (C) Three-color composite from Fc images - Red: R(965 nm)/R(750 nm); Green: R(750 nm); Blue: R(440 nm)/R(750 nm). (D) Absorption band depth at 2.72 μm. (E) Absorption band depth at 3.1 μm. (F) Two-dimensional color composite of the absorption band depth at 2.72 μm and at 3.1 μm. (G) Spectral slope between 1.090 and 1.250 μm. (H) Absorption band depth at 3.4 μm. (I) Two-dimensional color composite of the spectral slope between 1.090 and 1.250 μm and the absorption band depth at 3.4 μm.

Fig. 19. VIR IR spectra of lobate materials near the northeastern wall of Occator from LAMO observation 510,654,782. (A) I/F spectra of VIR IR. (B) Ratio of VIR VIS and IR spectra from HAMO observation 494,253,260 with respect to the average spectrum of that dataset. tion is a determinant characteristic that may compete with grain erals. On the other hand, VIR spectra do not show clear bands size effects. due to transitions of Fe 2 + or Fe 3 + anywhere on Ceres. These ap- parent contradiction is mitigated by gamma-ray spectroscopy measurements ( Lawrence et al., 2017 ) which, although it detected Fe 4.2.1.3. Hypothesis of thin layers of spectrally neutral components. in the first meter if Ceres’ regolith, it “found Fe-abundance values Alternatively, the coating of high albedo materials by a thin layer that are lower than what is typically seen for carbonaceous chron- of low albedo materials moving down the wall of Occator may be drites”. Consequently, the combination of low albedo, low amounts thick enough to appear opaque at short wavelengths and trans- of Fe-bearing minerals and small grain size on the topmost surface parent at long wavelengths, because the photon absorption path (which favors light scattering rather than absorption) may be suffi- lengths through the medium increases as a function of the pho- cient to explain why Fe-bearing minerals are not detected on Ceres ton’s wavelength. In this case, grain-size sorting may also play a with VIR. role, because large grains increase the probability of photon absorption and decrease the efficiency of surface scattering. As a result, this simple stratification of the first micrometers of the re- 4.2.2. Composition diversity in the wall of Occator golith may create a positive spectra slope in reflectance, without Compared to the floor, ejecta and most of Occator’s northern the need for materials with a positive spectral slope. In this sce- wall, the existence of a relatively deep absorption band at 3.4 μm nario, the presence of a dark, spectrally neutral component could on the wall above the dark, lobate and smooth materials is not have a carbon-rich composition, although this simple spectral char- associated to a particularly deep 4 μm band depth, which likely acteristic is not sufficient for a clear identification. Many low- excludes higher abundances of carbonates, or to a more positive albedo minerals might contain Fe-rich phases but most of these spectral slope, which is expected for organic-rich materials. All actually show some absorption features (magnetite has absorp- the aforementioned hypotheses for the analysis of the floor do tions near 0.5 and 1 μm). Experimental analysis of Ceres-analog not seem relevant to explain the observations of Occator’s north- carbonaceous chondrite meteorites (e.g. Takir et al., 2015 ) indicate ern wall, because a stronger 3.4 μm absorption band depth is al- that the surface of Ceres should contain some iron-bearing min- ways associated with either an increase or no effect on the pos- J.-P. Combe et al. / Icarus 318 (2019) 124–146 143

Fig. 22. Reﬂectance spectrum of black-brown tholin from Bernard et al. (2006) , with C / N = 1 . 72 , C / H = 0 . 95 , [ C + N] / [H] = 1 . 50 . Fig. 20. Spectra of solid bitumens (as in Fig. 6 B), normalized at 2 μm, illustrate the variation of the CH absorption band centered at 3.4 μm as a function of the molecular structure, from high H/C and O/C ratios of wurtzilite to lower H/C and O/C ratios of kerite (adapted from Moroz et al., 1998 ).

tor (Ezinu was formed during the stratigraphically oldest period, itive spectral slope. Here, the higher values provided by the ab- whereas Occator was formed during the stratigraphically interme- sorption band parameter and the absence of a distinctive spectral diate period, with geological activities continuing as late as during slope at short wavelengths are not easily linked to any specific sur- the stratigraphically young period ( Scully et al., 2017 )). Size may face composition. Visual inspection and comparison with spectra be a factor, as the smallest crater showing these characteristics is from adjacent areas do not reveal dramatic changes in the spectral about 20 km in diameter. However, there are only three observed shape: the only noticeable difference is a more concave curvature occurrences within the limits of this quadrangle, which is not suf- between 2.8 and 4.1 μm on the wall of Occator compared to the ficient to analyze any correlation. If the observations were system- dark lobate materials of the floor. This characteristic, added to a atically of deeper absorption band depth on the rim than on the lower albedo, may be sufficient to explain higher values of absorp- wall, floor or ejecta, we could invoke more intense aqueous alter- tion band depth measured at 3.4 μm without major changes in the ation or grain size sorting as a likely explanation. However, we no- chemistry. tice one example with the opposite behavior (weaker absorption band depth on the rim than on the floor and ejecta) in the Fejokoo 4.3. Deeper absorption bands on the rims of a few large impact quadrangle ( Singh et al., 2017 ), which indicates that this hypothesis craters is not valid everywhere on Ceres. Compositional variations in the stratigraphy, on the other Rims of a few large impact craters (Ezinu, Occator) exhibit a hand, could fit to any observation, as suggested by Ammannito deeper absorption band depth, at 2.72 μm and 3.1 μm, than the et al. (2016) . An impact with a meteoroid results in excavating ma- floors of the surrounding terrains, forming a distinctive ring with terials, deeper near the center of the cavity, and the distribution brighter pixels ( Fig. 23 shows a close-up view of Ezinu). This ob- of the ejecta also reflects the stratigraphy, as materials projected servation is also distinct from impact crater floors that exhibit a at longer distances are provided from the deepest parts of the ex- strong absorption band at 3.1 μm and weak absorption band at cavation. Lateral and vertical abundance variations of hydroxylated 2.72 μm [e.g., studies of quadrangles Dantu and Kerwan, respec- and ammoniated phyllosilicates across the surface and in the crust tively by Stephan et al. (2017b) and Palomba et al. (2017a) ]. In of Ceres constitute an interpretation that is supported by observa- general, deeper absorption bands can be related either to higher tions of the mineralogical distribution at global scale ( Ammannito abundances of the absorbing material, or larger grain sizes. Deeper et al., 2016 ). It certainly requires the right thickness of stratification absorption bands on the rim do not occur for all craters, and it and depth of the impact-transient cavity (and thus a crater size) to is not observed for all absorption bands. It does not seem to be form a ring of distinctive composition on a crater rim; however, associated with crater age, as Ezinu is much fresher than Occa- this is a possible occurrence ( Fig. 24 ).

Fig. 21. Spectra from Moroz et al. (1998) illustrate that an increase in grain size of an asphaltite sample results in a smaller CH absorption band centered at 3.4 μm. 144 J.-P. Combe et al. / Icarus 318 (2019) 124–146

Fig. 23. Ezinu crater. (A) FC clear ﬁlter controlled mosaic from LAMO. (B) FC clear ﬁlter albedo mosaic from HAMO. (C) Three-color composite from Fc images Red: R(965 nm)/R(750 nm); Green: R(750 nm); Blue: R(440 nm)/R(750 nm). (D) Absorption band depth at 2.72 μm. (E) Absorption band depth at 3.1 μm. (F) Two-dimension color composite of the absorption band depth at 2.72 μm and at 3.1 μm.

Fig. 24. Schematic cross-section of an impact crater (Adapted from Melosh, 1989 ). This illustrates the inverted stratigraphy near the rim. As a result, the surface of the rim may have a distinct composition (white layer in the picture): from above, it may appear as a ring.

5. Conclusions or exposed at the surface. One main interpretation difference is that small, high-albedo areas exhibit absorption bands character-

We studied the surface composition of Ceres within the limits istic of carbonates and not H2 O ice, as Scully et al. (2016) initially of the Ezinu quadrangle by analyzing data from Dawn’s VIR map- suggested. This result is not likely the consequence of detection ping spectrometer between 1.0 and 5.1 μm and multispectral im- limitation, since exposed H2 O ice is identified in VIR spectra over ages from the Framing Camera. The surface of Ezinu quadrangle is small areas ( < 100 m 2 ) that are spatially unresolved by VIR and just largely homogenous, except small, high-albedo carbonate-rich ar- about the resolution of FC ( Combe et al., 2013 ). Absorption bands eas, and one zone on dark, lobate materials on the floor of Occa- of hydroxylated and ammoniated minerals (phyllosilicates) at 2.72 tor. The results are interpreted in accordance with global mapping and 3.1 μm respectively are weaker over the carbonate-rich areas, of phyllosilicates ( Ammannito et al., 2016; Frigeri et al., 2017 ) and which can be explained by the fact that higher abundances of car- geological interpretations of this quadrangle ( Scully et al., 2017 ), bonates at the topmost surface partially mask the absorption bands which highlighted the prominent role of H2 O in the geological evo- of phyllosilicates. lution from the frequent occurrence of lobate flows. On a dark lobate feature on the floor of Occator, a relatively In fact, the largest carbonate-rich regions seem to be associ- steep positive spectral slope in the visible and between 1.09 and ated with areas showing evidence of H2 O ice in the subsurface, 1.25 μm is observed. A number of mineralogical components or J.-P. Combe et al. / Icarus 318 (2019) 124–146 145 physical characteristics may cause reddening of the surface; how- Besse, S., Yokota, Y., Boardman, J., Green, R., Haruyama, J., Isaacson, P., Mall, U., Mat- ever, because of the lack of diagnostic absorption features, the sunaga, T., Ohtake, M., Pieters, C., Staid, M., Sunshine, J., Yamamoto, S., 2013. One Moon, many measurements 2: Photometric corrections. Icarus 226, 127– spectra cannot be unambiguously linked to one specific cause. We 139. doi: 10.1016/j.icarus.2013.05.009 . investigated the possible presence of aliphatic organic materials in Brown, et al., 2006. Observations in the Saturn system during approach and orbital this small area of Occator following their discovery in the Ernutet insertion, with Cassini’s visual and infrared mapping spectrometer (VIMS). A&A

446, 707–716. doi:10.1051/0004-6361:20053054. area of Ceres (De Sanctis et al., 2017), and because they have a pos- Buczkowski, D.L., Schmidt, B.E., Williams, D.A., Mest, S.C., Scully, J.E.C., Ermakov, A.I., itive spectral slope in the visible and near-infrared ( Schröder et al., Preusker, F., Schenk, P., Otto, K.A., Hiesinger, H., O’Brien, D., Marchi, S., Size- 2017 ). Since the 3.4 μm absorption band depth is not stronger in more, H., Hughson, K., Chilton, H., Bland, M., Byrne, S., Schorghofer, N., Platz, T., this area than on surrounding terrains, the presence of organic Jaumann, R., Roatsch, T., Sykes, M.V., Nathues, A., De Sanctis, M.C., Ray- mond, C.A., Russell, C.T., 2017. The geomorphology of Ceres. Science 353. doi: 10. components cannot be conﬁrmed. Even if they are present, their 1126/science.aaf4332 . nature is likely different from those discovered in Ernutet, such as Buratti, B.J., Cruikshank, D.P., Brown, R.H., Clark, R.N., Bauer, J.M., Jaumann, R., Mc- higher H/C, O/C ratios and lower aromaticity, or larger grain size. Cord, T.B., Simonelli, D.P., Hibbitts, C.A., Hansen, G.B., Owen, T.C., Baines, K.H., Bellucci, G., Bibring, J.-P., Capaccioni, F., Cerroni, P., Coradini, A., Drossart, P.,

Some tholin species synthesized as Titan and Triton’s analogs may Formisano, V., Langevin, Y., Matson, D.L., Mennella, V., Nelson, R.M., Nichol- exhibit a positive spectral slope and no absorption at 3.4 μm, which son, P.D., Sicardy, B., Sotin, C., Roush, T.L., Soderlund, K., Muradyan, A., 2005. corresponds better to VIR observations. It is, however, challenging Cassini visual and infrared mapping spectrometer observations of iapetus: detection of CO . Astrophys. J. 622 (2), 149–152. doi: 10.1086/429800 . 2 to explain the presence of any organic materials only on slumped Capaccioni, F., Coradini, A., Filacchione, G., Erard, S., Arnold, G., Drossart, P., De materials on the ﬂoor of Occator and not on the wall. Conversely, Sanctis, M.C., Bockelee-Morvan, D., Capria, M.T., Tosi, F., Leyrat, C., Schmitt, B., fresh slumped materials may be an easier way to explain the ob- Quirico, E., Cerroni, P., Mennella, V., Raponi, A., Ciarniello, M., McCord, T., Mo- roz, L., Palomba, E., Ammannito, E., Barucci, M.A., Bellucci, G., Benkhoff, J., servations: the mechanical process involved in material sliding off Bibring, J.P., Blanco, A., Blecka, M., Carlson, R., Carsenty, U., Colangeli, L., of the tallest wall of Occator is compatible with smaller grains, Combes, M., Combi, M., Crovisier, J., Encrenaz, T., Federico, C., Fink, U., Fonti, S., which may cause reddening of the surface. The difference in spec- Ip, W.H., Irwin, P., Jaumann, R., Kuehrt, E., Langevin, Y., Magni, G., Mottola, S.,

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