To emphasize the generality and power of the physics of scattering that has been the central theme of this chapter, we will remark in closing that the very same physical principles account for the high albedo of snow and ice. The point of commonality with scattering from cloud droplets is that any discontinuity in index of refraction will lead to scattering. In the case of snow, the discontinuity is between the crystals of particles and the voids between them. Given how much denser most solids are than gases, it matters little whether the voids are filled with air as on Earth, or filled with near-vacuum as they would be on Europa. Similarly, as long as the snow is made of a mostly transparent solid, it matters little just what it is made of. There are variations in index of refraction amongst different ices, but all are significantly different from unity. All snow is highly reflective, whether it be N2 snow on Triton or CO2 snow on Mars. For ice, the scatterers are air bubbles or brine pockets, and here it matters a bit more what the composition of the freezing fluid may be. To make gas bubbles in the frozen liquid, there must be a significant amount of some gas dissolved in the fluid, and to make brine pockets there must be some suitable solute (salt in the Earth case). The rate of freezing also makes a difference, to the albedo of ice, since slow freezing allows gas to be rejected before bubbles form, leading to clear, low-albedo ice. Things get even more interesting if one allows for an admixture of absorbing particles (dust or soot) with the snow or ice.
Since albedo has such an important effect on planetary radiation budgets, the physics of snow and ice albedo is a critical field of play for radiative transfer. It can be treated using essentially the same techniques that have been introduced in this chapter.
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