The composition and structure of Ceres' interior

Icarus 335 (2020) 113404

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The composition and structure of Ceres' interior Mikhail Yu. Zolotov

T

School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA

ARTICLE INFO

ABSTRACT

Keywords: Asteroid Ceres Composition Geophysics Interiors Organic chemistry

Results of Ceres' exploration with the Dawn spacecraft are modeled and discussed in terms of rock/organic/ elemental composition, density and porosity in the interior, and formation, migration and geological evolution of the body. Carbon-rich surface composition is used to assess phase and elemental composition of the interior. The consistent bulk density and surface composition suggest an abundant organic matter within the body. Ceres is modeled as a chemically uniform mixture of CI-type carbonaceous chondritic rocks and 12–29 vol% of macromolecular organic matter. Water ice, gas hydrates or high porosity (>10%) are not required to explain bulk density. Ceres may not have a partially differentiated interior structure because gravity and shape could be explained by compaction of chemically uniform materials. Gravity data suggest a two-layer structure with an abrupt density change. Gravity may not reflect the current global density distribution in the interior because the implied bulk porosity >9% and grain density > 2380 kg m−3 disagree with organic-rich compositions. In contrast, Ceres' polar flattening indicates mild density gradients that could be explained by two-layer and gradual compaction models. The flattening implies grain density of 2200–2350 kg m−3 that is consistent with the organic-rich interior. Viscosity of warmed rock-organic mixtures at depth could account for the observed relaxation of long wavelength topography. The organic-rich composition together with abundant surface carbonates and NH4-bearing phases suggests Ceres' formation at larger heliocentric distances and later than CI chondrites. Ceres-forming materials could have been more water-rich than parent bodies of CI chondrites and excessive water could have been lost from the body. A majority of Ceres' surface compounds could have formed through water-rock-organic reactions in a middle interior followed by collisional stripping of an upper interior.

1. Introduction Ceres is the largest body in the main asteroid belt with an ellipsoidal shape and a cratered rocky surface. Ceres' density (Table 1) implies a volatile-rich interior with abundant hydrated minerals, ices and/or organic matter. Before 2015, Ceres' mass, dimensions and spectral properties have been investigated with Earth's ground and orbital telescopes with limited spatial and spectral resolutions (e.g. Thomas et al., 2005; Carry et al., 2008; Rivkin et al., 2011; Li et al., 2016). Although the low bulk density and a dominance of phyllosilicate-rich surface materials became evident from telescopic data, both surface composition (Rivkin et al., 2011) and interior structure (McCord et al., 2011) remained uncertain until after arrival of the NASA Dawn spacecraft in 2015. Pre-Dawn data on Ceres' dimensions and mass provided inconsistent constraints on the interior structure (Thomas et al., 2005; Carry et al., 2008; McCord et al., 2011; Drummond et al., 2014) (Fig. 1). Thermal evolution models suggested a differentiated interior structure with water mantle (McCord and Sotin, 2005; CastilloRogez and McCord, 2010; Castillo-Rogez, 2011). Zolotov (2009) argued for an undifferentiated compacted interior in which the low density is

due to high porosity (≥10%) of hydrated CI-type chondritic rocks and/ or due to an elevated organic content. Exploration of Ceres with Dawn provided data on morphology, topography and composition of the surface together with the body's gravity and shape. Near infrared spectra of surface materials reveal the presence of Mg-rich and ammoniated phyllosilicates together with Mg/ Ca carbonates (De Sanctis et al., 2015, 2018). Sodium carbonates are detected in some geologically young craters and domes (De Sanctis et al., 2016; Carrozzo et al., 2018; Zambon et al., 2017). An abundant opaque component could be rich in carbonaceous matter altered by space weathering (Hendrix et al., 2016; Marchi et al., 2019). Data from the Dawn GRaND instrument for surface materials suggest higher C and H contents, and lower Fe and Ni concentrations than in CI carbonaceous chondrites (Prettyman et al., 2017, 2019). It is unclear if surface C-rich and ammoniated compounds reflect alteration of organic matter in chondritic materials or accretion of compounds from the outer solar system (De Sanctis et al., 2015, 2016; McSween et al., 2018; CastilloRogez et al., 2018). Dawn gravity data suggest a moderate concentration of mass toward Ceres' center (Park et al., 2016). The data indicate the lack of an upper

E-mail address: [email protected]. https://doi.org/10.1016/j.icarus.2019.113404 Received 17 January 2019; Received in revised form 18 July 2019; Accepted 2 August 2019 Available online 07 August 2019 0019-1035/ © 2019 Elsevier Inc. All rights reserved.

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gradients due to compaction. A two-layer model assumed an abrupt porosity collapse at lithostatic pressure of 20–60 MPa. In another compaction model, porosity decreased gradually as a function of pressure. The models with compacted CI chondrites did not require abundant low-density materials (ice, hydrated salts, gas hydrates and organic matter) in the lower and upper interior. The assessed interior structures agreed with Ceres' dimensions from Carry et al. (2008) and had the dimensionless moment of inertia factor (MOI = CM/R2) of 0.37–0.39. These high MOI values and two-layer structures without water shell appeared to be consistent with Dawn gravity-based interior models of Park et al. (2016). Zolotov's (2009) models used pre-Dawn data on shape and density and need to be revisited to address the controversial interpretation of Dawn gravity vs. shape data (Mao and McKinnon, 2018; Ermakov et al., 2017). Other ambiguities include relations between surface and interior composition, heterogeneity of the interior composition, density and porosity, and a necessity for the two-layer interior structure. This paper presents Dawn-based inferences and models for chemically uniform, supposedly organic-rich, slightly porous and ice-poor interior structures consistent with either gravity or shape data. Surface composition is used to assess the phase and elemental composition of the interior. We show that Ceres' shape is consistent with an organicrich interior and suggests mild density gradients in frameworks of twolayer and/or graduate compaction models. The envisioned interior composition and structure are discussed in terms of Ceres' formation in the outer solar system, current and initial water abundance, and rockwater-organic interactions in geological history. The presented views complement discussions of the Dawn team (e.g. Ermakov et al., 2017; Fu et al., 2017; McSween et al., 2018; Castillo-Rogez et al., 2018; McCord and Castillo-Rogez, 2018).

Table 1 Physical characteristics of Ceres. Parameter

Value

Source 20

Mass, kg Volume, km3 Volumetric mean radius, km Bulk density, kg m−3 Polar flattening, km Geometric oblateness Rotation period, hours J2 gravity moment

9.3839 × 10 (434.13 ± 0.5) × 106 469.72 2161.6 ± 2.5 36.15 ± 0.28 0.075 ± 0.001 9.07417 26.5 × 10−3

[2] [2] [2] [2] [1] [1] [3] [1]

Note: [1] Park et al. (2016), [2] Park et al. (2019), [3] Chamberlain et al. (2007). Ceres' flattening stands for the difference between mean equatorial a’ = (a + b)/2 and polar (c) ellipsoidal semi-axes. The error bar for polar flattening reflects the uncertainty of semi-axes of 0.2 km from [1]. Geometric oblateness corresponds to (a’–c)/a’.

40

Polar flattening, km

Hom

38 36

oge

n eo

us

bod

y

Park et al. (2019) Carry et al. (2008)

34 32 30 2000

Thomas et al. (2005)

Flattening consistent with measured J2

2. Composition, grain density and porosity of Ceres

2100

2200

2300

Bulk composition of Ceres is important in modeling of the interior structure because it constrains grain (zero-porosity) density. Without meteorite or returned samples, the composition could be assessed from elemental abundances and infrared spectra of surface materials, measured bulk density and assumed porosity of the body. Porosity could be estimated from compaction of material analogs at relevant pressures. In turn, porosity could be inferred from supposed grain density of Ceres' materials and bulk density.

Ceres' density, kg m-3 Fig. 1. The data on Ceres' shape and bulk density together with shape-density relations for a homogeneous body with the current spin period of 9.074 h. The curve for the homogeneous body corresponds to hydrostatic equilibrium. For Dawn polar flattening (Park et al., 2016), the smaller error bar (0.28 km) is from Table 1 and the larger error bar (0.6 km) is from Mao and McKinnon (2018). The data on polar flattening after Carry et al. (2008) and Park et al. (2016) indicate a mild density stratification and do not exclude homogeneous (undifferentiated) interior structure. Telescopic data of Drummond et al. (2014) that do not exclude homogeneous Ceres are not shown because of large error bars. The triangle symbol corresponds to the calculated polar flattening that is consistent with Ceres' J2, density and spin period from (Park et al., 2016) at hydrostatic equilibrium (this work). The figure is modified after Thomas et al. (2005) and Zolotov (2009).

2.1. Possible bulk composition and low-density compounds Ceres has compositional features of carbonaceous chondrites and outer solar system bodies such as Enceladus, Kuiper belt objects (KBO's) and comets. Near infrared spectra of Ceres' surface (Rivkin et al., 2011; De Sanctis et al., 2015, 2018), the detection of Mg-rich phyllosilicates in them (De Sanctis et al., 2015; Ammannito et al., 2016) and potassium content in surface materials (Prettyman et al., 2017) reveal a similarity with CI/CM carbonaceous chondrites (McSween et al., 2018). As in carbonaceous chondrites (Brearley and Jones, 1998; Zolensky et al., 2018a), Mg-rich phyllosilicates suggest an advanced aqueous alteration of initial anhydrous silicate-metal-oxide compounds (McSween et al., 2018). Ceres' gravity and bulk density indicate a dominance of hydrated rocks with abundant phyllosilicates throughout the interior (Park et al., 2016). At low latitudes, Ceres' surface materials do not contain water ice (Combe et al., 2019; Prettyman et al., 2019) and ice is not required in some interior structure models (Park et al., 2016; Bland et al., 2016; Zolotov, 2009). Ceres' dissimilarity with carbonaceous chondrites reflects abundance, speciation and redox state of C-, N-, S- and Na-bearing compounds. Ceres' surface C content is between that of CI chondrites and comets (Prettyman et al., 2017, 2018, 2019). Pervasive NH4-bearing phyllosilicates (King et al., 1992; De Sanctis et al., 2015; Ammannito et al., 2016; Ferrari et al., 2019) and local NH4Cl (Raponi et al., 2019b)

water layer, consistent with the high topography of impact structures with diameter < 150 km at low and high latitudes (Bland et al., 2016). The initial interpretation of gravity data in the framework of a twolayer interior structure implied the upper layer with thickness of 70–190 km and density of 1680–1950 kg m−3 (Park et al., 2016). The followed analysis of gravity and topography suggested a ~ 40 km thick upper layer with density of 1200–1360 kg m−3 (Ermakov et al., 2017). The presence of ice+porosity (<35 vol%), hydrated salts and/or gas hydrates has been invoked to account for the low density and high viscosity of the layer (Fu et al., 2017; Ermakov et al., 2017). Mao and McKinnon (2018) noted that Ceres' degree-2 zonal gravity J2 moment implies smaller polar flattening than observed. They discussed the inconsistency in terms of geologically recent de-spinning and deep uncompensated density anomalies. In the latter case, Ceres' shape (polar flattening) represents the current global interior structure. In pre-Dawn models of Zolotov (2009), Ceres was considered as a chemically uniform hydrostatically equilibrated body with density 2

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in surface materials are in contrast with mineralogy of carbonaceous chondrites (Brearley and Jones, 1998) and imply more than CI-chondritic abundance of accreted NH3. An elevated fraction of NH4-phyllisilicates in younger crater ejecta (Ammannito et al., 2016) suggests an NH4-rich interior. Although NH4-bearing solids have not been reported in chondrites, NH3 is present in comets (NH3/H2O = 0.3–1.4 × 10−2, Mumma and Charnley, 2011), in the plume of Enceladus (NH3/ H2O = 0.4–1.4 × 10−2, Waite et al., 2017), and at the surfaces of Quaoar (Jewitt and Luu, 2004) and Pluto's moons (Grundy et al., 2016; Cook et al., 2018; Dalle Ore et al., 2018). Ceres' abundant surface carbonates suggest an involvement of CO2/CO ices (De Sanctis et al., 2015, 2016) in larger proportions than in CI/CM/CR carbonaceous chondrites that contain <2–3 vol% carbonates (Alexander et al., 2015). In contrast to chondrites, CO and CO2 contents in cometary gases can reach 20–30% relative to H2O (A'Hearn et al., 2012; Mumma and Charnley, 2011). CO2 and CO ices are present at the surfaces of Triton (Cruikshank et al., 1993), a captured KBO body, and Pluto (Grundy et al., 2016), respectively. In contrast to sulfate-rich CI and CM chondrites (Brearley and Jones, 1998; Brearley, 2006; Zolensky et al., 2018a), the apparent lack or deficiency of sulfates among Ceres' surface salts supports the similarity of Ceres with bodies located beyond the orbit of Jupiter. Chondrites do not contain Na carbonates and Ceres' local occurrences of Na carbonates (De Sanctis et al., 2016; Carrozzo et al., 2018) suggest a similarity with Enceladus' plume Na-CO3/HCO3Cl particles (Postberg et al., 2009) and volatiles with cometary D/H, CH4/H2O and NH3/H2O ratios (Waite et al., 2009, 2017). Ceres' Na‑carbonates may match elevated abundances of Na in the dust of comets 1P/Halley and 67P/Churyumov-Gerasimenko (Jessberger et al., 1988; Bardyn et al., 2017). The apparent Ceres' enrichment in C and N makes its composition closer to that of the solar photosphere. These and possibly other low-mass elements (Na, Mg) in corresponding inorganic and organic nebular compounds could contribute to a low grain density of Ceres' materials. Organic compounds could have a major effect on lowering density. Condensed organic matter often considered as ‘CHON’ compounds is abundant in dust emitted by comets 1P (Kissel and Krueger, 1987; Jessberger et al., 1988; Fomenkova et al., 1994; Hanner and Bradley, 2005) and 67P (Bardyn et al., 2017). Remotely studied dust of comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp) could be rich in a similar material (Lisse et al., 2006, 2007). Anhydrous interplanetary dust particles (IDPs), porous chondritic IDPs and micrometeorites of likely cometary origin contain more organic matter than CI chondrites (Schramm et al., 1989; Thomas et al., 1993; Flynn et al., 2003; Dartois et al., 2013, 2018). Data from the Dawn GRaND instrument reveal the elevated C content in Ceres' equatorial areas (Table 2). It is worth noting that reflectance spectra do not indicate organic matter all over

the surface because the organic matter could be amorphous through space weathering (Rivkin et al., 2011; De Sanctis et al., 2015; Hendrix et al., 2016; Marchi et al., 2019). Irradiation and micrometeorite bombardment cause decomposition of surface organic matter that includes aromatization and release of escaping gases (CH4, etc.). On the surface of Mars, the deficiency of exogenic organic compounds could be due to radiolysis and formation of volatile reaction products (Shkrob et al., 2010; Schuerger et al., 2012; Moores and Schuerger, 2012). By analogy with 4 Vesta (Prettyman et al., 2012), a fraction of surface materials could be exogenic and similar to CI/CM carbonaceous chondrites. Such a material with 2–4 wt% C (Table 2) could dilute inherent C-rich materials exposed at the surface, though unabundant Fe and Ni (Prettyman et al., 2017) do no indicate much chondritic contamination. It follows that bulk Ceres may contain more C-N-H bearing organic matter than is present in the equatorial surface materials and carbonaceous chondrites (Table 2). Although abundant surface carbonates are consistent with the elevated C content, it is unclear if they represent the whole body. Carbonates could have accumulated through aqueous processes in deep (Zolotov and Mironenko, 2015; Castillo-Rogez et al., 2018) and shallow (De Sanctis et al., 2016; Zolotov, 2017) interior, decomposed through space weathering (Mukhin et al., 1996; Zolotov, 2017) and formed from organic matter and/or C oxides in impact processes (Zolotov, 2014). We do not consider carbonates as a separate component in first order rockorganic models for the whole body. Ceres' surface materials contain less Fe than CI/CM chondrites (Table 2). Surface concentrations may characterize bulk Ceres because a majority of Fe may not be mobilized and depleted through aqueous alteration, impacts or space weathering. Possible surface contamination by Fe-rich chondritic materials suggests bulk Fe content less than in surface materials. If the body contains 2–5 wt% less Fe than CI chondrites, the estimated grain density of Fe-depleted CI-type materials (2290–2420 kg m−3, Table S1 in Supplimentary data) is higher than Ceres' density and additional low-density phases and/or 6–11% porosity need to be invoked. Prettyman et al. (2017) show that Fe concentration in surface materials (Table 2) corresponds to addition of ~ 13 wt% of a Fe-free material to CI carbonaceous chondrites. Surface abundances of Fe, H and C are modeled by mixtures of CI chondrites with chondritic solvent insoluble organic matter, IOM, (Marchi et al., 2019) and by CI-IOM‑carbonate mixtures (Prettyman et al., 2019). A mixture of CI carbonaceous chondrites (60 vol%), amorphous carbon/ IOM (22 vol%), NH4-bearing phases and Mg/Ca carbonates provides a good fit for Ceres' near infrared spectrum and agrees with Fe/H/C concentrations in equatorial surface materials (Marchi et al., 2019). In addition to the elevated C content, these models are consistent with the presence of organic matter that was compositionally and structurally altered at the surface.

Table 2 The composition of Ceres' surface and its assumed building materials (wt%).

2.2. Models for bulk rock-organic interior

Material

Grain density, kg m−3

H

C

K

Fe

Ceres' surface IOM component CI chondrites CM chondrites CIR component

– 1300 ± 100 2420 ± 60 2960 ± 40 2563

1.7–2.1 4.06 1.55 1.0 1.55

8–14 69.15 3.48 2.2 3.48

0.37–0.45 – 0.546 0.307 0.546

15–17 – 18.66 21.3 18.66

Dawn compositional data on Vesta (Prettyman et al., 2012) and Ceres (De Sanctis et al., 2018; Marchi et al., 2019) suggest a limited surface contamination by carbonaceous chondritic materials. Therefore, we assume that Ceres' C-rich and Fe-poor surface represents the whole body. Rock-organic mixtures (Table 2) are used to model the composition and density of Ceres' interior. Compositions and grain densities of CI and CM carbonaceous chondrites represent the rocks. Another proposed rock (Ceres' Interior Rock, CIR) has the bulk composition of CI chondrites and an elevated grain density. The CIR end member component is chosen to reflect the apparent lack or deficiency of hydrated low-density Mg/Na sulfates. For the end member organic material, the used composition C100H70O20N3.5S3 represents moderately altered IOM in chondrites (Alexander et al., 2007, 2017). The IOM consists of a kerogen-like polymer that contains a majority of organic matter in carbonaceous chondrites. The composition of chondritic IOM is not much different from that of non-volatile cometary organic matter

Note: The composition of Ceres' surface material is for the equatorial region based on Dawn GRaND measurements (Prettyman et al., 2017, 2019). The IOM component stands for chondritic insoluble organic matter with composition C100H70O20N3.5S3. The CIR component stands for the Ceres' Interior Rock with bulk composition of CI chondrites and elevated grain density. Grain density of chondrites is from Macke et al. (2011) and Flynn et al. (2018). For CI chondrites, H content is from Alexander et al. (2012) and other elements are from Palme et al. (2014). For CM chondrites, the chosen H content reflects concentrations of 0.56–1.46 wt% (Alexander et al., 2012); other elements are from Lodders and Fegley (1998). 3

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(Kissel and Krueger, 1987; Jessberger et al., 1988; Fomenkova et al., 1994; Levasseur-Regourd et al., 2018) that may have a common origin (Alexander, 2011; Alexander et al., 2017). The difference between IOM and analogous cometary organic matter reflects aromatization and decreasing of H/C, N/C and O/C ratios through alteration on parent bodies of chondrites. Organic-rich dust of comets 1P and 67P (Levasseur-Regourd et al., 2018) has an IOM-like N/C atomic ratio (0.035 ± 0.011, Fray et al., 2017), slightly elevated H/C (≥ 0.8, Kissel and Krueger, 1987; Fray et al., 2016) and O/C ratios (0.2–0.3, Kissel and Krueger, 1987; Bardyn et al., 2017) and a CI-like S/C ratio (0.02, Kissel and Krueger, 1987). Therefore, the used IOM composition is applicable for mixtures of chondrites with non-volatile cometary organics. To the best of our knowledge, density of IOM has not been reported and the used value of 1300 ± 100 kg m−3 is typical for analog materials such as kerogen, bituminous coal and pyrene (Table S3). The assumed IOM density is between that of coal tar and anthracite. Zolotov (2009) used the same value to model a chondrite-organic composition of Ceres. The composition and grain density of rock-organic mixtures are constrained by surface abundances of C, Fe and H together with grain densities of end members and Ceres' bulk density (Table 2). The difference between Ceres' bulk density (ρCeres) and calculated density of rock-organic mixtures (ρmix) was accounted for by porosity (ω), Ceres / mix )

× 100%

Fe

Ceres' surface

Si

10

Concentration, wt%

Mg S Ceres' surface carbon

H Ca

Ni

1 Al

Na N Cr

(1)

CI chondrites

= (1 –

O

0.1

Mixtures that correspond to porosity of <8% are considered reasonable and consistent with previous estimations (Britt et al., 2002; Zolotov, 2009). 2.3. Results for bulk rock-organic interior

0

Mixtures of CI chondrites with IOM that agree with surface C content constrain the elemental composition of Ceres (Fig. 2, Table 3). Addition of IOM to chondrites dilutes rock-forming elements (O, Fe, Si, Mg, etc.) and increases concentrations of H and N in the mixture. Concentrations of Fe and H in the mixture agree with values detected in the equatorial surface regions (Figs. 2, S1 in Supplimentary data). Although measured Fe and H concentrations suggest slightly higher C contents (up to 17–18 wt%), the overall consistency of C-Fe-H data implies abundant C-H bearing materials at the surface and supports inferences of Prettyman et al. (2019) and Marchi et al. (2019). The lack of the match for potassium suggests a deficiency of K in the chondritic end member and/or K depletion through space weathering. Mixtures of CM chondrites with IOM are inconsistent with surface abundances of Fe, H and K (Fig. S2). A consistency requires a larger fraction of IOM that corresponds to >17 wt% C in CM-IOM mixtures. The measured concentration of C (Table 2) constrains a mass fraction of IOM in rock-IOM mixtures. The carbon content of 8–14 wt% corresponds to 6.9–16.0 wt% IOM in CI-IOM mixtures and to 8.7–17.6 wt% IOM in CM-IOM mixtures (Fig. 3a). For the CI-IOM mixture, an average value of 12 wt% C relates to 13 wt% IOM. These results are consistent with the interpretation of surface Fe content by addition a Fe-free component to CI chondrites (Prettyman et al., 2017): 11–15 wt% of added IOM corresponds to 10.7–13.3 wt% C in the CIIOM mixture (Fig. 3a). Fig. 4 illustrates the usefulness of CI-IOM mixtures to model bulk Ceres without invoking other low-density phases. The composition and density of CI-IOM mixtures match Dawn data. The figure shows that Ceres' bulk composition could be between that of CI chondrites and cometary dust as we know it from in situ studies of comets 1P and 67P (Levasseur-Regourd et al., 2018). In terms of the C/Si ratio, Ceres' materials are similar to anhydrous IDPs. Chondrite-IOM mixing models (Table 3) constrain the amount of H in hydrated and OH-bearing phases. At 8–14 wt% C in the CI-IOM mixture, 84–67% of H is present in the rock as a component of OHbearing and hydrated minerals. These values correspond to

Mn Cl K 5

10

15

20

C in CI-IOM mixture, wt% Fig. 2. The elemental composition of Ceres composed of a mixture of CI chondrites and the insoluble organic matter (IOM) as a function of C wt% C in the mixture. The vertical dash-dotted line represents C wt% in CI chondrites. The vertical dashed lines depict the range of C content in Ceres' surface materials (Table 2). The bold curves for Fe, H and K correspond to surface concentrations in the equatorial region of Ceres (Table 2). The data on C, Fe and H in surface materials are consistent with each other and suggest an organic-rich composition of surface materials (see also Fig. S1).

12.9–11.6 wt% of water-equivalent H in the mixture and constrain a lower limit for water abundance involved in aqueous alteration of rocks (Section 5.2). Volume fraction of IOM depends on adopted grain densities of end members (Fig. 3b, Table 3). At IOM density of 1300 kg m−3 and 8–14 wt% C, IOM volume fraction is 12–26% in CI-IOM mixtures, 13–27% in CIR-IOM mixtures and 18–33% in CM-IOM mixtures. In CIIOM and CIR-IOM mixtures, the uncertainty of IOM density corresponds to additional error bars of 0.7–1.6 vol% IOM (Table 3). With this uncertainty, the nominal values of 12 wt% C and 13 wt% IOM (Prettyman et al., 2017, 2019) correspond to 20.5–24.2 vol% IOM in CI-IOM and CIR-IOM mixtures. Grain density of rock-IOM mixtures is calculated from mass fractions and grain densities of end members (Supplementary data). At 8–14 wt% C, the assessed grain density of CM-IOM mixtures (2350–2700 kg m−3) exceeds Ceres' density (Figs. 4 and S3). The calculated grain density agrees with Ceres' density only at 19–25 wt% C in the CM-IOM mixture. The corresponding C/Si atomic ratio of 4.7–6.7 is closer to ratios typical of cometary dust than of chondrites (Fig. 4). For CI-IOM and CIR-IOM mixtures with 8–14 wt% C, grain density of 2080–2420 kg m−3 (Table 3) is largely below grain density of CI chondrites (Table 2). Models of Ceres' interior with CM-IOM mixtures containing 8–14 wt % C require either 7–20% porosity (Fig. S3) or additional low-density phases (water ice, gas hydrates). The models with CI-IOM mixtures do 4

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M.Y. Zolotov

Table 3 The composition of Ceres consisted of rock-organic mixtures. Component, wt%

8 wt% C

12 wt% C

14 wt% C

IOM H N O Na Mg Al Si S Cl K Ca Cr Mn Fe Ni Th U

6.87 1.72 0.469 44.0 0.462 8.89 0.782 9.965 5.36 0.0650 0.0508 0.848 0.245 0.178 17.4 1.02 2.8 × 10−6 7.5 × 10−7

13.0 1.89 0.623 42.3 0.432 8.30 0.731 9.31 5.37 0.0607 0.0475 0.793 0.229 0.167 16.24 0.950 2.6 × 10−6 7.0 × 10−7

16.0 1.95 0.700 41.5 0.417 8.01 0.705 8.985 5.38 0.0586 0.0458 0.765 0.221 0.161 15.67 0.916 2.5 × 10−6 6.8 × 10−7

Rock (CI chondrite) grain density = 2420 kg m−3 IOM, vol% 12.1+0.9 0.7

IOM content in rock-IOM mixture, wt.%

25

20

15 Surface organics from Fe content

10

5

12.7+1.4 1.2

26.2+1.6 1.4

0

2285+20 23 5.4+0.8 1.0

2176+35 38 0.66+1.5 1.8

2127+40 46 1.7+1.9 2.2

40

Rock (CIR) grain density = 2563 kg m−3 IOM, vol% 12.7+0.9 0.8

22.8+1.4 1.3

27.38+1.6 1.5

2403+22 25

2276+37 44

2218+44 49

10.0+0.8 1.09

5.0+1.5 1.8

2.5+1.9 2.2

Ceres' porosity, %

Rock-IOM density, kg m−3 Ceres' porosity, %

0

IOM content in rock-IOM mixture, vol.%

Rock-IOM density, kg m−3

a

Note: The rock has the composition of CI chondrites and the organic matter is presented by meteoritic IOM (Table 2). IOM data represent the values added to chondritic rock that contain some IOM. Carbon data correspond to the composition of Ceres' surface (Table 2). Error bars for calculated IOM vol%, rockIOM density and Ceres' porosity reflect the uncertainty of IOM density of 1300 ± 100 kg m−3. Negative porosity is inconsistent with Ceres' density and suggests higher grain density of the rock than that of CI chondrites. The composition and interior structures at 12 wt% C and rock grain density of 2563 kg m−3 (CIR component) are shown as diamond symbols in Figs. 3, 5b, 6 and 11, in Fig. 12c,d, and in Tables 4 and 5 (case TL-S3).

not require high porosity because their grain density is similar to Ceres' density (Fig. 5a, Table 3). At 12 wt% C, the required porosity is <2.2%. CI-IOM mixtures with >11 wt% C could be inconsistent with Ceres' data because grain densities could be less than the body's density. CIRIOM mixtures are slightly less denser than Ceres and corresponding interior structures require 0.3–10.8% porosity (Fig. 5b, Table 3). CIRIOM mixtures with 12 wt% C correspond to 3.2–6.5% porosity that is similar to that of least porous chondrites (Macke et al., 2011; Flynn et al., 2018; Ostrowski and Bryson, 2019). CIR-IOM mixtures provide best fits for Ceres' composition and density. Bulk properties of Ceres consisted of rock-IOM mixtures at IOM density of 1300 kg m−3 are shown in Fig. 6 and Table 4. The rock is presented by the composition of CI chondrites with variable grain density. Grain density and porosity of the mixtures are plotted as functions of grain density of end member rock and IOM content. Ceres' parameter space corresponds to 11–15 wt% IOM and 2–8% porosity. Grain density of calculated rock-IOM mixtures (~ 2200–2350 kg m−3) is less than that of CI and CM chondrites (Fig. 6a). Grain density of end member rocks (~ 2410–2740 kg m−3) is between that of CI and CM chondrites. The chosen parameter space corresponds to 18–27 vol% IOM (Fig. 6c). At zero porosity, grain density of the end member rock is similar to that of CI chondrites and the rock-IOM mixture has 18–25 vol % IOM (Table 4). The nominal CIR-IOM composition at 5% bulk porosity is depicted in Table 4 and as diamond symbols in Figs. 3, 5 and 6. Analogues evaluations for IOM density of 1200 kg m−3 correspond to 2.5% higher rock density and 8% larger volume fraction of IOM (Fig. S4, Table S4) than at the nominal IOM density. In summary, Ceres' bulk interior is best modeled by a mixture of

5

10 C content in rock-IOM15 mixture, wt%

20

b

30 Rock grain density -3 2960 kg m

20

Rock grain density -3 2420 kg m

CM chondrites

10 CI chondrites

Ceres' surface GRaND data

0 0

5

10

15

20

C content in rock-IOM mixture, wt% Fig. 3. Mass (a) and volume (b) content of IOM added to the composition of carbonaceous chondrites as a function of bulk C content in rock-IOM mixtures. Solid symbols on the X axis show C content in CI and CM chondrites; the vertical dashed lines represent the range of C content in Ceres' surface materials (Table 2). In (a), the dotted lines show the range of added organic matter inferred from the low Fe content in surface materials (Prettyman et al., 2019; this work). The diamond symbols represent the CI-IOM mixture with 12 wt% C. In (b), the curves are for grain densities of end member rocks that represent CI and CM chondrites (Table 2). Solid curves correspond to IOM density of 1300 kg m−3. Short-dashed and long-dashed curves are for IOM density of 1200 kg m−3 and 1400 kg m−3, respectively. The diamond symbol corresponds to the CIR-IOM mixture characterized by 12 wt% C and IOM density of 1300 kg m−3 (Tables 3 and 4).

chondritic IOM with rocks compositionally similar to sulfate-less CI chondrites with slightly higher grain density to allow several % microporosity in mineral grains. Neither water ice nor high porosity is needed to explain bulk Ceres consisted of such rock-organic mixtures. The composition and density of CM-IOM mixtures disagree with Ceres' data. 5

Icarus 335 (2020) 113404

Solar photosphere

2500

Porosity, %

CI chondrites

10

i ty

Anhydrous IDP

8

Dawn data

CI

2400

Gr d e a in nsi ty

4

Ceres' density

0

5

10

15

20

12

3000

10

Fig. 4. The effect of addition of organic matter (IOM) to carbonaceous chondrites on grain density of rock-organic mixtures. Solid curves correspond to IOM density of 1300 kg m−3. Short-dashed and long-dashed curves are for IOM density of 1200 kg m−3 and 1400 kg m−3, respectively. The vertical dotted lines show C/Si ratios in CM (Lodders and Fegley, 1998) and CI (Palme et al., 2014) chondrites, cometary dust (Jessberger et al., 1988; Bardyn et al., 2017), anhydrous IDPs (Thomas et al., 1993) and in the solar photosphere (Palme et al., 2014). The horizontal line stands for Ceres' bulk density (Table 1). The bold line at Ceres' density represents the C/Si ratio at 12+24 wt% C in Ceres' surface materials (Table 2) with CI/CM-like numbers of Si atoms.

P,

Gr d e ain nsi ty

4

0

2400 Ceres' density

5

10

15

2200 2000 20

C in rock-IOM mixture, wt% Fig. 5. Grain density and bulk porosity of Ceres consisted of rock-IOM mixtures as a function of C wt% in the mixture. Rock end members (solid circle symbols) are represented by CI chondrites and the CIR component (Table 2). The vertical dashed lines depict the range of C content in Ceres' surface materials. Solid curves correspond to IOM density of 1300 kg m−3. Short-dashed and long-dashed curves are for IOM density of 1200 kg m−3 and 1400 kg m−3, respectively. Diamond symbols correspond to 5% porosity and grain density of 2276 kg m−3 at 12 wt% C in the CIR-IOM mixture (Tables 3 and 4, Figs. 3 and 6). Neither high porosity nor additional low-density phases (e.g. ice) are required to match data on surface composition and density of Ceres.

We model density distribution in Ceres' interior at the state of hydrostatic equilibrium. The composition and grain density of condensed phases (minerals and organic matter) are assumed uniform. Changes in density with depth are attributed to compaction. Grain density corresponds to zero-porosity materials that match rock-organic mixtures (Section 2). No water ice or gas hydrates are invoked. Calculations of gravity, density and pressure profiles are constrained by Dawn data on Ceres' volumetric radius, mass and rotation period (Table 1). Other calculated values are bulk porosity (Eq. (1)), polar flattening, J2 gravity moment and a dimensionless moment of inertia factor (C/MR2). Porosity profiles are set by free parameters. Density and porosity profiles are constrained by solving the hydrostatic equilibrium equations for either J2 or polar flattening (Supplementary data). Two-layer (TL) and graduate compaction (GC) interior structure models are applied for porosity after Zolotov (2009). Two-layer structures with a core and a higher-porosity upper layer are characterized by free parameters: core porosity, upper layer porosity and pressure of porosity collapse. TL structures are calculated in the parameter space constrained by 0–15% core porosity, 10–55% upper layer porosity and 10–60 MPa pressure at the core's upper boundary. These pressures roughly match compressive strengths of material analogs (Table S5). In GC models, porosity linearly decreases with pressure (P), c

6

0

3.1. Hydrostatic equilibrium models with porosity

Pc

2600

CIR

2

3. Density and porosity distribution in the interior

s

8

ity

C/Si atomic ratio

2800

Density, kg m-3

8

Porosity, %

6

os

4

r Po

2

2000

b 10

0

s

2200

C in rock-IOM mixture, wt%

Addition of IOM

=

2800 2600

0

2000

3000

a

6

2

Ceres' density

Ceres' surface GRaND data

os

Grain density, kg m-3

12 r Po

1P/Halley CM chondrites 67P/C-G

3000

Density, kg m-3

M.Y. Zolotov

(2300 ± 100 kg m−3, Table 3) and bulk density of surface materials (1240 ± 100 kg m−3) determined from radar investigations of Ceres (Mitchell et al., 1996). 3.2. Results for interior structures with compacted center Calculations of hydrostatic equilibria for two-layer interior structures show that Ceres' density, gravity, shape and rotation period (Table 1) could be explained in terms of a compositionally uniform iceless interior with <10% porosity. Gravity and shape data cannot be described by a unique interior structure. Different sets of parameters of interior structures agree with gravity or shape. Gravity data suggest more mass concentration in the core than shape data, and interior structures correspond to C/MR2 of 0.377 and 0.395, respectively. Shape-consistent structures have J2 of 29.6 × 10−3 and gravity-consistent structures have polar flattening of 33.9 km (Figs. 1, 7). The observed polar flattening (Table 1) is much closer to that for homogeneous undifferentiated body than calculated flattening that agrees with measured J2. These inferences are consistent with results of Mao and McKinnon (2018). Parameters of two-layer interior structures are presented in Figs. 8–12, S5–S9 and Table 5. Although the results are shown for a broad range of parameters, below we discuss properties of most probable structures with bulk porosity of <10%, core porosity of <5% and upper layer porosity of 10–50%. Figs. 8 and S5–S7 show the effect of core and upper layer porosity at a specified pressure of porosity collapse. Dependent parameters are grain density, bulk porosity, polar

(2)

where ωs represents porosity of surface rocks, and ωc and Pc stand for porosity and pressure at the body's center, respectively. This approach has been used to model sandstone-rich sedimentary basins (Allen and Allen, 2013) and a chemically uniform ice-less Ceres (Zolotov, 2009). Porosities at the center (0–5%) and at the surface (5–51%) are free parameters of GC models. The maximal surface porosity reflected 46 ± 5% porosity inferred from the grain density of CI-IOM mixtures 6

Icarus 335 (2020) 113404

CM

3000

0 28

2800

2600

Grain density of rock-IOM mixture

s

0 c CI

00 26

Ddnsity

Grain density of rock, kg m-3

M.Y. Zolotov

CIR

24

ho

it e dr

Table 4 Properties of rock–IOM mixtures that may compose Ceres.

a

Ceres' bulk porosity, % −3

Rock-IOM mixture density, kg m Rock grain density, kg m−3 IOM, vol% IOM, wt% C, wt%

n

00

00

Ceres' bulk density

2200 0

5

20

10

15

00

180

X Data

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CM

b

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2600

Polar flattening, km

0

CIR CI

2400

-5 -10 -15

Ceres' bulk density

-20

X Data

5

10

15

20

25

30

25

Porosity, %

CM

2800

c

34

Dawn gravity

2400 0 Ceres' bulk density

-5 X Data

-10

10

15

28

30

32

Fig. 7. The inconsistency of Ceres' polar flattening and J2 measurements (Table 1) in the framework of hydrostatic equilibrium at the current spin period. The vertical dashed arrow points to J2 that is consistent with the current flattening. The horizontal dashed arrow points to flattening that agrees with the measured J2.

5

CI

26

J2 x 1000

CIR

5

2276 2563 22.8 13 12

35

32 24

10

2600

0

2206–2350 2411–2740 18.4–27.0 11–15 10.7–13.3

Dawn flattening

33

15

2200

2206–2276 2411–2623 18.4–26.2 11–15 10.7–13.3

36

20

Ddnsity

Grain density of rock, kg m-3

Wt.% of IOM added to rock 3000

2162 2355–2449 18.3–24.8 11–15 10.7–13.3

37

5

0

5

Hydrostatic equilibrium

10

2200

2–8

38

15

2800

Ddnsity

Grain density of rock, kg m-3

Wt.% of IOM added to rock 3000

2–5

Note: The rock end member has the bulk composition of CI chondrites and variable grain density. Density of the mixtures corresponds to the chosen porosity. Grain density of the rock and vol% of IOM correspond to IOM density of 1300 kg m−3. The range of rock grain density corresponds to chosen ranges of porosity and IOM wt%. The abundance of C reflects wt% of IOM, agrees with C measurements (Table 2; Fig. 3) and corresponds to 3.48 wt% C in the rock end member. At 2–8% porosity, the range of these parameters is shown as rhombuses in Fig. 6. The last column represents a nominal case for the CIR-IOM mixture shown in Figs. 3, 5b, 6 and 11 as diamond symbols, in Fig. 12c,d, and in Tables 3 and 5 (case TL-S3). Analogues data at IOM density of 1200 kg m−3 are presented in Table S4.

2400 22

0

20

25

flattening, J2 gravitational moment, C/MR2, depth of porosity collapse, upper layer density, core density and pressure in the body's center. Higher upper layer porosity (lower density) is related to higher values for upper layer thickness and core density, and to lesser values for polar flattening, J2 and C/MR2. Higher core porosity corresponds to higher values for polar flattening, J2, C/MR2 and upper layer density and to lower numbers for upper layer thickness and core density. Figs. 9 and S8 illustrate effects of upper layer porosity and pressure of porosity collapse at 0% and 5% core porosity. Lesser pressure of porosity collapse (thinner upper layer) corresponds to lesser bulk porosity, grain density, upper layer density and central pressure, and to higher values for polar flattening, J2 and C/MR2. In Figs. 8, 9 and S5–S8, the marked curves disclose parameters that agree with either measured J2 or polar flattening. The distinct positions of the curves reveal differences of corresponding interior structures in

30

Vol. % of IOM added to rock Fig. 6. Bulk properties of Ceres consisted of rock-IOM mixtures. The rock end member has the bulk composition CI chondrites and variable grain density. IOM grain density is 1300 kg m−3. (a), Grain density of the mixture (counter lines). The dash-dotted curves show grain densities of CI and CM chondrites. (b), Bulk porosity as a function of wt% of IOM. (c), Bulk porosity as a function of vol% of IOM. In (a)-(c), solid symbols on the Y axis show average grain densities of chondrites and Ceres' density (Table 2). The large rhombuses represent the parameter space at 2–8% porosity, 11–15 wt% IOM and 18.4–27 vol% IOM (Table 4). The diamond symbols stand for the CIR-IOM mixture at 5% porosity, 12 wt% C, 13 wt% IOM, 22.8 vol% IOM and rock grain density of 2563 kg m−3 (Tables 3 and 4, Figs. 3 and 5b). Analogues plots at IOM density of 1200 kg m−3 are shown in Fig. S4.

7

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M.Y. Zolotov

Upper layer porosity, %

280

40 CI c

30

ata ravity d Dawn g 26

ho n

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20

23

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25

dri

00

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ra Dawn g

Homogeneous body

0.385

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20 s ha Dawn

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c

33 vity data

80 vity data 0.3

0.390

hape

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ra Dawn g

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C/MR

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data

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ravity d

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10 15 55 10 15 Depth of porosity collapse, km

f

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pe da

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ata ape d

y s bod 30mogeneou Ho

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s Dawn

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hape

data

ody ous b gene Homo 5

3

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15 15

Core porosity, %

0 0

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10 10

15 15

Core porosity, %

Fig. 8. Parameters of the two-layer interior stucture at the pressure of porosity collapse of 20 MPa. The marked curves with symbols depict conditions consistent with Ceres' gravity or shape (Table 1). In (a), grain densities of CI chondrites are shown (Table 2). The solid symbols correspond to interior structures in Table 5 (cases TLG3, TL-G6 and TL-S5). Densities of the upper layer and the core are shown in Fig. S4. The grey lines correspond to conditions of a homogeneous body without an upper layer.

bulk and core porosity, grain density, and in thickness, density and porosity of the upper layer. Symbols on the curves stand for representative interior structures depicted in Table 5. Plausible TL structures that are consistent with gravity have bulk porosity of >9% and zero core porosity (Fig. 10, cases TL-G1 to TL-G3 in Table 5). Typical structures require unlikely high bulk porosity (Figs. 8d, 9d; cases TL-G6 to TL-G8) especially at higher pressures of porosity collapse (Figs. 10b, S6d, S7d; S8d; cases TL-G4 to TL-G8). Bulk porosity of 9–10% corresponds to grain density of 2380–2400 kg m−3 that is similar to that of CI chondrites (Figs. 9a,d, 10a, 12a,b) but is higher than density of CI-IOM mixtures (Table 3). The upper layer with 36–50% porosity has thickness of 30–47 km that corresponds to porosity collapse at 10–20 MPa (Figs. 9h, 10d). Structures with the upper layer thinner than ~ 30 km have an improbable upper layer porosity (>50%) that corresponds to density of <1190 kg m−3 (Fig. 9f, h; Table 5). Structures with the upper layer thickness of >47 km (porosity collapses at pressure > 20 MPa) have bulk porosity of >10% (Figs. 9d, 10, S8d; Table 5). Fig. 12a,b represents a probable interior structure

(TL-G2) that has a lowest reasonable porosity (9.4%) consistent with Ceres' gravity, density and spin period. This bulk porosity matches that of the Vigarano CV3 chondrite. The upper layer porosity (41%) is only slightly higher than that of Orgueil CI chondrite (34.9 ± 2.1%, Macke et al., 2011). In this structure, density (~ 1400 kg m−3) and thickness (40 km) of the upper layer agree with estimations of Ermakov et al. (2017). Shape-consistent two-layer structures have lesser bulk porosity (1.8–8%) and lesser upper layer porosity (9–20%) than gravity-consistent models. At core porosity of <5%, the upper layer porosity is <20%. At zero core porosity, the upper layer has 9–15% porosity and thickness of 20–37 km (Fig. 9h, Fig. 11b,d, Table 5). At 5% core porosity, the upper layer has ~ 10–20% porosity and thickness of 19–113 km (Fig. S8h, Table 5). Corresponding upper layer density (1870–2110 kg m−3) is higher than in gravity-consistent structures. A core with <5% porosity has density of 2200–2240 kg m−3. Grain density for shape-consistent TL structures (2200–2350 kg m−3) is below that of CI chondrites (Figs. 8a, S6a, S7a, 9a, S8a, 11a, 12c,d). The 8

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M.Y. Zolotov

Fig. 9. Parameters of the two-layer interior model at zero core porosity. The curves with symbols depict conditions consistent with Ceres' gravity or shape. In (a), grain densities of CI and CM chondrites are shown (Table 2). Solid symbols correspond to representative interior structures in Table 5 (cases TL-G1 to TL-G5 and TLS1). Empty circle symbols correspond to the interior structure TL-G2 at the upper layer thickness of ~ 40 km (Table 5, Fig. 12).

lowest possible grain density corresponds to zero core porosity and 1.8% bulk porosity; the higher end of grain density relates to 5% core porosity (Fig. 11a,b; cases TL-S1 and TL-S7 in Table 5). For structures with the upper layer thickness of 38–45 km (Ermakov et al., 2017), the maximal grain density is 2320 kg m−3 (Fig. 11, case TL-S5). The best agreement is achieved for grain density of 2200–2280 kg m−3 that corresponds to 1.8–5% porosity (cases TL-S1 to TL-S3 in Table 5). Fig. 12a,d shows porosity and density profiles for a plausible TL

structure with 5% bulk porosity and the upper layer thickness of 41 km (case TL-S3). Ceres' flattening suggests a lesser density/porosity change than in gravity-consistent TL structures (Fig. 12). Lesser density gradients in shape-consistent structures correspond to lower central pressures (148–150 MPa) than gravity-consistent cases (163–171 MPa). Interior structures with graduate compaction are different for gravity- and shape-consistent cases (Table 6). Gravity-consistent GC structures require bulk porosity of 25–30% and grain density of ~ 9

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M.Y. Zolotov

Fig. 10. Parameters of the two-layer interior model consistent with Ceres' J2 value. In (a) and (b), the dashed curves show the range of parameters limited by zero core porosity that sets lower limits for grain density (~2380 kg m−3) and bulk porosity (~ 9%). In (a), the error bars show the range of grain densities of CI and CM chondrites (Table 2). In (d), the thickness of the upper layer is after Ermakov et al. (2017). The symbols correspond to the interior structures in Table 5 (cases TL-G1 to TL-G3 and TL-G6). Empty circle symbols stand for the interior structure TL-G2 with the upper layer thickness of 40 km (Table 5, Fig. 12a,b).

3000 kg m−3 (Fig. S8). Although this grain density is similar to that of CM chondrites, such a high porosity disagrees with low compressive strengths of carbonaceous chondrites and organic-rich solids (Table S5). Both high grain density and bulk porosity do not imply a significant fraction of organic matter. It follows that the used CG model does not explain the measured J2 value. In contrast, shape-consistent GC structures require grain density of 2290–2400 kg m−3, bulk porosity of 5–10% and surface porosity of 9–14% (Fig. 13). These values are similar to those in shape-consistent TL models. The grain density is less than that of CI/CM chondrites and agrees with that of chondrite-organic mixtures (Table 3).

anomalies (Mao and McKinnon, 2018; Ermakov et al., 2017; Tricarico, 2018). The anomalies could be due to accretion of planetesimals with elevated rock/organic and/or metal/silicate ratios. Ceres' shape is more consistent with a low-density organic-rich interior structure than gravity. Shape-consistent interior structures with ~ 2–8% porosity suggest grain density (2200–2400 kg m−3, Tables 5, 6, S6) that agrees with that of rock-IOM mixtures (2160–2420 kg m−3, Tables 3, 4, S4) estimated from the surface composition and bulk density. In contrast, grain density of gravity-consistent structures (2380–2680 kg m−3, Table 5; 2426–2439 kg m−3, a zero-porosity core from Ermakov et al., 2017) only barely matches density of rock-IOM mixtures. Bulk porosity of 1.8–8% suggested for shape-consistent structures (Tables 5, 6) agrees with ~7% Ceres' porosity estimated through compaction of carbonaceous chondrites (Britt et al., 2002). This porosity is consistent with compaction of material analogs (e.g. shale sands) at corresponding pressures (Zolotov, 2009) and is typical for least porous chondrites with dominant microporosity (Fig. 14; Consolmagno et al., 2008; Macke et al., 2011; Flynn et al., 2018; Ostrowski and Bryson, 2019). An abundant low-viscosity organic matter in the interior (Table 3) does not imply macroporosity (except for the upper layer) but could not affect microporosity of mineral grains. Bulk porosity of >9% of ice-free gravity-consistent structures (Table 5) is inconsistent with an organic-rich interior. Ceres' shape suggests 9–20% upper layer porosity in the TL structures and 9–14% porosity of the surface layer in GC structures (Tables 5, 6, S6). These values are in the range of porosities of the lunar crust evaluated from GRAIL data and modeled by a graduate compaction (Wieczorek et al., 2013; Han et al., 2014). A structural collapse of a rock at the pressure of compressive strength does not eliminate porosity until pressure reaches ~300 MPa

4. The case for undifferentiated ice-deficient interior consistent with Ceres' shape Calculations of hydrostatic equilibria show that Ceres' gravity could be explained by two-layer interior structure models with contrast densities and bulk porosity of >9%. Ceres' polar flattening could be explained by more moderate density gradients in two-layer and graduate compaction interior structures with bulk porosity of <8%. Here we argue for a chemically homogeneous interior structure that agrees with Ceres' shape. Dawn data indicate that Ceres is in hydrostatic equilibrium (Park et al., 2016) that can be established in a geologically short time (Ermakov et al., 2017). The suggested presence of abundant organic matter in the interior (Section 2) also favors viscous relaxation and hydrostatic equilibration. The explanation of current polar flattening by recent de-spinning (Mao and McKinnon, 2018) could be inconsistent with the fast hydrostatic equilibration (Ermakov et al., 2017). If polar flattening represents the global interior structure at the current spin period, the measured J2 could reflect deep uncompensated density 10

Icarus 335 (2020) 113404

30 25

Y Data

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Y Data

on

20

24 00

Grain density, kg m-3 10

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15 2 % bulk porosity

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5 % core porosity -5

00 23

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Upper layer porosity, %

d

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c 10

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P of porosity collapse, MPa

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P of porosity collapse, MPa

ch

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a CI

P of porosity collapse, MPa

M.Y. Zolotov

-5

Ermakov et al. 2017 -1 Core porosity, % 0

10 20

10 25

30

35

40

45

50

55

Depth of porosity collapse, km

5

10

15

20

Upper layer porosity, %

Fig. 11. Parameters of the two-layer interior model consistent with Ceres' polar flattening. In (a) and (b), the dashed curves show the range of parameters limited by core porosity of 0–5%. Zero core porosity sets lower limits for grain density (~2200 kg m−3) and bulk porosity (~1.8%). In (a), (b) and (c), the large rhombuses correspond to conditions limited by the upper layer thickness of 37.8–44.7 km (Ermakov et al., 2017) and core porosity of 0–5%. In (d), the vertical dashed lines show thickness of the upper low-density layer after Ermakov et al. (2017). The symbols correspond to interior structures in Table 5 (cases TL-S1 to TL-S5). The diamond symbols correspond to the structure TL-S3 at 5% porosity and the upper layer thickness of 41 km (Table 5, Fig. 12c,d).

topography relaxation models set upper limits for the ground ice content (30–40 vol%, Bland et al., 2016; 25 vol%, Fu et al., 2017) but do not indicate the presence of ice. In analogy with the Moon and Mercury, the presence of ground ice in Ceres' polar and shadowed areas (Prettyman et al., 2017, 2019; Platz et al., 2017; Combe et al., 2019) could reflect cold trapping of water vapor released by impacts. The detection of ice in the upper ~1 m does not exclude ice-less interior. Ceres has no vivid fluidized impact craters that clearly signify the presence of ground ice on Mars. Some fluidized appearing ejecta (Hughson et al., 2019; Sizemore et al., 2019) could be flows of lowviscosity organic-rich materials warmed and partially volatilized by impacts. At least some lobate landslides (Krohn et al., 2016; Schmidt et al., 2017; Sizemore et al., 2019; Chilton et al., 2019) could be granular flows seen on dry solar system bodies (Kumar et al., 2012; Scaioni et al., 2018). A similar morphology of pitted terrains on Ceres and ice-free Vesta (Sizemore et al., 2017) does not support cryogenic origins of Ceres' pits. Some dome-like features could be diapirs of lowviscosity and low-density (Sori et al., 2018) organic materials (Section 5.4) rather than ices or hydrates. Terrestrial mud volcanoes could be analogs for domes. As many mud volcanoes (Mazzini and Etiope, 2017), Ceres' counterparts could be enriched in low molecular weight hydrocarbons that decrease viscosity and density of the organic-silicate ‘mud breccia’. Uneven global distribution of supposedly ice-related morphological features (Sizemore et al., 2019) could reflect compositional (e.g. organic fraction) and/or physical (strength, porosity, temperature) heterogeneity of the upper interior. The supposed icy composition of Ahuna Mons (Ruesch et al., 2016; Sori et al., 2017) is inconsistent with the positive gravity anomaly of the region (Ermakov et al., 2017; Tricarico, 2018), though gravity data have lesser spatial resolution than imaging. Signs of H2O(g) plumes (Küppers et al., 2014) have not been

(Wash and Brace, 1984; Yang and Aplin, 2004). Neither chondritic nor terrestrial sedimentary analogs (Consolmagno et al., 2008; Macke et al., 2011; Flynn et al., 2018; Zolotov, 2009) suggest zero porosity in the lower interior implied in gravity-consistent structures (Fig. 12a,b). A moderate compaction of shape-consistent structures (Figs. 12c,d, 13; Tables 5, 6) is more likely than abrupt (36–50%) changes in porosity/ density in gravity-consistent TL structures. An estimated ~ 10% porosity of the upper ~ 40 km thick layer (Fu et al., 2017) is more consistent with Ceres' shape than with gravity. Neither shape- nor gravity-consistent structures require abundant water ice in the interior. Ceres' shape does not suggest ice in the upper interior because a modest density gradient is easily explained by compaction (Fig. 12c,d, Fig. 13). In contrast, gravity-consistent models require an upper interior layer with abundant low-density phases (Ermakov et al., 2017; Fu et al., 2017) and/or high porosity (this work). Carbonaceous chondrites are porous (Fig. 14) and impacts introduce additional micro- and macroporosity to chondrites (Consolmagno et al., 2008; Piekutowski, 1980; Wiggins et al., 2018). As on the Moon (Besserer et al., 2014), a limited low-gravity compaction of disrupted and ejected materials could contribute to Ceres' crustal porosity. However, we agree that gravity data do not exclude volatile-rich solids in the upper interior. Fu et al. (2017) invoked ≤25 vol% water ice together with ≥36 vol% gas and/or salt hydrates to explain density and viscosity of the upper layer. Do we have unequivocal sings for abundant water ice or hydrates that will support a gravity-consistent interior structure? The high Ceres' topography that persists at all latitudes (Bland et al., 2016; Ermakov et al., 2017; Fu et al., 2017; Park et al., 2019) provides a strong sign for ice deficiency: the warmer equatorial material could be more relaxed in the presence of abundant ground ice. The crustal 11

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500

a

400

b

Upper layer

400

300

Porosity collapses at 16.1 MPa at depth 40 km

200

Core

300

Bulk porosity

100

200

Core

100

Grain Bulk density density

0

0

0

10 20 30 40 50

1500

Porosity, %

2000

2500

-3 Density, kg m

Consistent with shape

500

500

Surface, 470 km

400 300

Porosity collapses at 22.7 MPa at depth 41 km

200

Core

c

Bulk porosity

100 0

d

Upper layer

400

300

Core

200

100

Grain Bulk density density

CI chondrites

Radius, km

Surface, 470 km

Radius, km

warmed rock-organic mixtures at depth. The estimated viscosity of the upper layer (1021–1027 Pa s, Fu et al., 2017) does not imply a dominance of low-viscosity materials such as bitumen or water ice at corresponding temperatures (Fig. 15). However, an asphalt cement analog alone or in a mixture with additional bitumen has viscosity that matches Ceres' values. Lowering of grain size in the asphalt cement from mm to ~ μm (as in chondritic matrix) decreases viscosity and allows a better match with supposed temperature-viscosity conditions of Ceres' upper interior. Viscosity of organic-rich analogs decreases rapidly with increasing temperature (~ 10 times per 3.5 K). At the temperature of >160–220 K, viscosity of rock-organic mixtures becomes less than ~ 1021 Pa s and allows plastic deformation below certain depth (Fig. 15). A thermal gradient of 0.5–2 K km−1 in Ceres' upper interior corresponds to thickness of the upper low-viscosity upper layer of 18–120 km. This thickness agrees with the depth of porosity collapse in two-layer interior structure models (Table 5). It is possible that plastic deformation occurs at and below the depth of porosity collapse. A low thermal conductivity of the porous upper layer of organic-rich materials could contribute to an elevated temperature and low viscosity below the layer. Neither deep near-eutectic brines nor compositional changes with depth (Fu et al., 2017) are required to explain the viscous relaxation at depth. In summary, the possible lack of abundant ice in the upper interior agrees with a shape-consistent, isochemical and organic-rich interior structure with a moderate compaction and viscous relaxation at depth. Another problem with chemically heterogeneous gravity-consistent interior structures is formation of abundant water ice, gas hydrates and hydrated salts in the upper interior. Putative ground ice, gas hydrates and salts imply addition of water solution to existing pore spaces in the past. However, Ceres' upper interior is and was too cold to harbor water solutions (McCord and Sotin, 2005; Castillo-Rogez and McCord, 2010; Castillo-Rogez, 2011). Supposed addition of solutions/ices/salts to pore spaces increases density of the upper layer and is not needed to explain the supposed low density of the upper interior. If the current upper layer represents a deeper interior exposed by collisional stripping (Section 5.3), abundant ice (>5 vol%) is inconsistent with a minute porosity and a dominant microporosity of the interior at the time of aqueous filling of pore spaces. Even high-porosity CI/CM chondrites (Fig. 14) have a low permeability (1017–1019 m2, Bland et al., 2009) and μm-size pores could not be filled by solution to form ice-rich rocks. Both micro-pores in mineral grains (Consolmagno et al., 2008) and supposed organic-rich solids are almost impermeable for water solutions. It follows that an abundant organic matter (Table 3) rather than ice and/or gas hydrates could account for the low density of the upper and lower interior. Ceres' polar flattening could be explained by end member models of abrupt and graduate compaction in TL and GC structures, respectively (Figs. 12c,d, 13). On the one hand, abrupt compaction agrees with the observed collapse of Ceres' material analogs (carbonaceous chondrites, sandstones, coals) at compressive strengths (Table S5; Zolotov, 2009) that correspond to depth of ≤n × 10 km. A clastic sandstone-like structure of carbonaceous chondrites with a sub-μm matrix and mmsize chondrules (Brearley and Jones, 1998; Zolensky et al., 2018a, 2018b) suggests structural collapse at certain pressures. An abrupt compaction through structural collapse is consistent with the apparently high strength of the upper ~ 40 km layer that lies above a lowviscosity material (Fu et al., 2017). On the other hand, gradual compaction typical for fine-grained materials in terrestrial sedimentary basins (Allen and Allen, 2013; Zolotov, 2009) could explain density distribution in the lunar megaregolith (Wieczorek et al., 2013; Han et al., 2014). Impact craters are dominant surface features on Ceres (Hiesinger et al., 2016; Marchi et al., 2016) and the corresponding bombardment could have produced compositionally homogenized megaregolith with porosity gradient. Our results (Section 3.2) do not exclude a shape-consistent two-layer interior structure of Ceres with some gradual compaction in the upper layer and in the core.

Consistent with gravity

500

0

0

10 20 30 40 50

Porosity, %

1500

2000

Density, kg m

2500

-3

Fig. 12. Representative interior structures of the compositionally uniform Ceres in the framework of the two-layer interior model. In (a) and (b), the structure is consistent with Ceres' J2 value. In (c) and (d), the structure agrees with polar flattening. In (b) and (d), dash-dotted lines show the range of grain density of CI chondrites (Table 2). In (a)–(d), symbols stand for bulk properties of the body. These interior structures are presented in Table 5 (cases TL-G2 and TL-S3) and as empty circle symbols in Figs. 9–11. For the structure TL-S3, possible chemical composition is shown in Table 3 for 12 wt% C in the rock-IOM mixture.

confirmed (McKay et al., 2017; Roth, 2018). The fluxes of water inferred from telescopic data (Küppers et al., 2014) may not be explained by sublimation from either observed water ice or ice-exposed craters (Landis et al., 2019). If occasional water plumes are real (Küppers, 2019), they may not infer abundant ice in the upper interior level. The occurrence of high-albedo carbonate-rich deposits (Carrozzo et al., 2018) suggests impact melting of ground ice (Bowling et al., 2019; Ruesch et al., 2019; Zolotov, 2017) but does not inform about ice fraction in the rock. Clathrates of CO2 and CH4 are unstable with respect to a low pressure in a permeable regolith and could be decomposed irreversibly via impact warming and decompression. Putative impact volatilization of crustal clathrates could cause more fluidization of ejecta than fluidization of ice-bearing targets. CO2 clathrates are unstable with respect to Ceres' high-pH water solutions with dominant CO32– and HCO3– ions (Zolotov, 2012, 2017) and do not form in such solutions with a sparse concentration of CO2. The presence of Mg-rich and NH4-bearing phyllosilicates (De Sanctis et al., 2015) suggest alkaline solutions that prevent formation of CO2 hydrates. Abundant hydrated salts are inconsistent with the overall low surface albedo and with the apparent deficiency of Si/Al rich phases (silica, talc, montmorillonite, kaolinite) that form during through aqueous alteration of silicates to salts (Zolotov, 2017). The observed viscous relaxation of Ceres' long wavelength (> 150 km) topography (Fu et al., 2017) could be caused by plasticity of 12

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Table 5 Representative interior structures of Ceres in the framework of two-layer compaction hydrostatic model. Case

Density, kg m−3

Porosity, % Bulk

Core

Upper layer

Bulk grain

Core

Upper layer

Pressure of pore collapse, MPa

Upper layer thickness, km

J2 × 103

Polar flattening, km

CM/R2

Central pressure, MPa

Interior TL-G1 TL-G2 TL-G3 TL-G4 TL-G5 TL-G6 TL-G7 TL-G8

structures consistent with gravity 9.02 0 50.0 2376 9.43 0 41.0 2387 9.80 0 36.7 2397 11.9 0 27.5 2455 15.1 0 25.8 2546 14.3 5.0 39.9 2523 16.3 5.0 31.0 2583 19.3 5.0 29.4 2679

2376 2387 2397 2455 2546 2397 2454 2545

1188 1408 1517 1780 1889 1516 1782 1891

10.3 16.1 20.0 40.0 60.0 20.0 40.0 60.0

30 40 47 82 120 47 82 120

26.5 26.5 26.5 26.5 26.5 26.5 26.5 26.5

33.9 33.9 33.9 33.9 33.9 33.9 33.9 33.9

0.377 0.377 0.377 0.377 0.377 0.377 0.377 0.377

162.8 164.9 164.1 167.0 171.1 164.1 167.3 171.4

Interior TL-S1 TL-S2 TL-S3 TL-S4 TL-S5 TL-S6 TL-S7

structures consistent with shape 1.80 0 15.3 2202 1.95 0 9.1 2205 5.0 3.0 11.3 2276 6.71 5.0 19.5 2317 6.87 5.0 13.7 2321 7.37 5.0 11.0 2334 8.09 5.0 10.5 2352

2202 2205 2207 2201 2205 2217 2235

1867 2004 2019 1865 2003 2077 2105

10.0 20.0 22.7 10.0 20.0 40.0 60.0

20 37 41 19 37 73 113

29.6 29.6 29.6 29.6 29.6 29.6 29.6

36.15 36.15 36.15 36.15 36.15 36.15 36.15

0.395 0.395 0.395 0.395 0.395 0.394 0.394

147.8 148.0 148.1 147.8 148.0 149.1 149.6

Note: Ceres is modeled as a chemically homogeneous body. Grain density corresponds to all condensed materials (e.g. rock-organic mixtures). These interior structures are consistent with either Ceres' gravity (J2) or shape (polar flattening) data (Table 1) at hydrostatic equilibrium. These structures are depicted as symbols in Figs. 8-11, S4 and S7. For plausible cases TL-G2 and TL-S3, depth profiles are shown in Fig. 12. For the interior structure TL-S3, possible chemical composition is depicted in Table 3 for 12 wt% C in the rock-IOM mixture. Table 6 Representative interior structures of Ceres in the framework of gradual compaction hydrostatic model. Case

Density, kg m−3

Porosity, % Bulk

Center

Surface

Bulk grain

Surface

J2 × 103

Polar flattening, km

C/MR2

Central pressure, MPa

Interior structures consistent GC-G1 25.6 GC-G2 27.1 GC-G3 29.4

with gravity 0 2 5

38.5 39.8 41.7

2907 2967 3063

1786 1786 1786

26.5 26.5 26.5

33.9 33.9 33.9

0.377 0.377 0.377

177.6 177.7 177.7

Interior structures consistent GC-S1 5.57 GC-S2 7.46 GC-S3 10.3

with shape 0 2 5

9.1 10.9 13.7

2290 2336 2411

2081 2081 2081

29.6 29.6 29.6

36.15 36.15 36.15

0.395 0.395 0.394

150.2 150.1 150.1

Note: Ceres is modeled as a chemically homogeneous body. Gravity-consistent structures do not represent Ceres because they require very high bulk porosity. For cases GC-G1 and GC-S1, porosity and density profiles are shown in Figs. S8 and 13, respectively. Parameters of the model GC-S1 is roughly similar to those of the twolayer structure TL-S3 (Table 5).

500

b

400

300

300

200

200

Bulk porosity

100 0

Grain density

100

Bulk density

Ceres' bulk density and shape together with bulk composition suggested from surface remote sensing data could constrain formation and geological history of the body. CI carbonaceous chondrites are reasonable analogs for the mineral (silicates, oxides, sulfides but not sulfates) ingredients of Ceres because they provide decent fits in models for surface composition (Prettyman et al., 2019; Marchi et al., 2019; this work) and bulk density (this work). A presence of up to 30 vol% of high molecular weight organic matter similar to chondritic IOM and cometary refractory organic matter agrees with compositional data (Prettyman et al., 2019; Marchi et al., 2019), bulk density, polar flattening and viscous relaxation of long wavelength topography (this work). The possible above CI-chondritic abundance of organic matter indicates a significant contribution of organic and volatile (H2O, CO2, NH3, etc.) materials from the outer solar system. Abundant surface NH4-bearing phases and carbonates suggest accretion of CO2/CO ices and NH3 (De Sanctis et al., 2015) detected on bodies beyond the orbit of Jupiter. Several Ceres' characteristics (bulk density, supposed grain density, abundances of C, H and Fe) are between that of carbonaceous chondrites and comets. Here we discuss the suggested organic-rich and ice-deficient Ceres in terms of formation and evolution of the body.

CI chondrites

400

Radius, km

Surface, 470 km

Radius, km

5. Constraints on formation and geological history of Ceres

500

a

0

0

2

4

6

8 10 12

Porosity, %

1500

2000

Density, g cm

2500

-3

Fig. 13. Porosity and density profiles in a presentative shape-consistent interior structure with a gradual compaction. Other parameters of this structure are depicted in Table 6 (case GC-S1). In (b), the dash-dotted lines show the range of grain density of CI chondrites (Table 2).

13

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CI CM CR CK CO CV Upper layer consistent with gravity

Upper layer consistent with shape Bulk Ceres

0

10

20

30

40

50

Porosity, % Fig. 14. Porosity of Ceres' materials in the framework of two-layer model and porosity of carbonaceous chondrites (Macke et al., 2011; Flynn et al., 2018). The lower limit for bulk porosity (1.8%) corresponds to the two-layer shapeconsistent structure TL-S1 (Table 5). The upper limit for Ceres' bulk porosity of 10% is chosen based on previous estimations and discussions (Britt et al., 2002; Zolotov, 2009). For gravity-consistent structures, 10% bulk porosity and zero core porosity sets the lower limit for the upper layer porosity (~ 36%).

Fig. 15. Viscosity and temperature of Ceres' upper interior and material analogs. In (a), the rhombus corresponds to Ceres' low-viscosity upper layer and the horizontal dashed line with arrows shows viscosity of the underlying interior from Fu et al. (2017). The dynamic viscosity data for 0.1 mm grain sized water ice is from Fu et al. (2017). The data for asphalt cement and gas-free bitumen represent extrapolation of dynamic viscosity measurements performed at temperatures >283 K (Traxler, 1962; Mehrorta and Svrcek, 1982). Arrows at the asphalt cement curve show lower viscosity at lower grain sizes of silicate grains. The dotted curve represents a rock-organic mixture with an average viscosity of bitumen and asphalt cement. The plot shows that rock-organic mixtures could be analogs for the high-viscosity layer and the underlying viscous interior of Ceres. In (b), the solid lines show temperature profiles at supposed thermal gradients (Castillo-Rogez and McCord, 2010; Castillo-Rogez et al., 2018) at the equator with the surface temperature of 155 K (Hayne and Aharonson, 2015). The vertical dotted lines correspond to the temperature at which material analogs from panel (a) have viscosity of 1021 Pa s that allows viscous relaxation (Fu et al., 2017). The intersections of dotted lines with temperature profiles set thickness of the upper high-viscosity layer (18–30 km for 2 K km-1, 36–60 km for 1 K km-1, 70–120 km for 0.5 K km-1).

5.1. Place and time of formation and migration Ceres' formation in the main asteroid belt is unlikely because it implies highly selective accretion of volatile- and organic-rich material on several asteroids with Ceres-like near infrared spectra (Takir and Emery, 2012; Rivkin et al., 2014, 2019). At the radial distance of ~3 AU, nebula conditions may have never allowed condensation of CO2 and NH3. Formation places of main belt C-type asteroids, KBO's and comets are probably different from their current location because of scattering by giant planets (Morbidelli et al., 2015; Nesvorný, 2018). For example, the Grand Tack model suggests formation of C-type asteroids beyond a past orbit of Jupiter followed by ejection to the main asteroid belt through migration of the planet toward the Sun (Walsh et al., 2011; Raymond and Morbidelli, 2014). In other models an inward transfer of C-type asteroids is attributed to chaotic migration of Jupiter and Saturn (Izidoro et al., 2016) and/or to scattering of planetesimals at later formation stages of the giant planets (Raymond and Izidoro, 2017). Isotopic composition (O, Cr, Ti, Mo and Ni) and dating of meteorites agrees with formation of carbonaceous chondrites beyond Jupiter (Kruijer et al., 2017; Scott et al., 2018; Desch et al., 2018). The Sun-like composition of CI chondrites (except H, C, N, O and noble gases; Palme et al., 2014) suggests their formation at larger radial distances than other chondrites (e.g. Desch et al., 2018). At least a fraction of KBO's could have formed in a disk of planetesimals at 15–30 AU and then scattered to 30–50 AU and toward the Sun (Gomes et al., 2005; Morbidelli et al., 2008). Small bodies that form short-period Jupiter-family comets could be remnants of collided planetesimals ejected to the Kuiper belt at 30–50 AU (Morbidelli and Rickman, 2015). Density of largest KBO's (~ 1600–2600 kg m−3, Barr and Schwamb, 2016) and frozen H2O, CO2, CO, N2, CH4 and NH3 on their surfaces suggest formation of the bodies at larger radial distances than C-type chondrites and corresponding main belt asteroids. In comets, the elevated abundances of refractory (Jessberger et al., 1988; Fomenkova et al., 1994; Levasseur-Regourd et al., 2018) and volatile organic compounds together with H2O/CO/CO2/NH3 ices (Bockelée-Morvan et al., 2004; Mumma and Charnley, 2011; A'Hearn et al., 2012) indicate their

formation together with KBO's and at larger heliocentric distances. The lack of detection of sulfates at KBO's and comets suggests formation of those bodies at lower stellar irradiation than sulfate-bearing C chondrites that could have accreted irradiated ices with embedded strong oxidants, O2, O3 and H2O2 (Zolotov, 2016). The surface and supposed bulk composition of Ceres (Sections 2 and 3; Fig. 4) implies accretion of the body between formation regions of CI chondrites and KBO's together with Jupiter-family comets. Ceres could have formed within past orbits of giant planets and then ejected to the current position together with bodies with C spectral complex. The ejection could be caused by migration of giant planets (e.g. Walsh et al., 2011; Morbidelli et al., 2015; Izidoro et al., 2016) or by disturbances at later formation stages of the planets (Raymond and Izidoro, 2017) within several first Ma of solar system history and is supported by astronomical and meteoritic data. Near infrared spectra of many large (> 200 km) low-albedo asteroids are not represented in the meteorite collection, while spectral features of those asteroids (e.g. 324 Bamberga) are similar to that of comet 67P (Rivkin et al., 2019). B-type asteroids could be dormant comets (Nuth et al., 2019). The presence of active asteroids in the main belt (main-belt comets, Snodgrass et al., 2017; Nuth et al., 2019), and their low eccentricity and low inclination 14

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M.Y. Zolotov

orbits are consistent with an early sunward migration of those volatilerich bodies (Hsieh and Haghighipour, 2016). Migration of Ceres together with other bodies that formed the main asteroid belt does not imply a major angular momentum exchange with the massive asteroid belt to explain the current low-eccentricity of Ceres' orbit (cf. McKinnon, 2008, 2012). Although large Ceres-like samples are absent from the meteorite collection, meteoritic data suggest delivery of such materials to the inner solar system within several first Ma. Chondrites and brecciated differentiated meteorites (e.g. ureilites, howardites) contain C-rich clasts and inclusions with often elevated D/H, 17O/16O, 18O/16O, 15 N/14N and 54Cr/52Cr ratios that suggest outer solar system sources (Zolensky et al., 2018b; Chan et al., 2018; Goodrich et al., 2019; Kebukawa et al., 2019; Patzek et al., 2019). An absence of chondrules and Ca-Al rich inclusions (CAIs) in carbonaceous clasts agrees with the outer solar system sources. Some clasts could have formed within a few first Ma through collisions of Ceres, Ceres-like bodies and/or D/P type asteroids (Chan et al., 2018; Kebukawa et al., 2019) during their migration to the main asteroid belt. The presence of phyllosilicates, magnetite and carbonates in carbonaceous clasts suggests aqueous alteration of corresponding bodies by the time of their disruption. Ceres and Ceres-like asteroids (Rivkin et al., 2019) could have migrated in the framework of Grand Tack model (Fig. S10). The age of CB chondrites (4.8 ± 0.3 Ma after CAIs; Bollard et al., 2015) formed through a high-speed collision could mark the time of the Grand Tack (Johnson et al., 2016). A disruption of a parent body of ureilites 4–6 Ma after CAIs could be related to the Grand Tack (Scott et al., 2018). In an alternative scenario, Ceres could have formed in the transNeptunian region (McKinnon, 2008, 2012), possibly in the inner part of planetesimal disk just beyond the a orbit of Neptune at ~ 15 AU. The inner part of the disk could have harbored other large, high-density and compositionally distinct KBO's (Morbidelli et al., 2008). Some ices could have been lost from those KBO's because of collisions (Barr and Schwamb, 2016) that could have been more frequent and higher-velocity than in peripheral parts of the disk. In this scenario (McKinnon, 2008), Ceres was ejected to the main asteroid belt in the framework of Nice body migration model. The Nice model (Tsiganis et al., 2005; Gomes et al., 2005; Levison et al., 2009) suggests scattering of the transNeptunian disk of planetesimals with KBO's and Jovian-family comets from ~ 15–30 AU to higher and lower radial distances within ~ 350–1100 Ma after formation of the solar system due to an outward migration of outer three giant planets. This last scenario implies Ceres' implantation into existing low-mass asteroid belt and a decreasing of Ceres' orbital eccentricity may not be easily explained (McKinnon, 2008), especially in the light of skepticism about the Nice model and the interpretation of lunar heavy bombardment by the accretion tail model (Morbidelli et al., 2018). A massive inward migration of outer solar system bodies at ~ 4 Ga is inconsistent with the absence of chemical signatures of comets in lunar samples (Kring and Cohen, 2002; Galenas et al., 2011). Body dynamic models show that migration of a Ceres-size body from the trans-Neptunian region to the outer asteroid belt (>2.6 AU) is much less statistically feasible than a transfer of Jovian Trojans and D/P asteroids (Levison et al., 2009; Morbidelli et al., 2015; Vokrouhlický et al., 2016). It follows that Ceres' formation among past orbits of giant planets at ~ 4–15 AU followed by inward scattering together with C-type asteroids is a feasible scenario. Body dynamics and accretion models, isotope dating of chondrites, degree of alteration and thermal evolution models of chondrite parent bodies suggest later accretion of planetesimals at greater radial distances (e.g. Krot et al., 2009, 2015; Johansen et al., 2015; Desch et al., 2018; Scott et al., 2018). Isotopic dating of carbonates in CI/CM chondrites (Fujiya et al., 2013) implies accretion of their parent bodies 3.5–4.4 Ma after formation of CAIs. This dating sets a lower limit for Ceres' formation at greater radial distances than CI/CM chondrites. Ceres' density, gravity and shape (Park et al., 2016; Mao and McKinnon, 2018; this work) do not imply major metamorphic dehydration of

phyllosilicates and graphitization of organic matter in the interior. A mild interior heating (< 300–400 °C) suggests Ceres' accretion from 26 Al-poor planetesimals at >5–6 Ma after formation of CAIs. The lack of major dehydration also implies below CI-chondritic abundances of long-living radionuclides (e.g. <0.546 wt% K, Table 2), as one can infer from thermal evolution models (Castillo-Rogez and McCord, 2010; Castillo-Rogez, 2011). It follows that Ceres' composition and properties are consistent with a more distant and later formation than CI chondrites. 5.2. Amounts of accreted, consumed and lost water An initial amount of Ceres' water is a major unknown value. On the one hand, the composition of surface materials at low latitudes (Prettyman et al., 2019; Marchi et al., 2019) and interior models (Sections 2, 3) do not imply more water than is present in H-bearing minerals of CI chondrites. On the other hand, surface mineralogy (De Sanctis et al., 2015, 2018; McCord and Zambon, 2019) suggests pervasive aqueous alteration (McSween et al., 2018) at water-rich conditions. Comets and other outer solar system bodies are rich in both C and water ice, and an elevated C content on Ceres may suggest an above CIchondritic amount of accreted water. CI-IOM mixing models suggest 12+0.8 0.4 wt% of water-equivalent H in the inorganic fraction of Ceres (Table 7, Fig. 16). This value represents a fraction of accreted water that was consumed through aqueous hydration of inorganic nebular condensates. As in carbonaceous chondrites (e.g. Brearley, 2006; Zolensky et al., 2018a; Alexander, 2019), hydration of amorphous and crystalline Mg-rich silicates and Fe sulfides likely led to formation of serpentine, saponite, cronstedtite and tochilinite. Net reaction MgSiO3 (enstatite) + Mg2SiO4 (forsterite) + 2H2 O(l)

Mg3Si2 O5 (OH)4 (chrysotile)

(3) Table 7 Present and past water balance on Ceres. 8 wt% C 12 wt% C 14 wt% C Present H and water balance (CI-IOM mixture) IOM, wt% H, wt% H fraction associated with rock, % Water equivalent H associated with rock, wt% H2O-free rock, wt% H2O-free rock/IOM mass ratio

6.87 1.72 83.8 12.9 80.2 11.7

13.0 1.89 71.9 12.1 74.9 5.76

16.0 1.95 66.7 11.6 72.4 4.53

Water consumed in aqueous alteration H2O spent for oxidation of Fe0, wt% 3.7 H2O spent for oxidation of Fe0 and hydration, wt% 16.6

3.5 15.6

3.4 15.0

Ceres formed as a KBO (rock-IOM-ice mixture) Initial density = 1700 kg m−3 (Charon) Rock, wt% IOM, wt% Ice, wt%

58 5 37

56 10 34

55 12 32

Initial density = 1842 kg m−3 (Pluto-Charon) Rock, wt% IOM, wt% Ice, wt%

63 5 31

61 10 28

60 13 26

Initial density = 2060 kg m−3 (Triton) Rock, wt% IOM, wt% Ice, wt%

70 6 24

68 12 20

67 15 18

Note: The data for present Ceres copy and amend data from Table 3. The amount of water consumed through oxidation and hydration corresponds to complete Fe0-metal oxidation (Eq. (4)) and to water equivalent H associated with current surface rocks. The composition of Ceres formed as a KBO is constrained by C content, bulk density, and density of rock and organic constituents (Supplementary data).

15

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40 1700 k g

H2O, wt.%

Nebular condensates

formed as a KBO with initial density of 1700–2000 kg m−3, 8–14 wt% C and [dry rock]/IOM mass ratio of 4.5–11.7 correspond to 18–37 wt% ice in the body (Table 7, Fig. 16). At 12 wt% C in bulk Ceres and primordial density of Pluto-Charon binary, an initial KBO-like Ceres could have contained 28 wt% ice. Mass balance calculations show that the amount of water ice in lowtemperature nebular condensates depends on CO/CH4 gas ratio and on C fraction associated with non-volatile organic matter (Wong et al., 2008). Depending on these factors, condensates could contain 26–53 wt % ice and have density of 1450–2130 kg m−3 (Johnson and Estrada, 2009; Castillo-Rogez et al., 2012). Condensates from a CO-rich solar nebula are depleted in water ice, enriched in non-volatile C-rich matter and have higher density. Low-pressure and low-temperature conditions in an initial trans-Neptunian region do not suggest newly-formed methane (Lewis and Prinn, 1980; Prinn and Fegley Jr., 1989). The composition of carbonaceous chondrites (Alexander, 2019) and comets (Mumma and Charnley, 2011) indicates a high (CO + CO2)/CH4 gas ratio in their formation regions. Therefore, condensation from a CO-rich nebula could have contributed to the apparent water deficiency of highdensity KBO's (Wong et al., 2008). The dust-to-gas ratio in coma of comet 67P suggests 14–33 wt% water ice in the nucleus (Davidsson et al., 2016), consistent with a CO-rich nebula. The abundance of ice in 67P could be less than the value inferred for mineral dust-ice mixtures accreted on parent bodies of carbonaceous chondrites (32 ± 6 wt%, Alexander, 2019). If Ceres formed from water-depleted condensates of CO-rich nebula, a majority of accreted water could have been consumed through hydration and oxidation of inorganic (Eqs. (3), (4)) and organic compounds (Section 5.4). This scenario may not require water-rock differentiation and formation of water shell, especially if a fraction of ice has been lost through collisions and accretion of planetesimals and/ or if Ceres accreted less outer system volatiles than comets (Fig. 4).

-3

m

1800 kg

-3

m

-3

30

m 19 0 0 k g -3 m 2000 kg

20

Hydration & oxidation

Ceres at present

Hydration

10 2

4

6

8

10

12

14

Unhydrous rock/IOM mass ratio Fig. 16. Present and past water abundance on Ceres. The ‘hydration’ curve represents water equivalent H associated with H-bearing minerals on current Ceres (Table 7). The ‘hydration and oxidation’ curve corresponds to hydration (e.g. Eq. (3)) and water consumption through oxidation of Fe0 metal to magnetite (Eq. (4)). The vertical dotted lines show the range of [dry rock]/IOM ratio in initial Ceres' materials and the dashed line stands for that ratio at 12 wt % C (Table 7). Four upper curves correspond to water ice content in the rockIOM-ice system that represents solar nebular condensates with depicted density. The range of density corresponds to density of Pluto, Charon and Triton that could consist of such condensates. The used densities of anhydrous and reduced rocks, IOM and ice are, 3900 kg m−3, 1300 kg m−3 and 920 kg m−3, respectively. The triangle symbol corresponds to density of the Pluto-Charon system of 1842 kg m−3. The plot constrains the amount of water that could have been lost from Ceres or Ceres-forming planetesimals if the body formed in the transNeptunian region together with large KBO's.

5.3. Provenance of aqueous alteration and water removal

illustrates the formation of Mg-rich serpentine through hydration of Mg-rich silicates. Some additional liquid water could have been consumed through hydrolysis of accreted C oxides, refractory organic matter and other organic compounds (Section 5.4). Another major fraction of accreted water could have been consumed in oxidation of FeNi metal (kamacite)

3Feo (in kamacite) + 4H2 O(l)

Fe3 O4 (magnetite) + 4H2

Another unknown is whether aqueous alteration happened on planetesimals or after Ceres' accretion. On the one hand, a common occurrence of compositionally variable clasts in carbonaceous chondrites suggests re-accretion of materials from preexisting aqueously altered planetesimals. A greater contribution from altered planetesimals is expected for later-formed and larger bodies such as Ceres. On the other hand, small icy and organic-rich planetesimals formed after parent bodies of carbonaceous chondrites could not have sufficient 26Al and long-living radionuclides (due to dilution by ices and organics) to cause ice melting. Comets do not reveal clear evidence for in situ aqueous processes. Accretion models do not exclude formation of even Ceressized bodies from small (
(4)

(Krot et al., 1998; Alexander et al., 2010; Zolotov, 2012) and other phases (sulfides, phosphides, etc.). An elevated Mg/Fe ratio in Ceres' phyllosilicates (Ammannito et al., 2016) suggests rigorous oxidation of Fe0 to magnetite (McSween et al., 2018) and less abundant Fe2+bearing silicates. As on parent bodies of carbonaceous chondrites (e.g. Wilson et al., 1999; Alexander et al., 2010; Zolotov, 2012; Alexander, 2019), a major fraction of Ceres' H2 could have been lost to space, especially in a case of upward migration of fluids (Zolotov and Mironenko, 2015). The evaluated abundance of Ceres' H2O consumed in reaction (4) (3.4–3.7 wt%) reflects assumed accretion of Fe in kamacite and troilite (FeS) (Tables 7, S7). The sum of water consumed through hydration and Fe0 oxidation constrains a lower limit of accreted water (15–17 wt%) on Ceres consisted of chondrite-IOM mixtures (Table 7, Fig. 16). Another approach to constrain the initial water content is to interpret density of large low-porosity trans-Neptunian objects. Density of the Pluto-Charon binary (1842 kg m−3) could represent a typical transNeptunian nebular condensate, while high-density bodies (Eris, 2520 kg m−3, Haumea, 2600 kg m−3) may reflect ice removal by collisions (Barr and Schwamb, 2016). We have evaluated mass fractions in the rock-organic-ice system at the specified density of 1700–2000 kg m−3 that characterizes Pluto, Charon and Triton (Table 7, Fig. 16, Supplementary data). The rock is represented by reduced (Fe0-metal rich) and anhydrous condensate of solar composition and IOM (Table 2) is the proxy for refractory organic matter. If Ceres 16

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Unaltered rock-organicice mixture

Unaltered rock-organicice mixture

Water ice <15-17 wt.%

Water ice >15-17 wt.%

that observed on Ceres (Marchi et al., 2016). One possible explanation of Ceres' crater statistics is endogenic resurfacing (Marchi et al., 2016) at >1 Ga after the collisional stripping during first Ma. A possibility of late endogenic resurfacing through cryovolcanism and/or organic volcanism remains to be validated by thermal evolution models that include unabundant radionuclides (Table 3) and a low thermal conductivity of chondrite-organic mixtures. 5.4. Fate of organic matter: surface and interior

Sublimation

Ceres' typical near infrared spectra do not reveal organic compounds (De Sanctis et al., 2015) likely due to graphitization and amorphization of high molecular weight organics by space weathering (Hendrix et al., 2016; Marchi et al., 2019). One exception is aliphatic organic matter at the Ernutet crater region (De Sanctis et al., 2017, 2019; Pieters et al., 2018; Kaplan et al., 2018). The organic matter is more abundant than chondritic IOM and is spectrally similar to asphaltites, kerogen and IOM. Near infrared spectra of the organic material suggest abundant N-H bearing groups and a low O abundance (De Sanctis et al., 2019). It is unclear if the organic matter at Ernutet reflects a presence of exogenic (cometary?), pristine or altered endogenic material. The patchy appearance of the organic matter without a correlation with geology, the lack of obvious excavation features or vents and the presence of small fresh craters associated with the organic matter suggest exogenic sources (Pieters et al., 2018; Schröder et al., 2017). Some spatial correlations of aliphatic spectral features with signs of carbonate and ammoniated species (De Sanctis et al., 2017, 2019; Raponi et al., 2019a) do not exclude exogenic, likely cometary, sources. Carbonate spectral features are reported in impact-ejected dust of comet 9P/Tempel (Lisse et al., 2006), Na-C-O associations are notable in impact-ionized CHON particles of comet 1P/Halley (Clark and Mason, 2019), dust of comet P67 reveals a strong Na-C correlation (Stenzel et al., 2018) and comet P67 nucleus shows spectroscopic features of ammoniated compounds (Filacchione et al., 2019). A deficiency of O in Ernutet aliphatic signatures may reflect compositional heterogeneity within (Clark and Mason, 2019) and among (Mumma and Charnley, 2011; Brownlee, 2014; Levasseur-Regourd et al., 2018) cometary bodies. It follows that the organic‑carbonate association at Ernutet could reflect transformation of exogenic cometary materials through impacts and/or space weathering. The organic-rich locations at Ernutet have a prominent red slope (Schröder et al., 2017; Raponi et al., 2019a, 2019b) typical for space weathered organic matter. A similar slope is observed in the main highalbedo Na‑carbonate deposit in Occator crater (Cerealia Facula) (Schröder et al., 2017; Nathues et al., 2019). If organics in Cerealia Facula is masked by carbonate spectral features at 3.3–3.4 μm, red slopes at Occator and Ernutet may indicate geologically recent emplacements of organic matter. In the case of Cerealia Facula, organic compounds could have been deposited from plumes of icy grains formed through subsurface boiling of post-impact aqueous solutions (Ruesch et al., 2019; Zolotov, 2017). A similarity of refractory organic matter in cometary dust, anhydrous and chondritic porous IDPs, and carbonaceous chondrites (Alexander, 2011; Alexander et al., 2017) suggests accretion of alike materials on Ceres. In carbonaceous chondrites, the compositional diversity of IOM reflects temperature of parent body alteration rather than initial composition (Alexander et al., 2017). As in chondrites and comets, Ceres' initial refractory organic matter could have had an elevated aliphatic/aromatic ratio (atomic H/C > 0.7–0.8). As in carbonaceous chondrites and comets, Ceres' volatile (e.g. CH4, BockeléeMorvan et al., 2004; Mumma and Charnley, 2011) and solvent soluble organic species (hydrocarbons, carboxylic and amino acids, alcohols, amines, etc.) could compose <20–30 wt% of organic C inventory (Septhon, 2014; Alexander et al., 2017). As on parent bodies of chondrites (Alexander et al., 2007, 2010, 2017), a majority of Ceres' organic matter could not have formed

Water (ice) shell Unaltered rock-organic-ice Stripping

Aqueously altered rock-organic mixture

Aqueously altered rock-organic mixture

Fig. 17. Scenarios of geological history of Ceres that accreted different amounts of water ice. In both cases, accretion of rock-organic-ice mixtures (upper panels) was followed by aqueous alteration and collisional stripping of upper interior that exposed aqueously altered materials. In the water-rich scenario (right hand side panels), water shell formed through upward migration of solutions with water-soluble salts and organic compounds. Before collisional stripping, salts and organic compounds could have formed lag deposits through sublimation of ice.

solutions deposited Ca/Mg carbonates in the upper part of the silicate core while Na+-Cl−–HCO3−–CO32– solutions with dissolved organic compounds accumulated in water shell. A deficiency of Ni in surface materials (Prettyman et al., 2017) could reflect leaching by upwelling hydrothermal fluids (Zolotov and Mironenko, 2015). Subsequent loss of icy shell through sublimation could have led to widespread salt lag deposits that are not seen on the current low-albedo surface, however. Salt-rich lag deposits slowed sublimation (e.g. Fanale and Salvail, 1989) and collisional stripping of ice could have been more efficient than sublimation. Collisional stripping of putative icy shell and salt lag deposits is more consistent with the low surface albedo (~0.09) than sublimation. The widespread surface phyllosilicates and Mg/Ca carbonates (De Sanctis et al., 2015) suggest stripping of a rocky envelope because their formation requires liquid water that has never been stable in the vicinity of Ceres' surface. A deficiency of large impact craters (Marchi et al., 2016) could reflect collision(s) that removed an upper rocky interior and erased the crater record. Fig. 17 illustrates collisional stripping of upper interior that contained altered and pristine (past surface) rocks with or without volatile-rich shell of water ice, high-solubility salts and organic compounds. The apparent stripping of rocky upper interior could have occurred together or after less evident removal of a volatile-rich shell through sublimation, impacts and/or major collisions. The exposed aqueous mineralogy and the lack of Ceres' family suggest stripping collisions after aqueous alteration but before or during migration (Section 5.1) to the main asteroid belt. Both the Grand Tack model (Walsh et al., 2011; Raymond and Morbidelli, 2014) and a rapid late growth of giant planets (Raymond and Izidoro, 2017) imply high eccentricities of gravitationally destabilized bodies. Corresponding high-speed collisions led to disruption of planetesimals (Bollard et al., 2015; Scott et al., 2018) and an exposure of hydrated interiors of some C-type bodies. A Grand Tack collisional stripping scenario for Ceres suggests completion of aqueous alteration by 5–6 Ma after CAIs (Section 5.1). Collisions associated with late fast growths of giant planets (Raymond and Izidoro, 2017) suggest comparable time scales. The supposed early stripping of upper interior needs to be reconciled with dynamical evolution of the asteroid belt (Morbidelli et al., 2015) that implies more large craters 17

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through in situ synthesis from inorganic precursors. Experimental data do not suggest an efficient formation of methane and light hydrocarbons from inorganic C species and H2 (McCollom, 2013, 2016). A formation of formate (HCOO−) and methanol (CH3OH) via hydrolysis of dissolved (aq) CO and reduction of carbonate/bicarbonate ions (Seewald et al., 2006; Zolotov, 2012) by abundant H2 (Eq. (4)) could be exceptions,

CO, aq + H2 O(l)

HCO3– + H2 , aq HCOO–

+

H+

HCOOH (formic acid)

HCOO– + H2 O(l)

+ 2H2, aq

CH3OH + H2 O(l).

HCOO– + H+

Zolotov, 2017). By analogy with terrestrial bitumen (asphalt) and bituminous shales/sands, Ceres' high molecular weight organic matter could not have been mobilized to form lenses or diapirs, especially at low gravity. In contrast, low molecular weight organic species and water-soluble organic compounds could have accumulated in upper interior together with salt-bearing water. After stripping of an upper interior (Fig. 17), a fraction of remaining low molecular weight organic matter (‘oil’) could have accumulated in a sub-crustal layer and/or crustal niches with appropriate temperature, porosity and permeability. Such a low-viscosity organic matter could have contributed to viscous relaxation at long wavelengths and to formation of some surface morphological features (Section 4).

(5) (6) (7)

Aqueous alteration and thermal metamorphism on parent bodies of carbonaceous chondrites led to aromatization of IOM and decreasing of H/C, N/C and O/C ratios in it (Cody and Alexander, 2005; Alexander et al., 2007, 2017; Septhon, 2014; Bonal et al., 2016). Methane, aliphatic hydrocarbons, H2 and N-bearing organic compounds are likely products of these processes. Radioactive decay heating of deep Ceres' interior up to 300–400 °C suggests analogues transformations of IOM without graphitization. However, the elevated lithostatic pressure, abundant liquid water (Table 7) in pore spaces and a low permeability of clay/organic materials (Bland et al., 2009) limited separation, diffusion and escape of H2 and CH4 from Ceres (cf. Wilson et al., 1999), and possibly limited aromatization of IOM. Elevated H2 fugacity (Castillo-Rogez et al., 2018) favored stability and formation of aliphatic functional groups and compounds through hydrogenation of organic precursors (Fig. S11). Another possible process is organic-water interaction. Experimental hydrous pyrolysis of chondritic IOM reveals formation of low-molecular weight compounds (Sephton et al., 2000). On Earth, a ‘hydrolytic disproportionation of organic matter’ via illustrative reaction C128 H68O7 (kerogen) + 107. 5H2 O(l)

6. Concluding remarks Ceres' phase and elemental composition together with density and porosity distribution in the interior are estimated in the framework of a compacted, chemically uniform and organic-rich body. The composition and porosity are calculated to fit the elemental composition (C, H and Fe) of surface materials and Ceres' bulk density. Assuming hydrostatic equilibrium, vertical density and porosity gradients are estimated to fit either Ceres' gravity or shape (polar flattening) data. The interior composition is best modeled by a mixture of aqueously altered CIchondrite like rocks with C-H-O-N-S macromolecular organic matter. The estimated organic fraction (7–16 wt%, 12–29 vol%) and C content in the rock-organic mixture are more than two times higher than those in CI chondrites but lower than in comets. Ceres' surface materials are enriched in C and H and depleted in Fe compared to CI chondrites (Prettyman et al., 2017, 2019). Surface chemical data are consistent with a rock-organic mixture in which rock has the composition of CI chondrites and organic matter is similar to chondritic IOM (Marchi et al., 2019; this work). We assumed a surfacelike composition of the interior and constrained the parameter space for grain density of end member rocks, a fraction of IOM and bulk porosity. The modeling shows that elevated porosity (>10%), ice or gas hydrates are not needed to explain density of organic-rich Ceres. A decent fit is achieved for rock-IOM mixtures characterized by CI-like rocks with grain density of 2411–2623 kg m−3, rock-IOM mixture density of 2206–2276 kg m−3, 11–15 wt% IOM, 18–26 vol% IOM, 10.7–13.3 wt% C and 2–5% porosity (Table 4). The latter model constrains elemental composition of the body, including concentrations of long-living radionuclides (Table 3). Mixtures of CM chondrites with IOM are inconsistent with the surface composition and bulk density of Ceres. Based on Dawn gravity data, Park et al. (2016) stated that Ceres is partially differentiated. In planetary sciences, differentiation of a body implies a global separation of compounds according to their physical and/or chemical properties. In the case of Ceres, differentiation means water-rock separation. Our work shows that Ceres' gravity and especially shape data could be explained without water-rock separation in the current interior structure. Although gravity and shape data suggest different density distributions in the interior (Mao and McKinnon, 2018; this work), the apparent concentration of mass toward the center could be explained by compaction of chemically homogeneous solids. The current interior may not be differentiated, even partially. Gravity data could be explained in the framework of two-layer interior structure with thickness of upper low-density layer of <120 km (Park et al., 2016; Ermakov et al., 2017; Mao and McKinnon, 2018; this work). Our work shows that gravity is inconsistent with a gradual density increase toward the body's center. Gravity-consistent two-layer structures suggest abrupt density changes at the upper layer - core boundary and imply upper layer density of 1200–1900 kg m−3, in agreement with Ermakov et al. (2017) and Mao and McKinnon (2018). Gravity-consistent models do not require ice or gas hydrates in the upper interior but occurrences of these low-density phases will agree with gravity data. In our models, the apparent low density of the upper

70.75CH 4 (for aliphatic compounds) + 57.25CO2

(8) could be responsible for the formation of natural gas and oil from lignine and kerogen in sedimentary basins at ~150–200 °C and < 150 MPa (Seewald et al., 1998; Price and DeWitt, 2001; Helgeson et al., 2009). In addition to low molecular weight aliphatic hydrocarbons, reaction (8) could produce kerogens with elevated H/C ratios. Temperature-pressure conditions and the rock/organic ratio in Ceres' interior (Table 3) could be similar to those in sedimentary basins. A fraction of Ceres' light aliphatic hydrocarbons, IOM with elevated H/ C ratio and carbonates could have formed through disproportionation of C in accreted IOM-like polymers (Eq. (8)). If Ernutet organic matter is endogenic (De Sanctis et al., 2019), its composition is consistent with suppressed aromatization of accreted organic matter in H2-rich and low-temperature interior, and with hydrolytic disproportionation of C. However, C isotopic composition of alkanes in Murchison CM2 chondrite is inconsistent with their thermogenic formation from high molecular weight compounds (Yuen et al., 1984). On Earth and metamorphosed carbonaceous chondrites, sulfate-organic reactions produce sulfides and carbonates. These reactions are irrelevant to Ceres if it formed away from the Sun without irradiated ices with trapped oxidants, though a sign of O2 at comet P67 (Bieler et al., 2015) does not exclude accretion of strong oxidants on transNeptunian objects. Accretion of irradiated ices with oxidants on Ceres could have caused aromatization of IOM (Cody and Alexander, 2005), formation of carbonates from organic compounds and oxidation of sulfides to sulfates (Zolotov, 2016). In such a case, the lack of abundant sulfates on the surface could reflect leaching of high-solubility Mg/Na sulfates with upwelling fluids followed by collisional stripping (Fig. 17). Remaining low-solubility Ca sulfates could have been reduced by organic matter at temperature above 150–200 °C but below the temperature of NH4+ release from NH4-bearing clay minerals (270–285 °C, Bishop et al., 2002). This hypothetic scenario implies organic-sulfate reactions that produced some of Ceres' carbonates and sulfides (cf. 18

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layer is accounted for by >36% porosity of carbonaceous materials disrupted by impacts and poorly compacted at low gravity. In chemically homogeneous Ceres, gravity data imply bulk porosity (>9%) and grain density (>2380 kg m−3) that are inconsistent with properties of rock-organic mixtures (Tables 3, 4) inferred from the composition of surface materials and bulk density. It follows that Ceres' gravity (J2) could reflect deep uncompensated density anomalies (Mao and McKinnon, 2018; Ermakov et al., 2017). Shape-consistent interior structures are more probable than gravityconsistent models. Polar flattening suggests mild density gradients and does not exclude an almost physically homogeneous body (Fig. 1). For chemically homogeneous Ceres, shape data at 1.8–6% porosity suggest grain density of ~ 2200–2300 kg m−3 that agrees with an organic-rich (20–30 vol% IOM) interior implied from the surface composition. The flattening could be explained by two-layer, gradual compaction or combined hydrostatic models that do not exclude a few % porosity in deep interior. If porosity collapses at 10–20 MPa, as in analog materials, the upper layer of two-layer interior structure has 20–40 km thickness, 9–20% porosity and 1900–2000 kg m−3 density (Table 5). The observed relaxation of topography at long wavelength (Fu et al., 2017) could be explained by decreasing viscosity of rock-organic mixtures with increasing temperature at depth. No abundant water ice, gas hydrates or hydrated salts are required to explain viscosity and density of the upper interior on organic-rich Ceres. Low-viscosity surface features (flows, domes) could reflect the presence of abundant organic matter. Although there is no strong observational evidence for abundant ice/gas hydrates in the upper (20–100 km) interior layer, our models do not exclude some (<5–10 vol%) ground ice. Therefore, there is no strong contradiction between surface observations (e.g. Prettyman et al., 2017; Combe et al., 2019) and ice-less models for the interior structure. Mild density gradients in the interior do not indicate thermal dehydration of minerals and suggest a late (> 5 Ma after CAIs) formation of Ceres from materials depleted in short-living 26Al and long-living radionuclides of K, U and Th. Dilution of rocks by organic matter contributed to below CI-chondritic concentrations of rock-forming elements and radionuclides. Chemical and phase composition of surface materials and bulk density suggest a formation of Ceres beyond a past orbit of Jupiter and beyond a formation region of CI chondrites. The likely accretion of more than CI-chondritic amounts of refractory organic matter, CO2/CO ices and NH3 hydrate suggests accretion of above CI-chondritic amounts of water ice. If excessive water has been accreted it could have left the body. Post-accretional ice melting led to water consumption through hydration and oxidation of inorganic and organic compounds. Oxidation led to H2 that cannot be stored in the interior. Excessive water solutions migrated toward the surface followed by lowpressure boiling, degassing of low-solubility compounds (H2, methane, etc.), freezing and possible formation of icy envelope that has been removed by impacts and/or sublimation. The presence of H-bearing minerals at the surface and the deficiency of large craters suggest collisional stripping of the body's upper interior. The stripping could have affected aqueously altered materials, unaltered primordial crust, putative water shell and corresponding salt/organic lag deposits. A major stripping could have occured at the time of Ceres' migration to the main asteroud belt, possibly together with other C-type bodies at 5_6 Ma after formarion of CAIs.

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