arXiv subjects

T. Cavalié

Publications and source records attributed to T. Cavalié.

At least 19 recordsLinked to original sources

arXiv2026

Juice-SWI during the Lunar-Earth-Gravity-Assist (LEGA) - Part 2: Instrument operations

The Jupiter Icy Moons Explorer (Juice) embarked in 2023 on a 8-year interplanetary journey to Jupiter and its icy moons. The Submillimetre Wave Instrument (SWI) is one of the ten science instruments aboard the spacecraft. SWI is a sophisticated and first-of-its-kind payload visiting the outer solar system, featuring dual-band tunable receivers, two independent pointing mechanisms, and spectrometers capable of high resolution (up to a resolving power of 10^{7}). It is designed to support the diverse science objectives of the Juice mission targeting Jupiter's middle atmosphere, icy-moon's exospheres as well as near sub-surface thermophysical properties. For this purpose the Juice mission adopts a complex trajectory tour within the Jovian system, which further necessitates a sophisticated, mission-driven operations concept for SWI. This presents significant planning, operations and commanding challenges which are described in this paper in the context of the Lunar and Earth Gravity Assist (LEGA). After the development and ground calibration of the instrument, the SWI Team has designed a comprehensive calibration strategy applicable during the Cruise Phase of Juice. Among the various opportunities for calibration, including the Near-Earth Commissioning Phase and more than ten Payload Checkout Windows, the LEGA offers the means not only to improve the calibration of the instrument, but also to validate the operational strategy of future icy moon flybys.

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arXiv2026

Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?

The formation of Uranus and Neptune remains poorly constrained largely due to uncertain deep elemental abundances. Carbon monoxide (CO), a disequilibrium species in the upper troposphere, provides an indirect constraint on the deep oxygen abundance. We investigate the deep O/H and C/O ratios of the ice giants and their formation history by accounting for meridional variations in atmospheric structure and uncertainties in chemical kinetics. We extended a 1D thermochemical and diffusion model into a pseudo-2D model by including latitudinal variations in key model parameters. The O/H ratio was inferred by matching the modeled upper-tropospheric CO mole fractions to observations, and combined with the deep carbon abundance to derive the C/O ratio. For Uranus, varying tropospheric methane alone yielded O/H $\sim [47-57] \times$ protosolar, whereas allowing $K_{zz}$ to vary expanded the range to O/H $\sim [62-177] \times$ protosolar. For Neptune, the corresponding ranges are O/H $\sim [182-215] \times$ protosolar and O/H $\sim [222-342] \times$ protosolar, respectively, supporting lower oxygen enrichment in Uranus than Neptune. Chemical-network uncertainties have a more modest effect, amounting to at most $\sim 10\%$ of the retrieved oxygen ranges, comparable to the uncertainty associated with the measured CO abundance on Neptune, but larger on Uranus. We computed C/O latitudinal ranges and found C/O $\sim [0.06-0.52]$ on Uranus and $\sim [0.02-0.12]$ on Neptune. Comparison with a protoplanetary disk model suggests different formation or evolutionary pathways for the two planets. Our results highlight the dominant role of vertical mixing in constraining deep oxygen abundance and the importance of accounting for meridional variability and chemical uncertainties. This work also provides a framework for selecting the entry latitude of a future Uranus Orbiter and Probe mission.

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arXiv2026

New 12C/13C and 14N/15N isotopic ratio measurements in Jupiter's stratosphere revealed by ALMA

The collision of comet SL9 with Jupiter in 1994 changed the chemical composition of the Jovian stratosphere for decades. New molecules were detected minutes after the impacts (HCN, CO, CS, etc.) and some are still present today. They were deposited in the stratosphere at pressures lower than 0.1 mbar and were most probably formed by shock-induced chemistry recombining Jovian and cometary material. However, the question of the origin of these molecules is still not completely understood. One way to address this open question is to determine the isotopic composition of the new molecules. Isotopic ratios have long been measured in the Solar System. They present a variety of values depending on the object or the molecule and therefore trace different reservoirs of material. Derivations of carbon and nitrogen isotopic ratios in HCN four years after SL9 showed atypical depletions in the heavier isotopes that had never been observed before in the Solar System. These results suggested an unusual cometary composition or an unknown fractionation mechanism in the hot and shocked air parcels. We aim to measure carbon and nitrogen isotopic ratios in HCN to shed light on the puzzling results of 1998. With Atacama Large Millimeter/submillimeter Array data from 2017 and radiative-transfer calculations, we derived the abundance of two HCN isotopologues, H13CN and HC15N, at pressures probed from 0.03 to 1.8 mbar. We find 12C/13C = $73\pm5$ and 14N/15N = $245\pm29$, respectively (0.76-0.87) and (0.80-1.00) times the terrestrial references, and (0.69-0.87) and (0.42-0.70) times the solar-Jovian bulk values. In contrast to the strong depletions reported in 1998, our values are instead compatible with an enrichment in the heavier isotopes relative to the Jovian bulk. We interpret these enrichments as the direct signature of the cometary contribution in HCN and/or as 23 years of chemical evolution.

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arXiv2026

Detection of stratospheric HCN and tropospheric CO in Uranus and the implication for their sources

Uranus belongs to the category of ice giants that are common in our Galaxy. However, Uranus is one of the least explored and understood planets in our Solar System. This work aims to constrain the deep oxygen abundance of Uranus to better understand its formation. Another goal concerns the origin of exogeneous species, such as CO and HCN, found in the upper stratospheres of giant planets. We used spectral mapping observations of the CO (J=3-2) and HCN (J=4-3) rotational submillimeter lines obtained with ALMA in 2022 and 2024. We combined them with radiative transfer and thermochemical modeling to determine the tropospheric abundance of CO and the deep O/H ratio of Uranus. We used radiative transfer simulations with physical models of various sources of external CO and HCN to constrain the vertical and meridional distributions of these species and narrow down the nature of their external sources. We also applied a wind retrieval algorithm to search for zonal winds in the stratosphere of Uranus at the levels probed by the CO and HCN lines. We unambiguously detect tropospheric CO for the first time with a mole fraction of 5.8$\pm$0.3 ppb and stratospheric HCN with a mole faction of (1.8$\pm$0.2)$\times10^{-11}$ restricted to pressures lower than 0.2 mbar. Thermochemical calculations suggest that the deep interior of Uranus is enriched in oxygen with respect to the protoplanetary nebula by at least a factor of 52$^{+30}_{-20}$. We also find that the stratospheric CO is rather uniform over the observed latitudes and that the CO lines are best fit by an old comet impact model, in which a large comet hit the planet several centuries ago. CO therefore has a dual origin in Uranus. Finally, we do not detect stratospheric winds from these data, but the CO data indicate that zonal winds in the 10$^\circ$S-10$^\circ$N latitudinal range are likely retrograde at submillibar pressures.

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arXiv2026

Observations of the temporal evolution of Saturn's stratosphere following the Great Storm of 2010-2011. II. Latitudinal distribution of CO and stratospheric winds

Saturn's Great Storm of 2010-2011 has produced two stratospheric hot spots, the "beacons," that eventually merged to produce a gigantic one in April and May 2011. This beacon perturbed stratospheric temperatures, hydrocarbon, and water abundances for several years. We aim to assess whether the beacon induced any perturbation in another oxygen species, namely CO. A second goal is to measure how the vortex perturbed the stratospheric wind regime. We conducted interferometric observations of Saturn in the submillimeter range with SMA and ALMA to spatially resolve the CO (J=3-2) and (J=2-1) emissions, respectively. We used a previously determined CO vertical profile as a template, to search for (i) the meridional distribution of CO and (ii) variations of the CO abundance associated with the storm. The high spatial and spectral resolutions of the ALMA observations enabled us to retrieve the winds from the Doppler shifts induced by the winds on the lines. Despite limitations resulting from the removal of baseline ripples, we find a relatively constant meridional distribution of CO. The average CO mole fraction implied by the adopted and rescaled 220-year-old-comet-impact vertical profile is (1.7$\pm$0.7)$\times10^{-7}$ at 0.3\,mbar, i.e., where the contribution functions peak. We also find that the CO abundance has not been noticeably altered in the beacon. The winds measured at 1\,mbar show striking differences with those measured in 2018, after the demise of the beacon. We find the signature of the vortex as an anticyclonic feature. The equatorial prograde jet is 100 to 200 m.s$^{-1}$ slower, and broader in latitude, than in quiescent conditions. We also detect several prograde jets in the southern hemisphere. Finally, we detect a retrograde jet at 74$^\circ$N which could be a polar jet caused by the interaction of the Saturn magnetosphere with its atmosphere.

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arXiv2024

Impact of the transport of magnetospheric electrons on the composition of the Triton atmosphere

Due to its inclined orbit and the complex geometry of the magnetic field of Neptune, Triton experiences a highly variable magnetic environment. As precipitation of magnetospheric electrons is thought to have a large impact on the Triton atmosphere, a better understanding of the interaction between its atmosphere and the magnetosphere of Neptune is important. We aim to couple a model of the Triton atmosphere with an electron transport model to compute the impact of a varying electron precipitation on the atmospheric composition. We coupled a recent photochemical model of the Triton atmosphere with the electron transport model TRANSPlanets. The inputs of this code were determined from Voyager 2 observations and previous studies. The main inputs were the electron precipitation flux, the orbital scaling factor, and the magnetic field strength. The electron-impact ionization and electron-impact dissociation rates computed by TRANSPlanets were then used in the photochemical model. We also analyzed the model uncertainties. The coupling of the two models enabled us to find an electron density profile, as well as N$_2$ and N number densities, that are consistent with the Voyager 2 observations. We found that photoionization and electron-impact ionization are of the same order, in contrast to the results of previous photochemical models. However, we emphasize that this result depends on the hypotheses we used to determine the input variables of TRANSPlanets. Our model would greatly benefit from new measurements of the magnetic environment of Triton, as well as of the electron fluxes in the Neptune magnetosphere.

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arXiv2024

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

The observations made during the Voyager 2 flyby have shown that the stratosphere of Uranus and Neptune are warmer than expected by previous models. In addition, no seasonal variability of the thermal structure has been observed on Uranus since Voyager 2 era and significant subseasonal variations have been revealed on Neptune. In this paper, we evaluate different realistic heat sources that can induce sufficient heating to warm the atmosphere of these planets and we estimate the seasonal effects on the thermal structure. The seasonal radiative-convective model developed by the Laboratoire de Météorologie Dynamique is used to reproduce the thermal structure of these planets. Three hypotheses for the heating sources are explored separately: aerosol layers, a higher methane mole fraction, and thermospheric conduction. Our modelling indicates that aerosols with plausible scattering properties can produce the requisite heating for Uranus, but not for Neptune. Alternatively, greater stratospheric methane abundances can provide the missing heating on both planets, but the large values needed are inconsistent with current observational constraints. In contrast, adding thermospheric conduction cannot warm alone the stratosphere of both planets. The combination of these heat sources is also investigated. In the upper troposphere of both planets, the meridional thermal structures produced by our model are found inconsistent with those retrieved from Voyager 2/IRIS data. Furthermore, our models predict seasonal variations should exist within the stratospheres of both planets while observations showed that Uranus seems to be invariant to meridional contrasts and only subseasonal temperature trends are visible on Neptune. However, a warm south pole is seen in our simulations of Neptune as observed since 2003.

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arXiv2022

A photochemical model of Triton's atmosphere with an uncertainty propagation study

Triton is the largest satellite of Neptune and probably a Kuiper Belt Object that was captured by the planet. It has a tenuous nitrogen atmosphere similar to the one of Pluto and may be an ocean world. The Neptunian system has only been visited by Voyager 2 in 1989. Over the last few years, the demand for a new mission to the Ice Giants and their systems has increased so that a theoretical basis to prepare for such a mission is important. We aim to develop a photochemical model of Triton's atmosphere with an up-to-date chemical scheme, as previous photochemical models date back to the post-flyby years. This is done to better understand the mechanisms governing Triton's atmospheric chemistry and highlight the critical parameters having a significant impact on the atmospheric composition. We also study model uncertainties to find what chemical studies are necessary to improve the modeling of Triton's atmosphere. We adapted a model of Titan's atmosphere to Triton's conditions. We first used Titan's chemical scheme before updating it to better model Triton's atmosphere. Once the nominal results were obtained, we studied model uncertainties with a Monte-Carlo procedure. Then, we performed global sensitivity analyzes to identify the reactions responsible for model uncertainties. With the nominal results, we determined the composition of Triton's atmosphere and studied the main chemical processes. We highlighted key chemical reactions that are the most important for the overall chemistry. We also identified some key parameters having a significant impact on the results. Uncertainties are large for most of the main atmospheric species as the atmospheric temperature is very low. We identified key uncertainty reactions that have the largest impact on the results uncertainties. These reactions must be studied in priority in order to improve the significance of our results.

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arXiv2021

Mapping the zonal winds of Jupiter's stratospheric equatorial oscillation

Since the 1950s, quasi-periodic oscillations have been studied in the terrestrial equatorial stratosphere. Other planets of the solar system present (or are expected to present) such oscillations, like the Jupiter Equatorial Oscillation(JEO) and the Saturn Semi-Annual Oscillation (SSAO). In Jupiter's stratosphere, the equatorial oscillation of its relative temperature structure about the equator, is characterized by a quasi-period of 4.4 years. The stratospheric wind field in Jupiter's equatorial zone has never been directly observed. In this paper, we aim at mapping the absolute wind speeds in Jupiter's equatorial stratosphere to quantify vertical and horizontal wind and temperature shear. Assuming geostrophic equilibrium, we apply the thermal wind balance using nearly simultaneous stratospheric temperature measurements between 0.1 and 30 mbar performed with Gemini/TEXES and direct zonal wind measurements derived at 1 mbar from ALMA observations, all carried out between March 14th and 22nd, 2017. We are thus able to calculate self-consistently the zonal wind field in Jupiter's stratosphere where the JEO occurs. We obtain stratospheric map of the zonal wind speeds as a function of latitude and pressure about Jupiter's equator for the first time. The winds are vertically layered with successive eastward and westward jets. We find a 200 m/s westward jet at 4 mbar at the equator, with a typical longitudinal variability on the order of ~50 m/s. By extending our wind calculations to the upper troposphere, we find a wind structure qualitatively close to the wind observed using cloud-tracking techniques. Nearly simultaneous temperature and wind measurements, both in the stratosphere, are a powerful tool for future investigations of the JEO (and other planetary equatorial oscillations) and its temporal evolution.

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arXiv2021

First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter

Context. The tropospheric wind pattern in Jupiter consists of alternating prograde and retrograde zonal jets with typical velocities of up to 100 m/s around the equator. At much higher altitudes, in the ionosphere, strong auroral jets have been discovered with velocities of 1-2 km/s. There is no such direct measurement in the stratosphere of the planet. Aims. In this paper, we bridge the altitude gap between these measurements by directly measuring the wind speeds in Jupiter's stratosphere. Methods. We use the Atacama Large Millimeter/submillimeter Array's very high spectral and angular resolution imaging of the stratosphere of Jupiter to retrieve the wind speeds as a function of latitude by fitting the Doppler shifts induced by the winds on the spectral lines. Results. We detect for the first time equatorial zonal jets that reside at 1 mbar, i.e. above the altitudes where Jupiter's Quasi-Quadrennial Oscillation occurs. Most noticeably, we find 300-400 m/s non-zonal winds at 0.1 mbar over the polar regions underneath the main auroral ovals. They are in counter-rotation and lie several hundreds of kilometers below the ionospheric auroral winds. We suspect them to be the lower tail of the ionospheric auroral winds. Conclusions. We detect directly and for the first time strong winds in Jupiter's stratosphere. They are zonal at low-to-mid latitudes and non-zonal at polar latitudes. The wind system found at polar latitudes may help increase the effciency of chemical complexification by confining the photochemical products in a region of large energetic electron precipitation.

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arXiv2020

1D photochemical model of the ionosphere and the stratosphere of Neptune

Neptune remains a mysterious world that deserves further exploration and is a high-priority objective for a future planetary mission in order to better understand the formation and evolution of ice giant planets. We have developed a coupled ion-neutral 1D photochemical model of Neptune's atmosphere to study the origin and evolution of the hydrocarbons and the oxygen species. The up-to-date chemical scheme is derived from one used for Titan's atmosphere, which led to good agreements with the Cassini-CIRS observations for oxygen species and the main hydrocarbons. The main results we obtain are the following: The ion-neutral chemistry coupling produces aromatics (and benzene in particular) in the atmosphere of Neptune with relatively high abundances. Our model results are in good agreement with observations (taking model uncertainties into account). Two ionospheric peaks are present in the atmosphere located above the pressure level of 10$^{-5}$ mbar and around 10$^{-3}$ mbar. The influx of oxygen species in the upper atmosphere of Neptune has an effect on the concentration of many ions. We show that in situ exploration of Neptune's atmosphere would provide very interesting constraints for photochemical models concerning in particular the origin of oxygen species and the contribution of ion chemistry. A precise description of upper atmospheric chemistry is crucial for a better understanding of the internal composition and the formation processes of this planet.

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arXiv2020

18-year long monitoring of the evolution of H2O vapor in the stratosphere of Jupiter with the Odin space telescope

Comet Shoemaker-Levy 9 impacted Jupiter in July 1994, leaving its stratosphere with several new species, among them water vapor (H2O). With the aid of a photochemical model H2O can be used as a dynamical tracer in the jovian stratosphere. In this paper, we aim at constraining vertical eddy diffusion (Kzz) at the levels where H2O resides. We monitored the H2O disk-averaged emission at 556.936 GHz with the Odin space telescope between 2002 and 2019, covering nearly two decades. We analyzed the data with a combination of 1D photochemical and radiative transfer models to constrain vertical eddy diffusion in the stratosphere of Jupiter. The Odin observations show us that the emission of H2O has an almost linear decrease of about 40% between 2002 and 2019.We can only reproduce our time series if we increase the magnitude of Kzz in the pressure range where H2O diffuses downward from 2002 to 2019, i.e. from ~0.2 mbar to ~5 mbar. However, this modified Kzz is incompatible with hydrocarbon observations. We find that, even if allowance is made for the initially large abundances of H2O and CO at the impact latitudes, the photochemical conversion of H2O to CO2 is not sufficient to explain the progressive decline of the H2O line emission, suggestive of additional loss mechanisms. The Kzz we derived from the Odin observations of H2O can only be viewed as an upper limit in the ~0.2 mbar to ~5 mbar pressure range. The incompatibility between the interpretations made from H2O and hydrocarbon observations probably results from 1D modeling limitations. Meridional variability of H2O, most probably at auroral latitudes, would need to be assessed and compared with that of hydrocarbons to quantify the role of auroral chemistry in the temporal evolution of the H2O abundance since the SL9 impacts. Modeling the temporal evolution of SL9 species with a 2D model would be the next natural step.

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arXiv2020

Key Atmospheric Signatures for Identifying the Source Reservoirs of Volatiles in Uranus and Neptune

We investigate the enrichment patterns of several delivery scenarios of the volatiles to the atmospheres of ice giants, having in mind that the only well constrained determination made remotely, i.e. the carbon abundance measurement, suggests that their envelopes possess highly supersolar metallicities, i.e. close to two orders of magnitude above that of the protosolar nebula. In the framework of the core accretion model, only the delivery of volatiles in solid forms (amorphous ice, clathrates, pure condensates) to these planets can account for the apparent supersolar metallicity of their envelopes. In contrast, because of the inward drift of icy particles through various snowlines, all mechanisms invoking the delivery of volatiles in vapor forms predict subsolar abundances in the envelopes of Uranus and Neptune. Alternatively, even if the disk instability mechanism remains questionable in our solar system, it might be consistent with the supersolar metallicities observed in Uranus and Neptune, assuming the two planets suffered subsequent erosion of their H-He envelopes. The enrichment patterns derived for each delivery scenario considered should be useful to interpret future in situ measurements by atmospheric entry probes.

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arXiv2019

Herschel map of Saturn's stratospheric water, delivered by the plumes of Enceladus

Context. The origin of water in the stratospheres of Giant Planets has been an outstanding question ever since its first detection by ISO some 20 years ago. Water can originate from interplanetary dust particles, icy rings and satellites and large comet impacts. Analysis of Herschel Space Observatory observations have proven that the bulk of Jupiter's stratospheric water was delivered by the Shoemaker-Levy 9 impacts in 1994. In 2006, the Cassini mission detected water plumes at the South Pole of Enceladus, placing the moon as a serious candidate for Saturn's stratospheric water. Further evidence was found in 2011, when Herschel demonstrated the presence of a water torus at the orbital distance of Enceladus, fed by the moon's plumes. Finally, water falling from the rings onto Saturn's uppermost atmospheric layers at low latitudes was detected during the final orbits of Cassini's end-of-mission plunge into the atmosphere. Aims. In this paper, we use Herschel mapping observations of water in Saturn's stratosphere to identify its source. Methods. Several empirical models are tested against the Herschel-HIFI and -PACS observations, which were collected on December 30, 2010, and January 2nd, 2011 (respectively). Results. We demonstrate that Saturn's stratospheric water is not uniformly mixed as a function of latitude, but peaking at the equator and decreasing poleward with a Gaussian distribution. We obtain our best fit with an equatorial mole fraction 1.1 ppb and a half-width at half-maximum of 25°, when accounting for a temperature increase in the two warm stratospheric vortices produced by Saturn's Great Storm of 2010-2011. Conclusions. This work demonstrates that Enceladus is the main source of Saturn's stratospheric water.

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arXiv2017

Scientific rationale for Uranus and Neptune in situ explorations

The ice giants Uranus and Neptune are the least understood class of planets in our solar system but the most frequently observed type of exoplanets. Presumed to have a small rocky core, a deep interior comprising ~70% heavy elements surrounded by a more dilute outer envelope of H2 and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the lack of dedicated exploration missions, our knowledge of the composition and atmospheric processes of these distant worlds is primarily derived from remote sensing from Earth-based observatories and space telescopes. As a result, Uranus's and Neptune's physical and atmospheric properties remain poorly constrained and their roles in the evolution of the Solar System not well understood. Exploration of an ice giant system is therefore a high-priority science objective as these systems (including the magnetosphere, satellites, rings, atmosphere, and interior) challenge our understanding of planetary formation and evolution. Here we describe the main scientific goals to be addressed by a future in situ exploration of an ice giant. An atmospheric entry probe targeting the 10-bar level, about 5 scale heights beneath the tropopause, would yield insight into two broad themes: i) the formation history of the ice giants and, in a broader extent, that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. In addition, possible mission concepts and partnerships are presented, and a strawman ice-giant probe payload is described. An ice-giant atmospheric probe could represent a significant ESA contribution to a future NASA ice-giant flagship mission.

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arXiv2015

2D photochemical modeling of Saturn's stratosphere. Part II: Feedback between composition and temperature

Saturn's axial tilt produces seasons in a similar way as on Earth. Both the stratospheric temperature and composition are affected by this latitudinally varying insolation along the seasons. The thermal structure is controlled and regulated by the amount of hydrocarbons in the stratosphere, which act as absorbers and coolants from the UV to the far-IR spectral range, and this structure influences the amount of hydrocarbons. We study here the feedback between the chemical composition and the thermal structure by coupling a latitudinal and seasonal photochemical model with a radiative seasonal model. Our results show that the seasonal temperature peak in the higher stratosphere, associated with the seasonal increase of insolation, is shifted earlier than the maximum insolation peak. This shift is increased with increasing latitudes and is caused by the low amount of stratospheric coolants in the spring season. At 80$^{\circ}$ in both hemispheres, the temperature peak at 1d-2mbar is seen to occur half a season earlier than was previously predicted by radiative seasonal models that assumed spatially and temporally uniform distribution of coolants. This shift progressively decreases with increasing pressure, up to around the 0.5mbar pressure level where it vanishes. However, the thermal field has a small feedback on the abundance distributions. This feedback modifies the predicted equator-to-pole temperature gradient. The meridional gradients of temperature at the mbar pressure levels are better reproduced when this feedback is accounted for. At lower pressure levels, the thermal structure seems to depart from pure radiative seasonal equilibrium as previously suggested by Guerlet et al. (2014). Although the agreement with the absolute value of the stratospheric temperature observed by Cassini is moderate, it is a mandatory step toward a fully coupled GCM-photochemical model.

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arXiv2015

The Hera Saturn Entry Probe Mission

The Hera Saturn entry probe mission is proposed as an M--class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Hera's aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems.

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arXiv2014

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 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.

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