  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 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.
