The Microstructural Evolution of Water Ice in the Solar System Through Sintering

Ice sintering is a form of metamorphism that drives the microstructural evolution of an aggregate of grains through surface and volume diffusion. This leads to an increase in the grain-to-grain contact area (neck) and density of the aggregate over time, resulting in the evolution of its strength, porosity, thermal conductivity, and other properties. This process plays an important role in the evolution of icy planetary surfaces, though its rate and nature are not well constrained. In this study, we explore the model of Swinkels and Ashby (1981, ) and assess the extent to which it can be used to quantify sintering timescales for water ice. We compare predicted neck growth rates to new and historical observations of ice sintering and find agreement to some studies at the order of magnitude level. First-order estimates of neck growth timescales on planetary surfaces show that ice may undergo significant modification over geologic timescales, even in the outer solar system. Densification occurs over much longer timescales, suggesting that some surfaces may develop cohesive, but porous, crusts. Sintering rates are extremely sensitive to temperature and grain size, occurring faster in warmer aggregates of smaller grains. This suggests that the microstructural evolution of ices may vary not only throughout the solar system but also spatially across the surface and in the near surface of a given body. Our experimental observations of complex grain growth and mass redistribution in ice aggregates point to components of the model that may benefit from improvement and areas where additional laboratory studies are needed. Plain Language Summary Ice sintering is a process that occurs to fresh ice grains deposited onto a planetary surface, which causes them to stick to each other and diffuse together. The contact regions (or necks) between individual grains and the density of the aggregate increases, leading the ice to become stronger and more cohesive over time. This process plays an important role in how ice surfaces evolve, which has implications for predicting their surface characteristics, interpreting spacecraft and telescopic observations, and developing technology to land on and sample these bodies. In this study, we use a numerical model to calculate the rate that sintering occurs in ice grains of varying size and temperature. We compare the predicted sintering rates to experimental observations and calculate estimates of sintering timescales on planetary surfaces. Our results suggest that that ice on planetary surfaces can undergo significant modification over geologic timescales, even in the outer solar system where the cold temperatures result in slow sintering rates. We find that many bodies may develop a cohesive, but porous, surface crusts. Due to the temperature dependence of the process, the evolution of ices is likely to vary significantly throughout the solar system, as well as spatially across a given surface.