Comparisons between frost observations on Ralph/MVIC color images in the Cthulhu region reported by Bertrand et al. (2020) at Venera Terra (A), Virgil Fossae (C), Beatrice Fossae (E), and the southern part of the region (G) and the model outputs (right) assuming an albedo of 0.69 for methane ice to achieve better agreement with the observations. For the performance of the baseline model (albedo of 0.67); see Fig. A.1. Perennial accumulation occurs on south-facing slopes (not shown). Perennial accumulation occurs on south-facing slopes (not shown). Our simulations predict that the CH₄ frost deposits (white patches in panels A, C, E, G) observed on north-facing slopes correspond to seasonal CH₄ frost that gradually sublimates and should disappear shortly after the New Horizons flyby. No curves are shown for south-facing slopes, as no CH₄ frost is visible in panels A, C, E, G due to shadowing. These slopes also display only perennial accumulation, without seasonal variability (see Fig. 6).