Abstract
AbstractThe Visible‐Infrared Mapping Spectrometer (VIR) on board the Dawn spacecraft revealed that aqueous secondary minerals—Mg‐phyllosilicates, NH4‐bearing phases, and Mg/Ca carbonates—are ubiquitous on Ceres. Ceres' low reflectance requires dark phases, which were assumed to be amorphous carbon and/or magnetite (∼80 wt.%). In contrast, the Gamma Ray and Neutron Detector (GRaND) constrained the abundances of C (8–14 wt.%) and Fe (15–17 wt.%). Here, we reconcile the VIR‐derived mineral composition with the GRaND‐derived elemental composition. First, we model mineral abundances from VIR data, including either meteorite‐derived insoluble organic matter (IOM), amorphous carbon, magnetite, or combination as the darkening agent and provide statistically rigorous error bars from a Bayesian algorithm combined with a radiative‐transfer model. Elemental abundances of C and Fe are much higher than is suggested by the GRaND observations for all models satisfying VIR data. We then show that radiative transfer modeling predicts higher reflectance from a carbonaceous chondrite of known composition than its measured reflectance. Consequently, our second models use multiple carbonaceous chondrite endmembers, allowing for the possibility that their specific textures or minerals other than carbon or magnetite act as darkening agents, including sulfides and tochilinite. Unmixing models with carbonaceous chondrites eliminate the discrepancy in elemental abundances of C and Fe. Ceres' average reflectance spectrum and elemental abundances are best reproduced by carbonaceous‐chondrite‐like materials (40–70 wt.%), IOM or amorphous carbon (10 wt.%), magnetite (3–8 wt.%), serpentine (10–25 wt.%), carbonates (4–12 wt.%), and NH4‐bearing phyllosilicates (1–11 wt.%).
Highlights
Ceres, a dwarf planet and the largest body in the asteroid belt, is thought to have a unique mineral composition
Our work shows that mimicking carbonaceous chondrite meteorites (CCs) properties, in particular composition and/or texture of their diverse opaque phases is important to fitting Ceres' spectral properties with modest additional amounts of insoluble organic matter (IOM) or amorphous carbon (∼10 wt.%) and smaller amounts of magnetite (3–8 wt.%) that are fully consistent with both Visible-Infrared Mapping Spectrometer (VIR) and Gamma Ray and Neutron Detector (GRaND) data
Comparing the resulting elemental abundances of H, C, K, and Fe with those derived from GRaND data demonstrates that only including pure endmember phases in the models leads to overestimated abundances of C and Fe
Summary
A dwarf planet and the largest body in the asteroid belt, is thought to have a unique mineral composition. Telescopic measurements prior to the arrival of the Dawn spacecraft indicated that Ceres somewhat differed from CM and CI meteorites, notably through an unusual spectral feature at 3.06 μm. Such a feature is not found in most other dark asteroids and was attributed to multiple possible causes (Rivkin et al, 2006; Takir & Emery, 2012) with the most likely candidate considered to be ammoniated Mg-smectite clay V. King et al, 1992)
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