Microstructural evolution of solar system ices through sintering

Sintering is a temperature-dependent form of metamorphism whereby adjacent particles within an aggregate experience diffusion of material into their contact regions, forming necks between them, isolating pore space, and becoming denser over time. Terrestrial studies have shown that this process significantly modifies the microphysical structure (e.g., porosity, thermal conductivity, strength) of snowpack on Earth over very short timescales. While limited work has been done on sintering in planetary environments, existing research suggests it also plays a strong role in the surface evolution of icy satellites, comets, and even Mars. Understanding this process is critical to characterizing the thermal and mechanical properties of regolith at their surfaces, has important implications for the interpretation of remote sensing data, and is crucial for the development of future in-situ exploration of Ocean Worlds. Unfortunately, terrestrial studies typically utilize field samples for this research, and thus limited progress has been made in adapting available sintering models for ice. In recent work, PI Molaro implemented a widely used metal alloy sintering model to simulate ice and compared the results to existing measurements of ice sintering rates. While the model has good agreement with some studies, it disagrees with others. Overall, experimental data in the literature is sparse and reported values of sintering rates are inconsistent, highlighting the need for more laboratory studies on this topic.

The proposed work uses a combined laboratory and modeling approach to explore the role of sintering on the evolution of water ice on icy satellites, Mars, and across the solar system. 1) We will perform experiments to image the sintering of ice grains under a microscope throughout the process, allowing us to better quantify the evolving morphology of water ice grains. Measurements of pure water and salty ices will be performed over a range of temperatures and grain sizes under ambient pressure, as well as in CO2 and N2 environments. 2) With the results of these efforts, we will validate the model and make improvements to its treatment of ice. We will utilize the model to constrain the sintering rates and microstructural properties of water ice on Europa, Enceladus, and Mars. 3) To demonstrate how this model may be used, we will relate our model results to existing observations of Europa and Enceladus to create global sintering maps of their surfaces, and constrain their microstructural surface properties. 4) We will also apply the model to Mars, by exploring the role that diurnal and seasonal thermal cycling plays in sintering rates, and the formation of subsurface density gradients. We will compare the results to a model of vapor diffusion densification to explore how these processes may produce layering in martian ice deposits, and relate the results to spacecraft images that show such layering effects on Mars.

The proposed work is directly relevant to the objectives of the Solar System Workings program, as defined in Appendix C.3 of ROSES-2017, including: characterize and understand the chemical, mineralogical, and physical features of planetary surfaces. […] Develop theoretical and experimental bases for understanding these features in the context of the varying conditions through time of formation (Surfaces and Interiors item 6, C.3-2). Furthermore, it addresses key questions and specific objectives associated with the science goals outlined by the 2013-2022 NRC Planetary Science Decadal Survey, including: how have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time (p. 71, question 10), and determine the effects and timing of secondary processes on the evolution of primitive bodies (p. 89, objective 3, goal 1), and characterize planetary surfaces to understand how they are modified by geologic processes.