Organic Material on Ceres: Insights from Visible and Infrared Space Observations

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Article Organic Material on Ceres: Insights from Visible and Infrared Space Observations

Andrea Raponi 1,* , Maria Cristina De Sanctis 1, Filippo Giacomo Carrozzo 1 , Mauro Ciarniello 1 , Batiste Rousseau 1 , Marco Ferrari 1 , Eleonora Ammannito 2, Simone De Angelis 1, Vassilissa Vinogradoff 3, Julie C. Castillo-Rogez 4, Federico Tosi 1, Alessandro Frigeri 1 , Michelangelo Formisano 1 , Francesca Zambon 1, Carol A. Raymond 4 and Christopher T. Russell 5

1 Istituto Nazionale di Astrofisica–Istituto di Astrofisica e Planetologia Spaziali, 00133 Rome, Italy; [email protected] (M.C.D.S.); fi[email protected] (F.G.C.); [email protected] (M.C.); [email protected] (B.R.); [email protected] (M.F.); [email protected] (S.D.A.); [email protected] (F.T.); [email protected] (A.F.); [email protected] (M.F.); [email protected] (F.Z.) 2 Agenzia Spaziale Italiana, 00133 Rome, Italy; [email protected] 3 Physique des Interactions Ioniques et Moléculaires, PIIM, Université d’Aix-Marseille, 13013 Marseille, France; [email protected] 4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA; [email protected] (J.C.C.-R.); [email protected] (C.A.R.) 5 Earth Planetary and Space Sciences, University of California, Los Angeles, CA 90095, USA; [email protected] * Correspondence: [email protected]

Abstract: The NASA/Dawn mission has acquired unprecedented measurements of the surface of the dwarf planet Ceres, the composition of which is a mixture of ultra-carbonaceous material, phyllosilicates, carbonates, organics, Fe-oxides, and volatiles as determined by remote sensing instruments including the VIR imaging spectrometer. We performed a refined analysis merging Citation: Raponi, A.; De Sanctis, M.C.; visible and infrared observations of Ceres’ surface for the first time. The overall shape of the combined Giacomo Carrozzo, F.; Ciarniello, M.; spectrum suggests another type of silicate not previously considered, and we confirmed a large Rousseau, B.; Ferrari, M.; Ammannito, abundance of carbon material. More importantly, by analyzing the local spectra of the organic-rich E.; De Angelis, S.; Vinogradoff, V.; region of the Ernutet crater, we identified a reddening in the visible range, strongly correlated to Castillo-Rogez, J.C.; Tosi, F.; et al. Or- the aliphatic signature at 3.4 µm. Similar reddening was found in the bright material making up ganic Material on Ceres: Insights from Cerealia Facula in the Occator crater. This implies that organic material might be present in the source Visible and Infrared Space Observa- of the faculae, where brines and organics are mixed in an environment that may be favorable for tions. Life 2021, 11, 9. https://dx.doi. prebiotic chemistry. org/10.3390/life11010009

Keywords: astrobiology; organic material; Ceres; small bodies Received: 22 October 2020 Accepted: 21 December 2020 Published: 24 December 2020

Publisher’s Note: MDPI stays neu- 1. Introduction tral with regard to jurisdictional claims Ceres is the largest object in the asteroid belt. Because of its size (~950 km in diameter) in published maps and institutional and its spheroidal shape, it has been classified as a dwarf planet. Spectrophotometric affiliations. ground observations of Ceres have revealed a complex mineralogy from the absorption band depths and positions along the spectrum of the solar flux reflected by its surface. The low albedo and the relatively flat visible/near-IR (0.4–2.5 µm) spectrum suggest the presence of a dark component like magnetite [1,2]. Absorption bands at 3.3–3.4 µm and Copyright: © 2020 by the authors. Li- µ censee MDPI, Basel, Switzerland. This 3.8–4.0 m have been interpreted by carbonates like dolomite, calcite, and magnesite, article is an open access article distributed which is consistent with observations in the mid-infrared [1,2]. A narrow absorption at under the terms and conditions of the 3.06 µm has been attributed to different species, like ice frost [3], NH4-bearing minerals [4], Creative Commons Attribution (CC BY) irradiated organics and crystalline water ice [5], cronstedtite [6], and brucite [1]. license (https://creativecommons.org/ The Dawn NASA mission represents a breakthrough for the investigation of Ceres. licenses/by/4.0/). Dawn’s payload includes a visible and infrared mapping spectrometer (VIR) [7], a framing

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camera (FC) with seven wavelength filters [8] and a gamma ray and neutron detector (GRaND) [9]. The three instruments were complementary in inferring the surface properties of Ceres. The VIR imaging spectrometer is composed of two channels: the VIS channel covers the spectral range 0.25–1 µm, with a spectral sampling of ~2 nm, and the IR channel covers the spectral range 1–5 µm, with a spectral sampling of ~10 nm. The VIR obtained spectra of Ceres with unprecedented quality and quantity for an icy body, without any limitations due to atmospheric absorptions (which hide the range of 2.5–2.9 microns), and at a resolution of about a few kilometers to tens of meters. The first finding from the VIR-IR channel shows a global surface composed of a dark component (carbon and/or magnetite), and Mg-carbonates such as dolomite, confirming some of the previous studies based on ground observations. The discovery of a strong absorption at 2.7 µm (confirmed by data from the satellite AKARI [10]) was assigned to Mg-phyllosilicates such as antigorite, and a better definition of the 3.06-µm absorption was attributed to NH4-phyllosilicates [11]. Then, the GRaND instrument provided new constraints. Based on this elemental data, in particular the abundance of C, H, K, and Fe, [12,13] it was established that the dark material that makes up most of Ceres’ surface composition should be rich in carbon and it resembles carbonaceous chondrite material [14,15] which is associated with a thick volatile-rich crust, thus suggesting a large and diffuse presence of organic material. Recent improvement in the VIS channel calibration [16,17] allows for extending the range of the VIR instrument to shorter wavelengths (0.25–1.0 µm). Here, we further refine the calibration of the IR channel following Rousseau et al. [16,17]. The resulting VIR data in the visible range, together with the refined calibration in the IR channel provide new constraints on Ceres mineralogy. Using this new capability, we performed spectral analysis and modelling to infer further details on the composition of the dwarf planet. In the present work, we focus on the average spectrum of Ceres. We also revise local spectra from the Ernutet and Occator crater regions, where the VIR-IR instrument detected clear signatures of aliphatic organics [18] and a mixture of dry and hydrated salts, respectively, with the possible presence of organics [19].

2. Average Ceres Spectrum 2.1. Calibration Refinement We collected calibrated VIR data (available through the Planetary Data System (PDS) online data archive at: https://sbn.psi.edu/pds/resource/dawn/dwncvirL1.html). The data investigated in this work were acquired during different phases of the Dawn mission at Ceres, and they provide almost complete coverage of Ceres’ surface. The calibrated dataset was further refined with the procedure described in Carrozzo et al. [20] and Rousseau et al. [16], in order to correct for the odd-even effect and systematic artifacts [20], and spurious spectral variations due to the detector temperature in the VIS channel [16]. Data were then photometrically corrected to standard viewing geometry (incidence angle = 30◦, emission angle = 0◦, phase angle = 30◦) by means of Hapke modeling [21,22] according to the photometric parameters derived by Ciarniello et al. [23]. Moreover, we applied correction factors derived from ground-based observations to correct for fictitious slopes on spectra of both VIR channels. These correction factors were first derived by Carrozzo et al. [20]. In the present work, we used the updated version for the VIS channel derived by Rousseau et al. [17], and an updated version for the IR channel, here proposed, which was derived following the same procedure of Rousseau et al. [17]. Here we summarize the main steps to produce the ground correction. 1. We collected ground observations of Ceres, which are mutually consistent in the spectral range, where they overlap [2,24–30]. 2. For each ground full-disk observation (point 1), bidirectional reflectance was con- verted to standard viewing geometry (incidence angle = 30◦, emission angle = 0◦, Life 2021, 11, x FOR PEER REVIEW 3 of 14

Life 2021, 11, 9 3 of 14 2. For each ground full-disk observation (point 1), bidirectional reflectance was con- verted to standard viewing geometry (incidence angle = 30°, emission angle = 0°, phase angle = 30°) by means of Hapke modeling [22] according to the photometric parametersphase angle derived = 30◦) by [23]. means of Hapke modeling [22] according to the photometric 3. Basedparameters on the derivedground-based by [23]. spectra (point 2), we calculated a smooth average spec- 3. trumBased that on thecovers ground-based the whole spectraspectral (point range 2), of we the calculated infrared a channel smooth averageof the VIR spectrum spectrometer.that covers the whole spectral range of the infrared channel of the VIR spectrometer. 4.4. WeWe calculated calculated the the average average spectrum spectrum of VIR-IR of VIR-IR calibrated calibrated data, data, after afterartifact artifact and pho- and tometricphotometric correction, correction, as described as described above. above. 5.5. WeWe calculated calculated the the ratio ratio between between the the average average spectrum spectrum from from ground ground observations (point(point 3) 3) with with the the average average spectrum spectrum obtained obtained from from VIR VIRdata data (point (point 4). This 4). ratio This spec- ratio trumspectrum is used is as used a multiplicative as a multiplicative correcti correctionon factor factor on every on every single single VIR spectrum. VIR spectrum. MergingMerging the the global average spectrumspectrum obtainedobtained in in the the VIS VIS channel channel [17 [17]] with with the the average aver- ageIR spectrumIR spectrum derived derived here, here, we we obtained obtained a single a single global global average average spectrum spectrum of Ceresof Ceres in thein therange range of 0.25–5.0of 0.25–5.0µm. µm. Then, Then, we we removed removed the thethermal thermal emissionemission affecting the spectrum spectrum longwardlongward of of 3.5 3.5 µm,µm, as as discussed discussed in in Section Section 2.22.2 (Figure(Figure 11).).

FigureFigure 1. 1. AverageAverage Ceres Ceres spectrum. spectrum. The The VIS channel (black) and IR channelchannel (gray)(gray) havehave beenbeen merged mergedto obtain to aobtain unique a spectrum.unique spectrum. The spectral The spectral range longward range longward of 4.1 µm of (not4.1 µm shown (not here) shown is affectedhere) is by affectedhigh noise by high level noise after thelevel subtraction after the subtraction of the thermal of the emission. thermal emission.

2.2.2.2. Spectral Spectral Modeling Modeling ToTo obtain obtain information information on on the the abundance abundance of of the the surface surface minerals, minerals, we we used used a a quantita- quantita- tivetive spectral spectral analysis analysis of of the the composition composition using using Hapke’s Hapke’s radiative radiative transfer transfer model model [21,22]. [21,22]. SimilarSimilar modeling modeling has has been been descri describedbed by by De De Sanctis Sanctis et et al. al. [11], [11], Marchi Marchi et et al. al. [14], [14], and and RaponiRaponi et et al. al. [31]. [31]. The The whole whole formulation formulation of of the the bidirectional bidirectional reflectance reﬂectance (r) (r) is: is: r = (SSA/4π) µ0/(µ+ µ0) × K [BSH p (g) + H (SSA, µ/K) H (SSA, µ0/K) − 1] × S (i, e, g, θ)BCB r = (SSA/4π) µ0/(µ+ µ0) × K [BSH p (g) + H (SSA, µ/K) H (SSA, µ0/K) − 1] × S (i, e, g, θ) BCB

wherewhere i, e., g i,are e, gthe are incidence, the incidence, emission emission,, and phase and angles, phase angles,respectively, respectively, and µ0, andµ areµ 0the, µ cosinesare the of cosines the incidence of the incidence and emission and emission angles. angles.These parameters These parameters come from come the from shape the modelshape and model position and position of the spacecraft of the spacecraft at the time at theof observation. time of observation. The parameters The parameters that con- tainthat most contain of the most spectral of the information spectral information are the single are scattering the single albedo scattering (SSA), albedo and the (SSA), related and Ambartsumian–Chandrasethe related Ambartsumian–Chandrasekharkhar functions H (SSA, functions µ/K), H which (SSA, describeµ/K), which the multiple describe scat- the teringmultiple components. scattering Other components. parameters Other that parameters describe the that photometric describe the be photometrichavior as a function behavior ofas the a function viewing ofgeometry the viewing are: the geometry single partic are: thele phase single function particle p phase (g); the function shadow p (g);hiding the shadow hiding opposition effect B (g); the coherent back-scattering opposition effect opposition effect BSH (g); the coherentSH back-scattering opposition effect BCB (g); the shadow θ θ functionBCB (g); themodeling shadow large-scal functione modelingroughness large-scaleS (i, e, g, θ roughness), with θ being S (i, e,the g, average), with surfacebeing slope;the average and the surface porosity slope; parameter, and the K porosity linked parameter,to the filling K factor linked [22]. to the These filling photometric factor [22]. These photometric parameters are fixed after Ciarniello et al. [23], who defined the average parameters are fixed after Ciarniello et al. [23], who defined the average scattering prop- scattering properties of Ceres’ regolith. The spectral properties are mainly affected by the erties of Ceres’ regolith. The spectral properties are mainly affected by the SSA. The latter SSA. The latter have been modeled for intimate mixing between different minerals, which have been modeled for intimate mixing between different minerals, which implies that implies that the particles of the endmember materials are in contact with each other and are all involved in the scattering of a single photon. The SSA of each mineral is defined starting from their grain size and their optical constants, as described in [18]. The optical

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the particles of the endmember materials are in contact with each other and are all involved in the scattering of a single photon. The SSA of each mineral is defined starting Life 2021, 11, 9 from their grain size and their optical constants, as described in [18]. The optical constants 4 of 14 are derived from laboratory measurements (Table 1) with the method described by Carli et al. [32]. The average SSA of the regolith is defined through the weight pi, which is the cross sectionconstants of all aregrains derived of the from ith laboratorymineral as measurementsa fraction of the (Table total1 area.) with To the simplify method the described model and minimize the number of free parameters, we assigned the same grain size to by Carli et al. [32]. The average SSA of the regolith is defined through the weight pi, which all endmembers.is the cross In sectionthis way, of allthe grains relative of cross the ith section mineral of aseach a fraction endmember of the is total identical area. To to simplify the relativethe volumetric model and abundance. minimize the The number best result of free is obtained parameters, by comparison we assigned of the the same model grain size with the tomeasured all endmembers. spectra by In applying this way, a the curve-fitting relative cross procedure. section ofThe each free endmember model param- is identical eters to beto retrieved the relative are: volumetric (i) abundance abundance. of the endmembers The best result and the is grain obtained size, by which comparison define of the the SSA; model(ii) a multiplicative with the measured constant spectra and additional by applying slope a curve-fittingto the SSA of procedure.the model in The order free model to accountparameters for uncertainties to be retrieved in the radiometric are: (i) abundance and photometric of the endmembers accuracies, and as well the grain as other size, which unknowndefine neutral the minerals SSA; (ii) amaking multiplicative up the su constantrface; and and (iii) additional temperature slope toT and the SSAbeaming of the model functionin ∧ order[33]. The to accountlatter is formultiplied uncertainties by the in directional the radiometric emissivity and photometricεd [22] to give accuracies, the as effectivewell emissivity as other (εeff unknown). The directional neutral mineralsemissivity making is a function up the of surface; the SSA and (wavelength- (iii) temperature T dependent)and and beaming the emission function angle.∧ [33 The]. The beaming latter is function multiplied accounts by the for directional the fact that emissivity a mac- εd [22] roscopicallyto give rough the surface effective does emissivity not emit (ε radiationeff). The directionalin the same emissivity way as a macroscopically is a function of the SSA flat surface,(wavelength-dependent) since there are a variety and of thelocal emission normals angle.across Thethe surface beaming [33]. function Interpretation accounts for the of the modeledfact that thermal a macroscopically emission is roughoutside surface the scope does of not this emit work. radiation The total in radiance the same is way as a modeledmacroscopically by accounting for flat both surface, the contribution since there are of athe variety reflected of local sunlight, normals and across the thermal the surface [33]. emission:Interpretation of the modeled thermal emission is outside the scope of this work. The total radiance is modeled by accounting for both the contribution of the reflected sunlight, and Rad = r × Fʘ/D2 + εeff × B(λ, T) the thermal emission: 2 where r is the Hapke bidirectional reflectance,Rad = r × Fʘ /Dis the+ solarεeff × irradianceB(λ, T) at 1 AU, D is the heliocentric distance (in AU), εeff is the effective emissivity, and B(λ, T) is the Planck func- where r is the Hapke bidirectional reflectance, F is the solar irradiance at 1 AU, D is tion. Thus, the estimation of the thermal emission is done simultaneously with the reflec- the heliocentric distance (in AU), ε is the effective emissivity, and B(λ, T) is the Planck tance modeling to yield a consistent result effbetween these two contributions to the total function. Thus, the estimation of the thermal emission is done simultaneously with the signal measured.reflectance Here, modeling we show to yield the measured a consistent spectra result after between the subtraction these two contributions of thermal to the emissiontotal to isolate signal the measured. contributi Here,on of we the show reflectance the measured spectra. spectra after the subtraction of thermal We emissiondiscarded to the isolate spectral the contributionrange shortward of the of reflectance 0.36 µm from spectra. the analysis because it has a high levelWe of discarded uncertainty the spectraldue to the range low shortward sensitivity of of 0.36 theµ detector,m from the and analysis the spectral because it has range longwarda high level of 4.1 of µm, uncertainty becausedue it is toaffected the low by sensitivity a high noise of the level detector, after the and subtraction the spectral range of the thermallongward emission. of 4.1 µm, because it is affected by a high noise level after the subtraction of the Thethermal SSA was emission. modeled starting with minerals that have already been discussed in De Sanctis et al. The[11,34] SSA (see was Table modeled 1). However, starting to with account minerals for the that spectral have already shape in been the discussedVIS in channel,De weSanctis added etthe al. spectrum [11,34] (see of lizardit Table1).e measured However, by to accountHiroi and for Zolensky the spectral [35] shapewho in the heated theVIS sample channel, in a we vacuum added at the 600°C spectrum to remove of lizardite water contaminating measured by the Hiroi sample. and Zolensky Such [35] a spectrumwho presents heated a the peak sample close in to a 0.8 vacuum µm, which at 600 resembles◦C to remove the Ceres’ water average contaminating spectrum. the sample. However,Such it is a significantly spectrum presents different a peakfrom closethe lizardite to 0.8 µ m,spectrum which resemblesmeasured theat ambient Ceres’ average conditions.spectrum. Along with However, dehydration it is significantly caused by the different reduction from in thethe lizarditewater absorption spectrum atmeasured 3 µm, the highat ambient temperature conditions. could have Along signific withantly dehydration dehydroxylated caused bythe thesample, reduction thus alter- in the water ing its mineralogyabsorption and at3 structure.µm, the highThe extent temperature of this couldalteration have deserves significantly further dehydroxylated investiga- the tion. Heresample, we refer thus to alteringthis sample its mineralogy as “heated lizardite”. and structure. The extent of this alteration deserves further investigation. Here we refer to this sample as “heated lizardite”. Table 1. Minerals considered for spectral modeling. Table 1. Minerals considered for spectral modeling. Mineral Type RELAB Sample ID/Reference Antigorite Mg-phyllosilicate AT-TXH-007RELAB Sample Mineral Type Heated Lizardite Mg-phyllosilicate [35] ID/Reference NH4-montmorilloniteAntigorite NH4-phyllosilicate Mg-phyllosilicate JB-JLB-189 AT-TXH-007 DolomiteHeated LizarditeMg-Ca-Carbonate Mg-phyllosilicate CB-EAC-003 [35] NatriteNH4 -montmorilloniteNa-carbonate NH4-phyllosilicate CB-EAC-034-C JB-JLB-189 Dolomite Mg-Ca-Carbonate CB-EAC-003 Magnetite NatriteIron oxide Na-carbonate [36] CB-EAC-034-C Magnetite Iron oxide [36] Kerite Organic material MA-ATB-043 Amorphous Carbon Organic material [37]

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Life 2021 11 , , 9 Kerite Organic material MA-ATB-043 5 of 14 Amorphous Carbon Organic material [37]

Marchi etet al.al. [ 14[14]] and and Kurokawa Kurokawa et et al. al. [15 [15]] found found that that a large a large fraction fraction of the of composition the compo- ofsition Ceres of shouldCeres should be composed be composed of material of materi similaral similar to carbonaceous to carbonaceous chondrites chondrites (CC), when(CC), takingwhen taking into account into account both the both IR the global IR global average average spectrum spectrum and the and elemental the elemental composition compo- fromsition thefrom GRaND the GRaND measurement. measurement. In particular, In particular, [14] obtained[14] obtained best ﬁtsbest by fits adding by adding the CCthe IvunaCC Ivuna (CI type)(CI type) or LAP or LAP (CM (CM type) type) to the to modeled the modeled mixtures; mixtures; [15] obtained [15] obtained best ﬁts best using fits Ivunausing Ivuna (CI type), (CI type), Tagish Tagish Lake (CMLake type),(CM type), or Cold or Cold Bokkeveld Bokkeveld (CM (CM type). type). However, However, our analysisour analysis revealed revealed that thethat above-mentioned the above-mentioned CC cannot CC cannot match match both the both VIS the and VIS IR regionsand IR ofregions the average of the average spectrum spectrum of Ceres. of Instead, Ceres. Instead, we considered we considered MAC 87300 MAC (CM 87300 type), (CM which type), iswhich already is already mentioned mentioned in [2], in whose [2], whose spectral spectral shape shape is most is most similar similar to the to Ceres’ the Ceres’ spectrum spec- acrosstrum across the two the spectral two spectral regions regions (Figure (Figure2). 2).

Figure 2. AverageAverage Ceres’ Ceres’ reflectance reflectance spectrum spectrum (black (black)) and and reflectance reflectance spectra spectra of carbonaceous of carbonaceous chondrites (in standard viewingviewing geometry)geometry) usedused toto produceproduce thethe bestbest fitfit withwith Ceres’Ceres’ mineralogymineralogyand elementaland elemental composition. composition.

The retrieved mineral abundances are listedlisted inin TableTable2 .2. BestBest ﬁtfit and and endmembers endmembers scaled for their abundances are shown inin FigureFigure3 3.. ElementalElemental constraintsconstraints from from the the GRaND GRaND measurements [[12,13]12,13] (7(7 wt.%wt.% << CC << 2020 wt.%,wt.%, HH == 1.91.9± ± 0.20.2 wt.%,wt.%, FeFe == 1616 ±± 1 wt.%) areare matched by the modelmodel (C(C == 16.5 wt.%,wt.%, HH == 1.81.8 wt.%,wt.%, FeFe == 16.5 wt.%),wt.%), accordingaccording toto thethe densities and elemental compositions ofof the endmembers reportedreported inin [[14]14] usedused toto convertconvert the abundance (vol.%)(vol.%) ofof mineralsminerals retrievedretrieved intointo thethe elementalelemental compositioncomposition (wt.%).(wt.%).

Table 2. Retrieved abundance and grain size of endmembers. In brackets the values retrieved for Area B have been reported when different from those of Area A.

Mineral Area A (Area B) Global Average MAC 87300 56% 60% Heated Lizardite 12% 10% Antigorite 1% 1% NH4-montmorillonite 5% 6% Dolomite 2% 2% Natrite 1% 0% Magnetite 1% 1% Kerite 2% 0% Amorphous Carbon 20% 20% (a) (b) Grain size 270 µm 210 µm Figure 3. Panel (a): measured (black)Multiplicative and modeled factor (red) average spectrum 0.72 (0.74)of Ceres in terms of I/F (r × π 0.63). Error bars − − − − indicate uncertainties on absolute calibration.Additional The Slope best fit including7.8 all (7.6)endmembers 10 3 µm is1 discussed in De7.8 Sanctis 10 3 etµm al. 1[11] with the addition of heated-lizardite (see Table 1), and MAC 87,300 (see Figure 2). In panel (b), the single scattering albedo

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Kerite Organic material MA-ATB-043 Amorphous Carbon Organic material [37]

Marchi et al. [14] and Kurokawa et al. [15] found that a large fraction of the composition of Ceres should be composed of material similar to carbonaceous chondrites (CC), when taking into account both the IR global average spectrum and the elemental composition from the GRaND measurement. In particular, [14] obtained best fits by adding the CC Ivuna (CI type) or LAP (CM type) to the modeled mixtures; [15] obtained best fits using Ivuna (CI type), Tagish Lake (CM type), or Cold Bokkeveld (CM type). However, our analysis revealed that the above-mentioned CC cannot match both the VIS and IR regions of the average spectrum of Ceres. Instead, we considered MAC 87300 (CM type), which is already mentioned in [2], whose spectral shape is most similar to the Ceres’ spectrum across the two spectral regions (Figure 2).

Figure 2. Average Ceres’ reflectance spectrum (black) and reflectance spectra of carbonaceous chondrites (in standard viewing geometry) used to produce the best fit with Ceres’ mineralogy and elemental composition.

The retrieved mineral abundances are listed in Table 2. Best fit and endmembers scaled for their abundances are shown in Figure 3. Elemental constraints from the GRaND measurements [12,13] (7 wt.% < C < 20 wt.%, H = 1.9 ± 0.2 wt.%, Fe = 16 ± 1 wt.%) are matched by the model (C = 16.5 wt.%, H = 1.8 wt.%, Fe = 16.5 wt.%), according to the Life 2021, 11, 9 densities and elemental compositions of the endmembers reported in [14] used to convert6 of 14 the abundance (vol.%) of minerals retrieved into the elemental composition (wt.%).

(a) (b)

Figure 3. Panel ( a): measured (black) and modeled (red) averageaverage spectrumspectrum ofof CeresCeres inin termsterms ofof I/FI/F (r(r ×× π). Error Error bars indicate uncertainties uncertainties on on absolute absolute calibration. calibration. The The best best fit ﬁt in includingcluding all all endmembers endmembers is is discussed discussed in in De De Sanctis Sanctis et et al. [11] [11] with the addition of heated-lizardite (see Tabl Tablee1 1),), andand MACMAC 87,30087,300 (see(see FigureFigure2 ).2). In In panel panel ( (b), the the single single scattering scattering albedo albedo

(SSA) of each endmember scaled for their respective relative abundance: MAC 87300 (orange), NH4-montmorillonite (dark green), heated lizardite (red), amorphous carbon (blue), dolomite (cyan), antigorite (purple), magnetite (pickle green).

The need for a multiplicative factor and an additional slope (see Table2) would suggest the presence of additional and/or different endmembers with respect to those assumed: the observed spectrum is darker and redder than the model derived from the endmembers listed in Table1. However, unknown endmembers should be featureless in the spectral range considered. On the other hand, this discrepancy could also be explained by the effect of space weathering [38].

3. Organic-Rich Region 3.1. Data Reduction Close to the Ernutet crater (53.4 km diameter, at latitude ~53◦ N, longitude 45.5◦ E), a region of ~1000 km2 exhibits a prominent and complex 3.4-µm band, composed of several bands: near 3.38–3.39 µm, 3.40–3.42 µm, and 3.49–3.50 µm. These spectral features are characteristic of the symmetric and antisymmetric stretching frequencies of methyl (CH3) and methylene (CH2). The main candidates for these bands are organic materials containing aliphatic hydrocarbons [18,39], probably with some aromatic hydrocarbons and heteroatoms [34]. Mapping the band depth at 3.4 µm [18,34], the organic material is found mainly in two broad areas: one close to the northwestern portion of the crater, and one located on the southwestern part. Here, we produced two average spectra considering rectangular portions of these two regions (A and B, respectively), delimited by the coordinates shown in Table3 (Figure4). Moreover, we accounted for a test area outside the organic-rich regions to rule out any possible instrumental effects that may affect the speciﬁc data we collected for the analysis of the organic-rich areas (Figure4). We accounted for acquisitions in the VIS and IR channel performed simultaneously, which covered the whole area of interest (spacecraft event time: 487085891). For each area, the average spectra obtained from data from the VIS and IR channels were merged to obtain a single spectrum from 0.25 to 5.0 µm.

Table 3. Range of coordinates of two rectangular areas inside the organic-rich regions, and a test area. The number of spatial pixels inside the areas used to perform the average spectra is provided.

Area Lon (◦) Lat (◦) N. Pixel VIS N. Pixel IR A 39.0–41.5 54.5–56. 22 22 B 40.0–44.0 48.5–52.5 184 180 Test 42.5–44.0 60.5–62.0 127 145 Life 2021, 11, x FOR PEER REVIEW 7 of 14

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Figure 4. Map of organics’ abundance reported from Raponi et al. [31], projected over a Framing Camera mosaic. The organic-rich areas A and B, and the test area have been marked. Coordinates ofFigureFigure the areas 4. 4. MapMap are of shown of organics’ organics’ in Table abundance abundance 3. reported reported from from Ra Raponiponi et etal. al. [31], [31 projected], projected over over a Framing a Framing CameraCamera mosaic. mosaic. The The organic-rich organic-rich areas A and B, andand the testtest areaarea havehave beenbeen marked.marked. CoordinatesCoordinates of ofthe the areasDifferences areas are are shown shown between in in Table Table 3the. 3. spectra of the organic-rich areas (A and B) and the test area with respect to the average Ceres spectrum, were highlighted by their normalization (Fig- ure 5).DifferencesDifferences A distinct between betweensignature the the of spectra spectraaliphatic of of theorgani the organic-rich organic-richcs is visible areas areas at 3.4(A (A andmicrons. and B) B) and and Moreover, the the test test area areathe absorptionwithwith respect respect at to to4 theµm the average shows average aCeres larger Ceres spectrum, abundance spectrum, were wereof highlighted carbonates highlighted byin thetheir by organic-rich theirnormalization normalization regions. (Fig- Suchure(Figure 5). results A5). distinct A have distinct signaturealready signature been of aliphatic ofdiscussed aliphatic organi in organics [3cs1]. isThe visible is spectrum visible at 3.4 at of 3.4microns. the microns. test Moreover, area Moreover, does notthe presentabsorptionthe absorption any at relevant 4 atµm 4 µ showsm differences shows a larger a larger with abundance abundancerespect toof ofthecarbonates carbonates average in global in the the organic-rich organic-richspectrum, exceptregions. regions. a slopeSuchSuch resultsthat results is slightlyhave have already already bluer inbeen been the discussedvisible discussed spectr in in [3al [311]. range.]. The The Thisspectrum spectrum average of of thetest the test testarea area areaalso doessamples not thepresentpresent typical any any background relevant relevant differences differences over which with with organic-ri respec respectt chto regionsthe average stand. global As a spectrum, result, an exceptunusual a positiveslopeslope that that slope is is slightly slightly (redder) bluer bluer with in in therespect the visible visible to thespectr spectral globalal range. range. spectrum This This average averageappears test testin thearea area visible also also samples samples part of thethe typicalorganic-rich typical background background spectra, over overeven which which more organic-ri organic-richso if we chconsider regions the stand. test Asarea As a a spectrumresult, result, an an asunusual background.positivepositive Thisslope slope confirms (redder) (redder) the with with previous respect respect analysis to to the the global performed spectrum spectrum with appears appearsframing in incamera the visible data [40]. part To of interpretthethe organic-rich this spectral spectra,spectra, reddening, even even more more we so so if perf weif we considerormed consider spectral the testthe areamodelingtest spectrumarea spectrum(Section as background. 3.2),as back- and checkedground.This conﬁrms Thisfor spatial confirms the previous correlations the previous analysis of the analysis performed organics performed withwith framingother with endmembers cameraframing data camera (Section [40]. data To 3.3). interpret [40]. To this spectral reddening, we performed spectral modeling (Section 3.2), and checked for interpret this spectral reddening, we performed spectral modeling (Section 3.2), and spatial correlations of the organics with other endmembers (Section 3.3). checked for spatial correlations of the organics with other endmembers (Section 3.3).

(a) (b)

FigureFigure 5. 5. PanelPanel ( (aa):): Average Average spectra spectra of of area area A A (blue), (blue), B B (red) (red) and and the the test test area area (green). (green). For For comparison, comparison, shown shown is is the the average average (a) (b) CeresCeres spectrum (black). Panel Panel ( b):): same same spectra spectra of of panel panel ( (aa)) in in the the VIS VIS range, range, normalized normalized at at 0.8 0.8 micron. Figure 5. Panel (a): Average spectra of area A (blue), B (red) and the test area (green). For comparison, shown is the average Ceres spectrum (black). Panel ( b): same spectra of panel (a) in the VIS range, normalized at 0.8 micron. 3.2. Spectral Modeling We performed spectral modeling following the approach in Section 2.2, using an intimate mixture of the endmembers listed in Table1.

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3.2. Spectral Modeling Life 2021, 11, 9 We performed spectral modeling following the approach in Section 2.2, using an8 of in- 14 timate mixture of the endmembers listed in Table 1. Spectral fits of the two organic-rich areas are shown in Figure 6. Retrieved abundances and grain size are listed in Table 2. The results are very similar to those discussed Spectral ﬁts of the two organic-rich areas are shown in Figure6. Retrieved abundances in [31], except for the larger content of Mg-phyllosilicates due to the novel introduction of and grain size are listed in Table2. The results are very similar to those discussed in [ 31], the heated lizardite to match the visible spectral range: the lower depth of the absorption except for the larger content of Mg-phyllosilicates due to the novel introduction of the band at 2.7 µm in the spectrum of lizardite with respect to antigorite is compensated by heated lizardite to match the visible spectral range: the lower depth of the absorption band its larger abundance. at 2.7 µm in the spectrum of lizardite with respect to antigorite is compensated by its larger abundance.

(a) (b)

Figure 6. Panels (aa):): measuredmeasured (black)(black) and and modeled modeled (red) (red) average averag spectrume spectrum of of Ceres Ceres in termsin terms of I/Fof I/F (r x(rπ x) ofπ) theof the organic-rich organic- richspectra spectra from from area Aarea and A areaand B,area respectively. B, respectively. Error barsError indicate bars indicate calibration calibration uncertainties. uncertainties. In panel In (panelb), the (b relative), the relative SSA of SSAeach of endmember each endmember scaled for scaled their for respective their respective abundance abunda (relativence cross(relative section): cross MACsection): 87300 MAC (orange), 87300 NH4-montmorillonite(orange), NH4-montmorillonite (dark green), heated lizardite (red), amorphous carbon (blue), dolomite (cyan), antigorite (purple), natrite (dark green), heated lizardite (red), amorphous carbon (blue), dolomite (cyan), antigorite (purple), natrite (light green), (light green), magnetite (pickle green), kerite (black). magnetite (pickle green), kerite (black).

AsAs in in the the case of the global spectrum, a further multiplicative constant constant and and an an addi- addi- tionaltional slope slope are are needed needed to correctly fit ﬁt the spectra (Table2 2).).

3.3.3.3. Spatial Spatial Correlations Correlations ToTo attribute the the changing changing slope slope (Figure (Figure 5b)5b) to to mineral mineral or or organic organic material material requires requires a comparisona comparison of the of the spatial spatial distribution distribution of the of slope the slope in the in VIS the range VIS rangewith the with spatial the spatialdistri- butiondistribution of compounds. of compounds. The slope The slopeis calc isulated calculated following following Rousseau Rousseau et al. [17]: et al. [17]:

S0.48–0.80µm = [r (0.80 µm) − r (0.48 µm)]/[r (0.48 µm) (λ0.80µm − λ0.48µm)] S0.48-0.80µm = [r (0.80 µm) − r (0.48 µm)]/[r (0.48 µm) (λ0.80µm − λ0.48µm)] Being: r (0.80 µm) the median of the reflectance over the range 0.75–0.85 µm (~50 bandBeing: of the rVIS (0.80 channel),µm) the and median r (0.48 of µm) the reﬂectancethe median overof the the reflectance range 0.75–0.85 over theµm range (~50 0.43–0.53band of theµm VIS (~50 channel), band of the and VIS r (0.48 channel).µm) the median of the reﬂectance over the range 0.43–0.53The mappedµm (~50 abundances band of the VISof minerals channel). have been reported by Raponi et al. [31], who used Thethe mappedsame spectral abundances modeling of minerals discussed have in been Section reported 2.2, bybut Raponi considering et al. [31 only], who the used IR channel.the same Thus, spectral the modelingS0.48–0.80µm and discussed the mineral in Section abundances 2.2, but came considering from two only independent the IR channel. datasets.Thus, the S0.48–0.80µm and the mineral abundances came from two independent datasets. ComparisonComparison of of panel panel (a) (a) and and (b) (b) of ofFigure Figure 7 7reveals reveals a spatial a spatial correlation correlation between between the Sthe0.48–0.80µm S0.48–0.80 andµ mtheand abundance the abundance of organic of organic matter. matter. We also We note also that note other that minerals other minerals do not presentdo not presentthe same the degree same degreeof spatial of spatialcorrelation. correlation. In particular, In particular, carbonates carbonates are only are more only abundantmore abundant in the insouthern the southern area (B), area and (B), Mg-phyllosilicates and Mg-phyllosilicates are mostly are mostly anticorrelated. anticorrelated. No spatialNo spatial correlation correlation can canbe noted be noted for the for other the other minerals minerals (see (see[31]). [31 ]).

Life 2021, 11, x FOR PEER REVIEW 9 of 14 Life 2021, 11, 9 9 of 14

(a) (b)

FigureFigure 7.7. (Panel(Panel aa):): projected slopeslope inin thethe rangerange 0.48–0.800.48–0.80 µµm.m. Stripes visiblevisible onon thethe mapsmaps areare artifactsartifacts duedue toto thethe lowlow spatialspatial resolutionresolution ofof the the data data considered. considered. For For comparison, comparison, the the retrieved retrieved abundance abundance of organicsof organics (panel (panelb), carbonatesb), carbonates (panel (panelc), and c), and Mg-phyllosilicates (panel d) reported from [31]. Mg-phyllosilicates (panel d) reported from [31]. 3.4. Cerealia Facula 3.4. CerealiaThe most Facula outstanding geological features on Ceres’ surface are the bright areas knownThe as most the Ceralia outstanding and Vinalia geological Faculae features in the on Ceres’geologically surface young are the Occator bright areascrater known (15.8– as24.9° the N, Ceralia 234.3–244.7° and Vinalia E, diameter Faculae ~90 in the km). geologically The faculae young have Occatoran albedo crater 5–10 (15.8–24.9 times higher◦ N, 234.3–244.7than the average◦ E, diameter surface ~90[23], km). with The Cerealia faculae Facula have being an albedo the brightest 5–10 times at the higher center than of thethe averagecrater. These surface faculae [23], show with Cerealiadistinct spectral Facula being differences the brightest from the at typical the center crater of thefloor. crater. Ab- Thesesorption faculae at ~3.5 show µm distinctand ~4 µm spectral increase differences toward fromthe brightest the typical areas crater and floor. are consistent Absorption with at ~3.5enrichmentµm and in ~4 carbonates.µm increase A toward shift of the the brightest carbonate areas band and center are consistent from 3.95 withto 4.01 enrichment µm indi- incates carbonates. a change A in shift the ofmineralogy the carbonate from band Mg-carbonate center from to 3.95 Na-Carbonate to 4.01 µm indicates [41]. A acomplex change inabsorption the mineralogy at 2.20–2.22 from µm Mg-carbonate is observed toonly Na-Carbonate in the brightest [41 areas.]. A complex Raponi et absorption al. [42] have at 2.20–2.22shown thatµm ammonium is observed salts, only and in more the brightest specifically areas. ammonium Raponi et chloride al. [42] have(NH4Cl), shown provide that ammoniumthe best match salts, with and this more spectral specifically feature. ammonium The overall chloride band area (NH between4Cl), provide 2.6 µm the and best 3.7 matchµm increases with this from spectral the crater feature. floor The to overall the brightest band area pixels, between and 2.6severalµm and minima 3.7 µm at increases 2.88, 3.2, from3.28, 3.38, the crater and 3.49 floor µm to theemerge brightest and become pixels, and deeper, several revealing minima the at presence 2.88, 3.2, 3.28,of hydrohalite 3.38, and 3.49[19].µ Moreover,m emerge according and become to deeper,spectral revealingmodeling, the aliphatic presence carbons, of hydrohalite and aromatic [19]. Moreover,hydrocar- accordingbons may be to present spectral in modeling, a small percentage aliphatic carbons,[19]. and aromatic hydrocarbons may be presentThe in novel a small possibility percentage of extending [19]. the spectral range to the VIS channel can yield new constraintsThe novel on the possibility mineralogy of extending of the faculae. the spectralHere, we range took tointo the account VIS channel an average can yieldspec- newtrum constraints of Cerealia on Facula, the mineralogy calculated of from the faculae. VIS and Here, IR high we took spatial into resolution account an data average (VIS spectrumSpacecraft of Event Cerealia Time: Facula, 524703945, calculated IR Spacecraft from VIS Event and IR Time: high 521436704) spatial resolution that covered data (VIS the Spacecraft Event Time: 524703945, IR Spacecraft Event Time: 521436704) that covered the whole region of Cerealia Facula (lat: 19.25°◦ N–20.0°◦ N, lon: 239.25°◦ E–240°◦ E, Figure 8). wholeThe spectra region in of the Cerealia VIS channel Facula presents (lat: 19.25 a posiN–20.0tive slopeN, lon:shortward 239.25 ofE–240 1.0 µm,E, confirming Figure8). µ Thethe previous spectra in analysis the VIS channelperformed presents with FC a positive data by slope Nathues shortward et al. [43]. of 1.0 Them, red confirming spectrum the previous analysis performed with FC data by Nathues et al. [43]. The red spectrum

Life 2021, 11, x FOR PEER REVIEW 10 of 14 Life 2021, 11, 9 10 of 14 Life 2021, 11, x FOR PEER REVIEW 10 of 14

detected in the VIS range is similar to those observed in the organic-rich spectra coming detected in the VIS range is similar to those observed in the organic-rich spectra coming fromdetected the Ernutet in the VISregions range discussed is similar above to those (Figure observed 9). in the organic-rich spectra coming fromfrom thethe ErnutetErnutet regionsregions discusseddiscussed aboveabove (Figure(Figure9 ).9).

Figure 8. Image of Cerealia Facula obtained by combining framing camera images acquired during LAMOFigureFigure phase 8.8. ImageImage with ofof three CerealiaCerealia images FaculaFacula using obtainedobtained spectral byby filters combiningcombining centered framingframing at 438, cameracamera 550, and imagesimages 965 nm. acquiredacquired VIR pixel duringduring LAMO phase with three images using spectral filters centered at 438, 550, and 965 nm. VIR pixel footprintsLAMO phase inside with the threered rectangle images using(lat: 19.25° spectral N–20. ﬁlters0° N, centered lon: 239.25° at 438, E–240° 550, E) and were 965 averaged nm. VIR to pixel obtainfootprints the reflectance inside the spectrumred rectangle [39]. (lat: 19.25° N–20.0° N, lon: 239.25° E–240° E) were averaged to footprints inside the red rectangle (lat: 19.25◦ N–20.0◦ N, lon: 239.25◦ E–240◦ E) were averaged to obtain the reflectance spectrum [39]. obtain the reﬂectance spectrum [39].

(a) (b) (a) (b) FigureFigure 9. 9. PanelPanel (a (a):): Cerealia Cerealia Facula Facula average average I/F I/F spec spectrum’strum’s VIS VIS and and IR IR channels channels (gray) (gray) were were merged merged to to obtain obtain a a single single spectrum.spectrum.Figure 9. Spectral SpectralPanel (a range): range Cerealia longward longward Facula of of average4.1 4.1 µmµm (not (notI/F shownspec showntrum’s here) here) VIS is is affected affectedand IR bychannels by a a high high noise(gray) noise level levelwere after aftermerged the the subtraction subtractionto obtain a of ofsingle the the spectrum. Spectral range longward of 4.1 µm (not shown here) is affected by a high noise level after the subtraction of the thermalthermal emission. emission. Panel Panel (b (b):): focus focus on on the the visible visible spectral spectral range; range; the the spectrum spectrum of of Cerealia Cerealia Facula, Facula, the the global global Ceres Ceres spectrum, spectrum, andthermal the organic emission. rich Panel regions (b): (Area focus A on and the B) visible were spectral normalized range; at the0.8 spectrumµm. of Cerealia Facula, the global Ceres spectrum, and the organic rich regions (Area A and B) were normalized at 0.8 µm. and the organic rich regions (Area A and B) were normalized at 0.8 µm. 4. Discussion 4. Discussion 4.1.4. DiscussionCeres Average Spectrum 4.1.4.1. CeresCeres AverageAverage SpectrumSpectrum The analysis of the average global spectrum of Ceres performed in this study confirms ThetheThe previous analysisanalysis ofresults of the the average ofaverage Marchi global global et al. spectrum [14],spectrum who of combined Ceresof Ceres performed performedmineralogy in this in (from studythis VIRstudy conﬁrms data) con- andthefirms elemental previous the previous resultscomposition results of Marchi of (from Marchi et al.GRaND [et14 al.], who [14],data) combinedwho to derive combined mineralogy Ceres’ mineralogy average (from surface (from VIR data) VIRcompo- data) and sition:elementaland elemental the large composition compositionabundance (from of(from GRaND carbon, GRaND data)which data) to exceeds derive to derive Ceres’ that Ceres’ of average carbonaceous average surface surface composition:chondrites, compo- foundthesition: large on the a abundance volatile-richlarge abundance of body carbon, ofimplies carbon, which that exceedswhich widespread exceeds that of organic carbonaceousthat of chemistry carbonaceous chondrites, could chondrites, have found oc- curredonfound a volatile-rich on on Ceres. a volatile-rich body implies body implies that widespread that widespread organic organic chemistry chemistry could havecould occurred have oc- oncurred Ceres.The on analysis Ceres. of the average spectrum of Ceres was extended here to the visible part of the TheThespectrum, analysisanalysis taking ofof thethe into averageaverage account spectrumspectrum the VIS ofofchannel CeresCeres wasofwas the extendedextended VIR instrument, herehere toto the thewhich visiblevisible is now partpart ofof the spectrum, taking taking into into account account the the VIS VIS channel channel of the of the VIR VIR instrument, instrument, which which is now is

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now available after the setup of the calibration for that channel [16]. The overall shape of Ceres’ spectrum implies that an additional mineral is needed on top of those suggested in De Sanctis et al.[11] and Marchi et al. [14] in order to match the spectral shape in the range 0.3–1.0 µm. Here, we suggested the heated lizardite of Hiroi and Zolensky [35] as a possible analog. Lizardite, like antigorite, is a polymorph of serpentine; however, heating at 600 ◦C can lead to partial dihydroxylation and structural change, and thus to one or more different silicates. The fit with those thermally processed silicates does not necessarily imply that Ceres’ surface experienced similar temperatures. Instead, it is possible that these silicates are of primary origin, i.e., they were accreted from planetesimals that were thermally processed. Further laboratory measurements are needed to investigate the properties of this silicate, whose applicability to Ceres, if confirmed, would bring new constraints on Ceres’ origin. Moreover, the need for a different carbonaceous chondrite (MAC 87300) than those considered in previous works [14,15], requires further analysis to explain which specific mineral phases determine its spectral shape and match with Ceres’ spectra. The spectral modeling presented in this study requires an additional slope and a multiplicative constant, which reddens and darkens the spectrum, respectively, in order to match the observed average spectrum of Ceres. This would compensate for the possible role of space weathering [39].

4.2. Ernutet Organic-Rich Region The introduction of the VIS channel data provided new possibilities for refining the modeled composition of the organic rich regions in the Ernutet crater area. Our estimates of organic abundance do not differ significantly from the analysis of De Sanctis et al. [18,34] and Raponi et al. [31]. We point out that the amount of organic material retrieved is strongly dependent on the type of organic that is used for spectral modeling. The limit is repre- sented by the absorption depth of the laboratory spectrum, which cannot be less than the absorption depth measured on Ceres’ surface. Hence, considering different endmembers to match the organic signatures could lead to much larger abundance estimates, e.g., areas matched with 6% of kerite (Figure7b) would instead require 25% of shocked asphaltite (AS- LXM-004, RELAB) [34]. The retrieved abundance can be even larger (45%–65%) if samples of insoluble organic matter extracted from carbonaceous chondrites are assumed [44]: the less pronounced band at 3.4 µm of the laboratory samples can be compensated by a larger abundance retrieved from the spectral fit. However, we found that kerite provides the best fit with data, both for the shape of the absorption at 3.4 µm and the degree of redness [39]. A significant finding obtained from the VIS channel analysis is the detection of an unusual red slope in the VIS range, which has a strong spatial correlation with aliphatic organic abundance. This correlation suggests that this slope is produced by the same organic compound that produces the aliphatic stretching at 3.4 µm, or by a different compound that is closely linked to organics in their formation, conservation, and/or transportation on the surface.

4.3. Cerealia Facula

Sodium salts (NaCl, NaHCO3, Na2CO3, NaCl·H2O) identiﬁed on Cerealia Facula have also been detected in Enceladus’ plume, suggesting they are sourced from a similar alkaline environment. Furthermore, the presence of hydrated/hydroxylated salts indicates recent exposure of brines, as these compounds are not stable on Ceres’ surface [19]. The current occurrence of brines in Ceres attests to the presence of liquid throughout Ceres’ history. As with Enceladus, one expects that organic chemistry could have been ongoing in Ceres [45,46]. The addition of the visible spectral range to the average spectrum of Cerealia Facula adds new constraints on its mineralogy. Here, we highlighted a red slope shortward of 1.0 µm that appears very similar to the absorption over the same wavelength range detected in the Ernutet organic-rich regions. This similarity strongly suggests the presence of organic material in the faculae material, as already suggested by De Sanctis et al. [19]. However, Life 2021, 11, 9 12 of 14

the red slope of the facula with respect the average Ceres terrain could also be attributed to other physical properties of the material, such as different particle size, and/or space weather acting differently with respect to the rest of the surface because of its recent age and unique mineralogy [38].

5. Conclusions Based on the amount of data returned by the NASA/Dawn spacecraft and its scientific payload, Ceres is now recognized as a target of astrobiological significance. In particular, Ceres is one of the few bodies where abundant water and organic matter have been found. Here, we reported further indications of an environment in Ceres that is favorable to prebiotic chemistry during the course of its history, and potentially, at present. The analysis presented here may open new avenues of investigation on Ceres’ mineralogy, in particular, the attribution of the red slope in the VIS range to organic matter and the nature of the silicates that make up most of Ceres’ regolith (together with the carbon component). Indeed, phyllosilicates can protect organics from space weathering effects because they efficiently absorb organic molecules [47,48]. Moreover, detailed spectral modeling (including the visible range) of the bright material making up the faculae would be desirable in order to improve our knowledge of this hydrated salt-rich mixture and for the information it provides on Ceres’ brine environment. The prospect that organic matter may be present in brines for extended periods of time further supports the value of Ceres as a target of astrobiological significance and calls for follow-up exploration.

Author Contributions: Conceptualization, A.R. and M.C.D.S.; methodology, A.R.; software, A.R.; resources, C.T.R., C.A.R. and M.C.D.S.; data curation, A.R., B.R., M.C., F.T., F.G.C. and F.Z.; writing— original draft preparation, A.R.; writing—review and editing, A.R., M.C.D.S., F.G.C., M.C., B.R., M.F. (Marco Ferrari), E.A., S.D.A., V.V., J.C.C.-R., F.T., A.F., M.F. (Michelangelo Formisano), F.Z., C.A.R. and C.T.R.; visualization, A.R. and F.G.C.; supervision, M.C.D.S.; project administration, C.T.R., C.A.R. and M.C.D.S.; funding acquisition, C.T.R., C.A.R. and M.C.D.S. All authors have read and agreed to the published version of the manuscript. Funding: The VIR is funded by the Italian Space Agency-ASI and was developed under the leadership of INAF-Istituto di Astrofisica e Planetologia Spaziali, Rome-Italy. The instrument was built by Selex- Galileo, Florence-Italy. The authors acknowledge the support of the Dawn Science, Instrument, and Operations Teams. This work was supported by ASI-INAF n. I/004/12/0 and NASA. Conflicts of Interest: The authors declare no conflict of interest.

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