Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p41a..08r&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P41A-08
Mathematics
Logic
[5744] Planetary Sciences: Fluid Planets / Orbital And Rotational Dynamics, [5749] Planetary Sciences: Fluid Planets / Origin And Evolution, [5770] Planetary Sciences: Fluid Planets / Tidal Forces, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Mercury's surface exhibits specific compressive features, called lobate scarps, that suggest that Mercury has experienced a change of shape during its history. These compressive features indicate global contraction and their apparent N-S preferred orientation suggests a possible effect of tidal despinning. The analysis of the terrains associated to the lobate scarps provide evidence for a formation after the end of the Late Heavy Bombardment (LHB). Another particularity is the presence of an intrinsic magnetic field intrinsic. This can be a proof for a liquid layer in the core. By adapting a model initially developed for Iapetus, we propose to evaluate the thermal evolution of Mercury and the associated despinning, contraction and core evolution. We perform 3D numerical simulations for a wide range of plausible initial conditions to evaluate: (1) the evolution of the temperature structure, (2) the resulting despinning rate, (3) the change of Mercury's shape, (4) the associated lithospheric stress field, (5) the onset of the core crystallization, and (6) the possibility of a liquid layer in the core at the present day. Thermal convection equations are solved for a fluid with temperature-dependent viscosity in a spherical geometry using the numerical tool OEDIPUS. Cooling and crystallization of the core are also taken into account. Different values for the activation energy, initial mantle temperature, mantle density, core radius and sulfur content are tested. The horizontally averaged temperature profiles and the radius of inner core obtained from the 3D internal model as a function of time can be used to compute the evolution of Mercury's rotation and shape. We use a visco-anelastic transient rheological model initially developed for the Earth’s uppermost mantle. We consider different values of eccentricity (taken constant during the evolution), initial rotation period and grain size. A coupling between mantle and core is also investigated. Four different parameterizations corresponding to different melting curves of an iron-sulfur alloy are proposed for the growth of the solid inner core. For all the simulations, despinning is completed before the LHB and before the onset of convection. Among the various parameters tested in our study, despinning is completed between 7.4 Myr (for a hot initial mantle temperature of 2100 K) and 84.5 Myr (for a fast initial rotation of 10 h). The most important source of variation is the initial rotation period. Surface stresses due to the despinning are acquired when the current value of rotational period is obtained. The stresses are oriented north-south between 60° N and 60°S and east-west at higher latitudes. Convection begins at about 200 Myr and the parameter giving the most important variation is the initial internal temperature. In all of our simulations, crystallization of the core occurs and a liquid layer in the core still exists at present day. The onset of the core crystallization is about 100 Myr after the onset of convection (at about 300 Myr). An increase of the sulfur content tends to inhibit and delay crystallization.
Cadek Ondřej
Choblet Gaël
Mocquet Antoine
Robuchon Guillaume
Tobie Gabriel
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