Stability of hydrated carbonates on Ceres

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Stability of hydrated carbonates on Ceres C. Bu a,∗, G. Rodriguez Lopez a, C.A. Dukes a, L.A. McFadden b, J-Y. Li c, O. Ruesch b a

University of Virginia, Charlottesville, VA 22904, United States NASA Goddard, Greenbelt, MD 20771, United States c Planetary Science Institute, Tucson, AZ 85719, United States b

a r t i c l e

i n f o

Article history: Received 15 September 2017 Revised 11 December 2017 Accepted 22 December 2017 Available online xxx Keywords: Ceres Dehydration Uv–visible spectroscopy Carbonates X-ray diffraction

a b s t r a c t Deposits of carbonates have been observed and definitively identified by Dawn’s Visible Near-Infrared Mapping Spectrometer (VIR), particularly in the faculae that lie within the central portion of Occator, Oxo, and Haulani craters, implying geologically recent cryo-volcanism or extrusion with sub-surface CO2 and H2 O. Carbonate composition varies from primarily sodium at the Cerealia and Vinalia Faculae and at Oxo crater, where carbonate deposits are most abundant, to magnesium and calcium for most other bright regions. The formation of hydrated salts is expected from the aqueous alteration of silicates; however, VIR measurements of the faculae show no water signature, potentially the result of dehydration after exposure to Ceres’ surface conditions. We investigate the stability and decomposition pathway for hydrated sodium-carbonate, natron (Na2 CO3 . 10H2 O), grains in the laboratory under Ceres’ cryogenic, lowpressure environment by UV–vis–NIR reflectance spectroscopy and X-ray powder diffraction. H2 O-loss begins simultaneously with vacuum-exposure, altering natron’s spectral signature by attenuation of the water bands, enhancement of the carbonate features, and concurrent reduction of the NIR blue spectral slope. We find that the water absorption features in natron reduce below VIR’s detection limit (< 2%) within a time scale of < 6 days at temperatures ≥ 200 K, eliminating hydrous sodium carbonate from Ceres’ surface mineralogy in the equatorial region and the mid-latitudes without continuous rehydration. A temperature-dependent systematic shift of the 1.9-μm band center to lower wavelengths is observed with vacumm-exposure time at 200 and 240 K. In Ceres’ polar-regions (∼120 K), natron retains water longer, depleting the 1.9-μm water band to < 2% within a few hundred years (∼300 years). No significant changes in the visible relative reflectance or spectral slope result from vacuum-exposure of hydrous or anhydrous sodium carbonate, which does not match the observed red-slope in the Framing Camera (FC) measurements for Occator Crater’s faculae. In the UV, extended exposure to vacuum in all sodium carbonates examined here causes significant reddening due to an increase in short-range crystallographic defects and reduction of the conduction band energy; in addition, the development of new UV electronic transition features at ∼ 275 and 235 nm is observed in hydrous and anhydrous sodium-carbonates with vacuum-processing. Similar transitions in carbonates and organics may contribute to the unidentified 280 nm absorption feature on Ceres. © 2017 Elsevier Inc. All rights reserved.

1. Introduction The aqueous alteration of volatile-containing silicates, with elemental compositions similar to carbonaceous meteorites, is expected to form a viscous brine below dwarf planet Ceres’ solid crust. Hydrated salts from this icy reservoir are deposited on the dwarf-planet surface by extrusion through vents and fissures or co-ejected by jets of sublimating sub-surface water ice (CastilloRogez & McCord, 2010; Neveu & Steven, 2015; McCord et al., 2017; Zolotov, 2017; Ruesch et al., 2017 (this issue)). Alkaline

∗

Corresponding author. E-mail address: [email protected] (C. Bu).

conditions are implied by the presence of ammoniated phyllosilicates (De Sanctis et al., 2015) and encourage the formation of carbonates (Rivkin et al., 2006), as well as brucite (Milliken and Rivkin, 2009), beneath the Cerean surface (Zolotov, 2016). Thick (∼400 m) deposits of sodium carbonate have been definitively identified by Dawn’s Visible Near-Infrared Mapping Spectrometer (VIR), particularly in the bright, central portion of Occator crater in Ceres’ northern hemisphere (20°N, 239°E), where the carbonate signature is strongest (De Sanctis et al., 2016; Nathues et al., 2017). Calcite and magnesite/dolomite have also been identified over much of the Cerean landscape, as well as within the majority of impact-crater-associated bright regions, generally mixed with the global phyllosilicates (De Sanctis et al., 2015; Ammannito et al., 2016; Palomba et al., 2017a (this issue); Stein et al., 2017 (this

https://doi.org/10.1016/j.icarus.2017.12.036 0019-1035/© 2017 Elsevier Inc. All rights reserved.

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issue); Raponi et al., 2017 (this issue)). Less heterogeneity occurs in Ceres’ sodium carbonate deposits – near-homogeneous deposits are observed at Cerealia and Vinalia Faculae, as well as at Ahuna Mons (10°S 316°E) and Oxo crater (43°N 0°E) (Ruesch et al., 2016; Zambon et al., 2017); in other locations, including Ezinu (61°N 221°E), Kupani (41°S, 171°E), and Hualani craters (17°N 135°E) (Palomba et al., 2017a (this issue)), Na-carbonates are mixed with minor amounts of Mg/Ca carbonate minerals. Sodium carbonates such as natron (Na2 CO3 ·10H2 O) and natrite (Na2 CO3 ) occur with some frequency as mineral evaporites and, more rarely, in volcanic material on Earth. However, they are particularly unusual on airless bodies, having only previously been detected on Europa (McCord et al. 1998) and in the plumes of Enceladus along with NaCl and NaHCO3 (Postberg et al., 2011). Within the CO2 -rich atmosphere of Mars, where water was abundant, sodium carbonate is expected, but not yet identified in the panoply of carbonate phases seen by thermal emission, optical reflectance, and identified via scanning calorimetry across the planetary surface, as well as in the ubiquitous Martian dust (e.g., Bandfield et al., 2003; Boynton et al., 2009; Ehlmann et al., 2008; Ming et al., 2008). Indeed, laboratory analysis of Martian meteorites show aqueous weathering products which include endogenous deposits of carbonates such as calcite, dolomite, and siderite, although natrite remains elusive (Niles et al., 2013). Similarly, magnesite and siderite are inferred within the emission spectra and dust of comets 1P/Halley and 9P/Tempel 1 (Fomenkova et al 1992; Lisse et al. 2006) and in carbonaceous meteorites (CM, CI, Tagish Lake), along with calcite, suggesting the presence of water on asteroidal parent-bodies (e.g., Zolensky et al., 2002; de Leuw et al., 2010). Definitive remote observation of carbonates in the near-infrared (NIR) typically utilize the combination of absorption features at 3.9 μm (attributed to CO3 2- ν 1 + ν 3 stretch combination band) and at 3.4 μm (ν 3 overtone) in anhydrous materials, since overlap with O–H bands at these wavelengths is negligible. The relative intensities and shape of these two bands are then used to identify the chemistry of carbonate: natrite, magnesite, dolomite, calcite, etc. (Palomba et al., 2017a (this issue), Palomba et al., 2017b). Less intense CO3 2− absorptions may also be apparent near 2.3 μm (ν 1 + 2ν 3 ) and 2.5 μm (3ν 3 ) and can be differentiated from the 2.7–3.0 μm O–H stretch-overtone/combination in minimallyhydrated materials. The 3.4-μm and 3.9-μm absorptions in VIR spectra of Ceres confirm the presence of carbonates, and spectral models identify anhydrous sodium carbonate (natrite, Na2 CO3 ) as the best match for Occator crater’s bright material (De Sanctis et al., 2016; Palomba et al., 2017a (this issue)). Carbonates are expected to have formed in an endogenous, slushy brine below Ceres’ solid crust (Nathues et al., 2017; Park et al., 2016) by the interaction of subsurface water and/or ice with silicates in the presence of carbon dioxide. The resulting saline solution, composed of carbonate anions (CO3 2− ) formed in the CO2 -rich aqueous environment and positive ions (Na+ ; but also Mg+ , Ca+ , NH4 + , etc.) leached from native silicates by the mildly alkaline solution (pH ≈ 10), produces Na2 CO3 ·10H2 O, as well as NaHCO3 and other salts (Zolotov 2017). Hydrous precipitates are later driven to Ceres’ surface by high pressure or exposed by meteoritic impact. Measurement of high stability for hydrous salts at Ceres’ near-surface temperatures may indicate that the natrite at Occator was emplaced by extrusion, with its sluggish flow and extended vacuumexposure, rather than cryo-volcanism or post-impact ballistic deposition (Ruesch et al., 2017 (this issue); Stein et al., 2017 (this issue)). Based on thermodynamic models (Zolotov and Shock, 2001; Zolotov 2017), subsurface aqueous salts of natron and sodium bicarbonate, along with ammonium bicarbonate (NH4 HCO3 ) and ammonium chloride (NH4 Cl), should dehydrate at Ceres’ surface pres-

sure, even at cryogenic temperatures that lie between 240 and 160 K for the mid-latitudes and equatorial region (Formisano et al., 2016). The precise surface temperature is both a function of latitude and heliocentric distance, with minimal (< ± 15 K) day/night variation (Tosi et al., 2015; Formisano et al., 2016). Temperatures in Ceres’ polar and permanently shadowed regions are expected to be lower: 115–125 K (Formisano et al., 2016). The stability of hydrated carbonates depends strongly on temperature and pressure. Laboratory measurements at relatively high temperatures (300–365 K) imply the efficient, endothermic decomposition of natron (Na2 CO3 · 10H2 O) to thermonatrite (Na2 CO3 · 1H2 O) then to natrite (anhydrous Na2 CO3 ) through the release of molecular H2 O (Hartman et al., 2001a & 2001b). However, cryogenic measurements simulating environmental conditions on Europa at ∼100 K under vacuum (10−9 Torr) show that natron is quite stable on a low-temperature planetary surface (McCord et al., 2001). Using a first-order kinetic model based on thermal desorption data, McCord et al., (2001) extrapolated the lifetime for natron at low pressures to 108 years on the surface of Europa where temperatures remain below 130 K. Ceres surface temperature falls between these two thermal extremes. Thus, we determine the inherent stability and correlated spectral effects of emplaced hydrated and anhydrous sodium carbonates exposed to Ceres’ surface environment by measuring the temporal evolution of UV–vis–NIR spectra acquired under vacuum as a function of temperature. To our knowledge, no previous laboratory measurements of the isothermal kinetics of vacuum-dehydration for natron at cryogenic temperatures appear in the literature. Our objective is to measure the rate of dehydration for natron at Cereslike temperatures to elucidate the planetary deposition mechanism, determine dehydration pathways, and use the hydration extent or NIR spectral slope of this material to infer the age of observed hydrated carbonate deposits. 2. Experimental details Reagent-grade sodium carbonate decahydrate (Na2 CO3 ·10H2 O, natron), anhydrous sodium carbonate (Na2 CO3 , natrite) and sodium bicarbonate (NaHCO3 ) powders were purchased directly from chemical suppliers (Sigma Aldrich). Natron is not stable under ambient conditions (pressure≈1 atmosphere = 760 Torr; temperature = 296 ± 2 K); thus, received material was processed (ground and dry-sieved, and in some cases pressed) in a cold room maintained at 4 °C (277 K), transported in a cooler with dry-ice (195 K), and subsequently stored in a refrigerator at − 7 °C (266 K). Natron and bicarbonate powders were ground and dry-sieved to a grain size fraction of 45 − 83 μm, within the range of grain sizes indicated by Ceres’ albedo (De Sanctis et al., 2016). Optical microscopy shows irregularly shaped particles generally within the expected size range. Some smaller fragments, as well as elongated grains, are also present with no significant dust ( < 10 μm) observed on the primary grains. Natrite, as purchased, had a grain size fraction less than 45 μm, thus no grinding nor sieving was required. Due to its hygroscopic nature, natrite samples were prepared in a glove box under continuously flowing, dry nitrogen (N2 ) at room temperature (296 ± 2 K) and transported in a dry-N2 purged vessel with desiccant. Subsequent X-ray powder diffraction (XRD) analysis and spectral measurements confirmed that sample preparation caused no significant modification to sample composition or crystallographic structure. XRD spectra of the powder samples were acquired under ambient conditions using a Panalytical X’Pert Pro MPD Diffrac˚ which took ∼15 minutes for tometer with Cu Kα X-ray (λ = 1.54 A), each measurement. Crystallographic structure was determined by comparing the diffraction patterns to the PDF-4+ library database. X-ray fluorescence spectroscopy confirmed the purity of our materials, with trace level (< 3 ppm) impurities.

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Fig. 1. (a) Reflectance spectra are acquired using a Harrick Praying Mantis (PM) diffuse reflectance accessory (with specular rejection) seated inside the sample compartment of a dual-beam (orange-lines) Perkin Elmer Lambda-1050 UV/vis/NIR spectrometer. Powder samples are placed in a low-temperature reaction chamber (LT-RC) within the PM. The LT-RC is evacuated via a port under the sample with an oil-free turbo pump. The spectrometer, PM, and LT-RC are purged by dry nitrogen (N2 ) gas as appropriate and shielded from ambient light. (b) The optical path (orange-lines) is shown through the PM diffuse reflectance accessory. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Dehydration extent was quantified by reflectance spectra, covering a wavelength (λ) range of 220–2500 nm and acquired using a Perkin Elmer Lambda-1050 UV/Vis/NIR spectrometer, equipped with deuterium / halogen lamps and photomultiplier (175–860 nm)/InGaAs (860–2500 nm) detectors. This dual-beam instrument references the primary reflected spectral-signal to a secondary internal beam to eliminate environmental absorption features (e.g., H2 O, CO, CO2 , etc.), as well as changes in detector response or variation in lamp intensity. The incident light is monochromated and chopped, minimizing sample heating during spectral measurement. A Harrick Praying-Mantis (PM) diffuse-reflectance accessory with specular-component rejection was mounted in the spectrometer bench, and a lowtemperature reaction chamber (LT-RC) was used for cryogenic and low-pressure measurements (Fig. 1a, b). Samples were loaded into a temperature-controlled sample cup mounted in the LT-RC; the sample temperature was monitored by a thermocouple inserted into the powder. The LT-RC, with the sample inside, was evacuated to ∼10−3 –10−5 Torr (≈ 10−6 –10−8 Earth atmosphere), via a port below the sample using a Pfeiffer 60 l/s, dry turbo pump through a 10-μm grid. The LT-RC pressure was limited by the outgassing rate of the hydrated salts (Bu et al., 2017); thus, dehydration time is primarily a function of the diffusion of H2 O to the grain-surface/vacuum interface, providing a reasonable upper limit for Na2 CO3 . 10H2 O lifetime on Ceres’ surface. In-situ timesequenced spectra (constant resolution λ = 4 nm) were taken every five minutes. Spectral measurements were performed primarily at 240 (± 4), 200 (± 4) and 122 (± 1) K, simulating Ceres’ surface temperatures at the mid-latitudes and equatorial region, as well as in polar and permanently shadowed regions (Tosi et al., 2015; Formisano et al., 2016; Hayne and Aharonson, 2015; Schorghofer et al., 2016). For measurements at cryogenic temperatures, materials must be “flash-frozen” at the required temperatures prior to evacuation to retain hydration, as discussed in earlier studies (McCord et al., 20 01; Dalton, 20 03; Bu et al., 2017). Thus, the spectrometer, PM,

and LT-RC were all purged with dry-N2 during target cooling and sample loading to eliminate condensation of ambient water vapor onto the optics or samples. Relative reflectance spectra were normalized and corrected for instrumental error using the ratio of the data to the spectrum for a PTFE (polytetrafluorethylene) powder standard, taken under similar experimental conditions. Reference data was corrected for the minor 2.3-μm PTFE absorption feature by substitution with a continuous, polynomial curve. PTFE is a typical reference material for laboratory diffuse reflectance measurements at λ < 2.50 μm, due to its bright, featureless spectrum in this range and chemical stability. The relative reflectance of carbonates can exceed unity at a given wavelength if signal is brighter than the PTFE standard. Na2 CO3 bi-directional (i = 0°, e = 30°) reflectance spectra in the mid-IR region at cryogenic temperatures were taken using a Thermo-Nicolet 670 Fourier-transform infrared spectrometer (FTIR) with XT-KBr beam splitter and Mercury-Cadmium-Telluride (MCT-A) detector (1.5–16 μm), coupled to an external vacuum chamber (base pressure P ≈ 10−9 Torr) by KBr windows through N2 -purged optics. Vertically mounted, pressed powder samples of the grains ( < 45 μm grains for Na2 CO3 ) were cooled to 200 K in N2 within the vacuum chamber, then pumped to a sample-limited pressure of ∼10−6 Torr. Water vapor could also be leaked into the vacuum chamber to examine any spectral effects due to vapor deposition. Because PTFE has absorptions at wavelengths beyond 2.5 μm, a diffuse gold standard (Labsphere Infragold) was used in the infrared range. 3. Results 3.1. Visible–NIR spectral reflectance of hydrated sodium carbonates Currently, much of the available laboratory data for planetary minerals and salts used for spectral fitting and remote compositional identification has been acquired at room temperature under atmospheric pressure. However, these materials, particularly

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Fig. 2. For natron (Na2 CO3 ·10H2 O, 45–83 μm grains), the sample temperature influences the spectral signature. As the temperature increases, the loss of incorporated water molecules is accelerated, attenuating the water absorption features at ∼1.49 and 1.98 μm over the data acquisition period (∼5 min). Reflectance spectra for natron were acquired in dry-N2 atmosphere at 122, 240 and 296 K, referred to a powder PTFE standard, and scaled to one at λ = 0.55 μm. The x-axis gap, 0.820– 0.870 μm, contains instrumental artifacts induced by detector switching.

hydrous salts, are unstable at low-pressure (Wang et al., 2006; Bu et al., 2017) and/or have modified absorption features at lowtemperatures (e.g., Dalton et al., 2001; De Angelis et al., 2017; Bu et al., 2017). For natron with its ten incorporated H2 O molecules, this is illustrated in Fig. 2 by relative reflectance spectra (scaled to one at λ = 0.55 μm) acquired at 296, 240, and 122 K, at atmospheric pressure (∼760 Torr) in dry-N2 . Dehydration during the data acquisition, ∼5 minutes, is qualitatively demonstrated by the reduction of water bands at ∼1.49 and 1.98 μm, together with band shape changes, which proceeds more slowly with decreased temperature. Therefore, explicit remote identification of a mineral species requires the understanding of the chemical, physical and spectral modifications that occur as a consequence of the space environment. All hydrous salt spectra have dual, broad, asymmetric absorption-features at ∼1.49 and 1.98 μm from the incorporated molecular water, attributed to the first overtone of the O–H stretching fundamentals (aν 1 + bν 3 ; a + b = 2) and the superposition of the O–H stretching and the H–O–H bending fundamentals (aν 1 + ν 2 + bν 3 ; a + b = 1), respectively (Hunt and Ashley, 1979; Crowley , 1991; McCord et al., 2001; Dalton, 2003). Other relatively minor H2 O-related absorption features are the ∼1.2 μm combination-band for the stretching first-overtone and the bending-fundamental (aν 1 + ν 2 + bν 3 ; a + b = 2), and the ∼1.0 μm, a combination of the stretching (aν 1 + bν 3 ; a + b = 3) and bending overtones (aν 1 + ν 2 + bν 3 ; a + b = 3) (Hunt, 1979). The shape of these water absorption features depends on temperature, and particularly the one at ∼1.49 μm shows fine structure at low temperatures, attributed to degenerate vibrational energies of the incorporated water molecules at low temperatures (Dalton, 2003). The spectra for hydrated carbonates are relatively flat over the visible region (0.4–0.8 μm), while they have negative (blue) slopes in the NIR region. 3.2. Dehydration of natron (Na2 CO3 ·10H2 O) under Ceres-like conditions No water absorption features have been detected by Dawn’s VIR spectrometer at the Occator crater, where diagnostic carbonate absorption features are identified (De Sanctis et al., 2016). The paucity of H2 O is likely due to surface (or sub-surface) process-

ing, such as dehydration (e.g., Wang et al., 2006; Bu et al., 2017), impact-induced heating (Bowling et al., 2017 (this issue); Stein et al., 2017 (this issue)), or space weathering (e.g., Noble et al., 2007; Dukes et al., 2016; Stein et al., 2017 (this issue)), all of which result in the loss of the water bands. Fig. 3(a)–(c) shows the temporal evolution of the relative reflectance for natron grains (45–83 μm) exposed to low pressure (∼ 10−5 Torr) at 240 (± 3) K, 200 (± 3) K, and 122 (± 2) K. The attenuation of the water absorption features with increasing vacuum-exposure time (t), together with changes of the band shape, signifies the loss of incorporated water molecules. Spectral changes occur more slowly as the temperature decreases. An absorption feature at ∼2.35 μm, likely from the CO3 2- group but initially masked by the strong water absorption bands (∼3 and 1.98 μm) becomes more obvious as the dehydration continues at 240 and 200 K. The extent of the dehydration at 122 K within our experimental duration is insufficient to reliably deconvolve this carbonate feature from the absorption background.

3.3. Anhydrous Na2 CO3 and NaHCO3 under Ceres-like environmental conditions VIR measurements from Occator crater’s faculae show distinct absorptions at ∼3.4 and 3.9 μm, indicative of enrichments in carbonates (De Sanctis et al., 2016; Nathues et al., 2017; Palomba et al., 2017b), particularly anhydrous Na2 CO3 (De Sanctis et al., 2016; Palomba et al., 2017b), the expected end-product for both hydrated sodium carbonate (Na2 CO3 ·nH2 O, n > 0) and sodium bicarbonate (Zolotov 2017). We confirm the stability of natrite (anhydrous Na2 CO3 ) exposed to low pressure at 296 K in the visible and near infrared range (0.36–2.50 μm) (Fig. 4a). Measurements were taken at 296 K, rather than a more Ceres’ typical temperature (∼200 K), as the rate of chemical change increases as a function of temperature – with spectral modifications observed more clearly and quickly at higher temperature (e.g., Hartman et al., 2001b; Bu et al., 2017). No significant changes occur within the NIR spectral range (∼1–2.5 μm) for natrite, over a 312-h (13days) experimental duration. Due to its hygroscopic nature, the nominally anhydrous Na2 CO3 shows a small absorption band at ∼1.98 μm due to water physisorbed during sample preparation. Similar data for NaHCO3 , over 308- h’ (12.8 days) low-pressure exposure, show no spectral change in the NIR region (Fig 4a). We note that NaHCO3 has a particularly broad absorption at wavelengths > ∼1.5 μm, characteristic of bicarbonate (Fig. 4a). The observed spectral stability for natrite and NaHCO3 is consistent with XRD measurements which show no change in the crystallographic structure, above instrumental error, after about 300-h and 60-days vacuum-processing, respectively (Fig. 4b). Similarly, in the near-mid infrared region, bi-directional reflectance of natrite pressed powders show no significant temperature or pressure-induced spectral change, before and after lowpressure exposure at 296 K and 200 K (Fig. 4c). The CO3 2− anion has mid-IR, fundamental symmetric (ν 1 ) and asymmetric (ν 3 ) stretching modes at ∼9.4 and 7.1 μm, respectively, and fundamental out-of-plane (ν 2 ) and in-plane (ν 4 ) bending modes at ∼11.4 and 14.7 μm (Lane and Christensen, 1997). Combinations and overtones of the carbonate fundamentals result in characteristic features at ∼3.9 μm and ∼3.4 μm. Additional bands at shorter wavelengths are due to higher-order combinations and overtones at ∼2–2.5 μm, e.g., 2.55 μm and 2.35 μm (Gaffey, 1987; Crowley 1991; Lane and Christensen 1997; Clark, 1999; Cloutis et al., 2008; Harner & Gilmore, 2015). The precise position and shape of these carbonate features depend on the specific mineralogy, temperature, and crystalline structure, and can be distorted or even masked by the OH/H2 O

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Fig. 3. Temporal evolution of the relative reflectance (referred to PTFE standard) for 45–83 μm natron grains shows dehydration due to vacuum-exposure at (a) 240 (± 3) K, (b) 200 (± 3) K, and (c) 122 (± 2) K, as inferred by the reduced water bands at ∼1.49 and 1.98 μm. At t = 0, powder samples were cooled to the experimental temperature under dry-N2 at ambient pressure (∼760 Torr), and subsequently exposed to continuously decreasing pressure (P), with the exposure-time (t) as shown. Pumping speed remained constant. The spectra were taken every five minutes, but for clarity only a few representative spectra at conditions (t, P) are presented.

features in the hydrous carbonates (Crowley 1991; McCord et al., 2001), e.g., the 2.35-μm feature is not apparent for the spectra of natron in Fig. 2. On Ceres, sublimation of water vapor from surficial ice or sub-surface reservoir has been hypothesized and tentatively identified by Dawn’s Framing Camera and the Gamma Ray and Neutron Detector (GRaND), as well as by the Herschel Space Observatory (Küppers et al., 2014; Nathues et al., 2015; Prettyman et al., 2016; Thangiam et al., 2016; Li et al., 2016). We simulate the effect of water vapor deposition on cryogenic anhydrous Na2 CO3 by leaking water-vapor into the vacuum chamber, and find that detectability of carbonate absorption features, particularly the one at ∼3.4 μm, strongly depends on water-vapor exposure (blue curve in Fig. 4c).

4. Discussion 4.1. Isothermal kinetics of vacuum-dehydration of natron (Na2 CO3 ·10H2 O) at ∼300 K Natron has a monoclinic crystal structure, consisting of Na2 (H2 O)10 2+ and CO3 −2 units. The Na2 (H2 O)10 2+ units, pairs of Na(H2 O)6 octahedra with one sharing edge, are hydrogen-bonded to CO3 −2 anions in a NaCl-like arrangement to form crystalline natron (NaCO3 ·10H2 O) (Taga, 1969). At low temperatures (∼110 K) the CO3 −2 anions within natron appear well-ordered, but become increasingly dynamic with rising temperatures (Libowitzky and Giester, 2003). At room temperature, natron is generally unstable in Earth’s ambient atmosphere and dissolves in its own crystal water at ∼306 K (33 °C) (Groenvold and Meisingset, 1983). Above 288 K, natron dehydrates endothermically from decahydrate

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other phase transitions occur at temperatures significantly above 300 K (Dušek et al., 2003; Arakcheeva et al., 2010). We investigate the structural and chemical kinetics of natron’s cryogenic isothermal (200–240 K) vacuum-dehydration, as this remains uncertain for deposits within Occator crater on Ceres’ surface. Since facilities for cryogenic XRD measurements were not available, we probed for structural transformation and/or compositional variation in natron grains (45–83 μm) using XRD at 296 K, as a function of low-pressure exposure-time. We acquired companion optical-dehydration measurements for natron grains (45–83 μm) at 296 K, similar to Fig. 4. Also, measurements at 296 K are more indicative in laboratory aspect, since spectral or structural change will occur at a faster rate and are more likely to be detected within the experiment’s duration. X-ray powder diffraction patterns for natron prior to vacuum exposure show that the crystal structure is well matched to library models for sodium carbonate decahydrate (Fig. 5a, top panel), based on the angular peak position, confirming the material identity and crystallography. Diffraction peak positions for sodium carbonate are a function of the hydration-level and related to atomic distances in the lattice, thus the good correlation in 2θ angle suggests no significant change is induced by sample preparation nor XRD experimental acquisition time. The relative peak heights vary to some extent, due to the preferred crystallographic orientation of the grains (Suryanarayana & Norton, 1998) and the dynamic structural-disorder of the CO3 anions constrained by hydrogenbonds within the crystallographic structure found at room temperature (∼295 K) (Libowitzky and Giester, 2003). NaCO3 ·10H2 O samples (∼4 g) for diffraction measurements were loaded into glass vials and placed under vacuum for varied times; the XRD patterns for samples with various vacuum-exposure are presented in Fig. 5a. We match each sample to the best combination of spectra from the standard library for sodium carbonate compounds, including natron, sodium bicarbonate (NaHCO3 ), hepta-hydrate (Na2 CO3 ·7H2 O), monohydrate (Na2 CO3 ·1H2 O), and natrite (Na2 CO3 ). Based on the observed systematic change in diffraction pattern (Fig. 5a), isothermal vacuum-dehydration of natron proceeds as:

Na2 CO3 ·10H2 O(s) → aNa2 CO3 ·7H2 O(s) + bH2 O(g)

(1)

Na2 CO3 ·7H2 O(s) → cNa2 CO3 ·1H2 O(s) + dH2 O(g)

(2)

Na2 CO3 ·1H2 O(s) → eNa2 CO3 ·1H2 O(s) + fNa2 CO3 (s) + gH2 O(g) → Na2 CO3 (s) (3) Fig. 4. (a) No spectral change is observed in the relative reflectance (λ = 0.36– 2.5 μm range) of natrite (Na2 CO3 , grains of < 45 μm) and NaHCO3 (grains of 45– 83 μm) with about 300-h’ exposure to low pressure at 296 K, implying their stability on Ceres. (b) XRD measurements for natrite and NaHCO3 show no crystallographic change after ∼300 h’ and two months’ exposure to low pressure. (c) Bidirectional reflectance (λ = 1.6–4.5 μm) of natrite exposed to low pressure at 296 K and 200 K show similar stability under vacuum-exposure. Exposure of natrite to water vapor at 200 K shows significant overlap of water features with the carbonate above 2.7 μm, obscuring the carbonate absorptions at 2.8 and 3.4 μm. The O–H stretch overtone absorption at ∼3 μm saturates with H2 O exposure.

to monohydrate over the course of a few hours in a batch, fluidized-bed reactor (Hartman et al., 2001a). During dehydration Na2 CO3 ·10H2 O undergoes crystallographic transformation as molecular H2 O is removed, to orthorhombic lattice-structure in Na2 CO3 ·1H2 O, with concurrent increases in porosity and surface area but reduction in particle size (by ∼20%) (Hartman et al., 2001a). Synthetic anhydrous sodium-carbonate (natrite, Na2 CO3 ) has four different temperature-dependent phases with a phase transition from incommensurately-modulated, monoclinic Na2 CO3 γ to commensurately-modulated, monoclinic Na2 CO3 -δ at ∼170 K;

Where molar constants a –g depend on time, pressure, temperature, grain size, etc., and molecular water is removed by pumping. We note that sodium bicarbonate is not required to match the XRD patterns within the measured time frames, thus any formation of sodium bicarbonate during dehydration appears insignificant. These experimentally determined dehydration pathways are well matched to the theoretical, post-depositional, thermodynamicdecomposition-pathways proposed for Ceres by Zolotov (2017). Incorporated-water-molecule loss occurs instantaneously once natron is exposed to low pressure. H2 O-loss continues with exposure time, as indicated by the decreased depth of the water-band absorptions in Fig. 5(b). We use the area A(t) of the OH/H2 O absorption feature at ∼1.98 μm (in %, scaled to the value at t = 0) as an indicator of the sample hydration level. A(t) was quantified by converting the relative reflectance spectra in Fig. 5(b) at the corresponding time (t) to optical depth units, – ln(R), where R is the relative reflectance; the integrated area of the absorption feature between 1.82 and 2.25 μm is then calculated after subtraction of a polynomial background, and divided by the area of the feature prior to vacuum-exposure (at t = 0) (Bu et al., 2017). While the 1.49-μm water band is better separated from the car-

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Fig. 5. (a) Normalized XRD patterns (in black) for natron powder (Na2 CO3 ·10H2 O, 45–83 μm grains) with varied time-exposure to ∼10−6 Torr at 296 K describe the evolutions of crystal structure from natron to natrite over 48 h. Vacuum-exposure time increases from top to bottom; standard library spectra for natron, thermonatrite, and natrite overlay the data. (b) Temporal-evolution of the relative reflectance (referred to the PTFE standard) for 45–83 μm natron grains with vacuum-exposure at 296 (± 2) K shows attenuation of the water bands at ∼1.49 and 1.98 μm and the appearance of carbonate feature at ∼2.35 μm. (c) The isothermal dehydration kinetics for vacuum-exposed natron show that desiccation proceeds over two general stages. In our Avrami-type model, the double natural logarithm representation of natron hydration (A, the H2 O band area at ∼1.98 μm in %) is derived from Fig. 5(b) and shown as a function of vacuum-exposure time t (minutes). In this model, each linear region indicates a distinct phase in the dehydration process, and the slope is indicative of the local dehydration rate. The error bars, if not seen, are smaller than the size of the data point. (d) Reduction of the intensity of the XRD patterns as the vacuum-exposure time increases (from blue to green curve) indicates the increased local disorder within the crystalline structure of the dehydrated natron, most likely due to the dynamics of CO3 groups at 296 K. The XRD data is identical to that in Fig. 5(a) for 60 and 135 min vacuum exposure, except that they remained un-normalized.

bonate overtones and dehydration curves using the area of this band has also been derived, we present the 1.98-μm band data for comparison with VIR spectra, which display instrumental artifacts at ∼1.4–1.6 μm due to filter junctions in this region. We confirmed that measured dehydration rates are identical using both bands. To describe natron’s isothermal dehydration kinetics, we plot the A(t) vs. t in a double-logarithm representative (Fig. 5c), following an Avrami-type model, A(t) = exp( − K × tn ), where K is a temperature-dependent dehydration-rate constant, and n is the Avrami exponent, to simulate the isothermal dehydration process. Note that A(t) represents the remaining natron water content, not the water loss (1 - A(t)) measured in thermogravimetric experiments (e.g., Hartman et al., 20 01a & 20 01b). Avramitype equations typically describe crystallization kinetics, but can also be applied to systems undergoing isothermal phase-change, such as dehydration (Satava and Sestak, 1973; Wang et al., 2015; Bu et al., 2017). In a double natural logarithm representation, ln[ − ln(A)] = ln(K) + n × ln(t), each linear region indicates a distinct dehydration-phase and the slope is indicative of the local dehydration rate. In multiply-hydrated sodium carbonate material, the dehydration rate, A(t)/t, is determined by the bonding energy for each water molecule within the crystal lattice, as well as their individual diffusion rates within the material and then into the vac-

uum, both of which are dependent on grain size, crystalline structure, temperature, etc. Each dehydration pathway indicated by XRD measurement can be correlated to the concurrent linear regions in the Avramiplot (Fig. 5c) based on vacuum-exposure time. The first linear region (Fig. 5c, left of the dashed-vertical line), where dehydration occurs rapidly from t = 0 through t ≈ 27 min, arises from the decomposition of sodium carbonate decahydrate to heptahydrate (Na2 CO3 ·7H2 O) with the loss of the first three hydrogen-bonded H2 O as in Eq. (1). Sodium carbonate heptahydrate is a metastable state, which decomposes rapidly to thermonitrite (sodium carbonate monohydrate, Na2 CO3 ·1H2 O) in the manner described by Eq. (2). The subsequent region – delineated between the dashedand dotted-vertical lines – derives from the removal of the remaining H2 O. In dehydrated natron, edge-sharing water molecules have two coordination sites: the first with an oxygen atom coordinated with two Na cations (Na–O–Na) and the second hydrogen-bonded to two neighboring CO3 anions (Taga, 1969; Hartman et al., 2001a), and undergo the slow removal described in Eq. (3). The conversion of the thermonatirite to natrite contains two linear regions (or sub-phases) with distinct slopes – an initial shallow slope (shaded) followed by a steeper slope (hatched) (Fig. 5c). The double-slope may result from the dynamic disorder of the CO3 anions, which is observed above 270 K and increases with temperature, potentially interrupting the orderly removal of the water molecules due

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Exposure Time t (min) 27

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ln(t) (min) Fig. 6. Isothermal vacuum-dehydration of natron at Ceres’ relevant temperatures can be modeled by the Avrami equation, A(t) = exp( − K × tn ). The double natural logarithm representation of hydration (A, the H2 O band area at ∼1.98 μm in %) is plotted as a function of vacuum-exposure time t (minutes) at 240, 200 and 122 K (derived from Fig. 3a–c, respectively), where each linear region describes a distinct phase in the dehydration process, and the slope of the linear regions relates to the local dehydration rate. Error bars are smaller than the size of the data point, if not seen. All the dehydration curves share a general trend – a fast-increasing linear region, where loosely-bounded H2 O is easily lost with simultaneous changes in crystallographic symmetry, followed by a slowly-increasing region where, again, both water-loss and crystallographic changes occur. As expected, the rate of dehydration decreases significantly at lower temperatures.

to their fluctuating positions within the crystalline structure; no correlated change in space group symmetry has been observed with this disorder (Libowitzky and Giester, 2003). Based on normalized experimental XRD patterns, natron with vacuum-exposure for t ≈ 60 and 136 mins (within the shaded and hatched region, respectively, Fig. 5c) both match the crystalline structures of Na2 CO3 ·1H2 O (Fig. 5a). However, the intensity of the diffraction peaks diminish with increased vacuum-exposure, by ∼14% for the two strongest peaks (Fig. 5d), which cannot be due solely to formation of natrite ( < 8%, Fig. 5a) but also must include the formation of local disorders, e.g., position fluctuation of CO3 anions, amorphization, etc. The positional fluctuation interrupts the H2 O loss as molecular-bonds repeatedly break and reform, resulting in the relative flat slope in the shaded region (Fig. 5c). A similar interruption mechanism has been described for dehydration of amorphous calcium carbonates (Ihli et al., 2014). To some extent, the two separate slopes may also reflect the different H2 O-bond sites: Na-O–Na coordination for the edge-sharing water molecule in the monohydrate rather than the Na–O coordination in the higher hydration level (Taga, 1969), and/or the significantly different lengths/strengths of the two hydrogen bonds (Dickens et al., 1970; Wu & Brown, 1975). 4.2. Cryogenic isothermal dehydration of natron and implications for detection on Ceres The attenuation of the hydration features with low-pressure exposure, together with the consequent spectral effects, is a potential chronometer for natron deposits on Ceres’ surface. We derive rates of isothermal dehydration for natron at 122, 200, and 240 K in Fig. 6, using the area of the water band at ∼1.98 μm by analogy with the method for room-temperature dehydration described above for Fig. 5c. The dehydration curves at lower temperatures show similar trends as the one at 296 K – a fast-increasing region,

followed by a slowly-increasing region (Fig. 6, left and right of the dashed-vertical lines, respectively). For 240 and 200 K, dehydration of natron proceeds rapidly for t < ∼27 min upon exposure to vacuum, as loosely-bounded (hydrogen-bonded) H2 O is removed from the material as eq. (1) & (2), analogous to the case at 296 K (Fig. 5c, left of the dashed-vertical line). However, at lower temperatures (200 and 122 K), additional phases appear in the rapidly-increasing region, rather than the single slope observed at 296 K. This may be the result of incorporated water-molecule bond-energy degeneration at cryogenic temperatures (McCord et al., 2001; Dalton, 2003), coupled with the reduced dehydration rate, which allows differentiation between the dehydration phases described in Eqs. (1) & (2). The slowly-increasing water-loss region (right of dashedvertical lines) at 240 and 200 K, attributed to the removal of the edge-sharing water molecule in the dehydrated natron, shows a shallow, relatively constant slope(s), significantly different than the dual-phase curve at 296 K (Fig. 5c, shaded and hatched). This is due to the increased order for the CO3 groups at low temperatures, which lie below the gradual temperature threshold (∼250 K) for dynamic disorder (Libowitzky and Giester, 2003). Therefore, dehydration at cryogenic temperatures (240 and 200, and 122 K) experiences no interruption and proceeds continuously after Eqs. (1) & (2) are complete. It should be noted that at 122 K, the dehydration is extremely slow and doesn’t proceed to Eq. (3) within our experiment duration (∼ 77 h). We extrapolate the time for the water absorption feature to attenuate to 2% of the initial value, rendering it undetectable by VIR, since observations at ∼ 2-μm have a signal-to-noise level with a standard deviation of 0.02 − 0.03. Measurement of VIR’s 2% instrument sensitivity follows a standard methodology used for other hyperspectral datasets (e.g., Ruesch et al., 2012). For natron at 240 and 200 K, dehydration times are relatively brief, ∼4 h (245 min) and 5.6 days (8103 min), respectively, for the 1.98-μm band to attenuate below the VIR detection limit (2%), indicated by the horizontal dashed-line in Fig. 6. Thus on Ceres, for latitudes between + 45° and − 45°, total dehydration of natron deposits occurs within a matter of days without an additional source of rehydration, becoming natrite. Without continuous observation, the rapidity of dehydration of natron (NaCO3 ·10H2 O) at Ceres-relevant temperatures makes it unlikely that this material will be observed in-situ in any state of hydration. However, should hydrous sodium-carbonate be observed on Ceres or other airless body of similar surface temperature and regolith grain size, it has the potential to act as a geologic-clock by comparison with laboratory spectra. Since on a planetary surface the mineralogy is often mixed and the band area for a particular component, such as water, is difficult to quantify, we consider the position of the 1.9-μm water absorption-band minimum to provide a rough estimate of the sodium-carbonate hydration extent. The minimum of the 1.9-μmband in each of the relative reflectance spectra (Fig. 3a–c) is determined following the method used by De Angelis et al. (2017) – a linear background is removed between the band edges (∼1.8– 2.3 μm) prior to fitting with a second-order polynomial. The band center is the wavelength of the resulting minimum. Fig. 7(a) shows the band center as a function of the corresponding hydration level (A, derived from Fig. 6) for 240, 200, and 122 K. The position of the 1.9-μm minimum may be alternatively plotted against vacuumexposure time (min) (Fig. 7b). A small shift toward shorter wavelength is observed with dehydration as a function of vacuumexposure time at 200 and 240 K. The magnitude of the shift in minimum-position increases with natron temperature, due to the enhanced dehydration rate. For the duration of the experiment, no change in position is noted for T = 122 K within experimental error due to the attenuated dehydration rate. Assuming natron is initially deposited on Ceres or other Main Belt asteroid surface with 45-

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port that the deposits are: 1) recent, 2) the temperatures at these locations are always significant lower than 122 K, and/or 3) a continuous water source is available. Our measurements for the stability of natron at 200 and 240 K, determined by NIR optical reflectance, are consistent with the 1/elevel lifetimes of 10−2 and 10−5 years, respectively, derived from kinetic models for dehydration at low temperatures (McCord et al., 2001). However, contrary to our isothermal measurement below 130 K, McCord et al., (2001) found natron to be stable to 108 years. This discrepancy may be a result of the low-temperature extrapolation of thermal desorption data in McCord et al. (2001), measured above 220 K, or more likely, is due to the large difference in grain size – we use 45–83 μm, compared with ∼500 μm. Grain size plays an important role in the rate of dehydration of salts, as evidenced by our previous measurements of MgSO4 ·6H2 O dehydration (Bu et al., 2017); in that work, Bu et al. (2017) show that the dehydration rate decreases with larger grain size, due to the reduced surface area to volume ratio for enhanced diffusion. 4.3. Dehydration-Induced spectral slope change over the visible and NIR range

2000 1990 1980

122 K

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240 K

10

100

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Vacuum-exposure Time (min) Fig. 7. (a) For natron grains, a shift toward shorter wavelength is observed for the 1.9-μm water absorption-band center (see text for details) with hydration level as a function of vacuum-exposure. (b) The same data (a) is shown as a function of vacuum-exposure time (min). The magnitude of the shift in wavelength of the band center increases with natron temperature over the same time period; any changes in position at 122 K are within experimental error over the experimental duration. Thus, the position of the 1.9-μm water-absorption band can be used as a reasonable chronometer for the age of hydrated sodium carbonate deposits on Ceres or other airless body with similar surface temperatures of 200–240 K and regolith grain size of 45–83 μm. The dashed-lines are to guide the eye.

83-μm regolith grain-size and surface temperatures of 200–240 K, the position of the 1.9-μm water absorption-band minimum can be compared with laboratory measurements to infer hydration level (Fig. 7a), and thus the age of the deposit (Fig. 7b). At lower temperature (122 K), expected above ± 80° latitudes near Ceres’ poles (Tu et al., 2014), near-infrared optical evidence for the presence of natron should persist for an estimated 320 years, based on linear-extrapolation of the Avrami-plot dehydration curve (solid-red line in Fig. 6). Observation of hydrated sodium carbonate on Ceres or other Main Belt asteroids, with known surface temperature ∼120 K and regolith grain size of 45–83 μm, can be compared with laboratory measurements of the 1.9-μm water bandarea (Fig. 6) and used to approximate salt deposit age. Alternatively, a rough determination of planetary surface temperature or regolith grain-size may be inferred by comparison between laboratory and in-situ natron dehydration rates. For Ceres, VIR detection of stable features at ∼1.5 and ∼2.0 μm with also 3.9 μm, definitive confirmation of hydrous sodium carbonate salts, would sup-

With increasing distance from the central faculae, Dawn FC measurements show a ∼4-times reduction in absolute reflectance (albedo) of the color spectra, with overall spectral slope gradually changing from strongly-red (positive slope) at Cerealia Facula to almost neutral for the crater floor, similar to Ceres average spectrum (Nathues et al., 2015 & 2017). This visible red-slope was originally interpreted as an indicator for magnesium sulfate hexahydrate, and the systematic darkening with location was hypothesized to result from dehydration of hydrated salts and/or mixture with the dark floor material (Nathues et al., 2015). The bright material within the faculae has since been convincingly identified as carbonates based on the diagnostic features at ∼3.4 and 3.9 μm in the VIR spectra, rather than hydrated sulfates - although sulfates have not been conclusively eliminated (De Sanctis et al., 2016; Bu et al., 2017). However, the question still remains why the material within the faculae reddens across the visible wavelength range. We investigate the extent to which the dehydration affects the Vis-NIR spectral-slope of natron. For natron under low pressure at 200 K, Fig. 8 shows the average slope, defined as (R1 – R2 )/(λ1 – λ2 ), for two selected wavelength regions – one in the visible and the other over the NIR – as a function of the corresponding hydration level A, where Ri (i = 1, 2) is the relative reflectance at wavelength λi derived from Fig. 3(b), and A is from Fig. 6. As dehydration continues to A ≈ 10%, the slope in the NIR region changes by a factor of two from ∼ − 6 × 10−4 /nm to − 3 × 10−4 /nm, while the change in the visible region is insignificant, remaining ∼ − 1 × 10−4 /nm within experimental error. Thus, dehydration results in a reduction of the NIR blue-slope, derived from water-band attenuation, but is unlikely the dominant reddening mechanism in Cerealia Facula’s visible region. The measured dehydration rates (Fig. 6) show that dehydration of natron occurs within a short time scale under Ceres’ conditions, with the resulting natrite deposits further exposed to the low-pressure environment. We explore the spectral effects – darkening or brightening – due to long-term exposure of dehydrated natron to low pressure, after the water bands reduce below the instrument’s detection limit. Fig. 9(a) shows the relative reflectance of natron for eight selected wavelengths as a function of lowpressure-exposure time (t), derived from Fig. 5(b). At 296 K, the water bands reduce below our spectrometer sensitivity within ∼5 h. This study was performed at the relatively high temperature of 296 K, as dehydration rate is fast and spectral changes, if any, are likely to be observed within a practical experimental timeframe. The five wavelengths in the visible region were selected to

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Slope(*10-4/nm)

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80

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Hydration Level A (%) Fig. 8. Dehydration of natron at 200 K with extended exposure to vacuum results in a reduction of the blue slope in the NIR range (1152–2200 nm), while showing no significant effect on the spectral slope in the visible range (452–752 nm). Therefore, dehydration of sodium carbonate is unlikely the source of the visible-red slope observed at Cerealia Facula by Dawn’s Framing Camera. The slopes (see definition in the text) were derived from Fig. 3(b), as a function of the corresponding sample hydration level from Fig. 6. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

roughly match the wavelengths where the FC measurements were available (Nathues et al., 2015 & 2017). Fig. 9(b) shows the corresponding slope for the selected NIR and visible regions, following a similar method in Fig. 8 where data was taken at 200 K. We find that continuous exposure from ∼5 to 260 h to low pressure (∼10−3 – 10−4 Torr), after the x-axis break in Fig. 8, results in minor changes ( < 10%) in the overall relative reflectance across all the selected wavelengths, Fig. 9(a). The slope in the visible region remains ∼ − 1 × 10−4 /nm within experimental error, and the positive change in slope is within the error of our measurements (Fig. 8b). Therefore, extended vacuum-exposure of dehydrated natron (natrite) to low pressure cannot be the dominant mechanism governing spectral darkening at Cerealia Facula, which darkens by ∼75%. Other mechanisms, such as space weathering (Dukes et al., 2016; Stein et al., 2017 (this issue)) and mixture with the dark background materials (Nathues et al., 2015; De Sanctis et al., 2016; Palomba et al., 2017a (this issue); Stein et al., 2017 (this issue)), are required. In addition, Cerealia Facula’s red slope over the region 0.4–0.9 μm, ∼ + 4 × 10−4 /nm (at dome center) (Nathues et al., 2017), is not the result of natron dehydration nor long-term exposure of the dehydrated natron or Na2 CO3 to low pressure.

Fig. 9. (a) During vacuum-induced dehydration of natron at 296 K (left of the xaxis break), the relative reflectance (referred to PTFE standard) at 2.2 μm brightens significantly, while no significant changes are observed at 1.152 μm or within the visible region (λ = 800, 752, 652, 552, and 452 nm); continuous vacuum-exposure after complete dehydration (right of the x-axis break) results in no significant changes in the relative reflectance over all the wavelengths above. Therefore, extended vacuum-processing of natron (natrite) cannot be the dominant mechanism governing spectral darkening at Cerealia Facula. Reflectance measurements are derived from Fig. 5(b). (b) The change of slope in the visible range (452–752 nm) for natron with vacuum-exposure, derived from Fig. 5(b), is insignificant compared with VIR observations for Ceres, suggesting that, besides the vacuum-processing, other mechanisms must account for Cerealia Facula’s red slope over the region 0.4– 0.9 μm (Nathues et al., 2017). Extended vacuum-exposure after the water bands deplete (right of the x-axis break) does not causes significant spectral slope change in the NIR region of 1152–2200 nm.

4.4. UV spectral change for sodium carbonates in low-pressure environments The effect of long-term vacuum exposure on materials in the UV is of particular interest with respect to Ceres, where shortwavelength absorption is unusually strong when compared with most C-type asteroids, which have a minor red slope and are spectrally flat from the visible region through 0.3 μm, the edge of ground based observations (McCord & Gaffey, 1974; DeMeo & Carry, 2013). Hubble Space Telescope data show that Ceres’ absorption edge is located at a significantly longer wavelength, ∼0.45 μm, with strong absorption in the vis–UV relative to other carbonaceous materials (Li et al., 2006; Li et al., 2009; Hendrix et al., 2016). The source of Cere’s enhanced absorption in the UV–visible is unknown, but spectral matching of the 0.45 μm absorption edge suggests the presence of sulfur and sulfurous compounds (Hendrix

et al., 2016; Bu et al., 2017) in addition to the phyllosilicates or carbonates already identified by molecular vibrations in the infrared region. We examine the UV spectral effects of extended vacuumexposure for sodium carbonates, since shifts in the absorption edge have been observed in other salts (e.g., Bu et al., 2017). The optical effects at short wavelength (210–500 nm) for natron at 296 K with vacuum-exposure are shown in Fig. 10(a), as a function of low-pressure exposure time. Vacuum dehydration is begun at ambient pressure (∼760 Torr) and room temperature (296 K). Pumping speed remains constant over the spectral evolution. For unprocessed natron, an enhanced absorption is observed below wavelength of ∼220 nm, where systematic band-broadening in the UV is apparent over time under vacuum, with a systematic shift of the absorption edge toward longer wavelength, from

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Fig. 10. (a) Temporal evolution of natron grains (45–83 μm) exposed to low pressure shows a systematic shift in the absorption edge from ∼220 nm to longer wavelength, more similar to Ceres’ UV–vis spectral slope (c & d), as a result of exposure to low pressure. Vacuum dehydration is begun at t = 0 from at ambient pressure (∼760 Torr) at 296 K, with samples subsequently exposed to continuously decreasing pressure (P), with the exposure-time (t) as shown. Pumping speed remains constant. The spectra were taken every five minutes, but for clarity only a few representative spectra at conditions (t, P) are presented. Dehydration, as evidenced by loss of the 1.4 and 1.9 hydration bands in the corresponding spectra shown in Fig. 5b, occurs within roughly ∼5 h, while the shift in absorption edge and new spectral features develop over days. (b) Vacuum-exposure-induced shifting of the UV absorption edge was observed in various carbonate materials at 296 K, though at different rates (the exposure time shown in brackets). The grain size fraction was 45–83 μm for natron and NaHCO3 but < 45 μm for natrite. Spectra were scaled to 1 at λ = 550 nm. (c) & (d) Measurements of Ceres using HTS from Li et al. (2006) and Hendrix et al. (2016), respectively.

∼220 to 340 nm over ∼13 days. As exposure continues, two new absorption features appear at ∼275 and 235 nm, superimposed on the continuous background of the spectra. Similar trends in the UV region were observed in spectra of natrite and NaHCO3 under vacuum exposure, though at slower rates (Fig. 10b). Given that dehydration, as evidenced by the loss of the 1.4 and 1.9 hydration bands, almost completes within ∼5 h (Fig. 5), these UV spectral

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changes do not appear to be linked specifically to H2 O-loss, but occur with extended vacuum exposure. The observed step-function-like spectral feature at short wavelength in Fig. 10 is characteristic of interband transitions in insulating materials (Clark et al., 2007). For unprocessed natrite and natron, the absorption edge at 220 nm is consistent with bandgap calculations of ∼5.2 eV (Fedorov et al., 2006) and ∼ 5.35 eV (Duan et al., 2012) for natrite, similar to the 5–6 eV measured for CaCO3 (Baer and Blanchard, 1993; Ghadami Jadval Ghadam and Idrees, 2013). We hypothesize that extended-vacuum-exposure of natron, as well as natrite and NaHCO3 , initiates the production of short-range local defects (e.g., pore formation, dislocations, etc.) which appear to broaden the conduction band, shifting the edge toward longer wavelength (Welnic et al., 2006; Ramo and Bristowe, 2016). The transition between volume-scattering-dominant and surface-scattering dominant spectral reflectance is also expected to occur in the near-UV, observed as a minimum in reflectance for geologic powders at ∼150–450 nm (Hapke, 1981; Wagner et al., 1987). The intensity of volume-scattered light depends strongly on the grain size diameter, roughly proportional to exp(-α D), where α = 4π k/λ is the absorption efficiency, k is the imaginary component of the refractive index, λ is the wavelength, and D is the grain diameter. Shrinkage (20%) of grain diameter has been measured during dehydration of natron (Hartman et al., 2001a), shifting the volume-scattering edge toward shorter wavelength; thus, we eliminate volume-scattering as the source of our absorption edge shift toward longer wavelength and attribute the measured UV absorption edge to the conduction band in the sodium carbonates. However, in no case does the conduction band shift up in wavelength sufficiently to explain the unusually strong UV absorption at Ceres starting at ∼ 0.45 μm. Absorption in the UV (and much of the visible) results from electronic transitions in the solids due to d or f level excitations, molecular orbital transition, or charge transfer, rather than from molecular vibrational excitations seen in the infrared region. In particular, electrons involved in conjugated π -bonding and σ bonding orbitals, as well as lone pairs (non-bonding electrons, n orbital), have the potential to be promoted to the excited states (antibonding π ∗ and σ ∗ orbitals) by the absorption of UV light. The σ → σ ∗ and n → σ ∗ transitions requires greater energy and typically absorbs light in the deep UV (< 200 nm) – below the experimental spectral range. Only n → π ∗ and n → σ ∗ transitions are potentially identifiable above 200 nm. Measurements of organic carbonyl compounds, such as ketones, typically absorb light at ∼280 nm, due to excitation of an oxygen lone-pair electron into to an unoccupied π ∗ level (n → π ∗ transition), and at ∼250 nm, due to excitation of an electron from the π level to an unoccupied π ∗ level (π → π ∗ transition) (West 1968; Knowles & Burgess, 1984). In sodium carbonates, the carbonate anion (CO3 )−2 has a similar electronic configuration – one double-bond between carbon and oxygen atoms with a single π -bond and two lone-pair oxygen electrons (n electrons). Therefore, we correlate the new vacuumformed UV absorptions at 275 nm and 235 nm in sodium carbonates to carbonate anion n → π ∗ and π → π ∗ transitions, respectively. It remains uncertain what causes these absorption features to deepen with continued vacuum exposure, although it is likely that increasing vacuum-induced defects in sodium carbonates facilitate their occurrence. HST observations of Ceres show a strong, unidentified, UV absorption feature centering at 280 nm (Fig. 10(c), Li et al., 2006), with a position similar to ∼275 nm absorption observed in vacuum-processed sodium carbonate. However, the depth of Ceres’ band is significantly more prominent with a width extending ∼150 nm, thus this band does not appear to match vacuumprocessed sodium carbonate. The carbonyl-like n → π ∗ transition in sodium carbonate, as discussed above, is expected to occur in

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all carbonate species (Palomba et al., 2017a (this issue)) and organic materials (De Sanctis et al., 2017) on the surface of Ceres. The convolution of these electronic transitions may result in Ceres’ broad 280-nm feature, since the various cation groups for Cerean carbonates (Na, Ca, NH4 , etc.) can result in slight variation of band positions, effectively broadening (or deepening) the band. We note, however, that the Li et al. (2006) broad-band observations were not identically replicated in the higher-resolution data (Fig. 10(d), Hendrix et al., 2016), where a minima was observed at ∼210 nm along with small features at ∼260 and 310 nm. The discussion above may also be applied to these features. 5. Summary and conclusion Natron (Na2 CO3 ·10H2 O), deposited by either brine fountaining or as precipitates with sublimating water, will dehydrate within a few days at Ceres-relevant temperatures (200–240 K) across the equator and middle latitudes due to the low pressure environment. Without a continuous source of rehydration – on Ceres’ this includes areas exposed to exospheric water vapor from sublimation, ice grain ejection, or subsurface H2 O upwelling – the rapidity of dehydration for natron makes it unlikely that this material will be observed in any state of hydration across Ceres’ equator and middle latitudes. Indeed, definitive observation of hydrated salts should be a strong indicator for the presence of surficial water or very-recently deposited material emplaced by a fast-transport mechanism from the sub-surface such as cryo-volcanism. In the unlikely case that hydrous sodium-carbonate is detected within Ceres’ mid-latitudinal region, the position of the 1.9-μm waterabsorption feature can be used to both determine the hydration state and estimate the age of the deposits. In the high latitudes and polar regions, where temperatures are ∼120 K or lower, dehydration proceeds significantly more slowly - such that hydrated phases of sodium carbonate may persist and retain detectable water for up to several hundred years. In such case, the age of the hydrated sodium carbonate deposits, if observed on Ceres, can be approximated using the experimental dehydration rate derived from our Avrami model, given a similar surface temperature and regolith grain size. XRD patterns, in conjunction with the isothermal vacuumdehydration curves, were used to identify the vacuumdehydration pathway for natron: Na2 CO3 ·10H2 O → Na2 CO3 ·7H2 O (metastable) → Na2 CO3 ·1H2 O → Na2 CO3 . This progression is well matched to the theoretical, post-depositional, thermodynamicdecomposition pathway proposed for Ceres by Zolotov (2017). Exposure of hydrous and anhydrous sodium-carbonates to the vacuum conditions of space have spectral and physical effects over two regimes, the first (R1) is concurrent with dehydration and the second (R2) occurs after dehydration with continued exposure to vacuum. Thus, effects observed during R2 cannot be correlated with dehydration explicitly. We summarize these findings: (1) Over the NIR (1.0–2.5 μm), vacuum-dehydration (R1) attenuates the water absorption-bands, resulting in a NIR slope that becomes progressively less-blue. After dehydration, extended exposure to vacuum (R2) has no additional spectral effect in the NIR. (2) In the visible region (0.4–1.0 μm), extended vacuumdehydration (R1 and R2) of natron (and natrite) results in no significant spectral changes ( < 10%). Thus, vacuum exposure of sodium carbonates alone cannot account for the red slope observed by Dawn’s FC at Cereal Facula, nor can dehydration of salts explain the progressive change in slope between dome center and crater background. Other mechanisms, e.g., mixture of with Ceres’ global mineralogy or space weathering, must be invoked.

(3) In the UV range (0.22–0.40 μm), extended vacuumprocessing (R2) of sodium-carbonates (hydrous and anhydrous) results in enhanced UV absorption, with the absorption edge shifting from UV toward longer wavelengths (from ∼0.22 to 0.34 μm over ∼13 days), better matched – but not identical – to Ceres’ UV-visible spectral slope. (4) The formation of new features in the UV at 275 and 235 nm is observed with extended vacuum-exposure (R2) of natron and natrite, potentially due to increased n→π ∗ and π →π ∗ transitions within the CO3 2− group. The 275-nm absorption may be a factor in the ∼280 nm feature observed in broadband HST spectra of Ceres. (5) Sodium bicarbonate remains crystallographically stable with extended (2 months) vacuum-exposure at 296 K, with spectral change only occurring within the UV – similar to natron and natrite. Thus, sodium bicarbonate seems unlikely to be the source of Ceres’ abundant natrite deposits at Occator and other craters. The spectra of sodium carbonates resulting from exposure to the cryogenic, airless conditions existent on Ceres are not static, and quality laboratory characterization is particularly important for spectral matching and mineralogical identification. While this study focuses on the environment relevant to Ceres, these findings also have implications for the interpretation of observational data from other planetary objects under similar conditions. Acknowledgements We acknowledge the Dawn team for constructive discussions of this work, and Dr. David Brautigan, director of UVa’s Center For Cell Signaling, for allowing us access to his facility’s cold room for sample preparation of hydrous materials. C.A.D and C.B. thank the NASA SSW program (NNX15M38G) for support. A grant from the NSF Astronomy program supports the research of G.R.L. L.A.M. was supported by the Dawn mission through NASA’s Discovery Program, and O.R. through NASA’s Postdoctoral Program at Goddard Space Flight Center administered by Universities Space Research Association. J.-Y. Li’s work is supported by a UCLA subcontract (NNM05AA86) under NASA’s Dawn Discovery Mission. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.icarus.2017.12.036. References Ammannito, E., DeSanctis, M.C., Ciarniello, M., Frigeri, A., Carrozzo, F.G., Combe, J.P., et al., 2016. Distribution of phyllosilicates on the surface of Ceres. Science 353 (6303), aaf4279. Arakcheeva, A., Bindi, L., Pattison, P., Meisser, N., Chapuis, G., Pekov, I., 2010. The incommensurately modulated structures of natural natrite at 120 and 293 K from synchrotron X-ray data. Am. Mineral. 95 (4), 574–581. Baer, D.R., Blanchard, D.L., 1993. Studies of the calcite cleavage surface for comparison with calculation. Appl. Surf. Sci. 72 (4), 295–300. Bandfield, J.L., Glotch, T.D., Christensen, P.R., 2003. Spectroscopic identification of carbonate Minerals in the Martian dust. Science 301, 1084–1087. Bowling, T., et al., 2017. Impact modeling of Occator crater on Ceres. Icarus this issue. Boynton, W.V., Ming, D.W., Kounaves, S.P., Young, S.M.M, Arvidson, R.E., Hecht, M.H., Hoffman, J., Niles, P.B., Hamara, D.K., Quinn, R.C., Smith, P.H., Sutter, B., Catling, D.C., Morris, R.V., 2009. Evidence for calcium carbonate at the Mars phoenix landing site. Science 325, 61–64. Bu, C., Rodriguez Lopez, G., Dukes, C.A., Ruesch, O., McFadden, L.A., Li, J.-Y., 2017. Search for sulfates on the surface of ceres. Meteori. Planet. Sci. doi:10.1111/ maps.13024. Castillo-Rogez, J.C., McCord, T.B., 2010. Ceres’ evolution and present state constrained by shape data. Icarus 205 (2), 443–459. Clark, R.N., 1999. Chapter 1: Spectroscopy of rocks and minerals, and principles of spectroscopy. In: Rencz, A.N. (Ed.), Manual of remote sensing, Volume 3, Remote Sensing for the Earth Science. John Wiley and Sons, New York, pp. 3–58.

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