Resolving Sub-Surface Ice Pebbles in Cometary Simulants using THz Time-Domain Spectroscopy

Planetary formation theories aim to explain how planets form from the proto-planetary disk through processes such as classical hierarchical growth or pebble accretion. Comets are considered primitive remnants of this early phase, bodies that never coalesced into planets and now reside in the Oort Cloud and Kuiper Belt. Investigating their internal structure provides crucial tests for competing formation models. Previous in-situ missions have used infrared (IR) spectroscopy, ground-penetrating radar and other non-optical techniques to study cometary nuclei. While radar probes below the surface but with low resolution, IR offers finer detail but limited penetration. Terahertz time-domain spectroscopy (THz-TDS) provides a promising middle ground, achieving centimeter-scale depth with millimeter-scale resolution. We are assessing the suitability of THz-TDS for in-situ cometary exploration using COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy), a laboratory setup that simulates cometary conditions in a thermal vacuum chamber and houses a commercial THz spectrometer. Using comet analogs, we perform proof-of-concept experiments to evaluate the technique’s ability to resolve subsurface structures. In particular, we demonstrate the potential to detect icy pebbles beneath a dust layer. The presence–or absence–of such pebbles might well challenge the pebble accretion paradigm. Plain Language Summary Planets are thought to form from disks of gas and dust around young stars, but scientists still debate how this process works. Two ideas exist: planets may grow as small particles collide and stick together, or they may grow rapidly by collecting pebble-sized icy particles. Comets are useful for testing these ideas because they are leftover building blocks from the early Solar System that have remained largely unchanged for billions of years. Looking inside comets can provide clues about how planets formed. Space missions have already studied comet surfaces using radar and infrared light, but each method has limitations. Radar can see deeper below the surface but with poor detail, while infrared light provides sharp detail but cannot penetrate far. In this study, we explore a different technique called terahertz time-domain spectroscopy, which can probe a few centimeters below the surface while still resolving small features. –2– We tested this method in the laboratory using artificial comet materials under spacelike conditions. Our experiments show that this technique can detect centimeter scale icy pebbles hidden beneath a millimeter thick dust surface. Finding such pebbles inside real comets would support certain planet formation theories and help scientists better understand how planets like Earth came to exist.