Thermal State and Physical Properties of Water Ice in Ceres' Oxo Crater: Implications for Surface geomorphology and Evolution

Introduction

Ceres provides key insights into volatile-rich body evolution. Dawn showed Ceres' impact-modified surface involves subsurface volatiles. Recent craters reveal the shallow subsurface and ongoing geological activity. Oxo crater, a small (~9 km diameter) and geologically young feature (~1-10 Myr old; Combe et al., 2016; Nathues et al., 2017) located in Ceres' northern hemisphere (42.20°N, 359.75°E), is notable for its prominent exposures of water ice on its southern crater wall.

We integrate spectral/geological analyses to characterize ice properties (thermal state, grain size, abundance, distribution) and constrain its origin and role in recent surface evolution.

Data and Methods

• Spectral Analysis: We used Dawn VIR data (1.0–4.2 µm; De Sanctis et al., 2011), covering diagnostic water ice and carbonate absorption features. Pixels in/around Oxo crater were classified into four compositional types based on 2.0 µm ice and 4.0 µm carbonate absorption strengths: 1A (water ice and carbonates present), 1B (water ice present, carbonates weak), 1C (carbonates present, water ice absent), and 1D (Ceres’ average surface composition).

• Radiative Transfer Modeling: To quantify ice unit (1A, 1B) properties, we modeled spectra via Hapke theory (Hapke, 2012) using ice optical constants (Mastrapa et al., 2009) from 50-150 K. Areal mixing models (using 1C/1D and ice endmembers) yielded ice T, granulometry, and abundance. A linear correction addressed residual continuum slopes.
• Geological Mapping: Geomorphological mapping used Dawn FC images in QGIS, aided by a high-resolution DEM produced via ISIS stereo. This involved identifying and delineating structural (fractures), surface textures and features such as crater walls, taluses slump blocks, flow-like features, and the distribution of high-albedo materials potentially associated with ice deposits.

Results

• Spectral Modeling: Our analysis indicates that areal mixing models provide better fits for VIR spectra of ice-bearing units compared to intimate mixing models in accordance with previous studies (Combe et al., 2016, 2019; Raponi et al., 2016;). The best-fit parameters for the two ice-bearing classes under areal mixing are:
• Class 1A: effective ice temperature of T ≈ 110 K,  grain size of ~125 µm, and ice abundance of ~1.4%.
• Class 1B: also T ≈ 110 K, but a distinct grain size of ~105 µm and ice abundance of ~2.0%.
• Geological Analysis: Preliminary mapping reveals complex wall structures with evidence of slumping and terracing, particularly on the pole-facing slopes where ice exposures are concentrated. The spatial distribution of the spectrally classified units (1A, 1B, 1C, 1D) is being actively correlated with these mapped geomorphic features.

Discussion

The derived effective temperature of ~110 K for water ice in both dominant spectral classes (1A and 1B) is noteworthy. This temperature is significantly lower than Ceres' average daytime surface temperature (Combe et al., 2016; Formisano et al., 2019) but may reflect diurnal temperatures in pixel-scale micro-shadowed regions. The distinction between Class 1A and 1B, primarily in grain size, suggests potential differences in formation mechanism, exposure age, or subsequent surface processing (preferential sublimation, regolith mixing).

The slightly higher abundance and smaller grain size of Class 1B might indicate relatively fresher or purer ice exposures compared to Class 1A, where ice and carbonates appear more co-located. Additionally the presence and distribution of Class 1C pixels are also significant. This observation could represent several scenarios: areas where water ice, perhaps initially associated with carbonates as in Class 1A, has subsequently sublimated away, leaving a carbonate-enriched lag deposit; locations where carbonates were emplaced via a distinct process (impact-related, endogenic activity) not directly linked to the currently observed primary ice exposures; or surfaces where ice might persist below VIR's detection limits. Understanding the origin of these carbonate-rich, ice-poor areas, together with ice rich areas, is key to interpreting the evolution of volatile exposures and the driving mechanism for geology within Oxo.

The geological mapping aims to determine if these different spectral units (1A, 1B, and 1C) correlate spatially with morphological features. Are slumps predominantly associated with a particular ice type (1A vs 1B), suggesting a causal link between ice properties (abundance, perhaps influencing mechanical strength) and mass wasting? Furthermore, analyzing the spatial relationship between the ice-bearing units (1A, 1B) and the carbonate-dominant units (1C) can provide constraints on transport mechanisms, relative emplacement timing, and the longevity of exposed ice. Integrating spectral data with geomorphology constrains Oxo's evolution and how near-surface materials modify Ceres' craters, informing volatile-rich small body surface dynamics.

Conclusions

Our integrated analysis of VIR spectroscopy and geological mapping reveals distinct compositional units, including populations of water ice, within the young Oxo crater on Ceres. Characterized by an effective temperature of ~110 K but differing grain sizes, abundances, and significant variations in association with carbonates, these surface materials likely reflect complex emplacement and modification histories. Ongoing work correlating these spectral units with detailed geomorphological mapping will further understanding about the role of water ice and associated non-ice components in controlling surface evolution and mass wasting processes on Ceres. This study highlights the synergy between spectral modeling and geological investigation in deciphering the activity and evolution of volatile-rich small bodies.

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