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New clues on the interior of Titan from its rotation state
- Benoît Noyelles, Francis Nimmo
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- Journal:
- Proceedings of the International Astronomical Union / Volume 9 / Issue S310 / July 2014
- Published online by Cambridge University Press:
- 05 January 2015, pp. 17-20
- Print publication:
- July 2014
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- Article
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The Saturnian satellite Titan is one of the main targets of the Cassini-Huygens mission, which revealed in particular Titan's shape, gravity field, and rotation state. The shape and gravity field suggest that Titan is not in hydrostatic equilibrium, that it has a global subsurface ocean, and that its ice shell is both rigid (at tidal periods) and of variable thickness. The rotational state of Titan consists of an expected synchronous rotation rate and an unexpectedly high obliquity (0.3○) explained by Baland et al. (2011) to be a resonant behavior. We here combine a realistic model of the ice shell and interior and a 6-degrees of freedom rotational model, in which the librations, obliquity and polar motion of the rigid core and of the shell are modelled, to constrain the structure of Titan from the observations. We consider the gravitational pull of Saturn on the 2 rigid layers, the gravitational coupling between them, and the pressure coupling at the liquid-solid interfaces.
We confirm the influence of the resonance found by Baland et al., that affects between 10 and 13% of the possible Titans. It is due to the 29.5-year periodic annual forcing. The resonant Titans can be obtained in situations in which a mass anomaly at the shell-ocean boundary (bottom loading) is from 80 to 92% compensated. This suggests a 250 to 280 km thick ocean below a 130 to 140 km thick shell, and is consistent with the degree-3 analysis of Hemingway 26 et al. (2013).
2 - The tectonics of Mercury
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- By Thomas R. Watters, Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC, Francis Nimmo, Department of Earth and Planetary Sciences, University of California, Santa Cruz
- Edited by Thomas R. Watters, Smithsonian Institution, Washington DC, Richard A. Schultz, University of Nevada, Reno
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- Book:
- Planetary Tectonics
- Published online:
- 30 March 2010
- Print publication:
- 17 December 2009, pp 15-80
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Summary
Summary
Mercury has a remarkable number of landforms that express widespread deformation of the planet's crustal materials. Deformation on Mercury can be broadly described as either distributed or basin-localized. The distributed deformation on Mercury is dominantly compressional. Crustal shortening is reflected by three landforms, lobate scarps, high-relief ridges, and wrinkle ridges. Lobate scarps are the expression of surface-breaking thrust faults and are widely distributed on Mercury. High-relief ridges are closely related to lobate scarps and appear to be formed by high-angle reverse faults. Wrinkle ridges are landforms that reflect folding and thrust faulting and are found largely in smooth plains material within and exterior to the Caloris basin. The Caloris basin has an array of basin-localized tectonic features. Basin-concentric wrinkle ridges in the interior smooth plains material are very similar to those found in lunar mascon basins. The Caloris basin also has the only clear evidence of broad-scale, extensional deformation. Extension of the interior plains materials is expressed as a complex pattern of basin-radial and basin-concentric graben. The graben crosscut the wrinkle ridges in Caloris, suggesting that they are among the youngest tectonic features on Mercury. The tectonic features have been used to constrain the mechanical and thermal structure of Mercury's crust and lithosphere and to test models for the origin of tectonic stresses. Modeling of lobate scarp thrust faults suggests that the likely depth to the brittle–ductile transition (BDT) is 30 to 40 km.
7 - Tectonics of the outer planet satellites
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- By Geoffrey C. Collins, Wheaton College, Norton, William B. McKinnon, Washington University, Saint Louis, Jeffrey M. Moore, NASA Ames Research Center, Moffett Field, Francis Nimmo, University of California, Santa Cruz, Robert T. Pappalardo, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Louise M. Prockter, Applied Physics Laboratory, Laurel, Paul M. Schenk, Lunar and Planetary Institute, Houston
- Edited by Thomas R. Watters, Smithsonian Institution, Washington DC, Richard A. Schultz, University of Nevada, Reno
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- Book:
- Planetary Tectonics
- Published online:
- 30 March 2010
- Print publication:
- 17 December 2009, pp 264-350
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Summary
Summary
Tectonic features on the satellites of the outer planets range from the familiar, such as clearly recognizable graben on many satellites, to the bizarre, such as the ubiquitous double ridges on Europa, the twisting sets of ridges on Triton, or the isolated giant mountains rising from Io's surface. All of the large and middle-sized outer planet satellites except Io are dominated by water ice near their surfaces. Though ice is a brittle material at the cold temperatures found in the outer solar system, the amount of energy it takes to bring it close to its melting point is lower than for a rocky body. Therefore, some unique features of icy satellite tectonics may be influenced by a near-surface ductile layer beneath the brittle surface material, and several of the icy satellites may possess subsurface oceans. Sources of stress to drive tectonism are commonly dominated by the tides that deform these satellites as they orbit their primary giant planets. On several satellites, the observed tectonic features may be the result of changes in their tidal figures, or motions of their solid surfaces with respect to their tidal figures. Other driving mechanisms for tectonics include volume changes due to ice or water phase changes in the interior, thermoelastic stress, deformation of the surface above rising diapirs of warm ice, and motion of subsurface material toward large impact basins as they fill in and relax.