Physical properties of spectrally distinct surface units on Ceres

Introduction

Our previous work proposed a two-stage spectral evolution for Ceres. During the early stage within 10⁶ years, fresh materials on Ceres become spectrally blue, the 2.7 μm band strength weakens, and the band center shifts toward longer wavelength. Then the late-stage after 10⁶ years is characterized by spectral reddening and band strengthening without the band center shift [1]. Based on these spectral characteristics, the flanks of the cryovolcanic dome Ahuna Mons on Ceres [2] can be divided into three spectrally distinct surface units (Fig. 1):

(1) Young: the 2.7 μm band center shifts to shorter wavelengths;

(2) Intermediate: an unshifted 2.7 μm band center and spectrally blue;

(3) Old: an unshifted 2.7 μm band center and spectrally red.

To test whether the physical properties of these units contribute to the observed spectral variations, we analyze their photometric and thermophysical properties.

Fig.1 (a) 2.7 μm absorption center map, (b) visible/near-infrared slope map, and (c) image of Ahuna Mons. The black, white and red ROIs indicate the three spectrally distinct units, corresponding to young, intermediate, and old units, respectively.

 

Photometric properties

We used all Framing Camera (FC) color images covering the Ahuna Mons region acquired during the Rotational Characterization 3 (RC3), Survey, High Altitude Mapping Orbit (HAMO), Low Altitude Mapping Orbit (LAMO), and Extended Mission Orbit (XMO) phases, with phase angles ranging from ∼5° to ∼85°. We adopted a five-parameter Hapke model [3]. The amplitude parameter (B₀) and width parameter (h) of the shadow-hiding opposition effect are fixed since there are not sufficient data available at low phase angles [4]. The remaining three free parameters, including the single-scattering albedo, the asymmetry factor of the single-term Henyey-Greenstein function, and the roughness parameter, are fitted using a Bayesian approach Markov chain Monte Carlo. Preliminary results show that the single-scattering albedo and surface roughness appear to vary systematically with surface exposure age (Fig. 2).

Fig.2 Hapke model parameters of the three spectrally distinct surface units on Ahuna Mons. The B₀ and h parameters are fixed to 1.6 and 0.06, respectively.

 

Thermophysical properties

The photometric analysis is further complemented by thermal modeling of the Ahuna Mons region. Surface temperatures are retrieved from Visible and Infrared Spectrometer (VIR) data in the infrared range dominated by thermal emission [5]. Ongoing work applies a thermophysical model to derive the thermal inertia and surface roughness parameters of the different units, which could also constrain their surface physical properties.

 

References. [1] Zhang, Q., & Li, J. Y. (2025). EPSC-DPS Joint Meeting 2025, EPSC-DPS2025-570. [2] Ruesch, O., et al. (2016). Science, 353(6303), aaf4286. [3] Hapke, B. (2012). Cambridge university press. [4] Li, J. Y., et al. (2019). Icarus, 322, 144-167. [5] Simon, A. A., et al. (2020). Science, 370(6517), eabc3522.