The M experiments will be performed after the S experiments in the first half of the second year of the project. Therefore, the main vacuum chamber will be installed at the TU Braunschweig. This vacuum chamber (see Sect. 5.5 and Fig. 6) will be provided by the Max-Planck-Institut für Sonnensystemforschung and will be transported to its final destination at the beginning of the project. Installation of the main vacuum chamber will be the task during the first year of the project. This includes, transport of the chamber, installation of vacuum equipment and cooling system as well as performing testing runs.
In the M-experiment phase the equipment of the D-A-CH members will be moved to Braunschweig and equipped to the main vacuum chamber. The reason for performing the M experiments is threefold. The first goal of the M experiments is to test and calibrate the installed instruments. A smooth transition from the S to the L experiments is the second objective of the M experiments. This objective will be realized by an investigation of the scalability of the S experimental results to larger sample sizes (the sample size will be increased by a factor of three in the M experiments; thus, cylindrical samples with a diameter of 24 cm will be used). The results of the S experiments will be used as a calibration benchmark for the M experiments and, hence, for the freshly installed L chamber. The third aim of these experiments is to prepare the L experiments by using samples with a lower complexity than those planned for the L experiments (i.e., samples containing just one instead of two volatile constituents; see sample preparation below). Because the L-experiment campaigns are each assigned to one of the top level science objectives (see Sect. 2.2), we plan to perform also five M-experiment runs.
Care will be taken to not vary too many parameters, so that a proper interpretation of the experimental results will be possible. The results of the M experiments will be discussed in the following D-A-CH meetings (M4 – M7) and will be published in peer-reviewed journals.
We plan to perform the following M experiments:
- Release of surface material (D-A-CH): This experiment will be used to study under which physical conditions the release of the surface material is possible. Care will be taken to scale the experiments on Earth to the situation on the comet nucleus by adjusting the required parameters (e.g., aggregate size and gas pressure). For example, studying the ejection of dust aggregates will require aggregate sizes smaller than ~200 μm, so that cohesion and not gravity controls the tensile strength (see Fig. 1b). The temperature of the sample will be adjusted accordingly to achieve realistic gas-pressure-to-tensile-strength ratios and realize gas and dust emission during the comet simulation experiment.
- Diffusion of volatiles (D-A-CH): During the second experimental run, the diffusion of volatile components inside the surface layers of the sample will be investigated. Therefore, we will equip the sample with temperature sensors located at different depths inside the material. This will enable the measurement of the temperature distribution inside the sample and, therewith, the redistribution of the volatiles inside the porous material. Modeling support in combination with the experimental data will be mandatory to understand the diffusion processes inside the material.
- Role of the organic material (D-A-CH): One of the major goals of this M experiment is to test several organic materials surveyed during the first year of the project. One key question is if phase changes, or redistribution of the organic material due to diffusion can alter the thermophysical and, mechanical properties and the probability of particle ejection. These experiments will also be used to study whether the organic material causes an increase of the stickiness of the sample material.
- Surface texture (D-A-CH): The fourth set of experiments is dedicated to the formation and evolution of surface textures. During these experiments, we will mimic observed surface structures (observed by the OSIRIS camera onboard the Rosetta spacecraft and by Philae’s cameras) by producing and modelling the surface of our comet analogue material. Camera observation of the surface before, during and after the experiments will show whether the surface texture and its evolution can influence the physical properties of the analogue material (e.g., outgassing rate, or optical properties). In addition, we will study whether self-organizing surface textures appear.
Formation and evolution of comets (D-A-CH): This M experiment is dedicated to the formation of comets and their evolution. Therefore, we will investigate materials with different morphologies motivated by the different formation scenarios currently debated in the cometary science community (i.e., aggregate layers versus consolidated-dust layers consisting of micrometric particles). During this experiment, we will study whether the differences in morphology can lead to a measureable change of the thermophysical, mechanical, or dust-emission properties of the sample.