The aim of the S experiments is to prepare the planned M and L experiments by investigating the basic physical processes occurring in the cometary surface layers. However, these experiments will also provide scientifically new results which will be published in peer-reviewed journals. A realistic expectation is to perform two experiments per group (the planned timeframe for these experiments is half a year) with small-sized cylindrical samples (8 cm in diameter and ~8 cm in height; however, details of sample size and geometry can change with the experimental requirements). The following S experiments are planned:

  1. Outgassing rate (D): The outgassing rate of the different samples will be measured by using an existing experimental setup which has been designed to study the outgassing properties of H2O-ice and CO2-ice (funded through DFG project GU 1620/1). The vacuum chamber is equipped with a chopper-system (see Gundlach et al., 2011 for details) and a mass spectrometer, which allows the simultaneous measurement of the total outgassing rate and the contribution of the different volatiles to the gas flux. The samples will be used to calibrate the measurement system. In comparison to the DFG project GU 1620/1, the addition of the organic and the dusty component will provide new scientific results important for understanding the activity of comets.
  2. Tensile strength (D): The tensile strength of cometary analogue samples can be measured by the differential pressure method already available in the laboratory (see Blum et al., 2014 for details). This technique provides the possibility to derive low tensile strength values by pre-compression of the material by a gas differential. Extrapolating the experiment data to the case in which no compression is applied yields the tensile strength of very-weakly-bound sample materials. This experimental method can also be used to determine the gas permeability of the sample materials (see Gundlach et al., 2011a, for details).
  3. Thermal conductivity (A): The thermal conductivity is one of the key parameters needed to understand the energy and mass balance of a cometary surface when it interacts with the solar radiation. One way to determine this property accurately is to use the so-called “hot needle” method. In the frame of previous work, IWF has designed and procured custom-made needle sensors (see Fig. 4), which can measure thermal conductivity accurately and which have a mechanical strength large enough to use them also for material strength measurements. Measuring thermal conductivity in this direct way avoids the need to evaluate this parameter by iterative modeling of the whole system, as it was done in the KOSI experiments (Kömle et al., 1991).
  4. Compressive strength (A): The unexpected surface hardness of the cometary nucleus, as suggested by the MUPUS penetrator aboard the lander Philae, has raised many questions about the processes leading to such mechanical properties in the topmost layers of a comet. One way to determine the mechanical strength of surface layers (i.e., the compressive strength) is to use cone penetrometry (Zöhrer and Kargl, 2006). From the measured force profile and corresponding physical models, the mechanical properties of the penetrated layers can be derived. Such measurements were already investigated in the laboratory in Graz for a variety of surface materials from icy to dry regoliths.
  5. Ejection of surface material (CH): The morphology and size of the fragments ejected are highly dependent on the nature of the dust as well as the mode of mixture between dust and ice. We will perform a series of sublimation experiments varying the nature and concentrations of minerals and organics in the dust to derive trends between the different parameters and compare the size distributions and morphologies to Rosetta observations.
  6. Redistribution of ices (CH): Using the same experimental setups as for the S5 runs, we will study how water might have re-condensed and/or sintered below the dust layer. We will dissect the samples at the end of the sublimation experiments to characterize visually (with microscopes) the vertical re-distribution of the ice and quantify the associated changes in mechanical properties by cone penetrometry (see experiment S4).