Thermophysical Modelling of the CoPhyLab Experiments

Introduction

After the visit of comet 1P/Halley by the Giotto mission, “KOSI” (comet simulation) – the first large scale laboratory campaign for cometary research – investigated various physical phenomena occurring on comets by using analogue materials. The experiments were also accompanied by intensive modeling activity, which yielded several thermophysical models for the description of comets [1]. Following the revolutionary findings of the Rosetta mission, however, a new laboratory campaign became necessary to adapt existing models and to incorporate the new knowledge acquired. Thus, the “CoPhyLab – Comet Physics Laboratory” project was initiated in 2018 by an international consortium (www.cophylab.space).

In the framework of this project so far, we have used several individual experiments to characterize various materials as potential ingredients for a new cometary analogue. In parallel, the construction of a large cryogenic vacuum chamber (“L-Chamber”) was completed, enabling the measurement of refractory-volatile mixtures by multiple instruments simultaneously. While cometary activity involves various physical processes, such as sublimation and gas diffusion in porous materials, understanding the comet’s thermal behavior is crucial. A rigorous thermophysical model (TPM) that considers the porous structure of the cometary material, as well as the volatile phases, and processes connected to them (sublimation, phase changes etc.), would significantly improve the understanding of the evolution of comets.

Here, we present a strategy for the development of a new TPM that in the first phase is able to describe the laboratory experiments and ultimately can be scaled to cometary environments to assist the interpretation of data collected by observations and space missions.

 

Strategy

Previous modelling activity related to the “KOSI”-campaign was successful in the sense that the experiments could be well described by the models [2, 3]. However, a large variation of parameters between individual experiments impeded a proper understanding of phenomena that occur differently on comets than in the laboratory. Therefore, we pay special attention in the CoPhyLab campaign to repeating the most important baseline experiments. Moreover, we evolve the experiments from a simple form, where well characterized albeit idealized samples are used (e.g. glass beads), to more advanced iterations with more realistic analogue materials. On the one hand, this is to ensure that we understand the fundamental physical processes before considering complex interactions. On the other hand, the simple experiments provide us with the functional dependencies of thermophysical parameters (e.g. thermal conductivity) on the material properties, such as porosity, grain size distribution, grain shape and temperature.

 

Numerical Model

We approach the TPM analogously to the experiments, by starting with a very simplified macrophysical model using the finite element method (FEM). Thereby, the material-specific input parameters, which contain the microphysical relations, are taken from the characterization experiments. While most TPMs for comets are one-dimensional [2, 4], the consideration of special spatial features (e.g. sample boundaries, sensor hardware etc.) may require a 3D model [5]. To this end, we use the commercial FEM simulation software “COMSOL Multiphysics”, as it is ideally suited for our application. After the verification of our basic TPM with COMSOL, we will translate and evolve the model as a 1D code in python. This will provide a complementary, resource efficient method to simulate processes such as material ejection.

 

First Phase

In the first phase, we will establish the mathematical model that is required to describe the temperature evolution in a dry, well-defined sample (e.g. glass beads), illuminated in vacuum. A schematic of the corresponding experiment is shown in Figure 1. After satisfactory agreement between the model and experiment, more complex iterations will follow, for example by adding volatile phases and modelling their sublimation.

These investigations lead to a better understanding of effects in the cometary analogue material during measurements, and in addition, of effects related to the measurement setup and its interaction with the sample. As an extra benefit, a higher accuracy of future laboratory experiments in this context can be achieved.

Acknowledgements

This work is carried out in the framework of the CoPhyLab project funded by the D-A-CH programme (DFG GU 1620/3-1 and BL 298/26-1 / SNF 200021E 177964 / FWF I 3730-N36).

 

References

[1] Sears D. W. G. et al.: Laboratory simulation of the physical processes occurring on and near the
surface of comet nuclei, Meteoritics & Planet. Sci., 34, pp. 497-525, 1999.

[2] Spohn, T. and Benkhoff, J.: Thermal history models for KOSI sublimation experiments, Icarus Vol. 87, pp. 358-371.

[3] Benkhoff, J. and Spohn, T.: Thermal histories of the KOSI samples, Geophys. Res. Letters Vol. 18, pp. 261-264, 1991.

[4] Steiner, G. and Kömle, N. I.: A model of the thermal conductivity of porous water ice at low gas pressures, Planet. Space Sci. Vol. 39, pp. 507-513, 1991.

[5] Macher, W et al.: 3D thermal modeling of two selected regions on comet 67P and comparison with Rosetta/MIRO measurements, Astron. Astrophys., Vol. 630, id. A12, 2019.