publications

2025

1. THz Image Explorer - An Interactive Cross-Platform Open-Source THz Image Analysis Tool

   Linus Leo Stöckli , Arnaud Demion , and Nicolas Thomas

   Journal of Open Source Software, 2025

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2. Evolution of granular salty ice analogs for Europa: Sublimation and irradiation

   Rafael Ottersberg , Antoine Pommerol , Linus Leo Stöckli , and 5 more authors

   Icarus, 2025

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   We study the evolution of the Vis-NIR reflectance spectrum of salty granular ice analog samples in a simulation chamber under conditions relevant to the surface of Europa. A novel application and custom calibration of a thermopile sensor enabled the measurement of the surface temperature of the samples in far infrared emission. This allows the kinetics of the observed changes to be scaled to equivalent timescales on Europa. We observed significant changes in the depth and shape of the broad water absorption bands for all samples on timescales of a few thousand years of equatorial conditions on Europa. This effect should be taken into account if quantitative predictions about bulk composition are made based on remote-sensing data. A narrow absorption feature attributed to hydrohalite formed during the sublimation of the sodium chloride sample. We used near-infrared spectroscopy in an irradiation chamber to assess the stability of this narrow feature under electron irradiation. We find that the radiation environment present on Europa dehydrates the hydrohalite on short timescales. Therefore, we expect hydrohalite not to be present on the surface, unless erupted very recently (<10 yr) or located in thermal anomalies (> 145 K), so a detection of hydrohalite clearly indicates recent activity.

3. Frequency-Dependent Deconvolution for Enhanced THz-TDS Scans: Accounting for Beam Width Variations in Time Traces

   Arnaud Demion , Linus Leo Stöckli , Nicolas Thomas , and 1 more author

   IEEE Transactions on Terahertz Science and Technology, 2025

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   Terahertz time-domain spectroscopy (THz-TDS) is a powerful tool for nondestructive testing, but the quality of scans is limited by beam spreading effects, which vary with frequency. We propose a deconvolution method that accounts for the changing beam width as a function of frequency. By incorporating frequency-specific Gaussian beam profiles, determined through the knife-edge technique, the proposed method enhances the spatial resolution of THz-TDS scans while keeping the ability to analyze the phase of the signals. Our approach significantly improves contrast and sharpness in both reflectance and transmission modes. This technique offers a more accurate and robust enhancement of THz-TDS imaging.

4. Design and commissioning of a THz time-domain spectro-goniometer in a cryogenic comet simulation chamber

   Linus Leo Stöckli , Mathias Brändli , Daniele Piazza , and 4 more authors

   Review of Scientific Instruments, Mar 2025

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   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.

2024

1. Metal substrate printed circuit boards as low-cost, robust temperature sensor mounts for cryogenic thermometry

   Linus Leo Stöckli , Rafael Ottersberg , and Nicolas Thomas

   Engineering Research Express, Dec 2024

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   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.

2. The Comet Interceptor Mission

   Geraint H. Jones , Colin Snodgrass , Cecilia Tubiana , and 231 more authors

   Space Science Reviews, Feb 2024

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   Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s FClass call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms−1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

3. Gas permeability and mechanical properties of dust grain aggregates at hyper- and zero-gravity

   Holly L Capelo , Jean-David Bodénan , Martin Jutzi , and 13 more authors

   Monthly Notices of the Royal Astronomical Society, Aug 2024

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   Particle–particle and particle–gas processes significantly impact planetary precursors such as dust aggregates and planetesimals. We investigate gas permeability (\\κ\) in 12 granular samples, mimicking planetesimal dust regoliths. Using parabolic flights, this study assesses how gravitational compression – and lack thereof – influences gas permeation, impacting the equilibrium state of low-gravity objects. Transitioning between micro- and hyper-gravity induces granular sedimentation dynamics, revealing collective dust–grain aerodynamics. Our experiments measure \\κacross Knudsen number (Kn) ranges, reflecting transitional flow. Using mass and momentum conservation, we derive \\κand calculate pressure gradients within the granular matrix. Key findings: (i) As confinement pressure increases with gravitational load and mass flow, \\κand average pore space decrease. This implies that a planetesimal’s unique dust-compaction history limits subsurface volatile outflows. (ii) The derived pressure gradient enables tensile strength determination for asteroid regolith simulants with cohesion. This offers a unique approach to studying dust-layer properties when suspended in confinement pressures comparable to the equilibrium state on planetesimals surfaces, which will be valuable for modelling their collisional evolution. (iii) We observe a dynamical flow symmetry breaking when granular material moves against the pressure gradient. This occurs even at low Reynolds numbers, suggesting that Stokes numbers for drifting dust aggregates near the Stokes–Epstein transition require a drag force modification based on permeability.