nai

Project Progress

NAI team member Meech, and postdoc Pittichova, have been collaborating with Akiva Bar-Nun and Gila Notesco (University of Tel-Aviv), to understand the role of water ice in the activity of comets on their inbound orbital trajectories at large heliocentric distances. A thorough understanding of cometary activity is closely coupled with knowledge about the formation of comets, the physical and chemical structure in the nucleus, and its subsequent evolution, and this has implications for delivery of volatiles and organics to the terrestrial planets. The traditional view that sublimation of water-ice is the driver of activity in comets is changing considerably as observations with new telescope facilities are showing that comets are often active at very large distances from the sun, where the ambient temperatures are much lower than the sublimation temperature of water-ice. Thermal models have been able to explain some of the activity post-perihelion, due to slow transport of the heat from perihelion into the interior, however activity at large distances before the comet is heated has been difficult to explain. Work carried out on amorphous, gas-laden ice samples suggests that the cause of distant cometary activity is from gas release during annealing processes in the water ice between 35 and 120K. The experimental set up involves flowing different mixtures of gases onto a cryogenically cooled plate at 50K in a pumped chamber at 10⁸ torr, forming 5 to 200-micron-thick ice layers. The ice deposition plate is then heated while monitoring the emanation of trapped gases, water vapor and ejected ice grains. The experiments show that the gases trapped in the amorphous ice during its formation are partly released from the ice during its annealing as it warms up. A paper is in progress describing this work.