MODELING THE THERMAL-PROPERTIES AND THE GAS FLUX FROM A POROUS, ICE-DUST BODY IN THE ORBIT OF P WIRTANEN/

Sublimation of gases from ices in porous bodies is an important, obser vable phenomenon in the solar system, Dust mantle build-up and surface erosion a on a comet nucleus and coma formation are processes related to the flux of sublimating gases. Computer simulations are performed to model the gas flux from volatile, icy components in the surface lay er of am porous, Jupiter-family model comet assuming a porous body, co ntaining dust and up to four components of chemically different ices ( e.g. H2O, CO, CO2, CH4) The mass and energy equations are solved for t he different volatiles simultaneously. Inflowing and outflowing gas wi thin the body, dust mantle build-up, depletion of the most volatile ic es in outer layers, and recondensation of gases in deeper layers are i ncluded in the model. The calculations start with a homogeneously mixe d body at a constant temperature and a constant mass density at apheli on of an orbit. The internal heat of the body increases due to insolat ion and conduction. As a result, of sublimation of the minor, more vol atile components, the initially homogeneous body differentiates into a layered body. The depths of the the boundaries between the layers (su blimation fronts of the corresponding volatile phases) change with tim e and are on the order of tens of meters. Temperature, porosity, relat ive chemical abundance, and pore size distributions are obtained as a function of depth, and the gas flux into the interior and into the com a for each of the volatiles at various positions of the comet in its o rbit. The ratio of the gas flux of minor volatiles to that of H2O in t he coma varies by several orders of magnitude throughout the orbit and cannot be simply related to the mixing ratio of the ices in the body. It is believed that the mixing ratio of the minor constituents of fro zen gases in the ice-dust conglomerate of the nucleus is a very import ant clue to the original composition of the frozen gases in the solar nebula, but it is not well understood. To address this important issue , the present coma chemistry model is combined with the nucleus surfac e layers model for a more physically realistic gas production rate fro m which to begin the coma calculations. The combined incorporates gas production into the coma from three sources: Volatile sublimation at t he nucleus surface, subsurface sublimation of volatiles from the inter ior, and release of gas from the dust grains in the coma (distributed sources). Copyright (C) 1996 Elsevier Science Ltd