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The pressure torque by a liquid core that drove Mercury to the nominal Cassini state of rotation is dominated by the torque from the solid inner core. The gravitational torque exerted on Mercury's mantle from an asymmetric solid inner core increases the equilibrium obliquity of the mantle spin axis. Since the observed obliquity of the mantle must be compatible with the presence of a solid inner core, the moment of inertia inferred from the occupancy of the Cassini state must be reduced to compensate the torque from the inner core and bring Mercury's spin axis to the observed position. The unknown size and shape of the inner core means that the moment of inertia is more uncertain than previously inferred.

Although Earth is the only known planet on which plate tectonics operates, many small- and large-scale tectonic landforms indicate that deformational processes also occur on the other rocky planets. Although the mechanisms of deformation differ on Mercury, Venus, and Mars, the surface manifestations of their tectonics are frequently very similar to those found on Earth. Furthermore, tectonic processes invoked to explain deformation on Earth before the recognition of horizontal mobility of tectonic plates remain relevant for the other rocky planets. These connections highlight the importance of drawing analogies between the rocky planets for characterizing deformation of their lithospheres and for describing, applying appropriate nomenclature, and understanding the formation of their resulting tectonic structures. Here we characterize and compare the lithospheres of the rocky planets, describe structures of interest and where we study them, provide examples of how historic views on geology are applicable to planetary tectonics, and then apply these concepts to Mercury, Venus, and Mars. Key words: planetary tectonics, planetary geology, Mercury, Venus, Mars. Bien que la Terre soit la seule planete connue sur laquelle il y a une tectonique des plaques, de nombreuses formes de relief tectoniques de petite et grande envergure indiquent que des processus de deformation se produisent egalement sur d'autres planetes rocheuses. Si les mecanismes de deformation sur Mercure, Venus et Mars different, les manifestations en surface de leurs tectoniques respectives sont souvent tres semblables a celles observees sur la Terre. En outre, des processus tectoniques invoques pour expliquer la deformation sur la Terre avant la reconnaissance de la mobilite horizontale de plaques tectoniques revetent toujours une pertinence pour les autres planetes rocheuses. Ces connexions soulignent l'importance d'etablir des analogies entre les planetes rocheuses pour caracteriser la deformation de leurs lithospheres et pour decrire, en appliquant la bonne nomenclature, et comprendre la formation des structures tectoniques en resultant. Nous caracterisons et comparons les lithospheres des planetes rocheuses, decrivons les structures d'interet et les lieux ou elles sont etudiees, et presentons des exemples d'application de notions historiques a la tectonique planetaire pour ensuite appliquer ces concepts a Mercure, Venus et Mars. [Traduit par la Redaction] Mots-cles: tectonique des planetes, geologie des planetes, Mercure, Venus, Mars., Introduction Without a theoretical framework, such as the plate tectonic models, of the spatial relations of rock bodies to one another in an orogenic belt, however wrong they may be, [...]

To gain insight into the thickness of the crust of Mercury, we use gravity and topography data acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft to calculate geoid-to-topography ratios over the northern hemisphere of the planet. For an Airy model for isostatic compensation of variations in topography, we infer an average crustal thickness of 35 ± 18 km. Combined with the value of the radius of the core of Mercury, this crustal thickness implies that Mercury had the highest efficiency of crustal production among the terrestrial planets. From the measured abundance of heat-producing elements on the surface, we calculate that the heat production in the mantle from long-lived radioactive elements at 4.45 Ga was greater than 5.4 ×10-12W/kg. By analogy with the Moon, the relatively thin crust of Mercury allows for the possibility that major impact events, such as the one that formed the Caloris basin, excavated material from Mercury's mantle.

The combination of the radio tracking of the MESSENGER spacecraft and Earth-based radar measurements of the planet's spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density \rho, moment of inertia C, and moment of inertia of the outer solid shell Cm. This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parametrized by the tidal potential Love number k2. We investigate the tidal response for sets of interior models that are compatible with the available constraints (\rho, C, and Cm). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell, and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain-size of 1~cm, we calculate that the tidal k2 of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.

The recent determination of the gravity field of Mercury and new Earth-based radar observations of the planet's spin state afford the opportunity to explore Mercury's internal structure. These observations provide estimates of two measures of the radial mass distribution of Mercury: the normalized polar moment of inertia and the fractional polar moment of inertia of the solid portion of the planet overlying the liquid core. Employing Monte Carlo techniques, we calculate several million models of the radial density structure of Mercury consistent with its radius and bulk density and constrained by these moment of inertia parameters. We estimate that the top of the liquid core is at a radius of 2020 ± 30 km, the mean density above this boundary is 3380 ± 200 kg m-3, and the density below the boundary is 6980 ± 280 kg m-3. We find that these internal structure parameters are robust across a broad range of compositional models for the core and planet as a whole. Geochemical observations of Mercury's surface by MESSENGER indicate a chemically reducing environment that would favor the partitioning of silicon or both silicon and sulfur into the metallic core during core-mantle differentiation. For a core composed of Fe-S-Si materials, the thermodynamic properties at elevated pressures and temperatures suggest that an FeS-rich layer could form at the top of the core and that a portion of it may be presently solid. Key PointsNew MESSENGER and Earth-based radar data provide Mercury's moments of inertiaMercury's core-mantle boundary is 420 +/- 30 km below the planet's surfaceThe core may be compositionally segregated ©2013. American Geophysical Union. All Rights Reserved.

Current geophysical knowledge of the planet Mercury is based upon observations from ground-based astronomy and flybys of the Mariner 10 spacecraft, along with theoretical and computational studies. Mercury has the highest uncompressed density of the terrestrial planets and by implication has a metallic core with a radius approximately 75% of the planetary radius. Mercury's spin rate is stably locked at 1.5 times the orbital mean motion. Capture into this state is the natural result of tidal evolution if this is the only dissipative process affecting the spin, but the capture probability is enhanced if Mercury's core were molten at the time of capture. The discovery of Mercury's magnetic field by Mariner 10 suggests the possibility that the core is partially molten to the present, a result that is surprising given the planet's size and a surface crater density indicative of early cessation of significant volcanic activity. A present-day liquid outer core within Mercury would require either a core sulfur content of at least several weight percent or an unusual history of heat loss from the planet's core and silicate fraction. A crustal remanent contribution to Mercury's observed magnetic field cannot be ruled out on the basis of current knowledge. Measurements from the MESSENGER orbiter, in combination with continued ground-based observations, hold the promise of setting on a firmer basis our understanding of the structure and evolution of Mercury's interior and the relationship of that evolution to the planet's geological history. © 2007 Springer Science+Business Media B.V.