Heat and Gas Diffusion in Comet Nuclei (pdf file 5.5 MB) - ISSI

22 2. The Structure of Comet Nuclei
entiation of the samples. The sublimated gas escapes from the surface of
the sample and also diffuses into the interior where it recondenses. Smoluchowski
(1982) was the first to recognize the importance of heat transport
via to the vapour phase in a porous comet nucleus. This idea was verified
during the KOSI experiments (Spohn and Benkhoff, 1990; Benkhoff and
Spohn, 1991a,b). Benkhoff and Spohn (1991a,b) showed that in a porous
matrix heat transport into the interior by the vapour phase is more effective
than heat conduction by the matrix. The dominant heat mechanism is
transfer of latent heat. This strongly influences the gaseous diffusion of the
volatile ices and the temperature profile within the body. Convex shapes
of the temperature profiles below energy sinks (sublimation fronts) were
observed as a result of the heat-carrying, inward-flowing vapour and subsequent
freeing of latent heat after condensation of the gas at cooler, deeper
layers.
The main differences between the various simulation experiments were
the duration of the insolation period, the intensity of the insolation, the
time profile of the insolation, and the composition of the samples. In ten
of the experiments, porous water-ice and dust samples together with one or
two volatile ices (CO 2 , CH 3 OH) were used. The experiments were aimed at
investigating the influence of the sample material on the sublimation, the
formation of a dust layer (mantle) attenuating the gas flux, the dust mantle
erosion through entrainment by the escaping gas, and the thermal history
of the sample. Most of the comet nucleus material was a porous mixture of
water ice and dust; therefore the physics of sublimation of water ice from
the porous samples was an important process.
In order to investigate the contribution of the gas diffusion to the coma
as well as to the heat transport into the interior in more detail, a simulation
experiment with a pure, porous water ice sample (KOSI-8) was performed
(Benkhoff et al., 1995). The thermal behaviour, the sublimation, and the
thermal infrared emission of this sample were investigated in response to
irradiation by an artificial Sun.
The theoretical model, which described the thermal evolution of the
laboratory samples, became the basis of the thermal comet nucleus model
of Benkhoff and Huebner (1995). An energy analysis for an ice and dust
sample given by Grün et al. (1991b) shows that the main contributions to
the power balance are radiative input, reflected light, thermal reradiation,
energy consumed for gas sublimation and diffusion, increase of the internal
energy of the sample, and energy flow through the walls of the sample
container.