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Le Touquet – Paris-Plage, France

Tobie G.,CNRS Nantes Laboratory of Planetology and Geodynamics | Gautier D.,LESIA | Hersant F.,University of Bordeaux 1 | Hersant F.,French National Center for Scientific Research
Astrophysical Journal | Year: 2012

In the present report, by using a series of data gathered by the Cassini-Huygens mission, we constrain the bulk content of Titan's interior for various gas species (CH4, CO2, CO, NH3, H 2S, 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 Titan's 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 Titan's atmosphere appears higher than the ratio in carbonaceous chondrites. © 2012. The American Astronomical Society. All rights reserved.. Source


Bockelee-Morvan D.,LESIA
Proceedings of the International Astronomical Union | Year: 2011

Comets are made of ices, organics and minerals that record the chemistry of the outer regions of the primitive solar nebula where they agglomerated 4.6 Gyr ago. Compositional analyses of comets can provide important clues on the chemical and physical processes that occurred in the early phases of Solar System formation, and possibly in the natal molecular cloud that predated the formation of the solar nebula. This paper presents a short review of our present knowledge of the composition of comets. Implications for the origin of cometary materials are discussed. © 2011 International Astronomical Union. Source


Doressoundiram A.,LESIA
Journal of Geophysical Research E: Planets | Year: 2015

Volcanism on Mercury has been indisputably identified at various locations on the surface, by means of both effusive and explosive volcanism. Its characterization is crucial to understand the evolution of the planet, in particular the thermal evolution of the mantle, and the volatile content of the planet. This analysis presents a detailed view of the pyroclastic deposits of the Caloris basin. Observations from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) are used to understand the spectral characteristics of the pyroclastic deposits, both in the visible and near-infrared. Additional calibration steps are proposed to reconcile the difference of absolute reflectance between the visible (VIS) and near-infrared (NIR) detectors. These calibration steps allow the use of the full spectral range of the MASCS instrument. Pyroclastic deposits exhibit a redder spectral slope in the VIS and NIR. This spectral slope diminishes toward the edge of the deposits to match that of Mercury's average surface. Spectral properties in the ultraviolet (UV) also change as a function of distance to the vent. Only the UV properties unambiguously separate the pyroclastic deposits from Mercury's average spectra. The spectral variations are consistent with a lower iron content of the pyroclastic deposits with respect to the average surface of Mercury, similar to what has been proposed for pyrolcastic deposits on the lunar surface. Nonetheless, given the limited illumination conditions diversity of the MASCS instrument, other causes such as grain size, space weathering, and bulk composition could also be accounted for the spectral variations. Variability of the pyroclastic deposits' properties within the entire basin are potentially identified between the three main clusters, and could be related to space weathering of deposits of different ages. ©2015. American Geophysical Union. All Rights Reserved. Source


Von Papen M.,University of Cologne | Saur J.,University of Cologne | Alexandrova O.,LESIA
Journal of Geophysical Research: Space Physics | Year: 2014

We analyze the statistical properties of magnetic field fluctuations measured by the Cassini spacecraft inside Saturn's magnetosphere. We introduce Saturn's magnetosphere as a new laboratory for plasma turbulence, where background magnetic field is strong (1 nT = 0.07), and the ion plasma βi is smaller than one. We also show that the dissipation of these magnetic field fluctuations has important implications for the magnetosphere. In a case study of the second orbit of Cassini around Saturn we show that at MHD scales, the spectra and the nature of fluctuations are characterized by large-scale nonstationary processes. The spectral slope varies between -0.8 and -1.7. At higher frequencies we observe a steeper spectrum with nearly constant power law exponent. A spectral break correlating with ion scales separates the two frequency ranges. We carry out a statistical study of high-frequency range fluctuations using the first seven orbits of Cassini. We find that the energy density of raw frequency spectra depends on radial distance from Saturn and thermal and magnetic pressures. However, normalized spectra depend only on ion plasma βi. Closer to Saturn the spectral slope is about -2.3 and for radial distances r>9 Rs the average slope is -2.6. The fluctuations have probability density functions with increasingly non-Gaussian tails and a power law increase of flatness with frequency, which indicates intermittency. We estimate the total energy flux contained in the turbulent cascade as 60-100 GW, which is on the same order of magnitude as needed to heat an adiabatically expanding plasma to the temperatures measured in Saturn's magnetosphere. Key Points A kinetic range turbulent cascade is found throughout the middle magnetosphere Plasma heating through turbulence is important for magnetospheric energy budget The turbulent cascade is intermittent and rather strong with spectral index 2.6 ©2014. American Geophysical Union. All Rights Reserved. Source


Thebault P.,LESIA
Celestial Mechanics and Dynamical Astronomy | Year: 2011

HD 196885 Ab is the most "extreme" planet-in-a-binary discovered to date, whose orbit places it at the limit for orbital stability. The presence of a planet in such a highly perturbed region poses a clear challenge to planet-formation scenarios. We investigate this issue by focusing on the planet-formation stage that is arguably the most sensitive to binary perturbations: the mutual accretion of kilometre-sized planetesimals. To this effect we numerically estimate the impact velocities dv amongst a population of circumprimary planetesimals. We find that most of the circumprimary disc is strongly hostile to planetesimal accretion, especially the region around 2.6 AU (the planet's location) where binary perturbations induce planetesimal-shattering dv of more than 1 kms-1. Possible solutions to the paradox of having a planet in such accretion-hostile regions are (1) that initial planetesimals were very big, at least 250 km (2) that the binary had an initial orbit at least twice the present one, and was later compacted due to early stellar encounters (3) that planetesimals did not grow by mutual impacts but by sweeping of dust (the "snowball" growth mode identified by Xie et al., in Astrophys J 724:1153, 2010b), or (4) that HD 196885 Ab was formed not by core-accretion but by the concurrent disc instability mechanism. All of these 4 scenarios remain however highly conjectural. © 2011 Springer Science+Business Media B.V. Source

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