Penitente Spire Geometry as an Anisotropic Thermal Boundary Condition on Europa: A Pin-Fin Analogy and Vapor-Phase Pseudocapillary Hypothesis

Current models of heat transport on icy moons, including the recent ocean circulation framework of Zhang et al. (2025/2026), treat the ice-ocean interface and ice surface as homogeneous boundary conditions. No existing model incorporates meter-scale surface texture into heat flux estimates. This paper proposes two related, testable hypotheses concerning Europa's predicted penitente field. First, that penitente spires function as inverted pin-fin heat exchangers: in Europa's reversed thermal gradient (cold exterior, warm interior), high-surface-area-to-volume spire geometry preferentially channels ascending internal heat toward the surface, while vapor-filled inter-spire troughs act as insulating gaps, producing anisotropic solid-state thermal conductivity across the surface layer. Second, that vapor pressure gradients between spire tips and troughs drive a sublimation-transport mechanism structurally analogous to capillary action - a process unmodelled in existing penitente formation theories, which assume meltwater as the working fluid. Both hypotheses are conceptual and quantitative modeling is invited.