An approach to challenging pebble accretion using THz spectroscopy

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.