The transport of super-volatiles through cometary nuclei by radio-active heating

Investigation is made of the transport of volatile molecules such as N 2 or CO through cometary nuclei as they are heated by radio-active elements. Coupled equations of heat and gas transport in the presence of gas sublimation and recondensation as well as heat source are numerically solved. For the sake of numerical stability, it is necessary to take a small value for the effective surface area where sublimation and condensation take place. It is shown that the super-volatiles such as N 2 or 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 is expected to have observational consequences on activities at large heliocentric distances of comets such as P/Schwassmann-Wachmann I and Bowell.