Karen Meech
Institute for Astronomy, University of Hawaii
 

"What Exploration of Small Icy Bodies Tell Us About the Physics and Chemistry of the Early Solar System"
 

Small icy bodies in the outer solar system have had an enormous influence on the architecture of the solar system, and have probably played a role in the delivery of organics and volatiles to the early Earth.  Moreover, they preserve a record of the physics and chemistry in our Solar System's protoplanetary disk.  Observations of cometary activity in these objects when they are far from the sun are especially useful for making inferences about both these early conditions and the dynamical history of the solar system.  A summary of distant comet observations spanning 20 years will be presented.  All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant on the inbound (coldest) portions of their orbits. In conjunction with this database, we have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated above the temperature of the amorphous to crystalline ice phase transition (T~135K).  We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System.  Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases.  These results are consistent with both historical observations of distant comet activity and with the data presented here.  The ice annealing process can only release gases at temperatures above the temperature at which the amorphous ices condensed. Thus, if observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed.  New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive. Additional thermal models have been run to explore the heliocentric light curve behavior of these comets, and our results show that we may be seeing evidence of different physical properties in the surface materials of comets showing different formation locations.