Atypical planetary lavas: rheological evolution of cooling and crystallizing flows from lunar impact melts and cryovolcanic processes

Abstract

Volcanism is common to many of the solid planets and moons throughout the solar system. On Earth, volcanic research is mainly targeted at hazard assessment and prediction but volcanism on other worlds helps us understand how planetary bodies evolve and what that evolution means for the Earth and its future. Understanding the volcanic process on our world and others yields information about heat and mass transport processes, and about interior and surface evolution.One way of furthering our understanding of the volcanic process is by investigating the erupted products. Lavas in particular make up a large portion of planetary surfaces, however, some lavas in the solar system are very different to what we expect on Earth. Both impact events and ice volcanism (cryovolcanism) in the outer solar system can create molten material of very different compositions to the silicate volcanism on Earth, at very different conditions (e.g., temperature and pressure). Despite this, many planetary features share common morphologies with terrestrial volcanism, suggesting similar physical processes driving emplacement. In this work, I draw comparisons between composition and formation mechanism for impact melts, cryovolcanism, and silicate volcanism by investigating their rheology – the flow behavior that links material properties to morphology.I measured the rheology of lunar simulants for both highland and mare compositions to investigate how lunar impact melts evolve as they flow. Crystallization happens rapidly upon crossing the liquidus for highland compositions but mare compositions require undercooling before rapid crystallization occurs. This leads to shorter, thicker flows in the highlands and longer, thinner flows in the mare. This pattern may explain why more highland impact melt sheets are observed, because the thinner impact melts in the mare are more readily erased by impact gardening resulting in a preservation bias in the rock record. I also synthesized a wide range of aqueous solutions as analog cryolavas to measure their viscosity. I developed a new viscosity model, based on the Vogel-Fulcher-Tammann (VFT) equation commonly used in silicate rheology, to predict viscosity of aqueous solutions a function of both temperature and concentration for binary systems. This model provides better extrapolation down to cryogenic temperatures than previous models and can be scaled up to more complicated multicomponent systems. I then developed a new model for cryovolcanic flow evolution to investigate emplacement. This model simultaneously tracks the physical, chemical, and thermal state of the flow and allows entrainment of the solid fraction rather than surface accumulation. These are all improvements over several previous models. I found that the heat loss from vaporization of the flow in the low-pressure environment of many icy worlds was the dominant heat flux and that aspect ratios predicted match well with observed features.