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Project Progress

Many comets have been observed to have activity at large heliocentric distances on their inbound orbital legs at distances too far from the sun to be caused by sublimation of pure water ice. 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. It is unlikely that the dominant form of activity at large distances is caused by sublimation of pure ices which are more volatile than water because laboratory experiments have shown that as water vapor in the nebula condensed at low temperatures it would have trapped these more volatile materials as gases in an amorphous water-ice matrix. Release of the trapped gases would then be controlled by the physical properties of the water ice as it warms up. Understanding the causes of activity at large distances can provide information about the chemistry and small scale structure in primitive bodies, and this relates to constraints on the chemistry, physical conditions and planetesimal formation processes in the young solar nebula.

NAI team member Meech, and postdoc Pittichova have been collaborating with Akiva Bar-Nun and Gila Notesco (Univ. 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 has implications for delivery of volatiles and organics to the terrestrial planets. 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 10^-8 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.

We have been comparing model results to activity in comets at large distances, in particular on their first inbound passage into the inner solar system. Work has continued this year on the observing and calibration of the comet data.

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