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JWST NIRSpec IFU (2.9-5.3 mu) and MIRI MRS (4.9-28.5 mu) observations were performed on both the leading and trailing hemispheres of Ganymede with a spectral resolution of ~2700. Reflectance spectra show signatures of water ice, CO2 and H2O2. An absorption feature at 5.9 mu is revealed and is tentatively assigned to sulfuric acid hydrates. The CO2 4.26-mu 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 leading northern polar cap, the CO2 band is dominated by a high wavelength component at ~4.27 mu, consistent with CO2 trapped in amorphous water ice. At equatorial latitudes (and especially on dark terrains) the observed band is broader and shifted towards the blue, suggesting CO2 adsorbed on non-icy materials. Amorphous ice is detected in the ice-rich polar regions, and is especially abundant on the leading northern polar cap. 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 nano/micrometre-scale texture, of diurnal or geographic origin, is observed in both hemispheres. Ice frost is observed on the morning limb of the trailing hemisphere, 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-/Mg-sulfate salts, sulfuric acid hydrates, and possibly phyllosilicates mixed with fine-grained opaque minerals, having an highly porous texture. Mid-IR brightness temperatures indicate a rough surface and a very low thermal inertia of 20-40 J m-2 s-0.5 K-1, consistent with a porous surface., Comment: 35 pages, 34 figures

A new line list for hydronium (H$_3$$^{16}$O$^+$) is computed. The line list is based on a new ab initio dipole moment surface (CCSD(T)/aug-cc-pVQZ) and a new empirical potential energy surface (PES). The empirical PES of H$_3$O$^+$ was obtained by refining an ab initio surface through a global fit to the experimentally determined ro-vibrational energies collected from the literature covering the ground, $\nu_1^{\pm}$, $\nu_2^{\pm}$, $2\nu_2^{\pm}$, $\nu_3^{\pm}$ and $\nu_4^{\pm}$ vibrational states. The line list covers the wavenumber range up to 10 000 cm$^{-1}$ (wavelengths $>1 \mu$m) and should be complete for temperatures up to $T=1500$ K. This is the first comprehensive line list for H$_3$O$^+$ with extensive wavenumber coverage and accurate transitional probabilities. Prospects of detection of hydronium in spectra of solar system giant planets as well as exoplanets are discussed. The eXeL line list is publicly available from the ExoMol and CDS databases.

Since its discovery in the aurorae of Jupiter ~30 years ago, the H$_{3}^{+}$ ion has served as an invaluable probe of giant planet upper atmospheres. However, the vast majority of monitoring of planetary H$_{3}^{+}$ radiation has followed from observations that rely on deriving parameters from column-integrated paths through the emitting layer. Here, we investigate the effects of density and temperature gradients along such paths on the measured H$_{3}^{+}$ spectrum and its resulting interpretation. In a non-isothermal atmosphere, H$_{3}^{+}$ column densities retrieved from such observations are found to represent a lower limit, reduced by 20% or more from the true atmospheric value. Global simulations of Uranus' ionosphere reveal that measured H$_{3}^{+}$ temperature variations are often attributable to well-understood solar zenith angle effects rather than indications of real atmospheric variability. Finally, based on these insights, a preliminary method of deriving vertical temperature structure is demonstrated at Jupiter using model reproductions of electron density and H$_{3}^{+}$ measurements. The sheer diversity and uncertainty of conditions in planetary atmospheres prohibits this work from providing blanket quantitative correction factors; nonetheless, we illustrate a few simple ways in which the already formidable utility of H$_{3}^{+}$ observations in understanding planetary atmospheres can be enhanced., Comment: 14 pages, 9 figures, part of Philosophical Transactions A special issue following workshop entitled "Advances in hydrogen molecular ions: H3+, H5+ and beyond"

Emission from the molecular ion H$_3^+$ is a powerful diagnostic of the upper atmosphere of Jupiter, Saturn, and Uranus, but it remains undetected at Neptune. In search of this emission, we present near-infrared spectral observations of Neptune between 3.93 and 4.00 $\mu$m taken with the newly commissioned iSHELL instrument on the NASA Infrared Telescope Facility in Hawaii, obtained 17-20 August 2017. We spent 15.4 h integrating across the disk of the planet, yet were unable to unambiguously identify any H$_3^+$ line emissions. Assuming a temperature of 550 K, we derive an upper limit on the column integrated density of $1.0^{+1.2}_{-0.8}\times10^{13}$ m$^{-2}$, which is an improvement of 30\% on the best previous observational constraint. This result means that models are over-estimating the density by at least a factor of 5, highlighting the need for renewed modelling efforts. A potential solution is strong vertical mixing of polyatomic neutral species from Neptune's upper stratosphere to the thermosphere, reacting with H$_3^+$, thus greatly reducing the column integrated H$_3^+$ densities. This upper limit also provide constraints on future attempts at detecting H$_3^+$ using the James Webb Space Telescope., Comment: 8 pages, 6 figures, published in Monthly Notices of the Royal Astronomical Society

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn's atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn's bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn's upper troposphere would constrain its bulk O/H ratio. We highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn's stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations., Comment: Submitted to Planetary and Space Science

Saturn's ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation. At low latitudes the latter should result in a weak planet-wide glow in infrared (IR), corresponding to the planet's uniform illumination by the Sun. The observed low-latitude ionospheric electron density is lower and the temperature higher than predicted by models. A planet-ring magnetic connection has been previously suggested in which an influx of water from the rings could explain the lower than expected electron densities in Saturn's atmosphere. Here we report the detection of a pattern of features, extending across a broad latitude band from ~25 to 60 degrees, that is superposed on the lower latitude background glow, with peaks in emission that map along the planet's magnetic field lines to gaps in Saturn's rings. This pattern implies the transfer of charged water products from the ring-plane to the ionosphere, revealing the influx on a global scale, flooding between 30 to 43% of the planet's upper-atmospheric surface. This ring `rain' plays a fundamental role in modulating ionospheric emissions and suppressing electron densities., Comment: 10 pages, 3 figures. Published in Nature, April 2013