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.

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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.

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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’.@en

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.@en

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.

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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.

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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.

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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.

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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.@en

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.

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Ganymede is the only satellite in the solar system known to have an intrinsic magnetic field. Interactions between this field and the Jovian magnetosphere are expected to funnel most of the associated impinging charged particles, which radiolytically alter surface chemistry across the Jupiter system, to Ganymede's polar regions. Using observations obtained with JWST as part of the Early Release Science program exploring the Jupiter system, we report the discovery of hydrogen peroxide, a radiolysis product of water ice, specifically constrained to the high latitudes. This detection directly implies radiolytic modification of the polar caps by precipitation of Jovian charged particles along partially open field lines within Ganymede's magnetosphere. Stark contrasts between the spatial distribution of this polar hydrogen peroxide, those of Ganymede's other radiolytic oxidants, and that of hydrogen peroxide on neighboring Europa have important implications for understanding water-ice radiolysis throughout the solar system.

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We observed Io with the James Webb Space Telescope (JWST) while the satellite was in eclipse, and detected thermal emission from several volcanoes. The data were taken as part of our JWST-ERS program #1373 on 15 November 2022. Kanehekili Fluctus was exceptionally bright, and Loki Patera had most likely entered a new brightening phase. Spectra were taken with NIRSpec/IFU at a resolving power R ≈ 2,700 between 1.65 and 5.3 µm. The spectra were matched by a combination of blackbody curves that showed that the highest temperature, ∼1,200 K, for Kanehekili Fluctus originated from an area ∼0.25 km² in size, and for Loki Patera this high temperature was confined to an area of ∼0.06 km². Lower temperatures, down to 300 K, cover areas of ∼2,000 km² for Kanehekili Fluctus, and ∼5,000 km² for Loki Patera. We further detected the a¹Δ ⇒ X³Σ⁻ 1.707 µm rovibronic forbidden SO emission band complex over the southern hemisphere, which peaked at the location of Kanehekili Fluctus. This is the first time this emission has been seen above an active volcano, and suggests that the origin of such emissions is ejection of SO molecules directly from the vent in an excited state, after having been equilibrated at temperatures of ∼1,500 K below the surface, as was previously hypothesized.

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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.

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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].

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ESA’s Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cassini, and Juno missions, alongside ground- and space-based observatories. We focus on remote sensing of the climate, meteorology, and chemistry of the atmosphere and auroras from the cloud-forming weather layer, through the upper troposphere, into the stratosphere and ionosphere. The Jupiter orbital tour provides a wealth of opportunities for atmospheric and auroral science: global perspectives with its near-equatorial and inclined phases, sampling all phase angles from dayside to nightside, and investigating phenomena evolving on timescales from minutes to months. The remote sensing payload spans far-UV spectroscopy (50-210 nm), visible imaging (340-1080 nm), visible/near-infrared spectroscopy (0.49-5.56 μm), and sub-millimetre sounding (near 530-625 GHz and 1067-1275 GHz). This is coupled to radio, stellar, and solar occultation opportunities to explore the atmosphere at high vertical resolution; and radio and plasma wave measurements of electric discharges in the Jovian atmosphere and auroras. Cross-disciplinary scientific investigations enable JUICE to explore coupling processes in giant planet atmospheres, to show how the atmosphere is connected to (i) the deep circulation and composition of the hydrogen-dominated interior; and (ii) to the currents and charged particle environments of the external magnetosphere. JUICE will provide a comprehensive characterisation of the atmosphere and auroras of this archetypal giant planet.

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Context. The subsurface oceans of icy satellites are among the most compelling among the potentially habitable environments in our Solar System. The question of whether a liquid subsurface layer can be maintained over geological timescales depends on its chemical composition. The composition of icy satellites is linked to that of the circumplanetary disk (CPD) in which they form. The CPD accretes material from the surrounding circumstellar disk in the vicinity of the planet, however, the degree of chemical inheritance is unclear. Aims. We aim to investigate the composition of ices in chemically reset or inherited circumplanetary disks to inform interior modeling and the interpretation of in situ measurements of icy solar system satellites, with an emphasis on the Galilean moon system. Methods. We used the radiation-thermochemical code ProDiMo to produce circumplanetary disk models and then extract the ice composition from time-dependent chemistry, incorporating gas-phase and grain-surface reactions. Results. The initial sublimation of ices during accretion may result in a CO2 -rich ice composition due to efficient OH formation at high gas densities. In the case of a Jovian CPD, the sublimation of accreted ices results in a CO2 iceline between the present-day orbits of Ganymede and Callisto. Sublimated ammonia ice is destroyed by background radiation while drifting towards the CPD midplane. Liberated nitrogen becomes locked in N2 due to efficient self-shielding, leaving ices depleted of ammonia. A significant ammonia ice component remains only when ices are inherited from the circumstellar disk. Conclusions. The observed composition of the Galilean moons is consistent with the sublimation of ices during accretion onto the CPD. In this scenario, the Galilean moon ices are nitrogen-poor and CO2 on Callisto is endogenous and primordial. The ice composition is significantly altered after an initial reset of accreted circumstellar ice. The chemical history of the Galilean moons stands in contrast to the Saturnian system, where the composition of the moons corresponds more closely with the directly inherited circumstellar disk material.

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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.

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Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus’s origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the plume. The proposed Moonraker mission consists of an ESA-provided platform with strong heritage from JUICE and Mars Sample Return and carrying a suite of instruments dedicated to plume and surface analysis. The nominal Moonraker mission has a duration of ∼13.5 yr. It includes a 23-flyby segment with 189 days allocated for the science phase and can be expanded with additional segments if resources allow. The mission concept consists of investigating (i) the habitability conditions of present-day Enceladus and its internal ocean, (ii) the mechanisms at play for the communication between the internal ocean and the surface of the South Polar Terrain, and (iii) the formation conditions of the moon. Moonraker, thanks to state-of-the-art instruments representing a significant improvement over Cassini's payload, would quantify the abundance of key species in the plume, isotopic ratios, and the physical parameters of the plume and the surface. Such a mission would pave the way for a possible future landed mission.

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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.

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Context. Comets are small celestial bodies made of ice, dust, and rock that orbit the Sun. Understanding their behavior as they warm up at perihelion unveils many pieces of information about the interior and general morphology of the ices hidden under the dust. Aims. The goal of this research is to study the sublimation of CH4 through amorphous solid water (ASW), with a focus on the structural changes in water and the influence of a layer of indene (as a proxy of the crust) during a period of thermal processing, which we use in a controlled laboratory setting to simulate cometary environments. Methods. Ices at a CH4/H2O abundance ratio of about 0.01 are deposited and layered, or co-deposited, at 30 K and are heated until 200 K (or 140 K) with a ramp of either 1 or 5 K min-1. We use mass spectrometry and infrared spectroscopy to analyze the results. Results. Depending on the heating ramp and type of deposition, the sublimation of methane through ASW varies, being lower in intensity and higher in temperature when the co-deposited structure is considered. When two temperature cycles are applied, the second one sees less intense CH4 desorptions. When indene is placed above the ice mixtures, we find that the thicker its layer, the later the methane desorption. However, this later desorption sees a greater quantity of methane released due to water reorganization and higher desorbed material pressure. Conclusions. The structural changes of water ice drive volatile and hyper-volatile desorption because of the transition from high to low intrinsic density and transformation from amorphous to crystalline. This desorption indicates that such material has been deposited at low temperatures in agreement with previous theories on cometary ices formed in the pre-stellar cloud. During the two temperature cycles of our experiments, most of the released material is seen to be pristine and the processed part, if any, is of a negligible quantity, in agreement with dust-rock cometary studies.

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