The cometary activity of Chiron: a stratigraphic model

A theoretical model of a comet nucleus composed of [H.sub.2]O ice, a highly volatile ice, and nonvolatile material is used to simulate the recently observed comet-like activity of minor planet Chiron. It is found that even at Chiron's range of heliocentric distances, a continuous low level of activity can be sustained by the sublimation and diffusion of CO and/or C[H.sub.4] ices beneath a water ice and dust mantle and would be sufficient for possible emission of any loose submicrometer [H.sub.2]O ice grains. Due to Chiron's large size this continuous, low level gas flux can result in a significant loss of mass that can produce comet-like activity. The receding volatile ice never reaches an equilibrium depth but continues to recede slowly with time. The gas diffusing through the [H.sub.2]O ice mantle also decreases slowly with time due to the increasing mantle thickness. The mantle keeps the volatile ice interface nearly isothermal throughout an orbit after the first few orbits. This seems to explain the lack of correlation of outburst activity with heliocentric distance as is observed for most comets if Chiron typically contains a large fraction of water ice and if the water ice possesses sufficient strength to resist blowoff. We attempt to show that several processes suggested as a trigger for the apparently stochastic outbursts are not possible such as simple accumulation of highly volatile molecules, impacts, or thermal cracking of an [H.sub.2]O crust. We speculate that the outbursts may be caused by occasional exposure of more active regions on an object of high obliquity. The possible influence of impacts and temperature induced stress cracks on outbursts is discussed. Our results are compatible with previous findings that Chiron must have been inserted into its present orbit very recently.