Four decades of snowpack sintering theory are at odds with observations and with theory for other materials. Vapor diffusion from ice grains to necks, controlled by surface radii of curvature, has long been considered the dominant sintering mechanism in dry snow. However, the detailed geometry of bonds and the shrinkage of snow during sintering are inconsistent with vapor diffusion and suggest combined grain boundary and surface diffusion as the dominant mechanisms. Snow contains minute quantities of soluble impurities, and scanning electron microscopy confirms their concentration on pore surfaces and at the boundaries separating bonded snow grains. Activated sintering of metals employs small quantities of dopants to reduce the powder melting temperature, activation energy and grain boundary energy, thereby increasing the rate and strength of sintering. Is sintering in snow a similar activated sintering process? Contrasting sintering theories lead to predictions about geometry that offer clear tests for identifying relevant processes under different conditions. For both pure and impure ice, we may distinguish between operative mechanisms and identify the controlling processes with a decisive set of sintering experiments across a wide range of temperatures, densities, particle sizes and geometries. Scanning electron microscopy will produce time-series observations of the sintering process from the compacted snow level down to the micrometer scale for snow crystals gathered in the field as well as those grown and allowed to sinter in the laboratory. Atomic force microscopy will carry the topographic component of the investigation down to molecular scale. Optical and electron scattering properties of ice allow us to track grain orientation, grain boundary geometry and migration, and pore evolution from initial adhesion through final stage sintering. Additional laboratory measurements will characterize changes in mechanical, bulk and microstructural properties due to sintering. This work has implications beyond sintering theory, from understanding snowpack stability and avalanche hazard during winter deposition to identifying factors affecting solute concentrations in the spring runoff.
