Scientific Rationale of Saturn's In Situ Exploration

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 illustratedby 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 scienti-c 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 discussedthroughout this paper : rst, 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 isotopiccomposition 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 may help constraining its bulk OH ratio. We compare predictions of Jupiter and Saturn's bulk compositions from different formation scenarios, and 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 mostextrasolar 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. Dierent mission architectures are envisaged, which would benet from strong international collaborations, all based on an entry probe that would descend through Saturn's stratosphere and troposphere under parachute down to a minimum of 10 bars of atmospheric pressure. We rally discuss the science payload required on a Saturn probe to match the measurement requirements

Atmospheric chemistry on Uranus and Neptune

Comparatively little is known about atmospheric chemistry on Uranus and Neptune, because remote spectral observations of these cold, distant ``Ice Giants'' are challenging, and each planet has only been visited by a single spacecraft during brief flybys in the 1980s. Thermochemical equilibrium is expected to control the composition in the deeper, hotter regions of the atmosphere on both planets, but disequilibrium chemical processes such as transport-induced quenching and photochemistry alter the composition in the upper atmospheric regions that can be probed remotely. Surprising disparities in the abundance of disequilibrium chemical products between the two planets point to significant differences in atmospheric transport. The atmospheric composition of Uranus and Neptune can provide critical clues for unravelling details of planet formation and evolution, but only if it is fully understood how and why atmospheric constituents vary in a three-dimensional sense and how material coming in from outside the planet affects observed abundances. Future mission planning should take into account the key outstanding questions that remain unanswered about atmospheric chemistry on Uranus and Neptune, particularly those questions that pertain to planet formation and evolution, and those that address the complex, coupled atmospheric processes that operate on Ice Giants within our solar system and beyond

The first submillimeter observation of CO in the stratosphere of Uranus

Context. Carbon monoxide (CO) has been detected in all giant planets and its origin is both internal and external in Jupiter and Neptune. Despite its first detection in Uranus a decade ago, the magnitude of its internal and external sources remains unconstrained. Aims. We targeted CO lines in Uranus in the submillimeter range to constrain its origin. Methods. We recorded the disk-averaged spectrum of Uranus with very high spectral resolution at the frequencies of CO rotational lines in the submillimeter range in 2011-2012. We used empirical and diffusion models of the atmosphere of Uranus to constrain the origin of CO. We also used a thermochemical model of its troposphere to derive an upper limit on the oxygen-to-hydrogen (O/H) ratio in the deep atmosphere of Uranus. Results. We have detected the CO(8-7) rotational line for the first time with Herschel-HIFI. Both empirical and diffusion models results show that CO has an external origin. An empirical profile in which CO is constant above the 100 mbar level with a mole fraction of 7.1-9.0 × 10-9, depending on the adopted stratospheric thermal structure, reproduces the data. Sporadic and steady source models cannot be differentiated with our data. Taking the internal source model upper limit of a mole fraction of 2.1 × 10 -9 we find, based on our thermochemical computations, that the deep O/H ratio of Uranus is less than 500 times solar. Conclusions. Our work shows that the average mole fraction of CO decreases from the stratosphere to the troposphere and thus strongly advocates for an external source of CO in Uranus. Photochemical modeling of oxygen species in the atmosphere of Uranus and more sensitive observations are needed to reveal the nature of the external source. © ESO, 2014

The photochemical response to the variation of temperature in Saturn’s 2011-2012 stratospheric vortex

Context. A hot vortex formed in the stratosphere of Saturn following the 2010−2011 Northern Storm. Huge temperature increases have been measured in the vortex around the millibar level. Enhancements in hydrocarbon abundances have been observed at the millibar level in 2011−2012 inside this vortex. Aims. We model the time-dependent photochemistry inside the vortex by accounting for the temperature variability over the period from January 2011 to March 2012 to assess whether photochemistry alone can explain the enhancements seen in the hydrocarbon abundances. Methods. We used a 1D time-dependent photochemical model of Saturn and adapted it to the perturbed conditions of the vortex after validating it in quiescent conditions. Results. Our model predicts non-variability for ethane (C2H6) and acetylene (C2H2) and an increase in ethylene (C2H4) by a factor of 3 in the mbar region. Heavier hydrocarbons show a stronger variability than the lighter ones. We are unable to reproduce the increase seen in C2H2 , and we significantly underestimate the increase seen in C2H4. Conclusions. Pure photochemistry does not explain the variability seen in the abundance of most hydrocarbons. This means that dynamics (eddy diffusion and/or advection) must have played a significant role in shaping the vertical profiles of the main hydrocarbons

Sub-millimetre spectroscopy of Saturn's trace gases from Herschel/SPIRE

Aims. We provide an extensive new sub-millimetre survey of the trace gas composition of Saturn's atmosphere using the broad spectral range (15-51 cm -1) and high spectral resolution (0.048 cm -1) offered by Fourier transform spectroscopy by the Herschel/SPIRE instrument (Spectral and Photometric Imaging REceiver). Observations were acquired in June 2010, shortly after equinox, with negligible contribution from Saturn's ring emission. Methods. Tropospheric temperatures and the vertical distributions of phosphine and ammonia are derived using an optimal estimation retrieval algorithm to reproduce the sub-millimetre data. The abundance of methane, water and upper limits on a range of different species are estimated using a line-by-line forward model. Results. Saturn's disc-averaged temperature profile is found to be quasi-isothermal between 60 and 300 mbar, with uncertainties of 7 K due to the absolute calibration of SPIRE. Modelling of PH 3 rotational lines confirms the vertical profile derived in previous studies and shows that negligible PH 3 is present above the 10-to 20-mbar level. The upper tropospheric abundance of NH 3 appears to follow a vapour pressure distribution throughout the region of sensitivity in the SPIRE data, but the degree of saturation is highly uncertain. The tropospheric CH 4 abundance and Saturn's bulk C/H ratio are consistent with Cassini studies. We improve the upper limits on several species (H 2S, HCN, HCP and HI); provide the first observational constraints on others (SO 2, CS, methanol, formaldehyde, CH 3Cl); and confirm previous upper limits on HF, HCl and HBr. Stratospheric emission from H 2O is suggested at 36.6 and 38.8 cm -1 with a 1σ significance level, and these lines are used to derive mole fractions and column abundances consistent with ISO and SWAS estimations a decade earlier. © 2012 ESO

Radiative-convective models of the atmospheres of Uranus and Neptune: heating sources and seasonal effects

Radiative-convective models of the atmospheres ofUranus and Neptune: heating sources & seasonal effectsGwenael Milcarecka,b, Sandrine Guerleta,c, Franck Montmessinb, Aymeric Spigaa, Jeremy Leconted, EhouarnMilloura, Noe Clementd, Leigh N. Fletchere, Michael T. Romane, Emmanuel Lellouchc, Raphael Morenoc,Thibault Cavalied,c, Oscar Carrion-Gonzalezca Laboratoire de M´et´eorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Universit´e, CNRS,´Ecole Polytechnique, Institut Polytechnique de Paris, ´Ecole Normale Sup´erieure (ENS), PSL Research University, 4place Jussieu BC99, 75005 Paris, Franceb Laboratoire Atmosph`eres, Milieux, Observations spatiales (LATMOS), IPSL, Observatoire de VersaillesSt-Quentin-en-Yvelines, Universit´e de Versailles St-Quentin-en-Yvelines, CNRS, 11 boulevard d’Alembert, 78280Guyancourt, Francec Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS,Sorbonne Universit´e, Universit´e Paris-Diderot, Meudon, Franced University of Bordeaux, CNRS, LAB, UMR 5804, Pessac, Francee School of Physics & Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, United KingdomAbstractThe observations made during the Voyager 2 flyby have shown that the stratosphere of Uranus andNeptune are warmer than expected by previous models. In addition, no seasonal variability of the thermalstructure has been observed on Uranus since Voyager 2 era and significant subseasonal variations have beenrevealed on Neptune. In this paper, we evaluate different realistic heat sources that can induce sufficientheating to warm the atmosphere of these planets and we estimate the seasonal effects on the thermalstructure. The seasonal radiative-convective model developed by the Laboratoire de M´et´eorologie Dynamiqueis used to reproduce the thermal structure of these planets. Three hypotheses for the heating sources areexplored separately: aerosol layers, a higher methane mole fraction, and thermospheric conduction. Ourmodelling indicates that aerosols with plausible scattering properties can produce the requisite heating forUranus, but not for Neptune. Alternatively, greater stratospheric methane abundances can provide themissing heating on both planets, but the large values needed are inconsistent with current observationalconstraints. In contrast, adding thermospheric conduction cannot warm alone the stratosphere of bothplanets. The combination of these heat sources is also investigated. In the upper troposphere of bothplanets, the meridional thermal structures produced by our model are found inconsistent with those retrievedfrom Voyager 2/IRIS data. Furthermore, our models predict seasonal variations should exist within thestratospheres of both planets while observations showed that Uranus seems to be invariant to meridionalcontrasts and only subseasonal temperature trends are visible on Neptune. However, a warm south pole isseen in our simulations of Neptune as observed since 2003

Regions of interest on Ganymede's and Callisto's surfaces as potential targets for ESA's JUICE mission

The JUpiter Icy moons Explorer (JUICE) will investigate Ganymede's and Callisto's surfaces and subsurfaces from orbit to explore the geologic processes that have shaped and altered their surfaces by impact, tectonics, possible cryovolcanism, space weathering due to micrometeorites, radiation and charged particles as well as explore the structure and properties of the icy crust and liquid shell (Grasset et al., 2013). The best possible synergy of the JUICE instruments is required to answer the major science objective of this mission and to fully exploit the potential of the JUICE mission. Therefore, the JUICE team is aiming to define high priority targets on both Ganymede's and Callisto's surfaces to support the coordination of the planning activities by the individual instrument teams. Based on the science objectives of the JUICE mission and the most recent knowledge of Ganymede's and Callisto's geologic evolution we propose a collection of Regions of Interest (RoIs), which characterize surface features and terrain types representing important traces of geologic processes, from past and/or present cryovolcanic and tectonic activity to space weathering processes, which are crucial to understand the geologic evolution of Ganymede and Callisto. The proposed evaluation of RoIs is based on their scientific importance as well as on the opportunities and conditions to observe them during the currently discussed mission profile

Giant Planet Observations with the James Webb Space Telescope

This white paper examines the benefit of the upcoming James Webb Space Telescope (JWST) for studies of the Solar System's four giant planets: Jupiter, Saturn, Uranus, and Neptune. JWST's superior sensitivity, combined with high spatial and spectral resolution, will enable near- and mid-infrared imaging and spectroscopy of these objects with unprecedented quality. In this paper, we discuss some of the myriad scientific investigations possible with JWST regarding the giant planets. This discussion is preceded by the specifics of JWST instrumentation most relevant to giant-planet observations. We conclude with identification of desired pre-launch testing and operational aspects of JWST that would greatly benefit future studies of the giant planets

Effect of Metabolic Inhibition on Couplon Behavior in Rabbit Ventricular Myocytes☆☆☆

We investigated the effect of combined inhibition of oxidative and glycolytic metabolism on L-type Ca2+ channels (LCCs) and Ca2+ spikes in isolated patch-clamped rabbit ventricular myocytes. Metabolic inhibition (MI) reduced LCC open probability, increased null probability, increased first latency, and decreased open time but left conductance unchanged. These results explain the reduction in macroscopic Ca2+ current observed during MI. MI also produced a gradual reduction in action potential duration at 90% repolarization (APD90), a clear decline in spike probability, and an increase in spike latency and variance. These effects are consistent with the changes we observed in LCC activity. MI had no effect on the amplitude or time to peak of Ca2+ spikes until APD90 reached 10% of control, suggesting preserved sarcoplasmic reticulum Ca2+ stores and ryanodine receptor (RyR) conductance in those couplons that remained functioning. Ca2+ spikes disappeared completely when APD90 reached <2% of control, although in two cells, spikes were reactivated in a highly synchronized fashion by very short action potentials. This reactivation is probably due to the increased driving force for Ca2+ entry through a reduced number of LCCs that remain open during early repolarization. The enlarged single channel flux produced by rapid repolarization is apparently sufficient to trigger RyRs whose Ca2+ sensitivity is likely reduced by MI. We suggest that loss of coupling fidelity during MI is explained by loss of LCC activity (possibly mediated by Ca2+-calmodulin kinase II activity). In addition, the results are consistent with loss of RyR activity, which can be mitigated under conditions likely to enlarge the trigger