POLARIZATION BY FROST FORMED AT VERY-LOW TEMPERATURES, AS RELEVANT TOICY PLANETARY SURFACES

Optical polarization by water-ice frost formed at temperatures down to 100 K is measured, and found to be significantly different from that previously observed for terrestrial ice and frost; the difference is g reatest for frost with very small - possibly subwavelength - ice cryst als, formed by very rapid cooling of a small amount of water vapour: t his is found to give deep negative polarization at a small phase angle , and a high value of slope. Assembled ice and frost data give a new s lope/albedo relationship, significantly different both from the well-e stablished relationship for terrestrial and planetary rocks, and from the intermediate band for opaque light-coloured powders. This new rela tionship is relevant to the interpretation of observations of icy plan etary satellites. The UMIST polarimeter, with its low-temperature samp le holder, is described.