Outgassing of icy bodies in the Solar System – I. The sublimation of hexagonal water ice through dust layers

Abstract

Our knowledge about the physical processes determining the activity of comets were mainly influenced by several extremely successful space missions (Giotto, Deep Space I, Stardust, Deep Impact and EPOXI), the predictions of theoretical models and the results of laboratory experiments. However, novel computer models should not be treated in isolation but should be based on experimental results and should be verified and calibrated by experimental work. Therefore, a new experimental setup was constructed to investigate the temperature dependent sublimation properties of hexagonal water ice and the gas diffusion through a dry dust layer covering the ice surface. We show that this experimental setup is capable to reproduce known gas production rates of pure hexagonal water ice. The reduction of the gas production rate due to an additional dust layer on top of the ice surface was measured and compared to the results of another experimental setup in which the gas diffusion through dust layers at room temperature was investigated. We found that the relative permeability of the dust layer is inversely proportional to its thickness, which is also predicted by theoretical models. However, the measured absolute weakening of the gas flow was smaller than predicted by models. This lack of correspondence between model and experiment may be caused by an ill-determination of the boundary condition in the theoretical models, which further demonstrates the necessity of laboratory investigations. Furthermore, the impedance of the dust layer to the ice evaporation was found to be similar to the impedance at room temperature, which means that the temperature profile of the dust layer is not influencing the reduction of the gas production. Finally, we present the results of an extended investigation of the sublimation coefficient, which is an important factor for the description of the sublimation rate of water ice and, thus, an important value for thermophysical modeling of icy bodies in the solar system. The achieved results of this laboratory investigations demonstrate that experimental works are essential for the understanding of the origin of cometary activity.

Pre-perihelion activity of comet 67P/Churyumov-Gerasimenko

“…Any ice buried under a dusty crust does not sublimate freely, but follows a diffusion regime where gas escapes through the crust. The mass loss rate from such a buried layer can be written as (assuming an ideal gas law, (Fanale & Salvail 1984;Schorghofer 2008;Gundlach et al 2011;Guilbert-Lepoutre 2014):…”

Single Particle Measurements of the Optical Properties of Small Ice Crystals and Heterogeneous Ice Nuclei

“…GLEN AND S. D. BROOKS the sublimation rate using the classical Hertz-Knudsen formula (Gundlach et al 2011). Given a residence time through the zone of possible sublimation in our apparatus of at most 0.5 s, our calculations indicate that at -50 C, the sublimation rate is »3.3 £ 10 ¡9 kg m ¡2 s ¡1 and the estimated ice loss corresponds to a decrease in radius of less than 1%.…”

Is near-surface ice the driver of dust activity on 67P/Churyumov-Gerasimenko

“…For the results presented in Sect. 4 the experimental dependence (Gundlach et al 2011) is also used. The model explicitly treats the solid and radiative part of the effective conductivity, which leads to a temperature dependence of the effective conductivity (Gundlach & Blum 2012).…”

“…Strictly speaking, this is only physically reasonable if the back flux and corresponding inward momentum can be ignored. However, it is known through experiments (Gundlach et al 2011) as well as from theory , and references therein) that a porous media has a diffusion resistance that rapidly grows with increasing thickness. For example, only about 10% of the sublimating molecules enter the coma from a layer with dimensionless thickness of 20 R A .…”

“…Two different cases are shown that vary in their dependence on the assumption about the layer permeability. The first case is for a modified Clausing function , and the second case considers an experimental function evaluating a Knudsen diffusion through a mono-disperse random porous layer (Gundlach et al 2011). The Clausing formula (dashed curves) predicts a more rapid decrease of the outflow intensity, and the dimensionless pressure drop is always smaller than the saturation vapor pressure.…”