Establishing an approach to determining the physico-chemicalproperties at sub-centimetre scales: The SUBICE Project

1 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 revealed

complex 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 1–5, 2025.

[3] Arnaud Demion, Linus Leo St¨ockli, Nicolas Thomas, and Silvan Zahno. Frequency-dependent deconvolution for enhanced thz-tds scans: Accounting for beam width variations in time traces. IEEE Transactions on Terahertz Science and Technology, 15(3):505–513, 2025.