Dominik Belousov

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.

Context: Comets preserve primordial material essential for understanding planetesimal formation. However, characterizing their internal structure remains a challenge: infrared spectroscopy offers high spatial resolution but lacks penetration, while ground-penetrating radar penetrates deeply into the interior but suffers from meter-scale wavelength limitations. We propose Terahertz time-domain spectroscopy (THz-TDS) as a novel intermediate technique capable of centimeter-scale penetration with sub-centimeter-scale resolution.Methods: We developed COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy), a laboratory facility integrating a commercial THz time-domain spectrometer within a controlled thermal-vacuum environment. Using cometary analog materials, we simulated realistic surface and subsurface conditions to evaluate penetration depth and spatial resolution.Results: Our proof-of-concept experiments demonstrate that THz-TDS successfully resolves embedded structural heterogeneities, specifically icy pebbles, which are inaccessible to current radar or infrared systems. Figure 1 illustrates the successful reconstruction of an ice pebble buried in a cometary dust analogue, validating the technique's ability to map subsurface morphology.Conclusions: THz-TDS represents a powerful complementary tool for future in-situ space missions. By bridging the gap between surface spectroscopy and deep-penetration radar, this technology offers a pathway to revolutionize the characterization of small body interiors, providing critical constraints on planetary growth models ranging from hierarchical accretion to pebble-based formation scenarios.

Planetary formation theories aim to explain the evolution of planets from the proto-planetary disk. Comets are thought to be primitive remnants that never accreted into planets and now reside in the Oort cloud and Kuiper belt. Their highly elliptical orbits shield them from solar radiation for most of their lifetimes, preserving their original composition. As such, comets offer a unique window into the early Solar System. Investigating their internal structure and composition can provide critical tests for models of planetary formation.Previous in-situ missions have employed infrared (IR) spectroscopy and ground-penetrating radar to study cometary nuclei. While radar can probe below the surface, its spatial resolution is limited by long wavelengths. Conversely, IR provides high spatial resolution but lacks subsurface penetration. Terahertz (THz) time-domain spectroscopy offers a promising middle ground, combining centimeter-scale penetration with millimeter-scale resolution. Moreover, key molecular species, such as amino acids detected on comets, exhibit distinctive absorption features in the THz range.As part of the SUBICE project, we are investigating the suitability of THz time-domain spectroscopy for in-situ space applications. For that purpose, we built a laboratory setup called COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy) [1]. COCoNuT can simulate the conditions found on comets in a thermal vacuum chamber and houses a commercial THz time domain spectrometer. Using cometary simulants, we conduct proof-of-concept experiments to assess the viability of this technique for future space missions [2].Figure 1: Pebble simulant printed out of Cyclic-Olefin-Copolymer (COC) and covered with cometary dust analogue (SiO2/charcoal, 9:1 mixture). The 3D THz scan with the reconstructed pebbles in red is shown to the right.Figure 2: A slice of the cometary simulant scan. The cross section of the pebble simulant is shown below and also overlayed. The sample holder top surface is visible on the sides of the scan as a straight line. The pebbles are clearly visible as voids in the scan.Initial measurements of ice-analogues and SiO2/charcoal dust shows that we can reconstruct pebbles located underneath the dust layer as shown in Figures 1 and 2.We will present measurements with porous ice and ice pebbles along with olivine dust analogues. References:[1] Linus Leo Stöckli, Mathias Brändli, Daniele Piazza, Rafael Ottersberg, Antoine Pommerol, Axel Murk, Nicolas Thomas; Design and commissioning of a THz time-domain spectro-goniometer in a cryogenic comet simulation chamber. Rev. Sci. Instrum. 1 March 2025; 96 (3): 034502. https://doi.org/10.1063/5.0252742[2] Pommerol, A., Jost, B., Poch, O. et al. Experimenting with Mixtures of Water Ice and Dust as Analogues for Icy Planetary Material. Space Sci Rev 215, 37 (2019). https://doi.org/10.1007/s11214-019-0603-

Introduction The composition and structure of cometary nuclei provide critical insights into the early solar system’s formation processes. Observations from the Rosetta mission at comet 67P/Churyumov–Gerasimenko revealedcomplex interactions between water ice and refractory materials, challenging existing models of cometary com-position. The SUBICE project aims to investigate the physical connections between these components at sub-centimeter scales to enhance our understanding of cometary evolution and the conditions prevalent in the early solar system. 2 Objectives The primary objectives of the SUBICE project are:1. To develop experimental and numerical techniques for analyzing the structures of cometary nuclei.2. To determine how water ice and refractory materials are physically connected within the nucleus at sub-centimeter scales.3. To constrain the processes involved in planetesimal formation during the early epochs of the solar system.4. To interpret and contextualize measurements obtained by the Rosetta spacecraft, particularly those that suggest complexities beyond current models. 3 Methodology The SUBICE project is driven by the long-term objective of developing a compact, robust instrument capable of being deployed on a space mission — ultimately landing on a comet to analyze its subsurface structure. This ambitious goal requires advancing both our scientific understanding and the technology needed to operate under space conditions. To meet these challenges, SUBICE follows a multidisciplinary approach that combines experimental simulations, advanced imaging techniques, and numerical modeling. The key methodological pillars are:• Laboratory Simulations: We reproduce cometary surface and subsurface conditions in controlled cryogenic vacuum environments to study the interaction between water ice and refractory materials. These experiments simulate thermal cycling, sublimation, recondensation, and structural evolution under near-cometary conditions.• THz-TDS Imaging: We use Terahertz Time-Domain Spectroscopy (THz-TDS) to non-destructively probe the internal structure of comet analog samples. This technique provides spatially-resolved spectral data that reveal ice distribution and porosity variations. Figure 1 shows a typical THz-TDS setup. It highlights one of the core challenges: the current systems are large, complex, and not suited for space deployment. A major focus of SUBICE is the miniaturization and ruggedization of this setup to meet the constraints of space missions.• Numerical Modeling: We develop computational models to simulate the physical processes relevant to cometary nuclei formation and evolution. These include thermal conductivity, phase transitions, sintering, fracturing, and mass redistribution in ice–dust mixtures. The models are informed by experimental results and help predict observable signatures for in situ missions.Figure 1: Principle of time domain THz spectroscopy using photo-conductive antennas (PCA) and a pulsed laser. These methodologies enable a comprehensive analysis of the physical connections between ice and refractory materials at sub-centimeter scales. 4 Results The newly developed COCoNuT setup [1] has been successfully commissioned and tested, demonstrating its capability to perform THz-TDS spectroscopy on cometary analog materials under cryogenic and vacuum conditions. The system achieves a spectral range of 0.1–5.5 THz with a resolution of up to 0.005 THz, operating at pressures down to 10−7 mbar and temperatures as low as 50 K. Two-dimensional scans reached a spatial resolution of 0.3 line-pair/mm, enabled by the precision-controlled x/y stage. In parallel, significant progress has been made in miniaturizing the THz path [2] for use in space-limited environments. By investigating silicon-based waveguides, antenna arrays, and compact reflective optics, the team has shown that system size can be reduced without degrading performance — a key step toward deploying THz-TDS on planetary landers or in borehole instruments. Finally, improvements in THz-TDS data processing [3] have been achieved by developing a frequency-dependent deconvolution method. This technique accounts for the beam width variation across frequencies, using Gaussian beam profiles derived from knife-edge measurements. As a result, it significantly enhances the spatial resolution and contrast of THz-TDS scans, while preserving the ability to analyze signal phase. A comparison of different deconvolution approaches is shown in Figure 2.Figure 2: Comparison of Point Spread Function (PSF) deconvolution techniques. The first column(a, e) shows the original THz scan of a resolution target. The second column (b, f) presents results from the standard Richardson–Lucy (RL) algorithm. The third (c, g) and fourth (d, h) columns show results from frequency-dependent variants of the deconvolution method developed in this work. 5 Conclusion The SUBICE project represents a significant step forward in understanding the structure of cometary nuclei. By developing innovative experimental and numerical techniques, SUBICE provides new insights into the physical connections between water ice and refractory materials, offering a more nuanced perspective on the processes that shaped our solar system. These advancements are crucial for interpreting current and future space mission data and for refining models of planetary formation. References [1] Linus St¨ockli, Mathias Br¨andli, Daniele Piazza, Rafael Ottersberg, Antoine Pommerol, Axel Murk, and Nicolas Thomas. Design and commissioning of a thz time-domain spectro-goniometer in a cryogenic comet simulation chamber. Review of Scientific Instruments, 96, 03 2025.[2] Valentin Meier, Marc Nicollerat, Joseph Moerschell, and Christoph Ellert. Thz path miniaturization in thz-tds. In 2025 International Conference on Mobile and Miniaturized Terahertz Systems (ICMMTS), pages...

Comets are believed to be relics of the early Solar System, preserving pristine material that has undergone minimal thermal processing since planetary formation. Understanding their composition and internal structure—particularly the relationship between ices and refractory materials—is essential for testing current models of planet formation, such as pebble accretion [1]. While ground-penetrating radar (GPR) allows for 3D mapping of subsurface structures, and Raman spectroscopy offers detailed chemical analysis of surface materials, neither is ideally suited for non-contact, high-resolution analysis at centimeter-scale depths in opaque granular media. Terahertz time-domain spectroscopy (THz-TDS), with its capacity to penetrate non-metallic media, offers a promising alternative by enabling both structural and spectroscopic characterization in a single pulse.As part of the SUBICE project, this work explores the miniaturization of THz-TDS systems for borehole applications on comets, focusing on simulated designs that could be integrated into future lander missions or robotic probes. The goal is to maintain high energy efficiency and spectral fidelity while minimizing optical complexity to reduce internal reflections (echoes), pulse distortion and energy loss, as well as preserving a large bandwidth.Three conceptual designs were considered: Off-axis reflection compact system: (Figure 1) A pair of conventional THz emitter and receiver modules are placed face to face, aligned such that a focusing THz beam is formed using aspherical silicon lenses. Two flat mirrors are used to deflect the beam sideways, enabling analysis of the borehole wall and underlying structures, and redirect it toward the receiver. Antenna array with programmable emission delays: (Figure 2a) A lensless solution is explored where an array of THz emitters (blue dots on the figure) is driven with variable time delays to create constructive interference at a desired focal point. This electronically controlled beam-steering approach enables 2D scanning across the borehole wall without moving parts. In THz-TDS, each emitter requires optical excitation by a femtosecond laser pulse between two electrodes. This requirement can be met by integrating a microlens array to distribute the excitation beam to the emitter array — a technique demonstrated in prior literature [2]. Monolithic emitter–receiver design with shared lens: (Figure 2b) The third approach aims to reduce both volume and alignment complexity by combining emitter and receiver into a single optical block. An aspherical lens focuses the beam from the emitter toward the target. After perpendicular reflection (e.g. on a borehole wall and one underlying materials), the returning pulse is captured by receiver antennas placed adjacent to the emitter. These antennas are positioned to intercept the returning beam, which remains within a ~1 mm diameter spot. Since THz antennas have typical dimensions on the order of hundreds of microns, this design is geometrically feasible. A combination of variant 2. and 3. is also examined, without additional THz lens. The simulations were performed using MEEP (MIT Electromagnetic Equation Propagation) with a spatial grid optimized for subwavelength resolution and absorbing boundary conditions to limit artifacts. In addition to flux maps, we evaluate pulse integrity by inspecting time-domain waveforms at the receiver. Maintaining pulse shape is essential to preserve spectroscopic resolution.As a representative example, a snapshot of the pulse propagation and the flux distribution at the focus point of the off-axis reflection compact system configuration are illustrated (Figure 1b and 1c).In this scenario, the emitter is characterized by a broadband Gaussian dipole pulse centered at 1 THz with a bandwidth of 4 THz. The pulse propagates through a silicon lens (refractive index 3.41) and is deflected toward the borehole wall using two flat mirrors. The focal spot at 1 THz reaches a FWHM of approximately 1 mm, consistent with efficient focusing. The flux distribution confirms that the energy remains concentrated along the intended path with limited dispersion. It should be noted that, due to normalization at the source, the y-axis values do not reflect absolute energy efficiency, however results are comparable to those of conventional THz setups employing spherical lenses and off-axis parabolic mirrors, which were also simulated.These simulations assess not only the energy transfer efficiency but also the preservation of the pulse shape, which is critical for accurate spectroscopic analysis. Pulse integrity affects the clarity of spectral features, making beam shape and minimal distortion essential.These concepts aim to guide the future integration of THz-TDS in borehole probes for cometary exploration. Unlike GPR, THz-TDS can resolve fine-scale structures and provide spectral fingerprints of volatiles and organics. Its depth range (cm-scale) and non-destructive nature make it a compelling complement to existing tools, especially for layered or porous cometary soils. Figure 1: Off-axis reflection compact system compact system: (a) optical schematic, (b) beam propagation snapshot, and (c) pulse flux at focal point. Figure 2: Schematic views of alternative designs: (a) antenna array and (b) monolithic emitter–receiver design.References[1] Lambrechts, M., & Johansen, A. (2012). Rapid growth of gas-giant cores by pebble accretion. Astronomy & Astrophysics, 544, A32. https://doi.org/10.1051/0004-6361/201219127[2] Ata Akatay, Caglar Ataman, and Hakan Urey, High-resolution beam steering using microlens arrays. Optics Letters, 31(19), 2861–2863 (2006). https://doi.org/10.1364/OL.31.0028

Radiometry is a highly accurate technique of gas spectroscopy, widely used in observations of stellar objects (e.g., molecular clouds, accretion discs), planetary atmospheres, comets and icy satellites. Since observations are usually made far from the object, the spectrum detected is a combination of many gas layers lying between the gas source and the observer. Thus, gas models are needed to properly fit observations, which becomes especially challenging in optically thick layers and with large gradients in gas profiles. In addition, optically thick layers can mask the properties of the gas source which have a direct connection with chemical and physical conditions of subsurface layers. Our project, called WEEVIL (the Water Emission of Vapour from Ice in the Laboratory), concerns the spectroscopy of gas plumes arising from the sublimation of icy, porous and dusty media in controlled laboratory experiments, following [1]. Project objectives are: 1) verification/correction of subsurface models of icy bodies by comparing laboratory and space observations; 2) studying the capabilities of radiometry for determining subsurface material properties. A heterodyne radiometer operating primarily at the frequency of 557 GHz (water rotational line) is being used to investigate column densities, production rates, temperatures, and outflow gas velocities of sublimed icy samples. An internal cooling system using liquid nitrogen and a cryocooler ensures stable temperature of the sample and radiometer compounds, and mitigates the impact of the vacuum chamber on the gas plume. The radiometer has been developed/procured and is undergoing the necessary sensitivity tests. The vacuum chamber is in the final design/production stage. Measured brightness temperatures by the radiometer will be compared with DSMC (direct simulation Monte Carlo) of gas and radiative transfer calculations. The radiative transfer model includes time dependence due to gas expansion and non-LTE due to non-collisional background, following [2]. [1] O. Auriacombe et al. 2022 MNRAS 515 2 [2] M. A. Cordiner et al 2022 ApJ 929 38

Aims. The objective of this work is to study the influence of a highly non-isothermal porous dust layer on the formation of a comet’s inner coma. We studied the water gas activity of comet 67P/Churyumov-Gerasimenko to find a link between the gas properties around the comet and the properties of the dust surface crust. The effects on the radiative transfer spectral lines were studied and compared with MIRO remote sensing observations. Methods. For cases of spherical and complex nucleus shapes, we validated surface boundary conditions for gas flow obtained from the two-layer consistent thermophysical model. This model accurately estimates the properties of the sublimation products as the gas diffuses through the layer. The gas expansion was then modeled using a 3D parallel implementation of a direct simulation Monte Carlo algorithm. A multi-beam linear interpolation was used to extract the gas density, velocity, and temperature profiles along a given line of sight. Finally, the radiative transfer equation was used to calculate the brightness temperature of the water vapor. Results. The presence of a porous layer results in an increase in gas temperature and a decrease in gas density at the surface. The gas has a greater acceleration due to the higher initial temperature and increased conversion of translational energy to kinetic energy. This reduces the difference in density between the different models, with the densest gas being the coolest, and increases the terminal expansion velocity of the hotter gas. While the gas density differences are small at large distances, the observable water absorption lines are significantly affected. Conclusions. The presence of a porous layer has a large effect on the properties of the gas in the coma, which can be seen by comparing the spectral lines. This demonstrates the potential interest of the approach in improving surface activity models and placing physical constraints on the dust layer.