Modeling Effusive Cryolava Flows: Reevaluating Emplacement & Feasibility of Tubes

While cryolava tubes have been mentioned casually in the literature, previous studies do not establish their feasibility, consider thermal and mechanical properties separately, or make incorrect simplifying assumptions. We present a preliminary model that tracks the evolution of a cryolava flow during emplacement, including coupled thermal and material properties. We find that cryolava flows are initially turbulent, entraining crystals rather than fractionating them to form an ice crust/roof. At such low viscosities, turbulence may persist to large crystal fractions. This suggests the evolution is more like slushie formation than a freezing river, and crust formation is not as simple as an ever-thickening ice layer. Therefore, we focus on the physical state of the flow at the transition to the laminar regime, which results in rapid crust formation from floatation, to inform where tube formation is likely. In this preliminary study we focus on H2O-NaCl liquids erupting into a vacuum onto pure water-ice substrate. The thermal budget is dominated by heat loss from vaporization and warming of eroded substrate material (xenoliths). Material properties (e.g., viscosity, density, solid fraction, velocity, etc.) are tracked until the transition to laminar flow occurs. The physical state of the flow is then used as input for modeling the laminar regime, handled separately. Preliminary results suggest that transition to laminar flow occurs at >~60 vol% crystals, typically reached at the eutectic for compositions >~12 wt% NaCl. If this holds for all initial conditions, it suggests yield strength development may be possible which would oppose buoyancy-driven floatation to a degree making formation of a tube roof more difficult. Further modeling is required for other chemical systems to constrain formation and feasibility of cryolava tubes. Our long-term goal is to explore the relationship between surface morphology and physical conditions of the flow at a given time, temperature, or distance from the vent. This will enable inferences about flow texture, albedo, and/or geometry, aiding in photogeology and interpretations of features observed during upcoming missions to ocean worlds. Some of this research was carried at JPL-Caltech under a contract with NASA.