31st Annual Meeting of the DPS, October 1999
Session 18. The Moon and Mercury Posters
Poster Group I, Monday-Wednesday, October 11, 1999, , Kursaal Center

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[18.02] New thermal evolution models suggesting a hot, partially molten Mercurian interior

V. Conzelmann, T. Spohn (Institut f\"ur Planetologie, WWU M\"unster, Germany)

New thermal evolution models of Mercury have been calculated using an axisymmetric convection code for the silicate mantle and an energy balance equation for the iron core. The model allows to estimate the strength of the magnetic field produced within the core. Various Newtonian rheologies of mantle rock, including pressure dependent viscosities, have been tested as well as different initial conditions and core sulfur contents. It is assumed that the planet is refractory in composition, in particular with respect to core sulfur and mantle heat producing elements (cf. Cameron et al. in Mercury, pp. 692--708, 1988). For activation energies characteristic of mantle rock (430 to 460 kJ mole-1 K-1) Mercury's sublithosphere mantle remains at supersolidus temperatures up to the present day. This suggests a partially molten mantle with early partial melt abundant enough to explain early volcanic activity. A partially molten mantle can only be avoided if the assumed activation energy for creep and/or the heat source density are significantly reduced from the values used here. A thick rheological lithosphere grows quickly and retards cooling of the interior aswell as it frustrates continued volcanic activity. A solid inner core grows quickly early in the evolution; its growth rate decreases significantly over the first Ga. Nevertheless, growth of the inner core releases enough energy to drive an intrinsic dynamo by chemical convection until present. Earlier models based on simple parameterized convection codes show much faster cooling and therefore require Mercury to have a chondritic composition to keep part of the core liquid and produce a present magnetic field. Mercury's present state with a magnetic field and a radius decrease of 2 km since the period of heavy bombardment about 3.9 Ga ago due to thermal contraction can be explained by the dynamical models presented here even if a rather refractory composition with little sulfur in the core and with a silicate layer depleted in radioactive elements in accordance with cosmochemical models is used.

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The author(s) of this abstract have provided an email address for comments about the abstract: conzelm@uni-muenster.de

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