The volatility of ice below 160K

Amorphous solid water (ASW) is probably the most abundant form of water in the universe. It forms when water vapor deposits at low temperatures. At temperatures above about 130K ASW crystallizes on time scales below one day, thereby forming nano-crystalline ice. Up to now, the saturation vapor pressure of ASW and nano-crystalline ice were not well characterized, but are crucial to describe the sublimation and growth of interstellar ices or cold ice cloud formation in planetary atmospheres, e.g. polar mesospheric clouds on Earth and H2O clouds on Mars. In this contribution, we present results from a laboratory experiment which is designed to study ice nucleation and growth processes on nanoparticles at temperatures below 150K. We use the ice growth-rate on nanoparticles at low and high supersaturation to evaluate for the saturation vapor pressure of ice phases deposited below 160K. We conclude, that ASW initially deposits, followed by temperature activated crystallization to nano-crystalline ice with crystal sizes between 7 and 19nm. We show that the saturation vapor pressure of ASW is substantially higher than previously assumed. Previous parameterizations for the saturation vapor pressure of ASW were in part based on differential scanning calorimetry measurements of the heat released during the crystallization process of ASW. We reconcile these measurements with our results by acknowledging the fact that the saturation vapor pressure of nano-crystalline ice is 200 to 300% higher than for macro-crystalline ice. Finally, we present new parameterizations for the saturation vapor pressure of ASW and nano-crystalline ice.