Optical properties of deep ice at the South Pole: scattering

Abstract

Recently, absorption and scattering at depths 800-1000 m in South Pole ice have been studied with transit-time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At 800-1000 m, scattering is independent of wavelength and the scattering centers are air bubbles of size ? wavelength. At 1500-2000 m it is predicted that all bubbles will have transformed into air-hydrate clathrate crystals and that scattering occurs primarily at dust grains, at liquid acids concentrated along three-crystal boundaries, and at salt grains. Scattering on decorated dislocations, at ice-ice boundaries, and at hydrate-ice boundaries will be of minor importance. Scattering from liquid acids in veins at three-crystal boundaries goes as ~lambda(-1) to ~lambda(-2) and should show essentially no depth dependence. Scattering from dust grains goes as ~lambda(-2) and should show peaks at depths of ~1050, ~1750, and ~2200 m in South Pole ice. If marine salt grains remain undissolved, they will scatter like insoluble dust grains. Refraction at ice-ice boundaries and at hydrate-ice boundaries is manifested by a multitude of small-angle scatters, independent of wavelength. The largest contribution to Rayleigh-like scattering is likely due to dislocations decorated discontinuously with impurities. Freshly grown laboratory ice exhibits a large Rayleigh-like scattering that we attribute to the much higher density of decorated dislocations than in glacial ice.