THERMAL-PROPERTIES OF COMETARY ICES AND SUBLIMATION RESIDUA INCLUDINGORGANICS

The simulation of cometary surface conditions by laboratory experiment s has proven to be an efficient way towards a better understanding of physical phenomena at and below the surface of a comet nucleus (Grun e t al., Geophys. Res. Lett. 18, 245-248, 1991). A question which has no t yet been answered by the comet simulation (KOSI) experiments perform ed so far is the influence of organic matter on the physical propertie s of the sublimation residua. Therefore, a number of experiments perfo rmed in a small vacuum chamber cooled by liquid nitrogen are reported on, which were dedicated to study the influence of organics on the the rmal properties of a cometary analogue sample. Using aliphatic hydroca rbons of low volatility (paraffins) as model substances for the organi c compounds, the formation of a several centimetres thick cohesive res iduum in response to heating of the sample was observed. In one of the experiments the evolution of an originally homogeneous multi-componen t sample (containing water ice, organics, and minerals) to a residuum containing (finally) only minerals and organics was followed. During t his evolution the thermal properties changed dramatically. The heat co nductivity of the cohesive residuum was found to be at least an order of magnitude larger than the typical value for a loose dust mantle con taining no organic material. Thus the evolution of comets with the sam e thermal history containing a considerable amount of organics might b e quite different from that of a comet consisting only of ices and min erals. Copyright (C) 1996 Elsevier Science Ltd