Gabriel Tobie

Strong Tidal Dissipation in Saturn and Constraints on Enceladus' Thermal State from Astrometry

Tidal interactions between Saturn and its satellites play a crucial role in both the orbital migration of the satellites and the heating of their interiors. Therefore, constraining the tidal dissipation of Saturn (here the ratio k2/Q) opens the door to the past evolution of the whole system. If Saturn’s tidal ratio can be determined at different frequencies, it may also be possible to constrain the giant planet’s interior structure, which is still uncertain. Here, we try to determine Saturn’s tidal ratio through its current effect on the orbits of the main moons, using astrometric data spanning more than a century. We find an intense tidal dissipation (k2/Q = (2.3 ± 0.7) × 10 −4 ), which is about 10 times higher than the usual value estimated from theoretical arguments. As a consequence, eccentricity equilibrium for Enceladus can now account for the huge heat emitted from Enceladus’ south pole. Moreover, the measured k2/Q is found to be poorly sensitive to the tidal frequency, on the short frequency interval considered. This suggests that Saturn’s dissipation may not be controlled by turbulent friction in the fluid envelope as commonly believed. If correct, the large tidal expansion of the moon orbits due to this strong Saturnian dissipation would be inconsistent with the moon formations 4.5 Byr ago above the synchronous orbit in the Saturnian subnebulae. But it would be compatible with a new model of satellite formation in which the Saturnian satellites formed possibly over a longer timescale at the outer edge of the main rings. In an attempt to take into account possible significant torques exerted by the rings on Mimas, we fitted a constant rate da/dt on Mimas’ semi-major axis as well. We obtained an unexpected large acceleration related to a negative value of da/dt =− (15.7 ± 4.4) × 10 −15 AU day −1 . Such acceleration is about an order of magnitude larger than the tidal deceleration rates observed for the other moons. If not coming from an astrometric artifact associated with the proximity of Saturn’s halo, such orbital decay may have significant implications on the Saturn’s rings.

Accretion of Saturn's mid-sized moons during the viscous spreading of young massive rings: Solving the paradox of silicate-poor rings versus silicate-rich moons

Abstract The origin of Saturn’s inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn’s rings is debated. Charnoz et al. [Charnoz, S., Salmon J., Crida A., 2010. Nature 465, 752–754] introduced the idea that the smallest inner moons could form from the spreading of the rings’ edge while Salmon et al. [Salmon, J., Charnoz, S., Crida, A., Brahic, A., 2010. Icarus 209, 771–785] showed that the rings could have been initially massive, and so was the ring’s progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested by Canup [Canup, R., 2010. Nature 468, 943–946]. However, the process driving mid-sized moon accretion from the icy debris disks has not been investigated in details. In particular, Canup’s (2010) model does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea’s mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Q p

Global circulation as the main source of cloud activity on Titan

Clouds on Titan result from the condensation of methane and ethane and, as on other planets, are primarily structured by circulation of the atmosphere. At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole and at mid-latitudes from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere. General circulation models predict that this distribution should change with the seasons on a 15-year timescale, and that clouds should develop under certain circumstances at temperate latitudes (∼40°) in the winter hemisphere. The models, however, have hitherto been poorly constrained and their long-term predictions have not yet been observationally verified. Here we report that the global spatial cloud coverage on Titan is in general agreement with the models, confirming that cloud activity is mainly controlled by the global circulation. The non-detection of clouds at latitude ∼40° N and the persistence of the southern clouds while the southern summer is ending are, however, both contrary to predictions. This suggests that Titan’s equator-to-pole thermal contrast is overestimated in the models and that its atmosphere responds to the seasonal forcing with a greater inertia than expected.

The Origin and Evolution of Titan

Titan was formed as a regular satellite in a disk that was the outgrowth of the formation of Saturn itself. Unlike the Jovian system, Titan is alone in terms of its size and mass, notart of a system gradational in density and hence rock abundance,erhaps reflecting a smaller disk and greater importance of stochastic events during satellite assembly. Accretional heating of Titan was enough to melt an outer layer of water (a water “magma ocean”) and sustain for a shorteriod an environment in which exposed water or water-ammonia liquid was in contact with organic molecules. Initial warm surface conditions are supported by direct samplings of Titans atmosphere by the mass spectrometers on board Cassini and Huygens, whichrovide circumstantial evidence that ammonia (NH3) is therimordial source of Titans atmospheric molecular nitrogen. Ammonia can be extracted from the liquidhase only if the surface temperature is above the meltingoint of the mixture, thus implying warm accretion.

Coupling mantle convection and tidal dissipation: Applications to Enceladus and Earth‐like planets

[1] Anelastic dissipation of tidal forces likely contributes to the thermal budget of several satellites of giant planets and Earth‐like planets closely orbiting other stars. In order to address how tidal heating influences the thermal evolution of such bodies, we describe here a new numerical tool that solves simultaneously mantle convection and tidal dissipation in a three‐dimensional spherical geometry. Since the two processes occur at different timescales, tidal dissipation averaged over a forcing period is included as a volumetric heat source for mantle dynamics. For the long‐term flow, a purely viscous material is considered, whereas a Maxwell‐like formalism is employed for the tidal viscoelastic problem. Due to the strongly temperature dependent rheological properties of both mechanisms, the coupling is achieved via the temperature field. The model is applied to two examples: Enceladus and an Earth‐like planet. For Enceladus, our new 3‐D method shows that the tidal strain rates are strongly enhanced in hot upwellings when compared with classical methods. Moreover, the heat flux at the base of Enceladus’ ice shell is strongly reduced at the poles, thus favoring the preservation of a liquid reservoir at depth. For Earth‐like planets, tidal dissipation patterns are predicted for different orbital configuration. Thermal runaway is observed for orbital periods smaller than a critical value (e.g., 30 days for an eccentricity of 0.2 and 3:2 resonance). This is likely to promote large‐scale melting of the mantle and Io‐like volcanism.

New constraints on Saturn's interior from Cassini astrometric data

Using astrometric observations spanning more than a century and including a large set of Cassini data, we determine Saturns tidal parameters through their current effects on the orbits of the eight main and four coorbital Moons. We have used the latter to make the first determination of Saturns Love number from observations, k2=0.390 ± 0.024, a value larger than the commonly used theoretical value of 0.341 (Gavrilov & Zharkov, 1977), but compatible with more recent models (Helled & Guillot, 2013) for which the static k2 ranges from 0.355 to 0.382. Depending on the assumed spin for Saturns interior, the new constraint can lead to a significant reduction in the number of potential models, offering great opportunities to probe the planets interior. In addition, significant tidal dissipation within Saturn is confirmed (Lainey et al., 2012) corresponding to a high present-day tidal ratio k2/Q=(1.59 ± 0.74) × 10−4 and implying fast orbital expansions of the Moons. This high dissipation, with no obvious variations for tidal frequencies corresponding to those of Enceladus and Dione, may be explained by viscous friction in a solid core, implying a core viscosity typically ranging between 1014 and 1016 Pa.s (Remus et al., 2012). However, a dissipation increase by one order of magnitude at Rheas frequency could suggest the existence of an additional, frequency-dependent, dissipation process, possibly from turbulent friction acting on tidal waves in the fluid envelope of Saturn (Ogilvie & Lin, 2004; Fuller et al. 2016).

Titan's Bulk Composition Constrained by Cassini-Huygens: Implication for Internal Outgassing

In the present report, by using a series of data gathered by the Cassini-Huygens mission, we constrain the bulk content of Titans interior for various gas species (CH4, CO2, CO, NH3, H2S, Ar, Ne, Xe), and we show that most of the gas compounds (except H2S and Xe) initially incorporated within Titan are likely stored dissolved in the subsurface water ocean. CO2 is likely to be the most abundant gas species (up to 3% of Titans total mass), while ammonia should not exceed 1.5 wt%. We predict that only a moderate fraction of CH4, CO2, and CO should be incorporated in the crust in the form of clathrate hydrates. By contrast, most of the H2S and Xe should be incorporated at the base of the subsurface ocean, in the form of heavy clathrate hydrates within the high-pressure ice layer. Moreover, we show that the rocky phase of Titan, assuming a composition similar to CI carbonaceous chondrites, is a likely source for the noble gas isotopes (40Ar, 36Ar, 22Ne) that have been detected in the atmosphere. A chondritic core may also potentially contribute to the methane inventory. Our calculations show that a moderate outgassing of methane containing traces of neon and argon from the subsurface ocean would be sufficient to explain the abundance estimated by the Gas Chromatograph Mass Spectrometer. The extraction process, implying partial clathration in the ice layers and exsolvation from the water ocean, may explain why the 22Ne/36Ar ratio in Titans atmosphere appears higher than the ratio in carbonaceous chondrites.

Tidally induced thermal runaways on extrasolar earths : impact on habitability

We study the susceptibility of extrasolar Earth-like planets to tidal dissipation by varying orbital, rheological, and heat transfer parameters. We employ a three-dimensional numerical method solving the coupled problem of mantle convection and tidal dissipation. A reference model mimicking a plate tectonic regime and reproducing Earths present-day heat output is considered. Four other models representing less efficient heat transfer regimes are also investigated. For these five initial models, we determine the orbital configurations under which a positive feedback between tidal dissipation and temperature evolution leads to a thermal runaway. In order to describe the occurrence of thermal runaways, we develop a scaling that relates the global dissipated power to a characteristic temperature and to the orbital parameters. For all numerical experiments sharing the same initial temperature conditions, we show that the reciprocal value of the runaway timescale depends linearly on the global dissipated power at the beginning of the simulation. In the plate tectonic-like regime, Earth-like planets in the habitable zone (HZ) of 0.1 M ☉ stars experience thermal runaways for 1:1 spin-orbit resonance if the eccentricity is sufficiently high (e>0.02 at a 4 day period, e>0.2 at a 10 day period). For less efficient convective regimes, runaways are obtained for eccentricities as low as ~0.004 at the inner limit of the HZ. In the case of 3:2 spin-orbit resonance, the occurrence of thermal runaways is independent of eccentricity and is predicted for orbital periods lower than 12 days. For less efficient convective regimes, runaways may occur at larger orbital periods potentially affecting the HZ of stars with a mass up to 0.4 M ☉. Whatever the convective regime and spin-orbit resonance, tidal heating within Earth-like planets orbiting in the HZ of stars more massive than 0.5 M ☉ is not significant.

Clathrate Hydrates: Implications for Exchange Processes in the Outer Solar System

Clathrate hydrates have unique physical and chemical properties because their crystalline structure contains voids that are filled by gas molecules. Their occurrence in many natural environments on Earth (deep-sea sediments, ice sheets) suggests their potential formation elsewhere in the Solar System where suitable pressure – temperature – gas composition conditions occur. This raises the question of their putative role in the storage and release of volatiles. This chapter first reviews the structure of clathrate hydrates, compares their thermo–physical properties with those of water ice, and addresses their stability and methods for estimating equilibria. Then we discuss their likely occurrence in Solar System objects and focus on the geophysical implications of their presence, from the perspectives of both the internal structure of and thermal transfer in icy Solar System bodies, as well as outgassing processes. A particular emphasis is placed on Titan and Enceladus, respectively the largest and the most geologically active satellites of Saturn, where the dissociation of clathrate hydrates could participate in atmospheric formation (Titan) and plume activity (Enceladus).

Titan's Hidden Ocean

Data from the Cassini-Huygens mission indicate that an ocean may exist beneath the solid surface of Saturns moon Titan.