THE TRANSPORT OF SUPER-VOLATILES THROUGH COMETARY NUCLEI BY RADIOACTIVE HEATING

A study has been made of the transport mechanism of volatile molecules such as N-2 and CO through cometary nuclei as they are heated by radioactive elements. Coupled equations of heat and gas transport in the presence of gas sublimation and recondensation, as well as a heat source, are numerically solved. It is assumed that the effective surface area where sublimation and condensation take place is so large that the gas pressure is always very close to the saturation pressure. It is shown that super-volatiles such as N-2 and CO are transported through the pores of the nucleus, and consequently the volatile molecules become more abundant near the surface than deep inside the nucleus. Such a modification of structure probably gives rise to the activity of large comets at large solar distances, as a portion of the surface is excavated by small-body impacts, while small comets which have not evolved significantly may last for a long time, because the sublimation is largely due to H2O.