Albedo Variations and Bright Spot Formation on Comet 67P: Microphysical Scenarios

The temporary exposure of volatiles on comet 67P/Churyumov-Gerasimenko, driven by sublimation-induced erosion, has been extensively discussed in previous studies. Bright spots observed on the surface have been attributed to the appearance of water ice [1, 2, 3]. These ice exposures, or bright features, were detected in optical OSIRIS/NavCam images through time-series analysis, appearing as high-albedo spots in several geomorphological regions. Temperature and spectral analyses of data from the VIRTIS-M instrument further confirmed the presence of water ice. Subsequent research [4] revealed a broad spatial distribution of these bright features. Notably, surface exposure of volatiles is not unique to 67P but has also been observed on other comets, such as 9P/Tempel 1 [5].

The formation of these bright spots on the surface of 67P may be the result of sublimation-driven erosion, likely involving subsurface volatiles. Mapping studies [4] and our own findings suggest a concentration of bright spots in the equatorial region, with a slight shift towards the southern hemisphere. These spots are predominantly located in rugged terrains with consolidated material. Additionally, the locations of faint jet outbursts and previously detected jets near the perihelion may be associated with some of these bright spots.

Albedo variation, a key parameter for detecting bright spots, was analyzed using the single-scattering albedo derived from the radiance factor and illumination data from OSIRIS images. By applying the algorithm from [6] and the Hapke reflectance model, we were able to extract albedo variations, revealing local discrepancies between the target features and the surrounding surface. Time-series analysis of the OSIRIS dataset confirmed the presence of numerous bright spots, supporting the hypothesis that these features result from sublimation-driven erosion, although the underlying physical processes may vary.

In our study, we focus on the microphysical processes that could lead to temporary increases in surface brightness. We introduce a novel heat transfer model for the near-surface layer, grounded in the medium’s microphysical properties [7]. This model consistently accounts for various heat transfer mechanisms, the permeability of the crust to gas flow, and other relevant factors. For the first time, we propose an evolving layer model that predicts the 'oscillating' behavior of the crust, where phases of accumulation (thickening) and discharge alternate repetitively.

In addition to the above model, we also examine the evolutionary model [8], which investigates the erosion of a heterogeneous layer composed of small ice particles and dust. This model introduces a unique approach to describing the evolution of such a mixture, providing estimates for both the removal of dust particles—while preserving exposed ice regions—and the formation of a temporary dry crust.

The computational experiments performed in our study allow for a comparison between the lifetime of bright spots and observational data. This provides us with quantitative constraints on the microscopic properties of the near-surface material.

References: [1] Pommerol A et al. (2015) A&A, 583, A25. [2] Barucci M. A. et al. (2016) A&A, 595, A102. [3] Filacchione G. et al. (2016a) Nature, 592, 7586, 268-372. [4] Deshapriya J. D.P. et al. (2018) A&A, 613, A36. [5] Sunshine J. M. et al. (2006) Science, 311, 5766, 1453-1455. [6] Davidsson B. J. R. et al. (2022) MNRAS, 516, 4, 5125-5142. [7] Xin Y. et al. (2025), A&A, 693, A123. [8] Schuckart , C. and J. Blum (2025) A&A, DOI: https://doi.org/10.1051/0004-6361/202553750