Substrates and the growth of crystals on for icy planetary surfaces

Experiment has shown that particle size is a strong function of growth temperature, and a weaker function of substrate, for deposition of amorphous ice in laboratory conditions. The size of the crystals remains stable over the time scales accessible in the laboratory (short - weeks). Additionally, the temperature regimes for outer planet satellites sufficiently low that ice should deposit in the amorphous form. This is contrary to observation spectral evidence that shows the outer planet satellites to be surfaced with hexagonal ice (which has a higher formation temperature than found on the surfaces of satellites) with reasonably large crystal sizes. This discrepancy between observed crystal form and deposition temperature has been explained variously with evolution of the crystal form from amorphous to hexagonal over geologic time; effects of radiation or impact; and low mobility of water vapor at temperatures extant on the outer planet satellites (< 140 K). Substrate effects may provide an additional mechanism for forcing crystalline deposition of ice at low temperatures. A crystalline ice substrate, once formed, may make deposition of ice in amorphous form energetically unfavorable. This mechanism, if operative, could imply a formation history for the substrate - though the timescales for which this record would be valid will be very difficult to establish experimentally. This work was supported by JPL under contract to NASA