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The standard approach to obtaining knowledge about the properties of the surface layer of a comet from observations of gas production consists of two stages. First, various thermophysical models are used to calculate gas production for a few sets of parameters. Second, a comparison of observations and theoretical predictions is performed. This approach is complicated because the values of many model characteristics are known only approximately. Therefore, it is necessary to investigate the sensitivity of the simulated outgassing to variations in the properties of the surface layer. This problem was recently considered by us for aggregates up to tens of microns in size. For millimetre-size aggregates, a qualitative extension of the method used to model the structural characteristics of the layer is required. It is also necessary to study the role of radiative thermal conductivity, which may play an important role for such large particles. We investigated layers constructed from large aggregates and having various thicknesses and porosity and evaluated the effective sublimation of water ice at different heliocentric distances. For radiative conductivity, approximate commonly used models and the complicated model based on the dense-medium radiative transfer theory were compared. It was shown that for millimetre-size aggregates careful consideration of the radiative thermal conductivity is required since this mechanism of energy transfer may change the resulting gas productivity by several times. We demonstrate that our model is more realistic for an evolved comet than simple models parameterizing the properties of the cometary surface layer, yet maintains comparable computational complexity.

CaSSIS (Colour and Stereo Surface Imaging System) is the main imaging system for the ExoMars Trace Gas Orbiter (TGO) mission. The instrument was completed in October 2015 and launched in March 2016 [1]. This abstract describes the current status of CaSSIS and provides a first assessment of its observations from the start of the primary science mission.

We present a pre fracture incrementally non linear elastic hydric model to constrain the models of formation of giant (spacing of 20 300 m) desiccation cracks on Earth and possibly intermediate sized (80 to 350 m) polygonal networks of cracks located in many impact craters on Mars which have been interpreted to form by desiccation as opposed to thermal contraction (El Maarry et al. 2010). The results of the model show that tensile stresses rise monotonically with desiccation. However in soils with diffusivities below 10 4 m2/s it is not possible to generate high enough stresses to cause fracturing. On the other hand intermediate values between 10 2 and 10 4 m2/s create optimum conditions for the formation of cracks at the time scales suggested for the formation of giant desiccation polygons on Earth. Our model clearly shows that for typical diffusivity values of clayey soils with a considerable amount of smectites enough stress can build up to stimulate cracking of various spatial scales. These results corroborate earlier assumptions that giant desiccation polygons on Earth occur through lowering of the water table rather than surface evaporation. Extending the model to Mars shows that soils would crack within similar diffusivity limits but in slightly longer periods of time owing to the lower gravity. Finally a model for formation of desiccation cracks on Mars is presented that shows that two main conditions need to be fulfilled for desiccation cracks to occur under current martian climatic conditions namely that the thermal and soil diffusivity conditions 1) allow for the formation of an unsaturated zone with a considerable thickness while at the same time 2) limiting the growth of the permafrost downward which would disrupt the cracking process.