RADIOACTIVE HEATING OF POROUS COMET NUCLEI

Radioactive heating is the main energy source of comets residing in th e distant parts of the Solar System. It should determine whether liqui d water could have existed in comets and whether comets, presumably fo rmed of amorphous ice, could have retained the ice, at least partly, i n this pristine form. Thermal evolution calculations of relatively lar ge (over 10 km in radius), porous comet nuclei are performed for many different initial parameter combinations. The radioisotopes considered are K-40, Th-232, U-235, and U-238, in meteoritic abundances, as well as Al-26, in various initial abundances. We allow for heat conduction through the ice-dust matrix, as well as advection by flowing gases. C rystallization of the amorphous ice accompanied by release of occluded gases, and sublimation/condensation from/ onto the pore walls are tak en into account. We find that porous comet nuclei may emerge from the long-term evolution in three different configurations, depending on th e thermal conductivity, porous structure, radius, etc.: (a) preserving their pristine structure throughout; (b) almost completely crystalliz ed (except for a relatively thin outer layer), and (c) having a crysta llized core, a layer of frozen gas (originally occluded in the amorpho us ice) and an outer layer of unaltered pristine material. Liquid core s may be obtained only if the porosity is negligible. The extent of su ch cores and the length of time during which they remain liquid are ag ain determined by initial conditions, as well as by physical propertie s of the ice. If, in addition to the very low porosity, the conductivi ty were extremely low, it should be possible to have both an extended liquid core, for a considerable period of time, and an outer layer of significant thickness that has retained its original pristine structur e. (C) 1995 Academic Press, Inc.