The repeatable production of well-characterized cometary analogue samples is a key challenge in performing a realistic, state-of-the-art, comet simulation experiment. In the sample preparation phase, the CoPhyLab team will establish a comprehensive list of possible materials (volatiles, organics and silicates) and sample-production protocols that can be used as basic components for the production of cometary analogue materials:

• Volatiles: H₂O ice 

In recent years, CO₂ ice has become a promising candidate to explain the activity of comets. Due to its super-volatility and its relatively high abundance in cometary nuclei (~10% relative to water ice; Bockelée-Morvan et al., 2004), CO₂ ice is a perfect basic component for our comet analogue material.

• Organic materials: Cometary nuclei are very rich in carbonaceous compounds, which among other interesting properties, are responsible for their extremely low albedo. Cometary missions have shown the existence of a broad range of complexity of the organic molecules, from amorphous carbon to low-weight semi-volatile species, as well as to heavy refractory polyaromatic molecules (Quirico et al., 2016). These compounds have a wide range of physical and chemical properties, some of them being peculiar and able to strongly affect the properties of the nucleus material. In recent years, organic materials have become a very prominent cometary analogue. However, detailed knowledge about well-suited organic analogue components is scarce. One goal of the CoPhyLab project is to select a few of these organic compounds for use in comet simulation experiments This selection will be based on the comparison of their properties to those of the more representative tholins and complex insoluble organic matter found in some carbonaceous chondrites, which seems closely related to cometary organic matter (Fray et al., 2016).
• Silicates: The mineral component of cometary dust was measured by several space missions starting with the Giotto and the Vega missions, but also by the Stardust, the Deep Impact and the Rosetta missions. Cometary dust is dominated by crystalline magnesium-rich silicates, iron sulfides, and glassy components (Engrand et al. 2016). The bulk composition is comparable to micrometeorites and interplanetary dust particles (Hilchenbach et al. 2016).Analogue materials considered for laboratory comet simulations should therefore reflect these findings (see Seiferlin et al., 2008 and Poch et al., 2016a, for reviews of silicate materials used in laboratory experiments).

Table 1: List of suitable cometary analogue materials. This list is meant as a starting point to test possible analogue materials during the project. Further materials will be added during the project.

The philosophy of this project is to create and investigate the samples with a step-by-step approach and to carefully increase the sample complexity by the addition of further components starting with the least number of basic components possible for the respective experiment, or by changing the morphology of the sample material (i.e., by changing the porosity, the arrangement of the different materials inside the sample, particle/aggregate size, or the structure, i.e. aggregate samples versus homogeneous samples). Mixing of the different components will be performed on three different levels:

1) on the molecular level (intra mixture[2])

2) on the particle level (inter mixture[3])

3) on the aggregate level (macro mixture[4]).

The particle size of the basic components is determined by either the production method (dispersion of liquids, crushing, or milling of bulk materials) if produced in the laboratory, or by availability if bought from industry.

In order to increase the quality of the planned experiments, it is mandatory to analyse and characterize the produced comet analogue materials. Therefore, we have planned to start the project with sample preparation and characterization within the first half year (and later during the project if required). Sample characterization during this phase will be performed at the three project laboratories and those of our cooperation partners. The comet analogue materials produced will be analyzed with respect to the following properties:

1. Grain size: by using optical microscopy (D and CH), (cryogenic) scanning-electron microscopy (D[5] and CH), pycnometer tests (A) and optical coherence tomography (CH).
2. Porosity: by measuring mass and volume (D, A and CH), computer-aided tomography (D[6]), terahertz spectroscopy (D[7]) and pycnometer tests (A).
3. Surface roughness and topography: by using optical coherence tomography (CH).
4. Gas permeability: by using gas flow measurements (D and A).
5. Bidirectional reflectance distribution function: by using a gonio-radiometer (CH).

6.     Spectra of the pure components: by using a hyperspectral imaging system (CH[8]).

7.     Sintering: by using scanning-electron microscopy (D), neutron scattering (D[9]) and direct cone penetration tests (A).

[2] Intra mixture: the volatile constituent will be mixed with the non-volatile components before freezing (e.g., by mixing water with micrometer-sized dust particles before dispersion).

[3] Inter mixture: pure water-ice (or CO₂-ice) particles are produced first and then mixed with the other basic components. Vapor deposition inside the samples is also possible.

[4] Macro mixture: aggregates composed of only a single basic component, but mixed together with other aggregates of different composition.

[5]  Cooperation partner: S. Gorb (Christian-Albrechts-Universität zu Kiel, Germany).

[6]  The computer tomography scans will be perfomed at the “Institut für Partikeltechnik” at the TU Braunschweig (costs: 100.00 € per scan).

[7] Cooperation partner: H. Mutschke (Friedrich-Schiller-Universität Jena, Germany; additional scientific output: terrahertz spectra of the produced sample material).

[8] Cooperation partner: O. Poch (Université de Grenoble-Alpes, Genoble, France; additional scientific output: optical constants of the produced sample materials, reflectance spectra, phase curves, polarization phase curves).

[9] Cooperation partner: H. Fraser (The Open University, England; additional scientificoutput: crystallographic structure of the sample materials).