The last one and a half years of this project will be used to perform the L experimental campaigns to address directly the scientific objectives of the D-A-CH proposal by investigating the top level science questions raised in Sect. 2.2. Therefore, five L experimental campaigns will be conducted within this phase, each assigned to one of the five top level science objectives (see below). The difference between the L and the M experiments is the increase of the complexity of the sample and that the L chamber will be fully equipped with the entire suite of scientific equipment available to the D-A-CH group (see Sect. 5.5 for details). Several experiments per campaign will secure that the sample’s complexity can be carefully increased from its M experimental state until its final composition and morphology required for the respective experiment is reached. Per experimental campaign, we will reserve three months for experimental work and one and a half months for data analysis and interpretation.

The individual campaigns are structured as follows:

1.  Joint L-experimental run 1 (D-A-CH): release of surface material

Primary strategy: investigation of the release of surface material by using a high-speed imaging system for optical observations and the outgassing-rate detector for measuring the gas activity of the samples.

1. 1. How does the high dust-to-ice ratio affect the activity and evolution of surface properties? First experiments in this campaign will focus on the question of how the dust-to-ice ratio influences the activity and the surface properties (such as thermal conductivity and surface reflectance). Therefore, the ice-to-dust ratio will be varied; in the first experiments, water ice will be the only icy component; later experiments will also use CO₂ ice as proxy for super-volatiles. 
   2. How is a high dust-to-ice ratio capable of ejecting dust? Experiments will investigate under which circumstances dust ejection from a cometary surface is possible. Samples with different compositions and morphologies will be studied. The ejected material will be monitored by three cameras (two for the reconstruction of the three-dimensional velocity and one to observe changes of the surface).
   3. Can µm-sized dust particles (as found in the coma and tail) be separated by the outgassing processes despite their high (~kPa) tensile strengths? Experiments dealing with this objective will be divided into two different types: 1) experiments to study the release of µm-sized dust particles from a dry surface dust layer covering the ices beneath; 2) experiments studying how aggregates, which have been already released from the surface, can disintegrate to produce the µm-sized dust particles (the internal tensile strength of aggregates consisting of µm-sized dust particles is in the order of several kPa).  
   4. What controls the size distribution of the ejected dust grains, or aggregates? This question directly relates to the question of how dust grains and aggregates are ejected from a cometary surface. Experiments with different sample compositions and morphologies will be performed and the ejection rate will be measured by imaging and correlated with the respective outgassing rate.  
   5. Is ejection of water-ice-containing aggregates possible by the outgassing of water ice, or is a super-volatile (CO₂) necessary? At the end of this campaign, we will perform experiments in which water-ice aggregates will cover a water-ice layer. Observation of the sample surface will clarify whether water-ice aggregates can be ejected by the outgassing of water ice, or if a more volatile species is required. In a second experiment, the underlying ice layer will be replaced by CO₂ ice to investigate the influence of this super-volatile on the ejection of the water ice aggregates.

1.  Joint L-experimental run 2 (D-A-CH): diffusion of volatiles

Primary strategy: measurement of the diffusion of volatiles in the surface layer and their influence on the physical properties of the material by monitoring the temperature distribution inside the sample and by using thermophysical models describing the mass transport inside the sample material.

1. 1. Can the transport of water molecules via sublimation and condensation inside the surface material lead to the formation of a solidified ice layer? The second L experimental run will start with relatively simple experiments. Different samples will be placed inside the vacuum chamber and will be irradiated over a (not yet defined) period of time. The temperature evolution inside the sample as well as the outgassing rate will be continuously measured while the sample is irradiated. Together with thermophysical modeling, the temperature data can be used to derive the redistribution of the volatiles inside the sample material. After the irradiation phase, the sample compressive strength of the sample will be measured by the penetrometer installed inside the vacuum chamber. A comparison of the different samples investigated during this experimental run will yield the required conditions under which a solidified layer can be formed. Together with the thermophysical model, this experiment will also provide the timescale of this process.
   2. Is this layer capable of preventing sublimating gas molecules (originating from deeper layers) escaping to the exterior? Can this lead to a local pressure build-up beneath the ice crust, ultimately driving cometary outbursts? This question can be investigated by measuring the outgassing rate and possibly the ejection of surface material (e.g., by outbursts) during the experiments mentioned above (L2a). The temperature data, outgassing rate and observed ejection can be correlated in order to investigate whether the formation of a solidified crust can lead to a local pressure build-up which then triggers an outburst.  
   3. Is the sublimation and re-condensation process capable of increasing the local thermal conductivity and the mechanical strength in a low-pressure environment? The measured temperature profiles will be used to estimate the change of the sample thermal conductivity. The penetrometer will additionally be used during the experimental run to investigate the change of mechanical properties with time. Care will be taken to not use the same location twice because the intrusion of the penetrometer will alter the sample properties. If necessary, similar samples will be produced in order to provide fresh material for the strength measurements.

1.  Joint L-experimental run 3 (D-A-CH): role of the organic material

Primary strategy: studying the influence of organic materials on the mechanical and thermophysical properties of the sample material.

1. 1. How do organic materials affect the mechanical and thermophysical properties of the cometary surface? Samples of different compositions and morphologies will be produced in order to investigate how the additional organic component influences the physical properties of the material. The samples will be produced with and without an organic component so that in direct comparison can be made. The difference of the physical properties between the samples will be measured (compressive strength, thermal conductivity, outgassing rate, etc.). 
   2. Is the evolution of the cometary thermal environment suitable to make organic components a major contributor to the subsurface texture with respect to pore space, mechanical strength, and thermal properties? In this experimental campaign, the temporal evolution of the sample material due to phase changes of the organic component will be studied. The composition of the sample material will be systematically varied whilst different irradiation intensities can be used to control the thermal environment. Changes in of the mechanical and thermophysical properties of the sample material will be monitored and compared. After the experimental runs, the samples surface and subsurface structure will be studied by different characterization methods (see text below: sample preparation and characterization) to investigate the evolution of the subsurface texture and pore space. Sample characterization prior to the experiments will provide the possibility for a direct comparison with the initial conditions.

1.  Joint L-experimental run 4 (D-A-CH): surface texture

Primary strategy:modification of the sample surface to mimic the observed cometary terrain structures and observation of the surface with the hyperspectral imaging system, the mass spectrometer and the outgassing rate detector.

1. 1. Are the observed surface textures original or a result of cometary activity? Can the observed surface textures and morphologies be used in combination with laboratory experiments to deduce details about the formation and evolution of comets? In this stage we intend to investigate the formation and evolution of interesting cometary surface features as observed by OSIRIS camera (e.g., cliffs, pits, cracks, etc.). Therefore the sample will be modified prior to the experiments to approximate the situation on the cometary surface. During the experiments, changes of the surface textures and morphologies will be monitored by the hyperspectral imaging system and compared with the findings of the Rosetta mission. Furthermore, the influences of the artificial surface structures on the physical properties of the sample material (e.g., the outgassing rate, or the temperature distribution) and their temporal evolution will be investigated. For example, this experimental run will be used to investigate if periodic day-night cycles can lead to formation of cracks on the sample’s surface. Additionally, the mass spectrometer will monitor the changes of the volatile composition during the experiments.
   2. How do the chemical composition and the physical surface texture influence the spectro-photometric properties of the surface?During this campaign, we will prepare different samples with variable surface textures (roughness, compaction) and composition. Both parameters will be varied in steps. We will then acquire hyperspectral images and mass spectra of these surfaces as they evolve to study which parameters have the strongest influence on the spectro-photometric properties of the sample material. We will pay particular attention to the relative influence of the porosity and concentration in carbon-rich organic compounds on the albedo and the spectral slope of the samples.

1.  Joint L-experimental run 5 (D-A-CH): formation and evolution of comets

Primary strategy: investigating the influence of different mixing types and chemical compositions on the sample’s properties and application of these findings to comet formation scenarios.

1. 1. Is it possible to understand how the icy and dusty components were mixed prior the formation of the comet nucleus?In order to investigate this question, we intend to create sample materials possessing different morphologies (i.e., aggregate layers versus homogeneous consolidated dust layers consisting of micrometric particles). During this experimental campaign, we will study if the different morphologies can lead to a measureable change in the sample properties. Furthermore, we will study whether different mixing types (e.g., intra- versus inter-mixture) can be used to create analogue materials with distinguishable properties. This campaign will be used to investigate how the different components were mixed prior to formation of cometesimals.
   2. Can the measured D/H ratio be used to gather information about the formation and evolution of cometary nuclei?At this point, we intend to use ices with a known D/H ratio in order to investigate under which circumstances a change of the D/H ratio (measured by the mass spectrometer) of the surface layers occurs. A comparison with ROSINA measurements (Altwegg et al., 2015) can then be used to interpret possible formation scenarios.