Modelling Voluminous, Rapid, Lava Flow Emplacement on Io

Io's voluminous and powerful "outburst" eruptions are characterised by lava fountains feeding extensive lava flows [1], an eruption style likely similar to that of ancient lunar eruptions [e.g., 2]. Spacecraft observations, primarily from Galileo, have provided intermittent snapshots of this activity at moderate spatial resolution in the visual and infrared, and ground-based observations [3] have provided more detailed temporal coverage at much lower spatial resolution. To maximize the extraction of information from these data we develop physical models of eruptions in a vacuum to fit to the available data. The heat output from the most energetic Io eruptions is hard to understand unless high initial magma discharge rates generate fast-growing lava flows whose initial motion is fully turbulent. We therefore numerically model radiative heat loss from turbulent lava of a range of compositions. Critical to understanding the time-variation of the total heat output is knowledge of when, and how far from the vent, lava motion becomes laminar. We therefore track how cooling of turbulent lava causes growth of phenocrysts and a progressive onset of non-Newtonian rheology. Increasing viscosity causes the Reynolds number, Re, to decrease, and increasing yield strength causes the Hedstrom number and the critical Reynolds number for turbulence, Re_crit, to increase. When Re becomes less than Re_crit a rapid transition to laminar flow allows the growth of a cooling crust and cooling basal thermal boundary layer. For mafic and ultramafic lava compositions, the cool crust thickness is always less than the depth of the unsheared plug controlled by the yield strength. When the lower thermal boundary layer grows upward to meet the base of the plug, all motion ceases. This is a very different basis for determining maximum flow length from the Gratz number criterion commonly used for flows that are laminar at all distances from the vent. Integration of the heat output from all parts of the resulting flow as a function of time is being compared with ground-based telescope data [3] to select viable eruption scenarios. This work is supported by the NASA SSW Program. References: [1] Davies, A. G. (1996) Icarus, 124, 45-61. [2] Wilson, L. and Head, J. W. (2018) GRL, 45, 5852-5859. [3] de Pater, I. et al. (2014) Icarus, 242, 352-364.