Probing Ceres' Internal Structure Using Spectral Anomalies: A Machine Learning Approach

(1) Ceres is a relict ocean world in the inner Solar System, located in the middle asteroid belt at ~2.8 AU (Russell et al., 2016). Ceres has attracted the attention of the scientific community for several reasons—but the primary being the huge amount of water it contains. Remnants of recent endogenic activities (e.g. cryovolcanism) on its surface suggests that the water might still be in a liquid state, as evidenced  by large-area mudflows, pitted terrains, and the bright faculae material, likely originating from extrusions of brine (Nathues et al., 2016, 2017; De Sanctis et al., 2020; Scully et al., 2021; J. Castillo-Rogez et al., 2022). However, the distribution of liquid water within Ceres depends on the degree of differentiation the body underwent—a question that is not yet fully resolved. For example, it is not clear whether Ceres underwent a differentiation that segregated the water in a global ‘mantle’ layer or multiple discrete brine reservoirs, or if this water is pervasive across its volume (J. C. Castillo-Rogez et al., 2019; J. Castillo-Rogez, 2020; Neumann et al., 2020; Daswani & Castillo-Rogez, 2022; Nathues et al., 2022; Ruesch et al., 2019).

The recent discovery of a “yellow bright material” (yBM) at Consus crater has revealed that ammoniated phyllosilicates are produced by endogenous activities, as carbonaceous chondritic (CM/CI) materials can contain significant amounts of NH₄⁺ (Nathues et al., 2024). However, yBM is only one of many spectrally anomalous color units that differ from the global average Ceres spectrum. Other anomalous color units appear both bright and dark, and occur in a variety of geological settings. In the absence of atmospheric processes, igneous and tectonic activity, and extensive space weathering beyond the top few centimetres, these units are also possibly of endogenous origin. Therefore, systematically identifying all regions on Ceres that differ from the average spectrum, and grouping them spectrally and spatially is essential to shed further light on the evolution of Ceres. Such a study can help us gain deeper insights into the state of Ceres’ differentiation and its phase state of water. In a differentiated (or partially differentiated) Ceres, spectral anomalies should show systematic depth dependencies (i.e., a layered structure) of exposed materials, with regional variations related to localized brine pockets. On the other hand, in an undifferentiated Ceres, the liquid is distributed throughout the interior in pore spaces and therefore, the spectral anomalies should display stochastic patterns. Moreover, correlating spectral features with crater ages could reveal changes in the water state and endogenic processes over geological time. In addition, spatial distribution patterns such as clustering at specific latitudes or longitudes, or in and around impact craters, or a random distribution can provide further geological insights into the origin of these spectrally anomalous sites.

With this objective, we are developing an automated pipeline to systematically identify and characterize spectrally anomalous sites across Ceres' surface using NASA Dawn's Framing Camera (FC) data (Sierks et al., 2011). Our pipeline uses an autoencoder to detect FC pixels that are spectrally different from the global average surface spectrum of Ceres, followed by morphological clustering and hierarchical clustering to group the sites based on spectral similarity. This approach allows us to identify and classify all major spectral units, including yBM sites and other yet undetected anomalous regions on Ceres.

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