Christophe Sotin

Episodic outgassing as the origin of atmospheric methane on Titan

Saturns largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time. Titan fly-by observations and ground-based observations rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titans history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titans atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.

Perennial water ice identified in the south polar cap of Mars.

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

The identification of liquid ethane in Titan's Ontario Lacus

Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan’s dense, nitrogen-dominated atmosphere. Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan’s Ontario Lacus.

Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan

Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar (ref. 1) and, similar to the Earth, N2 is the main component of the atmosphere. Methane is the second most important component, but it is photodissociated on a timescale of 107 years (ref. 3). This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure ∼30 km in diameter that does not resemble any features seen on other icy satellites. We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titans atmosphere.

Computation of seismic profiles from mineral physics: the importance of the non-olivine components for explaining the 660 km depth discontinuity

The recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals. The updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles. These profiles are chosen to stress out the influence of non-olivine minerals on seismic parameters, and to represent cold and horizontally averaged temperature profiles of the Earths mantle. In a first part, starting compositions of pyrolite and piclogite and a single layer convection are assumed. The results clearly point out the importance of the non-olivine part of the mineralogy. Two scenarios are found to explain the 660 km depth discontinuity, .

Three-dimensional thermal convection in an iso-viscous, infinite Prandtl number fluid heated from within and from below: applications to the transfer of heat through planetary mantles

Numerical experiments have been carried out to explore the efficiency of heat transfer through a three-dimensional layer heated from both within and below as it is the case for the mantle of earth-like planets. A systematic study for Rayleigh numbers (Ra) between 105 and 107 and non-dimensional internal heating rate (Hs) between 0 and 40 allows us to investigate the pattern of convection and the thermal characteristics of the layer in a range of parameters relevant to mantle convection in earth-like planets. Inversion of the results for the mean temperature and non-dimensional heat flux at the top and the bottom boundaries yields simple parameterization of the heat transfer. It is shown that the mean temperature of the convective fluid (θ) is the sum of the temperature that would exist with no internal heating and a contribution of the non-dimensional internal heating rate (Hs). As predicted by thermal boundary layer analysis, the non-dimensional heat flux at the upper boundary layer can be described by Q=[(Ra)/(Raδ)]1/3θ4/3 with θ=0.5+1.236[(Hs)3/4/(Ra)1/4], and Raδ being the thermal boundary layer Rayleigh number equal to 24.4. In agreement with laboratory experiments, this value slightly increases with the value of the Rayleigh number. This value is identical to that obtained for fluids heated from within only. In most cases, the hot plumes that form at the lower thermal boundary layer do not reach the upper boundary layer. No simple law has been found to describe the heat transfer through the lower thermal boundary layer, but the bottom heat flux can be determined using the global energy balance. The thermal boundary layer analysis performed in this study allows us to extrapolate our results to 3D spherical geometry and our predictions are in good agreement with numerical experiments described in the literature. A simple case of spherical 3D convection has been performed and provides the same thermal history of planetary mantles than that obtained from 3D numerical runs. Compared to previous parameterized analysis, this study shows that the behaviour of the thermal boundary layers is much different than that predicted by experiments for a fluid heated only from below: at similar Rayleigh numbers, the mean temperature is larger and the surface heat flux is much larger. It seems therefore necessary to reconsider previous models of the thermal evolution of planetary mantles.

Compositional maps of Saturn's moon Phoebe from imaging spectroscopy

The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed. By contrast, Saturns regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed. Here we report imaging spectroscopy of Phoebe resulting from the Cassini–Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO2, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebes surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin.

The surface of Syrtis Major: Composition of the volcanic substrate and mixing with altered dust and soil

Syrtis Major is an old, low relief volcanic plateau near the equatorial regions of Mars. It is a persistent low-albedo feature on the planet and is thought to contain a high abundance of exposed bedrock and/or locally derived surface material and debris. Spatially resolved variations in surface spectral properties, and therefore composition, are investigated with data from the Imaging Spectrometer for Mars (ISM) instrument. ISM obtained 128 wavelength channel spectra from 0.76 to 3.16 μm for contiguous pixels approximately 22 × 22 km in size across much of the plateau. The value and spatial distribution of four primary spectral variables (albedo, continuum slope, wavelength of the ferric-ferrous band minimum, area of the ferric-ferrous absorption) are mapped and coregistered to Viking digital photomosaics. Analysis of these maps shows that although there is a high degree of overall spectral variability on the plateau, the key indicators of mafic mineralogy are relatively homogeneous. Detailed examination of reflectance spectra from representative areas across the plateau indicate the volcanic surface is dominated by augite-bearing basalts and the pyroxene composition in the basalts is estimated to be 0.275± 0.075 Ca/(Ca+Fe+Mg) and 0.3± 0.1 Fe/(Fe+Ca+Mg). Additional mineral components may include olivine, feldspar, and glass. Most of the spectral variability on the plateau is interpreted to result from mixing of volcanic bedrock and/or locally derived surface material and debris with highly altered dust and soil. In western Syrtis Major the altered material is a transient component on the surface or occurs in large spatially coherent patches (e.g., crater rims). In eastern Syrtis Major it is apparent that the dust components are firmly fixed to the basaltic substrate as a stable oxide rind or coating.

Europa: Tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting

Europa. We show that tidal energy can be preferentially focused in rising plumes for viscosities in agreement with laboratory experiments. When the plume cores reach the base of the outer cold brittle layer, they spread laterally, causing shallow melting, disruption, and formation of terrain similar to lenticulae and chaos. We show that this mechanism can readily explain the major characteristics of chaos and lenticulae even if the average outer solid ice layer thickness is larger than 20 km.

A STUDY OF THE ACCURACY OF MASS-RADIUS RELATIONSHIPS FOR SILICATE-RICH AND ICE-RICH PLANETS UP TO 100 EARTH MASSES

A mass-radius relationship is proposed for solid planets and solid cores ranging from 1 to 100 Earth-mass planets. It relies on the assumption that solid spheres are composed of iron and silicates, around which a variable amount of water is added. The M-R law has been set up assuming that the planetary composition is similar to the averaged composition for silicates and iron obtained from the major elements ratio of 94 stars hosting exoplanets. Except on Earth for which a tremendous amount of data is available, the composition of silicate mantles and metallic cores cannot be constrained. Similarly, thermal profiles are poorly known. In this work, the effect of compositional parameters and thermal profiles on radii estimates is quantified. It will be demonstrated that uncertainties related to composition and temperature are of second order compared to the effect of the water amount. The Super-Earths family includes four classes of planets: iron-rich, silicate-rich, water-rich, or with a thick atmosphere. For a given mass, the planetary radius increases significantly from the ironrich to the atmospheric-rich planet. Even if some overlaps are likely, M-R measurements could be accurate enough to ascertain the discovery of an earth-like planet .The present work describes how the amount of water can be assessed from M-R measurements. Such an estimate depends on several assumptions including i) the accuracy of the internal structure model and ii) the accuracy of mass and radius measurements. It is shown that if the mass and the radius are perfectly known, the standard deviation on the amount of water is about 4.5 %. This value increases rapidly with the radius uncertainty but does not strongly depend on the mass uncertainty.