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Project Progress

Work was completed and published presenting the observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous-crystalline ice phase transition (T˜137 K). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. New HST observations were obtained to image the nuclei of several of these comets in order to combine the observations of activity with dust-dynamical modeling and thermal models to better explore the causes of activity. Preliminary models for some of the comets show that contrary to our expectations, it may be that the differences in behavior are more closely tied to different primordial microphysical properties in the outer layers of the nucleus (which affect the heat flow) than they are to major compositional differences. In the upcoming year we will begin the modeling combining the heliocentric light curve data, and the HST nucleus size estimates with dust and thermal models.