Detailed observations of Enceladus by the Cassini spacecraft revealed its astrobiological potential and transformed our perception of ocean worlds in the Solar System. Beneath Enceladus’ icy crust lies a warm ocean sustained by tidal heating. This ocean expels subsurface material through fissures at the south pole region into space as plumes. The particles in these plumes reaccrete on Enceladus’ surface, while some of the volatiles present in the sub-surface ocean diffuse through the ice shell to reach the surface. In this study, we irradiated thin Enceladus ice analogues in an ultra-high vacuum chamber optimised for ice chemistry at a surface temperature of 70 ± 2 K and compared the resulting composition with ices typical to the ISM (15 K). We studied the irradiation of ices composed of H₂O, CO₂, and NH₃ mixtures as a function of wavelength by using two different radiation sources that cover high and low photon energy ranges: The microwave-discharge hydrogen-flow lamp (MDHL) generating vacuum-ultraviolet (VUV) light, that is, between 115 - 180 nm, and the solar radiation Xe-arc lamp (SRL), simulating the solar broadband radiation from 200 nm - 1800 nm. Upon irradiation, solid-state photoproducts were identified using a Fourier-transform infrared (FTIR) spectrometer in the mid-infrared range (4000 - 700 cm⁻¹ or 2.5 - 14.3 μm). Sublimating gas-phase species were tracked using a quadrupole mass spectrometer (QMS). At 70 K, energetic photons from the MDHL formed new species such as O₃ and CO₃ in an H₂O:CO₂:NH₃ ice matrix due to the clustering of CO₂ at elevated temperatures. Hereby, dissociation of segregated CO₂ provides the necessary oxygen atoms to form O₃ via the enhanced mobility and addition reaction of O-atoms. At 15 K, CO,OCN⁻,H₂CO,CH₃OH, HCOOH and possibly NH₂OH were induced by VUV-photons. Similarly, these species were detected at 70 K with a tentative assignment for HCOOH and NH₂OH. The SRL caused no chemical evolution of the ice due to insufficient photon energies. In conclusion, we predict the formation of ozone by gardening of CO₂-rich or mixed CO₂:H₂O ice, found, for example, in-between the tiger stripes on Enceladus or on other icy bodies in our Solar System with surface temperatures cooler than 88 K.

In this study, plume experiments were conducted to mimic the thermodynamic conditions on Saturn's moon, Enceladus. The icy moon's subsurface ocean and cracks in the surface have been simulated using a liquid water reservoir and a narrow channel, while the low-pressure environment at Enceladus’ surface was achieved with a vacuum chamber. We aimed to examine how channel temperature affected the plume's temperature, solid fraction and velocity, testing two models with differing wall temperatures: room temperature and near 0 °C. The colder setup better replicated Enceladus’ plume, producing a saturated flow in which nucleation of icy particles is possible. A conservative 1.5%–3% minimum solid fraction is estimated from measurements and modelling. Pitot-tube measurements indicated velocities around 400–500 m/s at the channel outlet. Flow temperature and velocity are closely correlated with wall temperature, indicating effective heat transfer. With a plume model based on the energy conservation law, we concluded that supersonic plume velocities observed on Enceladus cannot be achieved with straight channels, i.e. without requiring extreme expansion ratios. Additionally, the research provides evidence of the relationship between the crevasse's expansion ratio and the temperatures of flow and crevasse walls.

Context. Infrared observations of water ice in the interstellar medium are hindered by uncertainties in the band strength, density, and porosity, which introduce considerable errors in the estimated column density of the ice on a line of sight. Aims. We revise the infrared band strength values and band positions of water ice grown under simulated interstellar conditions at different deposition and warm-up temperatures. We also explore other physical ice parameters: density, refractive index, and porosity. Methods. We grew water ice in simulated interstellar conditions in ultra-high vacuum with temperatures between 10 and 150 K. We used infrared spectroscopy and laser interferometry to obtain updated values for the band strengths. With these updated values, we calculated and report the density, refractive index, porosity, and infrared band position for water ice. Results. Previous measurements of these properties were inaccurate because ice was considered non-porous. Our results show that the band strength for the O-H stretching vibration varies significantly with deposition temperature and should not be considered constant. There is also measurable variation of the band strength during the warm-up of the ice after deposition. Conclusions. Previous ice density values were overestimated due to inaccurate band strengths and the omission of porosity, which can only be considered negligible at very slow growth rates and deposition temperatures close to sublimation. The infrared band position shifts with varying porosity.

We report observations of stripe-like features in Enceladus’ plumes captured simultaneously by Cassini's VIMS-IR and ISS NAC instruments during flyby E17, with similar patterns seen in VIMS-IR data from flyby E13 and E19. These parallel stripes, inclined at approximately 16°to the ecliptic and 43°to Saturn's ring plane, appear continuous across images when projected in the J2000 frame. A bright stripe, most visible at wavelengths around 5μm, acts as the zeroth-order diffraction peak of a reflection grating with an estimated groove spacing of 0.12–2.60 mm, while adjacent stripes are attributed to higher-order diffraction peaks. We suggest that this light-dispersing phenomenon originates from an inclined periodic structure within Saturn's E ring. This structure, constrained between Saturn's G ring and Rhea's orbit, likely consists of fresh ice particles supplied by Enceladus’ plumes.

The discovery of vast subsurface oceans beneath the thick ice crusts of icy moons in our Solar System has ignited global interest in their potential habitability and in the processes shaping these celestial bodies. With upcoming missions set to explore the Galilean and Cronian moons in the coming decades, experimental studies are essential for optimising mission planning, selecting and testing scientific instruments, and maximising the scientific return from future observations. In this paper, we present the Plumes and Ices Simulation Chamber for Enceladus and other moonS (PISCES) — a novel experimental setup designed to replicate the extreme environmental conditions of icy moons, with pressures reaching down to 3 × 10⁻⁵ mbar and temperatures as low as 80 K. PISCES enables controlled laboratory investigations of plume dynamics and surface interactions using a suite of integrated sensors and instruments. We describe the vacuum chamber setup, its capabilities, and its adaptability to various experimental configurations. To demonstrate its potential, we detail experiments simulating Enceladus’ plume activity with the Crevasse Laboratory Analogues for Moons (CLAM), an experimental apparatus employing 3D-printed cylindrical channels positioned above a liquid water reservoir within the vacuum chamber. This approach allows us to examine plume behaviour — including vent velocity, temperature, and particle size — in relation to subsurface conditions such as wall temperature, conduit dimensions, and expansion ratios. Ultimately, PISCES provides a groundbreaking platform for experimentally reproducing icy plumes under conditions analogous to those on Enceladus, advancing our understanding of plume physics and informing future planetary exploration efforts.

Enceladus, one of Saturn’s moons, is considered one of the most promising places in the solar system to find life. The Cassini mission discovered organic-rich water plumes from Enceladus’s subsurface ocean, prompting new lander mission planning. We developed a mechanically simple ice sampling system for autonomous life detection on lander missions. The system is controlled by a single rotary actuator that samples, liquefies, and prepares ice for microscopic observations. Sample acquisition uses a novel conical boundary layer pump that delivers samples to a microfluidic disk. A digital holographic microscope detects microorganisms without mechanical focusing. The single-actuator design enables closed-loop control of velocity, position, and torque, with an operational sequence controlling fluid dynamics in a centrifugal microfluidic disk. Testing demonstrated system feasibility and effectiveness across all subsystems. Open-source software was developed for automated onboard hologram processing, including organism motility detection to assess presence of life. This single-actuator design reduces mechanical complexity for lander missions. Future work focuses on adapting the technology for terrestrial applications and achieving required technology readiness levels for space deployment.

Context. The surfaces of icy moons are primarily composed of water ice that can be mixed with other compounds, such as carbon dioxide. The carbon dioxide (CO₂) stretching fundamental band observed on Europa and Ganymede appears to be a combination of several bands that are shifting location from one moon to another. Aims. We investigate the cause of the observed shift in the CO₂ stretching absorption band experimentally. We also explore the spectral behaviour of CO₂ ice by varying the temperature and concentration. Methods. We analyzed pure CO₂ ice and ice mixtures deposited at 10 K under ultra-high vacuum conditions using Fourier-transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) experiments. Laboratory ice spectra were compared to JWST observation of Europa's and Ganymede's leading hemispheres. The simulated IR spectra were calculated using density functional theory (DFT) methods, exploring the effect of porosity in CO₂ ice. Results. Pure CO₂ and CO₂-water ice show distinct spectral changes and desorption behaviours at different temperatures, revealing intricate CO₂ and H₂O interactions. The number of discernible peaks increases from two in pure CO₂ to three in CO₂-water mixtures. Conclusions. The different CO₂ bands were assigned to ν_3,1 (2351 cm^-1, 4.25 μm) caused by CO₂ dangling bonds (CO₂ found in pores or cracks) and ν_3,2 (2345 cm^-1, 4.26 μm) due to CO₂ segregated in water ice, whereas ν_3,3 (2341 cm^-1, 4.27 μm) is due to CO₂ molecules embedded in water ice. The JWST NIRSpec CO₂ spectra for Ganymede and for Europa can be fitted with two Gaussians attributed to ν_3,1 and ν_3,3. For Europa, ν_3,1 is located at lower wavelengths due to a lower temperature. The Ganymede data reveal latitudinal variations in CO₂ bands, with ν_3,3 dominating in the pole and ν_3,1 prevalent in other regions. This shows that CO₂ is embedded in water ice at the poles and it is present in pores or cracks in other regions. Ganymede longitudinal spectra reveal an increase of the CO₂ ν_3,1 band throughout the day, possibly due to ice cracks or pores caused by large temperature fluctuations.

Context. The James Webb Space Telescope (JWST) has captured the most detailed and sharpest infrared (IR) images ever taken of the inner region of the Orion Nebula, the nearest massive star formation region, and a prototypical highly irradiated dense photo-dissociation region (PDR).

Aims. We investigate the fundamental interaction of far-ultraviolet (FUV) photons with molecular clouds. The transitions across the ionization front (IF), dissociation front (DF), and the molecular cloud are studied at high-angular resolution. These transitions are relevant to understanding the effects of radiative feedback from massive stars and the dominant physical and chemical processes that lead to the IR emission that JWST will detect in many Galactic and extragalactic environments.

Methods. We utilized NIRCam and MIRI to obtain sub-arcsecond images over ~150″ and 42″ in key gas phase lines (e.g., Pa α, Br α, [FeII] 1.64 µm, H2 1−0 S(1) 2.12 µm, 0–0 S(9) 4.69 µm), aromatic and aliphatic infrared bands (aromatic infrared bands at 3.3–3.4 µm, 7.7, and 11.3 µm), dust emission, and scattered light. Their emission are powerful tracers of the IF and DF, FUV radiation field and density distribution. Using NIRSpec observations the fractional contributions of lines, AIBs, and continuum emission to our NIRCam images were estimated. A very good agreement is found for the distribution and intensity of lines and AIBs between the NIRCam and NIRSpec observations.

Results. Due to the proximity of the Orion Nebula and the unprecedented angular resolution of JWST, these data reveal that the molecular cloud borders are hyper structured at small angular scales of ~0.1–1″ (~0.0002–0.002 pc or ~40–400 au at 414 pc). A diverse set of features are observed such as ridges, waves, globules and photoevaporated protoplanetary disks. At the PDR atomic to molecular transition, several bright features are detected that are associated with the highly irradiated surroundings of the dense molecular condensations and embedded young star. Toward the Orion Bar PDR, a highly sculpted interface is detected with sharp edges and density increases near the IF and DF. This was predicted by previous modeling studies, but the fronts were unresolved in most tracers. The spatial distribution of the AIBs reveals that the PDR edge is steep and is followed by an extensive warm atomic layer up to the DF with multiple ridges. A complex, structured, and folded H0/H2 DF surface was traced by the H2 lines. This dataset was used to revisit the commonly adopted 2D PDR structure of the Orion Bar as our observations show that a 3D “terraced” geometry is required to explain the JWST observations. JWST provides us with a complete view of the PDR, all the way from the PDR edge to the substructured dense region, and this allowed us to determine, in detail, where the emission of the atomic and molecular lines, aromatic bands, and dust originate.

Conclusions. This study offers an unprecedented dataset to benchmark and transform PDR physico-chemical and dynamical models for the JWST era. A fundamental step forward in our understanding of the interaction of FUV photons with molecular clouds and the role of FUV irradiation along the star formation sequence is provided.

Context. JWST has taken the sharpest and most sensitive infrared (IR) spectral imaging observations ever of the Orion Bar photodis-sociation region (PDR), which is part of the nearest massive star-forming region the Orion Nebula, and often considered to be the 'prototypical'strongly illuminated PDR. Aims. We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the H II region to the atomic PDR -crossing the ionisation front (IF) -, and the subsequent transition to the molecular PDR -crossing the dissociation front (DF). Given the prevalence of PDRs in the interstellar medium and their dominant contribution to IR radiation, understanding the response of the PDR gas to far-ultraviolet (FUV) photons and the associated physical and chemical processes is fundamental to our understanding of star and planet formation and for the interpretation of any unresolved PDR as seen by JWST. Methods. We used high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science programme. We constructed a 3″ × 25″ spatio-spectral mosaic covering 0.97-5.27 μm at a spectral resolution R of ~2700 and an angular resolution of 0.075″-0.173″. To study the properties of key regions captured in this mosaic, we extracted five template spectra in apertures centred on the three H₂ dissociation fronts, the atomic PDR, and the H II region. This wealth of detailed spatial-spectral information was analysed in terms of variations in the physical conditions-incident UV field, density, and temperature -of the PDR gas. Results. The NIRSpec data reveal a forest of lines including, but not limited to, He I, H I, and C I recombination lines; ionic lines (e.g. Fe III and Fe II); O I and N I fluorescence lines; aromatic infrared bands (AIBs, including aromatic CH, aliphatic CH, and their CD counterparts); pure rotational and ro-vibrational lines from H₂; and ro-vibrational lines from HD, CO, and CH⁺, with most of them having been detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. In addition, we observed numerous smaller-scale structures whose typical size decreases with distance from θ¹ Ori C and IR lines from C I, if solely arising from radiative recombination and cascade, reveal very high gas temperatures (a few 1000 K) consistent with the hot irradiated surface of small-scale dense clumps inside the PDR. The morphology of the Bar, in particular that of the H₂ lines, reveals multiple prominent filaments that exhibit different characteristics. This leaves the impression of a 'terraced'transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. We attribute the different characteristics of the H₂ filaments to their varying depth into the PDR and, in some cases, not reaching the C⁺/C/CO transition. These observations thus reveal what local conditions are required to drive the physical and chemical processes needed to explain the different characteristics of the DFs and the photochemical evolution of the AIB carriers. Conclusions. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star and planet formation as well as galaxy evolution.

The discovery of vast subsurface oceans hidden under kilometers of ices on icy moons in our Solar System has sparked worldwide interests in ascertaining their potential habitability. In the case of Saturn’s moon Enceladus, supersonic plumes of water vapour and icy grains have been observed by the Cassini mission spewing from the surface, giving us indirect knowledge of the composition of this subsurface ocean. The exact mechanisms of the plumes however, and their effect on the composition of the ejected matter has yet to be clearly understood. The focus of this study is to experimentally investigate physical characteristics of the plumes located at the South Polar Terrain (SPT) of Enceladus. Using facilities at TU Delft faculty, we simulate experimentally the topology of the ice crevasses and the subsurface ocean with a narrow channel mounted atop a liquid water reservoir placed inside a vacuum chamber. We inquire upon the dependence of the channel walls temperature on the plume’s exhaust velocity. Using a straight channel, our results show that colder wall temperatures enable a saturated water vapour flow with a minima 1.5-3 % solid fraction and vent velocities reaching around 400-500 m/s. The data ranges for velocities and solid fraction extrapolated from the Cassini data (550-2000 m/s and 7-70 %) thus cannot be explained by straight channel models. Using a channel with an expansion ratio of 1.73 however, the measured supersonic plume velocity becomes comparable to some of the in situ Mach number determined at Enceladus. Using a method based on the energy conservation law, it is possible to extrapolate from our experimental data some plausible geometries of the ice crevasses of Enceladus. This work lays the ground work for a coming comprehensive parametric study of the channel geometry and its effect on exhaust Mach number, temperature and solid fraction.

Supersonic plumes of water vapour and icy particles have been observed by the Cassini spacecraft during several flybys over Enceladus. These plumes originate from the Tiger Stripes located in the South Polar Terrain (SPT), and indicate the presence of a subsurface ocean under the icy crust which is salty and contains complex organic molecules. Other characteristics of the plumes, such as the vent temperature, mass flow rate, velocity and mass fraction of icy particles can be used to determine the conditions in the channel, linking the subsurface ocean to the icy surface. In this paper, we developed a fluid dynamics model that accounts for nucleation, particle growth, wall accretion and sublimation. The channel behaves similarly to a converging–diverging nozzle, which forms supersonic plumes due to a pressure difference between the reservoir where the subsurface ocean is located and the exosphere. The geometry of the channel and its evolution with accretion of gas and sublimation of ice are studied to reproduce the characteristics of the plumes observed by Cassini. We first performed a parameter study on the channel geometry to determine how it influences the plumes’ velocity, solid fraction and exit temperature. Our results show that the size of the icy particles is primarily dependent on the length of the channel, indicating that large particles (∼75μm) must originate from within a kilometer below the surface, while smaller particles (∼3μm) can originate from only hundreds of meters below the surface. We further show that the velocity of the flow, exit temperature and nucleation depend directly on the exit-to-throat size ratio. We find that the channel geometry evolves within a few tens of hours until an equilibrium is reached, when considering the accretion of gas to the walls, or sublimation of ice from the walls. As the channel closes due to accretion, the flow becomes thinner, which in turn reduces accretion. After around 70 h, the accretion is sufficiently slowed such that the geometry does not evolve anymore. This equilibrium geometry produces higher Mach numbers and a larger particle size and solid fraction compared to the initial geometry.

Context. Mid-infrared emission features are important probes of the properties of ionized gas and hot or warm molecular gas, which are difficult to probe at other wavelengths. The Orion Bar photodissociation region (PDR) is a bright, nearby, and frequently studied target containing large amounts of gas under these conditions. Under the “PDRs4All” Early Release Science Program for JWST, a part of the Orion Bar was observed with MIRI integral field unit (IFU) spectroscopy, and these high-sensitivity IR spectroscopic images of very high angular resolution (0.2^′′) provide a rich observational inventory of the mid-infrared (MIR) emission lines, while resolving the H II region, the ionization front, and multiple dissociation fronts. Aims. We list, identify, and measure the most prominent gas emission lines in the Orion Bar using the new MIRI IFU data. An initial analysis summarizes the physical conditions of the gas and demonstrates the potential of these new data and future IFU observations with JWST. Methods. The MIRI IFU mosaic spatially resolves the substructure of the PDR, its footprint cutting perpendicularly across the ionization front and three dissociation fronts. We performed an up-to-date data reduction, and extracted five spectra that represent the ionized, atomic, and molecular gas layers. We identified the observed lines through a comparison with theoretical line lists derived from atomic data and simulated PDR models. The identified species and transitions are summarized in the main table of this work, with measurements of the line intensities and central wavelengths. Results. We identified around 100 lines and report an additional 18 lines that remain unidentified. The majority consists of H I recombination lines arising from the ionized gas layer bordering the PDR. The H I line ratios are well matched by emissivity coefficients from H recombination theory, but deviate by up to 10% because of contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni. We show how the Ne III/Ne II, S IV/S III, and Ar III/Ar II ratios trace the conditions in the ionized layer bordering the PDR, while Fe III/Fe II and Ni III/Ni II exhibit a different behavior, as there are significant contributions to Fe II and Ni II from the neutral PDR gas. We observe the pure-rotational H₂ lines in the vibrational ground state from 0–0 S(1) to 0–0 S(8), and in the first vibrationally excited state from 1–1 S(5) to 1–1 S(9). We derive H₂ excitation diagrams, and for the three observed dissociation fronts, the rotational excitation can be approximated with one thermal (∼700 K) component representative of an average gas temperature, and one nonthermal component (∼2700 K) probing the effect of UV pumping. We compare these results to an existing model of the Orion Bar PDR, and find that the predicted excitation matches the data qualitatively, while adjustments to the parameters of the PDR model are required to reproduce the intensity of the 0–0 S(6) to S(8) lines.

Jupiter's icy moon Ganymede has a tenuous exosphere produced by sputtering and possibly sublimation of water ice. To date, only atomic hydrogen and oxygen have been directly detected in this exosphere. Here, we present observations of Ganymede's CO₂ exosphere obtained with the James Webb Space Telescope. CO₂ gas is observed over different terrain types, mainly over those exposed to intense Jovian plasma irradiation, as well as over some bright or dark terrains. Despite warm surface temperatures, the CO₂ abundance over equatorial subsolar regions is low. CO₂ vapor has the highest abundance over the north polar cap of the leading hemisphere, reaching a surface pressure of 1 pbar. From modeling we show that the local enhancement observed near 12 h local time in this region can be explained by the presence of cold traps enabling CO₂ adsorption. However, whether the release mechanism in this high-latitude region is sputtering or sublimation remains unclear. The north polar cap of the leading hemisphere also has unique surface-ice properties, probably linked to the presence of the large atmospheric CO₂ excess over this region. These CO₂ molecules might have been initially released in the atmosphere after the radiolysis of CO₂ precursors, or from the sputtering of CO₂ embedded in the H₂O ice bedrock. Dark terrains (regiones), more widespread on the north versus south polar regions, possibly harbor CO₂ precursors. CO₂ molecules would then be redistributed via cold trapping on ice-rich terrains of the polar cap and be diurnally released and redeposited on these terrains. Ganymede's CO₂ exosphere highlights the complexity of surface-atmosphere interactions on Jupiter's icy Galilean moons.

Context. We present the first spectroscopic observations of Ganymede by the James Webb Space Telescope undertaken in August 2022 as part of the proposal "ERS observations of the Jovian system as a demonstration of JWST's capabilities for Solar System science". Aims. We aimed to investigate the composition and thermal properties of the surface, and to study the relationships of ice and non-water-ice materials and their distribution. Methods. NIRSpec IFU (2.9-5.3 μm) and MIRI MRS (4.9-28.5 μm) observations were performed on both the leading and trailing hemispheres of Ganymede, with a spectral resolution of ∼2700 and a spatial sampling of 0.1 to 0.17″ (while the Ganymede size was ∼1.68″). We characterized the spectral signatures and their spatial distribution on the surface. The distribution of brightness temperatures was analyzed with standard thermophysical modeling including surface roughness. Results. Reflectance spectra show signatures of water ice, CO₂, and H₂O₂. An absorption feature at 5.9 μm, with a shoulder at 6.5 μm, is revealed, and is tentatively assigned to sulfuric acid hydrates. The CO₂ 4.26-μm band shows latitudinal and longitudinal variations in depth, shape, and position over the two hemispheres, unveiling different CO₂ physical states. In the ice-rich polar regions, which are the most exposed to Jupiter's plasma irradiation, the CO₂ band is redshifted with respect to other terrains. In the boreal region of the leading hemisphere, the CO₂ band is dominated by a high wavelength component at ∼4.27 μm, consistent with CO₂ trapped in amorphous water ice. At equatorial latitudes (and especially on dark terrains), the observed band is broader and shifted toward the blue, suggesting CO₂ adsorbed on non-icy materials, such as minerals or salts. Maps of the H₂O Fresnel peak area correlate with Bond albedo maps and follow the distribution of water ice inferred from H₂O absorption bands. Amorphous ice is detected in the ice-rich polar regions, and is especially abundant on the northern polar cap of the leading hemisphere. Leading and trailing polar regions exhibit different H₂O, CO₂, and H₂O₂ spectral properties. However, in both hemispheres the north polar cap ice appears to be more processed than the south polar cap. A longitudinal modification of the H₂O ice molecular structure and/or nanometer- and micrometer-scale texture, of diurnal or geographic origin, is observed in both hemispheres. Ice frost is tentatively observed on the morning limb of the trailing hemisphere, which possibly formed during the night from the recondensation of water subliming from the warmer subsurface. Reflectance spectra of the dark terrains are compatible with the presence of Na- and Mg-sulfate salts, sulfuric acid hydrates, and possibly phyllosilicates mixed with fine-grained opaque minerals, with a highly porous texture. Latitude and local time variations of the brightness temperatures indicate a rough surface with mean slope angles of 15° - 25° and a low thermal inertia Γ = 20-40 J m^-2 s^-0.5 K^-1, consistent with a porous surface, with no obvious difference between the leading and trailing sides.

The reservoir of sulphur accounting for sulphur depletion in the gas of dense clouds and circumstellar regions is still unclear. One possibility is the formation of sulphur chains, which would be difficult to detect by spectroscopic techniques. This work explores the formation of sulphur chains experimentally, both in pure HS ice samples and in HO:HS ice mixtures. An ultrahigh vacuum chamber, ISAC, eqquipped with FTIR and QMS, was used for the experiments. Our results show that the formation of HS species is efficient, not only in pure HS ice samples, but also in water-rich ice samples. Large sulphur chains are formed more efficiently at low temperatures (10 K), while high temperatures (50 K) favour the formation of short sulphur chains. Mass spectra of HS, x = 2-6, species are presented for the first time. Their analysis suggests that HS species are favoured in comparison with S chains. Nevertheless, the detection of several S fragments at high temperatures in HS:HO ice mixtures suggests the presence of S in the irradiated ice samples, which could sublimate from 260 K. ROSINA instrument data from the cometary Rosetta mission detected mass-to-charge ratios 96 and 128. Comparing these detections with our experiments, we propose two alternatives: (1) HS and HS to be responsible of those S and S cations, respectively, or (2) S species, sublimating and being fragmented in the mass spectrometer. If S is the parent molecule, then S and S cations could be also detected in future missions by broadening the mass spectrometer range.

Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µm. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. These high-quality data allow for an unprecedentedly detailed view of AIBs.

Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H2 dissociation fronts), the atomic PDR, and the H II region.

Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here.

Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm with well-defined profiles. In addition, the spectra display a wealth of weaker features and sub-components. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the H II region template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters (2022, A&A, 390, 1089), except for the 11.2 µm AIB profile deep in the molecular zone, which belongs to class B11.2. Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µm AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3/11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the H II region, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the effects of photochemistry. The observed broadening of the bands in the molecular PDR is consistent with an enhanced importance of smaller PAHs since smaller PAHs attain a higher internal excitation energy at a fixed photon energy.

Conclusions. Spectral-imaging observations of the Orion Bar using JWST yield key insights into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 µm AIB emission from class B11.2 in the molecular PDR to class A11.2 in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a “weeding out” of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called ‘grandPAHs’.

Molecular hydrogen (H ₂ ) is the most abundant molecule in the Universe. Its formation involves catalytic reactions occurring on the surface of interstellar dust grains, but also involving polycyclic aromatic hydrocarbons (PAHs). Experiments and theoretical calculations have been performed to determine the processes governing the formation of H ₂ on dust and involving PAHs. These studies were recently brought in a review paper (Wakelam et al. 2017), from which this contribution is based.

Context. Gaining a full understanding of the planet and moon formation process calls for observations that probe the circumplanetary environment of accreting giant planets. The mid-infrared ELT imager and spectrograph (METIS) will provide a unique capability to detect warm-gas emission lines from circumplanetary disks. Aims. We aim to demonstrate the capability of the METIS instrument on the Extremely Large Telescope (ELT) to detect circumplanetary disks (CPDs) with fundamental v = 1 0 transitions of 12CO from 4.5 to 5 μm. Methods. We considered the case of the well-studied HD 100546 pre-transitional disk to inform our disk modeling approach. We used the radiation-thermochemical disk modeling code ProDiMo to produce synthetic spectral channel maps. The observational simulator SimMETIS was employed to produce realistic data products with the integral field spectroscopic (IFU) mode. Results. The detectability of the CPD depends strongly on the level of external irradiation and the physical extent of the disk, favoring massive (∼10 MJ) planets and spatially extended disks, with radii approaching the planetary Hill radius. The majority of 12CO line emission originates from the outer disk surface and, thus, the CO line profiles are centrally peaked. The planetary luminosity does not contribute significantly to exciting disk gas line emission. If CPDs are dust-depleted, the 12CO line emission is enhanced as external radiation can penetrate deeper into the line emitting region. Conclusions. UV-bright star systems with pre-transitional disks are ideal candidates to search for CO-emitting CPDs with ELT/METIS. METIS will be able to detect a variety of circumplanetary disks via their fundamental 12CO ro-vibrational line emission in only 60 s of total detector integration time.

Context. Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30-m Large Program aimed at determining the elemental abundances of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical star-forming filaments. In particular, the elemental abundance of S remains uncertain by several orders of magnitude, and its determination is one of the most challenging goals of this program. Aims. This paper aims to constrain the sulfur elemental abundance in Taurus, Perseus, and Orion A based on the GEMS molecular database. The selected regions are prototypes of low-mass, intermediate-mass, and high-mass star-forming regions, respectively, providing useful templates for the study of interstellar chemistry. Methods. We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO+, HCN, HNC, CS, SO, H2S, OCS, and HCS+ to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions from AV ∼ 3 mag to >50 mag, molecular hydrogen densities ranging from a few × 103 cm3 to 3 × 106 cm3, and Tk ∼ 10-35 K. We investigate the possible relationship between sulfur depletion and the grain charge distribution in different environments. Results. Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry, t = 0.1 Myr, ζH2 ∼ (0.51) × 1016 s1, and [S/H] ∼ 1.5 × 106. On the contrary, most of the positions in Orion are fitted with t = 1 Myr and ζH2 ∼ 1017 s1. Moreover, ∼40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H] ∼ 1.5 × 105. We find a tentative trend of sulfur depletion increasing with density. Conclusions. Our results suggest that sulfur depletion depends on the environment. While the abundances of sulfur-bearing species are consistent with undepleted sulfur in Orion, a depletion factor of ∼20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution might explain these variations. Grains become negatively charged at a visual extinction of AV ∼ 3.5 mag in Taurus and Perseus. At this low visual extinction, the S+ abundance is high, X(S+) > 106, and the electrostatic attraction between S+ and negatively charged grains could contribute to enhance sulfur depletion. In Orion, the net charge of grains remains approximately zero until higher visual extinctions (AV ∼ 5.5 mag), where the abundance of S+ is already low because of the higher densities, thus reducing sulfur accretion. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H].

Planetary Radio Interferometry and Doppler Experiment (PRIDE) is a multi-purpose experimental technique aimed at enhancing the science return of planetary missions. The technique exploits the science payload and spacecraft service systems without requiring a dedicated onboard instrumentation or imposing on the existing instrumentation any special for PRIDE requirements. PRIDE is based on the near-field phase-referencing Very Long Baseline Interferometry (VLBI) and evaluation of the Doppler shift of the radio signal transmitted by spacecraft by observing it with multiple Earth-based radio telescopes. The methodology of PRIDE has been developed initially at the Joint Institute for VLBI ERIC (JIVE) for tracking the ESA’s Huygens Probe during its descent in the atmosphere of Titan in 2005. From that point on, the technique has been demonstrated for various planetary and other space science missions. The estimates of lateral position of the target spacecraft are done using the phase-referencing VLBI technique. Together with radial Doppler estimates, these observables can be used for a variety of applications, including improving the knowledge of the spacecraft state vector. The PRIDE measurements can be applied to a broad scope of research fields including studies of atmospheres through the use of radio occultations, the improvement of planetary and satellite ephemerides, as well as gravity field parameters and other geodetic properties of interest, and estimations of interplanetary plasma properties. This paper presents the implementation of PRIDE as a component of the ESA’s Jupiter Icy Moons Explorer (JUICE) mission.