Axel Murk

The Submillimetre Wave Instrument (SWI) onboard the Jupiter Icy Moons Explorer (Juice) spacecraft consists of a 29 cm off-axis paraboloid antenna and two double-sideband heterodyne receivers operating in the frequency ranges of 530–638 and 1066–1286 GHz. This instrument is designed to study Jupiter's atmosphere and the exospheres of the Galilean moons. We used the Lunar-Earth-Gravity-Assist (LEGA) data from August 2024 to characterise this antenna using several SWI observations pointing towards the Earth. The data analysis of the LEGA observations dedicated to the antenna beam allows us to characterise the antenna beam Full Width at Half Maximum (FWHM), sidelobes, absolute pointing and receiver coalignement. Moreover, an estimate of the calibration efficiency factor has been obtained from an optical model simulation.

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.

On 19 and 20 August 2024 the Jupiter Icy Moons Explorer (Juice) executed during its cruise phase towards Jupiter a combined Lunar Earth Gravity Assist (LEGA) maneuver. These close flybys of the Moon and the Earth provided so far the best opportunity to test the behavior, performance, and calibration of the Submillimetre Wave Instrument (SWI) onboard Juice. This paper shows typical data taken during the Earth Gravity Assist and the following few days. Data quality and problems resulting from unexpected behavior of the hardware are discussed.

The Zeeman effect affects millimeter-wave emission lines from atmospheric oxygen through the interaction between the molecular magnetic dipole moment and the geomagnetic field, leading to characteristic line splitting and polarization signatures. Depending on the total angular momentum quantum number, the split components may appear as distinct peaks or as effective line broadening when unresolved. These signatures are observable in molecular oxygen emissions from the stratosphere and mesosphere, where pressure broadening is weak compared to the troposphere. The resulting line shape5 depends on magnetic field strength, viewing geometry, and polarization. The impact of the magnetic field on the line-spectra provides a potential pathway to remotely sense geomagnetic field perturbations. In this study, we examine the feasibility of such measurements using two oxygen fine-structure transitions at 53.067 GHz and 53.596 GHz. Radiative transfer simulations and synthetic retrievals indicate sensitivity to magnetic field perturbations at altitudes between 30 and 75 km. The method is further evaluated using fully polarimetric observations from the Campaign Temperature Radiometer (TEMPERA-C) during a10 geomagnetic storm associated with a strong solar flare in January 2026. Applying the developed inversion scheme, we retrieve a perturbation of the vertical magnetic field component of −130 ± 62 nT for the event.

The Jupiter Icy moons Explorer (Juice) was the first spacecraft ever that performed a combined gravity assist using both the Moon and Earth in succession. The double flyby required highly precise navigation to succeed. The LEGA allowed Juice to make a shortcut through the inner solar system on its way to Jupiter, using less fuel than would have been otherwise required. On 19 August 2024, Juice had its closest approach to the Moon. This first part of the manoeuvre accelerated the spacecraft by approximately 0.9 km s ⁻¹ relative to the Sun. On 20 August 2024, the spacecraft swung past Earth. This second part of the manoeuvre reduced the spacecraft's speed by 4.8 km s ⁻¹ relative to the Sun. This was a unique opportunity for its payloads to observe the Moon and Earth from a close distance as both calibration and science targets. The Submillimetre Wave Instrument (SWI), a dual channel heterodyne spectrometer observed both targets in two far-infrared bands around 500 and 250 µm wavelength in order to characterize and calibrate the overall performance of the instrument, including its receiver frontend, spectrometer backend and telescope mechanisms. In addition, the commanding pipeline and operations processes of the instrument were also tested close to its full range of flexibility using the relevant pipelines. In this paper we provide a contextual description of physical and functional characteristics of SWI, its operational principles and in-flight calibration activities during LEGA.

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.

High-quality water vapor measurements from the ground to space are essential for assessing Earth's radiative balance. Water vapor contributes substantially to the uncertainty of the outgoing longwave radiation, emphasizing the need for accurate and reliable measurements. The current generation of commercially available operational radiosondes exhibits rapidly degrading reliability above the tropopause, leaving a critical observational gap. The Cryogenic Frost Point Hygrometer (CFH), a widely used instrument for water vapor profile measurements relies on the R23 coolant, which was banned under the Montreal Protocol.The Swiss H2O Hub addresses this challenge using two balloon-borne instruments and a newly developed cryogenic water vapor radiometer. These observations are complemented by the MeteoSwiss RALMO Lidar system. ALBATROSS is a compact mid-IR laser spectrometer that measures the water vapor absorption spectrum in an open-path multipass cell, providing SI-traceable water vapor data. The second balloon-borne instrument is the Peltier Cooled Frost Point Hygrometer (PCFH) that replaces the R23 coolant with a Peltier cooler and measures the frost point temperature via a thermocouple calibrated at METAS (SI-traceable). Stratospheric and mesospheric water vapor is measured with two passive radiometers: the already established MIAWARA instrument at the Zimmerwald Observatory near Bern and a newly developed cryogenic instrument operated at the laboratory building. In total, we conducted 29 balloon soundings with various payload configurations and performed detailed instrument intercomparisons for the troposphere and the lower stratosphere. Both balloon instruments achieve breakthrough requirements for the stratosphere and reach threshold requirements for the troposphere.These combined observations enabled the detection of increased water vapor volume-mixing ratios in the lower stratosphere and mesosphere above Switzerland. Both the passive radiometers and the balloon soundings revealed enhanced water vapor levels in the years following the eruption.

The Zeeman effect affects millimeter-wave emission lines from atmospheric oxygen through the interaction between the molecular magnetic dipole moment and the geomagnetic field, leading to characteristic line splitting and polarization signatures. Depending on the total angular momentum quantum number, the split components may appear as distinct peaks or as effective line broadening when unresolved. These signatures are observable in molecular oxygen emissions from the stratosphere and mesosphere, where pressure broadening is weak compared to the troposphere. The resulting line shape5 depends on magnetic field strength, viewing geometry, and polarization. The impact of the magnetic field on the line-spectra provides a potential pathway to remotely sense geomagnetic field perturbations. In this study, we examine the feasibility of such measurements using two oxygen fine-structure transitions at 53.067 GHz and 53.596 GHz. Radiative transfer simulations and synthetic retrievals indicate sensitivity to magnetic field perturbations at altitudes between 30 and 75 km. The method is further evaluated using fully polarimetric observations from the Campaign Temperature Radiometer (TEMPERA-C) during a10 geomagnetic storm associated with a strong solar flare in January 2026. Applying the developed inversion scheme, we retrieve a perturbation of the vertical magnetic field component of −130 ± 62 nT for the event.

The Submillimetre Wave Instrument (SWI) is a passive dual-beam heterodyne radiometer on the Jupiter Icy Moons Explorer (Juice) spacecraft that operates in the frequency bands 530 – 630 GHz and 1066 – 1280 GHz. This paper discusses the misalignment, surface deviations, and thermal deformation related to its optics and how they are included into the optical model of the instrument. The misalignment and surface deviations of the SWI protoflight model that are caused by the manufacturing and assembly tolerances were measured with a coordinate measurement machine. Thermoelastic deformation of the optics that is subjected to typical operational temperatures ranging from -70°C to -130°C is determined with finite element method simulations. The optical performance of the instrument is then simulated for several telescope pointing angles using the optical model containing the above-mentioned imperfections. The effect of each imperfection is investigated also separately.

The Jupiter Icy moons Explorer (Juice) was the first spacecraft ever that performed a combined gravity assist using 20 both the Moon and Earth in succession. The double flyby required highly precise navigation to succeed. The LEGA allowed Juice to make a shortcut through the inner solar system on its way to Jupiter, using less fuel than would have been otherwise required. On August 19, 2024, Juice had its closest approach to the Moon. This first part of the manoeuvre accelerated the spacecraft by approximately 0.9 km/s relative to the Sun. On August 20, 2024, the spacecraft swung past Earth. This second part of the manoeuvre reduced the spacecraft's speed by 4.8 km/s relative to the Sun. This was a unique opportunity for its payloads to observe the Moon and Earth from a close distance as both calibration and science targets. The Submillimetre Wave Instrument (SWI), a dual channel heterodyne spectrometer observed both targets in two far-infrared bands around 500 and 250 μm wavelength in order to characterize and calibrate the overall performance of the instrument, including its receiver frontend, spectrometer backend and telescope mechanisms. In addition, the commanding pipeline and operations processes of the instrument were also tested close to its full range of flexibility using the relevant pipelines. In this paper we provide a contextual description of physical and functional characteristics of SWI, its operational principles and in-flight calibration activities during LEGA.