Linus Leo Stöckli

Understanding how planets formed from the protoplanetary disk requires access to primordial material that has remained largely unaltered since the earliest stages of Solar System evolution. Comets, preserved in distant reservoirs such as the Kuiper Belt and Oort Cloud and exposed to significant solar heating only during brief perihelion passages, are among the most pristine remnants of planetesimal formation. Their internal structure and composition therefore provide key constraints for testing competing models of planetary growth, including classical hierarchical accretion and pebble-based formation scenarios. Previous spacecraft missions have relied primarily on infrared spectroscopy for surface characterization and ground-penetrating radar for probing the interior. While radar instruments like CONSERT have demonstrated the ability to explore subsurface layers, their spatial resolution is intrinsically limited by meter-scale wavelengths. Conversely, infrared techniques offer fine spatial detail but lack the ability to penetrate below the surface. Terahertz time-domain spectroscopy (THz-TDS) occupies an intermediate regime, capable of providing centimeter-scale penetration while achieving millimeter-scale spatial resolution, and additionally offers spectroscopic sensitivity to key molecular species, such as amino acids, through characteristic absorption features in the THz band. This thesis investigates the feasibility of THz-TDS as a novel method for in-situ subsurface analysis of cometary nuclei. To this end, we developed COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy), a laboratory facility that integrates a commercial THz time-domain spectrometer within a thermal-vacuum environment to simulate cometary surface and subsurface conditions. Using a suite of cometary analog materials, we perform proof-of-concept experiments to evaluate penetration depth, spatial resolution, data interpretation techniques, and the detectability of embedded structures and molecular signatures. The outcomes of this work establish the potential of THz-TDS as a complementary tool for future missions aimed at characterizing the internal structure and composition of small bodies in the Solar System.

We present our newly developed laboratory setup, called COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy), consisting of a THz spectro-goniometer in a vacuum chamber along with a cryo-cooler intended for cooling down samples rich in ices expected in cometary environments. This setup allows the use of THz time domain spectroscopy on analogs including refractory materials in the spectral range from 0.1 to 5.5 THz with a spectral resolution of up to 0.005 THz. Since the optical setup is mounted on a precision controlled x/y-stage, imaging or spatial averaging can be performed. In this work, we present the design decisions and selection of components. Furthermore, the commissioning of the setup is described and the capabilities are shown. The system can reach pressures of 10−7 mbar and temperatures of 50 K. For 2D scanning, a spatial resolution of 0.3 line-pair per mm is obtained.

Cryogenic experiments strongly depend on accurate temperature measurements. Many of the sensors used for these purposes are glued to the object to be measured to achieve adequate thermal contact. This calls for a more flexible, long lasting and robust bolt-mounted solution. Therefore, we present a low-cost (\textless5$), compact, PCB (printed circuit board) based sensor mount for RTDs (resistance temperature detector) and diodes showing reliable results at temperatures from 50 K to 400 K with possible modifications for an extended range down below 20 K. We anticipate that the robustness and low cost of the reported sensor mount design will strongly enhance the accessibility of reliable temperature measurements at a wide range of temperatures in laboratory applications.